Backpressure Question.

[QP4Me]

Hi,

I am wondering about the need, or otherwise, for exhaust backpressure im a 4.2 QP.

In my younger days of drag racing, I ran a 275bhp drag bike which sported an open exhaust, which, if run for more than a short time, would overheat and damage the exhaust/inlet valves in quick time. The reason, it seems, was a lack of backpressure due to the totally open exhaust.

Zip forward to the present day and I am toying with the idea of releasing the glorious noise of the V8 (in short, sharp sessions), through the use of electric exhaust cut-outs. While it is a fairly straight-forward job, the concern I have is the possibility ofdamage the lack of backpressure (while the cut-outs are open) may cause to the motor.

Any thoughts on this issue out there?

Slan,

Pat.

 

[BJL]

Bl00dy expensive if you get it wrong....

 

[CatmanV2]

Hi,

I am wondering about the need, or otherwise, for exhaust backpressure im a 4.2 QP.

In my younger days of drag racing, I ran a 275bhp drag bike which sported an open exhaust, which, if run for more than a short time, would overheat and damage the exhaust/inlet valves in quick time. The reason, it seems, was a lack of backpressure due to the totally open exhaust.

Pat.

HI Pat, what was the logic about lack of back pressure causing it to overheat? Was it going lean for some odd fluid dynamic reason?

C

 

[QP4Me]

I'm not totally sure as to the reasoning, but there could have been a number of factors; high compression, advanced timing. high octane fuel or an unrestricted exhaust. It was also running very lean ok, carb equipped, it also ran on Nitros.

These factors aside, how would a CPU managed engine cope with minimal backpressure I wonder?

 

[CatmanV2]

Ahh I see No idea how it would cope, pat. First thing that springs to mind is the Lambda is going to be going haywire, isn't it?

C

 

[QP4Me]

No idea whatsoever, until I was seduced by the Masserati, I was strictly a carb and old school type of guy! I had thought a Lambda was a dance craze.

 

[CatmanV2]

LOL. Yes, I love carbs. cf Mrs Catman's Sprint and its overrun

A device to deliver the wrong fuel and air ratio at all times, isn't it?

C

 

[QP4Me]

Yep thats it! Trying to get 4, 36mm Mikuni's to run as one, is enought to drive anyone to drink!

 

[CatmanV2]

The old 116 had DelOrtos that drifted terribly as well. Sounded lush, though

C

 

[alfatwo]

LOL. Yes, I love carbs. cf Mrs Catman's Sprint and its overrun

A device to deliver the wrong fuel and air ratio at all times, isn't it?

C

Carburettor's,there all just calibrated leak's...... sound good though

 

[Emtee]

Surely if the valves were burning out with a truly open flowing exhaust, then it doesn't stack up that the exhaust would be the culprit, unless the exhaust wasn't flowing correctly and was actually creating a back pressure? Bike exhaust runs are pretty short compared to cars, so you need to know the exhaust is truly unrestricted. Just because there isn't any physical obstruction in the way doesn't mean it's actually free flowing.

 

[Almichie]

The exhaust is used not just to get the burnt fuel from an engine but also to scavenge - if designed correctly it will pull the vapour from the engine as much as the engine pushes it out by creating vacuum between pulses in the headers.

There is more to an exhaust than meets the eye!

 

[Emtee]

The exhaust is used not just to get the burnt fuel from an engine but also to scavenge - if designed correctly it will pull the vapour from the engine as much as the engine pushes it out by creating vacuum between pulses in the headers.

There is more to an exhaust than meets the eye!

That's what I was getting at Almichie, though you described it far better than I did!

I'm having an exhaust built for my Exxy (also a short exhaust run being lateral mid-engine layout) that is designed with steps in it's pipe run (also with a step to the manifold) to break the pulses. The result is a 'free flowing' exhaust , but with a lot of thought into the design of the piping.