Race Engine Forum UK

Hi Guy,

I'm after a bit of advice on CR for the twin cam engine I'm building for my Lancia Montecarlo. The car is used for 'spirited' road driving mainly, with the odd trackday / sprint.

The rest of the engine spec is as follows-

Gasflowed Volumex head (52cc chambers)
300deg 40/80 10.3mm cams
42DCNFs on Alquati manifold
Ansa 4-2-1 manifold, single outlet performance silencer
84.6mm flat top pistons
Block decked 0.9mm for pistons to protrude 0.5mm

The problem I have is that when calculated, the CR only comes out at 9.5:1. I was hoping for over 10:1, and ideally 10.5:1. Do you think this relatively low CR will be enough with the rest of the engine spec? Would I be better off leaving in the std cams with this CR?

Thanks in advance,
Nick

You will be fine with 9.5/1 and it certainly wouldn't worry me, but much will depend on the performance of those camshafts - if they have a powerful lift integral (I have written much on this) it will be fine even at 9.5/1, the gasflowed head (if done right - check out my forthcoming DVD, see GC V/W) will more than compensate for that low-ish CR value with good cylinder filling.
I'd never run over 11/1 on the 8V TC - they are no more powerful at higher figures due to impaired burn rate - but I have run single inlet and twin GC IIIA cams at 9.5/1 and had excellent results.

BTW - HC cast pistons that I wrote about 10 years ago or more are almost gone now, all my units use my own forged. I say this simply because I am still frequently asked about using HC pistons (where the owner makes no reference to what kind they might be - or have to be to) and the truth is most of the stocks of say, 1600 TC pistons - that would give the CR you're looking for have been junked - or are simply exhausted.

GC

Thanks Guy, you've put my mind at rest. I'll crack on and get it built using the new pistons and block that I already have then.

The cams I'm getting are an unknown quantity. I'd love to be getting the cams from yourself, but unfortunately my limited budget mean I'll have to look elsewhere. I'm taking advantage of the favourable exchange rate and ordering them from the US. They're pretty cheap, so I haven't got much to lose really. If I'm not happy with the results I can always swap them at a later date and I'll only have myself to blame.

I'll let you know how I get on.

Thanks again,
Nick