Valve to piston clearances
 

I have been looking at the different opinions on valve to piston clearances.

1.5mm on the inlet valve seems common but some say that 1.0mm is OK.

Webcam seem to suggest that 1.25 on the inlet and 2.0mm on the exhaust.

Bruce Andersons book suggests 1.5mm and says Porsche suggested 0.8mm but on an early 2.0 litre.

I don't really follow as to why exhaust valve clearance should be greater than the inlet - any thoughts?

I want to build a couple of engines to rev to 8500 rpm and would be interested in what would be sensible values.

Great questions!

I am not sure what testing goes into making the recommendations. When one says 1.25 is suggested did they test 1.0 and have an issue? What was the safety margin, or do they just know that no one has had and issue with 1.25?

I am pretty sure it comes down to data points. As in known cases where X clearance worked or did not work at certain RPM (based on specific cam and spring data). Measuring the flex and stretch in the crank, bearings, rod, pin, piston, valve and spring, heat expansion etc would make empirical guessing difficult.

Should make a good discussion, especially if we can get it backed up with true empirical data!

Exhaust valve gets hotter (the same old CTE consideration) and builds up more carbon? Just a guess..

1.5mm intake and 2.0mm exhaust is what I like to see.

I have had customers run as tight as .65mm on the intake and .8mm on the exhaust. You may get some contact at this number. The valve relief angle has to match the valve perfectly or the valve will bend. Aftermarket springs, Ti retainers and valves and low RPM only until the engine heat soaks and expands.

Why more on the exhaust? At TDC on the exhaust stroke when both valves are closest to the pistons, the intake valve is opening and the exhaust is closing. IF the exhaust valve lofts, or doesn't follow the camshaft profile on closing and hangs open you will have interference.
For the intake valve to touch, it would have to bounce off the seat when it closes. Typical valve seat closing in the 100-110 degree btdc range gives it time to settle down and close.

Thank you - very helpful.

The reason for my question concerns a customer's engine fitted with 'stock' JE pistons (90MM dia) and 'GE60' Cams.

With a 5mm overlap setting the exhaust valve clearance is only 0.5mm which is worrying and I suspect that the cams - which are of unknown origin - may be incorrect but we don't normally use stock JE pistons.

I will ask a separate question.

Exhaust lobe is mild compared to an RSR type cam. strange that it would be so close. You would think the JE piston would be set up for RSR type lift at overlap.

I agree and suspect the cams may have an incorrect lobe separation.

Seen that a couple of times back in the early 90's. one bank 92 degrees, the other 108.

I have a set of race cams done by a very reputable grinder (not camgrinder), and the lobe separatation is different on the right bank than on the left. I discovered it when I decided to measure the cam duration and lift, and found them to be different by 2-3 degrees between the right and left banks.

Scott