To be safe, I didn’t even use the motor.  I removed the belt and just turned the mill/drill by hand.  Only took a couple of minutes to do each relief.

Now, both the intake and exhaust valves had more than enough room.  There was ample clearance, both radial & vertical.  Lancer is working with Wiseco to resolve any interference issues.  The valve-to-piston clearance is critical on the flat-top installations.  It’s affected by the cam, quench clearance, valve size, and cam timing.  YOU MUST CHECK THIS CLEARANCE, EVERY FLAT-TOP BUILD.

The rest of the 97mm flat-top assembly was straight forward.  I used a .042” copper head gasket and applied a thin, uniform coat of Permatex Optimum Grey 27036.  Since I had “zero” deck, the quench is about .042” to .045”.  The DR650 cam was set exactly to the factory timing marks.  The valves were adjusted to .005”.  I filled it with Rotella T4 for break-in.  It looked pretty good too.

Break-in went well.  No leaks.  It ran perfect.  I hardly had to spend any time dialing in the carb.  The engine starts easy and idles smooth.  It didn’t use a drop of oil during break-in.  It’s quiet but the vibration levels are robust.  The interesting thing about the vibes is they are heavy at low rpm but tend to smooth out as the rpm rises.  At freeway speed, it doesn’t shake any more than the stocker.  The mirrors are clear.  It has tremendous power and torque.

After 500 miles I changed the oil.  It was time for the usual dose of Mobile 1 V-Twin 20W-50.  A quick clearance check showed the valves were all within .005” - .006”.   Things were lookin good.

Note: Consult the old posts for the performance evaluation of the 97mm pop-top engine.  I had some problems with an oil leak.  It took me a while to figure that out, but it was worth the effort.  The 97mm pop-top was a great motor, lots of power and torque.  The stock clutch couldn’t hold it.  I will provide comparative data in the acceleration results of this post.  Keep in mind that the pop-top motor was running a different cam & carb.

Switch to Stock Header

I wanted to see how much the stock header would affect performance on the big flat-top engine.  I installed the stock header and dialed in the jetting.  It took a lot less main jet to achieve 12:1 A/F ratio.

Second Gear 4K to 7K: 2.37 seconds            0.08 seconds slower than Mac header

Third Gear 4K to 6.5K: 4.14 seconds            0.35 seconds slower than Mac header

Third Gear 4K to 7K: 5.78 seconds            0.59 seconds slower than Mac header

Fifth Gear 3.5K to 5K: 3.80 seconds            0.10 seconds slower than Mac header

Fifth Gear 3.5K to 5.5K: 6.56  seconds      0.61 seconds slower than Mac header

Wow!  That really sucked the wind out of the sails.  This engine puts the big Mac header to good use.  It’s still plenty fast with the stock header, and it feels great.  It’s very responsive.  But you give up a lot.  I guess the 1.79” port likes to be matched to the 1.79” head pipe.

Fuel Economy

Unfortunately, I did not check fuel economy on the 97mm Pop-Top.  I have no idea how much fuel it used.   All I can say is that it ran fine on 92 octane.

Fuel economy for the 97mm flat-top averaged about 67 mpg.  I checked it eleven times.  Worst  was 62 mpg; best was 74 mpg.  The flat-top was efficient, just like the 94mm piston.   I also checked it three times with the 36mm carb and stock header.  It was about the same, 67 mpg.


Operating Temperatures

Cylinder head temperature (CHT) was always higher on the engine with the 97mm pop-top.  CHT ran around 310°and would routinely approach 330°-340° CHT.   The 97mm flat-top piston always ran around 280°-300° CHT.   We’re talkin cruise, not stuck in traffic.
 
Oil temperature followed the trend.  The pop-top would motor along with oil temp around 200°-220°.  The flat-top runs around 180°-200°oil temp.


Noise & Vibration

Like the 94mm Wiseco pistons, audible noise was highest on the flat-top piston.  It had this rattle at light load.  Nothing alarming.  Just a faint rattle.  The pop-top had a hint of the same rattle.  The forged pistons need more clearance and it’s not uncommon to get a bit of slap.  The fins on the air-cooled engine amplify the noise.  I am also running the 97mm flat-top on the loose side.

The vibration levels on the 97mm pop-top piston are very close to stock.  That makes sense since they are almost the same weight.
 
The 97mm flat-top is significantly heavier than the stock piston.  As a result, the flat-top shakes a bit.  I refer to it as Harleyesque.  It’s got a pronounced shake at low rpm but get it up around 4000 rpm and it smooths out.  On the freeway it’s about the same as stock.  IMO, this (the freeway) is the area of concern.  Long hauls at a constant rpm can take their toll.  Lucky for us the heavier flat-top doesn’t cause high vibration in this critical range.  The low rpm stuff makes the bike feel more muscular.  I don’t see vibration as a problem with this piston.  

Here’s another view of the carbon accumulation.  It seems a little heavy to me for only 1583 miles.

Driveability

Both engines run great.  They start right up, idle smooth, have good throttle response and broad useable power bands.  But the flat-top piston is better at everything.  More horsepower, more torque, broader powerband,  faster acceleration, instant response.  Pump gas (92 octane) works fine, no hint of detonation.  The only advantage the pop-top has is ease of installation.
 
The cams behave exactly the same.  The Web 402 doesn’t seem to have any advantage over the factory DR650 cam, but you have to run stiffer high-lift springs with the 402.  I don’t think it’s worth it.

The carburetion is superb with either carb (VM or PWK).  They both run great.

The part throttle performance is very good on the pop-top and excellent on the flat-top.  As I mentioned on the prior report, this is the area where the higher compression really shines.  The flat-top with its tight quench steals the show again.

These bigger high-compression engines pull like crazy in 5th gear.  You can lug it all the way down to 2500 rpm, whack the twist grip, and it will accelerate hard all the way to 6500.  It really pulls.  The 5th gear performance is stellar.