Back Pressure - Understand the Balance

[Phantom Mark]

So I am looking at modifying my standard exhaust here, I have the welding ability just want to make sure I get the principles right.

I removed both back boxes just to see how it would sound with straight pipes, sounded rough as fek and when I went for a quick spin felt like the lack of back pressure from the back boxes had reduced the throttle response, no idea if the power went down of course.

So with this in mind I am planning on removing the centre box but retaining the standard back boxes for the moment.

Just wanted to ask how you work out how much back pressure you need, or indeed if you need back pressure ???

The only reason I ask is that I was looking at a Corvette L88 the other day with race pipes on, litrally straight through side exits with no baffles I could see..........I assume this has no "back pressure?" so this left me wondering ????

By the way, the centre box on the R8, looks like both sides come together into a centre collector ?? must this be retained, or should I be ok to simple replace that box with straight through individual pipes, I remember reading another thread here about "X" sections and how they do something or another, cant remember what now ??

TIA

[snowwolf]

Hi Mark, will keep an eye on this thread, I too have welding and workshop facilties to do a system, just need to know how!

[ringram]

Forget back pressure. None is best.
However what you can do is lose exahust flow. For example some mufflers act as an area of low pressure, thereby creating a pulse of low pressure back up the exhaust which when sited right helps make the system work as a unit and extract the gasses out. If you pull a muffler out you may be removing a tuned location of low pressure. Result might be crap flow.

This is a good read. http://www.superchevy.com/technical/engines_drivet...

Basically size headers on exhaust port flow, rest of the exhaust for engine bhp. (2.2cfm required per fwhp, formula's on in the article)

Edited by ringram on Thursday 8th October 22:30

[stevieturbo]

anyone who says an engine wants backpressure in the exhaust for performance, is an idiot.

[k15tox]

with some help i 'made my own'........i took the middle silencers out and left the back box on, but it sounded like a tractor!

this is my new 2.5inch with x pipe www.youtube.com/watch?v=rv8YXJSnV78

and one with the middle boxes removed (sounds good underload, but like a tractor on idle!) www.youtube.com/watch?v=xJrW0NnykSw

oh and a little read for you www.ls1.com/forums/showthread.php?p=1391200

Edited by k15tox on Thursday 8th October 22:37

Edited by k15tox on Thursday 8th October 22:38

[ringram]

stevieturbo said:

anyone who says an engine wants backpressure in the exhaust for performance, is an idiot.

[Phantom Mark]

Thanks guys......

Wonder if I would get the same result by removing my middle box then, the exhausts appear quite different to the Ro ? from what I have seen anyways.

Anybody else done this mod ??

Just having trouble justifying nearly 1200 quid for some bloody tunes out of my V8 !!

I had a full custom made stainless system for my 6cyl BMW made for around £300 quid and that was gorgeous with lifetime warranty !!!

[GreenV8S]

stevieturbo said:

anyone who says an engine wants backpressure in the exhaust for performance, is an idiot.

My engine wants backpressure in the exhaust for performance.

Under some conditions.

Edited by GreenV8S on Thursday 8th October 23:44

[ringram]

GreenV8S said:

My engine wants backpressure in the exhaust for performance.

Under some conditions.

Explanation and justification please Sir!? Ill suggest that statement is false.

[madmunky]

Don't turbo cars require exhaust back pressure? when I had my Supra () if you removed all the exhaust back pressure the turbo would over boost so restrictor rings where fitted in the exhaust to prevent this when the cats and bit where removed.

[ringram]

Nope. They require high pressure before the compressor to get it spinning hard, after that they require none either. Pressure differentials!
If its overboosting then thats because it was so st before. Therefore the setup was inefficient and the overboost was the symptom of a problem not the cause. My 2p.

[madmunky]

ringram said:

Nope. They require high pressure before the compressor to get it spinning hard, after that they require none either. Pressure differentials!
If its overboosting then thats because it was so st before. Therefore the setup was inefficient and the overboost was the symptom of a problem not the cause. My 2p.

Well maybe but when the car is mapped from standard and you start adding things like FMIC your increasing the pressure before the compressors so without remapping it and making the ECU control the boost you can fit restrictor rings in the exhaust to prevent the over boost.

[eliot]

I've already done this on the weekend. I cut around seam in an attempt to remove the end
can. It wont come off because of the inlet. The exhaust enters (and passes through) and goes all around the box, finally coming back into that rear chamber by another pipe (just near where I have wedge that metal in) and finally goes out via another hole in the middle out to the rear tips.


My cunning plan with the holes was allow the exhaust to bypass all the internals and enter the rear chamber:

Once all tig welded back together - it didn't make any difference.

So the MK2 plan - if I can be bothered is to cut the end off again and also cut through the feed pipe (from the side, where those holes were drilled) and make it so the feed, only goes into the end chamber.

Or for £500 you can get the Magnaflow (pn 16883) back boxes from Nemesis uk.

Edited by eliot on Friday 9th October 09:15

[ringram]

madmunky said:

Well maybe but when the car is mapped from standard and you start adding things like FMIC your increasing the pressure before the compressors so without remapping it and making the ECU control the boost you can fit restrictor rings in the exhaust to prevent the over boost.

Exactly, problem is elsewhere.. thats the general trend when people say they need backpressure! The wrong answer to the wrong question. Kind of like the Labour Government.

[Boosted LS1]

^ He said government, snigger, lol.

[cv8 mike]

ringram said:

Nope. They require high pressure before the compressor to get it spinning hard, after that they require none either. Pressure differentials!
If its overboosting then thats because it was so shit before. Therefore the setup was inefficient and the overboost was the symptom of a problem not the cause. My 2p.

Off topic I know but anyway. Japanese import supras overboost when decatted because the turbo wastegates arent big enough to reel in the boost pressure unlike the UK spec Supra. Thats why a restrictor ring has to be placed in the system.

Edited by cv8 mike on Friday 9th October 12:43

Edited by cv8 mike on Friday 9th October 12:46

[ringram]

Its nice when you have to make an inefficient BSFC system even less efficient just to get it to not blow up!

[higgster5.7]

I'm lookin in to mofifing my exhaust so I don't have to keep removing the centre boxes I've just come across a company called waldron exhaust and they've a similar idea to wortecs quiet to loud but all the trickery is in the centre boxes, anyone know anything about these?

[GreenV8S]

ringram said:

GreenV8S said:

My engine wants backpressure in the exhaust for performance.

Under some conditions.

Explanation and justification please Sir!? Ill suggest that statement is false.

It's supercharged (not turbo) so there is an inherent pressure imbalance across the engine. At low RPM on boost I get >100% scavenging, in effect the boosted charge is blowing straight through the cylinder on overlap so I'm losing boost and power (and the exhaust gets quite hot). At higher RPM the inertia is sufficient to stop this.

[ringram]

Yeah ok, Ill accept that excuse with regards to performance. But Id say it was the tail wagging the dog. Your issue is cam timing not exhaust.