Rubrignitz

I thinktheproblems that you're mentioning have moreto do more with tube diameter and flow rate. With my 1 5/8' header primaries and 2.5' exhuast diameter I should have the proper flow rate for my setup. And the mufflers should provide enough backpressure themselves. I'd like to hear some other folks chime in on this one...

99flhr

I`m feeling positive that valves are closed by springs... unless of course you have a desmo Ducati.

GTstangB.F.D.M.

ya they are.. by the cam i believe.. but i also think the that exhuast has something to do wiht the pressure in pushing back

99flhr

Negative, the cam only opens the valve, with the exception mentioned above

Derf00

So many things wrong with that post I won't even point them all out...

Backpressure has nothing to do with 'valves closing' or 'compression' And the fire you see on drag cars or any type of vehicle that you see running straight headers and are on a drag strip is because of the type of fuel they are running. Hi methane based with alchol mix. You could get fire out of a Stock mustang exhaust if you simply hook up a spark plug to the tail pipe and wired it.

I didn't want to write it all out... Oh and valves are closed by springs...NOT PRESSURE IN THE CHAMBER!! I copy and pasted this explanation below from another forum who had cut and paste it as well... I've bolded the meat.

There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp.

So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels.

Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity.

Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger.

But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting.

Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it.

Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe.

I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that.

This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished.

So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application.

To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.

The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect.

Rubrignitz

Thank you! for digging that up. I'm going to research this more but ALL I've ever known or heard in my years of working on and driving cars is that TOO MUCH backpressure is bad and can cause a MULTITUDE of issues. I also know that running the WRONG exhaust setup (diameter tubing, etc) can also cause a multitude of problems. As long as you have the right setup for your engine (and I do) you're all good.

Let me edit this to say that I'm not an ASE mechanic and there may be some smog or computer controlled devices on these cars that are backpressure reliant. I know that some EGR valves on newer cars will not open without the correct amount of backpressure. I don't think this would cause any of the aforementioned problems though.

99flhr

Ah, the voice of logic & reason,Thank you Derf00 !

Rubrignitz

One of the supporting vendors over on modularfords said:

"If anyone brings up not enough backpressure and there being a problem with it just don't listen to them and make sure they don't work on your car.

There is a huge difference between backpressure and exhaust velocity and some people tend to confuse them."

Exactly.

GTstangB.F.D.M.

wow.. good post man.. glad thats straightened out.. just i have always heard that crap about backpressure.. thanks for informing us all.. and to hear the guy from modularfords say it too...

so will open headers be more benficial than headers wiht straight pipes?.. or the other way around.. or what.. jw... very cool article

and rubrignitz.. what kina setup do you have/are you doing?

Rubrignitz

lol! I'd never run open headers. Ran mine when the new ones went on and it was soooo freaking loud. You need some pipe back there. And that goes back to optimum exhaust gas velocity, etc. It would probably run like crap with open headers. The dragsters you see with open headers are running really high compression engines/alternative fuelsand that offsets somewhat the need for proper exhaust sizing/length.

I have JBA 1 5/8' long tube headers, JBAoff-road h-pipe and 2.5' exhaust tubing all the way back to the JBA Evol axle backs.

It runs (and sounds) great, but I think a bit rich. I'm going to bring it to a local tuner to see what they can do with it.