Exhaust tube size

[Safety Guy]

Thanks for the link Nate.

Lots of variables here:

-engine type and size
-stock vs. custom engine work (including intake variables probably)
-exhaust manifold vs. header (and type of header)
-exhaust pipe diameter
-any differences in pipe diameter vs. component in/out diameters
-sizing and types of cat
-sizing and types of muffler

To really do a good test, you'd have to take your car to a dyno or a dragstrip and do several runs for a baseline score. Then change one variable at a time to see if you make more power or not. Again, three to five or more runs on each variation to eliminate chance.

To do a good job, you'd have to do many many tests with different combinations.

I guess if you had a programmer who knew enough, and a supercomputer, you might be able to crunch enough numbers to begin with and eliminate some of the variables. You'd still want to do some real world testing to confirm your hypotheses.

Now figure in costs and any weight differences between pipe sizes and components.

That's how I see it anyway.

Karl

[Pocket_Rocket]

Well on my Swift GTi I had a complete 2" exhaust including a cat, no resonator and a 13" long stainless steel canister muffler, and it kinda felt slower...I think I've read that too big an exhaust can slow down flow, I.E. make the gases lose their inertia, and prevent proper cylinder scavenging which can cause reversion. And since you lose that vacuum effect given when the gases have enough inertia, induction suffers a bit, 'cause that vacuum isn't there anymore to help suck that intake air in during valve overlap. I'm not 100% sure about this, maybe Jim could straighten this out?

[skeeters_keeper]

^funky, thats what I did, have you seen my latest "skeeter update thread"? i have a custom 2 1/8" DP + exhaust, only thing to disrupt flow is a cat (nececary for inspection).



Geneva... if you want to learn try reading the link i posted. Fancy that, it answers your question! whodathunkit?

As exhaust gasses are allowed to expand, they cool, and slow down, decreasing scavanging effect of the exhaust pulses... so a smaller diamiter pipe is more efficient for small displacement engines. larger displacement engines (or turbo/supercharged engines which are moving more air) benefit from larger pipes. There is a lot more to it than that too, i suggest you read a little if you want to learn. But back pressure is still bad, your not increasing back pressure with your smaller pipe, your actaully in effect decreasing it because of the increased exhaust gass velocity.

for further reading after you read the article I posted, google "exhaust theory", there is a lotta good info out there!

[funky technician]

the most ideal size (friction free scenario)is to keep it the same size the whole length of the exhaust, right form the down pipe, everytime you change the diameter of the pipe, you disrupt the flow and loose potential energy, (bernoulli's principal) you can usually go slightly larger then the down pipe size for your exhaust to make up for the extra restriction caused by the friction of the exhaust on the tubes walls as it travels the lenght of the exhaust, so roughly a 2 inch exhaust is a good size for our given down pipe, or make a new 2" down pipe and keep it a 2" exhaust, that way your opening up the exhaust and not changing the shape throughout the whole lenght

[GenevaDirt]

if it does not need the backpressure then why does it matter what size pipes and muffler you put on it? I am not trying to argue, I am trying to learn. I put a bigger exhaust and cherry bombs on my car and the low end torque was gone along with what little passing ability these cars have, not to mention my gas milage dropped off. Once I put it back to stock exhaust the cars pep was back and so was my gas milage. If it is not back pressure that causes this then what is it?

[skeeters_keeper]

back pressure is always bad, on modern engines.. on carb'd cars it can cause the engine to run a little lean, but that's b/c they need to be jetted to compensate for the better air flow through the engine.
here is a decent not too in depth article on exhaust theory... keep in mind this just skims the surface! i've read some much more in depth articles, and still am no where near understanding everything. I would say for a B3 no larger than 2", probably 1 5/8 would be better. a B6 with PnP/header might benefit from 2", which is what i have on my B6, but even then it may be over kill. Toss in a turbo though, and you need to move a much larger volume of air, and so the larger pipes are more efficient.

http://www.nsxprime.com/FAQ/Miscellaneous/exhausttheory.htm

[funky technician]

I would not go any larger then a 2.0 and use the best flowing muffler and cat you can get ( affford), needing back pressure in an exhaust system is a myth.

In the old days guys would put on headers and free flowing pipes and the engine would run lean, any damage was caused by not adjusting the carb to compensate with the new free flowing exhuast. So they would blaim "lack of backpressure" as the cause of the problem and not the failure ( of them) to properly tune the carb afterwards.

that is correct, I got in an arguement on this forum before about this, few people know this fact

[alpaccino]

by the way what would be the recommended size for the b5 (i wanted to know this for sometime now)

[alpaccino]

from my personal experience i did a simple muffler pan change and the loss to the back pressure was so significant that in forth and fifth gears the car would just go to sleep on me. so i think i wouldn't want to go so big prob go with billthekat

[NovaSS]

I would not go any larger then a 2.0 and use the best flowing muffler and cat you can get ( affford), needing back pressure in an exhaust system is a myth.

In the old days guys would put on headers and free flowing pipes and the engine would run lean, any damage was caused by not adjusting the carb to compensate with the new free flowing exhuast. So they would blaim "lack of backpressure" as the cause of the problem and not the failure ( of them) to properly tune the carb afterwards.

[nonamekid]

I would go 2 inch and then get a restrictive muffler. Later if you put a bigger motor in, switch to a less restrictive muffler.

[billthekat]

i just ported my b3 manifold to match my b6 head. and went with a 2" downpipe and 1 and 7/8" tail pipe after a glass pack mufler. and it flows plenty well. i belive that for a stock b3 i wouldent want to go biger than inch and 7/8 or you would lose to much bottom end.

[Fordverde]

So, should I go with the full 2" or an 1 5/8" tube??? What's the original tube size???

[GenevaDirt]

^^yes....but until you do a swap you will notice a performance drop and maybe a decrease in fuel milage. I know that when I had the larger exhaust with 2 cherry bombs on it the car had no power at all...and milage fell off. I put it back to original exhaust and it was a lot more peppy and milage went up by about 3-4 mpg

[sasquatch]

isnt the fms exhaust 2.25 inch. and dont they say for stock and modded applications or motor swaps. if you do put a 2.25 inch on it then you can do a motor swap and no worry about doing the exhaust later.