This is an engine that I had for sale that has just been fitted into a Dyna. A mild, mid range unstressed build that was designed to be tuned easily and multi purpose from traffic light drag racing to riding long motorway distances. It turned out very well - 129 ft/lb of torque and 126 HP ...
It's misleading in Harley performance terms to quote maximum HP. The reason for this is that the maximum HP occurs at a point in the RPM that is hardly ever used. Take the graph above for example - 126 HP is only realised at 5830 RPM so it's essentially a pointless measurement. Nobody ever rides at that RPM. My priority is torque production when I'm doing heads or designing a build. Torque is far more important as it's torque that you can feel when you accelerate. Having 129 ft/lbs of torque there at 4300 RPM as this Dyna has is perfect for snapping the throttle open at 70 mph and disappearing into the distance ...
Alex whats the max compression on that "mild unstressed build" im still looking at doing somit to mine poss in Autumn now (96 to 110 ci) Just interested in what compression increase is considered not over stressing the engine ? Or wether to go standard compression to save any probs with crank/ general ware & tear etc. cheers o/
There's no 'max compression', it doesn't vary. The static compression on this one is 10.6:1 and the Wood 777 cam profile has an intake closing point of 42 degrees after bottom dead centre which gives a corrected compression of 9.67:1.
It's more riding habits that contribute to over stressing the engine such as lugging. This one has a welded crank so it won't shift no matter how much it's abused. Static compression ratios don't mean much as the corrected compression changes depending on which cam is used. Cranking pressure over say 215 psi will demand better engine planning regarding cylinder head work, fuel and timing mapping and some kind of decompression release mechanism to aid hot starts. Piston speed has it's limitations with longer stroke engines but a mid range build will never get near any problems in that area.
Again it depends on how it's ridden. Stock engines are low compression and that's why they produce low torque and power levels. Compression is power. Your stock 96 is only about 9:1 static compression but because the stock cam closes the intake valve at 30 degrees after bottom dead centre the corrected compression is around 8.68:1. This gives a cranking pressure of about 176 psi. The only way to get more torque out of an otherwise stock engine is to fit a cam with an earlier intake closing point than 30 degrees. Anything later will bleed off pressure and decrease performance. High cranking pressure and high RPM engines demand better than OEM quality for some components such as lifters which get a particularly hard life. Crankshafts are hit and miss - you never know if they're going to slip even in a stock output engine ...
Ahh right, yes by max comp I did mean corrected sorry. Good info so my 96 ci is around 8.68.1 when it?s running and 9.1 cranking (static).
Thanks for the info once agin. I?ll get there one day.
No - The static compression is simply a calculated relationship between the volume of the cylinder with the piston at bottom dead centre and the volume of the combustion chamber. This is very different from cranking pressure which is the pressure developed in psi from the point where the intake valve closes to the point where the piston stops at top dead centre on the compression stroke. The static compression is fixed as long as the cylinder dimensions, crankshaft stroke, piston dome and the combustion chamber volume remain the same. If you did nothing but fit a Feuling 594 cam in your current engine the cranking pressure would drop to about 150 psi but the static compression ratio would remain the same.
