Macgyver 90 #1 Posted March 22, 2018 I have built several P 216 & 220 motors over a period of time and one thing I would like to know has anyone thought about raising the compression after all 6.5 & 7.0 is kind of lame. I have a spare set of heads and thought about giving them a haircut. What are your thoughts.o Share this post Link to post Share on other sites
lynnmor 6,762 #2 Posted March 22, 2018 A flat head air cooled is more prone to knocking than any other design. If you give it a try, the use of higher octane fuel should then be used. Fuel efficiency and power should increase, but engine life will decrease. 1 Share this post Link to post Share on other sites
Macgyver 90 #3 Posted March 23, 2018 Why do you think engine life will decrease? I suppose that depends on how hard you beat the engine and not take care of it. I took about .020 of the heads. Share this post Link to post Share on other sites
953 nut 51,536 #4 Posted March 23, 2018 3 hours ago, Macgyver said: Why do you think engine life will decrease? I suppose that depends on how hard you beat the engine and not take care of it. I took about .020 of the heads. As a kid we always wanted a little more than the factory had given us so having the head shaved (or replaced with racing heads) was the least expensive and easiest performance enhancement available. Theoretically it improved the performance, but it wasn't very noticeable unless it was complemented by other power equipment. How much of an increase in compression ratio do you feel this .020" will give you? Share this post Link to post Share on other sites
Macgyver 90 #5 Posted March 24, 2018 I figure that the compression should be between 8 to 9 to 1 any would be an improvement over 7 to 1 also hoping for better combustion with less carbon buildup. These are good motors well built but only 20hp from a 47cid is not very Impressive by my thinking. 1 Share this post Link to post Share on other sites
953 nut 51,536 #6 Posted March 24, 2018 19 minutes ago, Macgyver said: figure that the compression should be between 8 to 9 to 1 Hate to be the bearer of bad news, but that isn't going to happen with shaving .020" off the heads. The 16 HP has a stroke of 2.620 and CR of 6.5 to !. The 18 HP has the same bore and stroke of 2.875 resulting in a CR of 7 to 1. You might get all the way up to 6.55 to 1 with the shaved head. 1 Share this post Link to post Share on other sites
oldredrider 2,547 #7 Posted March 24, 2018 This is just my opinion. While I am not a fan of Onans in garden tractors, they do have their place. They are great, long lasting engines if utilized as stationary units. They were built for power and longevity. Proof in the fact there are some old Onans still kicking, running generators, log splitters etc. Raising the compression ratio would probably increase horse power but decrease longevity due to the internal stress on rods, cranks etc. It is what it is. I had heard a while back that some of the 20hp Onans actually dynoed out at 28hp. Not too shabby. If you recall several years ago, many engine manufacturers were sued because listed horse power did not match actual horse power. Onan was NOT included in the lawsuit because they actually produced more horse power than their stated rating. If you want to pull with the motor with shaved heads, fine. I wouldn't expect a long life out of it. Share this post Link to post Share on other sites
Macgyver 90 #8 Posted March 24, 2018 That's interesting about the 20hp actually putting out more than 20 hp. The engine I'm working on is a 20 so the starting compression is 7 to 1 if I get to 8+ that's fine with me. This motor is just a backup that can go in my 520H or my Sears FF20 Share this post Link to post Share on other sites
DennisThornton 4,769 #9 Posted March 24, 2018 8 hours ago, Macgyver said: That's interesting about the 20hp actually putting out more than 20 hp. The engine I'm working on is a 20 so the starting compression is 7 to 1 if I get to 8+ that's fine with me. This motor is just a backup that can go in my 520H or my Sears FF20 You got a FF20! What do you think of it? Share this post Link to post Share on other sites
Macgyver 90 #10 Posted March 24, 2018 The FF 20 is an impressive tractor it's about the size of a D 200. What attracted me to it and I use it is it has the original Sears/ Kwik Way loader. I'm in the Midwest a few miles south of Rockford IL. I will take it to AJs show in May and the big 4 day show at Portage Wi in July. Share this post Link to post Share on other sites
pfrederi 17,129 #11 Posted March 24, 2018 Horsepower ratings in small engines are, shall we say"Highly Variable" . Depends who is rating them. My Mule has a 2 cylinder Overhead valve Hercules air cooled engine 42 Cubic inch. it is rated a 14hp Share this post Link to post Share on other sites
lynnmor 6,762 #12 Posted March 24, 2018 On 3/23/2018 at 9:23 AM, Macgyver said: Why do you think engine life will decrease? I suppose that depends on how hard you beat the engine and not take care of it. I took about .020 of the heads. Increasing power simply means that pressures increase. More pressure on pistons, wrist pins, rod bearings and main bearings will obviously increase wear and stress those parts more. It is all a balancing act and what one is willing to accept. It might be that increasing power a reasonable amount will not impact the useful life of the engine for a particular person, but then one might find the limits in a bad way. Share this post Link to post Share on other sites