How much power/torque will i gain by having a 3" 5 stud down pipe over v band?

Not sure of exact figures but you would need to have a 3" system all the way through to get any benefits.

No, it's perfectly ok to have say 2.5" from the cat. Everything's cooled down (less volume) back there.
Also I suspect the benefits will vary with the design of the pipe. Also how big catalyst etc.
One thing is for certain, it WILL be better than std downpipe!

depends entirely on the turbo - the standard downpipe will strangle most turbos, but the amount of power you'll get depends on the potential of the turbo

I'm assuming you have at least a GT28R or GT28RS - anything less than that would always use the stock V-Band adapter

A lot also depends on what other supporting mods you have fitted

For example - if you have JUST a GT28R and everything else is standard, you might go from 240 to 260 bhp

However, if you have a GT28RS and boost control, good intercooling, straight intake and filter, you could easily go from 250 to over 300bhp, especially if you have it mapped at the same time

You WILL loose a bit of low-down torque, but it will be more than made up by the fact that the turbo can allow a lot of exhaust gas past at high revs - it will carry on pulling hard all the way to the redline

With the correct mapping, you get the best of both worlds - bigs slugs of turbo torque in the mid range, and the top-end reviness that you normally associate with normally aspirated engines

Its addictive

Hi people
I have a gt28rsr with supporting mods,my car is not mapped though but it made 297bhp and 275lbt(surrey rolling road) on a gtec2 @1.2bar with it only being a bit rich.

I am on a std clutch(i will go organic next time),i don't really want to start pushing std internals,i know there is a big debate on how much torque std internals can take,so i want to keep the boost quite low and try and keep a safe-ish margine.

i brought a 0.6bar actuator with my turbo and im holding about 0.9bar @ the red line is that about right for 1.2bar peak? or should i go for a stronger actuator?

I don't want to go to mad with the power but want a quick spooling fairly powerfull car thats not on the edge of the internals.

I am just thinking what to do next and if a dp is worth while upgrade, what do you guys think?

Jon

yes, with your spec, a 3" downpipe with 5-stud fixing will certainly free up some top-end power

However, this is where the fuelling is most critical and where engine damage can happen very quickly

I reckon you should factor in a remap from FC Perfomance as well as the cost of the DP - you'll get a good strong car AND you'll know its running safely

nigel
i think your right,i do plan to see flea in the future,do you think a 2.5"dp is enough or go straight for the 3"?

2.5" will be fine for 300bhp, but not much more - go straight for the 3", as you'll never need another DP, unless you're going for stupidly high power (ie well over 500)

I have my down pipe on at last,it was a bit of a pavalova though,because the 5 stud holes did not match the gasket very well and we had to enlarge the holes to make it fit!
Does anyone have trouble with the flexi section on the 3"h&s downpipe touching the gearbox plate thing?
I had trouble with the original dp hiting my lower brace and the gearbox plate but it was down to the rear gearbox mount.
The mount has a plate that starts to bend over time allowing the gearbox/engine to sag,I have put a 10mm thick steel plate on the mount now so that baby is going nowhere
After doing the mount it has lifted everything up higher so the dp does not touch the lower brace now and my drive shaft has stopped hitting the wishbones so my knocking noise has finely gone!
Its just the big flexi section is about 4" and i have had to space the gearbox plate thing right out and its still hitting,Is everyones the same?

can someone explain why having a 3 inch downpipe would lose low end torque if the the turbo spools faster with it, surely if it spools faster it will be on boost quicker therefore it will give more torque lower down, cheers

Less backpressure will cause less torque. Someone will be able to explain this in a more technicle way I'm sure.

I hope someone can explain that theory, not one I've heard before.

hmmm im pretty sure im right on this ill find an explanation

small exhause = more backpressure = more vacumm low down rpm witch will give you more low down torque because of sucking out gasses from cylinder

large exhaust = less low end vacumm. but as rpms rise there will be more gasses so once it gets to a certain prssure there will be more vacume causing more torque higher up.

hope that helps. im pretty sure thats right.

Sorry, that just isn't the case. You have a turbo in the way remember, it's not a NA engine. THE evil of turbo charged cars is back pressure, less is better.

If you go with a larger exhaust system then inevitably this will have some impact on exhaust velocity at low rpms, this is where the torque will likely be reduced. As Nick has said, a smaller exhaust at this point will allow for greater exhaust velocity which in turn increases vacuum helping pull the exhaust gasses along. When the turbo is on boost and at high rpms you also want to keep the gas velocity high. This is where the bigger exhaust becomes useful so that it doesn't bottleneck leading to increased back pressure and reversion of gasses. Of course generally with turbo cars you go bigger for more power.

