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The Tecumseh Star, Alternate Racing Power
By Ken Weber
In motorsports, the person who spends the most usually wins the most – unless he spends it on beer. The ongoing search for a competitive advantage gets more technical and more costly every year. In order to combat this cost, some karting sanctioning bodies resort to spec engines, spec tires and, in some cases, complete vehicles to reduce or eliminate the necessity to continuously upgrade the hardware, theoretically holding down the cost of racing.
There is still another popular format – the "run what you brung" crowd. In this format, horsepower per dollar is the name of the game, and bigger displacement means more horsepower. But, there is only so much you can do in terms of physical engine size and weight without eliminating a place to sit or destroying the handling characteristics of the kart.
Nationally, in terms of numbers, four-cycle go kart racing and junior drag racing are dominated by the 5 hp Briggs & Stratton engine in various states of tune, from pure stock to fully prepared fuel burners. Parts availability is good, and a lot of shops have become deeply involved in this market. There are also classes for the Tecumseh 5 hp and Star series engines that are popular.
As in other types of racing, horsepower costs money, both in initial outlay and in higher maintenance and shorter life. In the modified arena, some of the high horsepower, stroker Briggs engines are going out the door for prices significantly over $5,000, and more than one is needed to race an entire season. Where the rules do not permit alternative engines, the racer is forced to march to the same music as everyone else, and the more limited the choices, the more expensive it is to be a consistent winner. But, in those markets where engine choice is allowed, the Tecumseh Star series engines are a popular, cost effective alternative to the Briggs.
The Tecumseh Star is larger in displacement, physical size and weight than the Briggs, but that also translates into similar power with more reliability and less cost, or more power for comparable money. Basic racing preparation is straightforward, and being an industrial flathead, there is plenty of opportunity to increase the power levels. Aftermarket crankshafts and side covers are also available that will narrow the engine to near Briggs dimensions.
Stock Star specifications are 3.312" bore, and a 2.530" stroke, making 21.8 cubic inches, compared with a stock Briggs at 12.56 cubic inches. The Star’s valve sizes are 1.340" intake and 1.160" exhaust. In stock trim, this package averages around 14 to 15 horsepower, about twice what a stock Briggs develops.
Follow me through the preparation of a mid-level modified Star that will make 25 horsepower at 7,500 rpm and sell for under $2,000.
I start with a Tecumseh Motorsports block, because it has increased webbing between the bottom of the cylinder and the flywheel-side main bearing boss, and a beefed up lifter boss.
The first operation is to install a "DU" main bearing bushing in the block. This type of bushing is a self-lubricating steel sleeve with a mixture of lead and Teflon fiber as a bearing surface. It provides a low friction bearing surface that is just about bulletproof. The case must be bored to accept the bushing, and it’s best to use the side cover that will be used on the engine to ensure main bearing bore alignment.
Next, I install brass guide inserts to reduce the valve stem size from 5/16" to 1/4", followed by the installation of a 1.530" O.D. by 1.300" I.D. by .250" deep intake seat for a 1.500" intake valve. Installing the guides is a simple “drive them in and ream them to size” operation. Installing the seat is done on a vertical mill, using either a conventional seat cutter, or a boring head to cut the block. Cut the block for .010", press on the seat ring and heat it to about 300° F before driving the seat in.
The porting is the foundation for whatever power level you decide you need. In this case, I am using a VKE Motorsports tapered intake manifold, so I Helicoiled the 5/16" manifold bolt holes in the block down to 1/4" to match the manifold.
In porting a flathead engine, remember that the cylinder head sits right on top of the valves, so, as the valve approaches to within about .150" to .200" of the head, flow is reduced around the short turn and what I call the back side of the valve. Consequently, I try to maximize the flow across the underside of the valve on its way to the cylinder, with only minimal attention to flow around the back side of the valve next to the head.
The short turn is fairly tight with a smooth radius, and I widen and flatten the port here. In widening the short turn, most of the material is taken out of the side toward the exhaust port, until I break through into the end of the intake manifold bolt hole. Very little material is taken out of the left side of the port.
The bolt pattern on the VKE intake manifold is designed to offset the manifold downward on the block. It comes close to lining up with the stock port at the top, and when matching the manifold to the block, leaves considerable material to remove on the port floor. Some blocks will just break through on the bottom when properly done.
