torque affected by exhaust velocity

[beef_bourito]

I heard a couple of people mention the fact that higher flowing exhaust pipes make less low-end torque because the engine pushes out the gases as fast as they come in. I was just wondering how this would work, i always thought that the easier it is to push the gas out, the more power can be spent on spinning the crank.

Someone please clear up how exhaust gas velocity/exhaust tubing size affects torque.

[s14tilo]

I have a hotshot header, gutted cat, and a huge 80mm inlet - 115mm tip Apex N1 exhaust, and I had no loss in torque.

[curtis73]

Its very complicated, but I'll try to sum it up. Those of you who know me are familiar with my usual verbal proliferation.

The exhaust gasses as they exit the cylinder create an inertia. The faster the exhaust gasses flow (to a point) the more they can effectively evacuate the cylinder. They also (at a certain RPM range) create a suction that helps pull new intake charge in.

At a certain RPM, this effect is maximized and torque will tend to peak there assuming all the parts work well together. All things remaining equal, switching to a larger exhaust slows the exhaust flow, reducing this effect until you reach higher RPMs. Its not so much that a larger exhaust reduces torque, it just takes it away from the low RPM range and makes it peak at a higher RPM.

As I said above, the faster the exhaust flows (to a point) the better. The trouble is that most factory exhausts are great for low end torque, but reach a point where velocity gets too great and restriction happens. There is usually a benefit to switching up a little bit if the current exhaust has proven restrictive. It is pointless to go too large if the rest of the engine doesn't need it, since you would be further shifting the peak scavenging way above where the rest of the engine is tuned. These mismatches are what make sluggish engines.

[curtis73]

Originally Posted by s14tilo

I have a hotshot header, gutted cat, and a huge 80mm inlet - 115mm tip Apex N1 exhaust, and I had no loss in torque.

You didn't have any torque to lose ha ha just kidding. I like to make fun of the "little" guys. I currently am building two V8s that will have a total of 1200 lb-ft of torque. One's a chevy 454 and the other is a twin turbo Caddy 500.

Its not torque unless you have to replace a billet 10.5" ring gear after each run.

[s14tilo]

You V8 guys lol j/k man

the sad thing is that I beat a lot of V8's(stock of course)

[curtis73]

I give full marks to the specific output you 4-banger guys get out of those little things. There is some definite fun about screaming engines making for a fun drive.

I thought of one more thing to say about exhaust velocity, too. beefbourito, when designing an exhaust, often it is just one or two components that offer the restriction. Usually stock manifolds suck, as do stock mufflers. Often times, stock piping isn't so bad, but sometimes its just too small to be effective. Putting an aftermarket "cat back" won't be as effective as a whole re-do. You want things to be well matched throughout the exhaust, so don't just go big tailpipe and big muffler and expect tons of power. Anytime you ask the velocity to change in the exhaust, it gets ticked off. Also, further down the, exhaust flow means less. As the exhaust travels down the tubing, it loses lots of heat and therfore volume. I actaully like to drop tailpipe size one step after the muffler to A) keep velocity up, and B) keep it quieter. That's a fine-tuning step that isn't really necessary, but something to consider.

[Zgringo]

What Mr. Curtis73 is saying is absulate fact and not theory. Were talking about N/A engines.
The same thing holds true for the intake. If you have tubes or runners the right size you can adjust the point where your engine peeks at by changing the size and length of the intake runners. Newton's law say's,"Things in motion tend to stay in motion". Which means, a mass of air is rushing in the intake tube and the intake valve closes, the air continues to rush in against the valve, and if timed right, when the intake valve opens you'll have a positive pressure there. Tuned intake and exhaust has been developed to a high point in motorcycles for years but often overlooked in the automobile field.

[beef_bourito]

thank you all for your responses, my next question was going to be how intake size affects toeque but you already answered it. thanks

[curtis73]

Tomorrow's lesson; valve event dynamics

just kidding.

[Kurtdg19]

Good exhaust flow as noted is very important to a cars output. I'd also like to mention valve overlap as it is also important. Valve overlap is the period in camshaft degress in which both the intake and exhaust valves are partially open to draw burned gases out of the cylinder, and help pull a fresh charge into the cylinder. This effect extremely helps engine breathing especially during high engine speeds as there needs to be an even lesser restriction on the flow of gases into/out the combustion chamber.

Another type of system that increases output is a system called the Miller-Cycle. This type of engine is designed to gain maximum benefits equipped with a supercharger. This system keeps the intake valves open for a longer time. This longer duration in the intake stroke can lead to the charge of air/fuel being pushed back into the intake port, therefore; the supercharger is needed to keep the intake manifold pressurized in preventing the charge being pushed back into the manifold.

[Zgringo]

Originally Posted by curtis73

Tomorrow's lesson; valve event dynamics

just kidding.

Your mean, you had everyone shiting there pants, then say JK.

The fact is guys tomarrow 20 question test on the thermo dynamics of the internal combustion engine- part A, and the second part, block flex and it's effect on performance.

Albert

[public]

How about Fluid Dynamics of the intake manifold?