849cc IJSBA “Stock Class” Upgrade for SXR 800s

The Short Version

For the 2008 racing season, IJSBA has permitted big boring of stock class SXR800s to 850cc.

 Group K has specially constructed 85.0mm (839cc) and 85.4mm (847.3cc) SXR pistons for this big boring.  These piston sizes result in a finished clearance cylinder bore diameter that is slightly under the 850cc maximum allowed.  The IJSBA has approved these pistons (and accompanying large-bore head-gasket) for use in all divisions of the IJSBA “800 Stock” stand-up classes.

  Group K been building and selling our 849cc Sleeper kits since 2005, so we have considerable experience in setting up these big bores in a reliable fashion.  Only minor screw adjustments need to be made to the stock jetted carbs, and no lower end or cylinder-head modification at all is required (or permitted).  Needless to say, the “850 Stock” top end upgrade offers considerably better acceleration and peak speeds over a stock machine.

 

The Parts Needed for Modification:

Your SXR 800 Cylinder (with all the top cylinder head studs)         Your oem SXR 800 Head Gasket

 

The Pricing

849cc IJSBA Stock Class Upgrade       $498.00            

Includes:    Cylinder big-boring     Cylinder top deck lapping     Installation of cylinder-head studs

IJSBA legal port chamfering      Final bore clearance sizing      Boring of stock oem head gasket 

2- 85.0mm pistons with rings, pins & clips      3-Bond 1211 Sealer

 

SXR Cylinder Prep only – Includes big-boring, top deck lapping, stud installation, IJSBA legal chamfering, & final bore clearance sizing

170.

 

 

85.0mm or 85.4mm piston  (with rings , wrist pin, & clips)

169. each

 

 

New 85mm diameter head gasket

79.*

Boring of stock OEM head gasket

35.

OEM SXR 800 head gasket core

39.*

 

 

 

The Detailed Version

About Cylinder and Sleeve Strength

  To be sure, boring an SXR cylinder from 82mm to 85mm removes a significant amount of sleeve wall thickness.  The SXR800 cylinders do have a considerably thicker sleeve wall thickness than the earlier 743cc cylinders.  But even with these thicker SXR sleeves do have some limits.

  Super-stock SXR800s are routinely run at 7400-7800rpms.  These very high rpms are very hard on both the pistons and sleeves (as well as many other parts).   In a high-output high-rpm super-stock 849 engine setup, the relatively soft stock sleeve material can be at risk of developing small fractures between the transfer port windows.   This problem is normally resolved by installing sleeves made of a much tougher high-nickel steel alloy.  Unfortunately such re-sleeving is not legal in stock class.

  Fortunately however, IJSBA Stock-Class SXRs only turn peak rpms between 6900-7000 rpm.  At these relatively low rpms, the loads on the pistons and sleeves are dramatically reduced to the point where there are very good prospects of reliable long-term piston and sleeve life.

 

About the Cylinder Studs

  The removal of the sleeve material for the big boring slightly reduces the overall rigidity of the cylinder casting.  Most importantly, it reduces the amount of material between the cylinder head studs, and the bore diameters.  The cylinder casting mass in this area becomes so thin that merely installing the studs into the cylinder creates a high spot on the bore diameter at each stud location.  In fact, the actual installation torque applied to the studs makes a difference in how much bore diameter deflection there is.  If a cylinder is merely bored and sized to 849 without the studs torqued in, there is a high likelihood that the cylinder will experience a non-stop series of piston seizures caused by these cylinder-stud high-spots.

  To eliminate the effects of this problem, we mandate that your top cylinder studs be furnished to us with every 849 Kit.  During the final preparation of each Group K 849 cylinder, we will torque in the cylinder studs, and then do the final cylinder sizing to eliminate the high spots in the bore.  Note that we will not produce any 849 kit or cylinder unless we get the top studs for this operation.  If a stud is broken off while removing it from a cylinder, we will machine out the broken stud and install a thread insert for $45 per broken stud.

 

About Compression

  From a theoretical standpoint, any time the bore diameter is slightly increased, compression ratios are also slightly increased.  In the case of big boring, the compression ratio is increased considerably.  This increase in compression ratio is, in large part, one of the features that allows the big performance increases from the big bore upgrade.  During our testing, we found this compression ratio to be easily 91-octane safe, so long as the stock ignition timing and stock exhaust are in place.

 

About the Head & Gasket

  In a perfect world, the domes of the cylinder head would be cut to be the same diameter as the cylinder bores, at an angle that matches the piston crowns.  Unfortunately, the letter of the IJSBA rules do not permit head dome machining of any kind …. And that’s actually okay on this particular engine setup.

