Is it worth the effort

[denicholls2]

That depends on your definition of "wrong".

Clearance and timing settings are always a compromise intended to cover the dynamics of the operating environment. But like most people do not have the ability to set their valve clearances hot, so their cam timing.

The important part of any specification is that it includes the conditions under which it it measured. In most cases, this will be a cold engine. Because someone else took time making the specification work optimally, you can be confident that it is also optimal at running temperatures without worrying too much about the physics (which often has multiple variables working in different directions) of it all. Sometimes thinking too much about things can get you in a bad place.

"Is top dead center different on the compression stroke than the exhaust?"

[AussieJohn]

Thanks for that, I think I may be looking too deeply, John

[rgh0]

Rohan, does the fact that the valve clearances change when running mean that if you try to set the engine up with offset dowels to the spec as per the manual then this will be wrong because the exhaust cam duration will be less than stated in the manual because of the larger clearance? I am now getting confused! cheers, John

If you set up the valve timing to be correct at the maximum opening point using a degree wheel and offset dowells the fact that the clearance change when hot does not affect this timing. The duration of valve opening is affected with the duration lengthening by a few degrees as the clearances close at the start and the end of the opening period but the set MOP does not change due to the valve clearance.

However the MOP does change with temperature a fraction due to the growth of the alloy head plus cast iron block being greater than the growth of the steel chain so this advances the timing a fraction for a hot engine. However to counter this the dynamic loads will cause the chain to stretch a little versus the static timing causing the timing to retard a little. Both affects are small and opposite each other so if you set the timing to the required MOP with a static, cold motor you will be near enough with a running hot engine.

cheers
Rohan

[AussieJohn]

Thanks for the info Rohan, explained well; cheers John