Forum » General Engine Building Discussion » setting valve clearances - what does it do to cyl pressure and the onset of knock?

setting valve clearances - what does it do to cyl pressure and the onset of knock?

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So i had my valve clearances set to the low end of factor specs - the car now idles with less ticking on a cold start etc - but its going quite a lot better now... the butt dyno says "wow it made a noticeable difference"...

So if we tighten up the clearances to min advised by factory - wouldn't this hold the cams open a little bit further (increase lift) and cause a drop in cyl pressure compared to a looser setting - would this also affect the onset point of knock in any way - eg make it more prone or less prone to knock?

ok - so a little bit less than minimum due to not having the exact feeler gauge available eg 0.21-0.25mm recommended - set to 0.214mm sorta thing.

Hello it does have a huge effect on all the things you have noted above, the difference on say a Datsun a12 engine can be up to 10 percent power gain by .002 tho difference in the valve to rocker arm clearance. this has been proven many many times on my dyno. i have also seen similar power gains on chev and ford v8 engines

Regards Ross

After an hour or so reading about cams and lift/duration/overlap stuff after work today it seems that power comes from the cyl pressure driving the pistons down (badly explained on my part) so if i have more power - i have increased my VE - so I'll be seeing more cyl pressure - so I'm increasing the chances of knock occurring.

Would this be a correct guesstimate?

Time to hook up the audio knock monitor again....