Change in Boost Threshold
Turbo Mini Forums

So the Sprint used to make a few psi boost at a little over 2.2k, then full boost by 3k. since I changed the head gasket It only makes a bit of boost at 2.8k and then full boost at 3.3k (all approximate).

Fuelling and ignition are unchanged (although I can't check fuelling 100%, the carb and settings are the same as it was all left connected).

The only thing I need to double check is the tappet clearance.

But of a puzzler. any thoughts?

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I'm thinking aloud here, but could it be that the head gasket compressed thickness is larger, increasing the volume and decreasing the compression ratio?

In the 13's at last!.. Just

Did you "de-coke" it whilst the head was off ???

Same logic, decreased CR.

Schrödinger's cat - so which one am I ???

Check, double check, and triple check for a boost leak somewhere.
I've been convinced my turbo was knackered on more than one occasion only to find a boost leak.

Boost leak is a good idea, on a similar thought, I'll check the wastegate.

The head has only don a few thousand miles, so no build up of crap. It is a different gasket, but the volume is the same (smaller if anything)

Cheers

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

valve clearence .......... also as tom says boost leak

Although I haven't been for a test drive yet, the valve vclearances were all very tight (7 thou - me being a laxy bastard). Now they are all 15 thou.

Found no boost leaks, but I'm always sceptical about the dump valve, so will experient with that if it's still not right.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

Right. The resetting of the tappets made a remarkable differance. It's not quite as good as it was but now driveable.

I'd still like to lower the threshold as much as I can by fuelling and ignition. If I'm WOT at 2k it's a pretty ponderous experience, so i think richening it up and some ignition advance here will help. Any thoughts?

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

I found putting more advance on the van when off boost made a massive difference. Also brought the boost threshold down slightly it would seem too?

If something is worth doing, it's worth doing half of.

try tappets at 18 ?

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

I was thinking the same thing Robert I run Tappets at anything between 18 and 20 but bear in mind running tappets much over 15 will increce cam wear

Out of curiosity, opening the tappets is like opening the valves a little later and somehow decreasing the opening time of the cam (by a very very small amount, I know).
But I remember reading in Vizard's book that advancing the cam is beneficial to low end torque whereas retarding it as an influence on top end.

The way I see it, it's the other way round.

So what shall I believe ?

Or maybe it is to do with the exhaust rather than the inlet ?

On the exhaust it would be equivalent to some extra power stroke time and a bigger "gulp" of exhaust gas to drive the turbine. But still less exhaust duration

Can anyone try different tappet clearance between exhausts and inlets please ? (so that I don't have to do it )

Rusty by nature

By opening both tappet clearances you effectively decrease the overlap between inlet and exhaust, which at slower speeds will help drive the turbine into boost.

If you do only the inlet, or only the exhaust, you will just find you get "half" the amount of difference as you will not decrease the overlap as much.

Excellent, this is the type of thinking and advice that I was hoping for. I would never have tried bigger gaps.

Personally I think the 266 was the wrong choice of cam. Would be good in the race car, but it does compromise low end.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

wil have a look at the KENT md sports injection cam. will suite your baby spot on wil

Oki doki so it was all about overlap !

Thanks Tom.

Edit :
I hope I made a good choice with the MD 256 I bought.
I have been fearing for a while seeing Alpa or Wil going for a 266.

Thanks Wil, you cheered me up !

Edited by Advantage on 15th Sep, 2010.

Rusty by nature

Yeah, 256 or MG cam would be better from past experience. I only used the 266 as I had one.

opening up the gaps to 18 has dropped the boost thresholed 200 rpm, so happy. More tossing around with timing and I think we'll be sorted.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

inlet at 20 ,and ex at 12 ? ...

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