View Full Version : Timing my engine

25-10-2006, 07:07 PM
Just replacing the head gasket on my engine, which, as it turns out, didn't have the head bolts done up to the right torque, and so ruined my trip to nurburg! I think sabotage took place on the ferry! At least no major damage has been done, although there is the potential for major damage in the next few days!

Anyway, just timing the engine, and the engine builder says that at TDC on cylinder 1, the inlet valve should be open 3.6mm, and the exhaust valve open 3mm. At first I thought this sounded wrong, but I am guessing that the exhaust valve is actually closing (shouldn't it be closed at TDC?) from the exhaust stroke, and the inlet valve is starting to open for the inlet stroke? Does this sound right?

Also, to get the valves at the right place, I have turned the head upside down, setup a dial gauge on the valves, then measured them till there open to the right amount, then locked the cam's against each other. I can't find any other way of doing it, as you can't get a dial gauage to sit on the top of the lifters, which was my original plan!

So, does this sound right? Many thanks to anyone who can help.

Uncle Benz
25-10-2006, 09:16 PM
What you are describing is overlap. This is where the exhaust valve remains slightly ajar whilst the inlet valve starts to open. The exhaust gas rushing out creates a slight vacuum effect which drags in more inlet charge. More racy cams have a larger degree of overlap to make the most of this effect. At idle, this overlap does not create a vacuum, since the exhaust gas speed is too low and this results in pulsing in the inlet manifold, which gives you the classic 'cammy idle' that sports camshafts create. Are you using hydraulic lifters? If you are, these measurements may not be so easy to set up as you don't know if the lifter is pumped up to take out all the clearance. I tend to set cams up using the crank position at maximum valve lift, but if you really can't get a dial gauge on the lifter what you descibe may be your only option. Proper nerd stuff, this!!

25-10-2006, 09:42 PM
Cheers UB! Just been researching it and discovered overlap, although there looks to be about .4mm of clearance between the valves so I guess I need to get it spot on. It is on solid lifters, so the measurements I take should be exact(ish!)

I was hoping they give me a full lift at x degrees, but unfotunately not. If I can find a gauge with a long thin extension, I may be able to do it direct on the lifters, I'll have to ask all my machinist mates (all 2 of them), tomorow and see if I can borrow one.

Thanks for the help!