OutdoorKing Repair Forum Forums Restoring, Customising and Modifying Tech Talk Modifing B&S motor

Has anyone modified a briggs and stratton ie for a gokart how did you do it and what was the result? I've done it and had some good fun. Thought it may make an interesting post

I dont own one as such but I helped with the building of a 5hp briggs junior drag kart motor.

About the only original bit left on it was the sticker on thwe cowling. had the following done to it...

-used the points and had a high voltage transformer type ignition coil
-Shaved the head as far as we could without the valves running into the head and polished the chamber
- ran a crankshaft from a much bigger engine (i honestly cant remember what) had to grinf the inside of the crankcae out and tig new covers over the sides to compensate for the stroke.
- Ran a much larger piston (pretty sure it was from the same engine as the crank) had to have a custom billet conrod made
- Big mikuni dirt bike carb
- Large (for engine size) exhaust with a little hotdog in it.


Now power figures may never be known, I stopped talking to the guy at the end of the build due to personal differences, however during testing the thing was absolutely off the chain.

If you are trying to produce a reasonably powerful small engine at low cost, any side-valve model is an odd choice. They inherently have low efficiency and low horsepower per litre, and with the best possible modifications will lag well behind a modestly tuned overhead valve engine. In most cases an overhead camshaft engine will do even better because it can achieve higher speeds.

If you particularly want to work on a side valve engine for some reason (for example, because the racing formula you intend to race under requires it) then some things can be done, and they may go further than Joe's example did, but notice that Joe's engine had to have its awful aluminium connecting rod replaced in order to operate at higher crankshaft speeds. A billet con rod (that is, one milled out of a solid billet of metal, normally structural steel) is a very expensive item. If your rules just require you to use a lawnmower engine, you could get a great deal more power by using, say, a Kawasaki or Suzuki 2-stroke engine as the starting point. If it has to be a 4 stroke, an OHV or OHC Honda seems a lot more promising as a starting point than a side valve engine of any kind.

If you insist on using a side valve engine, and want to make it as cheaply as possible from common parts, a B&S is quite a good starting point since the parts are commonplace. The main problems you have to address then are poor breathing, low compression ratio, and weak connecting rod. You can improve the breathing somewhat by eliminating the air filter and muffler, and increasing the size of the intake port and intake valve. The larger valve will require machining work, to fit the valve and valve seat from a larger Briggs engine. The camshaft lobes will need to be altered to increase the duration of lift of both intake and exhaust valves. This can be done by hand with great difficulty, or by a hot-rod shop more easily. The compression ratio on side valve engines used to be maximized (about 8.5:1 is as high as you can go even with a lot of modification) by milling away the bottom surface of the head until the combustion chamber above the piston was non-existent, just a flat machined surface, and allowing for the valves to clear the head by end-milling a recess above each valve, the same diameter as the valve head. Watch that the spark plug electrode is not hit by the piston. If you want the engine to run in a narrow speed range, you can tune the exhaust pipe length, but you will lose as much power at other speeds as you gain at the tuned speed.

The weak connecting rod is a problem, since a stronger one would be ridiculously expensive. The solution that seems to be used by some of the B&S mower racing enthusiasts is to make some minor modifications to improve the high speed lubrication, minimise the piston weight by removing unnecessary metal, then expect that the rod will last one or two races at a maximum speed of 6,000 rpm for a very short time. Of course you replace the rod after each race.

To get into the right class for drag racing we had to run a "5hp briggs" there was no specific ruling on the modifications allowed, aftermarket rods are available made from billet alloy but they do pull a premium.

Ok so our engine wasnt exactly a 5hp briggs anymore but it did use the original block

If they allow things like increased capacity, they most likely allow modified camshafts and oversized inlet valves Joe, and I think you could probably have gone a lot further with the compression ratio increase. I did it with a car engine when I was a kid, and to get to 8:6 to one I had to go to a nominal 0.025" clearance over the piston, to cover carbon accumulation on both the head and the top of the piston. (That was about the thickness of the head gasket, in other words). You could run a higher compression ratio than that as far as the fuel is concerned (it was still 98 octane leaded at the time) but unfortunately you can't get it with side-valve geometry, you run out of places to put the valves when they are open.

Quite a lot can be done with camshaft modification. You have to grind down the base circle of the cams, then grind away the top of the lobes to return to the same valve lift. The outcome is considerably more duration of the open periods. You have to polish the lobes very carefully then get them flash-chromed in most cases because you've ground away the hardened surface. The result can be a bigger power increase than you get from the compression ratio increase, but to go that far you have to destroy the low speed torque, and have quite a high idle speed.

