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Old 08-24-2007, 04:58 PM   #11
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Default RE: what cam do you recommend?


What Is Piston-to-Valve Clearance And How Does It Affect Me?

Piston-To-Valve clearance is just what it says ... the minimum distance between the engine exhaust and intake valves and the top of the piston at the closest given position of the reciprocating cycle. Piston to valve contact is probably one of the most common causes of damage to a racing engine. Piston to valve interference can be caused by many items, including: Cam Lift, Cam Timing, Piston Design, Timing Chain/Drive Failures, Deck Height, Cylinder Head Modifications, Valve Diameter, Broken Valvetrain Parts (valve locks, springs, retainers, lifters, pushrods and cam lobe failures), Valve Diameter, Connecting Rod Stretch, etc. In this article I will simply be addressing cam timing, cam lift, and basically anything that has to do specifically with the valvetrain.
Anytime you make a change to your engine in relation to the cam lift, cam timing, pistons, cylinder heads, rockers arms, or anything else that can change, even so slightly, the position of the valve in relation to the piston, you MUST verify that you have the minimum clearances safely met. This procedure consists of setting up your engine and measuring the clearance at the valves closest point to the piston and validating a "safety margin" clearance. This is typically .080" on Intake valves and .100" on Exhaust valves when using steel connecting rods. With aluminum connecting rods this dimension changes to .100" on Intake and .120" on Exhaust. The exhaust always needs more clearance because of the added COE (Coefficient of Expansion) of the hotter exhaust valve. These are standard numbers, in some cases you have to look at your entire combo and make your own decision as to the minimum clearance values.

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I agree that ptv issues are not always a result and or deciding factor in ptv problems....and I know duration plays a major factor in ptv issues......but come on....were dealing with a bone stock block and stock pistons ...you won't get 600 lift with 1.72 rockers to not have ptv issues even if your duration was on the light side.....I seriously doubt the guy is going to degree in the cam and or advance it or retard it to achieve maximum valve timing and efficiency.....most people install their cams straight up to be safe so this is why I reccommended what I did.......I agree with you Joel 5.0 that irregardless of cam choice ptv should always be checked....
Cam lift during the overlap cycle (when both valves are open at the same time and the piston is @TDC)..... YES.......total valve lift ....NOPE....and yes, you can have a .600" total lift cam with 1.72 rockers and not have PtV issues on a stock block/pistons......not that you need it, but I'm simply trying to explain this.

How can total valve lift be a factor, if it happens when the piston is down the cylinder >1.5" ? .......

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