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Home 1998 Editions April, 1998

Most businesses have the sameproblems and each must look for ways to remain competitive byreducing costs and exploring new markets for existing productsor services.

And so it is in engine building. There is alwayssomeone that is able to provide more features per dollar thanyou (or at least make someone believe they can). Therefore, tocontinue in business, a shop must continually make more efficientuse of its equipment and operating expenses.

More and more shops today are actively exploringalternative markets for existing shop equipment and employee expertise.One of these markets is the junior dragster classes that NationalHot Rod Association (NHRA) started a couple of years ago. An "all-out"Briggs and Stratton motor can cost $4,000 and up, and a superlight chassis with all the "gee whiz" items can doublethat. Add in designer driver’s apparel, an enclosed trailer withan upscale truck to tow it, and the total investment could top$35,000-$40,000.

However, since this is bracket racing, completewith break outs, the aspiring Kenny Bernstein "wanna-be"can also start at the grass roots level with a bone stock 5 hpBriggs, centrifugal clutch instead of torque converter drive,home built chassis, and haul it in the back of the old Chevy pickup.

This approach allows you to be as competitiveas the next guy and can probably be done for $1,000-$3,000 (ifyou already own the pickup). The difference is that the $4,000Briggs can be expected to turn 1/8th mile elapsed times in thenine second range while the stock 5 hp will probably be in themid to high teens, depending on the total weight of car and driver.

One benefit of the junior dragster market isthat the parts and components for the driveline are, for the mostpart, borrowed from the go kart market. So if you build theseBriggs engines, you can market in both the junior drag and kartingperformance arenas.

Another offshoot is the new Bandolero full-bodiedspec car program which seems to be patterned after the successfullegend cars. These vehicles use the Briggs Vanguard V-twin andare reported to be sanctioned by the same INEX organization. Inkarting, another engine comes into the picture, the Tecumseh Starmotor. Bigger in both physical size and displacement than theBriggs (21.8 cu. in. versus 12.56 cu. in.) it is capable of about20% to 30% more horsepower. However, in junior dragsters NHRArules specify only the Briggs be used.

Peripheral equipment is also a large segmentof the market. With every engine there must be some means of transmittingthe power to the rear wheels. Clutches or torque converters, belts,chains, sprockets, axles and hubs are all necessary components.Other support equipment includes a battery-powered starter insteadof the rip cord if a highly modified engine is used. The listgoes on and on. This segment of the performance market is growingrapidly and could offer profitable business for your existingshop capabilities.

The other day I was talking to a dad that wasthinking about getting his child a junior dragster. He had beentalking to someone who told him that "they" were getting75 horsepower out of a Briggs motor. "Oh yeah?" I asked,"Who’s building those motors?" I then questioned himif 6 hp per cubic inch from a 7 to 1 compression air cooled flatheadmotor sounded realistic? And if it were true, I’m sure one ormore of the major automotive companies is probably conductingaround-the-clock surveillance of that person’s shop.

Imagine roughly two-and-one-half times morehorsepower per cubic inch than the best Dick Maskin pro stockmotor is capable of. WOW! Think of it; a 500 cu. in. flatheadair-cooled pro stocker making 3,000 hp, or a 2,100 hp WinstonCup car. I’ll bet I could win every race of the season with that.Jeff Gordon move over!

Realistically, though, a Briggs with a .200"over bore and a .500" stroker (17.6 cu. in.), on a 30% loadof nitro methane will make low to mid 20s hp at 9000+ rpm. A Tecumsehcan make over 30 hp at about 8000 rpm. To get to that point inthe Briggs, however, requires major surgery, and the only originalpart in either motor will be a severely modified block.

To put a .200" over piston in the Briggsrequires boring the block almost into the head bolt threads andresleeving it. Then with the stroker crank, the rod hits the cam,the lifters, and the bottom of the cylinder, and the rod dipperhits the front of the crankcase requiring more custom fitting.When you install a high lift cam, you must clearance the bottomof the lifter bores to allow the lifters enough room to reachmaximum lift, and the bigger lobe makes for more conflict withthe connecting rod and crank counterweight.

