Martin J Stirling

As you may have noticed from previous posts, I am trying to get my head around the basics of 'exhaust science' so that I can make an informed decision when buying a new complete system for the P1 (turbo back).

Of all the systems available, most are being offered in either 2.5" or 3" bore. The justification behind each is that the smaller bore system will have higher back pressure allowing exhaust gases to exit more quickly through a 'scavaging' effect giving greater torque gains lower down the rev range. Whereas the larger bore system will 'breathe' better allowing the turbo to spool up quicker due to reduced back pressure.

Ok, I get these basic principles but I am no nearer to deciding which exhaust bore would be best for me. Surely there must be a precise exhaust science which dictates the effects of large vs small bore systems.

I am looking for the best gains in terms of driveability, so the best torque gains lower down the rev range. I understand that with a larger bore system, peak torque will be increased but will be pushed further up the rev range - is this the case? I want the best torque gains but I don't want to loose low end grunt.

I know some of the big bore systems can claim the best bhp gains on paper, but this doesn't mean alot when you're driving. Peak torque and its location on the rev range is what I'm after.

I find it hard to believe that a small bore system such as the scoobysport can be as effective on a 280bhp car as it is on a 215 bhp. Surely the greater the powern the greater the bore required to operate successfully? Yet the Soobysport seems to outsell any other system. As far as I know all group N rally cars use 3" or 3.5" systems so surely they know whats best?

Can anyone clear this up for me, it's driving me in circles?

Cheers,

Martin Stirling.

Sam Elassar

hi martin
you need to see through all the hype surrounding the different exhaust systems. you have hit the nail on the head their with the 215 and 280bhp cars. it really depends on your turbo though and not the power of the car. if you have and import running one of the bigger VF series turbos you will gain more with the bigger diameter system. the only thing though is your system will be as big as the biggest restriction you have ( ie flanges ) so if you are going for a 3" daimater it will have to be 3inch all the way through. that in turns mean comes MOT time you can't just swap the midsection with a catted one. so it becomes more of hassel.


another thing to think about is BHP is actually calculated from the torque figures ( can't remember the formula ) so if an exhaust is giving you more bhp it should have given more torque as well maybe slighly later in the rev range. also the more quickly you get that turbo spinning on full boost the quicker your car will go regardless what every one says.


i hope this helps.

GP

scavenging on overlap is really 95% a function of the manifold, so it's this which is critical on non-turbos - on turbo cars I would've thought this was much less important though as I'd guess the turbo itself spoils this effect to a certain extent..

oh I don't know, NA cars are my thing - only replying cause noone else has

Martin J Stirling

Hmmm,

never thought about MOT consequences as the car is only 2 months old. I'll worry about that when the time comes. So Sam, you wreckon a 3" system would be better? I have a P1, so I suppose it has the larger turbo as it's basically an STI?

MS

Novark N Goode

Back pressure is a bit of a black art.

On highly strung N/A engines (like motorbikes) it can have a staggering effect on outputs.

On Forced Induction low revving things like turbocharged cars it shouldn't be as important. However, different engine configurations like different things. Cosworths for example like big free flowing pipes, whereas the Calibra Turbo engine (almost identical bore/stroke/port lengths) doesn't respond in a similar fashion. This makes me think its down to Manifold design - as the two examples above have very different exhaust mainfolds.

Turbochargers vary back pressure with the wastegate, so, it'd be fair to assume that back pressure after the turbocharger is fairly irrelevant for the engine because anything after the turbine (with the wastegate closed) is not going to affect the exhaust gas speed too much. But.... back pressure could affect turbine speeds and acceleration with the wastegate closed, which is a different subject entirely.

Most modern Turbo engines spend very little time with the wastegate fully closed, so back pressure will probably play a part on the road.

That doesn't answer the question.

Adam M

sam, i dont understand this.

All the three inch systems I know of mate to a 2.5 inch system. Yes there is a step, but this only affects the performance of the exhaust.

The centre cat will still help with the mot surely? it just wont make a great exhaust system.

steve McCulloch

Adam

(I'm not having a go here) - just wondering as you've commented before

The BPM system I had was 3 inches all the way thru - including the downpipe

I know cos I had a 2.5 inch section from the downpipe to the rear pipe with the ends smoothed out to fit over the larger diameter bits (The exhaust maker Long Life can only work up to 2.5 inches so I know that the rest was over this amount.

Which part of the system is 2.5 inches?

(BPM coming off and standard exhaust going back on - ok loss of performance but the noise is driving me up the wall!)

I have never had a proper answer on this question. The exhaust maker I used said yuou must have back pressure or you'll lose power. Others have said you dont want back pressure at all, as its a restriction!

