Originally Posted by automotive breath
Nice work, did you get it running? Results?
Oh yea, I've had it running for most of the second half of this summer (it turns out it had other problems that caused it to blow the headgasket besides the obvious problem with the head, also realted to crappy assembly/machining practices which I fixed also).
Right now I have the best running "6.75 peak torque" Briggs L head powered lawnmower that I've ever seen. It was obviously different from the first start, where the governor couldn't keep the rpm down to anything that I felt was reasonable and had to bend the spring bracket to lighten the tension on the governor spring to keep the speed down.
It starts first pull every time, it runs much more smoothly than it did before, has much more power and I get more work done from a tank of gas.
To give an example, now I can run it full steam into overgrown (and I'm mean overgrown, the first run was after the mower was down a few weeks and it was 8-10" high and drizzling at the time) wet grass at it's fastest self propelled rate without it bogging down, what used to take 1:20-1:30 to do I typically get done in :45-:55min and before it took about a tank and a half of gas, now it takes 2/3-3/4 of a tank.
Honestly, the improvement overall is so dramatic that I'm really giving numbers on the conservative end of what I'm seeing, I'm not sure that I'd believe what I'm seeing if I wasn't tinkering with it myself, but part of the deal here is that those engines run all the time in the RPM range that these changes make a big difference in. The nice thing with doing it on a lawnmower is that I can make bigger changes faster on it (it takes only a few minutes to pull the head... there's only one cylinder...) so it's much easier to experiment with and figure out what works and doesn't quickly and get actual numbers (I measured, cc'ed, IR temp gunned everything which could have taken days on a car engine if this was my full time job, and weeks or more "just tinkering"). This lawnmower engine (as much as I absolutely hate them) taught me more about this stuff than messing with a dozen or more car cylinder heads.
I'm actually tempted to get another head for it and try to take this all a step further, since I'm THRILLED with the results and what I've learned from it, and now I'm thinking about what I could have done differently and how to test that/wonder how much gain there still is to be found. If for no other reason because I've found some much better inserts for the cutting head that I used to mill the head with that I want to play with (OTOH, I just used it to modify the supercharger for my brother's car...)