  Given that the stock SXR pistons are virtually flat crowned, and that the squish clearance of the stock SXR engine is so generous, the shape of the resulting big-bore squish band is easily within the “technically acceptable” range.  In fact, it’s so acceptable that KMC didn’t even bother changing the SXR800 head dome casting to have an 82mm dome.  All stock SXR heads actually have an 80mm diameter dome … same as all the 750s back to 1992.  In contrast, what would be “technically unacceptable” would be the pre-ignition risks of the thin metal 82mm OEM SXR head gasket “hanging” free over the top of the 85mm big bore piston.   For this reason, the IJSBA has also permitted the big boring of the stock head gasket to 85.5mm.  While some “tuners” might opt to keep the 82mm head gasket in the interest of having a slightly higher compression ratio, the gains of that tiny bit of compression are certainly not worth the risks of pre-ignition …. besides, the big bore already has “plenty” of compression just from the bore size increase.

 

About Head Gasket Sealing

One of the design problems that comes with any Big Bore modification is sealing the head gasket surface.  This head gasket sealing issue is a two fold problem.  The first issue is the sheer reduction of sealing area between the head studs and the new larger bore diameters.  The second issue is the tendency of the top cylinder surface to deflect slightly and “move around” from the constantly varying temperatures of operation.  The big boring removes a considerable amount of sheer mass from the cylinder casting, and thereby further increases the likelihood of irregular casting expansion, and gasket surface “movement”.  For our 849 upgrade, we opted to use a stock OEM metal head gasket bored out to the larger 85mm bore diameters.  These gaskets (along with a thin application of 3-Bond 1211 sealer) offer excellent long tem sealing.  To assure compliance with the IJSBA rules regarding head gasket thickness, we will only bore and supply stock OEM Kawasaki head gaskets. 

  To further improve head surface sealing prospects, every 849 big-bore cylinder has it’s top deck “lapped” to assure a perfectly flat sealing surface.  “Lapping” is “surface restoration” operation that dresses the high spots down to the original surface.  As such, this operation does not remove any measurable amount of material from the top deck of the cylinder.  The intent of lapping is to remove random “high spots” from the gasket sealing surface.  The most common of these high spots are those around each threaded stud hole.  The mere installation of a cylinder stud, and repeated head torqueing can result in elevated high spots around each threaded stud hole.  These high spots will reduce the gasket pressure on the sealing area of the gasket, and eventually result in a head gasket leak.    For owners that wish to confirm the perfect flatness of the sealing surface of their cylinder head, we also offer optional lapping services for the stock head.

 

About Ignition Timing

  The IJSBA stock-class rules allow the use of aftermarket ignition boxes and timing advancers.  On a stock SXR (that has a very mild compression ratio), these advancing devices may certainly improve overall acceleration with little or no risk of detonation.  However the 849 big-bore has considerably higher compression than a stocker.  In addition, detonation becomes more difficult to control as bore diameters increase.  The higher detonation risks of a big-bore are “usually” abated by reducing squish clearance and altering the combustion chamber shape …. Both of which are not allowed in IJSBA stock class rules.

  In the end, the sheer increase in torque and acceleration offered by the 849 big bore, is greater than that offered by most ignition modifications.  Owners that want to keep the ignition advancers on their 849 stock big bore will likely find that they’ll need to run 100+ octane fuel to stave of detonation and overheating…. And they’ll also find that the advancers don’t help the 849cc setup as much as they helped the stock 782cc setup.

 

About Carburetion

Contrary to popular thought, the 849 big-bore does not require richer jetting.  The reason is … “signal”.  The 849 pistons create a much stronger crankcase vacuum (aka “signal”) in the inlet tracts and carb throats.  This stronger “signal” actually draws more fuel through the jets (per stroke) than a stock displacement 782cc engine does, and this stronger signal feeds the 849 the added fuel that it needs.  The end result is that the big bore runs happily on virtually stock carb jetting.  Like the stock 782cc setup, lighter pop-off springs are a benefit when free breathing flame arrestors are installed.  Barring that, the 849 requires only minor high and low speed screw adjustment with stock jetting.

 

About Exhaust Modifications

  IJSBA has allowed the use of a restrictor sleeve in the stinger tube of the stock exhaust pipe.  From a theoretical standpoint, reducing stinger diameter has two general effects.  1) It can significantly increase peak rpm power (very good), and 2) it significantly increases piston crown temperatures (not good at all).  The very mild-compression stock 782cc SXR may response well to this stinger mod while running happily on 91-octane pump gas.  However the higher-compression 849cc big bore needs the sheer volume of the stock stinger tube to properly “exhale” the greater volume of exhaust gases, and lacks the need for the added “stinger sleeve” back pressures that are beneficial to the stock 782.  Here again, owners that wish to retain their stinger sleeves on their stock-class 849 may find that the octane will need to be increased considerably to stave off detonation.

 

About Pump & Props

  It goes without saying that the 849 offers the brute power to pull a steeper prop than the 782cc setup.  There are numerous folks that strongly embrace their particular custom prop/pump setup as the “ultimate best” setup …. And we won’t try to enter that fray.  That said, during all of our 849 testing we consistently got the best acceleration and peak speed results using the Skat-Trak C-75 9/17 Swirl impeller on a stock pump case.  Is it the “ultimate”? …. Who knows.  What we do know is that our SXR 849 (and all the test riders) liked it better than any other off-the-shelf impeller.