valve to piston clearance in hi-performance engine

[Rustang]

Has anyone had experience with piston to valve clearance on a hi-performance gas engine? (ford 302 v8). I know the rule of thumb is .080 intake and .100 exhaust. I've only got .028 on the intake, which I know is not enough. However I want to take the minimum amount of material off the piston so I don't lose too much compression. I figure if I mill the valve reliefs another .040 that will equate to about .3 loss of compression ratio.

I've heard of guys running as little as .035 clearance in a high rpm(7000-8000) ohv v8.

 

[magnograil]

As a reference point, a BSA 750cc racing triple with aluminum rods 5.75" c-c, 285 gm piston, 70mm stroke, 10000 RPM uses 0.050" of clearance. The squish band was made 0.032". The rods stretched 0.030" at maximum RPM.

 

[Tmoose]

Rod stretch is only part of what gets pistons and valves closer together.

As a young punk my cousin bent most of his Mom's brand new stock automatic Mustang's valves with a Fast-n-Furious "downshift to go faster" thereby inducing a severe over-rev.

 

[icrman]

Otherwise known as valve float. The reason for stiffer valve springs.

Magnograil, how did they or you measure rod stretch at the given rpm?

 

[magnograil]

They ran the squish band closer and closer until they could hear the piston/head contact and maximum RPM. The squish band was critical to getting more power.
If you have problems with valve float then you are way off on your cam/valve train design. Go with high performance decals instead.

 

[TomOstrowski]

Valve float doesn't necessarilly result in piston to valve contact. Actually, it rarely does. Valve float is simply lift(and everything it encompasses- ramp design, etc.) outrunning, if you will, spring tension and most production engines will eventually experience it at one time or another when the occassional over-rev occurs, whether mistakenly or purposefully, without any real damage to the valves and/or slugs. An improper valvetrain design can provide plenty of float without the valves ever coming anywhere near the piston crowns, as long as ample clearance was originally established, just as valve-to-piston contact can occur without any instance of valve float ever occuring if clearance(too little) was improperly engineered.

Materials thermal expansion, compression ratio, spring rates, valvetrain stability(use shaft mounts? girdles?), camshaft design, rod stretch, etc. will all play a role when considering piston-to-valve clearance in any particular engine. There's no real rule of thumb when it comes to maximum performance.

I believe it was Smokey Yunick who stated at one time or another that he builds his top ends to where the valves were just "kissing" the pistons and then backs them off 5 thousandths or so and called it good.

Whatever works, I guess.

 

[ShaunT]

Excessive piston to bore clearance will hurt the V.P clearance when the piston rocks out of the bore at TDC from the lack of support.

Another area of valve train lingo you need to check if you havn't already. That is the Intake and Exhaust Centerlines. I.E. where the cam is degreed to in the motor. For instance a cam with a 110 Lobe Seperation Angle that is 4 degrees advanced will have an Intake C/L of 106 ATDC and an Exhaust C/L of 114. Narrowing the Lobe Seperation Angle and Advancing the cam will gain Exhaust V.P. clearance, and reduce Intake V.P.clearance. If you have somehow radicly advanced the cam or your cam really failed quality control and got the Lobe C/L's messed up when it was ground, you will find it if you start checking it with dial indicaters off of the lobes and in concert with a degree wheel on the crankshaft in the same fashion as if you were trying to degree the cam into the motor.


Shaun TiedeULTRADYNE/LUNATI Austin,TX(stiede@ev1.net)