Callum Ferguson

iTrader: (1)

Can anyone explain why some EGBP may, in certain circumstances, increase bottom end power?

I appreciate Pat's knowledgeable opinion that zero back pressure is best, however, there appears to be conflicting opinion, some of which may be based on experience?

AG Bell in "Forced Induction Tuning" agrees with Pat in that high power 2.0l's, circa 400bhp, cars may benefit from an exhaust 0.5" to 1" larger than the normally recommended 2.75" / 3" systems.

The latest APS website shows a graph comparing three systems and their respective EGBP's with the 3" system showing 3.5psi @ 7000rpm against a standard system at 9.6psi.

See here:- http://www.airpowersystems.com.au/wrx/exhaust.htm

I realise that gas velocity slows as it cools and that heat maintainance will be important pre turbo but my brain cannot get around how performance can be improved by restricing flow post turbo. An explanation would be greatly appreciated

Many thanks,
Callum

john banks

Neither can mine.

NotoriousREV

I've often heard the "backpressure is needed" argument and never understood it. Pre-turbo, I guess it's possible to use exhaust length tuning to help with scavenging, but post turbo, no chance, any return flow would impede the turbine, you just want to get rid of the gasses with no obstruction.

nom

Here's my guess at where the rumour came from (and no, I can't see how a blockage can help flow either ) -
If the system behind the exhaust side of the turbo is restricting flow, it is meant to allow the turbo to spool up quicker (I think this works both technically and practically, but a complicated one...) so the effect is boost comes in earlier. However, as boost is a measure of how restrictve the system is (the higher the boost, the more restrictive 'cos boost is only there as the air can't 'get through' the engine), the level of boost doesn't demonstrate power - although it's assumed it does, and of course in like-for-like situations it does (keep the engine/exhaust system etc. exactly the same & higher boost will mean higher torque). But blocking the system to just get boost won't, of course, increase power. Just boost.
Anyway, that where I think it comes from, and it's most definately pants

Andy.F

As the EGBP at the cylinder head is a function of the complete ex system before and after the turbo, then increased BP could allow better cylinder filling (torque) at low rpm (off boost) by reducing the amount of charge escaping during the overlap phase.

The Yamaha R1 for example has a valve in the exhaust system to build back pressure and increase bottom end torque. This is of course a completely different animal with long duration cams and no turbo but the concept re cylinder filling is the same.

Andy

Pavlo

I don't beleive increased back pressure after the turbo does increase torque.

Many tests are flawed in that they don't fix things like ignition advance at the same RPM.

Before the turbo things can be different though. Smaller header tubes will increase pressure wave intensity and flow speed aswell as back pressure.

Paul

MurrayZA

I though EGBP was for NA cars only.

Pete Croney

Has anyone thought about how much of the low rpm exhaust gas energy is used in accelerating the larger volume of gas in a big pipe system? If you think of torque as the engine's ability to accelerate, the statement starts to make more sense.

For every power level, there is an optimum diameter, which balances exhaust gas speed with back presure. Being bigger than the optimum will have problems, just as being smaller than the optimum will.

hypoluxa

Agree with Andy, however it can be said that increased back pressure could do the exact opposite - hurt cylinder filling at low rpm due to reversion during overlap.

If you increase the volumetric efficiency at a particular rpm (by increasing or decreasing BP in this instance) then you will increase torque at that rpm. The trick is to be able to do it over the entire operating range of the engine

Pete - For spool up and response you want a big pressure differential across the exhaust side of the turbo. For maximum power you want as little back pressure (inc turbo & manifold) as possible. IMO a 3" exhaust would out perform a 2 3/4" one from say 2000rpm onwards.

Callum Ferguson

iTrader: (1)

Many thanks for the informative input - all interesting stuff

Pete,
Is it possible to provide test figures to support your theory? If there is an optimum diameter per power level, and I don't necessarily doubt this, then it has presumably been arrived at by R&D and the figures accessible; they would certainly be interesting Given your business this may be commercially sensitive - if it is no problem.

My interest in the subject has an alterior motive as I am trying to develop 320+ bhp ATW. Given a bigger turbo, more air & fuel, good mapping, etc., I am now wondering if a 2.75" or even 3" system will provide the best route to power across the range. Top end is unlikely to be a problem but bottom end might

More input welcome - any more exhaust suppliers lurking?

