suped-up briggs and stratton?

[hondahaulic]

Hey, has anybody out there ever had any experience with seriously modding an older briggs and straton small engine? The reason I ask is because we are currently working on small engines in an automotive class at highschool and me and a couple buddies decided that our 3 horse b&s needed hopped up a little bit lol.
So far, we have built up the lobes on the cam with weld and ground them smooth until we were satisfied with the new valve lift. Its a push rod style motor with the cam in the crank case, so now the valves hit the cylinder head and we are in the process of milling 2 pockets in the head to allow the needed clearance for the valves.
We are planing on doing a little portwork and polishing on the intake/exhaust/and head. Were probably gonna try to use a carb from a 5h.p. briggs to give it a little more fuel, and I am seriously thinking about trying to make our own stroker crank for it. I know we can get another 1/2" of stroke out of the cylinder, and even more if I dont use the bottom ring. We might have to do something about the compression now, because the pockets in the head will probably drop the compression a ways. Maybe build up a little aluminum weld on the piston or head? We also have a metal lathe and can lighten the flywheel, but it might take a little reasearch to figure out how much weight to take off. Our plan is to give this little motor as much ridiculous power as possible(for its size) without spending any money, oh and reliability is no issue either lol, heck i'd be satisfied if it ran like a raped ape for 5 minutes and blew to pieces!

[200x newby]

you did hard weld the cam right? if not harden then it will wear like no other and if you take out the bottom ring thats your oil ring it will smoke like no other also.

[Tri-ZNate]

Flat head correct? If so I'd just Stroke it and make sure you have no defined edges around your Ports. If its a domed head and you have someone who can Tig Exceptionally well have them Tig on the piston and chuck it up onto the lathe to make it a high comp Piston.

[hondahaulic]

Our shop does have the tools to heat-treat and harden the cam if it wears too quickly, last year we made center punches on a lathe and heat treated them. The engine probably won't be run alot, its basically just gonna be a guinea pig for us to mess with so we might not even really have to worry about it wearing.
Oh and for the bottom ring, I was under the impression that the cylinder would be lubricated less without it, and we could make it work by possibly using mixed gas.... maybe 50:1 at the richest? You might be right though, it might just smoke un-controlably.

[ProCarbine2k1]

I dont know much about the older briggs, but was told that a newer briggs 5.0 could be pushed out to 45 hp. Havent seen this in person, but heard it from a guy who used to race karts.

[hondahaulic]

45hp!!! thats crazy! wow. Hey by chance that we do get this motor running good with our mods, is there any easy way to figure out what it has for power? I dont really know alot about dynoing, and actually im kind of a noob to building up an engine anyway if you couldn't tell lol.

[Bryan Raffa]

check out some kart sites.. they make alum. blocks and everything for them..
I had a built one on my gokart.. Dad put it on a rototiller when I was through with karts ... should have seen him try and rototill the garden...ha 12" strait pipe and all..

check this out http://www.briggsracing.com/display/router.asp?docid=101376

[GoodKarma]

I've seen B&S motors with cams and alcohol carbs, and they will flat out get at it! I used to race karts on a 10th mile dirt track, and have seen some crazy stuff with these motors.

[hondahaulic]

hey, this might sound kinda crazy but do you guys think i can cut and weld on the crankshaft in order to get some more stroke? I figure either....

A: i won't be able top get good, solid welds on the crank
B: i wont be able to keep everything perfectly lined up and it might be "tweaked" a little bit
C: if i get good welds, brace it well, and everything stays straight when its all welded together it just might work.


i drew up a quick pic. in paint to kind of show what i mean....

http://i50.photobucket.com/albums/f344/mrswanson/crnkshft.jpg

[RedRider_AK]

Holy crap, another 1/4" of stroke... LOL.

[storm_impulse]

i think if u weld on that crankshaft u might unbalence it causing it to wobble wearing out the rod bearings. just my .02

[Bryan Raffa]

better off makeing the rod longer

[icp4life162005]

yeah but he said he didn't caRE ABOUT CATASTROPHIC DESTRUCTION OF THEN ENGINE.