The junction between the manifold and the block will have the smallest cross section of the entire port, with a smooth bend on the right side becoming tangent to the right side of the bowl at about the center line of the valve stem. After opening the port up just below the new seat ring to match the 1.300" I.D. of the seat, I remove most of the material from the exhaust valve side of the port. From the seat ring to the guide I make the port wall slightly wider at the bottom than at the throat. The goal here is to create a forward leaning "bowl" shape where the sides are slightly wider than the throat, and the floor (long side) ramps smoothly from beside the guide up to the seat.
You will notice that the left side of the port is already considerably wider than the throat, and I fill this with epoxy to make it more like the modified right side. The original port offset promotes swirl, which is bad when it comes to making horsepower. Again, I want to flow the intake charge across under the open valve to the cylinder. The long side from the guide to the seat, and the areas that have been ground through, will also be filled with epoxy.
When using epoxy, it is very important that the surface be sand blasted and then cleaned with acetone or brake parts cleaner and blown dry before laying in the epoxy. Your hands must be clean or be sure to wear rubber gloves. Any oil, even skin oil from your hands, will cause the epoxy to separate from the surface when the engine gets hot.
I use Moroso white epoxy in the intake side, and the high-temp blue on the exhaust side. The blue will not withstand direct exhaust gas exposure, but works well on the outside of the casting in thin areas to eliminate burn through; I usually put some of this in the fins behind the outer header bolt hole for that purpose.
The deck area between the valves and the bore is relieved and lowered to the top of the top ring at TDC. I use a stock cylinder head with a VKE compression release installed, and the valve pockets machined for the larger valves and high lift cam. A .050" aluminum head gasket is hand matched to the deck relief and then transferred to the head which I then hand match to the gasket.
I like to use the Briggs header bolt pattern on the Star, because when I port the exhaust, I frequently cut into the stock Star lower header bolt hole. To do this, I Helicoil the upper header bolt hole to 1/4"-20 thread like the intakes, then I install 5/16" aluminum all-thread in the lower right header bolt hole with red Loctite, and trim it to length. Using a Briggs header flange as a pattern, I drill and tap the lower hole to 1/4"-20, without a Helicoil. This change to the Briggs flange will move the centerline of the exhaust port out and down, allowing the port to be straightened somewhat when it’s ported.
The 1.500" intake valve comes with a 30 degree seat, which I change to 45 degrees, with a 60 degree break at the edge of the valve, a 30 degree undercut and a 10 degree cut from the 30 degree angle to the stem. To have any edge margin, the diameter of the valve may have to be reduced slightly. If you radius the edge of the valve grinding wheel, you can blend the 10 degree cut right into the stem without any further hand-work. I make the exhaust valve from a stock size stainless intake by cutting it from 1.340" to 1.200", with the same angles as the intake. The 10 degree back cut is intended to thin the underside of the valve (reduce the amount of tulip), which helps flow. The seats in the block mirror the valves, with a 15 degree top cut, then 30, 45 and 60 degree cuts. The 60 degree angles are then hand blended into the bowl.
The final operation on the cylinder before cleaning and assembly is to hone the bore to fit the piston. I prefer a JE piston because it runs with only .0025" to .003" skirt clearance, minimizing piston rock and providing longer ring life. VKE orders these pistons with a special cam (ovality) and taper on the skirt which helps distribute the load better.
With a new cylinder, I use a .005" over piston because the cylinders are pulled so far out of round when a side cover and torque plate are bolted on, that it is rare to be able to finish one standard. I hone a .005" choke in the cylinder from about the middle up, and plateau finish the cylinder with a 400 grit over 280 grit finish. The reason for this is because air-cooled cylinders run with a lot higher temperature gradient from top to bottom than a liquid-cooled cylinder, and the smaller diameter at the top allows for the higher expansion. Hopefully, the cylinder will be straight up and down when running.
The crankshaft in this engine will be a stock unit with the counter weight clearanced for the cam. VKE also offers billet cranks in various strokes in both standard and short widths that are used with a billet side cover to substantially narrow the engine. I polish approximately .001" off of the main bearing surface next to the cam gear so that the ball bearing is a thumb push onto the crank. The crank will be connected to the piston with a .500" longer VKE billet aluminum connecting rod, and rod bearing clearance is okay between .002" and .003". When installing the bearings, you should line up the oil holes in the bearing with the oil feed holes in the rod, but I did tear one engine down that had been assembled with the drilled bearing in the cap – with no apparent problems. Remember, this is a splash oiling system. Rod bolt torque is 240 in.lbs., dry. The 12-point bolts are treated with a dry lubricant that makes it unnecessary to use oil or moly paste.
