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Author Topic: Formula, please - valve timing/piston position  (Read 20241 times)

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Offline Dynoroom

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Re: Formula, please - valve timing/piston position
« Reply #15 on: December 08, 2011, 07:19:46 PM »
Another thing to consider is "piston rock". Most racing engines use pistons with more piston to wall clearance thus when cold the piston moves or "rocks" oven in the bore requiring more radial valve head clearance. YMMV.
Michael LeFevers
Kugel and LeFevers Pontiac Firebird

Without Data You're Just Another Guy With An Opinion!

Racing is just a series of "Problem Solving" events that allow you to spend money & make noise...

Offline fordboy628

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Re: Formula, please - valve timing/piston position
« Reply #16 on: March 18, 2012, 05:00:06 PM »
There are commercial programs out there that accomplish this, some expensive, some "affordable."  Almost all the useful ones however, require the utilization of an electronic cam profile file, such as "Cam Doctor".  You can accomplish what you need to "the old fashioned way" & accurately with the following:

1)  Degree wheel & pointer, both solidly attached to the mocked up long assembly
2)  .50"/1.00" travel dial indicator and stand solidly attached to cyl. head
3)  VERY LIGHT substitute valve springs for cylinder being checked
4)  A LIGHT TOUCH when rotating engine during overlap period if you suspect valve to piston clash  (As opposed to the "Clash" which never had a light touch,
     musically speaking.)
5)  Opening the valve lash to reduce lift @ overlap can give you somewhere to start calculating flycutting dimensions if the dreaded clash occurs straight off.
6)  Light springs on the valves & the indicator on the valve stem can be used to plot existing valve to piston clearance (from the seated valve to the piston top) from
     about 20/15 degrees BTDC to 15/20 degrees ATDC, WITHOUT ANY CAM LIFT.  DO NOT "ASSUME" BTDC & ATDC WILL BE THE SAME, you might be surprised.
     Then graph this out and you have an idea of what that particular setup will tolerate for NET cam lift.  You probably want to measure every 2 degrees or so.
7)  When building up an "orphan setup" the "engine builder" (as opposed to the "engine assembler") NEEDS TO VERIFY THIS INFO FOR HIMSELF, regardless of what
     any others may advise.  Unless the "builder/assembler has a "bottomless wallet".
8]  I like to see .080" inlet to piston clr. & .100" exhaust for pushrod engines running 9k redlines.  IF VALVE TRAIN MOTION IS VERY WELL CONTROLLED, (or " IF
     you're feelin' lucky, punk".) clearance can be as tight as .050" inlet/.070" exhaust.
9)  I reserve the right to change my opinion, at any time, without notice.

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Offline DND

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Re: Formula, please - valve timing/piston position
« Reply #17 on: October 16, 2012, 08:07:04 AM »

To know your piston to valve, put in a intake & exhaust valve with no springs and strech a rubber band around the valve stems to keep them in place

Install head and used gasket with a few bolts & zero indicator on the intake stem, crank a TDC put rubber band over guide and slide valve down to piston pocket and write down distance [ .400 ] lets say.

Rotate crank to 5 - 10 - 15 - 20 degrees , then the same on the exhaust valve.

Take off head install running valve springs and valve train, install head and adjust each valve with proper lash.

Put indicator on intake retainer and go to your crank settings at TDC etc on split overlap and write down the lift number under the free valve number of .400 [ say you have .300 lift then you have .100 piston to valve. this way you know what you have at each 5 degrees of crank rotation with no guess work & no CLAY.

If you are a little short on the exhaust side advance the cam a tick to pick up more clearence.

This is the proper way to know what you have, Don