I recently tore down my engine for a 20,000 mile inspection.  I had done extensive modifications to the lube oil system, and I wanted to do an inspection to see how things were holding up.  The engine had a 97mm pop-top with the compression ratio bumped up a little (9.5:1 to 10.8:1).  It was a test bed for my oil system mods.  It made excellent power and lots of torque; the perfect motor for testing the drive train.

Once the cylinder was removed, I noticed that my second piston ring (the oil scraper ring) was a little tight in the groove.  Not stuck, just snug.  I attributed that to carbon accumulation.  The rings looked fine.  I continued with the tear down.  With the cylinder out of the way, I had good visual access to the input gears.

All the stuff related to the lube oil system modifications looked great.  Gear teeth were in good condition, oil spray tube was intact, no evidence of main bearing failure, cam and rockers looked fine (same small pits I have been monitoring for several years), high-speed pump drive looked fine.

The oil system mods seemed to have solved my bearing and gear problems, so I was ready to add a bit more steam.  I got busy with other projects and put the Savage on the back burner for a while.  I was happy with the inspection, and was looking forward to building a new motor with a 97mm flat-top piston.

Several weeks later, I was reviewing photographs of the teardown and inspection.  I noticed an unusual dull grey condition on the rear of the piston.  It was on the circumference at the very top, just above the top piston ring.  It appeared as if the surface had been sand blasted with a course abrasive media.  The grooves around the circumference were badly eroded.  Those grooves are referred to as “Anti-Detonation Grooves” or “Detonation Suppression Bands”.  They are supposed to protect the ring lands from damage resulting from extreme pressure (i.e. detonation).

I inspected the cylinder head and there was a trench blasted into the head.  This trench looked like a cluster of tiny pits right in line with the crevice formed by the piston outside diameter and the cylinder bore.

This particular engine was an attempt to see if the 97mm Wiseco pop-top could perform as well as the 97mm Wiseco flat-top.  I wanted to see if the pop-top could match the performance and economy of the flat-top.  When installed in a standard height cylinder, the pop-top only increases compression to 9.5:1, but the flat-top can take compression all the way up to 11.4:1 (depending on quench clearance).  To level the playing field, I installed the pop-top in a shortened cylinder.  It was set up at .117” quench clearance which provided 10.8:1 compression ratio.
 
With quench clearance that large, I was concerned about detonation from the start.  My understanding of the detonation problem was that it was associated with large voids far away from the spark plug.  Fuel and air hide in these voids, and as pressure and temperature build and migrate toward the void, the hidden charge auto-ignites and explodes.   Anything to the inside of the quench zone can aggravate the condition by interfering with flame propagation.  For instance, the pop-top gets in the way; it can prevent even propagation of the flame and also trap fuel and air.  Given the very large quench clearance, and the pop-top configuration, I assumed it would be prone to detonation.  But the absence of any telltale indicators led me to believe that detonation was not occurring on this engine.  As I mentioned, there was no unusual noise, knocking, pinging, etc., no visual indications on the spark plug.

I decided to look at all my old photos and parts to see if I had missed anything on my other engine projects.  I had built six different engines with Wiseco pistons.  Did any of those engines exhibit similar sandblasting on the circumference of the piston?

I guess you could say that there wasn’t any sort of clean area on the top of the 94mm PT.

The top of the 94mm flat-top piston was clean as a whistle on the intake side.  Is that possibly a precursor to detonation problems?

The 97mm flat-top has evidence of sandblasting.  The detonation suppression bands are pitted across the intake side, but not as severely as my latest 97mm pop-top.  This piston was set up with a 11.2:1 compression ratio.  It was run with my Stage 3 head, a DR650 cam, a PWK40 carburetor, a 1.79” Mac header with LCGP HiFlow muffler.  The piston has 6390 miles on it.

The first 97mm pop-top with boosted compression suffered a seizure.  I attribute the seizure to inadequate running clearance.  Wiseco specifies .0019” clearance, but that’s simply not enough for this air-cooled dinosaur, especially when you jack up the compression ratio even more.   This piston was set up with a 10.8:1 compression ratio (short cylinder).  It was run with my Stage 3 head, a DR650 cam, a PWK40 carburetor, a K&N RD-0710 air filter, and a 1.79” Mac header with LCGP HiFlow muffler.  There is no evidence of sand blasting on the detonation suppression bands.  The piston has 2532 miles on it.

After the seizure, I restored the engine with another 97mm pop-top and continued testing.  It had the same configuration, I just added a bit more running clearance.  That build went through a main bearing and it was restored again, but I continued to run the same top end, so ultimately the current piston has about 27,000 miles in total.

The common denominator here seems to be compression ratio.  Looks like running the CR past 10.5:1 results in detonation trouble.  I’m gonna stick my neck out and say that I think you can get away with a little higher CR on the flat-top, but the safe bet would be to keep it below 10.5.