For supercharging the 5 hp Briggs & Stratton "Raptor" engine
The life of your engine will be dramatically improved by these relatively simple recommendations. Please note and follow them carefully! (Also consult the WKA Rule Book to confirm the latest rules for your class.) Supercharging eliminates the need to push the engine to stressful & extreme rpm levels.
Therefore, always choose axle sprocket ratios which will keep your engine speed below 7500 rpm!
a. After-market, three-ring type, forged "Superstock-type" unchromed (ie. Wiseco 1992 series)
Stock cast-type pistons cannot survive the pressures and temperatures, even briefly!!
b. Ring end-gap .008" to .012" measured while installed into the bore
c. Use two "top-rings" plus the bottom oiling ring. (The factory "middle-ring" is not used)
d. Piston-to-bore clearance .007" - .008" measured 90 degrees from wrist-pin at bottom of piston
(This clearance assumes "Superstock-type" 3-ring forged piston. Others may differ.)
e. Piston "pop-up" above the cylinder should follow stock spec's. (Unlike naturally aspirated engines, high compression ratios do not benefit supercharged engines!)
Rod: After-market "ARC" rod with "dipper-oil-feed" or Horstman "H-beam" (3.875" stock length)
(Note that the "ARC" rod has excellent lubrication for the bearing, but is not strong enough for extreme engine speeds. Builders of "open" class engines with larger-than-stock intake valves might exceed 7500 rpm and must use a stronger connecting rod!)
a. Clearance of wrist-pin to rod must be .0007" to .0013"
b. Clearance of crank-journal to rod must be .0030" to .0035" measured after rod cap is torqued to 150 in-lb. The bearing inserts must be chamfered to clear the radius of the crank-journal.
a. Cast iron sleeved bore
b. Flat and smooth as possible on deck surface.
c. Sintered bronze exhaust valve guide. Ream and hone to .2500" ID minimum.
d. Lapped valve seats.
e. Relieved cam-follower pockets, as cam dictates.
f. "Anchor-plate" trick (see "Headbolts" section below)--Dremel out region on block below the third fin at the one location identified in the related "Diagram #6.
g. Drill angled oil-feed hole to "magneto-side" journal bearing. Bearing life is extended dramatically! Use "EZ-Bore Bushing Kit EZTM-500" (Phone (757) 898-5645.)
a. Stock Briggs & Stratton, or stronger
b. Check clearance between crankshaft and cam-lobes while both are installed in the block, and grind relief-zones as required in the following two places:
i. Counter-balance farthest from the "flywheel-end" as required to clear exhaust cam lobe.
ii. Cast surface of crank next to con-rod bearing on "flywheel-end" to clear intake cam lobe.
a. Steel cams are more reliable than cast iron cams; additionally, they are usually available in a format which can be "re-phased" to match the tuning effects of your particular exhaust system. (Experiment to find phasing for best power at your engine speed, say 7000 rpm.)
b. Maximum lift about .380"
c. 25 to 40 valve overlap at .050" off-valve-seat (Closer to 25 for improved low-end torque. Supercharged engines operate better with less overlap than naturally aspirated race engines.)
d. 250 to 260dwell at .050" off-valve-seat (Closer to 250 for improved low-end torque.)
e. 111 to 114 phasing centerlines (Closer to 111 for improved low-end torque.)
f. Ask your cam supplier for help in selecting a cam grind for your particular type of racing.
g. Use a "short-reach" spark plug to clear the intake valve when using high-lift cams!
Valve Springs & Lifters:
a. At least "dual, medium-pressure" springs, as required for cam lift profile and rpm requirement.
b. Cam supplier can provide extra-length lifters which are suitable for a particular camshaft.
c. Some lifters require that a relief be provided in the block to allow full travel.
Exhaust Length: Less than 16" including muffler. (Otherwise backfires may occur in the intake system, depending on your specific camshaft and related valve-timing.)
Drive Ratio (Supercharger):
a. 18 teeth on the engine and 20 teeth on the supercharger will produce 10 to 12 psi boost at 6500 rpm, depending on your cam. (19 or 20 tooth sprockets may be substituted to obtain higher boost levels; however, engine life will likely be reduced as boost pressure is increased.)
b. With idle speed set to 2000 rpm; manifold vacuum on boost gauge will vary from 3" to 5" of mercury, depending on idle fuel mixture.
