3 replies to this topic

[Clarko]

Hi All,
I have a standard 1978 Mini Clubman Estate, 998cc, I have recently bought a 12G202 cylinder head, twin HS 2 SU carbs, and a Maniflow LCB manifold and Exhaust system, my question is will this setup give me a little more power? I thought this head had slightly bigger valves than the standard one, so would be suited to twin carbs? Anyone have any thoughts, thanks John.

[Cooperman]

So long as you do the measurements and the necessary subsequent machining to get the compression ratio to around 9.75:1 to 10:1 you will see a good improvement.

You will not see the possible improvement unless you gas-flow the head some and fit a better cam, with the cam accurately timed in.

The 998 is a lovely smooth engine and will give great results if altered to be more like the 998 Cooper, which gave about 55 bhp.

[Clarko]

Hi Cooperman,
Forgive me, but how do I work out what the compression ratio is on the new head with standard pistons? I have a machine shop that can do the machining if needed for me, what cam would you recommend for this setup? Ideally I would like a nice idling, nice sounding engine, that was a bit better than standard, I’m not really interested in a hugely tuned motor, if that makes sense! Best of all worlds.

[Cooperman]

When altering any engine in any way it is vital to take measurements and do calculations in order to get it correct. There are different types of pistons, some are flat top and some have a dish. This means that the volume of the combustion chambers for a given compression ratio will need to be different.

 

Now the 998 Cooper had special raised top, known as D-Top, pistons which had the top raised in the shape of a D to mean that the compression ratio would be higher and a standard Cooper head would then have the right comp ratio.

 

However, with a non-Cooper 998 first of all you need to establish the type of pistons fitted and the volume of the dish if they are dished pistons.

You then need to measure how far down the bores the piston tops are when the pistons are at Top-Dead-Centre (TDC).

 

From these dimensions the required volume of the combustion chambers in the head can be calculated in order to obtain the required CR.

Then you can put in a spark plug with the head upside down on a bench and set level with a spirit level and you put in the required volume of paraffin oil and measure how much needs to be skimmed from the head to get the CR correct.

 

If the head is being gas-flowed, this must be done before the head is skimmed as gas-flowing will increase the volume of the chamber. The valves must also be lapped-in and fitted prior to chamber volume measurements being taken, then stripped down again for final head skimming.

With the 998 it can be difficult to get the CR right if dished pistons are fitted.

 

I know this sounds complicated, but it is straightforward if approached in a logical manner. We are all here to help, but you will need some basic tools for measuring such as a Dial Test Indicator, a steel rule and a set of feeler gauges. You will also need a syringe calibrated in cc's (ml's) for putting the paraffin into the chambers and a vernier calliper to do other measurements.

 

If you cannot do this you will need to get someone to do it for you as it is very important to get this all correct.