Backpressure vs Turbo Lag

[cat007]

Hey all,

Ever since I've gone to a single I've always had a fair decent amount of lag. First with the Auci KKK K26 and now with a Turbonetics (I think) turbo, same spec as a GT35

The exhaust housing is quite small, around a 0.63 I think.

My exhaust consists of 2.5" mandrell bent, resonator in the middle and a rear muffler which isn't a straight through style one

Would this be causing my lag?

Exhaust manifold is stempipe - here's a picture of it:



Here's my dyno sheet:

http://www.supras.co.nz/pics/ecu/finished/dyno.jpg


Suggestions?[/img]

[Lith]

The exhaust housing is quite small, around a 0.63 I think.

My exhaust consists of 2.5" mandrell bent, resonator in the middle and a rear muffler which isn't a straight through style one

Would this be causing my lag? Suggestions?

That exhaust manifold looks nice but the exhaust system sounds like it will be a bit restrictive for the power you are making - freeing it up will help a little no doubt, but no miracles.

I'd hate to be captain obvious, but you are running a small displacement 6 cylinder with a large turbo - its hard to get all 6 pulses lined up nicely to a single scroll/single turbo setup and its never going to drive it as hard as a 4cyl of the same displacement, and even then a 600+hp plain bearing turbo is always going to be pretty laggy on a 2litre engine.

Short of changing to a better suited turbo to your power level, you may have to live with a reasonable amount of lag.

[cat007]

That exhaust manifold looks nice but the exhaust system sounds like it will be a bit restrictive for the power you are making - freeing it up will help a little no doubt, but no miracles.

I'd hate to be captain obvious, but you are running a small displacement 6 cylinder with a large turbo - its hard to get all 6 pulses lined up nicely to a single scroll/single turbo setup and its never going to drive it as hard as a 4cyl of the same displacement, and even then a 600+hp plain bearing turbo is always going to be pretty laggy on a 2litre engine.

Short of changing to a better suited turbo to your power level, you may have to live with a reasonable amount of lag.

You are right - however I even had a large amount of lag with the K26 - which isn't really a very big turbo by any means! And the turbo I have now is only a stage 2 exhaust wheel with a small compressor housing. I can take the exhaust off just after the down pipe - I might try that and see if it reduces lag a bit.....albeit loud lol

[barryogen]

Not having much experience with choosing a turbo myself(am learning though), but shite thats a big turbo for a 2L engine.

Just looking through the Garrett site, I would have chosen something more along the lines of the GT2860RS(might be at its limits) or a GT30(may still be a bit laggy)... or just for the middle road, the GT2871R.

This is quite a good calculator/plotter if you are not that way inclined.
http://www.squirrelpf.com/turbocalc/index.php

[cat007]

Not having much experience with choosing a turbo myself(am learning though), but shite thats a big turbo for a 2L engine.

Just looking through the Garrett site, I would have chosen something more along the lines of the GT2860RS(might be at its limits) or a GT30(may still be a bit laggy)... or just for the middle road, the GT2871R.

This is quite a good calculator/plotter if you are not that way inclined.
http://www.squirrelpf.com/turbocalc/index.php

You're right - it is a very large turbo - but only on the compressor side. The turbine side is quite small - I was hoping by having a smaller turbine side I might be able to do away with a bit of lag.....

[strx7]

that same exhuast side but with a smaller compressor wheel would spool faster.

A 6 cylinder will spool a turbo faster than a 4 cylinder of the same size, although the exhuast pulses theoretically contain less engery than a 4, there is more of them and therefore pressure is closer to being constant.

But yes turbo does seem pretty big for the engine. I'm assuming its a 1GGTE?

[cat007]

Yeah - the compressor side is a little large.

I was always under the impression a 2 litre 4 banger would spool up the same size turbo easier than a 2litre 6.....ah well - cant be right all the time hehe

oh, yeah it is a 1G-GTE

[Lith]

You're right - it is a very large turbo - but only on the compressor side. The turbine side is quite small - I was hoping by having a smaller turbine side I might be able to do away with a bit of lag.....

It can help a little but you are only going to vary spool by a few % realistically, and at the cost of backpressure. A turbo manufacturer is never (in their right mind) going to put too small a hot side on a big compressor as all you'll end up with is a surgey reversiony sack of poo to drive - the real way to end up with a nice responsive turbo car is to get one matched overall to what you want.

I was always under the impression a 2 litre 4 banger would spool up the same size turbo easier than a 2litre 6.....

As I said before, my vote on this one is that you are right - imho the pulses are stronger and easiest to tie together at the collector in such a way as they are not overlapping with each other.

The concept sounds nice that a 6cylinder has more constant pressure but the issue is the traffic jam at the collector, the constant pressure means that a exhaust pulse trying to get through into the turbine is having to fight with another one.

With 4cylinders you can get a closer and less restrictive path to the turbine wheel - this is one of the reasons 6cylinder cars work so well with twin turbos, you can have very short paths from the valve to the turbine and having only 3 pulses for each a nicely designed manifold should have each new pulse virtually following the low pressure tail of each last one. Not that I am too sure that it would work quite like that with a turbo car, but its going to be a lot more pleasant than 6 cylinders fighting for a small gap.

A twin scroll turbine housing would be pretty nifty, its a common trick for evening up the collection of pulses from a 6cylinder engine into a single turbine wheel - stops the collisions and provides you with better response.

