Piston Weights?
- Thread starter henris42
- Start date
BiTurbo228
New Member
- Messages
- 27
Yeah absolutely, though the Maserati is so oversquare that any step to improve that moves it in the right direction.
Another positive benefit (though again, small), is that longer strokes and shorter rod ratios are able to make better use of small ports and valves. The faster piston acceleration as it leaves TDC helps create a stronger pressure differential earlier in the intake stroke, increasing the amount of charge drawn in.
Is it enough to make it a viable proposition for the cost and effort involved? Probably not. But it's nice that it appears cost is the only negative, and all other aspects are improvements. Often with stroker engines you end up compromising on terrible rod ratios and borderline intolerable reciprocating/rotational forces. Check out stroked M20s if you want to see a situation where all of the pros and cons are flipped. They're also valve-limited, but M54 stroker cranks drop the rod ratio to a borderline-intolerable 1.51:1, make them significantly undersquare, and necessitate the use of forged components otherwise the reciprocating and rotational weight would limit the rpm significantly (say, 6000rpm max instead of 7000+). Their only redeeming feature is that M54B30 cranks are cheap, plentiful, and fit without significant modification.
An important consideration, and one I'd need to open a crankcase to discover. 2mm is not a lot of extra throw, but if the crankcase is already very tight it may cause issues. Potentially rectifiable, dependent on how bad they were, but that'd need investigation.
Liner length would be another consideration. I'm not sure how close the pistons get to the bottom of the liners during their normal operation, but 4mm extra downwards distance may cause issues if the liners are already short. Would need to be carefully measured (as would piston clearance to the crank throws). These are potentially rectifiable through shorter piston skirts, but again, would need to measure to find that out.
Haha I genuinely think I would enjoy that, though there might be a non-zero chance of getting murderedSounds like you two should go down the pub...
Eb
BiTurbo228
New Member
- Messages
- 27
Just to back up what Alpa was saying about the ports and valves being comparatively small on these engines. I've been sifting through the FIA Historic Database which is a treasure trove of technical information for cars up to the 80s/early 90s. They're homologation documents, so state the technical information of the standard road going cars (for the most part), which is useful.
Having comparatively small valves and ports for a given cylinder CC seems to be a bit of an Italian design philosophy in the 80s/90s, especially with multivalve engines (though Alfa bucks the trend). The FIA database only has information on the 18v GrpA Maserati Biturbos rather than the 24v later cars, but from what I gather the designs are comparable in their smallish ports and valves. This appears to be true for the 2.5l 18v Maserati engine, all 16v variants of the Fiat Twincam (e.g. Delta Integrale), and the 4v Ferrari V8s of the 80s (308 Quattrovalvole and 288 GTO).
The metrics I've looked at are valve area and port area (at cylinder head face) over cylinder volume (both in mm2 per cc). Often you see rules of thumb about valve sizes being dictated by bore diameter, but to take an extreme example a 90mm bore with a 2mm stroke is going to need a much smaller valve than a 90mm bore with a 200mm stroke. Personally I feel benchmarking it against cylinder volume is more appropriate, though I must admit I don't have the empirical knowledge of the folks who came up with the 85% of bore diameter rule of thumb.
Over the 84 multivalve engines I've recorded so far, the average of inlet valve area to bore volume is 3.6mm2/cc, and the port area is 2.7mm2/cc. Maserati 18v 2.5l is 3.0 and 2.0 respectively. Delta Integrale 16v is 3.8 and 2.0 (so bigger valves, but just as small ports). 308 QV is 3.6 and 2.3 (bigger valves, bigger ports but still on the smaller side). 288 GTO is 3.7 and 2.3 (bigger valves than average, but same size ports as 308 QV).
Small ports doesn't necessarily mean small power though. The two multivalve engines on my list so far with the smallest ports to cylinder displacement ratio are the BMW S14B23 (E30 M3), Mercedes 16v Cosworth (190E 2.3 16v), and Saab B202 (900 16v Turbo) all at 1.8mm2/cc. This is followed shortly by the Cosworth BDA and Vauxhall C20XE at 1.9mm2/cc. Not engines that are known for poor power density. From what I understand, the size of the port is more related to the maximum rpm you can run for a given cylinder volume as it'll push the incoming air to the point of stall which will kill flow. The ideal situation is to have small ports for good charge velocity (sized appropriately for your rpm), and really good flow. I wonder if anyone has had the Maserati heads on a flowbench to see what they're like.
