quick throttle position change not smooth
Turbo Mini Forums

Hi All,

I can't get the final bit of tuning right for my supercharged 1275 Pickup.
Specs:
eaton M45
MR2 intercooler
blow through metro turbo Hiff44
Mechanical dump valve (VW G40, connected to throttle)

The problem I have is the following:
Quick increase of throttle doesn't work well. Engine hesitates for a while and then suddenly it picks up and the wheels starts spinning due to the 160Nm waking up :) Hesitation is too short to see how the mixture is on the wideband lambda, but my ghess is too lean.

With smooth increase and decrease of throttle the mixture is almost perfect thanks to my LED piston height measurement in combination with wideband lambda and hours of needle sanding.

I already tried different oils in the dashpot. Carb has had a complete overhaul.

Anybody any suggenstions what to do/check?

Thanks in advance!

Regards,
Remko

Does putting thicker oil or a heavier spring in make any difference?

On a side note, any more info on your LED piston lift indicator?
I made a mechanical one, but a nice electronic one would be better.

Phil.

WWW.TURBO-MINI.COM

Wouln’t a havier spring or thicker oil make is more lean? I paid extra attention to my lambda. It goes in the direction of lean. I’ll try some different springs.
I think it has something to do with the suddenly rise of boost, due to supercharger instead of turbocharger. Is it possible that the pressure rise in carb is quicker then pressure increase in fuel bowl?

And I’ll post some pictures of the LED system

There is some info on the excellent LED system here: http://www.turbominis.co.uk/forums/index.php?p=vt&tid=617324

While searching for info for a side project recently it appears historically there has been mixed feeling on here with dashpot springs with some members stating that the (I believe green strongest) was sufficient, others saying that swapping springs made no difference. I recently read of members using two standard springs.
Are you using a plenum of any sorts to incorporate the restrictor ring?

Shane

Thx Metroturbo, saves me finding the pics 😊

I’m driving with the standard plenum with the g40 recirculation valve in front.

What have you done with the part throttle lean off pipe ? If you've still got it connected as standard try either disconnecting it and blocking it off. Or leaving it as is and restricting it, with a clamp.

Thicker oil will make it run momentarily richer as it restricts the rise of the piston on a snap increase in throttle opening. Restricting the upward movement after opening the throttle has the affect of increasing the air velocity over the bridge of the carb, which reduces the air pressure over the bridge, and means more fuel is drawn out of the needle.

Personally I'm not a fan of using a stronger spring to solve problems of running lean on acceleration unless you have exhausted all other possibilities. The stronger spring changes where the piston naturally sits in cruise conditions, which will mess up all the work you have already done with your needle.

Are you using the standard turbo fuel pressure regulator? If so, it should have no problem keeping the fuel pressure high enough in the bowl. Might be worth plumbing in a gauge temporarily to see what is happening to fuel pressure. If you have the gear, a differential gauge measuring fuel pressure and boost pressure makes it much easier to see that fuel pressure is keeping ahead of boost.

DILLIGAFs suggestion is definitely worth checking as the part throttle lean off can cause this exact issue. I installed a solenoid in the part throttle lean off pipe to solve the problem of my carb going lean as it came on boost, whilst still allowing the carb to lean off on cruise. Megajolt takes care of opening and closing it, but I do need to add a time delay relay so that it stays shut when accelerating through the gears.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Thanks Dilligaf and Metroturbo! Plugging this pipe solved my problem for 90%!
Next step is trying different thickness of oil to solve hopefully the last 10%, but I’m already very happy with this improvement!

Once this is solved I’ll switch over to a smaller pulley. Boost will rise from 0,5 bar to 0,75 bar!

Edited by remko on 2nd Mar, 2021.