Tech Q: GTIR Tarmac rally car using too much fuel.....and too laggy...................

Jon Olds

Well-Known Member
I have got half an idea why and would appreciate any tech input on these thoughts.
Background;
GTIR with 3071 and 34mm inlet restrictor. Closed loop control of boost working correctly 1.1 bar ish. Fuelling corrector (against revs) upping the fuel to suit the bigger turbo, crudely. Forge dump/vent valve opens on deceleration, to atmosphere. The airflow meter is in the standard position, before the turbo
Issue;
Does about 4mpg rallying, big lag after exiting chicanes. Going rich??????
What causes it;
I think every time I hit the brakes the dump/vent valve getting rid of all the compressed air that the airflow meter thinks is going into the engine.
Ideal solution;
Airflow meter downstream of the dump/vent valve? (ie blow thro) (hard to achieve, plumbing wise) OR get rid of
dump/vent valve
Proposed solution for discussion;
I'm going to fit a flow restrictor before the dump/vent valve, so only a small amount of air gets dumped to atmosphere. 1mm, 2mm, 3mm, 4mm, 5mm or something like that.
Hopefully it wont be so slow to re accelerate due to not having to build up new boost and not running stupid AFR's initially. Byproduct is a big saving in fuel. Risk is doing damage to the turbo (surge)
Anybody care to comment?
Jon
 

keastygtir

Well-Known Member
I am no expert but it does sound rich to me. I think you need a session with a wideband.

On all my Nissan turbo's I have owned I have ended up not running a dump valve as it makes it run badly.

Maybe check you ignition timing?
 

Jon Olds

Well-Known Member
Thanks for your input. I would love to delete the dump/vent valve completely but the unknown is whether the turbo may be damaged with possible surge due to all throttle butterflies closing down during braking? The mixture must be mega rich as the full volume of metered compressed air of the front mount intercooler and pipework vents through the dump/vent valve EVERY TIME the throttle shuts. This must be wrong. The AFM has assumed all this air is going to be burnt and will add the appropriate amount of fuel via the injectors.
My front mount is big, as you would expect, likewise the pipework, big volumes of compressed air.
Anybody else?
Jon
 

keastygtir

Well-Known Member
I know its a different setup and yours is not a road car but my T28 with 360 bearing has been fine for many years. I used to run a front mount with short piping (radtec) and it was better. On my 200sx with different types of DV it always ran rich and was worse between gear changes.
 

Jon Olds

Well-Known Member
The rallying thing will make you laugh. You steam along in 3rd/4th gear behind someone, brake stupid late to gain 4 car
lengths into a chicane, be right up behind them through the chicane then lose three car lengths on the exit.
Frustrating.
Eg: Caught a rally mini, 90 hp. Couldn't power past 100m later. LOL. Overtook anyway being brave on the brakes.
280hp GTIR against 90hp mini.? LOL
Passed quite a few at the weekend using cunning and 'rallycross' technique. Turbo lag killed me and cost me 20
secs a stage easily.
On a faster rally, pulsar rocks
Jon
 

keastygtir

Well-Known Member
The only argument for running one is it prolongs the life of the turbo (which I don't believe) so you are talking about the turbo lasting 5 years instead of 4. That said it only my advice and I am no mechanic or professional.
 

Jon Olds

Well-Known Member
Thanks. Hopefully a few more people will provide an opinion. The car has averages <0.6 miles a week for the last six years, with
multiple £50 a gallon oil changes thrown in, so maybe turbo life isn't the issue here.
Another thought, if I was to go the vent recirc route, rather than to atmosphere, where would the air feed back in?
Jon
 

fubar andy

Moderator & N/W Rep
Staff member
Remove blow off valve and try it out.

I don't run one on mine and never had an issues at 1.0bar

Also fit a wideband sensor to monitor your fueling, this is important.
 

