Exhaust issue

[flamingheads]

I have a 2.5" exhaust with only one muffler at the back, which is loud as all urine and Im wanting to quieten it down. Today I went to the muffler shop and the guy there reckons 2.5" is too big for a 4AG because it doesnt have enough back pressure and 2.25" is the maximum size he'd go.
I know that small engines need back pressure but my cams apparently have the inlet and outlet valves open at the same time and I don't know weather this is why there is such a large exhaust.
I also have an almost complete 2" system I could put on.
Does anyone know anything to help me out?

[.wolfwood]

i have full 3inch system on my 4agte. its turbo back and got no problems, maybe cuz its turbo so its different story

[flamingheads]

I think turbos and big engines are a different story like you said, little NA engines need back pressure to run properly.

[MAT713]

Originally posted by flamingheads@Mar 6 2006, 10:37 AM
I have a 2.5" exhaust with only one muffler at the back, which is loud as all urine and Im wanting to quieten it down. Today I went to the muffler shop and the guy there reckons 2.5" is too big for a 4AG because it doesnt have enough back pressure and 2.25" is the maximum size he'd go.
I know that small engines need back pressure but my cams apparently have the inlet and outlet valves open at the same time and I don't know weather this is why there is such a large exhaust.
I also have an almost complete 2" system I could put on.
Does anyone know anything to help me out?
yeah dude, defiently on the big side of things, 2.5 would be ideal for a ZE but for NA it is to big, every car runs overlap in their cam timing. even from factory... finsih the 2" system off and see how u go with that, unlessu add more mufflers it wont necesarrly be quiter

[flamingheads]

yeah dude, defiently on the big side of things, 2.5 would be ideal for a ZE but for NA it is to big, every car runs overlap in their cam timing. even from factory... finsih the 2" system off and see how u go with that, unlessu add more mufflers it wont necesarrly be quiter[/b]

Thanks for that.

Also I was wondering if I did have to use the 2.5" exhaust if putting more backpressure in it would work or would that just be stupid.
Thanks for the help.

[biggo]

Back pressure is bad!

End of story.

[monkeymajik]

You could create backpressure by putting more stuff in there, resonators or silencers etc..

Generally 2.25 is optimum for a worked 4age.

[mattysshop]

depends on flow... 2.5" press bent system can go less than 2" in some of the bends.. people will argue that mandril bends don't make that much of a difference.. i want to hit them in the head with a nice big pressbent system.... if you want a quiet system.. you should shut it up a bit with some mufflers.. like a s bend one.. so the flow physically blows into the side of the muffler.. rather than 'straight through' i'll tell you now though.. there is a certain limit you can go.. power or flow... to shut it up.. you kinda need to block the flow.. but as you said a bit of resistance can help torque..

[flamingheads]

The 2.5" system on at the moment is mandrel bent and the 2" one is just bent and theres a lot of difference in the bends. I know I'll have to lose a bit of power for the quietness but I think it'll be worth it. I guess I'll just take it to the exhaust guy and see what he says


ÂÂÂ# Back pressure is bad!

End of story.[/b]

Small NA engines need a certain ammount of backpressure to work properly. Do some research.

[Medwin_3sGTE_AE86]

Okay lets do some myth busting... Back pressure is needed for engine to run well= myth!!! (well i think so anyways)

From everything i've studied, back pressure is bad... To create maximum torque at any given rpm, u need to achieve maximum cylinder fill rate... back pressure is detrimental to achieving that... the whole point of having tuned length extractors etc. is to tune the pulsing of the exhaust wave so that a low pressure region will be generated at the exhaust valve when it opens to extract maximum exhaust gas. All the scavenging effect occurs inside the extractors and have not much to do with the rest of the exhaust... The only basis for exhaust too big is not the back pressure, but the increased cooling surface as well as the volume of exhaust to push... technically SOMETIMES in SOME applications it is possible to flow better with a smaller exhaust but thats in RARE instances!!!

I'm simplifying a lot of stuff here and not everything is quite true but thats the rough idea... with good tuning... u want to MINIMIZE back pressure to MAXIMIZE exhaust flow to produce MAXIMUM torque...

2.5" too big... i dont think so... TODA extractors measure 60mm at the collector which is bigger than 2.25" and closer to 2.5".... it would be stupid if u fitted a exhaust smaller than the collector...

Noise problem... well there's not a whole lot u can do except for adding more mufflers... noise cancelation is a difficult science and just because one muffler works well on one car doesnt mean it'll work well on another... mufflers and resonators target different frequencys and usually a combination will work better... but from my personal experimentations... 4ages are just a f***ing loud engine and theres f*** all u can do unless u use really restrictive mufflers... restricting gas flow is the easiest way to reduce noise....

[Medwin_3sGTE_AE86]

Just to clear a few things up... everything i said above applies to full throttle... partial throttle is a different story... When theres not a whole lot of high energy gas flowing through... having a bigger exhaust is actually a restriction... But the story is still the same... trying to minimize back pressure to increase flow...

Have u guys seen F1 exhaust.... not a whole lot of back pressure there... infact the exhaust "trumpets" are positioned in a way that the aerodynamics of the car can draw the gases away even faster... also similar with v8 super cars....JGTC and every other form of motorsport car that allows it....everyones trying to minimize back pressure and using aerodynamics to generate low pressure regions around the exhaust exit to create NEGATIVE back pressure!!

I personally think the myth of the back pressure is mainly based on road cars which have suffered a reduction in torque during partial throttle...and the cars ignition and fuel were not tuned to match the exhaust...

[rthy]

I totaly agree with Funkdoc, people who talk about backpressure are just people who don't understand the pricibles of harmonic resonances, air speed and heat loss.

[flamingheads]

Have u guys seen F1 exhaust.... not a whole to of back pressure there... infact the exhaust "trumpets" are positioned in a way that the aerodynamics of the car can draw the gases away even faster... also similar with v8 super cars....jgtc and every other form of motorsport car that allows it....everyones trying to minimize back pressure and using aerodynamics to generate low pressure regions around the exhaust exit to create NEGATIVE back pressure!![/b]

These examples are of high power cars with big capacity engines. I'm talking about not particularly high powered, small capacity engines. I don't actually know anything about back pressure but I have read that small engines need a certain ammount of it. I'll look into it and see if I can find a definite answer.

[flamingheads]

The standard for exhaust diameter is to not run too large of exhaust, for the engine needs a certain amount of backpressure. This is correct in the sense of one should not run too large of diameter exhaust tubing, but the statement of the engine needing backpressure is not. You need to have the least amount of backpressure possible to produce maximum power. Too large of diameter exhaust will cause a power loss and loss of low end torque because a larger pipe has less exhaust stream velocity than a smaller pipe. If the exhaust pipe is too large, then the exhaust flow will be slowed with less velocity. There are exceptions, forced induction or high-volume race engines require a larger diameter exhaust due to the flow characteristics of the engine. Supercharged and turbocharged vehicles can have an exhaust gas volume of 1 to 2 times more than an equivalent displacement naturally aspirated engine. In this case, one is able to use a larger diameter of exhaust for greater performance.[/b]
From http://www.proficientperformance.com/tech_back_pressure.php

And according to that page for around a 1600cc (100 cubic inch) engine a 2" system is good. So I guess I'll talk to the exhaust guy and maybe put on the smaller system.

[fullmetal]

If you're in SA I have a 2.5" resonator you can buy for cheap which is also attached to some 2" pipe which will altogether make up about half of a hachi exhaust system - part of my old system before I changed bits.

This old system of mine was 4-1 extractors -> 2" pipe -> 2.25" cat -> 2" pipe -> 2x2.5" resonators (about half the total exhaust). This system worked pretty well in terms of flow and reasonable noise level, only reason I changed was to ditch the cat because I thought it was getting clogged up for more power.

Anyway my point was have you thought about using part of the 2" system you have and part of the 2.5" system so you don't have to fork out for a whole new one? The smaller pipe will make some difference to noise level, and if it's still too loud add on a straight through mid-muffler or resonator.

[flamingheads]

Unfortunatly Im in QLD, where all the good deals seem not to be. But the 2" system only needs about 10cm more pipe and a bit for it to bolt onto the extractors, plus I'll get a resonator or muffler put on so it should be fine.

