It answers your question in detail,

Q: How much compression can I run and still use pump gas?

A: 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 compression or cylinder pressure of an engine. Example: two identical engines with 9:1 compression. The only difference is in the cam profiles. One has no overlap and the other has a lot. Overlap is the time in which both valves are open. 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. As the piston comes up and starts to squeeze the air and fuel in the cylinder, some of the air and fuel (charge) is being pushed out 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 squeeze thus creating less pressure. It also decreases the velocity of air feeding the engine as well as decreases manifold vacuum. 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.

If you did a compression test on these two engines, the engine with no overlap would probably have about 140 -150psi or so in the cylinders. The engine with the overlap may only have 100 psi or so, depending on how much overlap the cam has. Some racing engines with 13:1 compression only have 125psi or so of cylinder pressure when a compression test is done. It just means that the cam has a ton of overlap. Once the engine gets spinning, the cylinder pressure will greatly increase and this is why hot cams "come-in" or "start breathing" at higher RPM's. So, you can run 92 octane pump gas on 11:1 compression with the right cam but 10:1 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.

SteveW

p.s. The common rule is 10-1 w/iron heads and 10.5-1 w/aluminum heads. But the cam is the key.

Tightening the lobe seperation increases the cranking (effective) compression. The piston can only compress the air it traps in the cylinder. Long durations and wide lobe seperations push the intake closing event into the "compression" stroke. If air is escaping out the intake valve as the piston travels upward, it's not getting compressed, and cylinder pressure goes out the window! Narrowing the lobe seperation helps build more compression by making the intake valve close sooner, enabling the compression stroke to capture and compress more air. Advancing the camshaft will do this as well.

Some people mistakenly believe that compression is lost during the OVERLAP period... WRONG! Overlap occurs when the exhaust valve is closing and the intake is just starting to open... Nothing is being compressed yet! NARROWING the lobe seperation pulls the intake closing event out of the compression stroke. Advancing the cam will make the intake valve close sooner as well.

Here's another site with some lobe seperation info.

http://www.fordmuscle.com/archives/2001/04/camtech/

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

I won't dispute what you said, but I'll invite Arron to explain if you'd like. He's a little busy 'cause he's building 15 race engines a week now days. Maybe he does need a proof reader, but the "jest" of it is that the cam is a major contributor.

SteveW

Could it be that we are confusing lsa with overlap? shortening the lsa increases overlap. Now I'm getting confused Mike

Good Luck!

------------------
Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/carbedstangs/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367
http://www.mustangworks.com/cgi-bin/moi-display.cgi?220

[This message has been edited by n2oMike (edited 02-11-2002).]

If you study the graph, you can see how large cam lobes and wide lobe seperation push the intake closing event into the compression stroke... bleeding off valuable cylinder pressure.

Good Luck!