question for the carb tuners out there
Turbo Mini Forums

I'm just trying to get to a basic needle setup for the 8psi GT15, I have a standard BDD and Avonbar 10psi needles, the 10psi Is just too rich (in the 11 area) and the BDD is a little weak but one area worries me.
After taking it down a small amount over the whole profile its Just about right on boost, AFR around 12.5 and feels real good, cruise at 3000 and 4000 is still a little lean at 16, I was going to try blocking the part throttle lean pipe first, then adjust the needle at stations 3-6 which should be the cruise area.

What worries me is if I get below 2000rpm and try to pull away, it leans off really bad (17 or more and will not pull) when I have the 10psi in it does not do this, I am just not sure what station of the needle is involved in this situation--- is it also 3-6 (I hope!) or is that more towards the tip?

Any info appreciated..
Terry

website:- http://www.terryhunt.co.uk

I'm not sure what you mean by "station 3-6" but think of it this way; the part of the needle used is proportional to the airflow, at 2000rpm the airflow is low, so you will be using the "thicker" end of the needle.
What dashpot spring is in there, as remember this also effects transient response.

Hi,
What tom said + dashpot oil.
Cheers,
Gavin :)

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agree with tom , your doing well to get a good curve on cruise and power with the side lean off pipe connected and no air bleed on it tel .well done
regards robert

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

trying to get the correct AFR with all boost figures is near impossible with the SU carb setup!

but you have done very well so far hats off

suprisingley! my 1380 turbo motor, the AFR which me and tom fenton set up on a AEM lambda gauge,
we got spot on using a standard BDD needle upto 8 and 14psi of boost

i think the donald rumsfeld quote is in order here:---

There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.

Edited by BENROSS on 7th Jun, 2007.

Is there some kind of formula you can use to work out what kind of needle to use in any given any carb?

Just wondered if its just a case of trial and error, or whether you can mathematically work out needle profile to get somewhere close before fine tuning?

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1/4mile in 13.2sec @ 111 terminal on 15psi

Hi,
There's the WinSU program, don't know if it's any good for forced induction though.
Cheers,
Gavin :)

VEMs Authorised Installer / Re-seller. K head kits now available!

WB/EGT gauges. Click here for customers write-up

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The winsu program is no good for forced induction, however it is usefull for comparing needle profiles to find a close needle for your setup.

I would definately get the part throttle lean off blocked before you try and sort the correct needle.

website:- http://www.terryhunt.co.uk

Try thinning the dashpot oil out a bit, put a bit of light oil in there with your 30wt, that should allow the needle to rise more quickly, thus richening the mixture up.

16:1 is VERY lean not a little....

12.5:1 is not very safe either...

11.5-12 would be better.

Do you have mapped ign?

Alex

AlexF

Edited by Turbo Tel on 7th Jun, 2007.

website:- http://www.terryhunt.co.uk

yes tel thicker the oil richer accel mixture .

Medusa + injection = too much torque for the dyno ..https://youtu.be/qg5o0_tJxYM

Thinner oil = piston lifts more quickly = greater area of jet exposed = more fuel.....?

Not quite Tom,
Thicker oil reduces the rate of piston lift. If the throttle is open furthur than the steady state position more air will be drawn through the venturi (throttle bridge) which will increase the suction and draw more fuel.
The oil thickness is the SU equivalent of the accelerator pump, thicker=richer.
Regards
Dave

Double post...

Edited by Turbo Tel on 7th Jun, 2007.

website:- http://www.terryhunt.co.uk

I just looked this up in my SU book, Its a great book "how to build and power tune SU carbs" by Des Hammill

It doesnt actually say why, but it does say that if the vertical movement of the piston is not damped the piston will rise too fast and the mixture will not adapt quickly enough, this will result in a lean condition and hesitation.

Anyway all seems OK now I just went out for a blast and have the idle at 14, cruise from 2500 to 4000 at 14-15 and full boost at 12.5
Its a blast to drive and full boost (8.5 according to Mjolt) is fully in just under 3000, I'm limiting it to 5000 at the mo. I had one scare when it was hot and gunning it up a hill, I think it detonated, so the on boost advance has been reduced a couple of degrees.

As an experiment I plugged the partthrottle lean pipe and the idle and boost values stayed just about the same but cruise went down(up?) to 13 or so. Of course now this needle is adjusted for running with it connected but I must say that the values with it connected are MUCH more stable and I am going to stick with it because of that. Cruise at 2000 is a bit lean still and the pickup is MUCH improved from 1-2000 but still need some more polished off so Tom was correct, it seems to be stations 2-3 that affect that.

Back to the book, he gives a way to check the needle profile by taking out the dashpot piston and inserting a dowling marked up in increments just like the "stations" on the needle, you just increase the revs (slowly cos the piston dampers not there) and if it hesitates its lean so look at the marks and this gives you how high the piston is therefore what station to polish. works great, esp with a wideband but I found that after 4000 rpm it went lean even though I was pretty sure it was OK there. I think the reason is that the turbo is blowing a little by then and the missing piston cap upsets the pressure in the dashpot (the clue was the mist blowing out when you revved it hard!) this also is probably why when I revved hard the piston shot up above the top of the dashpot, making me suspect the spring was too light , It seems it isn't..because when I covered the top with my hand the 4000-5000 rpm was smooth so keep that in mind if you do it that way.

I may do a quick guide to this on my website waddya think??


whew.. that was along one!!

btw heres my map.. its 2degrees lower than real because my sensor is not quite right yet, i.e 8deg at idle is really 10 deg I'll get round to adjusting that later!!


Terry

Edited by Turbo Tel on 8th Jun, 2007.

website:- http://www.terryhunt.co.uk

Quick guide would be great as a starting point for discussion.
Regards
Dave

Edited by danboy on 7th Jun, 2007.

OK done! Jeez I must have lots of time.. Its this or crap American TV...or beer...


take a look at

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=159683

Terry

Edited by Turbo Tel on 8th Jun, 2007.

website:- http://www.terryhunt.co.uk