exhaust theory

[roaddigitydogg]

i'm am so sick of people hearing of people saying "if you get 2.5" catback over the 2.25 you are going to lose all your low end torque." so i'm bored and i'm gonna write this. hope i get some good responses



first i want to start and i say that i have NEVER seen a dyno show a loss in horsepower and more importantly torque (becauses thats what everyone seems to think you lose). now of course those seem to be somewhat common and recommended sizes for exhaust pipe diameter. i am not saying 3 inch exhaust is gonna make you gain any torque but i believe stock exhaust is undersized on most cars for many reasons. first of its cheaper, which is a HUGE when developing and producing a car. it wouldnt be worth it to sell to have to sell a car for a few hundred more if it were only going to gain the car 5 hp. also smaller exhaust is much quieter, because the sound waves collide with the walls of the piping many more times in a smaller pipe, and each time the sound wave collides with another object (even air molecules), it cause it to vibrate therefore losing its energy (sound in this case).



as far as large(r) piping what i dont think people ever understand is the idea of pulses. everytime an exhaust valve opens and the piston pushes out the gas, it creates a pulse. that pulse helps to carry out the exhaust gas; the scavenger effect i believe is what its called. now the bigger pipes you have (to a certain extent), the less effective the pulse is because there is more volume each pulse has to cover. therefore is your pipe is too big, although there is alot of volume for the exhaust to travel through (more volume means less pressure) the pulse is less effective when in turn RAISES backpressure. i think that is the common misconception. thats why i wouldnt never get 3" inch cat back exhaust even if i had headers, test pipe, and every other bolt on mod. but like i said earlier i have NEVER seen a loss in torque and definentely not horsepower from upgrading to .5" inches or less.



i can understand how people think larger pipes make less torque (to a certain extent), but what REALLY pisses me off is when they say "if you take off your muffler/cat/resonator you are gonna lose torque." all the those things are just restrictions in your exhaust which RAISE backpressure. now to me, it seems like there would be no advantage to having any backpressure whatsoever. when your exhaust valve opens and the piston is rising to top dead center, it must expel the gases from the cylinder. any pressure outside of the cylinder (the exhaust system) is going to want to force the gas back inside which makes your engine fight harder to push out the exhaust gas.



with this said, my ideal exhaust would be the shortest, least restrictive exhaust possible (no bends, no cat, resonator, muffler, anythign like that). i would choose a pipe size diameter that fits the best charasteristics of the engine that would allow sufficient volume for the exhaust to flow through but not destroy the pulse.



Oh and for turbos cars, I believe that the pulse is much less efficient because it has to pass through the turbine wheel which almost destroys the pulse. Reason being why turbocharge cars can get away with larger diameter exhaust, and also they put out a lot of exhaust also cause a lot of air is going in.



anyone else agree with me?

[neptune_51]

that's pretty in-depth man!


since i don't know a lot about exhaust systems and such i can't say if that is correct or whatever...

but very good write-up anyways!!!

digity, I was having this discussion with my neighbor, Where you are correct that you do not "lose " torque by going with a larger exhaust, you do however change the charactaristics of your torque curve.

In many cases such as mine where we went with 2-1/4 inch pipe and hi flow cat and dualies there was .03 sec loss in 60' foot times but picked up .02 in 1/4. without putting the car on a dyno I would estimate about we pushed the torque peak higher up the rev range.

In a nutshell I agree with you.

[Marcus]

I agree on some points... but not all of them.

[shiny]

Tell me if there's something wrong with this reasoning...

There's often some amount of intake and exhaust valve overlap. That said, the speed of the exhaust gas out of the exhaust ports is higher than the of speed of the intake charge through the intake ports because the pressure differential across the exhaust valve versus the exhaust gas is higher than the pressure differential across the intake valve versus the intake charge(note: the heads are designed such that exhaust gas is moved away from the intake valves by the exhaust valve's pressure differential creating an area of lower pressure near that intake valve to allow the pressure differential across the intake valve to result in the intake charge moving into the cylinder.)

If the exhaust gas is evacuated too quickly, some of the intake charge can be drawn out of the exhaust valve, resulting in lower fuel economy and possibly lower performance. The tendency for this to happen will be directly proportional to the ratio of the exhaust valve pressure differential to the intake valve pressure differential. As a function of engine speed this ratio will decrease, thus at low engine speeds the tendency to lose the intake charge to the exhaust will increase. To combat this, an exhaust system is designed such that more backpressure will exist to decrease the exhaust valve pressure differential. This will in turn decrease that ratio inhibiting the loss of intake charge at low engine speeds.

Of course, with very flexible variable valve timing you could combat this very easily by making the amount of valve overlap proporational to engine speed. And also... that backpressure does waste energy. A force is applied indirectly through the gas as a result of that backpressure, so the piston performs work against that backpressure over the distance it travels -- so backpressure is used as a necessary evil to allow low engine speed usability and economy(which for street going daily drivers is one of those necessary things), but robs some amount of power at high engine speeds.

