engine build confusion

[Phunyguy]

If I have a semi built motor with higher compression and redline, maybe pushing 250 horse to the crank, and i put header, high flow cat, stock resonator, and dual "a little better than stock" mufflers... what would that do for flow? because it doesn't sound much different than a header / catback system other than the high flow cat.

I guess my real question is how exhaust works. how does tuning affect engine performance when it comes to exhaust. that is one of the things that always throws me for a loop. I am trying to build a car that looks like it could have been made from the factory as a trim package. kind of like the Type S lude and the Type R integra where the engines are hand made / etc. The motor is also going to have a 10 grand+ redline. I am also trying to keep it as legal as possible. hence exhaust problems. I am even going to fabricate covers in the engine bay to keep everything looking smooth. Kind of like a sleeper. If it robs a little power i don't care. my ultimate goal here isn't to have the fastest car - it is to make a car that has the potential to be a production car, so it has to be (mostly) legal and reliable.

Also, what are the stock axles rated at for horsepower on a 98 SH? i may want to keep those - not gonna be draggin, but i don't wanna break them either if my foot gets a little heavy next to a mustang.

all educated replies would be greatly appreciated.

[94PreludeJDM]

I'm not an expert by any means so anyone feel free to correct me if I'm wrong. As fas as the headers, I've read a lot of things they say the prelude has good quality headers already, unless you want to spend the grand or so to get mugen or smsp headers (DC sports are apparently a waste for preludes). I've also seen that JDM headers are the best bang for your buck. Dual mufflers I think would hurt your performance, not help it, because you won't have any back pressure. As far as the high flow cat, I'm not sure, I've actually wondered the same thing, so I hope someone can answer that. So I guess a simple answer to your question is that a more freely flowing exhaust system is going to give you better performance, especially on a built engine.

[Phunyguy]

you just contradicted yourself - "because you won't have any back pressure" - and "answer to your question is that a more freely flowing exhaust system is going to give you better performance" - i think you mean delta pressure? i found some reading in the technical forum. free flowing exhaust on a smaller diameter pipe is good because the exhaust velocity is higher flowing through it and it will create a vacuum behind it that will help to suck the first short breath of intake air:fuel into the cylinder during intake/exhaust valve overlap. I'm pretty sure this is best for a high revving engine. Back pressure is not a good thing - delta pressure is. Depending on where the pipe splits for the two mufflers will decide performance gains as well. running a true dual exhaust will make you lose performance because of the loss of delta pressure whereas 2 mufflers near the end of the exhaust will not necessarily hurt performance - especially in a high revving engine. In fact - i don't think that it will make much of a difference at all. what will make the biggest difference is the header - and a 4-2-1 header would be the best bet in my most educated guess. anything after that should be more free flowing. Someone please help me out on this - point me in the right direction if I am facing the wrong way.

[94PreludeJDM]

Yeah you're right, my mistake on my wording, that's what I meant. For the headers though, I've read that a 4-1 header is better than a 4-2-1 header (Mugen and SMSP are 4-1). Do some research on the headers to find which one will best suit your needs and goals.

[Phunyguy]

yeah - i know people say things are better - but i like to have it drawn out and thoroughly explained to me as to WHY it's better. i'm just like that.

[crzyCollegeKid]

phunyguy was absolutely correct.

back pressure = bad
delta pressure = good

try this. get a skinny straw, and a fatter straw. fill your mouth with water, and try shooting the water as far as possible with both straws. with the fatter straw, you need to try harder than the thinner one. why?? the diameter of the thinner straw creates backpressure, which i said is bad. however, some back pressure is good, because it creates delta pressure, which is what helps you spit the water farther with less effort. the higher the delta pressure, the faster fumes are leaving the engine. your probably thinking, then we want to increase back pressure to increase delta pressure, right? wrong. too much back pressure begins to constrict flow. try spitting the water out through a coffee straw, now you need a whole lot more effort to get the water out than with either of the two other straws. (refer to the technical forum sticky for a much more scientific explanation)

the header design helps with scavenging. this is harder to explain. basically, they try to design them so that the air leaving one cylinder actually pulls the air coming out of the next by creating a vacuum behind itself.

