Compression: How Much Is Too Much?
High compression is always good for more power and more torque, but as with all things, you do run into a point of diminishing returns. There are several factors that put a limit on benefits of compression. One that is often overlooked is combustion chamber shape,but some of you didn't see that coming! See, we can't just go with the "all things being equal, more compression will", because in order to get more compression, we do have to change the shape and volume of the combustion chamber. One of the things that is VERY important for power production, as well as detonation resistance, is combustion chamber turbulence.
Terms:
Squish Velocity -- the speed that gasses are pushed from the tight spaces between the flat periphery of the piston's surface and the flat areas of the cylinder head, towards the center of the combustion chamber as the piston approaches the top of the bore.
Tumble -- during the intake stroke, the rolling wave of incoming fuel/air mix flows across from the intake ports, across to the cylinder wall on the opposite side, and rolls over on itself like a wave as it reaches the top of the piston down in the bore. This is most commonly associated with cylinder heads with more than 2 valves.
Swirl -- intake motion in the manner of tumble, but instead it is more of a port angle that sends the incoming mixture in a circular motion down the bore, sort of like the downward threads of a very coarse screw. This is mostly associated with 2 valve heads.
Now, if there is a swirling/tumbling torrent of a mixture vortex at the point of spark ignition, that is sort of equivalent to a bunch of pieces of fuse being swirled and chopped in a blender. Each and every piece is going to be lighting a whole bunch of other pieces in a very short time, giving MUCH faster and more efficient burn.
Now, back to compression. A tighter/shorter/flatter combustion chamber doesn't leave a lot of space for all of the swirl/tumble/squish motion to survive without being suppressed, partly by a higher (thicker) viscosity of a more tightly compressed mixture, and partly because there just isn't enough space to freely continue with motion. And that's where compression meets one of its limits -- you need a tighter combustion chamber (with less space) to get higher compression. At some point you are making the burn more stagnant and inefficient in its burn rate.
Another point is that compression itself is a load on the engine. You need to use HP (in the form of crankshaft inertia) to compress the mixture before each combustion stroke. There will be a point (typically more pronounced at higher RPM) where the added load of compressing the charge is not compensated for by a better combustion.
Heat is yet another limiter. At some point heat from compression will not be effectively handled by the cooling system, and some of this extra heat will find its way into the intake port and pre-heat the intake charge, doing 2 undesirable things: a heated intake charge expands, so less fuel/air is taken in in the same space, so a less dense/less powerful charge, and not only will that expanded charge be less potent, but the higher temp will make it more prone to detonation.
So, after all that, what's the answer for your engine?
From all of my testing, the point of diminishing returns happens when you go above the 14s for compression. Sometimes you can see a few dyno runs where there is short term benefit above the 14s , but then heat load kicks in on a fully up-to-temp engine, and those gains are short-lived. Maybe OK for a drag race only setup -- one run then a cool down sort of riding -- but not good for sustained riding like GNCC\MX\Desert.
From my testing, you can get very good power from a properly designed pump gasser in the 12 comp range, and have good safety margin for high loads and not so good fuel or incorrect jetting. From there, in my opinion, the 13s are the "no man's land" for compression -- not always safe for pump, and not high enough to take full advantage of race fuel. So I either go poweful and safe with 12.25:1, or go to 14.25:1 for best use of high octane fuels.
Compression itself is not a liability. As long as you have enough octane and no detonation, you could literally run as high a comp as you want. Compression pressures will usually run in the 200s for psi, but combustion pressures will usually be between 1200 to 1400 psi for a well built/high-specific-output engine.
It's the pressure of combustion, that translates into the HP level of an engine, not the static rating of the piston. A 55hp pump gasser is seeing more pressure and stress than a 54hp high comp engine. If they were both 55hp, they would be seeing the same combustion pressures. The difference from 12:1 to 14:1 compression comes in at right about 1.5hp, as long as everything else is kept as equal as possible. So sometimes it just isn't worth people wanting to be married to a race fuel budget for 1.5HP, and that's understandable. 60hp pulls an almost identical holeshot to 61.5hp. The dyno picks that difference up more than the drag race ever will.
Still, some want all they can get out of everything they've got, so for them it's high comp and high octane.
Everyone has a goal, and a budget that moderates it. Hope that clears some questions, .
Venom Performance
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