Stock spec v times

[JerodKing]

Originally Posted by Rowens V-Spec

So the optimum exaust tubing is dependent on where you want your power curve? Like a smaller tube for more low rpm power, and bigger for higher rpm curve. So midrange will optimize high and low end power?

Speaking in general, yes, but I can only really answer that if I was able to dyno test all sorts of different sizes, but I can guess that 2.5 is going to be the best for a Spec-V. You might lose a little bit off your bottom end, but for your powerband you are getting the optimal size, and you could go bigger and probably get a better top end, but lose too much low to mid power to make it worthwhile.

I forget what car it is right now, I think the new Corvette, but there is one that has baffles that restrict the exhaust at low RPM's to maintain exhaust velocity, but open up at higher RPMs, so you get optimal exhaust size and velocity for the entire powerband.

[nismo_pilot]

so i should take my whole exhaust off including my header and ill see maximum top end right? or do i need that backpressure to make my car run right?

[nismo_pilot]

the only car you dont need backpressure on its a forced induction car, like i said they get enough from the turbine, but mild backpressure keeps exhaust velocity CONSTANT, which creates a continuous vaccum of exhaust leaving the pipe which pulls the exhaust out of the cylinders, if you use too big of an exhaust piping to eliminate backpressure exhaust velocity will actually be slower because of the volume of the exhaust piping, which is why you should'nt go too big, try removing your entire exhaust system and get back to me and let me know if you need a little backpressure or not. what you really want is the perfect size, just like fugiot said, that will create the greatest exhaust speed and lowest amount of backpressure, but you dont want zero backpressure because that will cause slower exhaust speed, catch 22 huh?

[JerodKing]

Originally Posted by nismo_pilot

so i should take my whole exhaust off including my header and ill see maximum top end right? or do i need that backpressure to make my car run right?

It seems like you just want to argue with me here, but your not going to win, seeing as how you would still have backpressure, from a little thing I like to call THE ATMOSPHERE.

You can search it and every time you will see, the ideal exhaust has maximum exhaust velocity and ZERO backpressure. The reason a car runs shitty with a 3.5 inch pipe is because there is less exhaust velocity as well as zero backpressure. Lets say for a given car 2 inches is perfect at 3000 RPM's(since no exhaust is perfect at all engine speeds), it would have maximum exhaust velocity and no backpressure. If you were to decrease the exhaust size to say 1.75 inches you would get backpressure which WOULD PUSH THE EXHAUST GASES BACK TOWARDS THE CYLINDERS! and you would lose exhaust velocity. If you would go to say 2.25 inches, then you would still have no backpressure (GOOD!) but your exhaust velocity would suffer do to the larger exhaust size. You can use Bernoulli's equation to figure out why this happens. I haven't taken physics in 2 years now so I can't quite derive it out, but if you search I'm sure it can be done for you.

*edit* I should say however that all of this is in theory, in the real world you cannot eliminate all backpressure(ie atmosphere, pipe bends, etc...), but the ideal exhaust has the lowest possible backpressure possible (lets say 3-4PSI as apposed to 20-25PSI stock).

[nismo_pilot]

all exhaust is going to squeeze the gas some, unless you either remove your tubing or go with something like you see on a honda, you will have SOME backpressure, you just dont want a lot of it, i dont think this discussion is going anywhere though

[JerodKing]

Originally Posted by nismo_pilot

all exhaust is going to squeeze the gas some, unless you either remove your tubing or go with something like you see on a honda, you will have SOME backpressure, you just dont want a lot of it, i dont think this discussion is going anywhere though

Your right, all exhausts are going to have some backpressure, but your argument that you WANT backpressure is flawed, which I was pointing out to help inform other board members that it is not backpressure, but exhaust velocity that is important.

[nismo_pilot]

but is it not true that SOME backpressure will actually increase exhaust velocity, its the perfect match of exhaust size and engine flow

[Chiquae07]

you guys are just confusing me...i think imma pick up "cars for dummies" i get the backpressure thing, its just dumb to be arguing about....but w/e

[JerodKing]

Originally Posted by nismo_pilot

but is it not true that SOME backpressure will actually increase exhaust velocity, its the perfect match of exhaust size and engine flow

Actually backpressure will never increase exhaust velocity, but having too small of a pipe for your top end will increase exhaust velocity for your low end, not because there is more backpressure, but because the exhaust velocity at low RPMs is better suited to a smaller pipe. But at high RPMs when your flowing much more exhaust, you will get more backpressure, which will hinder the velocity and give you less HP.

I looked into it and it is the new Corvette with the baffles in the exhaust that restrict the exhaust at low RPMs, this is not to cause backpressure, but to give it a small pipe at low RPMs to have maximum exhuast velocity, and when it opens up at high RPMs it is to minimize backpressure as well. Some Ferraris and Porsches have this as well, but I can't name any specific model.

I am not trying to argue here, its just a point that many people are confused on. Even I used to be confused on the relationship between exhaust velocity and backpressure, from growing up with people that worked on muscle cars, they tend to be the ones that spread the myth about backpressure.

[slideways...]

Originally Posted by JerodKing

Actually backpressure will never increase exhaust velocity, but having too small of a pipe for your top end will increase exhaust velocity for your low end, not because there is more backpressure, but because the exhaust velocity at low RPMs is better suited to a smaller pipe. But at high RPMs when your flowing much more exhaust, you will get more backpressure, which will hinder the velocity and give you less HP.

I looked into it and it is the new Corvette with the baffles in the exhaust that restrict the exhaust at low RPMs, this is not to cause backpressure, but to give it a small pipe at low RPMs to have maximum exhuast velocity, and when it opens up at high RPMs it is to minimize backpressure as well. Some Ferraris and Porsches have this as well, but I can't name any specific model.

I am not trying to argue here, its just a point that many people are confused on. Even I used to be confused on the relationship between exhaust velocity and backpressure, from growing up with people that worked on muscle cars, they tend to be the ones that spread the myth about backpressure.

almost...
everything is right except that

but because the exhaust velocity at low RPMs is better suited to a smaller pipe.

is sort of backwards. when gas at a constant volume and velocity is fed through a smaller pipe, the gas is compressed by the pressure from behind, speeding up the flow of the gas. sort of like squeezing a water baloon. it will try and flow out both ends but since there is more exhaust coming from the engine the only way to go is out the back. the analogy is more for effect and not for cause, since gas is highly compressable and water isnt. but compressing gas will build pressure, and the high pressure will want to go to the low pressure area behind the exhaust.