[KASEY]

better off makeing the rod longer


BUT LIKE A 85 250R AND A 89 250R WITH DIFFERENT LENGTH RODS IT WILL NOT GAIN ANY CC'S WITH A LONGER ROD,,,,,,,DOH

[hondahaulic]

with another 1/4" on the crankshaft it will give me another 1/2" or stroke. The piston will now go 1/4" higher and a 1/4" lower (a total of 1/2"), but i will have to shorten the rod 1/4" so the piston doesnt hit the head (flat top head). If it gets unbalanced too much, it probably will wear out the rod in a short amount of time but im hoping that the brace welded in before i cut the crank will keep everything inline while i re-weld it.

[hondahaulic]

oh and does anyone know for sure that removing the bottom ring will cause it to smoke like no other? It makes sence but if i can leave it out and get a little more stroke i definately want to do it. I rememeber my dad mentioning not being able to use all of the piston rings on one of his extremely built-up pulling tractors he made back in the 80's. (it was bored and stroked to the max along with a long list of other things)

[hondahaulic]

i cant remember what the bore of the motor is, but its somewhere between 2 1/4-3 i think.... and if its only 2 1/4, i can get about another 24.6cc's from the motor (if my guesstimations are correct, im sure their a little off). right now im thinkin just go for it and see what happens. I have all kinds of parts for this engine, and so does the school so if it blows up or ruins something its no big deal.... i can just figure out why it failed, grab the parts i need, and go from there!

[RedRider_AK]

Bottom ring is your oil ring. Yes it will "smoke like no other", I've seen machines with slightly worn oil rings "smoke like no other". I can just imagine what the absence of it altogether would do.

[KASEY]

to build a stroker crank you would weld up the journal and turn it down at a further distance from center,,, not cutting the crank,,, you will never weld a cast iron briggs crank and get it straight or hold ,, i repeat never,,, and cutting and welding the rod???? never,,,, you shoould check into junior dragsters they hop those engines up alot,,, and if you look at a briggs piston,,, it has a smaller diameter where the rings are,, so if all the skirt is out of the bore what keeps it straight ? and on top of that,,, you shorten the rod and have to take some of the skirt off too................ :crazy: :crazy: :crazy:

[RedRider_AK]

Yeah, I'd just see what you can order from Jegs or something, I know they have a dedicated B&S engine hop-up section.

Another idea, hook a fan up to the intake... Supercharging, anyone?!

[hondahaulic]

yea i wasnt quite sure if welding onto the cast crank would work very well... oh, and i did check out the piston today and KASEY, you right about it being narrower on top. Looks like im gonna be best off leaving the stroke alone.

[hondahaulic]

well just a little update, we got the cam all welded up so the valve lifters nearly bottom out on the valve lifter guides and the cam has a longer duration too. We milled out pockets in the head so the valves wouldn't hit, and we also did quite a bit of grinding on the crankshaft so the cam lobes didn't hit it as the cam spun.
We also had to mill down a couple places on the crankcase cover, because we had to use permatex instead of a gasket (just because there wasn't a new gasket available) and the thickness of the gasket had to be made up for, otherwise it squeezed the cam and crank too tight.
Once we got everthing all together, it fired right up and ran beautifully. I have no way to tell how much of a power increase there was, but it definately sounds like a healthy running engine and it starts with 1-2 pulls.

When we put the piston in, we didnt put the oiler ring in. This was partially because we couldnt find it, but mostly because I wanted to see if the engine would smoke bad if it wasn't used. As I expected there was virtually no smoke or noticeable oil burning, even with the used compression rings and un-touched origional bore. I'm sure if it was ran hard for long periods of time it would slowly burn oil, but I couldn't notice anything for the time it ran.

[cr480r]

i think if u weld on that crankshaft u might unbalence it causing it to wobble wearing out the rod bearings. just my .02

What rod bearing?