10 replies to this topic

[beanboy450]

Just wondering what's the best cam/head combo (for power) to fit a 998 that won't need any modifations to fit?
I have a 998 with hif 38, stage one kit and electronic ign.

[chichestermini]

I believe the best 'bolt on' head without modification is a 12G295, and camshaft wise would have to depend on the usage of the vehicle. Will you be aiming to use the engine for 'fast road', rallying, racing?

 

EDIT - Although if unmolested, the 12G295 cylinder head will need skimming to raise the compression ratio.

Edited by chichestermini, 17 May 2013 - 09:40 PM.

[beanboy450]

What do you mean by unmolstered? And it will be for fast road use.

[coopdog]

A standard 12g295 head will make your CR too low,

You need to skim the head to make the CR low enough to use,

A mg metro can and a 12g295 head will give you about 60bhp

[mini=love]

Personally i would go for the 12g940 head and pocket the block.

[creakyjaws5533]

I've got a 12g295 head it's pretty nippy for a 998 with a 731 cam I rather like it no idea on bhp sorry
Jack

Edited by creakyjaws5533, 18 May 2013 - 09:22 PM.

[chichestermini]

What do you mean by unmolstered? And it will be for fast road use.

Unmolested, meaning, hasn't yet been skimmed to raise the compression ratio.

[beanboy450]

Thanks for the replies. How much would it need to be skimmed by?

[beanboy450]

Bump

[chichestermini]

The biggest issue with the 12G295 is that it is meant for engines, which have raised D-top pistons - i.e. the original 998 Cooper. Therefore the combustion chambers are too large for a basic, non-Cooper 998, which normally has dished pistons or, at best, flat top ones. It is necessary to measure what you have, including the 12G295 head, do the compression ratio calculations and have the head skimmed to give the correct compression ratio for your engine. I would suggest a ratio of 10:1.
Whilst the head is being skimmed, it could be a good idea to have unleaded exhaust valve seats fitted. You don't need unleaded inlet valve seats. If the guides are worn fit new ones as you will need to re-lap the valves in before fitting anyway. The steel guides are the best really. Fit valve stem oil seals to all 8 valves.

 

Cooperman's guide to calculating Compression-Ratio is here, as follows:

 

1.  Measure the average distance from piston top to block deck level with the pistons at TDC. Take the average for all 4 pistons.

 

 

2.  Calculate the capacity of this volume (Call it ‘A’)

For 0.010” piston top to block deck this capacity will be:
Standard bore at 1275 cc  =  0.99 cc
At +0.020” overbore  =  1.01 cc
At +0.030” overbore  =  1.02 cc
At +0.040" overbore  =  1.025 cc
At +0.060” overbore  =  1.04 cc

 

3.  Measure the volume of the dish in the top of the piston using the 5-cc syringe with paraffin, meths, or even clean water. With Hepolite 20250, 20251 & 20253 pistons this is 8.3 cc.
Call this ‘B’

 

 

4.  Set the head up to be absolutely level using a spirit level. Measure the combustion chamber volume in the head using the syringe and paraffin, meths, or clean water. Do this for a couple of chambers and take the average. Call this ‘C’

 

5.  There is a volume in the head gasket area and we’ll make an assumption here that with the average gasket, say a BK450 or AF460, it’s 4 cc. Call this ‘D’

 

 

6.  There is the ‘ring land volume’. That’s the volume down the side of the piston to the top of the top ring. Allow 0.75 cc for this. Call it ‘E’.

 

7.  Now divide the actual swept volume of your engine by 4 to get the swept volume per cylinder. Call this the swept volume ‘SV’

For a standard 1275 cc  =  318.75 cc
At +0.020” overbore, i.e. 1293 cc =  323.25 cc
At +0.040”  i.e. 1310 cc  =  327.5 cc
At +0.060”  i.e. 1330 cc  =  332.5 cc

 

 

8.  Add all the unswept volumes together i.e. A+B+C+D+E.

Call this the Total Unswept Volume ‘UV’

 

9.  The Comp Ratio (CR) = (SV+UV) divided by UV

So, if you have a standard bore 1275 cc engine with the SV = 318,75
And if the total of the other volumes, the total unswept vol. UV =, say, 36 cc,
The CR = (318.75 + 36) divided by 36
= 354.75 / 36 =  9.85 to 1

 

 

However, if that is not the CR you want, you have to work backwards to calculate what the combustion chamber volume in the head should be.
Say you have the figures we just used, but you want the CR to be 10.4 to 1.

 

The formula to work out what the UV should be is:

SV divided by (CR-1) = UV

So, in the case we are using:

UV = 318.75/(10.4 – 1) = 318.75 divided by 9.4. = 33.9 cc

 

Add A+B+D+E, as established earlier and subtract this from the new figure for UV, in this case subtract it from 33.9. The resulting figure is the actual volume you need each combustion chamber to be.

Using the syringe again, after setting the head up to be absolutely level, put EXACTLY this amount of fluid (paraffin, meths or clean water) into a combustion chamber.

The distance from the head face to the surface of the fluid is the amount to be skimmed from the head. To measure this put a steel ruler across the head above the chamber and measure down to the fluid surface with a digital vernier, subtracting the thickness of the ruler afterwards. In practice, this is the most difficult dimension to measure as the fluid has a ‘meniscus’ which sort-of ‘reaches up’ to the bottom of the vernier calliper. You have to make a judgement on this, but don’t be put off, it is not that hard.

 

 

Once the head is skimmed as necessary you just fit it and your CR is correct. Remember if you are increasing the CR you will need to check and adjust the timing.
 

[beanboy450]

Bloody hell that's what I call a reply. thanks mate ill do that !! Much Appreciated