Pop-top vs tight quench - Wiseco Piston Installation

There are two options for increasing compression on our LS650 engines.  One is to install a Wiseco pop-top piston.  The other is to reduce the height of the cylinder (tight quench setup).  Each has distinct advantages and disadvantages.

I had been running my engine with a tight quench setup.  It ran great but I got a little careless and melted the piston.  This was a good opportunity to compare the two approaches.

I installed a 94mm Wiseco.  It was easy.

I know I will probably get an earful, but please don’t use one of these if you are re-sizing the cylinder.  Spring hones are fine for de-glazing, but any re-sizing should be done with a rigid hone.    The spring hone follows the existing bore, so any existing taper or out-of-round will just be made worse.  Use a rigid hone and torque plates to re-size.

I monitor the cylinder size using an inside micrometer and a dial bore gage.  That way I can make sure I’m keeping it round and straight.  Then I verify clearance with feeler strips.

When you reassemble, it’s tough to support the piston while you drop the cylinder down over the rings.  It’s a juggling act.  The ring compressor can only do its job if the top of the piston remains exactly perpendicular to the cylinder.  That’s tricky with the piston wobbling all around on the wrist pin.  This simple piston support works great.  Just three chunks of mahogany (1”x 2”) held together with wood screws.

BTW, don’t get confused by those pics of the cylinder installation.  I forgot to take pics when I was installing the Wiseco piston, so I took some photos using spare parts.  I’m sure you get the idea.

Once the cylinder is in place, you can run the piston up to TDC and get a look at the deck height.  You can see that the Wiseco is situated well below the deck (.143” to be exact).  No effective quench on this motor.  With the Wiseco, you have to rely on port tumble/turbulence to get the mixture stirred up, not gonna be any squish working on it.

With the Wiseco piston, I installed the 340b cam on the factory timing marks.  The initial compression check showed 185 psi.  That improved to 195 psi after about about 500 miles of very easy break-in.  195 psi is right where I want to be.  I should be able to run 92 octane without ever having to worry about the dreaded spark knock.

Since installing the Wiseco, I have been able to do a lot of testing.  I have loads of comparative data.

One thing I noticed immediately is vibration.  The LS vibrates a lot more with the Wiseco piston.  The 94mm Wiseco is 52 grams lighter than the stock assembly.  The mirrors are blurred continuously, and long freeway hauls really put my hands and butt to sleep.   It’s manageable, but I think the 97mm Wiseco would be a better choice.  It’s only 11 grams lighter than stock and will most likely run much smoother.

In terms of performance, the tight quench setup kills the Wiseco.  The tight quench has a lot more compression and you really feel it.  It has torque that starts low and never stops.  Second gear acceleration from 4K to 7K is much faster (2.60 tight quench vs 2.90 Wiseco).  That’s a comparison using exactly the same cylinder head, camshaft, carburetor, airbox, exhaust system, etc., the only difference being the method of compression enhancement.   The additional compression is clearly an advantage.  I also believe that the tight quench setup yields much better combustion.  If you imagine that 0.3 second acceleration advantage over five gears, you start to see what a big difference it is.

From a practical perspective, you can’t beat the Wiseco.  You may not achieve the same all out performance, but the simplicity of installation makes it a better choice.  It’s still plenty fast, and you don’t need a machine shop to install it.  There are no cam chain tensioner hurdles, and valve-to-piston clearance isn’t a problem.  Installation is easy.  It really offers a lot of bang for the bucks.

Thanks so much for the detailed write up and sharing!

Kamel
With all due respect, there is no way a simple carburated air cooled motor can run 18-1 compression.
My guess is that 10:1 is about as high as you'd want to go, unless you want a hand grenade.