Piston Weights?

alpa

Member
Messages
241
Slightly tappered ports being the best was proved probably 40 years ago. Many bikes have such ports, honda as well.

I don't have machining tools to make the same taper in 12 ports so I'm already happy that I could make them straight and bigger. Took me 45 minutes per port, if I remember well.

Yes there are great discussions on Speed Talk, also because there are people from everywhere.
 

alpa

Member
Messages
241
The block deck height of all v6 blocks is 211.3mm
There are two types of v6 engines: the ghibli/qp4 2.0 24v (at least 2 different AMxxx reference) and all others.
The ghibli engines use special rods (same as on v8). All other engines use the same bigger rods.
Compression height of the ghibli piston: 34.2mm + deck clearance 0.3mm
Ghibli rod length: 145.3mm
Other engines rod length: 137.4mm
2.8 18v piston compression height: 40.5mm
 

BiTurbo228

New Member
Messages
27
Perfect, thanks. That's the missing bits of information. Definitely see what you mean about tall pistons. Not necessarily the worst thing for a turbo engine that likes good thick ringlands, but I can see the potential for improvement with longer rods. Considering those dimensions your 476g pistons are remarkably light! The first comparable piston I can think of is a Mitsubishi 6G72 piston which are about 1mm too short to use with Ghibli rods and the 67mm crank (31.6mm compression height, 92mm bore). JE forged ones of those are 432g with pin (347g bare, not sure if rings are included in the weight but they won't be more than 5g).

I wouldn't necessarily advise using those particular pistons though as they're dished so won't help the compression ratio (unless the Maserati pistons are also dished), but they're very similar bore and for a turbo engine so should be indicative of what weight reduction you could expect with shortening the compression height. Not nothing, but not night and day either (comparing forged to forged).

And yeah, 21.5-23.5mm consistently across 12 ports is good going! Enabling consistent work across ports is one of the major benefits of a flowbench I think. Make one good port, then you can incrementally creep up to the same flow on all of the rest. It's no substitute for careful workmanship, but it's useful to have some form of benchmark.

Edit: A quick google suggests the Maserati pistons are dished, though I can't easily tell what volume.
 
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alpa

Member
Messages
241
Only 2.8 pistons.are dished.
Are you deciding which piston to use without opening an engine ? What's the purpose of all this talk ?
 

BiTurbo228

New Member
Messages
27
Thanks, again that's useful. Could find pictures of 2.0l and 2.8l pistons but not 2.5l.

Not deciding, more coming up with a list of potential options. That way I can hit the ground running if and when the time comes.

It was more as a comparison point for weight loss anyway. Lots more to sort out before making any decisions.
 

alpa

Member
Messages
241
Weight does not matter as long as you didn't study the engine. You (neither me) have no idea about how these cranks were balanced. Think only that the 2.8 cranks are LIGHTER than 2.0 ghibli ones, while all moving parts are much heavier in 2.8, and a 2.0 crank can be installed in place of a 2.8 one: all interface dimensions are identical.
 

BiTurbo228

New Member
Messages
27
True, though in principle the balancing shouldn't be an issue. Balancing isn't complex, it just requires expensive machinery and a bit of diligence. You may know this already of course! I expect you'd need to look into it for your forged pistons.

First step is working out if the Maserati 6 is internally or externally balanced. This is only really a consideration as to whether you balance the crank in isolation or with the flywheel and pulley. Both function effectively. We can find this by whether there's any off-centre weights on the flywheel and/or pulley.

Next step is weighing the reciprocating and rotating weight of the piston and rod. Simple.

Then we make up some bob weights to attach to the crank while balancing. From what I've read this is easier for 90deg odd fire V6s than even fire ones, and is a simple 50% of the reciprocating weight plus 100% of the rotating weight (still need to check if that's for one or two piston and rods as they share a crankpin).

If you want to be more thorough, you can work out what balance factor Maserati used. Start with the un-bobweighted crank and progressively add bob weights until it reads as balanced. Dividing that by your measured weights will give you the balance factor Maserati used, which you can then apply to your new weights.

Balancing proceeds as normal from this point onwards. Not a difficult process. But one that will need someone to do for you that's familiar with bobweighted crank balancing. Unless you fancy spending £5000 on a crank balancing machine yourself!

Oh, and it might even be slightly easier with the Maserati engine than some others. If you need to add weight to make up for some imbalance, normally you need to insert Mallory metal slugs into your counterweights. However, for a V6 there's practically nil issue with adding properly located bolt-on weights to the pulley and flywheel to do that. Much easier.
 

BiTurbo228

New Member
Messages
27
Also, a potential reason for the 2.8l crankshafts being lighter could be different balance factors. For instance, Buick odd-fire V6s have been measured using the above method as having a balance factor of 36.6% from the factory. They are not known as smooth engines, but they do function acceptably. This shows that the 'ideal' balance factor is something manufacturers have compromised on before.

Racers often change this to a 50% balance factor, which helps them hold together at higher rpm.

Maserati may have deemed a lower-than-optimum balance factor to be a suitable compromise for the rev range of 2.8l engines, but when making the higher-effort Ghibli 2.0l engine decided that the extra counter weighting was worth it.

Who knows without testing it, but it's a possibility.
 
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alpa

Member
Messages
241
Obviously you know everything about what to do, and fortunately it's simple. I hope we'll see your project done soon.
 

BiTurbo228

New Member
Messages
27
Haha not even slightly! I know enough to know just how much I don't know. But balancing is something I've had a little experience with. Thought it might be helpful to anyone coming past here in the future.

I hope so too, though in reality it may be quite some time indeed.