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Discussion Starter · #1 ·
Can someone please explain to me the technical reason why removing back pressure from the exhaust system causes a so-called loss of torque low down in the rev range. I've tried thinking through various reasons but can't find any at the moment.

Anyone car to enlighten me?

Cheers

Andy

[Edited to say I'm not disputing that it happens, just interested to find out technical reasons why]
 
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Discussion Starter · #3 ·
Surely a larger bore would reduce exhaust gass velocities and therefore _under_ scavenge ?

I suspect that when driving off-boost (manifold in vaccuum) a change in gas velocity caused by different size exhaust bores (say 2.5 and 3 ) would make a noticible difference... however... as soon as you want to drive into and then on boost, you want minimum backpressure from the exhaust system from the turbo outlet onwards. Also bear in mind that any significant change in the exhaust system will alter the engine's fueling requirements. This will probably be compensated for _up_to_a_point_ by the oem ecu and it's MAF based maps.
 
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Discussion Starter · #4 ·
I don't think reducing back pressure reduces low down torque. I think because the improvement low down (off boost) is not as much (if at all) as at the high end it gives the impression the low end torque has dropped.
Check out 21st C Performance, quite a bit on exhaust back pressure testing in there.
 
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Discussion Starter · #5 ·
certainly any loss is not that great as hh6 says
best to datalog before and after unfortuately i fitted the link after exhaust instead of doing first
so i cant give you a definitive answer

but at low rpm and given a relatively high cam overlap then it is likely with less back pressure that some of the inlet charge is blown straight out the exhaust (over scavange)

obviously at higher rpm the effect reverses due to inertia of gasses, etc and then yes the less bp the better

andrew
 
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Discussion Starter · #6 ·
HH6,
I've looked at 21CP a while ago and read all about it. The reason I asked was because someone posted in the exhaust section about a loss of low down torque by decatting the exhaust and I was wondering if there was something I'd missed because I couldn't see any reason why this should happen.

I can understand the possible slowing down of the air theory but the reduction in back pressure is occuring after the turbo and therefore what you should see is an increase in the spool up time of the turbo as it is pushing against less pressure therefore will spin up quicker.

The way I see it is that the engine is effectively working as a pump pushing exhaust gas through the turbo and out through the exhaust. The turbo and the exhaust together provide the pressure drop that the pump has to overcome.
If you reduce the back pressure on the pump (engine) then it will work more efficiently and provide either a greater flow at the same pressure or the same flow at a reduced pressure.

So provided that you keep the boost pressure the same, you will get a higher flowrate of gas through the engine and a faster spool up of the turbo.

This should all work irrespective of revs.

The only thing that might change is the point at which the 'tuned' exhaust length scavenging happens.
The exhaust will be tuned to provide the correct reversion pulse to help draw exhaust gas from the engine. This can only be tuned for a particular rpm (usually peak torque) and is based on a certain gas flowrate etc...
When you change the gas flowrates due to changing exhausts you will change the revs at which this happens.


After all this rambling then, I think I've just answered my original question...... which is

You may lose low down torque because you will move the point at which peak torque occurs because you are changing the velocity through the exhaust and so are changing the point at which the 'tuned' exhaust scavenging effect happens.

Anyone care to agree or diagree????

Andy
 
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Discussion Starter · #7 ·
well i am not agreeing or disagreeing because i dont have any analytical data however it appears that the general consensus from people is that it might

the lower the exhaust resistance and the lower the rpm the more likely the charge is to come in the open inlet and dissappear straight out the open exhaust valve, so bmep will be lower and there will be less exhaust to spool up the turbo as well

max bmep moves to higher up the rpm range, max power increases
however spool up IS faster BUT happens later, ie higher a up the rpm range it comes on boost more suddenly
this makes sense because once the overscavange effect goes away there is a higher pressure drop across the turbo

to compensate this you need to:
change the way the boost control circut works so there is no signal to the waste gate until target boost achieved
doing this the engine should reach 1.6 bar at approximately 3500 cf 4500 holding degrommeted oem solnoid open
alter the cam timing, either retarding both cams will recover lost torque but will loose boost/power at the top end
or narrow the cams, this will increase effective compression ratio raising bmep however overscavanging is made worse hence bmep gain minor and motor losses power over 4000 and has false peak at 5800 approximatey
 
