Contour Enthusiasts Group Archives Forums Modifications Duratec Performance Torque Vs Backpressure...

FWIW,many people forget the motor is a system. Improving only one part of a systems can be detrimental to the system as a whole.

For example, on a motorcycle, a better header (less back pressure) will deteriorate performance if you don't rejet the carburetor to make the mixture richer. With a rejetted carb and headers, performance is better than original. A lower back pressure exhaust changes the nature of the mixture in the cylinder. If no compensation is made in the rest of the system, you may see a deterioration instead of improvement.

Look at fuelie dragsters, they use short pipes, one per cyclinder for the lowest retriction possible. If restrictions helped, I am sure they would have some.

I realized that it is not backpressure that I was really discussing in my posts but is exhaust velocity, as others have mentioned. My boo

If you have a large cam overlap, then lowering the back pressure to far can cause the incomming fuel charge to exit through the still open exhaust valve rather than filling the chamber rather than just scaveging the exhaust out. Too little flow however will mean the incomming fuel charge is blocked by the escaping exhaust

For each overlap/engine/fuelling/revs combination there is a perfect exhaust flow that allows the exhaust gas to exit and the fresh charge to fill the chamber. Its a bit of a suck it and see situation for each engine

Its not back pressure as such either, its reflected pulses.

"Modern Engine Tuning" and "Modern Exhaust and inlet manifold design" (both really good books) describe situations when increasing and decreasing flow can help, in reality there is a narrow band that is good for your engine

In general an increase in "backpressure" can raise the fuel charge (up to a point) and increase torque, whereas a decrease tends to aid gas flow (again up to a point)

So it is wrong to say that a particular pressure, or raising or lowering it is good, as every car will behave differently and you would need a dyno and a variable length/backpressure zorst to test

By the way F1 cars have variable length exhausts that can be altered for fast (speed) or tight (torque) circuits

Yamaha experimented for a while with a variable exhaust length gizmo that changed with the rpm on their 2 strokes

I'm gonna bite on this bait.
No one seems to be able to give you the answer you are looking for, and I don't expect to either.

However, I will take a stab and write what I think are the two important characteristics. I won't even use terms like backpressure. Backpressure is a consequence of the pipe diameter and the pressure drop that occurs between the cylinder and the atmosphere at the end of the pipe.
I would have to say that you would want exhaust components that will tend to keep the exhaust velocity high and still flow the required volume of exhaust at the appropriate rpm. Lets not forget that as rpm changes from low to high, there is an increase in the volume of exhaust per unit time. This means that you have to optimize pipe diameter based upon two variables, Volume and velocity.
I would attempt to do this by picking the rpm where I wanted to optimize those variables in the middle of the rpm range dictated by the shift points. In my car, I would pick around 5,500 rpm.
That volume of air going into the engine at this rpm would be on the order of 300 cfm. This isn't necessarily the same volume coming out, but I don't have any data on that.
I dug up some data from fluid dynamics.
The average viscosity (n) for gasses that make up combustion exhaust is roughly:
n=3.5*10^-5 N-s/m^2 or n = 0.035 centipoise
I have equations that are derived from the Navier-Stokes equations that will give max velocity for fluid in a tube, volume flow rate, and the velocity profile. Whats the point anyway....

I am not going to finish this analysis because I haven't found in the past that many people even care to go this in-depth. They usually want the answers handed to them, I know I sometimes do.
So at this point I have to say that the question is too complicated for me to answer accurately.
I do not have the experience to properly size an exhaust system from an engineering perspective.
However, I know that automotive engineers must have plugged and chugged through all of the fluid dynamics and come up with the 'solved' equations
that are now shortcuts for the rest of us.
So Terry, you must have access to them....What are they?

Tom

I love it when Terry makes us think! I'll admit I don't know the first thing about fluid dynamics (didn't have to take it, heck I barely passed thermo!). But this will be a thread I keep handy for future reference.

I wonder if they have threads like this on the Honda boards?

i can tell you this much, i had new pipes put on my tour from the cat back (2.25") and my big ole annoying 'RAAAAAR' muffler, which i'm sure has little to no restriction (as i can see right through it) and i can tell the cars throwin unburnt fuel into the exhaust after running it hard

PA 3L SVT:

They don't have threads like this on the Honda boards because they never worry about discussing torque.... :rolleyes:

Ed

Navier-Stokes **cringe**

Man, now I'm having flashbacks to college fluid dynamics that I've been spending all these years trying to forget!

That stuff is for the base engine guys. Glad I'm on the vehicle side. BTW, you'd be surprised how much engine design is NOT calculated from scratch, but rather dervied from experience.

Ahhhh...Autoeng is getting it...experience...so is it fair to say that 'most' of the experience re flow/torque etc etc is from 'thumping great V8's & not high revving multi valve DOHC's ????

BACKPRESSURE = TORQUE?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast-moving but free-flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of this scavenging effect, which has the greatest impact on low-end torque.

Low backpressure and high exhaust stream velocity can be achieved by running straight-through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.

....the above was taking from magnaflows website...it may be an excerpt from the nov 2k1 issue of sport compact car...