24 replies to this topic

[Jukesr]

Hi all this is my first post.

The problem I have is that I got a 1982 mini with what I was told a 1293cc engine. Kent 286 cam 3:4 diff etc...
I tock the car to SH engineering for Dave (Top bloke by the way) to cast he's expert eye over it.
When getting the car back Dave sad he thinks the compression ratio is to low

So I tock my cylinder head off to do the calculations and found that the pistons were .030 not .020 that wold be on 1293 so it's actually 1301 :/
The other thing is that my pistons have a 14cc dish !!!
So after doing the CR calculations I found out that it's running at 8.5:1...
The combustion chamber is 19.5cc...

My question is what is the lowest combustion chamber cc on a 12g940 ?

Thanks in advance

[Tamworthbay]

Do you mean what was the lowest as standard? Or what is the lowest it can be machined down to?

[Alex_B]

If you're thinking that or CR is low, have you thought about forced induction? I believe you need a CR of about 8.5:1 to run a supercharger or a turbo, and either could result in a very pokey engine.

[Jukesr]

Hi both

I would like to know what the head can be mâché and down to, so I can work out what sort of CR I can get with these pistons.

Also what would be the best CR for a 1301.

I'm not after a forced induction motor that's cheating haha :)..

[Jukesr]

* Machined....

[Tamworthbay]

Hi both
I would like to know what the head can be mâché and down to, so I can work out what sort of CR I can get with these pistons.
Also what would be the best CR for a 1301.
I'm not after a forced induction motor that's cheating haha :)..

I would PM Cooperman on here, he is brilliant on stuff like this and will be able to give you a definitive answer. There are limits due to water jackets etc and of course it depends on what has been done already.

[Jukesr]

Nice one pal

This is what I am worried about :/
I was sold a lemon and now I'm trying to make good what I have.
I don't really want to be tacking the short block to bits to change the pistons if I can help it.
I want to Try the simple things first.

[Alex_B]

I don't think you were sold a lemon, either someone wanted to leave it open to forced induction at a later date or didnt do the calculations with regard to CR when the engine was built.

I wouldn't say forced induction is cheating, its merely a very efficient way to maximise the volumetric efficiency of the engine.

[Cooperman]

First of all measure what you have and calculate what size the combustion chambers need to be. I did post a CR calculation explanation a while back.

Then lay the head flat and using a syringe or a burette put exactly that amount of paraffin oil into a combustion chamber.

The distance from the head surface to the paraffin surface will shoe how much needs to be machined.

Then come back on here and we'll tell you how far the head can be machined, but we need to know whether the head has ever been machined in the past, so measure the total thickness of the head as well.

You can certainly machine 0.080" from a standard head with no problems, maybe slightly more.

Do the maths and come back for more help.

I don't think you have bought a dud engine, rather that it has unusual pistons (0.030" giving 1301 cc are rare these days) with a large dish. If you have the part numbers off the top of the pistons it would help. Be careful to measure how far down the bores the piston top edge is at TDC.

[gazza82]

Standard 12G940 head is 21.4cc ... so it sounds like head has already been skimmed.

[Cooperman]

I've had a 16.5 cc chamber volume on a 12G940 and that was after gas-flowing.

However, with 14 cc piston dish it will be difficult to get a really good CR for a high-revving engine such as you'll have with a 286 cam.

A 3.44:1 FDR with a standard gearbox will be far from ideal with a 286 cam as the best power will be from around 5500 rpm to 6700 rpm.

Your best bet might be to bore to +0.040", fit a set of 21253 pistons and set a CR of around 10:1. Use a 266 cam which will work very well with that gearbox & diff. Then the engine will have super torque, very useable power and excellent gear ratios.

[Jukesr]

Thanks for the advice Copperman.

 

I have done the calculations with a 16.5cc combustion chamber and my CR is coming out at 9.3:1

 

I just need to fill the chamber to see how much needs to be skimmed, so what is the minimum thickness the head can be?,  just worried the valves might come past the face as well and interfere with the pistons ? 

 

calculation:

 

cylinder to deck (TDC) = 3.98cc

Piston dish vol = 14.0cc

cylinder head vol = 16.5cc

BK450 Gasket vol = 4.0cc

Ring land vol = 0.75cc

Total= (39.23 Uv) + (327.41 Sv) = (366.64 Tc) Divided by, 39.23 = 9.3:1 CR

 

Dose the calculation look Right ?

and will this CR be ok ?

[Jukesr]

Also is there a way of telling what cam you have without taking the engine out ??

 

with what this guy told me about the car its possible it hasn't got a 286 cam installed..

 

Thanks

[Cooperman]

The pistons sitting so far down the bores won't help. 3.98 cc indicates about 0.040" which is a lot.

If you want a really nice running Mini it would be best to strip and re-build the engine, but it is understandable if this is not an option at this stage.

If you are lucky it won't have a 286 cam, as that will make it very unpleasant to drive on the roads. The 286 is really a stage rally cam and is what I use on my Cooper 'S' competition car. On the road it is not nice, even with SC CR gearbox and low FDR, so the cam must be identified. First of all measure the maximum lift at the push rod top, then compare it to the Kent Cams data to see what the lobe lift is and which is the same as yours. That will not give a definitive answer, but it will tell you if you have the lift of a 286. If it is a 286, then best to 'bite the bullet, strip the engine and get everything right.

 

I hope this helps.

[Jukesr]

Ye any information helps Cooperman :) and thanks

 

so 1mm from deck to piston @ TDC is to much..

 

so tell me if im wrong if it's a 286 cam the pushrod will lift by 7.99mm on inlet and 8.22 on Exhaust ??