increasing compression, running on regular gas?

[Southcross]

I plan to increase engine compression through the use of a thinner head gasket... I've already done the math and know exactly what thickness gasket will yield what compression...

what I don't know, what is the threshold where I will need to start using Mid-Grade gasoline? I know every engine is different and trial & error may be the only way to know for sure... I'm expecting somewhere at or above 9:1 compression for mid, and 10:1 for premium

[shovel]

I really doubt there is a magic number you can apply to engines... don't those 4cyl's in ford contours run 11.2:1 or so? And they burn 85 octane Utah gas without complaint. Probably with drastically scaled back timing, but that's for the ECU to determine.

[Christ]

using a thinner head gasket isn't going to yield you but a few compression points, so you won't have to worry about changing fuel types.

[jesse.rizzo]

The different "grades" of gasoline are misleading anyway. Now that up to 10% ethanol is required nation wide (the pump doesn't have to say if it does contain ethanol), and since ethanol raises the octane of gas, the "regular" is often the same octane as the "premium". Some places have started charging the same price for all grades.

But, unfortunately, as you said, the only way to know for sure is testing.

[Southcross]

Originally Posted by shovel

that's for the ECU to determine.

actually.. older cars with low compression engines don't have knock sensors so no, the ECU if it has one, won't do squat to the timing. I know I can retard the timing to help prevent ping, but that kind of counteracts the reason for increasing the compression.

I can actually get some pretty significant increases in compression... the head gasket I'm having made will increase the engine compression over .7 pts (from 8.2 to 8.9:1). I could theoretically go as high as 9.1:1, but I fear excessive mechanical interference if I were to do so.

[Christ]

Ok, I'll say it this way - No.

The average car runs fine on 87 Octane fuel.

Factually, your static compression ratio has less to do with anything than your dynamic compression (engine running, actual volume of mixture taken in and compressed) and cylinder pressures, which is why you hear about cars that run 11.2:1 compression on pump gas easily (modified cars, very few OEM)

One very simple example is "Mini-Me" swaps. When done as prescribed, they yield a compression ratio of between 9.8:1 and 11.7:1, depending on combination of parts. I've only ever seen one of these builds that couldn't run on standard 87 octane under normal driving, and that had more to do w/ the fact that the guy changed a lot more things around than the standard swap to get over 12.5:1 compression.

[Southcross]

hmmm... interesting....

[Shawn D.]

Do you think that a minuscule increase in CR is going to increase MPG enough to offset the cost of the gaskets and the timing belt (yes, you will have to replace that)?

Do you have an adjustable cam gear? If not, you'll be slightly retarding the cam, which is worse for MPG.

[Southcross]

Originally Posted by Shawn D.

Do you think that a minuscule increase in CR is going to increase MPG enough to offset the cost of the gaskets and the timing belt (yes, you will have to replace that)?

Do you have an adjustable cam gear? If not, you'll be slightly retarding the cam, which is worse for MPG.

I've done enough engines to be aware that if the camshaft is closer to the crankshaft that it retards valve timing With the HG I looking at purchasing its only a change of -.86 degrees. The shift in power band won't be that significant. Combined with the camshaft I'm looking at (Euro Spec Cam), peak power happens so early that shifting it a little higher in the RPM range will help with highway driving. I'd have to check my storage unit... I have like 3 or 4 aftermarket camshafts laying around, not sure if I have any spare cam gears though (proverbial dime a dozen though)

even if the increase in r/c doesn't yield an equally significant increase in MPG... should increase the power output into the "fun level". Considering the same engine (Euro spec) with 2pts more pts of compression yields 30-35 more HP (also has larger valves, but not significantly larger). An increase of only 10hp over the present engine, especially at this altitude, is rather significant.

[Shawn D.]

Originally Posted by Southcross

even if the increase in r/c doesn't yield an equally significant increase in MPG... should increase the power output into the "fun level". Considering the same engine (Euro spec) with 2pts more pts of compression yields 30-35 more HP (also has larger valves, but not significantly larger). An increase of only 10hp over the present engine, especially at this altitude, is rather significant.

I'd bet the Euro version also has more agressive timing and required higher-octane fuel.