Crazy rototiller engine (Briggs Flathead) HP estimate??

Thomas1971

Member
So a few years ago I was scrapping a 70’s Sears rototiller that had a good looking Briggs flathead 5hp on it so I kept the engine. I did some research on the engine and it turns out it’s from 1974. I also found that it is basically a Briggs Raptor II engine. But this specific model has a governor, a pull choke carb, a points style ignition system (being a 74) and a 3/4” by 1 1/8” keyed pto from the rototiller application. But last year I decided I was going to get it running but add a “few” performance parts...so here is what I did...

Performance block work;
I ported and polished the intake and exhaust alittle, not to crazy. I shaved the eye brows just alittle to give it better flow. I tapped and plugged the Governor hole.

Performance parts;
94ss camshaft
ARC billet 3.875 rod
ARC billet stock length lifters
20lbs valve springs
Heat treated lower spring retainers
ARC billet flywheel
22mm mikuni carb+custom intake manifold
Robertson torque tubes header

Non-performance block work;
I honed the cylinder for a fresh cross hatch, I installed a new exhaust valve guide, cut the valve seats, lapped in new valves, and plugged the points plunger hole with a press fit plug (Briggs part).

Non-performance parts;
New OEM crankshaft (correct pto)
New ring set
New valves
Upper spring retainers
New coil (non-points)
Burris Racing ball bearing side cover
PMI breather assembly
All new seals and gaskets

So that’s basically all I did to it. If any one that dyno’s engines can give me an estimate of how much horse power this engine makes (@6000-7000RPM on gas) i’d like to hear your guesses. I don’t have access to a small engine dyno so... yeah..
 
Yea, I'd guess in the 8-10HP range, depending on dyno and type. Dynos are relative anyway...

You really did no HP upgrades except the cam and carb. Everything else didn't add anything to the power output really. Switch it over to methanol, add a Tillotson carb and fuel pump and put a long rod and wiseco piston in there and you'll be making a little more.
 
I know not every part I put in the engine is going to add horse power. Almost all the parts I put in didn’t help with horsepower at all. I added them for the durability and reliability of the engine.
 
No I didn’t weld the lifter boss. I didn’t really see a need with this specific cam. If it were a more aggressive cam I would of.
 
I'm thinking you may be a lil light on the springs,maybe up around 28lbs. or you might float the valves before you reach 6 or 7000rpms.
 
The springs recommended for the cam were the 20lbs springs. I don’t seem to get any valve float. I’ve never opened it up all the way yet but I’ve gotten to 5800ish rpm. I thought they sounded light for that rpm range too but so far so good.
 
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