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Re: Backpressure is vital for most modern non-turbo engines
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Posted by Justin VanAbrahams [Email] (#32) [Profile/Gallery] (more from Justin VanAbrahams) on Sun, 25 Nov 2001 20:47:02 Share Post by Email
In Reply to: Backpressure is vital for most modern non-turbo engines, BillJ, Sun, 25 Nov 2001 09:34:38
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Hmm... that is not my understanding at all. While I do understand the issue of overlap at low engine speeds, this should *only* be a problem for
a car with a very aggressive cam setup. On a non-turbo car that overlap is specifically tuned to work with the scavenging that's built into the intake/exhaust system. On a turbo car, there should be minimal if any overlap, since the requirement for overlap is *mostly* eliminated by the presence of forced induction.

Often times people confuse pressure with velocity, which is what created the backpressure myth. The two are related of course - as you lower pressure you also lower velocity - but what you want is velocity and NOT backpressure. I'm sure at some point in time someone observed that making the exhaust too big resulted in poor low-speed engine performance and associated the reduced backpressure with the decrease, but the reality is the big exhaust reduced exhaust velocity and *that's* what killed the performance.

Now, I don't purport to be a mechanical engineer by any stretch of the imagination, but from reading and my own admittedly casual experience, my belief is that the goal of any exhaust system is to provide as quick an exit for exhaust gasses as possible. Increased backpressure increases pumping losses which results in inefficiency and poor performance. You *don't* want it.

On a non-turbo engine, the goal is to create an exhaust system which is large enough to flow all the exhaust you're making and small enough to maintain velocity. High exhaust velocity helps create negative pressure in the cylinder which increases scavenging and helps suck fresh air into the cylinders. A properly tuned exhaust will also participate in creating resonance in the intake manifold, which can be used to actually achieve postive intake pressure even in a non-turbo engine. This is true at low engine speed and high engine speed. Exhaust backpressure REDUCES scavenging and cylinder vacuum for obvious reasons - the intake air is now fighting against the exhaust to get into the cylinder which flat-out doesn't work on a non-turbo engine.

On a turbo engine, you've got a whole different set of concerns. Scavenging is much less of a concern - the increased volume of exhaust in the cylinder is far more ready to exit than a non-turbo engine since the exhaust volume at atmospheric pressure will be greater than the cylinder volume. And, cylinder vacuum is less of a problem since you've got a turbo pressurizing the intake anyway. Cam overlap can be tuned to use the turbo to help push out exhaust gases, but you'd NEVER build in so much overlap that you'd actually want to rely on exhaust backpressure to keep your intake charge in the cylinder! On any car, turbo or not, that would kill your performance across the rev range.

Anyway, that's my understanding. Someone here may be able to confirm or deny the validity of this, but I believe it to be true...

-Justin


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