Drive Ratio (Rear Axle):
a. Set the kart final drive sprocket ratio to allow operation up to 7200 rpm maximum at the highest speed portion of the track. This allows for 7500 rpm when "drafting" another kart.
b. Supercharging eliminates the need to push the engine to stressful and extreme rpm levels; therefore, always choose axle sprocket ratios which will keep your engine below 7500 rpm!
c. A beginning "guess" to start the experimentation may be helpful until you gain your own experience with this powerplant: "Enduro" karts will usually utilize ratios around 2.5 to 1, and "Speedway" karts will usually utilize ratios around 5 to 1. Be careful to avoid overspeeding the engine during the experimentation laps; the point is to preserve your engine!
Flywheel & Ignition Timing, and Cooling Baffles:
a. Use lapping compound to match flywheel to crankshaft taper, then clean and mount securely.
b. Stock ignition timing provides good operating characteristics. However, if most of the time on the track is at engine speeds above 6500 rpm, then a #5 offset key may be preferred.
c. When re-installing the ignition coil, be sure to keep the air-turning vane installed because it is essential in providing air to the always-hot valve-guide region!
d. Depending upon the air access to the engine for a particular kart and the road speed of a particular race track, the engine may run cooler with the cylinder-shroud removed. (Experiment with this by first adjusting the carburetor to provide an acceptable cylinderhead temperature during trial laps with the shroud. Then, remove the shroud and run again without re-adjusting the carb, observing the cylinder head temperature carefully.)
Crankcase Breather: Stock breather with internal check-plate must remain, but internal coalescing mesh must be removed. (Otherwise, the mesh works its way into the breather tube, which obstructs the venting of the crankcase. This impairs the fuel-pump diaphragm performance, which causes "lean" engine overheating and possible engine failure.)
a. This is not "just an oil cover"! It is also essential to the strength of the cylinder / crankcase casting. Therefore, guard against vibration-induced bolt loosening in every way possible. Torque to 110 inch-pounds. Use serrated washers against locking "bent-tab" washers, and high-temperature thread-lock adhesive. (ie. "Lock-tite #262 red" thread-lock adhesive)
b. A billet-type side-cover with an additional bolt hole would be an inexpensive "insurance policy" against block failures.
a. Stock head relieved for valves as required by cam lift and gasket thickness.
b. Flat and smooth as possible at gasket surface.
a. Solid copper or aluminum type
b. .040" minimum thickness, but .060" or .090" will provide better performance. (Unlike naturally aspirated engines, high compression ratios do not benefit supercharged engines!)
c. Common gasket adhesives are not resistant to methanol, but Teflon-type coatings are resistant. "High-temperature RTV" silicone also works well.
d. Laminated or fibrous gasket materials cannot survive the pressures and temperatures!
Torque all headbolts to 150 inch-pounds, and re-check torque after 10 minutes of operation. (Torque value assumes the use of 5/16"-18 UNC threads. Note that this torque would be too high for commercially purchased "fine-threaded" studs!) The exhaust-corner headbolt is trouble-prone because of its elevated temperature; see "Special Attachment" procedure.
Special attachment to single exhaust corner location only:
(The engine's aluminum threads in this "hot-spot" are effectively replaced with steel threads in a new "anchor-plate" which will have better strength at the elevated temperatures.)
Drill out the threads of this single head-bolt hole only to 5/16" diameter, as shown in "Diagram #6". Use a hand-held Dremel tool to cut out clearance for the new "anchor-plate" immediately beneath the third cooling fin from the top. Fabricate the steel anchor-plate per the drawing below. Place the anchor-plate between the cooling fins, insert the threaded rod from above, and screw it through the anchor-plate. After positioning the head gasket and the cylinderhead, place one hardened flat washer (ie. Tecumseh #650691), followed by five (5) Belleville washers (ie. Tecumseh #650690) onto the threaded rod in an alternating sequence as shown in "Diagram #5." Torque nut to 150 inch-pounds, and re-torque every 10 minutes of operation during break-in period to help "settle" the steel anchor-plate into the softened hot aluminum fin material. Re-torque only when the cylinderhead temperature is below 200 F. (Torque value assumes 5/16"-18 UNC threads. Note that this torque would be too high for commercially purchased "fine-threaded" studs!) Occasionally a Belleville washer will crack shortly after initial installation. Therefore, it is good practice to visually inspect washers before each re-torquing.
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