regarding exhaust back pressure versus gas velocity

[bodyrollzero]

hey there ive recently prepped for a very short side exit exhaust, coming out just about a foot and a half or so after the two-one down pipe.
ive done this more so as a joke in preparation for the actual motor i would like to have, however i wont lie i think it sound kewl.

through out the course of my car modding career i have heard many things having to do with exhaust back pressure. many have said that cars need some amount of back pressure to run well, and that putting huge exhausts on cars could actually reduce HP.

well after hearing the way my car sounded with very little exhaust at all attached to it (ive made sure its enough to keep my valves warm on shut off) ive come to wonder how important back pressure really is. after doing some research ive found that back pressure really isnt what cars need to run well, it is just commonly a result of what cars DO need to run well- exhaust gas velocity.

in other words little 1.8 Hondas with 3 inch pipes decreases back pressure- wich is a good thing, but they also drop the exhaust gas velocity so low that the power that COULD be gained from less back pressure was negated by the slow exit of the exhaust gasses as they left the combustion chamber.

this being said what sort of header/ pipe selection would keep a 2jzge happy with very little piping after the header? (i do plan on keeping it NA). im guessing equal length headers would be the obvious route however in which config; 6-1, 6-2-1, what have you

I can add my own experience of the pipe on the car as it is (5mge) and they arnt as terrible as one may think. the car is obviously very loud, however it has no true running errors. it idles correctly, delivers power better then before (minus a few on the lower RPMs), and is quite fun to drive around in lol
the only only issue ive run into is a singe misfire under sudden compression breaking- which i believe is a cause of the turbulence in the stock exhaust manifold.

[CarlH]

I would do research on the term reversion, That is the effect of exhaust system length and the effect the returning pressure wave has in helping fill the cylinder with the air/fuel mixture.

I was at a car show recently and none of the high end cars, Lambos, Porche, Masterati or Ferraris had loud exhaust, It seems to me they didn't need to be posers and didn't have to prove any thing. Plus they probably wanted to enjoy driving the cars and not attract attention.

Even tracks today have noise limits, I'm glad no one lives in my neighborhood with loud pipes or I would go to war with them. Waking people up in the middle of the night or disturbing them during their off time is something I find in very poor taste.

[Donn29]

Way too much madness going on here. I'd expect that there is a gain vs loss happening. As you increase the flow of the exhuast(shortening/bigger piping), you gain power, until you hit a point. Once you hit that point, you start to have that gained power drop off. I highly doubt you'd drop more power than the inital gain though. Like if you gain 15hp over stock at 3" you might only gain 10hp over stock with the header firing into the engine bay.

Just do a turbo motor if you really want to go faster. As soon as you turbo it, all of the time, math, money you spent getting the exhuast perfect is a waste, as you now make 100+hp more than a 5M/7M/JZ would n/a. Or put in a large displacement motor.

[lechner]

The real effect of shortening/widening your exhaust is to tune your exhaust for a higher rpm. You trade gains at higher rpms for losses at lower rpms. This usually increases peak HP on a dyno (if your engine can use the higher rpms), but not necessarily useable power or driveability.

The ideal situation (and what you see in most supercars) is an exhaust pipe of sufficient length and width to actually create a siphon as the exhaust gasses flow out at peak hp rpm. If you don't have this siphon effect, then the engine has to work to push the exhaust out of the cylinder. If the engine is having to push the exhaust out, then a very short, wide pipe is a benefit because it reduces this wasted use of power. But it is far better to properly tune the exhaust so it siphons the exhaust out at peak hp rpm.

The big problem with a wide long pipe is that the exhaust cools in the pipe and slows down as it cools. The hot exhaust up by the engine is moving faster and compacting against the cooler exhaust towards the back of the car, and the engine is having to work on the exhaust stroke to shove all this exhaust out of the way. If you have to use a longer pipe and go all the way to the back of the car, then it is important to keep it narrow enough to keep the heat up all the way to the exit. Wrapping your exhaust to keep the heat in helps a lot regardless of length/width.

I wouldn't be surprised if there are dual systems out there that use only 1 pipe at lower cruise rpm and both pipes at full throttle, as this would be more ideal, allowing the system to be tuned for max power at cruise as well as at full throttle rather than a compromise between the 2.

[bodyrollzero]

many of the over priced super cars do infact have variable restriction exhaust systems to allow for a larger powerband, however my question pertains to a VVT 2jz were the power is mostly made in mid to higher RPMs. theres also a very big difference between a track driven porches exhaust and the shiny doctor driven one u likely saw at your precious car show.

my car is no louder than any1 of the many harleys or muscle cars which drive around my neighborhood on a regular basis. i also find it funny how convinced every one is that no one should go NA. goals with my car arnt always going to be that same as others, and thus shouting TURBO at the 1st mention of a NA motor doesn't help.

my car is fully gutted and as soon as the the funds for the motor swap come in it will stop being my DD and be a dedicated drift car. so again i ask of any1 with actual experience setting up NA exhaust systems on engines with similar displacement/head flow as a 2jz.

[Donn29]

TURBO!

LOL, sorry, just seems to be a waste to do a bunch of crazy maths to try and figure out what exhuast is best.
Most people go 2.25 or 2.5 with a N/A setup.
I just find it hard to spend so much time/money/effort for another 4hp. I understand going with the newer N/A motor, I really do, but trying to make max hp on it is silly, when it'll never keep up with a stock turbo.

[rce311]

TURBO!

LOL, sorry, just seems to be a waste to do a bunch of crazy maths to try and figure out what exhuast is best. Most people go 2.25 or 2.5 with a N/A setup. I just find it hard to spend so much time/money/effort for another 4hp. I understand going with the newer N/A motor, I really do, but trying to make max hp on it is silly, when it'll never keep up with a stock turbo.

If the man want to do math and the research on building a better mousetrap, let him. You never know, he may come up with something. Instead of just throwing in bigger motors, turbos, spray, superchargers, I think a perfectly tuned exhaust sound like an interesting project.

And I did some work at a plant that makes exhaust parts for BMW a few weeks ago. Many of those bimmers use a muffler with a proportioning valve to allow higher flow rate at higher rpms. I found it quite intriguing.

Good luck with your project.

[4SFED]

Try Search - this subject has been covered several times in years past in this forum. Try searching Scavenging, tuned exhaust,back pressure, and the likes.

[StanS]

Instead of wrapping, use SS catback/muffler. SS has 1/3 the heat conduction of carbon steel so SS should approach performance improvement close to wrapping. SS lasts longer too. I had wrapped my cs catback and got a ss muffler. Catback rusted out after 150Kmi but muffler is still good.