I'm really just ranting so my apologies

[strx7]

As I said before, my vote on this one is that you are right - imho the pulses are stronger and easiest to tie together at the collector in such a way as they are not overlapping with each other.

The concept sounds nice that a 6cylinder has more constant pressure but the issue is the traffic jam at the collector, the constant pressure means that a exhaust pulse trying to get through into the turbine is having to fight with another one.

With 4cylinders you can get a closer and less restrictive path to the turbine wheel - this is one of the reasons 6cylinder cars work so well with twin turbos, you can have very short paths from the valve to the turbine and having only 3 pulses for each a nicely designed manifold should have each new pulse virtually following the low pressure tail of each last one. Not that I am too sure that it would work quite like that with a turbo car, but its going to be a lot more pleasant than 6 cylinders fighting for a small gap.

A twin scroll turbine housing would be pretty nifty, its a common trick for evening up the collection of pulses from a 6cylinder engine into a single turbine wheel - stops the collisions and provides you with better response.

I'm really just ranting so my apologies

I'm pretty sure in the turbos tech article that steve murch wrote for NZPC magazine he said you could run the next size exhuast housing on a 2L 6 as apposed to a 2L 4 cylinder. From memory he was talking about a .48 on a 4cyl and a .63 on a 6 cyl

[Phothog]

i'll put my info here...

Hunt has a
.70 A/r Compressor cover
60-1 T04B Compressor Wheel (600+hp capable)
what looks to be sligtly bigger than a Stage 1 turbine shaft (51mm exducer)
and a .58 A/r Turbine housing ... (alot better than the 1.06A/r Housing that came off it.)

the turbine side is ok... but the down side is the compressor end, it should have some thing smaller on it .. like a 50trim T04E or simmilar.
or a GT2871R with the .64 A/r or a GT3071R with a .63A/r.

No point in having a small turbine shaft with a massive compressor that can do twice as much as he might ever want.

but for what you got hunt it was a score.

-----------
Chris.

[cat007]

Cheers for that - do you think the large compressor wheel would cause the lag?

[Phothog]

Cheers for that - do you think the large compressor wheel would cause the lag?

it might not be the sole cause, but it wouldnt be helping...

------
Chris.

[cat007]

I wonder if I put a bigger downpipe on (say, 3") and then have it change to the 2.5" for the rest of the way, if that'll help?

[barryogen]

not exactly in the same league, but a friend did that(3" dump and normal rest of exhaust) on his otherwise stock Caldina, and it seemed to bring positive boost on about 700RPM lower, and doesn't seem to give boost creep that seems to be an issue on
Caldinas with 3" dumps.

[Py7h0n]

2.5" should still be enough for the power you are making - Going any bigger will drop your low end torque.

Do you have a EBC on your wastegate?

[cat007]

2.5" should still be enough for the power you are making - Going any bigger will drop your low end torque.

Do you have a EBC on your wastegate?

Unfortunalty I haven't got the EBC set up correctly. Currently I'm only using the "bottom" of the wastegate. And the EBC is set to bleed off that lower main feed.

Does that even make sense? My minds a mash today haha.

Idealy I'd have it so the lower, "actuator" part of the WG would be hooked up to just after the compressor housing, then the top of the wastegate would be hooked up to the EBC through the intake manifold then it would pulse pressure to the top of the WG holding it shut.......

But in saying that - my WG is venting to atmosphere, so I'd hear it if it was leaking......wouldn't I?

[Lith]

2.5" should still be enough for the power you are making - Going any bigger will drop your low end torque.

Actually I couldn't disagree with that more - I had been meaning recommending going to 3" or so. Nothing (short of going bigger motor or smaller turbo) is going to make it a lot better but things like going to a bigger exhaust, getting cams or a set of adjustable cam gears can be done to try and improve it a little.

[strx7]

2.5" should still be enough for the power you are making - Going any bigger will drop your low end torque.

How about provide dyno sheets to prove that theory....

I've seen dyno sheets that prove otherwise. Think it was on a 7MGTE MA70 supra. a guy dyno'd with a 2 1/2 inch, then with a 3 inch, same boost levels, no other mods. Picked up a couple of HP down low and 10-20HP in some areas. The normally aspirated exhaust laws go out the window as soon as you put a turbo into the equation

[cat007]

2.5" should still be enough for the power you are making - Going any bigger will drop your low end torque.

Actually I couldn't disagree with that more - I had been meaning recommending going to 3" or so. Nothing (short of going bigger motor or smaller turbo) is going to make it a lot better but things like going to a bigger exhaust, getting cams or a set of adjustable cam gears can be done to try and improve it a little.

hmmm - Top end performance isn't a huge issue - I'm only running a low 16-17psi - so can always wind that up - its just getting the boost to come on is rediculous.

There's a road leaving my house - called "Fitzwilliam Drive". going up there in say 3rd gear - at 50kmph - if I put my foot down in 3rd at 50kmph (cant remember for the life of me what rpm I'm doing) boost will sit at ~1psi - and I'll just end up driving up the hill.....heaven forbid if I need to get past a bus or something that's doing 30 - I have to drop it into 2nd and get the rev's to around 4K before boost will come up and I'll be able to accelerate.....

Lou I'll have to take you for a drive - its pretty EPIC LAG!

[FANGIN]

... getting cams or a set of adjustable cam gears can be done to try and improve it a little.

I agree,

The more flow you get the quicker its going to come on to boost..

Why not swap out the 1G for something nicer like a 1JZ out of an MA70?