Quote:
Originally posted by brendon2k
BACK PRESSURE IS NEVER GOOD!!!
Now that I have that clear, you do want the exhaust gas to exit with some velocity because then it helps pull more exhaust out of the combustion chamber resulting in more power. With dual exhaust you will lose power and not gain anything. The exhaust gas won't be passing out fast enough to help gain power. That is why most people go with a single exhaust with piping from 2.25-2.5 inches maybe 3 if you are FI.
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Ok this is long, but please read the whole thing before commenting...thanks.
I agree backpressure is never good. Or else you would see professional dragsters having mufflers right? Well they don't so whats that tell ya?
But i don't agree on the exhaust "pulling" more exhaust gas out of the engine. Thats not true. The engine "pushes" exhaust out because the cylinder head comes up while the exhaust valve is open which forces gas out into the exhaust.
Anyways further on the topic, back to velocity; Exhaust is going to flow wheter you have a 2.5" pipe or a 6" pipe, it is still going to floow just not as fast because Pressure In to the exhaust is > pressure out which makes flow. So if you have dual exhaust, that is creating more volume of exhaust gasses to pass through the exhaust system. I.E. each pipe takes 1/2 of the gasses at 1/2 the rate which means 1/2 the back pressure. Now with single exhaust, one pipe is taking 100% of the gas all the time at 100% rate of the velocity the engine puts out.
Ok so lets say you have 2 setups:
#1: dual 2.25" exhaust
and
#2: single 3" exhaust
Now which is going to have more volume to move air with the least backpressure? The dual exhaust right? yes...because pressure in an exhaust system is bad because exhaust can't escape which causes the engine to not burn correctly in the combustion chamber and "sufficate" the engine. Now to prove there is less pressure in dual exhaust lets look at the math:
single 3"
ok so A=(pie)r^2 right?
A=(3.14)1.5^2
A=~7.01"
Dual 2.25"
A=2((pie)r^2)
A=2((3.14)1.125^2
A=~7.95"
So try this at home experiment: Grab two things: a straw and a paper towel roll. Blow as hard as you can through each. What should you observe:
Straw:
Air flows very fast through end, but it is very hard to blow through the straw that hard and takes a lot longer to exhale all of your air in your lungs.
Paper towel roll:
Air flows slower through the end, but it is a lot easier to blow through the roll and takes a lot less time to exhale all the air out of your lungs.
(please do not reply with sticking 2 straws in your mouth doesn't work better then the paper towel roll. If you want the point, i am saying that a system with less volume causes the engine to work harder. The volumes of the experiment are not really close in values to the dual exhaust and 3" single exhaust. Its just a simple experiment/example to show how air flows through different volumes of piping.)
Apply this to your engine. on the exhaust stroke, if you apply pressure back into the cylinder, that would make it harder for the cylinder to come up and force the engine to work harder to finish the stroke which means a loss of power for the other cylinder that is currently compressing. But more or less what is happening is that part of the exhaust gas is getting mixed in with the fresh air/fuel mixture that is coming in and is causing not as good combustion which is causing loss of power.
So in conclusion, the dual exhaust DOES make the exhaust flow slower, but the system can get rid of the same amount of exhaust as the single pipe, but the dual system gets rid of it with ~1/2 or less the pressure of the single pipe. But i do see what you guys are talking about. with bigger pipes the chance of reversion is greater, but you have to have a pretty large system for reversion to become a problem. Like dual 3" would most likely cause a problem, but running a dual 2.25" or 2" would be a little better or about the same as one 3" all the way back. And you won't too suseptable to reversion because the the backwards flow toward the engine (not pressure, flow) would be stoped by muffler(s), resonator(s) and the cat.
I will say that I totally love the look of dual exaust... and if there were a way to maybe use smaller pipe after the split to keep back pressure up it would be cool... just a thought.
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