Inliners International Forums General Discussion Hi Performance Camshaft Performance and Valve Lash

I was wondering if I could get more performance out of my GMC 302 Isky E2 Cam, for drag racing only, by reducing the valve lash. The E2 cam has a cam lift of 0.298, valve lift of 0.448 and duration is 264 degrees. Duration at 0.050 is 228 degrees and the hot valve lash is supposed to be set at 0.018. The engine runs great and idles great at 0.018 valve lash, but idle get rough if the valve lash is reduced a few thousanths. But, I am thinking if the lash is reduced the valves will open sooner and the lift will be increased by the amount of lash decrease and mid and high end performance will be improved. I would only do this for drag racing and readjust the lash back for normal driving. What do you think?

Changing the lash also increases/decreases duration and that is where the real gain will come from.

^ x2

Hi radar . . .

The only caveat would be the cam profile. Cam profiles include clearance ramps:


The grinder designs these ramps with a specific lash in mind. The first part of the ramp takes up that lash - while the back half of the ramp slowly begins to overcome the inertia of the rocker and the valve and then begins to accelerate both. Lashing to generously uses up more clearance ramp space:
moving the valve event closer to the flank. This can be very hard on components.

Lashing to tightly will extend the total duration slightly by getting the valve off its seat earlier along the length of the clearance ramp (and leaving it off the seat longer on the opposite ramp). But the overall lift is unaffected by lash. Moreover, the added duration will be at very low lifts (as most of a cams lift occurs on the flank).

There are potential downsides to overly tight lashing. Peak heat expansion could take up all the lash - leaving the valve off the seat all the time. This will make the compression stroke inefficient because the intake valve can fail to seal entirely. The same condition on the exhaust valve could cause them to burn because 'seat time' is where heat gets dissipated to the water jacket.

Have you degree wheeled the tighter lash spec you have in mind? How much additional duration are you getting?

regards,
stock49

Performance Trends website discusses lashing . . .

I have not degree wheeled the tighter lash spec. Not sure how I would do that without pulling the front end off my 40 Chevy and disconnecting the AC.

The Isky cam spec. shows the 264 deg. adv. dur. is at 0.020 lifter rise, and 228 dur. at 0.050 lifter rise. These figures might be helpful in figuring out what a few thousandths less valve lash might do to the duration.

Or, I could just experiment by running with say 0.015 valve lash.

Just make sure you make numerous back to back track passes to establish a baseline before you make any changes. Unless you can get your passes down to a consistency of a few hundreths of a second between them, you may mistake a gain from driver error as a gain in performance when it really isn't.