[Fot] piston/head clearance

[Greg Solow]

In all of the research and reading I have ever done the idea has always been to have the clearance between the head and the top of the piston at tdc as small as possible with no actual contact.  This normally results in the top of the piston under the "quench" or squish area being clean, with no carbon deposits after running. In an engine with a race prepared stock crank and modified stock TR-4 rods this distance is .026".  At this clearance using a rev limit of 6800 RPM there will be no contact, at .025" the pistons will touch the head.  This is also with Hepolite "Powermax 87mm" liners and pistons.  Forged pistons probably expand more  with heat than these pistons, billet rods & a billet crank probably stretch or flex less.  These days I usually shoot for about .028" clearance when setting up the engine.  
        The most powerful engine that we have built used HD LA Sleeve 87mm liners that are thicker then the Hepolite liners, Forged pistons, std. size valves, & a billet crank. It used Weber 45 DCOE carbs with 42mm chokes & carefully tuned 4 into 1 headers  with a tuned length exhaust pipe with a megaphone on the end. 
This engine made peak power at 7,000 - 7,200 RPM.  It had a 13.4:1 CR with a 306 degree duration cam giving and actual .525" lift at the valve. The piston to head clearance was set up to be .028" under the squish area.  This engine made 189 HP at the flywheel with a very broad torque curve that peaked at 5,000 rpm, if I remember correctly.  It was built to run on 110 Octane ERC Racing gasoline.  The total advance at 6,000 RPM was 34 degrees. 
       The compression ratio is clearly related to the camshaft duration and both are related to the Octane rating of the gasoline the engine is going to be run on.  
I would certainly not try to run this engine on 102 octane fuel. You would have to retard the timing way back and the engine would just not run well. To run on 102 Octane fuel, I would think that the compression ratio would have to be lowered to somewhere around 11:1 to avoid detonation & the engine would loose a lot of torque which could be only regained by shortening the cam duration.  
      Maybe the results the Christian Max got have to do with the lower octane fuel that he has to use. We do not have that issue here in the US.

                                                                                                                     Greg Solow


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/fot/attachments/20170904/90dc1bd6/attachment.htm>