8 replies to this topic

[IainNeon91]

Hi all i have been reading through a lot of threads about the 12g940 and the gains it has on a 998 engine. But most of the threads i have looked at, the 12g940 head has been ported, polished and skimmed etc.. Which means you have to make sure the valves don't hit the pistons..

My question is, do you still have to the same modifications the the block/head if you want to bolt on a standard 12g940 head onto a 998??

Many thanks,

Iain.

Edited by IainNeon91, 29 January 2013 - 06:21 PM.

[rally1380]

Fitted a large valve 12G940 head on a 998 years ago and had trouble with the valves hitting the block. Had to pocket the block to solve this.

[bmcecosse]

The valves don't hit the pistons - and most heads are not ported/polished etc - the gains on a 998 are such that the standard head is fine - although I do suggest a few minutes of work down the exhaust throat where the tight corner is very restrictive.... You need 320 thou between the exhaust valve faces and the head face. If you don't have this - the easiest thing is to sink the exhaust valves into the head until you DO have 320 thou (8mm). Don't use anything more exotic than an MG Metro cam - and do set inlets to 12 thou and exhausts to 15 thou. You won't be disappointed.....

[IainNeon91]

- the gains on a 998 are such that the standard head is fine -

Do i still need to pocket the block if i want to fit a standard 12g940?

[Cooperman]

Not unless when the valves are opened 0.320" they are protruding beyond the face of the cylinder head. Put the head on the bench on its side and clamp it in position so it can't move. Put a dial gauge on each valve head in turn and open the valve 0.320". This requires the valve springs removed. With the valves opened that amount in turn, soo if the face of the valve is proud of the head face. If it is proud, then either recess the valve into the combustion chamber by the amount it is proud, or pocket the block - the choice wil be yours. If the valve heads are not proud you can fit the head.
It's called 'engineering'.

[Kat7Racing]

Just a quick Q on this.... Doesnt the 0.320 assume that standard rockers and nothing more exotic than an MG cam are being used, If you used 1.5 rockers and an MG cam wouldn't you get more than 320 lift? I only ask as if someones going to the extent of considering pockets then maybe they have other bolt on mods........... My block has pockets, an MG cam and 1.3 Rockers, among other mods.....
Dave

[Cooperman]

Yes, the 0.320" assumes a cam which gives a lift of 0.320 with 1.3:1 rockers.
If one is doing modification engineering on an engine, then knowing what is fitted and the specification in terms of clearances is vital.
The cam specification will give the cam lobe lift which is then multiplied by the rocker ratio. Then the valve is opened that amount and the valve head face relative to the head surface is checked. If the valve head face is proud of the head face then either valve recessing or block pocketing is required.
Measuring must be done accurately.

[MRA]

Just a quick Q on this.... Doesnt the 0.320 assume that standard rockers and nothing more exotic than an MG cam are being used, If you used 1.5 rockers and an MG cam wouldn't you get more than 320 lift? I only ask as if someones going to the extent of considering pockets then maybe they have other bolt on mods........... My block has pockets, an MG cam and 1.3 Rockers, among other mods.....
Dave

Hi Dave,

Simply if the combination of cam, rockers etc increases the valve lift to a displacement higher than the distance from the head of the valves to the surface of the cylinder head then work has to be done....

In either case pockets or sinking the valves deeper will have implications that need to be investigated, ie sinking the valves will necessitate valve train geometry changes, pocketting the block will require said block to be stripped down fully.

[Vipernoir]

Recessing the valves also destroys the flow - the exact opposite of what you are trying to gain !

If you can get one, a 12G202 or a 12G295 is a better option with less machining involved.