This is the ninth in a series of reports intended to document the results of progressive modifications to the LS650 engine.  You can find Parts 1 through 8 here.

Part 1 Stock: http://suzukisavage.com/cgi-bin/YaBB.pl?num=1620523526

Part 2 Airbox:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1621150483

Part 3 Exhaust:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1623048749

Part 4 Carburetor: http://suzukisavage.com/cgi-bin/YaBB.pl?num=1625732492

Part 5 Cam:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1626391255

Part 6 Flywheel:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1626921647

Part 7 Head:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1627891507

Part 8 Compression:  http://suzukisavage.com/cgi-bin/YaBB.pl?num=1631491370

In Part 9, we will increase the displacement by installing a 97mm piston.  We will compare the Wiseco 97mm pop-top piston (9.5:1 CR) to the Wiseco 97mm flat-top piston (11.2:1 CR).

What I am about to describe worked good for me.  If you decide to try this stuff on your own, you assume responsibility for the outcome.  If you don’t have the skills, don’t do it.  If you don’t understand something, STOP and get help.  Get a manual.  Read up.  Comply with ALL the safety requirements outlined in the manual.  Make sure you know what you are doing before attempting any of these modifications.

Let’s get started.

Up to this point, we have increased power and economy by improving volumetric efficiency and thermal efficiency.  Now we are going to take the “bigger is better” approach.  There’s no substitute for cubic inches; no replacement for displacement.

The stock LS650 engine has a 94mm (3.701”) bore and stroke.  That works out to 652 cc (39.8 cubic inches).  If we increase the bore to 97mm (3.819”), the displacement increases to 695 cc (42.4 cubic inches).  That’s a 6.6% increase in displacement.  Bigger should be better provided we can supply the engine with enough air.

Increasing the displacement will also increase the compression ratio (CR).  The stock engine has an 8.3:1 CR.  The 94mm pop-top brings the CR up to 9.2:1.  The 97mm pop-top yields 9.5:1.  The 94mm flat-top steps it up to 10.4:1.  Throw in a 97mm flat-top and we’re lookin at 11.2:1.  I think that’s pushing the limit for premium pump gas.
 
We’re gonna see if the old Twin-Swirl Combustion Chamber (TSCC) can hack a CR over 11:1.  Up to this point, it has handled increased compression well.  The centrally located spark plug and turbulent combustion chamber handle pump gasoline quite well.  Adding the tight quench associated with the Wiseo flat-top also helps to stir things up.  At 10.4:1, the 94mm flat-top engine ran fine on 92 octane.  No problems with detonation.  Now we’ll see what happens with more displacement and a little more compression.

As mentioned earlier, I’m not going to give you a blow-by-blow on building the big engines.  The prior posts provide plenty of how-to.  I’m just gonna hit on some of the unique issues I encountered on the 97mm builds, and then provide the test results for comparison.

In the interest of full disclosure, we need to talk about the clutch.  Once I took the engine up to 42 cubic inches, the stock clutch wasn’t up to the job.  It just couldn’t handle the power.  The same holds true for the 94mm flat-top piston.  Although I never tested the 94mm flat-top with the stock clutch, the 94mm flat-top runs just as good as the 97mm pop-top.  That means the 94mm flat-top must produce a similar amount of horsepower and torque.  If the clutch couldn’t hold the 97mm pop-top, it probably won’t hold the 94mm flat-top.  If you wanna play you gotta pay.  See this post for clutch options.  You might need em.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1615547049

It ended up just a bit looser than I wanted.  I would have preferred .0030” clearance.  Looks like .0035” will have to do.  I set the ring gaps using Wiseco’s guidance for “Blown Race Only”.  This is an air-cooled engine with really high compression, so I’m taking the conservative approach.  I’ve learned my lesson.  I set the gap for the top ring at .025”, and the gap for the second ring at .027”.

The Dupli-Color went on easy and looked great.   Made a silk purse out of a sow’s ear.

In contrast, the Web 402 after only 1583 miles.  Same oil, Mobile 1 V-Twin 20W-50, Phosphorous 1600 PPM, Zinc 1750 PPM, full synthetic.

Let’s look at some of the differences between the 97mm pop-top and 97mm flat-top.
 
The pop-top has a shorter compression height.  At TDC, the top of the piston is situated about .125” below the cylinder head, there is in no tight quench zone.  It bumps up compression using a pop-top, and increases displacement with a 3mm increase in bore size.

The pop-top doesn’t have a full-circle skirt.

At 496 grams, the pop-top is only 4 grams lighter than the stock piston.