Valves are undeniably small though. There is only multi-valve one engine on the list with smaller valve area to cylinder displacement (Saab B234 at 2.8mm2/cc). Then it's the 2.5l 18v Maserati (3.0mm2/cc). If the valves are the same in the 2.8l then that's the smallest I have on the list at 2.7mm2/cc. This might suggest that an increase in valve diameter would be beneficial even if you're leaving the port diameter unchanged. 29.9mm BMW M50B20 valves would take that ratio to 3.4mm2/cc which is still on the smaller side of normal but much closer to the rest of the pack. That'd help with flow area under the curve as the valve curtain area would be bigger at low lift conditions (where the valve spends most of its time), barring any shrouding issues.
I have the port diameter of the 24v Maserati head at 22mm, but wasn't sure if that was at the port face (so comparable with the rest of the list), or diameter at the throat. If it is the right measurement, that'd put the 24v heads way below the rest of the pack for port diameter (1.6mm2/cc for the 2.8l, 1.8mm2/cc for alpa's frankenstein 2.5l).
Also, as a caveat, judging port area at the head face isn't the ideal metric as there will be different taper angles and lengths of port which will change what the narrowest point of the port might be, but as it's the only information that's available for lots of engines without tracking down a ton of heads to measure, it'll have to do.
Having comparatively small valves and ports for a given cylinder CC seems to be a bit of an Italian design philosophy in the 80s/90s, especially with multivalve engines (though Alfa bucks the trend). The FIA database only has information on the 18v GrpA Maserati Biturbos rather than the 24v later cars, but from what I gather the designs are comparable in their smallish ports and valves. This appears to be true for the 2.5l 18v Maserati engine, all 16v variants of the Fiat Twincam (e.g. Delta Integrale), and the 4v Ferrari V8s of the 80s (308 Quattrovalvole and 288 GTO).
The metrics I've looked at are valve area and port area (at cylinder head face) over cylinder volume (both in mm2 per cc). Often you see rules of thumb about valve sizes being dictated by bore diameter, but to take an extreme example a 90mm bore with a 2mm stroke is going to need a much smaller valve than a 90mm bore with a 200mm stroke. Personally I feel benchmarking it against cylinder volume is more appropriate, though I must admit I don't have the empirical knowledge of the folks who came up with the 85% of bore diameter rule of thumb.
Over the 84 multivalve engines I've recorded so far, the average of inlet valve area to bore volume is 3.6mm2/cc, and the port area is 2.7mm2/cc. Maserati 18v 2.5l is 3.0 and 2.0 respectively. Delta Integrale 16v is 3.8 and 2.0 (so bigger valves, but just as small ports). 308 QV is 3.6 and 2.3 (bigger valves, bigger ports but still on the smaller side). 288 GTO is 3.7 and 2.3 (bigger valves than average, but same size ports as 308 QV).
Small ports doesn't necessarily mean small power though. The two multivalve engines on my list so far with the smallest ports to cylinder displacement ratio are the BMW S14B23 (E30 M3), Mercedes 16v Cosworth (190E 2.3 16v), and Saab B202 (900 16v Turbo) all at 1.8mm2/cc. This is followed shortly by the Cosworth BDA and Vauxhall C20XE at 1.9mm2/cc. Not engines that are known for poor power density. From what I understand, the size of the port is more related to the maximum rpm you can run for a given cylinder volume as it'll push the incoming air to the point of stall which will kill flow. The ideal situation is to have small ports for good charge velocity (sized appropriately for your rpm), and really good flow. I wonder if anyone has had the Maserati heads on a flowbench to see what they're like.
Valves are undeniably small though. There is only multi-valve one engine on the list with smaller valve area to cylinder displacement (Saab B234 at 2.8mm2/cc). Then it's the 2.5l 18v Maserati (3.0mm2/cc). If the valves are the same in the 2.8l then that's the smallest I have on the list at 2.7mm2/cc. This might suggest that an increase in valve diameter would be beneficial even if you're leaving the port diameter unchanged. 29.9mm BMW M50B20 valves would take that ratio to 3.4mm2/cc which is still on the smaller side of normal but much closer to the rest of the pack. That'd help with flow area under the curve as the valve curtain area would be bigger at low lift conditions (where the valve spends most of its time), barring any shrouding issues.
I have the port diameter of the 24v Maserati head at 22mm, but wasn't sure if that was at the port face (so comparable with the rest of the list), or diameter at the throat. If it is the right measurement, that'd put the 24v heads way below the rest of the pack for port diameter (1.6mm2/cc for the 2.8l, 1.8mm2/cc for alpa's frankenstein 2.5l).
Also, as a caveat, judging port area at the head face isn't the ideal metric as there will be different taper angles and lengths of port which will change what the narrowest point of the port might be, but as it's the only information that's available for lots of engines without tracking down a ton of heads to measure, it'll have to do.
