Compression ratio vs. octane requirements

[hawk_sixsix]

For a stock '97 4.0L at sea level with factory ignition settings:

At what compression ratio would 89 octane be required? And 93?

I'm doing a top end rebuild using the thinner Mopar head gasket, and I want to mill the head for cleanup and a little more compression would be welcome. Maybe more than a little, I haven't decided yet. (I want to know at which points I'll be having to pay a quarter more per gallon).

Question can be answered either in terms of compression ratio or in terms of combustion chamber volume / inches milled / chamber cc, and I'll figure out the math.

Thanks!

 

[Mesh]

I'm at 10.6:1 on 92 octane but it's a 4.7 and a different cam.

 

[old_man]

Quench height is the factor you didn't figure in. You might also want to figure in altitude.

 

[O-Gauge Steamer]

What I have found in most of the vehicles I have ran here at ~7k altitude (north end of the Springs) is that the higher octane gives better mileage. Even on stock engines, my Wife finally broke down and tried it in her 96 Full Size Bronco (aka the "tank") and went from 11mpg (yah, I know...) to an astounding 14mpg(yah, I still know...). Three tanks in a row, same results. She is now experimentig with the middle grade to see what it does.

Thing with the Btonco is that she likes tanks, always has. Her previous vehicle was an 89 Grand Wagoneer, before that F150. See a trend?

 

[hawk_sixsix]

Quench height is the factor you didn't figure in. You might also want to figure in altitude.

Sea level. :wave1:


As for quench height, since that number isn't really a variable between similar heads (97-01?), I can assume that if Joe Snuffy living near the beach milled his factory heads X amount of cc's and can/must run whatever-octane, I can do the same with the same results - without ever worrying about quench height. It's apples to apples.

 

[hawk_sixsix]

I'm at 10.6:1 on 92 octane but it's a 4.7 and a different cam.

Still good info, thanks. Stock ignition curve?

Now THIS is where quench height might come into play, since a 4.0L with 10:1 will have a lot more whacked off the head than a 4.7L with the same CR.

 

[Mesh]

Stock ignition curve for now. I need a second vehicle so I can have some down time to put in my new computer. I’ll be interested to see how the stock engine turns out with the higher compression.

 

[Talyn]

Quench has nothing to do with how much you have milled off the head.

 

[Goatman]

Still good info, thanks. Stock ignition curve?

Now THIS is where quench height might come into play, since a 4.0L with 10:1 will have a lot more whacked off the head than a 4.7L with the same CR.

Yeah, since quench is the distance from the top of the piston to the bottom of the head, if you don't change pistons or rods or don't mill the block, then your quench will be the same whatever you do to the head. Except for the thinner head gasket, which will lower the quench by the difference in gasket thickness.

The guys who are running higher CR in strokers are lowering the quench height in the build to allow the lower octane gas. But, still a good question on how high you could go in a stock blocked 4.0 on regular gas.