[gtofreek]

Originally Posted by pastry_chef

Yes, exactly.

The intake valve close event is by far the largest factor. Second would be the exhaust valve open event.

I would disagree with that statement. I've made many engines quit detonating by advancing their cams.



Instead, I would restate that comment as the biggest factor in detonation would be the valve opening difference between the intake valve and the exhaust valve at TDC during overlap. Retarding the cam causes the exhaust valve to be open more, and the intake open less at TDC so when the piston starts down the hole, for the first few degrees it is sucking harder on the exhaust port and trying to suck hot exhaust gasses back in which I feel are the main ingredient for detonating. I like to see the intake valve open between .035 and .040" more than the exhaust at TDC during overlap on 110° LSA cams. Wider lobe seps will bring these numbers closer so with 112 LSA the difference will be more like .025"-.035". If your exhaust valve is open close to the same amount as the intake at TDC during overlap, then your engine will almost certainly detonate without low compression. Looking at it this way, I have built quite a few 10:1+ iron headed engines that ran fine on 91.

[pastry_chef]

Agree if an engine has issue with residual exhaust, advancing the cam can help reversion. Advancing also opens the exhaust valve earlier.

Originally Posted by gtofreek

I like to see the intake valve open between .035 and .040" more than the exhaust at TDC during overlap on 110° LSA cams. Wider lobe seps will bring these numbers closer so with 112 LSA the difference will be more like .025"-.035". If your exhaust valve is open close to the same amount as the intake at TDC during overlap, then your engine will almost certainly detonate without low compression

Some real numbers:

Magnum
intake lobe 5241 / exhaust lobe 5204
236 / 244

112 LSA - 108 IC / 116 EC
1.65 rocker.

lift at TDC overlap.
intake .137
exhaust .114

--------------------------

intake lobe 5241 / exhaust lobe 5204
108 LSA - 104 IC / 112 EC
1.65 rocker

lift at TDC overlap.
intake .161
exhaust .129


With 112 LSA and 111 intake centerline the lifts at TDC are close to the same. Straight-up at 112 IC the exhaust has more lift.


UDHarold - Dec 19, 2005

Use my 'Quick-n-Dirty' method to check if the cam is in correct.
The only disassembly you have to do is take off a valve cover.
I use a Philips screwdriver in the #1 sparkplug hole to find TDC, turning in normal rotation until I feel it touch the piston, marking the degree wheel, then turning in opposite rotation until it hits again. I then find the average of the 2 readings, and that's TDC. Do it several times if you're unsure.
I then turn the #1 piston to TDC, and, using a 6" Machinist's Steel Rule, I check from the top of the INTAKE retainer to the spring seat, writing down the measurement. I then check from the top of the EXHAUST retainer to the spring seat, writing down that measurement.
The intake valve should be at least .025" to .050" closer to the head than the exhaust valve, if the cam is in the engine advanced. If the exhaust is closer, particularly a lot, then the cam is retarded