Last edited:
BiTurbo228
New Member
- Messages
- 27
Thought something pictographic would help explain better than words:
For reference, a 29.9mm inlet on the 2.5l 18v would slot between the C20XE 16v and the Toyota 7M. 24v Maserati head is pretty much the same ratio as the 18v 2.5l Maserati.
For reference, if you ported the 2.5l head to the same diameter as the 2.8l head it'd slot in just below the S50B30, about equal to the Opel CIH. 24v Maserati head would be off to the right of the Saab B.
For reference, a 29.9mm inlet on the 2.5l 18v would slot between the C20XE 16v and the Toyota 7M. 24v Maserati head is pretty much the same ratio as the 18v 2.5l Maserati.
For reference, if you ported the 2.5l head to the same diameter as the 2.8l head it'd slot in just below the S50B30, about equal to the Opel CIH. 24v Maserati head would be off to the right of the Saab B.
The 18v heads have a usual port/valve ratio, the one suggested by the rules of thumb. I think I mentioned this.
You can't increase 18v seats because the heads already crack when both intake are of the same size. So you can only put 30mm valves and enlarge the passage in the seats (use bmw 6mm stem valves with custom guides).
People make about 320 (350?) whp on the 18v heads with boosts below 1.5 bars, it's not bad. Again the 18v heads are good enough but can't be largely improved. The 24v are a marketing BS, they need a major rework to make them good.
I'm not sure you can judge stock configurations from the grA numbers. I'm not sure you can trust their numbers.
My experience with old Alfa engines is that their ports were large enough but not good at all: sharp turns. The shape is very important.
24v head ports have the perfect shape: a staight line, like on ferrari heads.
The 18v heads have one straight and one curved ports, an interesting combination. Long nose injectors would allow to spray essentially into the straight one. However the 18v intake manifold suffers from the charge robing between the runners. Same problem observed on nascar intakes when they switched to injection but kept the carb manifold.
You can't increase 18v seats because the heads already crack when both intake are of the same size. So you can only put 30mm valves and enlarge the passage in the seats (use bmw 6mm stem valves with custom guides).
People make about 320 (350?) whp on the 18v heads with boosts below 1.5 bars, it's not bad. Again the 18v heads are good enough but can't be largely improved. The 24v are a marketing BS, they need a major rework to make them good.
I'm not sure you can judge stock configurations from the grA numbers. I'm not sure you can trust their numbers.
My experience with old Alfa engines is that their ports were large enough but not good at all: sharp turns. The shape is very important.
24v head ports have the perfect shape: a staight line, like on ferrari heads.
The 18v heads have one straight and one curved ports, an interesting combination. Long nose injectors would allow to spray essentially into the straight one. However the 18v intake manifold suffers from the charge robing between the runners. Same problem observed on nascar intakes when they switched to injection but kept the carb manifold.
Last edited:
BiTurbo228
New Member
- Messages
- 27
Thanks, useful stuff.
The Group A figures seem to tally with actual road car engines for all of the ones that I have real-world data on (about 20 or so out of the 84 4v heads), so seem pretty accurate. The manufacturers do seem to have built in a fudge-factor by stating the standard road-car port dimensions but adding huge tolerances (-2%,+4%), presumably to allow for porting on their race engines. "Honest guv, we just have really poor manufacturing tolerances, that's why this particular port is much bigger than a road engine." I know there was a lot of rules-bending in Group A so I've been on the lookout for suspect data!
The FIA data gives the 2.5l 18v intake valve size as 28.15mm, though I note Maseratinet offer a 27mm intake valve for 18v engines. Need to dig out my 228 workshop manual and cross-reference the data if it's in there (unless you happen to know this off the top of your head!).
Useful to know about the 18v seats. Larger valves in the same seat should still offer some low-lift benefits, but it would be nice if the seats could be enlarged too. Hey ho. Charge robbing doesn't sound like much fun. Shall have a look at the firing order to see which ones are likely to pose problems. If we're lucky a twin plenum setup might help alleviate that.
I think you're spot on with the 24v heads being a marketing exercise. Or if we want to be charitable, perhaps an engineering experiment in super-small but well flowing ports. I can't say it's worked too badly considering the output, but not much headroom for more as you say.
There was some very interesting (and slightly heated) discussion over on Speed Talk about port shape, with one camp being very much of the opinion that straight ports are better, and another camp being of the opinion that straightness is secondary to ensuring the short side radius is really generously curved. The two opposites being a ruler-straight port with no short side radius at all, and a port that is entirely made up of one gentle short side radius. From what I gathered, both showed the potential to flow very well under idealised circumstances, though that knowledge is of limited use when you get to the frequently flawed world of actually making a port in real life. I suppose the two design philosophies could be described as 'make sure your short side radius isn't terrible' and 'can't have a terrible short side radius if you don't have a short side radius'.