I doubt theres very little in it at the end of the day?, between a lot of the after market systems out there.

Monggose seems to do as well as most and that aint a twin dump design and is therefore supposed to be restrictive, where-as the likes of BPM never seem to give 'any' performance advantage at all on the rollers! - but it did feel marginally quicker on the road

Just like my other mods - I did not notice much difference in fitting a Link - but I certainly noticed when a VF22 went on! - going totally off thread

Martin J Stirling

Thanks for the comments so far.

So, if different engines respond to larger bore exhausts differently, is anyone on here prepared to state how well the impreza engine layout will react to a larger bore system and reduced back pressure?

Where do you stop, if larger bore system equals gretaer BHP and hence torque, then why not fit a 5 or 6" system? Where is the mid point between power gains and driveability?

Steve,

I've had exactly the same comments. Some people have said that the less back pressure you have, the quicker the turbo will spool up etc, whereas others have said that back pressure is neccessary for low down torque. Confusing isn't it!

Still confused

Somebody somewhere must know the deinitive answer..

MS

[This message has been edited by Martin J Stirling (edited 26 June 2001).]

EvilBevel

Martin, I don't think that there was ever a definite yes or no to this question on this bbs (unless I missed it of course .

NA cars need some backpressure. An Impreza will be an NA car up to a certain point in the rev range. Hence why some people say you need some backpressure in the exhaust. Others reply "but the turbo is already a big restriction providing backpressure"

Once the turbo is at full song, I don't think there is any discussion at all on "bigger is (probably marginally) better"

You obviously feel a serious improvement removing cats and have a full 2.5" exhaust. Some people (mostly with larger turbo's) say they benefit from 3" systems ...

Friend of mine has exactly the same setup as me, but went for a 3" Mongoose exhaust last week. He claims to feel a difference higher up in the rev range. May well be, but horses for courses, and I personally prefer the sound of the SS backbox above all the others.

Anyway, there is no straightforward answer: maybe changing the exhaust manifold to an equal length header one gives a bigger difference than the choice between 2.5 and 3" re. turbo spool up.

Theo

Adam M

All I have said ever on exhausts is that on MY car when I fitted the BPM, I noticed an improvement in performance, over the previous Scoobysport system.

My car is a 2.2 and as such has I would guess a 10% higher volume of exhaust gas. This means the turbo will spin up faster, than if I had a 2.0.

I dont know what difference it would make on a TD04 turbo, as I have tried it, but manufacturers seem to state the things you say, "big diameter is for top range power, small diameter is for low down torque".

I dont know if this is always true, when i changed exhausts, everything was better afterwards, low down torque was there in abundance and it continued to pull to the red line.

What that has to do with back pressure I dont know.

I remember someone once saying that in a pressurised system, back pressure stops the turbo from spooling up. But that is as far as I understand it. I would have thought that off boost, the car behaves as normally aspirated and therefore requires some back pressure.

Once again, the man with teh practical and theoretical knowledge here is Bob Rawle.

Perhaps he can suggest.

Plus I think this should be in drivetrain.

Sam Elassar

hi adam
if it mates with a 2.5" flange it is not 3" all the way, if there is a restriction in the middle of the pipe the gases are only going to flow as much as the restriction allows it. the gases will actually cause some turbulance around the side of the restriction. simple physics here. back box is an exception as it the last in the chain, so having a 3" back box will be better i think.

marin
your car has got a VF28 which is one of the smaller VF turbos so a small dia will do however because your car revs to 8000rpm a bigger one will be better. confused ? you should be

sam

Adam M

think flanges have elongated holes, therefore 2.5 inch will mate to 3 inch, and it does.

My BPM is without doubt 3 inch all the way, but it connected to a scoobsport centre section, which was without doubt 2.5 inch.

That is all I know, except that the SM is 3 inch all the way also at joins, and also mates up the 2.5 inch scoobysport system.

steve McCulloch

As I remember (if Bob does not mind?) is that a nice 3 inch free flowing system is best with as little back pressure as possible - after all its all about getting the gases out as quickly as possible - thats why people uprate the 'inlet manifold' by putting on those equal length manifold headers

Who knows - until someone proves theres a substantial difference in exhaust performance (of the aftermarket variety) then I've no idea - I guess you can feel some difference on the road as Adam says between systems - so why cant I feel the difference in mine and on the rollers, esp (rhetorical)

I have posted before that I cant see the point, except for the sound of uprating the difference is so marginal - I am, unfortunately after years of modifying Escort Cosworths and more recently minor mods to the my own Sti5 still of that opinion. This is my opinion and others will vehemently differ

I have siad to friends before that an exhaust complements other mods and does not do much by itself - again others will differ - and I am not wantig to start a debtae - just my opinion

I seem to follow the rest of the flock in wasting too much money (and moren importantly - time!) on all this rubbish

Since first having the car rolling roaded i've added performance wise:

Exhaust
HKS filter
Link
VF22 Hybrid
Forge Dumper
and a few other bits here and there, am runnign shed loads more ignition advance and 1.5 bar of boost (1.3-1.4 held), and have only seen a 7.4% increase in power stats at Well Lane and a very disappointing story at Powerstation.

