19 replies to this topic

[998kid]

How do you go about recessing the valves into the head?
I am very interested in upgrading my standard 998 but so far no where' is helping ? Cheers

[Cooperman]

As with most engineering exercises it's a matter of measuring and calculation to get things right.
A 12G295 head is very good as it's what is known as an 'open chamber' head and it is what was fitted as standard to the 998 Cooper. However, the 998 Cooper had raised D-top pistons, so when fitting to a non-Cooper 998 the compression ratio must be measured, calculated and the necessary machining done to get it correct at around 10:1.
With the 12G940, which is an excellent head, there is a risk of the valves hitting the block deck face. First of all the compression ratio calculations need to be done and any amount needed to be skimmed from the head arrived at. Then you need to know the maximum lift for the cam you have. The distance from the installed valve head surface to what will be the face of the head must be at least equal to the valve lift at full lift. If not then the valves seats must be sunk further into the head chambers until the cam valve head to head surface distance is greater than the max. valve lift. You can go around 0.040" extra sinking and if that does not give the required clearance, which it may not do if a 'hoitter' cam is fitted, then the engine must come out and to pieces for block to be decked to achieve valve clearance.
There is no way round this as the measurements are critical.
I hope this helps.

[bmcecosse]

With a 940 head on a 998 engine - you won't need to skim the head, in fact the CR may end up slightly on the high side. Measure from the face of each exhaust valve to the face of the head. You need 320 thou (8mm) to have good clearance with standard lift cam and rockers. If there is not 320 thou - you sink the seats down until there IS 320 thou. I did this in less than 10 minutes for all 4 seats using a pillar drill and a 45 degree countersink. Slow speed and a piece of emery cloth under the cutter to stop chatter - makes a lovely job of the seat. If you have to sink any serious amount (unlikely) - you can then 'unpocket' the valve by running round the edge of the seat with a fine point grindstone in a drill. But frankly - it's the exhaust gases - and the increase in power using the 940 head is so good that the unpocketing is really not necessary. Use 15 thou exhaust valve gaps - and 1275 rocker gear -and of course a 1275 copper gasket.

[mini13]

Edited by mini13, 29 July 2012 - 06:58 PM.

[racingenglishcars]

it says on the website inlet 29mm exhaust 25.4(which is standard for a 998)
so how can this be anygood

Seems to me that aside from the fact that the 12G295 is an excellent head for the 998, the original question wasn't really answered.

The reason for increasing the inlet valve diameter whilst leaving the exhaust valve the same size is because inlet has (usually) only 1 atm of pressure to force the air into the cylinder.

The exhaust on the other hand has a positive displacement piston to force the gasses out - at whatever pressure is necessary to reduce 293cc of pressurized gas to 30cc of exhaust pressure gas within 0.003 seconds. That can be a lot of pressure, and though it does reduce engine power somewhat, this loss is easily offset by allowing more fuel and air into the cylinder in the first place.

Edited by racingenglishcars, 29 July 2012 - 01:24 PM.