All beyond my current capability to verify of course, but interesting to know regardless.
The Group A figures seem to tally with actual road car engines for all of the ones that I have real-world data on (about 20 or so out of the 84 4v heads), so seem pretty accurate. The manufacturers do seem to have built in a fudge-factor by stating the standard road-car port dimensions but adding huge tolerances (-2%,+4%), presumably to allow for porting on their race engines. "Honest guv, we just have really poor manufacturing tolerances, that's why this particular port is much bigger than a road engine." I know there was a lot of rules-bending in Group A so I've been on the lookout for suspect data!
The FIA data gives the 2.5l 18v intake valve size as 28.15mm, though I note Maseratinet offer a 27mm intake valve for 18v engines. Need to dig out my 228 workshop manual and cross-reference the data if it's in there (unless you happen to know this off the top of your head!).
Useful to know about the 18v seats. Larger valves in the same seat should still offer some low-lift benefits, but it would be nice if the seats could be enlarged too. Hey ho. Charge robbing doesn't sound like much fun. Shall have a look at the firing order to see which ones are likely to pose problems. If we're lucky a twin plenum setup might help alleviate that.
I think you're spot on with the 24v heads being a marketing exercise. Or if we want to be charitable, perhaps an engineering experiment in super-small but well flowing ports. I can't say it's worked too badly considering the output, but not much headroom for more as you say.
There was some very interesting (and slightly heated) discussion over on Speed Talk about port shape, with one camp being very much of the opinion that straight ports are better, and another camp being of the opinion that straightness is secondary to ensuring the short side radius is really generously curved. The two opposites being a ruler-straight port with no short side radius at all, and a port that is entirely made up of one gentle short side radius. From what I gathered, both showed the potential to flow very well under idealised circumstances, though that knowledge is of limited use when you get to the frequently flawed world of actually making a port in real life. I suppose the two design philosophies could be described as 'make sure your short side radius isn't terrible' and 'can't have a terrible short side radius if you don't have a short side radius'.
All beyond my current capability to verify of course, but interesting to know regardless.
Last edited:
Your 2.8 18v has two 28.3 mm intake valves. The very first biturbo had one 27mm valve.
For 24v heads the workshop man states 33mm intakes. It's wrong, they are 30mm. All 24v heads are the same except the presence of the distributor holder in early ones and some minor changes in the casting and machining.
To know what ports are the best it's not complicated: look at the F1/indycar/LMP (24hdM)/wrc modern engines. All these categories are today limited in their fuel consumption, so they have to be the most efficient possible.
For 24v heads the workshop man states 33mm intakes. It's wrong, they are 30mm. All 24v heads are the same except the presence of the distributor holder in early ones and some minor changes in the casting and machining.
To know what ports are the best it's not complicated: look at the F1/indycar/LMP (24hdM)/wrc modern engines. All these categories are today limited in their fuel consumption, so they have to be the most efficient possible.
Last edited:
BiTurbo228
New Member
- Messages
- 27
Thanks, dead useful.
Can you remember if your 22mm 24v port measurement was at the cylinder head face?
You're right on looking at the top flight of motorsports, and according to the Speed Talk folk they seem to have as close to ruler-straight vertical ports as they can get away with. The alternative camp was arguing that that's all well and good with a ground-up engine design, but with a real-world head in front of you it's more of a trade-off around having a good generous short side radius (and other things that manufacturers of road cars care about, like bonnet clearance). They were stating that even a very small short side radius can be very disruptive if it's too sharp, and nullify the advantages of a straight port over a curved one.
Though, again, I lack the knowledge to verify either of these, or how they would apply to the 4v head. A flowbench would sort that, which is on the cards for me, but I've got a garage to build first!
Can you remember if your 22mm 24v port measurement was at the cylinder head face?
You're right on looking at the top flight of motorsports, and according to the Speed Talk folk they seem to have as close to ruler-straight vertical ports as they can get away with. The alternative camp was arguing that that's all well and good with a ground-up engine design, but with a real-world head in front of you it's more of a trade-off around having a good generous short side radius (and other things that manufacturers of road cars care about, like bonnet clearance). They were stating that even a very small short side radius can be very disruptive if it's too sharp, and nullify the advantages of a straight port over a curved one.
Though, again, I lack the knowledge to verify either of these, or how they would apply to the 4v head. A flowbench would sort that, which is on the cards for me, but I've got a garage to build first!
So I went to check the reamers I used on the 24v heads. 21.5 to 23.5 by 0.5mm steps.
Now this is what I wrote the 8th Dec 2021:
For a 30mm valve this makes 30 * 0.81 = 24.3mm
What do we have ? 22mm, 19% of surface less ! It's actually even less because it's not round, you have to start with a 21.5mm reamer when you try to enlarge them.
