1NRO i agree with the 3" all the way through. but where was the question asked about a 300bhp+ car? because i can't see it i thought this was the question?

just to interject the question wasnt about a 300bhp car as nick has said, i think most people would agree with you 1NRO that if you are going for big power then the bigger exhaust is the better but i would think that there are a lot of folk who dont want to sacrafice low range torque for top end power as in the real world driving around using the upper reaches of the coupe's rev range isnt always possible or desirable given the speeds you would be travelling at, i asked the question as i was slightly confused as to why if a turbo spooled faster at lower revs it would lose torque lower down but having listened to a few answers it would seem than on the coupe this is the case with the bigger exhaust, theorising is all very well but if people are telling you from practical experience about how the exhaust behaves on there cars then surely that is more relevant

After jonone asked the question which would be better he goes on to state

"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."

He only wanted to go one way, more power, hence my reply. I've posted due to the very wrong statement that a certain percentage of back pressure is desirable, this was wrong then and it's still wrong now.

Your all boost mad :-)

The problem seems to me to be people are blaming their loss of lower rpm drivability on the wrong things. It isn't due to puting on a big exhaust. It might just be something else that has had it's deficiency brought to the forfront by improving another part don't you think? Look at your inlet system, ALL of it.

Mess with a couple of areas and you'll get clouded opinions unless you think about it.

One constant... the exhaust gases the engine produces, reduce the amount of back pressure fighting the turbine wheel, what do you suppose that does? Make the turbine work faster maybe? what does the turbine wheel do? Turn the compressor don't you think? turn that faster would be my opinion. turn it faster what does it do? compress more air I'd say. Now where's the problem when low rpm power suffers? Ah, it must be in the exhaust! It's obvious isn't it.

So Lancia want to provide a more drivable car at everyday driving levels for the Evo 2, what do they do? Nip the exhaust down pipe, nah, smaller turbo but oh no, the back pressures causing problems, what should we do they say, some bright spark thought it would be a good idea to put a fair % bigger down pipe elbow on, of course we'll spent loads on providing that just for the hell of it.

Nik

I have a 3" downpipe+GT28R but the lag is almost gone due to Flea's remapping. My 5 cents...

These posts seem to be getting confused.....

1NRO you asked me if i can read....

When I posted I have a 5 stud down pipe now.

Plenty of people have said a 3"down pipe will give more power.

The most respected people in the country who tune/work on these cars have said i will lose a bit of low down torque in trade of for power with a 3" down pipe.

lancia did make the down pipe bigger on the evo 2 but the turbo was smaller.

now can some one please answer my original question of does your flexi section hit your gear box plate?

Thanks Jon

Mine don't (flexipipe is 200mm behind the gearbox), but if every 3" downpipe is custombuilt that doesn't say much.

I have to 100% agree with 1NRO on this one and here is why.

This is taken from my motorsport lecture notes/books:

“There is one major goal for the design of the exhaust system downstream of the turbocharger and that is low back pressure. The higher the back pressure the slower the wheel speed the lower the horsepower. There is no getting around this fact. Unfortunately, this message is lost with many “experts” who suggest bad designs. You should ignore advice that minor choking is required to cause turbine spool. This is an old wives’ tale that should but won’t die. There is also no such thing as cylinder scavenging downstream of the turbine. In other words, there is no reason to try to design for high exhaust velocities downstream by necking down any part of the exhaust or by any other means. You will only introduce backpressure which is bad for any turbocharged vehicle. The ideal exhaust would be a very short “megaphone” but for street application there is a minimum acceptable pipe diameter required to support a given gas flow rate. Air flow mass into the engine is directly proportional to exhaust gas outflow, horsepower is related to that inflow, so it is possible to establish a relationship between horsepower and exhaust pipe diameter.”

You may have 3” turbine outlet and 3” downpipe but if the system necks down to 2.75” anywhere then this is your effective diameter. If you have to neck down for any reason do it as far away from the turbo as possible.

2” for anything upto 200BHP
2.5” for anything upto 300BHP
3” for anything upto 400BHP
3.5” for anything upto 500BHP

I hope this clears thing up.




So to answer your question, o you already have

And before the turbo spools up?

Well that will all depend upon what's happening upstream of the turbocharger.

Thank you kind Sir, a voice of reason. I'd say 3" on anything over 300 bhp which is a bit lower than you mention but without checking some text books on the subject I think I'm right on that.

The bottom line is that anything that increases turbine inlet pressure is a big NO NO.

Nik

The impact on low down torque is in relation to off boost and turbo spool. A bigger exhaust does impact this, the significance depends on the driver and their style.

I think of trying to get everything apart from the exhaust down pipe to be as close to NA as I can within the restraints of the turbo configuration whilst allowing for the increase in mass air and fuel.

The turbo in the middle of the exhaust system doesn't allow conventional exhaust tuning even if you can pretend to know anything about the subject. The ability to harness the pulses is often said to be impossible by many respected people, possibly due to the fecking great big turbo near the start of it. I'd be in the camp that would say it's possible to do some good work on the head side of the turbo with relevent lengths and sections and unless you fancy running a megaphone or stepped pipe the area after the turbo is only aimed at less restriction. The megaphone route puts a reflective pulse into the turbo exhaust exit at a given rpm and might prove useful to nutters I try to hang onto sanity and will just be using as straight a 3.5" pipe with a silencer as I can muster.

