10 replies to this topic

[Minigman]

I have a 998 low compression engine (8.3:1).

I'm thinking of supercharging it so need to achieve 7.3:1.

The standard A+ head is 25.5cc chamber volume.

Can someone far cleverer than me tell me what cc I need to get the head to in order to get the 7.3:1 please?

[Cooperman]

Somewhere on here is the detailed calculation method which I posted a while ago.

 

It gives all the information you will need.

[Turbo Phil]

Why does the compression ratio need to be so low ? How much boost are you planning on running ?

Phil.

[Minigman]

Somewhere on here is the detailed calculation method which I posted a while ago.
 
It gives all the information you will need.

[Minigman]

I was told that's what I needed by Stuart at Vmax Scart.

So I found the ratio Calc details posted by Cooperman.

Hopefully I've got this right?
SV / (CR-1) = UV
998/4 = 249
249 / 7.3 = 34.1
Std head is 25.5cc so the reminder of gasket, piston dish and piston land height would = 8.6cc.
If I need 7.3:1 then that would be 249/6.3 which = 39.52-8.6 = 30.92cc

I'll have a look through the multitudes of small bore heads I have in my garage and see what I've got.

Thanks for your help.

[Cooperman]

Loks like a standard 12G295 head would be about perfect for this as the chamber volume is c.28 cc and the head is open-chamber.

[Minigman]

You'd be right but I've a feeling mines had about 80 thou taken off already.

So you think my maths make sense Peter?

[Cooperman]

Looks about right without actually doing the calcs.

You might want to check the piston crown to deck height unless you will be skimming the block to suit, which I guess you will.

[Minigman]

To be honest Peter the engine is an original 34000 miler. I recently cracked the 12G295 head (not sure how, no overheating, rubbish CR and not giving it any beans) so I wasn't going to strip it down. I was going to go down the replacement 1275 route but have thought about S/C instead. In 30 years of mini ownership I've never had one, and I do like a small bore engine. Something buzzy about them.

[Cooperman]

I have always quite fancied an 850 with a Shorrock 'blower' as they were quite popular in the '60's.

 

I did some work on a Mk.1 Sprite with a 1275 engine and a Shorrock blower a few years ago. I never got to test drive it, but I believe it was great to drive with such a light body, an LSD and oodles of lovely torque.

[Minigman]

Hard to find a good shorrock these days. Heave seen a couple at shows. Thought about it for my mk1 with its std 850 but will most likely build a standard 1098 for it just for the torque factor. The early single shoe brakes wouldn't allow much more. Will have a think on the 998 supercharger over the next couple of months.