Let’s check on the 18v (not Cos) head: valve 28.3, port: 25. -> 89%. And 18v camshafts have 1mm more lift. On these heads the valves are more shrouded, so it's too much.
So you should assume the stock 24v heads have 21.5mm ports.
And I enlarged up to 23.5mm
Now this is what I wrote the 8th Dec 2021:
For a 30mm valve this makes 30 * 0.81 = 24.3mm
What do we have ? 22mm, 19% of surface less ! It's actually even less because it's not round, you have to start with a 21.5mm reamer when you try to enlarge them.
Let’s check on the 18v (not Cos) head: valve 28.3, port: 25. -> 89%. And 18v camshafts have 1mm more lift. On these heads the valves are more shrouded, so it's too much.
So you should assume the stock 24v heads have 21.5mm ports.
And I enlarged up to 23.5mm
BiTurbo228
New Member
- Messages
- 27
Thanks, that's helpful.
Typically, ports have a small taper to them down their length. Some have enormous tapers, but considering the size of the Maserati ports I was assuming it would be a small taper. It's unusual to have ports with no taper at all, but not unheard of. I think a degree or two is ideal from a flow perspective, but I think we've decided that Maserati ports are far from ideal in many ways!
If a port is tapered, the point it becomes its narrowest is called the 'MSCA' (I think I mislabelled it as 'throat', but I've seen that used as well), usually just before it turns to meet the valve (maybe an inch into the port). I think the port sizing rules of thumb are meant to be applied to this narrowest section of the port, which can be misleading if you're applying it to the port entry (where the inlet manifold bolts to) if the port tapers.
If the 24v are slightly oval, so 21.5mm across one dimension but closer to 22mm across another I can factor that in. Not that it'll make a great deal of difference, they're still off the bottom of the chart (1.6mm2/cc for the 2.8l). Your 23.5mm ports on a 2.5l 24v would give 2.1mm2/cc, which is still on the smaller side but is about what a YB Cosworth is so isn't way off the mark.
Typically, ports have a small taper to them down their length. Some have enormous tapers, but considering the size of the Maserati ports I was assuming it would be a small taper. It's unusual to have ports with no taper at all, but not unheard of. I think a degree or two is ideal from a flow perspective, but I think we've decided that Maserati ports are far from ideal in many ways!
If a port is tapered, the point it becomes its narrowest is called the 'MSCA' (I think I mislabelled it as 'throat', but I've seen that used as well), usually just before it turns to meet the valve (maybe an inch into the port). I think the port sizing rules of thumb are meant to be applied to this narrowest section of the port, which can be misleading if you're applying it to the port entry (where the inlet manifold bolts to) if the port tapers.
If the 24v are slightly oval, so 21.5mm across one dimension but closer to 22mm across another I can factor that in. Not that it'll make a great deal of difference, they're still off the bottom of the chart (1.6mm2/cc for the 2.8l). Your 23.5mm ports on a 2.5l 24v would give 2.1mm2/cc, which is still on the smaller side but is about what a YB Cosworth is so isn't way off the mark.
BiTurbo228
New Member
- Messages
- 27
One more quick question! I don't suppose you know what the compression height (distance from the top of the piston to the gudgeon pin axis) is for any of the Maserati pistons? Can't seem to find any info on piston catalogues, the FIA pages, or the factory workshop manual.
Also, just want to correct something I mentioned earlier about rod ratios and effective use of ports, after a bit of further research into things.
Theoretically, a short rod ratio will make better use of ports that are too big. This is due to the faster piston acceleration early in the stroke, getting airspeed up in the ports. A long rod ratio will make better use of ports that are too small, as ours are. That's due to the more gradual draw of air being less likely to overspeed the air in the small ports and cause stall. Though these effects are really not very large, and rod ratios are generally one of the first things to be compromised on in engine designs, so clearly aren't of major concern for road engines.
Saying that, most of the discussion appears to be on the benefits of rod ratios between 1.6:1 as 'short' and 2.0:1 as 'long'. The 2.8l 18v is close to this upper long-rod bracket with 2.05, but all the rest of the combinations are in excess of it (2.18 for 2.0l and 2.5l 18v, 2.17 for 2.8l 24v, and 2.31 for 2.0l 24v). The general consensus is that this is too long to have good combustion efficiency, but there's less discussion and testing on rods of this length.
Also, just want to correct something I mentioned earlier about rod ratios and effective use of ports, after a bit of further research into things.
