Compression for a given octane?

[Dominic]

Is there an easy way to calculate the maximum compression ratio an engine may have for a given octane fuel at a given intake air temperature?

I'm trying to find the compression ratio limits of 93 octane gas...

[jughead]

Some food for thought... the 4.2L inline 6 Trailblazer runs 10.5 compression on 93....

[gonein60]

Hey Dominic, I have a magazine that goes over it. They said the highest they had ever seen an engine go was 220 psi on 93 unleaded. Unfortunately you can compensate the octane requirements of a higher compression engine by installing a cam that has a later opening exhaust port, but it also hurts performance to an extent. So in aswer to your question it really depends on the cam set up the car has. My car was showing a high of 196 psi on the stock 9.5:1 internals so you can figure a 10:1 configuration would probably be safe. Now on the other hand timing advance is also going to play a role in the amount of detonation the engine sees from higher compression, and also take into account the engine is going to use knock retard as a defense mechanism against damaging the engine. For maximum performance first go with the best cam you can get and then go with the fuel you plan on running and then tune the timing advance and fuel ratios to the highest possible level before you start to show some knock retard or detonation. Maft+ and a scan tool such as autotap would be invaluable in successfully achieving maximum numbers.

Also remember the 4.2 uses a 10.5:1 on completely forged internals and may not see as many degrees of timing advance as we do with our cars. It also utilizes a coiless spark plug system as is utilized on the newer ford expeditions, where the coil is built into the spark plug itself.

[Patrick Javert]

I would say that with 93 octane, the highest you could go safely is 11:1. That is with all of the right factors for the engine to run well such as intake, exhaust, cam, heads, ect.

[Guardsman]

"As a general rule, follow this chart: 9:1=90, 10:1=94, 11:1=98, 12:1=102, 13:1=106, 14:1=110. These are some pretty close desired octanes for the corresponding compression ratios." Got that offo of Chevelles.com.

[Wicked 3800 V6]

That chart may be true, but when you run a computer controlled engine (as opposed to a carb'd one), you can sense knock -- which is the crucial factor in octane selection. Now, if you put 87 in your new caddy (which is computer controlled to go on 87 though is best on 93), the computer will retard a boatload to compensate for the low octane when it experiences knock. Better computer can compensate for lower octane (though everything has its limits). But, if you don't want the loss of performance, go with the highest octane you can. Go read the rocket fuel article from the guy mixing in toluene with his gas.
-Rob

[Brian Dieckman]

The explorers aren't "coilless" they're COP (Coil on plug) Lots of newer engines are being designed this way. The coil actually sits down on the plug (or very very close) and the spark has about 1" to travel, as opposed to 10"-20." As a result, the coils can be smaller, lighter and the electronics are simpler as well. Buicks have had this setup for a while now. I personally think it's a great idea. This "waste spark" B.S. that's on our V6's is a waste of good spark, that's all. If there was enough room to do COP on our engines (and the electronics to do it) I would change over in an instant.

[gonein60]

4.2l will almost bolt into our cars and mounts up to a 4l60E as well. All you would need is a new k member and motor mounts. It also has a flatter torque curve than our cars do and has 275 hp stock which is the same amount as a porsche carrera. And yes you are correct the coil is mounted on the plugs, which the gmc envoy utilizes, but it isn't really a "coil" in the traditional sense anyway although the function is basically the same. It also doesn't leave any room for routing the wrong way and melting plug wires.