The current trend for increasing horsepowerin these motors is to weld angled intake and exhaust runners tothe block. This is necessary because the original layout consistsof a pair of cylinders intersecting at a 90° angle with onecylinder offset from the axis of the other; it’s designed to promotea very high swirl to the mixture. There is not enough materialin the block to simply grind away what you don’t need and havea good port.

You can achieve dramatic results by using someepoxy in the intake ports, but to get that last 4 to 5 hp requiresthe welded runners. Of course this elevates the cost of the engineconsiderably. On the bright side, Briggs & Stratton Motorsportshas just announced its "Blockzilla," a new cylindercast out of T356-T7 alloy with heavier casting sections and angledports that will offer more performance potential than the productioncasting.

Porting a flathead is a different can of wormsthan dealing with overhead valves. In the overhead valve layout,the name of the game is to get the mix turned from the axis ofthe port to the axis of the valve so that you utilize as muchof the valve window as possible. In the flathead, it doesn’t domuch good to turn the mix because the roof of the combustion chambersits right on top of the valve; it’s like pointing a hose at awall.

The approach here is to simply make the shortturn smooth so that it works well during the low lift part ofthe valve opening where the mix can get around and over the topof the valve, between it and the roof of the combustion chamber.For the higher portion of the lift, as the valve approaches thehead the flow path over the top of the valve is cut off. Consequently,you work to get the mix to slide across under the valve and shootacross the channel between the valve seat and the bore.

The exhaust port works the same way exceptin reverse. Fairly often, in motors that have had some portingdone by the owner, you’ll see that the first thing they’ve doneis to remove the exhaust pipe threads. There’s no measurable benefitto doing this. The most that should be done is to knock the peaksoff the threads on the bottom (long side) of the port.

In both the Briggs and Tecumseh engines, the"as cast" exhaust port is too large. You can achievesuccess with a half-round shoe in the roof (short side) to reduceport size and effectively straighten the center line of the port.This results in a "D" shape with the flat of the "D"at the top. It seems to be beneficial, but it’s still questionableif the rewards are worth the time and effort to do it becausemost of the development efforts have really been in other areas.

Since you can’t put epoxy in the exhaust port,and welding is difficult, you can cast a shoe for the Tecumsehthat is fitted into the port between the back wall and the guideto create a slope or curve on the long side of the port insteadof the 90° turn in the "as cast" configuration.

Most of the performance gain is in the approachto the seat, the window that the air sees, and the transitionfrom the seat to the deck relief. If you study the pictures comparing a pair of stock ports with the modified ports,you can see the dramatic improvement in the flow window, and thesmooth blend of the seats to the port and the deck.

Like most performance engines, these enginesrespond to ever larger valves. VKE Motorsports, Denver, CO, whichprovided much of the information in this article, currently usesup to 1.5" intakes and 1.160" exhaust valves, whichnecessitates installing intake and exhaust seat rings in the Briggs,and intake seats in the Tecumseh. If a very high lift cam is used,the stem lengths have to be tailored to the valve spring requirements.

There are a number of approaches to high compressionon the market, but it’s questionable as to whether any of themprovided measurable benefits. If you study the layout on a flatheadengine, you’ll see that the air flow has to move from the valves,across the deck to the cylinder where it has to make a 90°turn into the cylinder. In order to raise compression, you haveto make this area of the deck and portion of the cylinder smaller,which makes the flow area smaller, and in turn makes the enginelose power.

The stock Briggs combustion chamber is aboutas good as it gets. The only way to get more compression and notlose power is to make the displacement bigger. Along this line,VKE advocates the tightest piston-to-head clearance you can livewith.

On highly modified motors using solid copperor aluminum head gaskets, a copper wire O-ring around the cylindermay be necessary to stop head gasket seepage. Dual spark plugsare also a benefit because of the relatively long flame travelrelative to bore size in these engines.

VKE is into camshaft profiles that have createdmajor problems finding valve springs that will fit into the springbox and not coil bind before maximum lift. When you start dealingwith .500" lobe lift, the cowpie hits the fan. Remember,flatheads don’t have rockers to multiply the cam lift at the valve.

So be careful in this area, and make sure youdon’t coil bind the springs or drive the valve into the head,because if you do, it usually breaks the cam bearing boss outof the block. Unlike an overhead valve motor, when you break theblock you lose all the valve and port work you put into it, too!