Theoretically, a short rod ratio will make better use of ports that are too big. This is due to the faster piston acceleration early in the stroke, getting airspeed up in the ports. A long rod ratio will make better use of ports that are too small, as ours are. That's due to the more gradual draw of air being less likely to overspeed the air in the small ports and cause stall. Though these effects are really not very large, and rod ratios are generally one of the first things to be compromised on in engine designs, so clearly aren't of major concern for road engines.
Saying that, most of the discussion appears to be on the benefits of rod ratios between 1.6:1 as 'short' and 2.0:1 as 'long'. The 2.8l 18v is close to this upper long-rod bracket with 2.05, but all the rest of the combinations are in excess of it (2.18 for 2.0l and 2.5l 18v, 2.17 for 2.8l 24v, and 2.31 for 2.0l 24v). The general consensus is that this is too long to have good combustion efficiency, but there's less discussion and testing on rods of this length.
BiTurbo228
New Member
- Messages
- 27
Just a joke about me noticing I was asking lots of irritating questions! Nothing more than that.Why not to elaborating about the following statement you made:
Haha I genuinely think I would enjoy that, though there might be a non-zero chance of getting murdered
BiTurbo228
New Member
- Messages
- 27
True. I wouldn't say it was the best joke I've ever made! Always hard to tell tone over text. I felt I was being irritating, but if I wasn't that's a bonus!
Did you have any information on compression height? I feel like I've found it somewhere in the past but can't find it anywhere in my notes. Not a problem if not.
Also, I've been doing lots of reading on rod ratios and the jury is definitely still out on whether they matter at all, though if you want to talk about drawing conclusions from incomplete data that's an egregious example! Lots of people stating absolute facts, but most of the real-world testing has been insufficient to draw conclusions that are applicable across different engine configurations (and there hasn't been much published real-world testing at all). What I've learnt so far:
I think the only interesting thing for our particular use-case (Maserati engines) is the Harley Davidson information (if it can be trusted). We have a long rod/stroke ratio. We have very small ports. We have mild cams. These things appear to be matched (for a given understanding of what 'long' and 'small' is). How small is appropriate for a given rod ratio is unknown. There are incredibly few engines that run both a long rod/stroke ratio and tall pistons, which limits comparisons there. There is no good information freely available around changing rod ratios within the range of 2.1:1 and 2.4:1, which is where we are looking at. I suspect F1 teams would know, but they're not telling anyone.
It'll be very interesting to see what your 23.5mm ports do.
Did you have any information on compression height? I feel like I've found it somewhere in the past but can't find it anywhere in my notes. Not a problem if not.
Also, I've been doing lots of reading on rod ratios and the jury is definitely still out on whether they matter at all, though if you want to talk about drawing conclusions from incomplete data that's an egregious example! Lots of people stating absolute facts, but most of the real-world testing has been insufficient to draw conclusions that are applicable across different engine configurations (and there hasn't been much published real-world testing at all). What I've learnt so far:
- On small block chevys (where most of the discussion and testing is), the range of rods goes from 1.6:1 to about 1.9:1. One back-to-back test in Motor Trend with the same engine shows either minimal difference (3hp max), but failed to account for the differing port and cam requirements that the theory and other practical tests suggest is necessary. The other back-to-back test undertaken by GM showed precisely zero measurable difference in rod ratios from 1.6:1 to 1.9:1.
- On Harley Davidsons (where some other testing has happened), the range of rods goes from 1.6:1 to about 2.1:1. This is where information about the differing port requirements for long and short rods comes from. Apparently short rods mismatched with small ports had the power drop off a cliff midway up the rev range. Long rods mismatched with big ports made poor low-rpm torque. Short rods make more power with aggressive cam profiles with long duration. Long rods make more power with mild cam profiles with shorter durations. However, these tests were on the race track rather than in controlled conditions. Whether this makes them more or less valid probably depends on your perspective.
- F1 cars frequently run very long rods, ranging from 2.3:1 to 2.9:1. People who have had conversations with people in the industry have got very different answers. Packaging constraints around big bores and super-short strokes. Packaging around inlet port geometry. Packaging around getting the centre of gravity in the right place. Dwell times at TDC being important for not outrunning your combustion when you're turning 18,000rpm. Material strength and reliability issues when you're turning 18,000rpm for hours on end. Perhaps it's all of these. Perhaps it's different ones in different circumstances. Perhaps they're not giving straight answers as it's a very cagey industry around technical design. There's definitely some reason they're doing it though, as they have a strong incentive to reduce the height of the engine to improve aerodynamics. Either way, the use case is extreme and not necessarily applicable to road cars.
