How much compression can You run and still use pump gas?

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From the pages of Bad-Ass Race Engines

This is a good one with no single answer. The key here is cylinder pressure, not compression! Cylinder pressure is controlled by valve timing (overlap), not the piston. Although bore, stroke, compression height, deck height and combustion chamber size, among others, have a great deal to do with the whole ball of wax, they don't dictate the actual working or "effective compression" or cylinder pressure of an engine. Example: two identical engines both with 9:1 "static compression". The only difference is in the cam profiles. One has no overlap with particular valve timing on the intake valve, and the other has a lot of overlap with a completely different intake valve timing. Overlap is the time in which both valves are open as the piston pushes exhaust out and starts to suck new fuel and air into the cylinder. When the exhaust valve is closing and the intake valve begins to open, there is a time (on high performance and race cams) where both valves are actually open at the same time. A piston can not make cylinder pressure if a valve (or valves) are open, whether individually or at the same time. As the piston goes back down and starts to suck-in a new charge of fuel and air into the cylinder, it is also still sucking exhaust back in, as long as that exhaust valve is still open. Some of the air and fuel (charge) is being pushed back out either of the open valves, thus creating that "rumpity bump" sound everyone likes so much, but by letting some of the charge escape, there is less there to suck-in and/or squeeze, thus creating less pressure. Overlap and valve timing on performance cams also decrease the velocity of air feeding the engine at lower RPM's, which is why cams "come alive" at higher RPMs. Overlap also causes a decrease in manifold vacuum, making throttle response sluggish. When you have too much overlap, you end-up with a dog for an engine that has no throttle response. This is what's known as being over-cammed. Lots of engines out there are over cammed. This is why they sound good to some people, but are complete turds off the line or when they hit the quarter mile.

If you did a compression test on either of these two 9:1 static compression engines, the engine with no overlap (or even negative overlap) would probably have about 140 -150psi or so in the cylinders. The engine with more overlap may only have 100 psi or so, depending on how much overlap the cam has, how narrow the lobe separation angle is, and what the valve timing is (when the intake valve opens and closes, and when the exhaust valve opens and closes). Some racing engines with 13:1 or so compression only have 125psi or so of cylinder pressure when a compression test is done. That's less than what grama's old beat-up car with over 100k miles on it is supposed to have! It just means that the cam has a ton of overlap and has valve timing designed to work on a "scavenging" princible, which actually increases cylinder pressure at higher RPMs. This is why race engines have "power bands" and come alive at higher RPM's. It's because the cylinder pressure increased as the engine came-up in RPM, which resulted in more horsepower. So, you can run 92 octane pump gas on 11:1 compression with the right cam but 9:1 could ping (detonate) with the wrong cam. Way back in the early 80's, Crower had a fuel economy kit that was designed to run on pump gas (for gas mileage) yet had a compression ratio of 14:1!! Really! It's all in the cam profile. It worked but never caught-on so they dumped the idea. Bottom line, cylinder pressure, not compression dictates what kind of fuel you'll need to run.

On another note, consider this: There are crap loads of older 68 & 69 302 Z/28's that came stock with 11:1 compression. The 63 and 64 365HP and 375HP 327 Vette's came stock with 11.25:1 compression. Shelby's, Cobra's, big block Chevelle's, 340 and 440 Six Pack and "magnum" Chrysler's came with everything from 10:1 to over 11:1 depending on the options. BOSS 302's and BOSS 429's, the 70 351-4V Mach 1's and MANY, MANY more cars ALL came stock with 10:1 to over 12:1 compression on some limited models and they all ran premium pump gas. Many of these cars are still running today on today's fuels. Just so ya know, brand new Ferrari's, Lamborghini's, Non turbo Porsche's, M3 and M5 BMW's and so on all come stock with 11:1 compression and some even slightly higher. With the advent of computers and knock sensors, the timing can be controlled and detonation (pinging) can be kept at bay. Ignition timing also helps control cylinder pressure. Eitheer way, whether it's the cam timing or the ignition timing, the bottom line is, it ain't the "compression ratio" that dictates whether you can or can't run pump gas, it's how much cylinder pressure the complete package makes as to whether you can or can't run pump gas.