Excessive initial cam preload??

[cinquecento]

I will speak to V8D to get some advice tomorrow, however could I get some initial feedback on what I consider to be an excessive amount of initial cam preload.

With empty (new) lifters with my Stealth cam, I have an initial cam preload of 2.6mm. Given a target of 1 mm and a 1-1.6 rocker ratio, that means I need a 2.56mm shim(s) (2.6-1 * 1.6).

That sounds HUGE... almost .1 of an inch (100 thou)

I am unaware if the heads have been previously shaved.. however may I ask.

1. Is this amount of preload normal?
2. Are shims the answer or are shortened push rods better?

many thanks..

[chris52]

Just curious here

Too little preload will cause tapping yes and I presume poor running due to the valves not opening enough.

But what problems does exesive preload cause? I would imagine that the valves will open more and longer so at higher revs you would get valves staying constanly open reducing max RPM and power.

I know the preload on mine is about .090" which is too much but the car is running very well 269bhp 340torques at the growl but did drop off very quickly as the revs went above 4/5k

Chris

[chris52]

Quinny said:

Too much pre-load... wears the cam out quicker... All that pressure on the lobes

Better be next on my list of things to do then

Chris

[GreenV8S]

chris52 said:

But what problems does exesive preload cause?

The preload has to be enough to compensate for mechanical expansion as stuff heats up, but the more preload you have beyond that the more scope there is for the lifters to pump up at high revs. Hence you want just enough preload and no more.

[Griffit]

GreenV8S said:

chris52 said:

But what problems does exesive preload cause?

The preload has to be enough to compensate for mechanical expansion as stuff heats up, but the more preload you have beyond that the more scope there is for the lifters to pump up at high revs. Hence you want just enough preload and no more.

So what would expand most and cause excessive tappety noise when hot?

[GreenV8S]

Griffit said:

GreenV8S said:

chris52 said:

But what problems does exesive preload cause?

The preload has to be enough to compensate for mechanical expansion as stuff heats up, but the more preload you have beyond that the more scope there is for the lifters to pump up at high revs. Hence you want just enough preload and no more.

So what would expand most and cause excessive tappety noise when hot?

Not sure what the question means, but aluminium expands roughly twice as much as steel when heated, so the follower will need to expand to take up the slack. It's only a very small change though, and any extra preload will just give scope for the followers to pump up if they're over-revved.