- Top flight drag racers often run very short rods, ranging from 1.3:1 to 1.4:1. People who have had conversations with people in the industry have got answers that mostly tally against each other. Rod ratio here is not a concern at all. They use short rods to have a super-low deck height which improves packaging of near-vertical intake ports and runners. As engine run time is measured in seconds, increased wear is a non-issue. People are also cagey about technical design, but at least their answers are broadly consistent. Also a very extreme use case, and not necessarily applicable to road cars.
- Top flight endurance cars run a wide variety of rod ratios between about 1.6:1 and 2.1ish:1, even when regulations are free. Doesn't seem to have much if any impact on their success as race cars, though the engine is actually a very small part of what makes a successful endurance car. However, it was noted that manufacturers use whatever rod ratio and max rpm they need to keep mean piston speeds below a certain level (~25m/s). It's suspected this is to ensure adequate reliability from other factors (material strength, oil film consistency etc.). NASCAR is an outlier in this as they run for hours well above that piston speed, though the understanding is that they are on the bleeding edge of material science to do this.
- Road cars run a wide variety of rod ratios between 1.6:1 and 2.1ish:1, though long rod ratios are rarer than short ones within this range. This seems to be something manufacturers readily compromise on within this range. There are a very few road engines with longer rod ratios. There are practically no road engines with shorter rod ratios. Longer rods on a clean-sheet design make packaging harder. Too short rods risk reliability due to increased wear. These are probably more important considerations.
I think the only interesting thing for our particular use-case (Maserati engines) is the Harley Davidson information (if it can be trusted). We have a long rod/stroke ratio. We have very small ports. We have mild cams. These things appear to be matched (for a given understanding of what 'long' and 'small' is). How small is appropriate for a given rod ratio is unknown. There are incredibly few engines that run both a long rod/stroke ratio and tall pistons, which limits comparisons there. There is no good information freely available around changing rod ratios within the range of 2.1:1 and 2.4:1, which is where we are looking at. I suspect F1 teams would know, but they're not telling anyone.
It'll be very interesting to see what your 23.5mm ports do.
Yes I've got 2.0 24v and 2.8 pistons, I'll measure when I have time this week end.
My opinion about all these web discussions is that they are mostly full of ****. Especially US forums where there are a lot of non scientific theories. And all these books like Vizard & Co are based on the real experience but elaborate many false theories about ratios and how exactly things must be made. Observation is only the first step of the scientific approach, not the goal.
Some stable relations can be observed in fields like harley bikes or race categories, but they can't be just copied into other application fileds, they are more proofs than laws.
I read that today F1/24hdM/Indycar/wrc/... engine designers consider there are no more mysteries about the performance aspects of the engines. Btw hybrid F1 run max @ 12k rpm today, not 18k, and no hybrid versions just can't keep pace. Modern race engines are fully designed on computers from the combustion requirements, then the rest is here only to provide the proper environment. It's extremely rare to read an article where motorists would tell you that long rods are better than short because there are so many variables around. There are constraints : fuel consumption, weight, size, power (note, not the first one), elec part integration, cost, and so on. The rest will be a compromise (as usually).
It's obvious that everyone tries to fit the largest valves possible, not necessarily for the power. That a closed deck engine is more robust than an open deck (our). That a fast combustion is the goal and high CR is better. That light is right. That cooling is very important. And few others. All the rest is the consequence. Red cars don't run faster because they are red.
My opinion about all these web discussions is that they are mostly full of ****. Especially US forums where there are a lot of non scientific theories. And all these books like Vizard & Co are based on the real experience but elaborate many false theories about ratios and how exactly things must be made. Observation is only the first step of the scientific approach, not the goal.
Some stable relations can be observed in fields like harley bikes or race categories, but they can't be just copied into other application fileds, they are more proofs than laws.
I read that today F1/24hdM/Indycar/wrc/... engine designers consider there are no more mysteries about the performance aspects of the engines. Btw hybrid F1 run max @ 12k rpm today, not 18k, and no hybrid versions just can't keep pace. Modern race engines are fully designed on computers from the combustion requirements, then the rest is here only to provide the proper environment. It's extremely rare to read an article where motorists would tell you that long rods are better than short because there are so many variables around. There are constraints : fuel consumption, weight, size, power (note, not the first one), elec part integration, cost, and so on. The rest will be a compromise (as usually).
It's obvious that everyone tries to fit the largest valves possible, not necessarily for the power. That a closed deck engine is more robust than an open deck (our). That a fast combustion is the goal and high CR is better. That light is right. That cooling is very important. And few others. All the rest is the consequence. Red cars don't run faster because they are red.
BiTurbo228
New Member
- Messages
- 27
Thanks, that'd be very helpful.