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Discussion Starter · #8 ·
Andy,

this is a bloody good question and I can't think of a good reason why it should be. Like you i was sceptical when people who had fitted freer flowing exhausts said they lost mid range torque. As someone said (hh6?) it may be just down to the car going better at high rpm so the mid range doesn't feel so special. But i'm not convinced. It certainly feels to me that i've lost out at mid range.

The only way top be sure is to do a back to back test. I'd be up for this is someone knows we can get onto a rolling road cheaply.

It is interesting to note that many motorcycles (r1 for example) and Ferraris have exhaust systems that can adjust the exhaust path and hence back pressure via actuators. Whilst this doesn't directly prove that some back pressure helps at the mid range, they must have done this for a reason.

I still can't understand why though. I wouldn't have thought that you'd get much tuning effect given that there is a turbo stuck in the way, but i could be wrong. I suppose if the back pressure is reduced gas velocity will be increased which will affect the tuning characteristics of the system.
 
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Discussion Starter · #9 ·
Oli.
The Ferraris might have it to either help with emissions i.e. higher backpressure |EQU| more exhaust gas recirculation??? or it may change the tuned length of the exhaust i.e. if you have the exhaust pressure wave bouncing back off the actuated valve then you get a enhanced scavenge at a certain rpm and with the valve open, you get the reflection further down the exhaust and so get it at another rpm as well.

Andy

[Edited to add that I should shortly be in possesion of 3 different exhausts if someone is doing a r/r day soon for comparison purposes. A standard E6, a standard E6 TME and a HKS Hiper Muffler]
 
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Discussion Starter · #11 ·
how about the effect of mixtures ? are you talking about changing exhausts on standard cars i pressume? if the car runs leaner at the low end you will loose a little torque etc..

the exhuat is better as in freer flowing the turbo spool up should be better which in turns mean you will be on the power earlier.


also how about the effect the exhaust have on egt? the lower the back pressure the lower the egt which should allow more timing etc..


there is far to many variable i am sure they all may mix together an react differently between cars depending how it is setup.


a various exhaust tried on teh same car will be a great idea :D


sam
 
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Discussion Starter · #12 ·
Andy,

to test 3 exhausts back to back would be a great idea. Ideally we could post the results in the MLR mag for all to see. I'd certainly be up for any assistance in changing the pipes over etc. Where are you based? Maybe drop me an e-mail and we can organise something.
 
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Discussion Starter · #13 ·
Oli,
I'm based up in Warrington in the North West but I do tend to travel down to London quite regularly so I suppose it would be easier arranging something down there but how I get all the exhausts down there I don't know....... so it maybe better up here somewhere.
I'm getting good practice at changing over pipes. Fitted my HKS Hiper Muffler last night within an hour from wheels on the floor to wheels on the floor :D

Andy
 
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Discussion Starter · #14 ·
After each power run you would need to wait a reasonable time for the exhaust to cool or get a pair of asbestos hands! It could end up a long day!
 
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Discussion Starter · #15 ·
Simon,
I changed my exhaust last night having just driven 200 miles up the motorway from London.
It had cooled down for no more than 20 minutes before I had it up on the axle stands and getting started.
You have to remember that the main amount of bolts that need undoing are on the hangars and so you don't have to touch the exhaust. The only exhaust bolts to be undone are the ones attached to the cat and you can get at them with a socket and spanner and you don't have to touch the exhaust with your hands.
By the time you have undone all of the bolts, the back section is usually cold enough to be able to pull it away from the car for fitting the new one.

The main problem would probably be the cost of the day because you will be taking up 3 runs on the rollers and also the time spent in between changing the exhausts over. That may well prove to be prohibitive especially if it turns out not to prove too much.

Andy
 
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