I think the only way to make the engine "work" harder is to open the throttle and let more air IN. Not by preventing it from getting OUT (backpressure). It makes it harder for the engine to produce when you maintain resistance against it.

Well thats kinda my point. By increasing the back pressure, you require the car to combust more air and fuel to maintain the same rpm level. This is why the car must be tuned for these changes. If you add just resistance or backpressure its simply parisitic but if you add both more air and fuel to the mix along with the backpressure, the engine creates more force to overcome the resitance and translates that to the crank.

 

quote:
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Originally posted by DaveO
I believe exhaust gas velocities should be considered, as much if not more than backpressure. Some race cars make more power with high performance mufflers than they do with staight pipes.

Some exhaust systems are designed to develop torque as well as horsepower. A few manufacturers have taken advantage of the x-pipe's exhaust gas scavenging - intake charging characteristics.

Some may find this article interesting....

http://www.carcraft.com/techarticles/69238/


DaveO
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The reason I said that the velocities arent important is that truly the velocity of exhaust gases is the result of their acceleration out of the engine. They are accelerated out of the engine by a force so using either one I guess would be fine but I chose to focus on the force because it is the root cause of that velocity. Looking at their information the scavenging effect they describe seems to be another way to increase that force by creating a smoother exhaust flow allowing more of a vacuum effect inside the exhaust and thereby increasing the velocities. And to fly by Z's point the critical information is given in the article. They said, "The increased backpressure also richened up the air/fuel mixture by about two carburetor jet sizes." This goes to the point I made in my last post. Since the engine was running rich extra airflow was needed to counteract this. In any case the critical element here is the torque curve. Backpressure will change the torque curve and thats what we should really study to make a good comparison. Also to FairladyZ, I used to major in physics which is why the biggest problem I have with this discussion is that I dont have enough data to study to make a strong conlclusion. I wish I had more but for now I'm just going with my gut and some theory. Science on the fly.

 

I once ran my vfr800 w/o the slip on(muffler) and it was loud and SLOW. The motor struggled once it got to 7k rpm's, no torque at all. But once the slip on was reinstalled the power was right back.

 

I have cut-outs on the test pipes right after my headers!! I have dyno'ed with them open vs closed- no difference except the "noise"!

 

That's right because only open headers is not ALWAYS the best. When the enitre exhaust system is tuned to flow together you will probably lose power just taking out the catback or muffler. You can easily increase torque when you remove the exhaust system by simply using smaller primaries in your headers. If you run exhaust manifolds or headers that were designed for the stock exhaust or headers with primaries that are too big of course you will lose power until you rev it really high which our motors cannot take. The people who gain power when they put on mufflers or exhaust simply did not have an appropriate open header setup to begin with. The primaries were probably too big. We can easily change the charateristics of our race cars by using different lengths and diameters of primaries. It is true it is all about how it is tuned but BACKPRESSURE is not your friend. Exhaust flow rate is but that is totally different.