I haven't seen the detail of the modifications you make to the oil slinger and the sides of the rod to get the original rod to go to 6,000 rpm and live, at least for perhaps a minute or so, but if the racing guys are telling the truth they are achieving it. Keeping the piston weight down is crucial though, since overspeeded rods normally fail in tension and the tensile load is caused by the weight of the piston and the rod itself. My old car engine had pressure lubrication and despite the very spindly rods it had, reached 6,500 rpm several times for brief moments. Each time it did that it left chatter marks on the head where the bouncing valves had been hitting it.

You would be surprised at the power increase and how robust the small motors are, there is a gokart racing and a drag racing class that uses these side valve motors and they happily rev to 6000 rpm. Check out the link http://www.aviator.cc/briggs.txt
I followed most of this guide and got great results.
I did the following
1.ported block and flowed the valves
2.flowed the carb
3.milled the head and block (a lot)
4.straight through exhaust
5.synthetic oil
6.advanced timing
7.high flow air cleaner
8.one shot nitrous
and another link
http://www.hotrod.com/newsletter/hrdp_0308_racing_engine_jr_drag_racing_league/

Thanks PMB, I left out the ignition timing, which is one of the most important changes. It needs something close to 30 degrees BTDC ignition timing to give maximum power. That is a critical variable: power increases fairly quickly as you increase the advance, until you reach the limit, then high speed detonation breaks your piston almost instantly.

Interesting read.

Im just wondering how a few of those steps would work on an old 13ci IC motor on a mower. lol.


A hotted up colt 5 with 8+hp @6000rpm. lol.



Bob.

I have just finished restoring my colt 5 and when I had the valves out I could help but clean up the ports and flow the carb but it developed a flat spot mid range so I had to reverse the carb bit.
I would like to go further but I think the gear box won't it.
Does any know if the colt 8 uses the same box?

First, I think there is more than one Colt 5 and more than one Colt 8. So far as the ones in the manuals section are concerned, they both have the same single speed gearbox.

You may have messed up the carburetor when you modified it. It sounds as if you may have enlarged the venturi, which is always a bad idea, because it reduces the suction applied to the fuel discharge nozzle and therefore makes the mixture leaner. It does this in a non-linear fashion versus speed. If you want a higher-flow carburetor, use a bigger one, don't bore one out. Also, port work or valve work on a properly developed engine just about always reduces the low and mid speed torque more than it boosts high speed torque. It is useful when you are going to use the engine for performance work at high speeds only, or attach it to a lighter vehicle, but it just doesn't make any sense when you are going to leave it on the same lawnmower.

Your right about the venturi but if you look down the carb at the throttle butterfly you will notice it never fully opens and there is a large screw holding it in place. All you need to do is cut the throttle stop so it opens all the way and cut the excess tread off the screw. Some carbs have a swirl in the intake manifold that can be removed. Note I cannot guarantee a perfect running motor through all rev ranges but works well in a kart as most the time your full throttle.

I agree PMB, when you only need it to have serious torque when it's at high speed, there are many useful things to do that may not have any important disadvantages. You sometmes do need to watch the detail, though - I can remember a carburetor with a protruding lump built onto its interior because it caused a turbulent wake area over one of the discharge orifices just at one speed, and that cured a troublesome flat spot at one fairly low engine speed.

Some racing classes do not allow gearboxes or clutches, so the engines may need some torque at a fairly low speed to be able to get around the corners and still accelerate out of them. I can even remember in the early 1950s a British firm called BRM made a really clever formula one engine that turned out to be useless. The formula at the time allowed 1.5 litres supercharged or 3 litres naturally aspirated, and BRM calculated that they could get much, much more power from a very highly blown 1.5 litre than from any 3 litre that was feasible at the time. In practice they got 535 hp, and no one could get much more than 300 from the 3 litre naturally aspirated at the time, so it looked very clever. The downside was that to get the 535 hp they had to use a three stage centrifugal supercharger. The pressure developed by centrifugal superchargers is approximately proportional to the cube of the speed (volume is proportional to the square of the speed), so the engine had a power curve shaped like the top of the Matterhorn. The best available drivers would go around the first half of a corner bogged down, then the engine speed would climb just 100-200 rpm and they'd go around the second half in an uncontrollable tail-slide. Scratch one good idea.

The 5 and 8 used the same Fwd/Rev box.

Some colts had a 3 speed box, i doubt any colt 5's had it.
Would be a good option.


Im sure it would not be hard to adapt a 5 or 6 speed on either.


Im sure the Fwd/Rev box would handle 12 or more HP.



Im thinking that a hotted up 5hp would be pretty useless for mowing grass anyway. The more it revs the faster it goes, the more torque it needs to keep things spinning, spinning at a faster speed than any of it was designed to do.