backpressure is bad, you want an exhaust system to give the right exhaust velocity for the intended rpm range

 

The perfect motor build would be one where the cam is ground to match your low back pressure high velocity exhaust system as well the intake system for the intended motor use and rpm range. Lets not forget gearing, altitude, temperature and weight of vehicle.
The local muffler butcher shops seem to just slam in Flowmasters no matter the vehicle and charge you big money.

 

Yeah, unfortuantely the perfect system is nearly impossible to build at a price that would ever return your investment. Having more valves with variable valve timing improves flow and exhaust velocity, but adds complexity to the engines. This decreases reliability and also tends to increase combustion temperatures. Even then you would have the problem that an exhaust that produces maximum torque and high exahust velocities at low RPM becomes restrictive at higher RPM. Solutions to this can be made with very complex exhaust systems that use diverter valves, with a smaller exhaust sytem for low RPM, and once the exhaust pressure reaches a certain point, the valve switches to the larger exhaust system. This is not practical at all, but it would work.

 

Exactly, the existance of backpressure is not a myth but the idea that you need it for low end torque most certainly is.

 

When I put my long tube headers on my truck it did affect my A/F mix!
My tuner had to adjust for the headers.

 

The concepts being thrown around here are correct, but the mechanism behind them seems to be a bit shaky.

Fluid flow through a pipe can be explained in two ways, laminar (straight, high speed) and turbulent (low speed).

An automotive exhaust sytem functions most efficiently when the flow is laminar as it does aid in the scavenging effect within the exhaust and cylinders. Most factory exhaust are designed to provide laminar flow at low RPM since this IS where most of your driving time is spent. Now this does create a restriction on the top end as the motor will try to expell more gas than the exhaust can handle.

Now, when you put "too large" of an exhaust sytem on a vehicle the exhaust flow is turbulent at low RPM. So what happens is when the flow goes turbulent it actually CREATES back pressure. The gas starts to roll and more or less stays in one place creating a backup within the pipe and heads.

THAT is why people who put headers on stock motors notice a loss of off idle power, since at that point the back pressure within the system has actually been increased.

Now this same motor with the same exhaust will pull harder at higher RPM when the exhaust flow becomes great enough to re-establish laminar flow which re-establishes the scavenging effects within the motor.

So what you want is the smallest exhaust possible that will still support the RPM range in which you drive the most. I like staying conservative since I'd rather lose a little up top than alot down low.

Justin

 

How does one get "laminar" flow thru a cast iron log of a 2x4 bolted to the heads no matter the rpm's?

I have never seen logs that were replaced with headers that caused increased back pressure, this with a pressurse gauge test on ports in the exhaust manifold be it logs or header ports as well in the collectors. Granted down stream of the manifold / header back pressure could be from pipe diameter, length and muffler used.

There isn't a flow but rather a exhaust pressure pulse your dealing with plus a exhaust gas thermal temperature.
Pressure, temperature, volume, sonic speed all play into this, manufactures design a system with a trade off between performance, noise and the biggest
factor of all cost.
We performance thinking animals do not have that cost limit item in our exhaust systems. What is the best trade off for the vehicle manufactures isn't necessarily a trade off to a performance person.

 

Well said.

 

MYTH not mith.

 

zpmps2, get the headers. Your horsepower will increase. Your fun will increase. Food will taste better. The air will seem fresher, and you will have more energy and self-confidence than you ever thought possible!

 

I would go for the headers but don't get the cut outs unless you have a serious motor, I had a warmed up 289 in a 68 mustang cam was a little over .500 lift i'm wanting to say (don't remember for sure) 270 duration, long tube headers, and full length 2 1/2 duals with dynomax mufflers it would go 14.2s in the 1/4 when I opened up the cut outs it was well into the 16s. I cant help but think this tourque loss would hurt a heavy truck with big tires even more, and those things leaked all the time.
-Johnboy

 

I would go for smog legal headers too, even if you are not smogging the vehicle. Smog legal are easier to install, and have more attention to detail. They allow you to keep the smog equipment or bypass them, whichever your pleasure is.