The Tecumseh motor has a problem with the timinggears that is worth mentioning. The engine routinely breaks thefactory crank gear. VKE has made its own replacement gear whichis a very satisfactory solution to the problem, but the cam gearfails at the 2nd, 3rd and 4th teeth next to the timing dot. Thishappens just as the crank passes top center on the firing stroke.

The breakage is related to wear on these teeth.If, through regular inspection, the wear is monitored, the breakagecan be prevented by replacing the cam before the wear becomessevere enough to promote failure. Nitriding the gear would help,but some class rules for this engine don’t allow any modificationsto the cam gear. This failure is also rpm related. If the engineis run below 5700-5800 rpm it is seldom a problem, but the currenttrend toward middle 6000 rpms in stock class engines and over8000 in modifieds has made it more prevalent.

If anybody had said a few years ago that asplash-oiled connecting rod motor of any make could be reliablytwisted over 10,000 rpm, their mental stability would have beenquestioned. Amazingly, it does work, and with relatively littledeviation from the stock setup.

VKE puts the DU bushing on the flywheel sidein both the Briggs and Tecumseh blocks, and slip fits the bearingon the power take-off end. The stock Briggs rod is good enoughfor anything you can do with a stock motor, but not much else.The Tecumseh rod is suspect even in stock classes if it’s turnedover 5500 rpm.

When these engines are rebuilt, it’s almostalways necessary to replace the rod because of the wear in thejournal end. The older Briggs motors used a stamped steel roddipper that was installed under the rod bolts. Don’t use thesedippers. They fatigue and break off, and then the rod sticks tothe crank and breaks. The new Briggs rod and all aftermarket rodshave the dipper made as part of the rod cap.

There are various components on the marketthat offer solutions to the various problems encountered whenmodifying these engines, such as cranks with smaller journal diameterswhich use smaller profile rods. Cams and valve springs are availablefrom a variety of grinders such as Crane, Comp Cams, Lunati andothers.

VKE CNC machines all of its own rods from 7075T-6 billet aluminum, and uses replaceable bearing inserts in them.This resolves some of the problems that have been prevalent withthe stock rod that runs directly on the crank and reduces thecost of rebuilding the engine. Aftermarket rods are absolutelymandatory in any modified engine. Nitriding the journals on thecranks is also a good idea because it makes them last infinitelylonger.

If you are going into this market, you needto understand that there is a steep and expensive learning curve.A flow bench and a dyno are essential, and a vertical mill isdesirable, too. We’ll assume you already have normal shop equipmentto hone cylinders, grind valves and valve seats, etc.

VKE made its own flow bench using its shopvacuum. You don’t need a $30,000 dyno, just an absorption unitlike the one Stuska makes, with a torque meter and a tach, plusa few small items like a weather station to get correction factorinformation.

VKE uses a flow meter for crankcase blowby,and a cylinder head temperature gauge. If you can get it, airflow and fuel flow information is helpful to compute brake specificair and fuel data, but it is not absolutely essential to have.The basic torque and rpm will tell you whether you are gainingor loosing with your modifications.

Steady state dyno testing is preferred as opposedto the acceleration approach. As far as VKE is concerned, allthe computerized stuff is needless expense. When you considerthat what you really want with a dyno or flow bench is a "beforeand after" comparison, the particulars of the equipment arenot that important as long as the technique to gather the informationis the same, and the temperature, humidity and barometer are carefullynoted.

With an air-cooled engine, cylinder head temperatureis an important variable to control. The more you do to gatherdata at the same cylinder head temperature the more accurate yourtests will be. And remember, don’t stand in front of the motorwhen it’s running, broken parts usually exit the case in thatdirection. Stay well off to the side!

There is nothing about these engines that isearth shattering or cutting edge technology. An orderly approachto modifications combined with attention to detail will resultin a product that’s up to the task at hand whether that be juniordragster, go kart, or even rototiller.

The impact on your bottom line depends on yourability to establish yourself in the market and convince the publicthat your product is competitive in performance, reliability andcost. It also depends on your ability to control your costs. Butthe increase in equipment utilization and shop space for buildingthese small, higher horsepower engines could be the differenceneeded to make the ink run black instead of red.

Thanks to VKE Motorsports, 3750 WheelingStreet, Denver, CO, 303-371-5427, which provided much of the informationused in this article.

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