Thanks again,
Callum

Callum Ferguson

iTrader: (1)

bttt - just for Pete

Jack0

In my opinion, as a general rule:
Boost = Torque
Spark (+fuel) = Power

For Big Power = NO EXHAUST!
For Tuned Torque = Tuned Pressure differential across Turbine (Header/manifold and Downpipe Mods) which is in effect playing with the spool characteristics (Expansion ratio/energy equation) on a specific turbine.

Jacko .

[Edited by Jack0 - 5/29/2003 1:13:59 PM]

Pete Croney

Callum

Its not a simple answer as the gas density has to be considered and this is changing all the way along the pipe, as the gas cools.

We have done lots of testing and most of it is info that I don't want to pass on, but consider the pipework on the latest STi for some ideas. The pipe that goes from the last flange to the silencer is 1.75" internal diameter. The car makes 265bhp. If you increase this to 2.5", you create twice the flow capacity... a flow rate to cope with 530bhp.

Before anyone else says it, the standard STi system is very restrictive and this equates to a slow boost build. Therefore some of the extra capacity we have created has to be used for getting the flow up to what it should be at a lower back pressure, but most of the back pressure from original system will be created by the cats, not the pipe size.

If we look at the downpipe, the standard one is 2.25" internal diameter and has a cat. It still produces 265bhp. If we increase the diameter to 3" we add 77% more capacity, or a potential of 470bhp. Its actually more than this as we have removed the cat so the flow will be proportionally higher if the same back pressure were used.

The lower pressure across the turbo gives us our better spool up, but it is the downpipe that the greatest influence, not the silencer. An open neck downpipe has even more effect than its pipe diameter. The void just after the turbo acts as an expansion chamber, right at the point where the gas is hot (post turbo). In the length of the downpipe the gas has cooled from c800 degC to c300 degC. In fact just changing the downpipe makes a considerable difference to the spool up, even if the rest of the original system is retained, for this very reason.

With the figures above in mind, you need to be generating some serious power before needing to go to 3" after the downpipe.

The final thing to consider is how the large volume of slow moving gas in a big system behaves. Is it an easy exit for the gas coming through the turbo at low engine speeds, or is it a heavy plug of gas that must be pushed along by the low energy gas coming through the impeller and so creates a blockage/restriction?

Oh... and quite a few WRC cars are running 2.125" feeds to their silencers and 3" downpipes. They "only" have c300 bhp, but they have spool up and torque that we can only dream of

MurrayZA

Some interesting notes that come to mind:

"most of the back pressure from original system will be created by the cats" - how much does the cat in the up pipe of the 01/02s then affect the system - I think maybe not much since its a high flow item and it provides no real back pressure to the turbo, but maybe valuable back pressure to the engine when looking at the std ecu setup ?

"800-300 degrees lost over the down-pipe" So, is the idea of wrapping the down-pipe actually relevant - I assume the hotter gas will flow quicker out of the system but, cooler gas is more dense and more air will flow because the gas is more dense albeit slower.

Crikey. All to confusing for me !

SiHethers

I'm pretty sure most WRCs exhaust wrap the whole length of their exhausts (certainly Ford do). Physics would dictate that keeping the exhaust gas as hot as possible will reduce backpressure due to hotter gas being less dense and travelling at a higher velocity, although the realworld benefit of this will diminish the further you are from the turbine outlet.

I think

Simon

rex11

I would have to agree with Pete on his assesment. The amount of output that a turbo puts out is related to the pressure differental across the turbine housing. In theory the "best" exhaust system is a velocity stack shaped bell on the turbine outlet...that's it. Of course this doesn't work for the hearing so a system is installed. Since the gas is compressible, the farther downstream of the turbo you are the less important the 3in piping is. If you can creat a localized expansion on the outlet of the turbine your output will increase.

As for the backpressure before the turbo, I don't think that it has the same effect as in a NA car. IIRC back pressure in a NA car is used to control the length of the fuel burn, and the amount of charge leaving the cylinder during overlap. While this would work for a turbo car as well, I would have to guess that the turbo will be able to supply more torque then the BP tuning.

Just my 2 cents, but I could be full of it...wouldn't be the first time