Cam Timing

[HotRodderBob]

Help!

I have a problem with my engine after doing the cam timing! I had to use a DTI to get the timing right as the timing marks have gone.

Ive started it up, which it did quite easily but theres a noisy top end. Its not valve clearances (checked) or oil starvation, so I'm wondering whether my valves are touching pistons.

Its an interference engine with piston cut outs.

The timings are 107.5 exhaust, and 115 inlet.

How far out would cam timing have to be on a DOHC engine for this to happen? Is the engine knackered with any kind of piston valve contact?

[227bhp]

Set them up correctly and see what happens.

[DVandrews]

The inlet will be safe at 115 degrees, that is a little retarded which will reduce the lift at TDC, The exhaust is also retarded which will increase lift at TDC. The rate at which production cams lift per degree is quite small, usually 3-4 thou per degree so I'd say that if your peak lift timings are set as you say you should be safe as the running clearance will be substantial.

Dave

[Mignon]

Checking my database of cam profiles the lift rate of OHC flat tappet cams averages about 5 thou per crank degree once the cam is into its max velocity phase, depending obviously on lifter diameter. A stock engine is unlikely to have less than 3mm spare valve to piston clearance so about 24 crank degrees of misadjustment to take that up however since we have no idea what engine this even is, or if it's stock or modified, it's impossible to say much.

[GreenV8S]

HotRodderBob said:

Its not valve clearances (checked)

If there's something wrong in the way you set the clearances then you may be reconfirming your mistake rather than proving they're correct. I don't know the engine you're working on or what procedure you followed to set the clearances so I can only speculate about possible mistakes. For example you might be assuming you have solid lifters but actually have hydraulic ones or vice versa. You might have worn followers which give a false reading when checked with feeler gauges.

ETA: Noticed you stated OHC. I assume it would be obvious if you had hydraulic followers.

Edited by GreenV8S on Saturday 25th February 14:19