7 replies to this topic

[BoboGib]

~~~~~Please delete this text before posting the topic~~~~~~
Please give your topic a title that quickly describes your problem. Just putting 'Help my Mini doesn't work' is useless to anyone trying to help.
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Model:Jet Black
Year:1988

Any non-standard parts that might be involved with the problem?


Just a general query. I have a 12g295 head, measured the chamber at 28cc so its pretty much untouched. I have a 998 block which is overbored +.030 and the pistons have a (can't quite remember but i think) 6cc dish. I've used an online cal and it gives me a comp. ratio of around 8.5:1

I have been doing a little research and some of the older pre A+ 998's had a comb ratio of 8.3:1. If I were to build this engine as it was, what performance degradation would I have due to the low comb. ratio?

The plan is to eventually go forced induction so I don't want to be skimming the equivalent of a steel rsj of the head to bring the comb. ratio up. Would it be worthwhile building the engine and using it with the low comb. ratio or wait till I can afford to do it all at the same time. (bearing in mind I have a baby due in sept so it might be a while before I can afford to build the engine to the full forced induction beast I want to)

[BoboGib]

please, someone has to know. Seen a few of your more experienced engine builders on here this afternoon.

[Nightrain]

It's gonna be better than the old 998A series with that head, the real (dynamic) compression ratio will be better than the old 8.3:1 998's simply because the head will improve the VE (volumetric efficiency) of the engine. So it will increasing the real compression ratio.

[mini_matt_106]

those are the real expensive/recomended/sought after 998 heads so im guessing your going to like it, if your wanting to raise CR y not flat top pistons?

[BoboGib]

I don't really want to buy a set of flat tops pistons as the eventual aim with the engine is to go forced induction, so ideally i'd like to build the engine up and use it while saving for the charger bits. Then i'd like to fit the charger with minimal disturbance to the engine.

[Ethel]

I'm not saying 8.5 is wrong, but did you factor in the head gasket thickness?

You won't actually notice much power loss as it will only be really evident when the throttle is wide open. It will also be close to what you'll be getting out of that turbo before it spools up.

[BoboGib]

I'm not saying 8.5 is wrong, but did you factor in the head gasket thickness?

You won't actually notice much power loss as it will only be really evident when the throttle is wide open. It will also be close to what you'll be getting out of that turbo before it spools up.

Yes mate, all has been roughly calculated in, won't know all the exact measurements till I build up the short block but have measured the combustion chamber in the head relatively accurately and have done the same for the dish in the piston. Hopefully I'll be able to get all the bearings I need after pay day (mains, big end, cam and thrust washers) and get the short engine built up and work it out a little better but early calculations are showing between 8.5 and 9:1

[ginigwunkle]

out of interest what do you all put in for the compressed head gasket thickness?

im using payen a460......