Quick n dirty comp ratio calc

[RobThomas]

Can anyone with the figures to hand give me a quick calc for a 948 with a 12G295 head on it? Just a guide figure would be enough. Need to see if it would be a viable project to fit the Shorrocks to this engine. I couldn't find a suggested figure for the area above the piston inside the bore to use in the calcs.

948, +20 with (8.3:1) 14396 pistons.

About 1.5:1 less comp ratio????

[philthehill]

Providing the top of your block has not been skimmed and you have got slightly dished pistons I would suggest that the un-swept volume above the piston is around 6cc - 8cc plus an extra 3cc for the gasket. The 12G295 has a combustion chamber of 28.3cc.

Fitting a supercharger to the above should not raise any problems only the cylinder pressure.

[RobThomas]

Thaks, Phil. Works out at 7.3 comp ratio.

It's for the late 53 Series MM which I'm using as my main transport. This one would take an A-series without mods, other than bolt-on stuff. 27bhp isn't really enough for everyday transport round here.

[philthehill]

Rob
As you will already know supercharging is the easiest way to get an increase in power without too much effort.

The 948cc 'A' Series engine is ideal for supercharging and there is a long history of the 'A' Series being supercharged for both road and competition use.

Phil

[les]

Better than a 1275 apparently. Sorry if I’m diverting a bit but I was rather disappointed recently after being informed by a guy who was quite knowledgeable regarding Supercharging using a Shorrocks charger. I was a little baffled by the explanation but it appears that revs play a big part, and an alternative charger would be favourite for a 1275.

[philthehill]

Les
Revs do play a part in filling the cylinder but all Shorrock superchargers I have seen rotate at crankshaft speed or thereabouts i.e. the crankshaft and supercharger drive pulleys are the same diameter.

http://www.sebringsprite.com/pdf-files/ ... Instns.pdf

http://mk1-performance-conversions.co.u ... ck_cat.pdf

The CB75B has a recommended max rotational speed of 7000rpm and is suitable for engines up to 1300cc

Looking at the second link - a 1275cc Sprite/Midget which has all the same parts which effect performance i.e. head and camshaft as a 1275cc Marina, when fitted with a Shorrocks supercharger the 1/4 mile time is reduced from 25.6 secs to 14.8 secs with an increase in bhp from 59bhp to 78bhp.

I have to say that your knowledgeable friend may not be as knowledgeable as they make out. I would rather trust the data in the attached links.

Fitting the supercharger to your 1275cc Marina engine will make it more tractable, smoother and more enjoyable to drive - I would fit and appreciate the increase in power. Go on you know you want to

[les]

Thanks for the links Phil, I do have the paperwork from the first link along will several others covering dismantling and reassembling, as well as installation. You’re right I have been keen to go down this route but was given the information I mentioned by a chap who bought an engine from me, and it rather threw me, he didn’t strike me as the usual ‘Ive done it all type’. However, can’t argue with the figures, just need to get motivated.