The project I’m about to discuss is a bit over the top.  It’s difficult and risky, but the results have been more than gratifying.  There are still some bugs to work out, but I’m making progress.  Some of you will think it’s a totally crazy approach, others may like the idea.  Either way, it’s worth a look.

I have a thirst for quench.  If you Google search “cylinder head quench clearance”, you will find all sorts of experts advocating good, tight quench.  It’s the clearance between the flat portion of the piston top and the flat bottom of the cylinder head (around the combustion chamber).

As the piston nears TDC, fuel/air mixture trapped between the top of the piston and the cylinder head’s quench area squirts out at high velocity.  This increases turbulence, which promotes better air/fuel atomization and reduces the potential for detonation.

This is an illustration of a wedge combustion chamber.  The S40 uses a pentroof.  The concept is the same.  Squeeze & squirt.

Not only does our engine have a dismal 8.5:1 compression ratio, it also has no quench area because the piston never gets close enough to the cylinder head to create any useful turbulence.

Some of you are running a Wiseco high compression piston.  The Wiseco increases the compression by filling some of the combustion chamber with a pop-top dome, but the portions of the piston that overlap the flat surfaces of the head still remain a good .165” away at TDC.  Result, no high velocity squirt.  I’m not knocking the Wiseco.  It is an inexpensive and easy way to amp up your compression.  It just doesn’t create any quench, and I want some quench.

I present to you “The Solution”.

So, the negative deck height of .162” combined with the head gasket thickness of .028” results in the piston being situated exactly .190” from the head at TDC.

Since my favorite quench clearance is .040”, I would need to reduce the cylinder height by .150” to achieve my desired quench clearance.  
Another option would be to increase the piston compression height (distance from the centerline of the wrist pin to the flat top of the piston) by .150”.

Both options have tall hurdles to clear.

Lobbing .150” off the top of the cylinder requires machine tools and will result in the cam chain going loose as a goose.  Also, it will retard cam timing, possibly cause interference problems between valves & piston, and create fit-up issues with the forward chain guide, dowels, etc.  I have the machinery, correcting the cam timing is easy, resolving the fit-up issues is straight forward, but that loose cam chain might be a hard problem to solve.

Installing a piston with an additional .150” compression height will require a special order (minimum purchase, ten pistons).  Also, increasing the compression height affects the thrust action of the piston skirt which, IMO, would require engineering analysis (i.e. Wiseco engineering staff should be giving this modification a good look.)  Piston thrust is affected by placement of the wrist pin, the length of the connecting rod, the stroke, angular velocity, etc.  Way above my pay grade.

I’m not willing to throw down a big wad of cash on ten special pistons (nine of which I don’t need), but I really want to see how this engine will perform with proper quench.  I decided to satisfy my thirst for quench by lobbing off the top of the cylinder.  If I can figure out a way to deal with the loose cam chain, maybe I can prove that the engine runs great with tight quench, and in the process conjure up some additional forum members who might be interested in the special piston.

No guts no glory.

Their calculator for dynamic compression requires the static compression ratio (10.9), exact length of the connecting rod (centerline of rod bearing to centerline of wrist pin is 6.573”), intake valve closing point @ .050” lift (DR650 cam closes intake at 34 degrees ABDC, .050” lift), elevation above sea level (my garage is 650 feet), boost pressure (0), and the usual data on bore, stroke, etc.

The dynamic compression calculator predicts cranking pressure will be 217 psi with static compression at 10.9.  I think that’s gonna be too high.
Maybe I can make do with .060” quench.  Recalculating using .060” quench I get static CR 10.4:1 and cranking pressure predicted to be 205 psi.  I would prefer around 190 psi but I also prefer .040” quench.  It’s a compromise.

So now, instead of lobbing .150” off the cylinder to achieve .040” quench, I will be lobbing off .130” to achieve .060” quench.  That should be way better than .190” pseudo quench.

Looks like I’m shooting for .060” quench, 10.4:1 compression ratio, 205 psi cranking pressure.

What about the cam chain?

Way back when I was testing the Fastman’s (Fast650’s) HammerHead, I tried to figure out exactly what would happen if I lowered the head .125”.  I made an eccentric bushing that would simulate a .125” drop in cam centerline.

The bushing, when inserted into the cam sprocket and supported by a dummy camshaft, simulates the cam situated .125” lower, very close to how things would be if I lobbed .130” off the top of the cylinder.  

And of course, the situation at the cam chain tensioner was dismal at best.  Not only was the tensioner plunger extended to 27mm (very bad), but the drive & slack sides of the chain were dangerously close.  Look at the chain just above the tensioner.  This would have to be corrected.  The Verslagy tensioner mod can only do so much.  I’m fixin to drop this head another .005” from where this photo shows.

Then I installed a jack bolt such that it would push on the back of the tensioner when I screwed the jack bolt in.  Note that the tensioner plunger that was extended 27mm is now extended only 13mm.  The pawl is removed from the tensioner for test purposes.  You can screw the jack bolt in and out and watch the tensioner plunger move.  When the jack bolt is screwed in, the plunger retracts.  When the jack bolt is screwed out, the plunger extends.  It seemed to work good.

Here is a look at the jack bolt assembly installed on my Stage II head.  In this photo the jack bolt is backed out all the way.