Also, of course, you're right about the cost aspect. That's a major factor in all aspects of design.

Regarding your idea of a larger pipe increasing backpressure(or perhaps I read you wrong?) -- I feel that you're not quite correct on that. I think that due to that pulsed nature there's probably a good medium somewhere where the effective backpressure is the least. Also, it should be possible to take advantage of resonances in the exhaust system to help scavenge exhaust at some frequences. Maybe when I get more time I'll work it out or throw it in a CFD package... For now, all I can give is an analogy: Does it take more force to push and pull an air plunger pumping air through a small pipe or a large pipe(with the same plunger volume in each case)? the small pipe, so the "backpressure"(force over unit crossectional area at some point in the exhaust path) is higher.

All in all, I think the benefits of the exhaust system really depend on the specifics of the application. In some engines that have a lot of valve overlap at low engine speeds, the hit from having reduced backpressure on low rpm torque may not be worth the extra torque at high RPMs. If a variable valve timing configuration is setup such that the backpressure is unnecessary, it's likely that the exhaust system was already setup to take advantage of it by the manufacturer. Of course, on some applications, the exhaust system may have been designed in a different country where emissions & noise regulations were different... and instead of redesigning the entire thing different(possibly more restrictive) cat and mufflers may have been attached for that country.

Ok.. that was fun.

[roaddigitydogg]

good response shiny. about the pipe diameter, yeah i dont think the bigger pipe the better, there is a correct medium for the car. for most cars i've seen, its inbetween a 0-.5" increase in diameter. the part about overlap was awesome, i totally forgot about that and it makes perfect sense. but still like i said i wish i had a dyno and some money, cause every dyno of a car i've seen before and after exhaust (assuming its not a huge jump in size diameter) hasn't shown a loss in toruqe and definentely not horsepower, whether its a low or high reving engine (i usually agree that exhaust helps the high-revers more anyway because it fits their powerband better. not always the case though).

as far as Deep6's comment. again the torque curve charastics are changed, but usually only increased, and not pushed forward or backward. as far as your example from running your car at the track, you would have to do alot of runs to prove that your exhaust caused those times. the temperature at the track could have been lower that day causing a lower 1/4 ET but you just didnt launch as good. too many variables. if i was gonna judge something like that i would have to make atleast 10 runs to see if the data was consistent.

i'm gonna try to find some dynos that show before and after exhaust

[Mr.JasonBH]

I run a 2.25 pipe.
The torque is at a different level, at higher RPM's it picks up, before I got my exhaust done this weekend, I had more low end torque, with similar pipe size, but with a resinator, I think I went from a 2 inch pipe to a 2.5.

I agree on the torque change, but most NA cars rely on back pressure so mostly, a larger pipe isnt needed unless you go turbo.

[Thrillhouse]

The simple truth is that SOME backpressure is essential to proper engine functions. Go stick some stub pipes off your exhaust ports and tell me how well the engine runs. You probably won't even be able to start it.

Each pulse of exhaust helps pull the pulse from the next cylinder along with it. The reason larger exhausts are bad is because they decrease exhaust gas velocity. This means the engine must work harder to expel waste gases which is why you lose low end torque.

On the other hand you gain top end power as at high RPM the exhaust gases are travelling fast enough to make the larger diameter pipe efficient. Essentially changing exhaust diameter shifts the power curve of the engine to different RPM ranges. You open the exhaust, you get more of your power at higher RPM, but you lose some in the low RPM.

The best exhaust is not the shortest or largest diameter, the best exhaust is the exhaust that maximizes exhaust gas velocity at the RPM levels you need. IE: drag racers would want a large exhaust as they're at WOT all the time. Autoxers would want a smaller exhaust as low end power is essential to get out of a tight corner. Road racers need a compromise as they need top end power as well as low end power.

Finally, the blanket statement "backpressure is BAD" is completely wrong. Backpressure is necessary to proper engine operation, removing all backpressure would cause your engine to run like crud, or not at all.

[roaddigitydogg]

i must have a completely different experience then. although i cannot didnt dyno this, i ran a jimmy rigged side pipe for a while. it was just the headers to the flex pipe, then i put a wide 90 degree bend after the flex pipe and ran it out behind my back of the passenger front tire. oh man did that thing rev (and not to mention sound almost like a the mix between a mustang gt and a WRX. ever tried taking off your cat and just running headers? its an awesome sound). never had i been able to get my tires break loose from flooring it in first gear, but it did then. it did not feel worse at the low end either.

my wonder is why they dont have a variable exhaust by now, just controlled by some valves that open up to create backpressure at lower rpms and allow better flow at higher rpm's. i know there is such thing as the supertrap muffler which you can add/remove disks in the muffler to increase/decrease backpressure, but its a one time thing instead of changing with the powerband.