Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Relieving


Post Fri Sep 12, 2008 3:19 pm

Posts: 1
I concidder relieving the cylinders on my 1937 U. But what will I gain?

Not chasing 1/10 of seconds on the race track, just everyday useable torque to haul the bike up-hill against the wind with passeger and two weeks hollyday luggage at 70 mph.

Will relieving do the trick, will it be worth the work?

Old Eagle

Post Sat Sep 13, 2008 8:11 am

Relieving isn't going to do anything for torque, which almost entire a function of compression and displacement in mid-range. Breathing only helps torque when the engine is spinning relatively fast.
However: most cylinders have the seats recessed too far, and a light radius from the leading side of the cylinder surface blended into the factory relief cut is helpful, and only takes a few minutes. The exhaust is even more important - the head and cylinder temps go up when the exhaust valve is shrouded.

Post Wed Sep 24, 2008 8:06 pm

Posts: 767
Location: Pa. , USA
Just remember old eagle what you take off you can't replace.
Vintage roadracing, Class C, AHRMA # 335

Post Sat Sep 27, 2008 4:19 am

Posts: 103
Location: Switzerland

Relieving will reduce the compression if you don't do anything else to increase compression. Reduced compression results in reduced torque. Relieving helps more in high RPM. As described by Panic, rounding off sharp edges will help avoiding hot-spots in the combustion chamber and will reduce pinging when you climb a hill.

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