that said. heres my synopsis and advice. unless your going to spend a whole lot of money on headers, don't bother, my "performance" set looks exactly the same as the stock jdm ones and from what i've read, perform almost identically. secondly, if you haven't changed your stock exhaust, you might want to. if i remember right, the stock pipe is 1.75" in diameter (coffee straw). Most people change it to 2.25" (regular straw) [unless your boosting, then you get 3"]. anything wider than that, your loosing power. another thing, dual exhaust on a 4-banger = bad. why? ur drastically reducing delta-pressure because you are reducing back-pressure way too much. leave dual exhausts for V6+. its all about balance and tuning.

you need to strike a balance between too much backpressure and too little

[AcesHigh]

The reason that the OEM exhaust manifold is better than any header you can buy below $500 is that 1) it has a superior design and 2) it has a heat shield, which most aftermarket headers do not. The best thing you can do for your exhaust manifold is to bore it out and have it ceramic heat wrapped.

Dual exhaust only hurts because of its weight on your car; in a V cylinder setup, the exhaust ports exit on opposite sides, so it would be unbalanced and impractical to have the exhaust exit on one side. In an inline six or four, such as we have, balance does not matter as much as having a straight, adequately large sized pipe.

There is alot of exhaust theory, but in truth the effects of having a straight pipe and a B pipe with two 90 degree bends are minimal at such low horsepower.

[Phunyguy]

so a b pipe with 90 degree bends right at the end to support 2 mufflers would have almost no effect on performance, right? it almost seems as if preludes were meant to have dual exhaust because on the exact opposite side of the stock muffler there is the same body lines in the car as if to be a mirror image of the side with exhaust. Hence my reasoning for wanting to put 2 stock looking mufflers - one on each side, to have that 300zx look.

[AcesHigh]

I used to have a poorly done custom dual setup that I hastily slapped together. I also favored the dual muffler / quad pipe look of a Mitsu 3000gt or 300zx, but this system proved to be incredibly inadequate. The left bottom of the Prelude doesn't accommodate an exhaust line like the right side, so you would either have to make an h pipe or make the left pipe lower than the right.





Making the left pipe lower, or both of them lower, seriously puts you at a risk of scratching the pipes. And the h pipe idea (which I did) does not evenly distribute exhaust flow. Maybe if you can figure out a nice clean way of doing it, it would look great. But otherwise, I wouldn't suggest dual muffler exhaust on an inline engine.

[Phunyguy]

not in mine - the 5th gen has both sides just about equal... like i said - it's almost like it was made for it

[sgtii]

I was impressed with the amount of knowledge in the forum and with how you helped each other.

I have changed exhaust many times (headers, Pipe diameter, cat to test pipe and tested back-pressure along the way. I have come to the conclusion that 2/1/4 exhaust pipe with or without cat. With stock Exhaust Manifold has the best performance for my vehicle (92 lude JDM H22A, Spoon Ecu, Under Drive pullies Short ram intake Type S Cams). Good take off and awesome top end (41Mph 1st 2nd 65 mph 3rd 101mph 4th 132mph 5th peg +, Constant pull through 8000rpm.

When you think about it there is alot of time and engineering put into any stock piece so why would we want to change it with something some company slapped together. Honda's are built for performance from the factory.

[Phunyguy]

Originally Posted by sgtii

When you think about it there is alot of time and engineering put into any stock piece so why would we want to change it with something some company slapped together. Honda's are built for performance from the factory.

[AcesHigh]

Yes, and that unfortunately is our curse. It is difficult to extract more power from a tuned engine.

[Phunyguy]

true. if i go turbo... would the dual muffler setup with a high flow cat work? Because if i remember correctly - exhaust tuning isn't as anal with turbo, right? just as long as it's big enough.

[sgtii]

Sounds good to me.
But I have never tested it.