I’m using a Precision Cams PC 703 camshaft in this engine, with 265 degree intake duration, 275 degree exhaust duration, and .385" /.395" lift at .050", on 108 degree separation angle and 108 degree intake centerline, and a VKE dual spring kit. Remember, on a flat head, there are no rocker arms, so valve lift is the same as lobe lift. With this cam, I use VKE 1.100" diameter lifters. They are larger than the stock 1.030" lifters, and they are also stronger, which in this case is important because the high valve acceleration rates of these cams tend to break stock lifters. As mentioned before, high lift cams require that the flywheel side crankshaft counter weight be machined to clear the cam exhaust lobe.
After a hot water scrub, I start assembling the short block. After setting the top ring end gap at .012", and the oil rings at .008", I trim the oil ring expander to reduce ring tension. I have found that a piston with just the oil ring installed should ideally have about six to eight pounds drag in a finished cylinder. Less than six, and the engine will burn oil; more than eight pounds, horsepower will suffer.
Next, the valves and springs are installed, and the valve lash is set at .006" intake and exhaust, by grinding the tips of the lifters in the valve grinder. The tops of the spring retainers are close enough to the tips of the valves that I don’t cut the valves to set lash. Otherwise, the tip of the valve could be below the top of the retainer, making measuring valve lash difficult.
I use the side cover I am installing on the engine to locate the cam and I have used lifters of various lengths to get a starting point when I measure the lash. Once the lash is set, the seals are driven in and the crank, rod and piston are installed. Then the lifters, cam and side cover are installed. After a degree-check on the cam, the side cover bolts are torqued to 120 in.lbs. with blue Loctite. If it is desired to advance or retard the cam, a three keyway timing gear is available.
Next, the flywheel is installed and the ignition timing is set. I use .180" down on the piston, which equates to about 28 degrees before top center. In this case I am using a VKE billet flywheel for lighter weight and less inertia. VKE also offers a dual plug ignition kit, but this engine has just the single plug. I coat the head gasket with Permatex high temp gray silicone and torque the bolts to 200 in.lbs. If you use a stock head gasket, silicone is not necessary, and only apply 160 in.lbs. of torque. The intake manifold is also installed with gray silicone.
These engines come from Tecumseh as an unassembled kit. Included is a Tillitson HL 374 series carb with a 1" throttle bore. This is spec for Stock class engines, but really limits horsepower. I am installing a Tillitson HR 191 carb with a 1.350" throttle bore that has been recalibrated by Mike Sandburg at VKE to run on alcohol.
Depending on application, there are other choices, including Mikuni slide valves that may make even better horsepower. However, if a heavy carb is used, you should install a supplemental support for it. Unsupported, these set-ups vibrate badly and tend to cause the top half of the cylinder to separate from the bottom.
After installing the blower housing and the rest of the air management tin, the carb, header, spark plug and oil are added, and the engine is ready to go to the dyno. Remember, you must either index the spark plug to clear the valves, or use a surface gap plug.
The example engine produced 25.5 horsepower at 7,500 rpm with a really flat torque curve from 4,000 to 6,000 rpm. With a clutch set at 3,800 to 4,000, this engine will pull hard off the corner and produce good power to almost 8,000. A bigger carb and a different cam and header will make more power on the top end, with a sacrifice in off-the-corner performance. We have turned these engines close to 10,000 rpm, but like any other engine, reliability is sacrificed the higher you turn it.
In engines built for kart racing, the stock crank should be replaced after about 15 to 20 race days, as they tend to break if run much longer. I really think some form of a harmonic balancer built into the flywheel would go a long way toward reducing failures in these and other single cylinder engines.
The difficulty in making a lot of horsepower in a flathead-style engine stems from two factors. First is restricted breathing due to the proximity of the head to the valves, and second, any attempts to significantly increase compression further reduces airflow from the valves to the cylinder.
This engine has about an 8:1 compression ratio. In most cases I don’t even measure it. Breathing is much more important. Despite the low compression ratio, the engine does respond to alcohol and nitro methane. Some of the racers I know routinely use 30 to 40 percent, and 20 percent nitro is common in open classes.
Remember, when running these engines on alcohol and/or nitro, it is important to purge the fuel system after each race day by running the engine on gasoline with a strong dose of 2 cycle oil. Pull the fuel line off the pump, hook up a container with the mix, start the engine, and run until it starts running real rich. Shut off the fuel, but keep it running until it runs out of gas.
If you have an opportunity to build these engines, the VKE Motorsports catalogue is available online at vkemotorsports.com, or by contacting the shop at VKE Motorsports, 3750 Wheeling St., Unit 5, Denver, CO 80239.