Jon Olds

Well-Known Member
The AR ratio is not ideal on what I have, I know, but this mismatch between what the AFM measures and what the engine injests
must fundamentally be the issue. Why I haven't come to this conclusion years ago, I'm not sure. Well, in reality I know why, I've had that many parallel projects on the go...
Anyway, I need to do a trackday and do some back to back time comparisons, ideally with some datalogging on the AFR ratio.
Jon
Am not going to downgrade the turbo, as the original idea was to sprint the GTIR without the restrictor, as well as stage rallying it.
 

fubar andy

Moderator & N/W Rep
Staff member
Anyway, I need to do a trackday and do some back to back time comparisons, ideally with some datalogging on the AFR ratio.
Jon
I would 100% spend time on a dyno 1st before a trackday.

At least try to work out what is going on with your ECU/turbo/fuel before you hammer the engine!
 

Jon Olds

Well-Known Member
I agree, ideally a RR session would be nice, but the exhaust lambda readers are generally too slow
to pick up on rapid AFR swings, like between gearchanges.
Its been in use for at least 11 years in its current configuration.......and spent all its time flat out.....so I'm not too worried.
It would be nice, but doing consecutive step response testing on a dyno also has potential problems
So trackday easier maybe, definitely cheaper
Merry xmas
Jon
 

MarkTurbo

Well-Known Member
Which exhaust housing has your 3071 got, 0.64 or 0.86? And what rpm are you getting full boost at?

I would just blank the dump valve off completely, the general opinion is it'll shorten the turbo life a bit but not significantly. Has your turbo got a metal bearing cage or the standard shitty plastic one?

I think your problem is lack of boost. I found with my 3071 that it felt gutless at anything below 1.4-1.5 bar, at 1.1 bar I bet your car is producing a crap torque figure which is why it feels slow out of corners.

That's why standard practice on a lot of rally cars is to run loads of boost to get some torque.
 

Jon Olds

Well-Known Member
I think it has the 'wrong' A to R but was quoted £500 for a rebuild, verbally. Maybe I should have paid the cash..
I'm going to try and blank the vent valve, my pipework runs are very nice aerodynamically, but high volume. The IC is a big front mount
so litres in volume also.

The two issues I think are key;
AFR goes stupid rich due to 'metered' air dumped.
Once the vent valve empties the pipework and IC, the compressor has to then refill it to get the boost back.

Boost controller (proper closed loop electronic system) is doing its job, looking at the RRoad graphs. Step response good, minimal overshoot, good control at the setpoint
The torque and power curves look fairly good on the RRoad. The car runs strong once wound up. I have nothing to compare it too though.
Havent tried higher boosts, lower end engine is basically standard.
Thanks for the input
Merry xmas
Jon
 

johnny gtir

Well-Known Member
Which exhaust housing has your 3071 got, 0.64 or 0.86? And what rpm are you getting full boost at?

I would just blank the dump valve off completely, the general opinion is it'll shorten the turbo life a bit but not significantly. Has your turbo got a metal bearing cage or the standard shitty plastic one?

I think your problem is lack of boost. I found with my 3071 that it felt gutless at anything below 1.4-1.5 bar, at 1.1 bar I bet your car is producing a crap torque figure which is why it feels slow out of corners.

That's why standard practice on a lot of rally cars is to run loads of boost to get some torque.
I thought the restrictor on there was to fetch the torque up ?
 

Jon Olds

Well-Known Member
No, all the restrictor does is to prevent it rev properly past 5K and limit power/torque accordingly. Every body has
to run one. You can see it on the RR plots, with/without. This means I have a narrow power band due to the lag at the bottom end
meaning the right gear is much more important
Happy new year to all
Jon
 

MarkTurbo

Well-Known Member
I thought the restrictor on there was to fetch the torque up ?
No it's there to limit the airflow and therefore restrict the power the engine makes. That's why a lot of rally cars don't use huge turbo's but run about 3 bar boost, that way they still only make around 330bhp but have 450/500 ft lbs of torque from 2500-3000rpm.

Jon, what power and torque is yours producing? I'm guessing as your still using a standard bottom end you don't want to up the boost too much and risk ringland failure of the pistons!
 
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