7 replies to this topic

[babsbrown]

Hi guys

apologies for a much over discussed topic, but just before My head gets machined, are my calculations correct?

Using my DTI on the exhaust valves, my lift with an evo001 cam on my 1130 block is coming to 6.9 mm at the valve cap. Adding the valve lash of 0.14" (0.35mm). All together being 7.25mm

My depth of the 1275 head surface to the exhaust valve on my 12g940 head is 6.9mm

So in theory the gap from valve to block is -0.35mm or 13.78 thou too little.


So by sinking the exhaust valves in the head by 30-40 thou ish would give me the necessary clearance?

With in theory 20 thou ish safety margin (not including head gasket)

One thing that's surprised me is the evo001 exhaust valve lift, I thought it would be more.

Thanks in advance, Luke

[Shooter63]

It sounds like you have geometry problems to be that low on lift at the valve, by sinking the valves in you will make it worse, the best way to remove the problem is to shim the rocker posts up, but then you have the problem of the tappet adjuster screws being further down which reduces the rocker ratio ( valve lift ), depending on how much you are bothered, depends on how far you go to reduce the problem, as I'm a bit of a nerd about these things, so I've got big bore pushrods in a small bore unit, rocker posts heavily shimmed.

Shooter

Edited by Shooter63, 26 May 2025 - 08:27 PM.

[nicklouse]

Have you worked out your CR? In addition to the above.

[Shooter63]

Have you worked out your CR? In addition to the above.

With his engine set up, a Std 940 head gives around 9-1, with that cam he needs more to get the best out of it, or any camshaft really.

Shooter

[Cooperman]

Your valve lift seems low and not just a geometry issue. Maybe check it again.
Don’t forget to add the gasket thickness when calculating the valve to block deck clearance (or lack of clearance).
With the better cam, a CR of around 9.75:1 would be good, but skimming the head needs to be put into your clearance calculations.
Best advice would be to pocket the block, which also improves the gas flow a little (but not a lot).
Remember, recessing the inlet valve increases the chamber volume and reduces the CR which means more to be skimmed to get the CR correct.
Do all the calculations very carefully.

[babsbrown]

Sorry guys,

I'm late back to the party. Been so busy with half term.
Thanks for the advice. I've rechecked my rocker geometry and added some extra shims under the rocker posts. I'm more 8.3mm now.

To be honest I've given up on the idea of the 940 head. I completely forgot about a 295 head I had. After an 80 thou skim I have the chambers down to 22.4cc

With a 2.8cc head gasket. 13.5tho deck clearance. 7cc +40 pistons. My compression should theoretically be 9.49:1
Better than my 202 head at 9.2 with 23.6cc chambers.

I just need to chamfer the rough edgings of the chambers and water holes after the skim.

I've heard that reduces the risk of cracking?

[Cooperman]

Sounds good.

[babsbrown]

Sounds good.

Thanks cooperman,


Having only done 500 miles In the engine, it also gave me an opportunity to see the bores and check for borewash. The cross hatching looks OK so fingers crossed all good in that regard.

Sounds good.

Thanks cooperman,


Having only done 500 miles In the engine, it also gave me an opportunity to see the bores and check for borewash. The cross hatching looks OK so fingers crossed all good in that regard.

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Edited by babsbrown, 02 June 2025 - 06:24 PM.