12 replies to this topic

[whiskymini]

Hi guys,

I've done the literature. I have it under good authority that fitting my 12G1316 (the smog head equivalent of the 19G940, MG1300 head) onto my stock 998 engine WITHOUT modification will work fine.

But I need to confirm with you all about one thing in particular.

My engine is a 99H as in high-compression (flat-top pistons i think) and if I'm correct, then the compression ratio is at just over 10:1 as it sits now with a 998 head and 24.5cc combustion chambers.

Fitting the new head with 21.4cc combustion chambers will raise the CR. But by too much?

Would it be acceptable to fit a thicker gasket? Where can I find one?

 

Any help would be appreciated. Thanks guys.

[nicklouse]

Of the top of my head I don’t know what the CR would be as you would need to do some measuring of the head (it may have been skimmed) and the distance the piston is down the bore as the block may also have been skimmed. Then you will have to do some measuring of the distance between the valve and the block as some will need pocketing with some builds. Others don’t . Never assume anything.

[whiskymini]

It's safe to say that both the block and head are not skimmed and unmodified.

my math is as follows (but i might be missing a tad of volume between block surface and swept piston-head). also assuming flat-top pistons...

(24.5+3)*4=110.  CR=(998+110)/110=10.1:1

(21.4+3)*4=97.6  CR=(998+97.6)/110=11.3:1

and Des Hamill states that a CR of over 11:1 is a racing engine and needs 11studs minimum. 10 is ideal apparently.

Edited by whiskymini, 27 February 2020 - 05:36 PM.

[dotmatrix]

It's safe to say that both the block and head are not skimmed and unmodified.

my math is as follows (but i might be missing a tad of volume between block surface and swept piston-head). also assuming flat-top pistons...

(24.5+3)*4=110.  CR=(998+110)/110=10.1:1

(21.4+3)*4=97.6  CR=(998+97.6)/110=11.3:1

and Des Hamill states that a CR of over 11:1 is a racing engine and needs 11studs minimum. 10 is ideal apparently.

aren't you forgetting ringland volume and piston to top distance. you need to include this.

 

but I have tried it. I put a 940 head on my +20 flat top block with a single normal headgasket and I think I calculated this to 10.5:1 or something like that with a kent 266 cam. cant remember exactly and this made the engine ping/detonate on load under 3000rpm. I then removed 1 ccm from each combustion chamber and this helped but it still has a small tendency to ping at full throttle on low rpm so removing more would be the best. so my conclusion is, aim for those 9.6 or what ever it is that is recommended.

[blacktulip]

My guess is it will be way too high unless you are planning to run it on very high octain fuel. The standard unleaded from the pumps is best suited for around a 9.5:1 CR

[Cooperman]

You will need to grind out some metal from each combustion chamber to get the CR to an acceptable level which is no higher than 10:1 at most.
You also need to check to ensure that the valves don't hit the block deck at full opening.
Engine building is not about bolting bits together, it is about measuring, calculating and machining.

[Spitz]

If an A+ 998...probably flat top..... but an Aseries 99H still had dished pistons

[robert.hedges]

Try this https://jscalc.io/ca...RNEglVkKLQir11T I cant take any credit for it but I use it all the time. From my manual calcs, this includes a general ring volume assumption of around 0.25c.... which on most cases is there abouts. Hope it helps. My experience tells me that you should really measure everything yourself. Do not trust published figures or standard volumes. Nothing is every really 'standard'.

Edited by robert.hedges, 28 February 2020 - 09:23 AM.

[robert.hedges]

As a further thought you can get oversized gaskets from https://www.cometic....om-gaskets.html some UK online retailer's also stock some limited oversize cosmetic gaskets sizes but they are expensive. To my mind it would be easier and cheaper in your case to grind out some metal from the head if needed.

[robert.hedges]

As a further thought you can get oversized gaskets from https://www.cometic....om-gaskets.html some UK online retailer's also stock some limited oversize cometic gaskets sizes but they are expensive. To my mind it would be easier and cheaper in your case to grind out some metal from the head if needed.

[whiskymini]

Great thanks guys. Looks like I'll have to do some math myself. I do appreciate the reference material everyone's provided (esp. that compression ratio calculator holy moly what a find, Robert!)

My path forward (without having to remove the block from the car), will be to:

1. calculate the new head volume,
2. buy thickest head gasket I can find,
3. remove the current head and check the volume on that one (to confirm a few things here, namely want to check its current CR as further confirmation of block volume),
4. check for flat/dished pistons and if I have to, run the math once more,
5. install new head with stock rockers and turn the engine a bit to make sure the exhaust valve isn't stopping it from turning,
6. finish complete install and carb tune then bobs your uncle.

hope that's a good conclusion to this thread unless someone has more to add.

Cheers.

[gazza82]

Here's an alternative and works in good old Imperial .. I tried a couple of online CR tools and they both came out the same.

[nicklouse]

5 you need to check the clearance as it not touching when moved by had means nothing. First I would check without gasket. If that is ok then you will be fine.

But you will know more once head is off.