Optimal Compression Ratio for 91 Octane Fuel

[NICAT]

Hi.
Yesterday, i started a thread about coolant leak. And according to replies it is very likely that the head is warped and need to be resurfaced. (will have it checked)
In case the head is warped, i prefer to replace it rather than resurfacing it. (it is headache for me for some reasons)
Car's engine is G15MF, by default it had non-interferance "head" with 8.6:1 Compression ratio. But a few years ago, previous owner had its head replaced due to the crack. by another type engine"s (A15SMS) head, which is interference and 9.5:1 compression ratio. So there are 2 types of heads that i can buy.
My question is, if the fuel octane is 91, which compression ratio is the most optimal ? Would the engine lose significant performance with 8.6 compression ratio?
Somewhere i read that, we will not gain anything with higher compression ratio if we will use lower octane fuel, and have to retard the ignition timing in order to prevent knock.
I want non-interferance engine.

(Had some bad experiences even though replaced the belt regularly.)

 

[Chris142]

That all depends on much more than just a compression ratio. Combustion chamber shape,material etc.

With that said a higher compression ratio will always give more power and mpg than a lower compression ratio.

 

[SubieRubyRoo]

Biggest determiner of what is "optimal" is the cam timing which leads to figuring out the dynamic compression ratio, which gives you a more or less "hard limit" on what you can do without scrambling parts from detonation. I haven't seen many street engines run above 9.0:1 dynamic compression ratio. Considering either head was available on your engine, I'd likely go with the smaller-chambered one.

 

[NICAT]

Thanks for useful infos.

 

[k1xv]

Direct injection of fuel also allows a higher compression ratio than port injection.

 

[mazdamonky]

Mazda seems to have decided 13:1 for 87 octane and 14:1 for 91 octane are both extremely efficient. To the best of my knowledge those are the highest compression mass produced gas engines in existence.

 

[sloinker]

Originally Posted by mazdamonky
Mazda seems to have decided 13:1 for 87 octane and 14:1 for 91 octane are both extremely efficient. To the best of my knowledge those are the highest compression mass produced gas engines in existence.

Couplw more points of compression you could run kerosene without spark plugs.

 

[NICAT]

Originally Posted by mazdamonky
Mazda seems to have decided 13:1 for 87 octane and 14:1 for 91 octane are both extremely efficient. To the best of my knowledge those are the highest compression mass produced gas engines in existence.


How is it possible to run engine which has such high compression ratio with relatively low octane fuel ?

Just by opttimizing the combustion chamber design, using special material and so on (As stated above) ?

 

[Rand]

What country are we talking about and fuel 91 ron? or what.

 

[Shannow]

Chris142 nailed it.

To extend, there's squish, quench, swirl, tumble, the location of spark plug(s), direct/port injection.

It's a fascinating subject.

 

[OVERKILL]

Originally Posted by Chris142
That all depends on much more than just a compression ratio. Combustion chamber shape,material etc.

With that said a higher compression ratio will always give more power and mpg than a lower compression ratio.


This.

The 3.6L Pentastar, which spec's 87 octane has a compression ratio of 10.2:1
The 5.7L HEMI, which allows for 87 or 89 has a compression ratio of 10.5:1
The 6.5L HEMI, which only calls for 91 octane, has a compression ratio of 10.9:1

Was common on the 5.0L "HO" Ford engines, which were, IIRC, 8.8:1, to bump base timing from 10 degrees BTDC to 14 to increase power and throttle response, but which required 91 octane not to ping.

 

[NICAT]

A little off-topic, but before i knew what "detonation" really is, my car's engine was detonating everytime during acceleration in daily driving. I didn't take care of it, Luckily it never caused any significant problem in 2 years. As soon as i got some info, i fixed it

 

[Linctex]

Originally Posted by NICAT


How is it possible to run engine which has such high compression ratio with relatively low octane fuel ?

Just by opttimizing the combustion chamber design, using special material and so on (As stated above) ?


Not necessarily...... Most people think of the "Otto cycle engine" when they think of a 4-cycle engine with a piston and a head with valves.

Many new high compression engines, like the newest Toyota 14:1 engines, are NOT "Otto" engines, they are "Atkinson Cycle Engines"

The parts look the same, but how they operate is VERY very different.

 

[Chris142]

To add. The bigger the bore the more likely detonation will occure. Cast iron heads will cause detonation where aluminum won't.

 

[supton]

Don't forget, there is static and dynamic compression. Atkinson cycle runs high static compression ratio--but holds the intake valve open long and so it bleeds off some of the intake charge.

I don't know if there is following for your engine. Hot rod guys "know" what a given engine family can tolerate, given octane, boost (if used) and cam timing. But otherwise all the variables come into play, as listed above.

 

[Silverado12]

compression

 

[Silverado12]

 

[Astro14]

Originally Posted by Rand
What country are we talking about and fuel 91 ron? or what.


Azerbaijan- so the 91 fuel is 91 RON, at best.

In other words, regular, or lower, in the US. Accordingly, I would go with the lower compression (larger volume) head given the OP's location.