Absolutely with you on most of the web discussion. One of the few that seems to have technical discussion from at least a halfway decent proportion of people with appropriate experience is Don Terrill's Speed Talk. Still lots of people making strong conclusions from hunches and beliefs, but also a handful of people who spend the time to look through the evidence properly. Also have to be really careful about applicability of the information there. For instance, one of the benefits of a super-short rod ratio for pro-stock drag cars is stability of the valve train at high rpm from shorter pushrods. Not applicable to OHC engines at all. And 99% of the talk there is about Chevy small blocks, where a 'short' rod is 1.6:1 and a 'long' rod is 1.9:1. So not the window we're looking at.
Vizard is excellent and very useful (especially so as it's publicly available), but is still only working within a subset of engines. For instance, his experience on appropriate port sizes seems to be completely at odds with modern direct injection/VVT engine ports. That doesn't invalidate any of his findings, but demonstrates some limits to their applicability.
The discussion on F1 engines was about the 2002-2004 V10s rather than newer engines. Mainly through lack of available information at the time the posts were being written I expect.
I beg to differ on the lack of folks suggesting long rods are superior though. A quick google shows an inordinate number of web pages, magazine articles, and general tuning forums stating the theoretical benefits of long rods. From what I can gather, taking the theory as read and not matching it up with any empirical evidence at all. Someone on the Speed Talk website put it quite succinctly I thought. They say 'it's like saying you can theoretically prove that a racecar will be faster with one coat of paint compared to two as it'll be lighter, but in the real world practically every other consideration is going to be more important'.
I suppose the question I was looking for an answer to was 'if we want to lighten a Maserati V6 piston past what is available with forgings using the original compression height, would lengthening the rods be fine or detrimental?'. And the answer is 'I've got no idea I'm afraid. No-one's tested anything even close to the use-case we have'. Hey ho. I learnt some things while doing the research.
Absolutely with you on most of the web discussion. One of the few that seems to have technical discussion from at least a halfway decent proportion of people with appropriate experience is Don Terrill's Speed Talk. Still lots of people making strong conclusions from hunches and beliefs, but also a handful of people who spend the time to look through the evidence properly. Also have to be really careful about applicability of the information there. For instance, one of the benefits of a super-short rod ratio for pro-stock drag cars is stability of the valve train at high rpm from shorter pushrods. Not applicable to OHC engines at all. And 99% of the talk there is about Chevy small blocks, where a 'short' rod is 1.6:1 and a 'long' rod is 1.9:1. So not the window we're looking at.
Vizard is excellent and very useful (especially so as it's publicly available), but is still only working within a subset of engines. For instance, his experience on appropriate port sizes seems to be completely at odds with modern direct injection/VVT engine ports. That doesn't invalidate any of his findings, but demonstrates some limits to their applicability.
The discussion on F1 engines was about the 2002-2004 V10s rather than newer engines. Mainly through lack of available information at the time the posts were being written I expect.
I beg to differ on the lack of folks suggesting long rods are superior though. A quick google shows an inordinate number of web pages, magazine articles, and general tuning forums stating the theoretical benefits of long rods. From what I can gather, taking the theory as read and not matching it up with any empirical evidence at all. Someone on the Speed Talk website put it quite succinctly I thought. They say 'it's like saying you can theoretically prove that a racecar will be faster with one coat of paint compared to two as it'll be lighter, but in the real world practically every other consideration is going to be more important'.
I suppose the question I was looking for an answer to was 'if we want to lighten a Maserati V6 piston past what is available with forgings using the original compression height, would lengthening the rods be fine or detrimental?'. And the answer is 'I've got no idea I'm afraid. No-one's tested anything even close to the use-case we have'. Hey ho. I learnt some things while doing the research.
BiTurbo228
New Member
- Messages
- 27
Oh, one of the other things I'd found while reading around was some promising things about port taper. Ports with a gradual taper along their length (say 1-2deg) outflowed and outperformed straight ports of comparable cross-sectional areas.
I'm not aware of how much material you have around the walls of your 23.5mm ports, but if you're more limited close in towards the valves with comparatively more thickness further out, running a 1-2deg tapered reamer down the ports may eke out a little more flow. Or taking a stone to the port divider to taper that, if you don't mind a more laborious approach. Ideally this would be tested on a flowbench of course, rather than just taking this at face value. Even more ideally by running them back to back on the same engine setup, but that's quite a lot of effort compared to a flowbench!
I'm not aware of how much material you have around the walls of your 23.5mm ports, but if you're more limited close in towards the valves with comparatively more thickness further out, running a 1-2deg tapered reamer down the ports may eke out a little more flow. Or taking a stone to the port divider to taper that, if you don't mind a more laborious approach. Ideally this would be tested on a flowbench of course, rather than just taking this at face value. Even more ideally by running them back to back on the same engine setup, but that's quite a lot of effort compared to a flowbench!
Last edited: