back pressure for aftermarket exhaust

[AE85.6]

for a 2.25" exhaust on a stock 4age that has a meaty big stainless straight through rear muff would a big coby up front give enough back pressure or to free flow ??? would this slow it down a little because of having no resonator ???

also same question for someone that has just done this on a 3a engine

[strap-on]

cobys make awesome resonators dude, cos ive been told they dont disturb ur flow too much, as long as ur pipe aint too big u will have enough back pressure so stick to 2.25,

as far as im aware the smaller piper means faster velcoity of the air flowing thru it which draws more air out of the engine, get two cobys to quieten it down, and ur system will be like min which is real mint

[AE85.6]

sweet man thats what i thought just wanted to confirm cheers

[LowRydr]

paint your coby black lol i h8 the look of them

[Cyclone21]

paint your coby black lol i h8 the look of them

Yeah mine was red and looked HORRIBLE at the back of my DA6 Integra, got the high temp spray paint out, sussed

[10k 20v]

don't worrie yourself about backpressure, it's a total lie.
you have to worrie about tuning the pulses and keeping the exhaust gas temp up all the through the exhaust to the tip

[RedMist]

Ewan, I think the reason the "old wives tale" regarding backpressure came about from the fact that people were getting better results with restrictive exhaust systems over free flow open air systems. Little do they realise it's all to do with exhaust cam timing. If you can reduce the backpresure you need to either alter your exhaust cam timing. Factory engines are tuned for a certain exhaust system, when that is changed performance can suffer. It's a tricky subject thats another of the engine details that is goverened by mathematical flow calculations.
But your most certainly right, if you can pump more air out you get more HP! And your again right in the fact that he's also talking about velocity not only backpressure!

[Mr Revhead]

I think the reason the "old wives tale" regarding backpressure came about from the fact that people were getting better results with restrictive exhaust systems over free flow open air systems

i also think ppl get confused between back pressure and the exhaust pulse...
what you want is to tune the exhaust so the pressure wave from the exhaust pulse is in time with the opening of the next exhaust valve to help scavange (suck) the gases out of the nest cylinder.
when one cylinder fires the exhaust from that cyl races down the exhaust, and the pressure wave from this will bounce back towards the cyl head, so you want the length tuned so this wave is just rebounding off the backside of the valve as it starts to open there by sucking the gases out.
can get confusing as its a pressure wave, not the actual gases as such that you are worried about. and im not the best at explaining....

[RedMist]

Reversion pulse tuning, interesting topic.

[Mr Revhead]

yeah, good luck with the maths on that one!! makes my head hurt just thinking about it....

one thing i do know is to really develop a good exhaust it can take a LOT of work experimenting and mucking around.... unless your good at black magic

[RedMist]

Can only be done once and I'm NEVER EVER going to do it for anyone else!
So how do you get 32 foot of primaries under the bonet? SHYTE!

[Monsterbishi]

Reversion pulse tuning, interesting topic.

Yeah, something WRX's didn't think about until 2001...

"But it sounds cool"

"And costs 20hp"

[nite b]

Just one tip. When ordering your coby, ask for chambered reso style. Bout same price.

Have always used these on 4ages and they rid that tinny raspy note
a lot of N/A 4ages get.

[Loudtoy]

Can only be done once and I'm NEVER EVER going to do it for anyone else!
So how do you get 32 foot of primaries under the bonet? SHYTE!

Haha i can just imagine what that may look like, but yeah looks like a real prick to work out i had a look at it once but never got into it enough to start working lengths and things out, just enough to get the basic principle not realy any more it made my head hurt

[vvega]

just think of pulses as high pressure waves and what folows a high pressure wave ????

a low presure area know as a vacume

its not the high pressure wave you want its the effect the vacum after it has that you want

if you want to get into pulse tuning start with making 2 stroke expansion chambers
its pritty much the pinical of pulse tuning


v

[RedMist]

VVega, the principal is simple, but making it work most certainly isn't timing the pressure drop to coinside with an open exhuast valve is very tricky and only works for a very narrow rev band.

Good point about expantion chambers, but arent they only used on a single pot? With reversion pulse tuning your attempting to return a low pressure wave to another pot or slam a high pressure wave into a closed valve and then have it open as it rebounds back down the pipe.

[vvega]

this is where it gets fun:S
expansion chambers are used one PER cylinder
most multicylinder 2 strokes have this
the reason is pulses reflect of each other so any more than one pulse down a tube and you will end up with a mess

4 strokes are the same but what most people dont realise is they are tuned twice
once in the headers
once in the main exhaust

the headers are the most effective place to tun as once your pulse loses heat the pressure in the pulse drops and so becomes inefective
that is why oversized exhaust can hurt prformance as evertime the gas expands to a larger pipe it loses energy and volecity and thus loseing theh small scavaging effect that your main exhaust can have


so basically we come back to the headers
this is where most of your pulse tuning is done
the theory is simple
longer header tubes tend to increase power below the engine’s torque peak and shorter header tubes tend to increase power above the torque peak.
large diameter headers and collectors tend to limit low-range power and increase high range power. Conversely, small diameter headers and collectors tend to increase low-range power and limit high-range power.

delta pressure and velocity are the most important things to uderstand

bbl have to work

vvega