Valve adjustment specs

[86_6series]

The valve gaps for the US M30 B34 is 12 intake and 14 exhaust.

I've read that with a hot cam the gap can be like 20 & 22. The cam is 294 Hartge.

Could it be a valves clearance issue.

In my case I have the Hartge B35 head, B34 block with custom pistons so valve clearance is not an issue.

Can anyone educate me and explain the reason for the larger gaps and if that's the way to go.

Thanks
Bob V

[Brucey]

there are several factors which influence the recommended valve clearances

1) the extent to which the valves heat up and reduce the running clearance
2) the way in which the valves/valve seats are liable to wear before the next service interval
3) the shape of the 'lead-ins' on the cam profile (these are the gentle parts at the start and the finish of the cam profile)
4) the way in which the cam profile has been designed w.r.t. opening/closing events


the first two relate to the chances of the valves running with zero clearance in service; if the clearance goes to zero (e.g. very hot valves in a relatively cold head, or just wear) the valves/valve seats can start to burn very quickly.

3) means that different cam designers will design different running clearances and build lead-ins into the cam to suit. The lead-in is a relatively gentle ramp which means the cam follower doesn't see a big hit as the clearance goes to zero just before the valve opens. It also controls the valve 'landing' as it reseats itself. The danger here is that if you run with the valve clearance too great (which can also happen with wear, depending on what wears fastest), the lead-in may not extend far enough and there will be impact loadings passed through the rockers and valves which shouldn't normally exist.

4) most engines are sensitive to the exact timing of the valve opening and closing events; this impacts on the way in which pulses in the inlet and exhaust tract interact and help fill/empty the cylinder. If you stick with the recommended valve clearances and the cam sprocket is correctly timed (which is often not the case eg because the camchain is worn or the head has been skimmed) then the opening/closing events will be at the intended positions. If you have some freedom with the valve clearances, you can effectively make small adjustments to the cam timing which may affect engine performance slightly.

The one thing you wouldn't do is to alter the valve (lash) clearances in order to influence the valve to piston clearance.

BTW there's a whole seperate conversation to be had about whether it is worth fitting that cam into an otherwise stock US-spec M30B34 motor.

cheers

[86_6series]

thanks Brucey--appreciate the info.