Exhaust gas velocity is of benefit without doubt but on the other side of the turbo, in the manifold.
An example maybe from the integrale range, early 16v evo's had a smaller cast exhaust elbow than the later evo 2's even though they ran a smaller turbo. In my opinion they were masking the higher potential for back pressure by using the larger exhaust elbow.
I think this is a case of NA exhaust tuning theory trying to pass over into FI exhaust, it doesn't. Once it's in the exhaust pipe it's done it's job and getting rid of it without backing it up is the number one job. Just my opinion and not looking for a row, if anything I'd like to see some real world evidence to disprove my theory and confirm how necking down the exhaust would be of use.

Nick

Whilst it sounds like a strange theory, i think there is some truth to it all. My first turbo car, being the tipo with a coupe turbo engine in, i ran the standard tipo exhaust for a while and it used to pull great upto about 5000rpm where you would feel the torque drop off quite a bit. Once i sorted the exhaust out, the low down torque was gone, so having a smaller diameter exhaust helped with the initial torque at low revs. I don't know why, but there is some truth to what is being said.

so if you were looking for low and mid range power rather than top end response you would be better with a smaller exhaust, for instance using a vband with standard downpipe rather than a 3 inch downpipe on smaller gt series turbos or just standard downpipe if using a superspool or gti-r etc

This is exactly what Roger was telling me at the weekend. Having driven a few examples of both, he advised against the H&S downpipe as it loses torque low down. If going for maximum power then the H&S is the way forward, if you want a little more driveability in the bottom to mid (or you'll just skint ) then the v-band is the bettwe option.

Having just had my new GT28RS mapped by Flea, I was shocked to find it making serious power at just 3,600rpm. It is a LOT quicker than the GTiR was here. I was expecting a bit of a 'LagMaster5000(tm)' not making any serious gains over the GTiR until 5,000rpms. This is at 1.3bar too, the GTiR was making 1.35bar here!

I think the point is that until the turbo is properly spooling any turbo engine is operating as an NA engine. Hence this is why there is balancing act between the ideals of the a small downpipe for low down performance and top end (on boost) performance.

cheers guys, interesting reading, has helped me make up my mind about future tuning options, be interesting to see your rolling roads graphs trappy with the vband adapter

I'll be doing it at the same place as last time and so will transfer it all to excel before posting for the ultimate comparison with the GTiR (which was also mapped by Leighton).

Should be very interesting

I still think exactly the same as I've written already. No way is a turbo car getting anything from a restrictive exhaust. The loss of torque at lower levels isn't due to lost effectiveness of the exhaust but rather the inferior induction system rearing it's ugly head but thats another heated debate altogether, maybe you'll be telling me to keep the induction ID small to improve spool time

Some of you are confusing vacumm in the exhaust with pulse tuning. Turbo theory is so mixed up, some supposed rules formed in the early years which still cloud judgement. Having spent the amount of time I have reading / researching how mega powerful turbo engines are built I'll not be necking my exhaust down, no way.

Crap picture of a benchmark 800 bhp honda thats had a bit of effort put in http://i3.photobucket.com/albums/y85/1NRO/hondaturbo.jpg
Another decent results honda with a proper sized exhaust but a very poor 90 out of the turbo http://i3.photobucket.com/albums/y85/1NRO/hondaexhaust.jpg
Neither car has a deficiency of power at any rpm point.
Turbod bike engine built in the low back pressure stylee http://i3.photobucket.com/albums/y85/1NRO/turbobike.jpg

If anyone can show me a high power turbo car with a necked exhaust I'll be very interested, I've never seen such a thing and I've been looking very hard. Back pressure is THE evil imo closely followed by restrictive induction. calculate the volume of air a turbo capable of high power compresses at low rpm and you'll see the logic.

It will be intresting to see how quick yours spool up and how much torque you have low down as mine runs 1.3 bar

Ben

nobody has said anything about a high powered car with a small exhaust. we all know for maximum power a large exhaust is needed it just the torque at the lower end of the RPMs that will be helped and thats all. for anyone going for a high powered turbo car then the bigger the better.

Seems to me the question was about 300bhp + power levels even before a remap, this puts the pipe size at 3" imo rather than 2.5" Maybe 300bhp + isn't trying?

1nro have you been hitting the crack pipe? (3" or 2.5")

My car is very similar set-up to Jonone. BUT I still have the standard downpipe with v-band (or at least the previous owner did not change it). I have a decat and 2.5 pipe all the way.
My car was dyno'd and mapped by PO at 290 on 99 octane.
It is seriously quick, and keeps up with my mates' 911 turbo and he drives hard.
The torque of my car is exceptional with boost coming in strongly at around 2500rpm much more flexible than some with more pwerful coupes, as they have told me.
Strangely in view of previous posters here, though, my car pulls smooth and strong all the way to the red line/ limiter with no tailing off!

Have you? Can you not read? Look, I'll say it in simple words.

If you are at the level of power you are I would go with a 3" exhaust down pipe.

If you know better why ask? Just go ahead and do what the masses do / say.