9 replies to this topic

[turbominicooper]

Just read the bit about putting a 12g940 head on a 998 and wanted someone to confirm that a simple head swap will cause no issues?

Standard 998 - no mods

[samsfern]

you need to pocket the block to accomadate the valves, so its not as straight forward as just swapping heads.

[HARBER07]

Or apparently you can recess the valves. You will also need to mod the water pump I believe and fit a smaller pulley.

[liirge]

change the thermostat housing and top hose to 1275 type, and fabricate a radiator bracket!

[turbominicooper]

you need to pocket the block to accomadate the valves, so its not as straight forward as just swapping heads.

is this not only required when a high lift cam is used?

i will be running the standard cam and standard rockers

[scrumpymini]

you need to pocket the block to accomadate the valves, so its not as straight forward as just swapping heads.

is this not only required when a high lift cam is used?

i will be running the standard cam and standard rockers

I did read on here the valves will hit the top of the block unless you have pockets machined due to the smaller size of the bore in the block, I have heard of people doing a bit of a bodge and machining the block in place but not sure how this is done, why not go for a good gas flowed head with the largest size valves you can fit over the standard bore.

[lrostoke]

I've also read that with standard rockers and camshafts it isn't always neccesary to do any machining.
There are different flavours of the 12g940 some have bigger valves

[1984mini25]

It’s pretty pointless just bolting on a 12g940 head and sticking with the standard 998 cam, you won’t get the best from it.

The mg 1275 head does have larger valves, but with a standard ish 998 they are too big and you might find you’ll lose powered compared to the same engine fitted with the standard valve size head.

[miniobsessed]

You can fit a 12G940 head on two ways...

1) Sink the exhaust valves.
Measure the distance between the exhaust valve and the head face. With a standard cam and rockers this will need to be around 320 thou if you want to avoid a big mess. If it is not 320 thou you will need to cut the valve seats until they are low enough.

I don't really favour this method as it increases the shrouding of the exhaust valve which will greatly impact low lift flow figures.

2) Pocket your block.
This should be done with the engine apart and ideally by a machine shop. Inconsistent volume pockets will mean inconsistent compression ratios between cylinders. Pockets should give about 80 thou clearance around the valve where possible (being carefull not to go over the gasket line) and should be as deep as required plus 40 thou (minimum) being carefull not to go anywhere near the top compression ring.

A 12G940 head on a 998 will make more peak power but you may loose the odd ft-lb in the low to mid rev range due to the bigger ports having a slower velocity on the 'small' engine. A 12G202 or 998 head can be made to flow very well and make very good torque and hp figures but it would be more expensive than fitting a standard 12G940.

[liirge]

You can fit a 12G940 head on two ways...

1) Sink the exhaust valves.
Measure the distance between the exhaust valve and the head face. With a standard cam and rockers this will need to be around 320 thou if you want to avoid a big mess. If it is not 320 thou you will need to cut the valve seats until they are low enough.

I don't really favour this method as it increases the shrouding of the exhaust valve which will greatly impact low lift flow figures.

2) Pocket your block.
This should be done with the engine apart and ideally by a machine shop. Inconsistent volume pockets will mean inconsistent compression ratios between cylinders. Pockets should give about 80 thou clearance around the valve where possible (being carefull not to go over the gasket line) and should be as deep as required plus 40 thou (minimum) being carefull not to go anywhere near the top compression ring.

A 12G940 head on a 998 will make more peak power but you may loose the odd ft-lb in the low to mid rev range due to the bigger ports having a slower velocity on the 'small' engine. A 12G202 or 998 head can be made to flow very well and make very good torque and hp figures but it would be more expensive than fitting a standard 12G940.

Nicely summed up there!
to gain some of that lost torque lower down, stick some twin HS2's on, they have a smaller throat and therefore increased intake velocity.