I think I should quit now before I get even more depressed!

Steve (I dont think I slagged anyone off in this comment!)

Sam Elassar

i am totally confused now. how could it be 3" all the way and fit onto a 2.5 " flange any where in the system ? it just does not make sense!!

the DP i have seen all start big a taper very slightly usually the pipe extends further than the flange to form a seal. this pipe will have to be 2.5" or other wise it is not going to go through the 2.5" flange

what is the chance of SM explaining their system a bit on here. ?

Hyperex

I tried different combinations. 2.5" all the way turbo back. 3" all the way turbo back. 2.5" downpipe -> 2 3/4" mid section -> 3" back box. Also changed of exhaust manifold.

2.5" all the way gives good low end torque but turbo spools up not as fast. Street driving is most suitable, more enjoyable.

3" all the way doesn't provide as good low end torque but it rev up fast from 5000 rpm onwards to red (my car @ 8000 redline). Street driving is not so enjoyable, turbo lag...

2.5" -> 2 3/4" -> 3" provides very good compromise (with good low end and good high end) among all but louder (megaphone effect).

Personally i would highly recommand the changed of exhaust manifol regardless which combination you would finally go for.
2 types of exhaust manifol avaiable, 4 to 2 to 1 or 4 to 1. I highly recommand the first one as this gives very good low to mid range performance (especially mid range), good torque and fast spool up. To me, the only disadvantage would be the lost of subaru unique engine note.... no "broom broom" sound (like an NA)

You might wonder where can i get the 2 3/4" mid section? Japan.

The above setting was testing on my sti with 280hp modified until 330hp (with standard turbo). Heard from my friend this setup works very well on his 220hp model too.

Note: Some manufacturer narrow the pipe dimention in the backbox to provide higher back pressure (thus loose the purpose), its something that i would watch out before i purchase.

Hyper

Sam Elassar

steve
forget the stats i am sure your car will be pushing at least 80-100bhp more than when standard it wil have to at those boost levels and with the bigger turbo you should be running over 300bhp. isn't similar specs to Bob Rawle's car and that pushed 340bhp, ok PE ROLLER are the little on the over side but still that is impressive compared to the standard sti's

steve McCulloch

Sam

I'm sure a fairly standard car from the 'Evo' mag prodcued nearly this at Powerengineering (the silver sti4 customer car that was in the mag for about a year) so I aint got that much confidence

It was interesting at Powerstation that on upgrading to a VF22 from a VF28 and upping the ignition advance that the power at wheels went down by 18bhp! - very interesting - me thinks! at least there should be some damn consistency!

I'll see what the car produces once the water injection and manifold headers go on (and the Venturi pipe)

WREXY

Hyper,

I have the 2 3/4 exhaust on my car. The exhaust is Japanese. It's the 5 Ziggen, canonball. My friend in OZ is the sole importer of these exhausts. There are two, the fireball which is loud and the canonball which is quieter more expensive and produces the same power as the fireball. Brilliant exhaust and looks good too. Lots of people fit these to their STI's and the 218hp scoobs in OZ as an aftermarket exhaust as it is a good compromise exhaust like you mentioned.

WREXY.

[This message has been edited by WREXY (edited 26 June 2001).]

Hyperex

WREXY,

Good choice, 5 Zigen happened to be one of the hottest exhaust in Japan these days.
Yes, u're right, the fireball is loud... don't know much about Canonball before ur post, but Fireball have a strange charateristic though, when first installed it sounds really bad, but after running in it sounds good. This applys to ur Canonball too?

Hyper

WREXY

Hyper,

The 5 Zigen canonball sounds great from the start. It is a little louder though, when first fitted but gets quieter after a couple of hundred km. Not as wild or raw sounding as the fireball, but has the same power, that is why it's more expensive because it's quieter with the same power. 5 Zigen Canonball, has the best sound IMHO.

WREXY.

[This message has been edited by WREXY (edited 27 June 2001).]

mook

From what I've heard back pressure is not important for a turbo car, as the turbo itself will riun any kind of 'harmonic' setup within the exhaust. Thus, a bigger bore will allow easier breathing which will allow turbo to spin up quicker giving more power.

However, as stated above, the turbo isn't working at all times so for low revs I wouldn't have a clue what you want Hope that adds to the confusion