[Medwin_3sGTE_AE86]

Capacity dont really mean much... i can have a 10L engine that ticks over at 250rpm peak and it'll need a exhaust no bigger than 2"... It dependent on horsepower... Because hp is a measure of work done and 4 stroke internal combustion engines have roughly the same breakdown of thermal efficiency. ie. 33% exits through the exhaust...33% heats up the engine and is cooled by the radiator and 33% is power created to drive the wheels. This means if ur car creates a certain amount of power... the amount of gas required to be flowed will be similar. If i remember correctly 190hp requires 2.5"... so if ur not chasing that sort of power... i guess 2.25" is enough.... i'd definately stay away from 2" though.... just incase u want to make modifications in the future....

[biggo]

I suppose it all depends on the person and what THEY want. Its totally up to them if they want their back pressure.

As for me, in my GZE in going to run a minium restriction sytem as i want my heat out of the engine bay as fast as possible. However, to do so will require me keeping the pipe hot. Remembering the colder the air the moore dense it becomes, thus slower to exit the system.

So yair its sorta a roundabout way of doin things i suppose, but hey, whatever floats your boat http://www.ae86drivingclub.com.au/forums/emoticons/smile.gif

[b1gb3n]

JUST KEEP UR CURRENT SYSTEM N GET A FEW RESONATORS OR A QUIETER MUFFLER. FRM EXPERIENCE, THE BIGGER THE DIAMETER THE QUIETER IT IS.... UNLESS U GET A WOOSY 1.5 INCH OR SOMETHING LIKE THAT.


I ALSO AGREE WITH THE NO NO FOR BACKPRESSURE.


FORGIVE THE CAPS LETTERS, BLOODY WORK COMP http://www.ae86drivingclub.com.au/forums/emoticons/angry.gif

[DavisJD]

in the argument to do with back pressure, simple experiment.
1) Put on ear muffs
2) remove exhaust after extractors
3) Test drive or dyno
http://www.ae86drivingclub.com.au/forums/emoticons/laugh.gif
want to swap exhausts, i have a 2 inch system. Worked great with my old 4age but looking for a 2.25-2.5 system for my GZE?

[flamingheads]

want to swap exhausts, i have a 2 inch system. Worked great with my old 4age but looking for a 2.25-2.5 system for my GZE?[/b]
I already have a spare 2" exhaust if I go that way and if I do I'll be selling the 2.5". I think I'll just take the car to a muffler place and do what he reccomends.

[love ke70]

the size of the pipe doesnt effect how much noise you make, its all about the muffler.
get a good muffler and keep the 2.5"
my 2c.

[dori_86]

just i quickie... would a 4age with extractors exhaust fit onto 4ac, im guessing it wont be a straight bolt on as it had extractors, but would possible need a adapter made up? i dont wont to try and fit the thing for it to then not fit!

[.wolfwood]

here is a question i have about zorst, with a turbo back system its its 3 inch all the way to the end with out some milo can on the end adn with jsut a high floe cat it is going to be uber loud? or not really cuz i herd that turbo isnt as loud as n/a or am i way off

[rthy]

just i quickie... would a 4age with extractors exhaust fit onto 4ac, im guessing it wont be a straight bolt on as it had extractors, but would possible need a adapter made up? i dont wont to try and fit the thing for it to then not fit![/b]


the exhaust on a 4ac is the oppersite than that for a 4ag, so not even with heavy chopping would it fit

[dori_86]

thats why i couldnt put it on! i knew there was a reason why i hadnt put it on yet http://www.ae86drivingclub.com.au/forums/emoticons/blush.gif http://www.ae86drivingclub.com.au/forums/emoticons/paperbag.gif

[damo101]

i reckon keep the 2.5..and get like a s flow muffler..that should give u a bit of noise reduction as well as back pressure coz of the restriction of the flow in the s bends.. and depending on how many bends are in the muffler, the amount of noise reduction will differ as well..

basically the s flow mufflers have a series of quarter wave resonators or something like that cant remember the exact names but each of them are tuned to different frequencies and they absorb the noise off different frequencies..

but i agree with the 'no backpressure is good' thing..reason being gases flow from high presure to low pressure..

i think in your case its not the lack of back pressure..but too much of it? reason being for the same amount of volume air flow out of exhaust valves;the velocity of air will be slower in a big pipe compared to a small dia pipe..when velocity slows;local pressure rises + pipe friction + energy losses from bends etc .will give u some back pressure..maybe thats y the muffler shop said its too big..

sorry for technicality but dats my 2c..some1 correct me if im talking trash