1/14/2024 0 Comments Camshaft compression calculatorSince this speed is not constant throughout the engine’s rotation, piston speed is only an average. About 4,000 f/m is safe for good-quality cast pistons and up to 5,000 f/m for forgings, for peak power only - shifting, not cruising! Where “P” is piston speed in feet per minute, “S” is the stroke in inches, “R” is engine RPM, and 6 is a constant. The only 2 exceptions are 0° (TDC) where both strokes are zero, and 180° (BDC) where both strokes are equal & nominal.īy the way, the usual limit of engine speed based on the stress of the reciprocating components is not RPM, it’s piston speed. Two motors with the same stroke, but different rod lengths, will not have the piston in the same place at the same point of crankshaft rotation. The piston’s motion during crankshaft rotation is not symmetrical: the piston speed before & after TDC is faster than before & after BDC, but the difference is not constant - it varies with the rod ratio. Of intake valve closure partially depends on the motor’s rod ratio. The percentage of mixture captured, compressed, and burned in the cylinder at a specific point Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. 99% of all motors fall between these 2 extreme limits, with most standard production designs between 2.0:1 and 1.45:1.Īn “n” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design. The ratio of rod length to stroke length (usually represented by the symbol “n” ) is almost always between 2.1:1 on the “long” end, and 1.4:1 on the “short” end. The geometric relationship between the rod and the rod journal is one not generally understood, but which plays a key role in the motor’s breathing and overall power characteristics. The crankshaft’s connecting rod journal (or “throw” ) is offset from the main journal by exactly ½ the stroke length. This last factor is not fully appreciated by most mechanics. Intake valve closing point (generally the 2 nd number in the cam timing data, given in degrees after bottom dead center, or ABDC) Theoretical or mechanical compression ratio (advertised, static, or nominal), including corrections made for changes in the combustion chamber volume due to variations in piston compression distance, changes in piston dome shape such as valve reliefs and flame slots, chamber modifications such as larger valve installations, combustion chamber re-shaping, bore notches for valve relief, alternate gasket thickness, head and/or block milling, &c. This article discusses briefly how to roughly calculate how changes in eitherĬam timing, compression, or both affect low-speed torque and response. I think he shifts it around 5400-5600.The experienced engine builder (Mopar, Harley-Davidson, or Ferrari) knows that the “advertised” compression ratio (suggested by the owner’s manual, piston manufacturer, &c.) is only one of several factors involved in determining how much pressure is developed in the combustion chamber. The DCR is way low on that motor, but it still works well enough. He says the cam is about 4 degrees advanced. My dad's car runs a kb107 at a true 8.95:1, and it works well. That means that you aren't going to have much of a top end, so maybe it would be better to drop the compression back a little more and advance the cam to fatten up the midrange. Are your heads going to be ported? The 107 seems to work well with unported heads with a good valve job- it works by keeping the velocity up. You have to consider what your combination is. That is only a guess on my part, but the DCR would be around 7, which is on the low side. I would think you could get away with 10.5:1 with iron heads on pump gas with the 107, if you run it straight up, with everything else just right- quench, good cooling system, timing and jetting dead on. Everybody here said it wasn't a good idea. If you use the DCR calculator, you can theoretically get away with 12:1 compression with this cam. I was thinking about running a similar type of cam (poston 112- same as 107 on 112 center). I've been trying to get a straight answer to this question too.
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