Nik

Ok Jon, I've tried to write it as plainly as I can and hope there's enough hints to kindle thought, it's up to you to look into it deeper to judge for yourself where the truth lies, I can only offer you another angle on the subject. There's always two sides to a story, some are just wider known and repeated than others.

Nik

No, mine dosen't (refering to a 3" H&S DP). Make sure you have the plate the correct way round and it fits like a glove without any spacing.

That might well be your experience but the fix isn't the smaller down pipe size, thats a step in the wrong direction and just a band aid for a consequence.

Nik, it isn't just theory but based on real world results. Begbie has alluded to this in his previous post, I myself have demonstrated this too. You can loose a little torque low down and gain some at the top end, just by changing some aspect of the exhaust that affects back pressure. I'm not advocating restricting the exhaust or necking down, just that there is a cause and effect.

Ok, that'll do for now, I've said enough to at least spark thought and won't row about it.

Turbo j
the plate is the right way round and has to be spaced to the point were i can only just fit the nuts on! would it be a problem to fit longer studs?

ok bit strange I never encountered this problem. But yes longer studs will be your best solution.

Found this link again in my many saved links and thought I'd chuck it into this thread

http://www.flowmastermufflers.com/backpressure.html

Just don't buy the products, not as good as others.

The case for the other side http://www.automotivehelper.com/topic163228.htm
These fellas are discussing NA traits and this is where the myth in regard to turbo engines has it's foundation. The fact is a turbo sitting in the middle of the runners makes convential pulse tuning like people try to utilise in NA a non starter, I'm not saying it's impossible but the methods aren't convential.

My knowledge of pulse / wave tuning is far from strong, does anyone have input, theory, opinion or even a wild stab to share? Turbo or NA. Heavy question I know but there's some keen / knowledgeable people kicking about on here, shout up.

Nik

I cut my teeth tuning two stroke racing engines, two strokes are more critical than any other form of combustion engine when it comes to exhaust design and as such I have a pretty good understanding of back pressure and exhaust pulse control in exhaust design, but I'm no expert on the subject.

From my point of view introducing back pressure into a turbo charged system to increase low down torque is laughable, that includes before the turbo

garrett and other turbo designers spend huge sums of money and time developing the exhaust housing design and that should be the restriction point not the manifold running into it

I would love to see some EGT figures for a setup like this, especially when your running close to the limit of the turbo

The best way of reducing the back pressure pre turbo is to reduce it after the turbo, a reduction in the downpipe tallies over to providing a reduction before the turbo. There is a need to be careful with the sizing of the manifold runners, bigger isn't better in this case. The speed of the gases is all important here, the bigger you go the slower it goes, a target of 650 fps (feet per second) is a a worthy region to be in. Too much more than that and the port is likely to choke, much less and the port is lazy.

The speed of sound is approx twice that level (temp and density variable) and is where the complex subject of pulse tuning comes into play. It's complex only if you want to be an expert on the subject, the symptom of pulse action is present in all engines, it's just a case of trying to get them in the rpm region you consider most useful for your task. Very easy to drive yourself mad looking too closely at the subject.

Question - what is the main cause of pulse creation?

Im not sure i understand the question??

The pulse is a sound wave working at the same frequency as the engine RPM, does that answer your question?

This is a subject that i have spent a fair amount of time trying to understand, its very complex with two strokes and i can only conclude that its only more complicated in four stroke form

And the same with port flow, I understand that as a rule of thumb any more than about mach 0.6 and the port will start to choke and the only way to increase flow would be to enlarge the port/valve to increase flow

This is a subject i encroched on in a previous thread about porting heads and alot of tuners making the ports larger than they need to be.

Yes, badly worded.

What causes a pulse? there are a few but the main one. The answer would clear up the reason why a turbo won't respond to a smaller down pipe to improve low down torque.

Nik, im not sure if i can answer this in a way that clears up the theory, please, prompt me more if im moving away from the answer

The sound wave is caused by the combustion process and in a two stroke there is no exhaust valve, i think that is critical here

My understanding is that with an exhaust valve in the way you are limited to a very small time window for the sound wave to do its work

Also being that a turbo is in the middle of the exhaust system i cant understand how the sound wave would scavange the cylinder

As the down pipe is after the turbine housing i cannot see how any kind of pulse tuning in the down pipe will affect the performace of the engine or turbo

My understanding is that back pressure in the down pipe can only cause the exhaust wheel to stall and as such reduce effiency

To my understanding a change in cross section or an obstruction causes a pulse to be created. This in an exhaust manifold would be at the end of the runners where the collector starts if it was nicely made or off that big numb turbo thats in the way. This pulse travels in the opposite direction only to be hopefully met by a closed exhaust valve which in turn reflects it back again in the direction of the flow, so if the exhaust valve was to open at the moment the pulse set off back down the runner it would assist in drawing exhaust gases out. Very amatuer explanation which might well have some inaccuracy but maybe the point it there?

So to reflect a pulse back through the turbine from the down pipe would not in my mind be too difficult to discount. It's just not going to happen.

A down pipe might concievably have a pulse reflecting off the turbine but what is going to start the pulse action? A change in cross section or obstruction isn't normally there. A big step in the pipe might well provide a source but thats very rarely seen, more usually a magaphone pipe (like two strokes sometimes use) which is very short is tried, the change in cross section being when it changes to open atmosphere, sometimes a reversed cone megaphone is used to widen the range.

Pretty much a banging the head against a wall exercise but that depends on how keen you are, with effort it's possible to be in the right region of the rpm range and from there starts the experimentation.