motronic pistons

[Peter Kemos <peterk@domain.elided>]

>I am strongly considering the 10.0 Motronic piston 
>& liners.   My concern and questions to the Digest are these:  Will the 
>compression jump from 9.0 to 10.0 CR create a need to use the top octane 
>gas?  Will I notice a significant increase in predetonation? 
>

I responded to the original post off list.  Anyway, having a fairly 
fresh 2L GTV with motronic pistons and Spica i can weigh in on this:

I am running euro '48' cams which are fairly mild, but timing has been 
adjusted for increased overlap.  The head is stock and not shaved, 
though a few thousandths have been taken off over the years in the 
course of fixing a few blown head gaskets.

The car WILL ping or pre-ignite or detonate or whatever the correct 
non-technical term is, using premium pump gas, primarily when under 
heavy load, in overgeared, uphill type situations, but also when simply 
nailing the throttle in 5th from anywhere under 4,500 rpm.
I managed to alleviate it somewhat by blending in Marvel Mystery Oil, 
but ultimately, as the GTV is becoming less of a street driver and more 
of a track car, i started running blends of 110 leaded, left over from 
whatever the last track event was.  This has effectively cured the 
detonation.

If your head is milled it will only exaggerate the problem of 
detonation.  The cam lift and timing will also affect the issue as they 
determine how much cylinder fill one gets.  Finally, ignition timing is 
also critical, more advance=more detonation.  Piston clearance 'should' 
be fine, but this really depends on just how much the head was shaved.

In a nutshell, based on your parameters, i would say that you are quite 
likely to get detonation using only premium pump gas.  Using stock cam 
and ignition timing specs will help, but what's the point of having a 
high compression motor with fairly aggressive cams, that can't get the 
revs up because of boggy timing?

Peter Kemos


Peter Kemos

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