I am looking for opinions on what octane I should be running on my 340. I found an online calculator to help me with the following numbers:

"measured" static comp: 9.7:1
dynamic comp showed: 8.72:1
dynamic cranking comp showed 178psi (I have used a gauge and I showed an actual avg of 180psi)
my local altitude is apx. 800ft.


Early last year, I was using 89 octane without any ping but I was getting run-on at shut down. Then I swapped to the 91 octane and I have been running it ever since with no more problems. I am hoping to get some opinions to see if I could drop back down to the 89 or if 91 octane would be better suited for my engine.

Thanks

i'd run the 91. your dynamic compression seems low to me, based on your measured cranking i would have expected it to be higher.

I had Scott Brown spec out my cam and I told him that I needed to run on pump gas with the 9.7:1 compression. Could the cam design be why the cranking comp seems high with the 9.7:1 static?

Thanks for the reply.

I am trying to think back 7-8 years ago when I bought the cam and I do remember saying pump gas but I am not sure if I told him the grade of the pump gas. I just checked out the cam card and I don't show any old notes that I may have given Scott at that time.

Thank you for the reply.

the cranking compression is whats going to dictate what kind of gas you can run. with 178-180 psi your going to need to run premium fuel. you can retard the cam timing and hope to run 87 octane fuel, but you will sacrifice some performance. you can also put a larger cam in that will have a later intake closing time and bleed off some cylinder pressure

This is an interesting post, is there a chart showing cranking compression/octane requirements?

yes and no. the information i'm referencing is from texts that i read while doing my masters in engine design. these texts were from the 70s and 80s. new fuel types and chamber designs and piston top coating all play a part as to how the flame front proceeds and where there are hot spots. therefore with all the proper coating and attention to detail its possible to run an engine with 11.0:1 compression on 87 octane. is this a good idea? no, i wouldn't risk it with my engine. the octane plays a vital roll in minimizing preignition at all engine conditions at all RPM. if your not worried about gas mileage and emissions, which most aren't i'd run the higher octane and be safe.

my experience has been 180psi is nearing the limit for premium fuel. there may be some variables for different combos but i feel its safe to say at 180psi your getting close to the edge. if an engine has a static compression ratio of 9.7 when new; it will be higher than that after running and carbon build up. maybe as much as 10.25-10.5:1.

My 9.9:1 static has a lot lower cranking compression than yours and I still have a slight pinging issue at times using premium pump gas.

"" "measured" static comp: 9.7:1
dynamic comp showed: 8.72:1
dynamic cranking comp showed 178psi (I have used a gauge and I showed an actual avg of 180psi)
my local altitude is apx. 800ft.""

I read in another thread that the valve timing (Closed, INT) has allot do do with the dynamic compression. My current cam card shows:

Valve Timing @ .050
OPEN CLOSE
INT: 5.0 btdc 41.0 abdc
EXH: 55.0 bbdc 3.0 atdc
Lobe CL intake: 108 exhaust: 116
Lobe Sep: 112 installed at 108

Does my current 41.0 contribute to the higher than expected dynamic compression (8.72:1)? If so, can someone give me a brief description as to why?
I am not interested in changing cams, I just want to understand more about the one that I have.

Thank you all for your responses. It appears that I will continue using 91 octane.

basically the earlier the intake valve closes, the greater the cylinder pressure and dynamic compression for a given static compression ratio. the cam you have has 4 degrees of advance ground into it. you may not need 4 degrees advance with a short intake duration. if the engine will run fine on 91 octane then just continue with it. there should be some 93 octane in your area to experiment with. i know from experience that trying to get a lean a/f mix at cruise can contribute to detonation. also a short cammed engine may not like an aggressive ignition curve.

The 91 octane has been fine with my current setup. I was just trying to see if I could get away with less octane or not. My cruise A/F is in the 14.0-14.8 range and after some recent secondary adjustments, I have no audible detonation at any time (cruise or any WOT conditions).

Thank you for the reply.

EDIT:
In general, does the higher cylinder pressure (180psi) along with the higher dynamic compression (8.72:1) result in richer idle A/F ratios?

I have found that this motor idles with the best vacuum number when I am running 13.2-13.4 A/F in D. I have tried a few different times to lean it out to the upper 13's to low 14's and it does not like to idle in D at that leaner A/F ratio. I was just curious if the above is related to this.

Thanks again.

You want to use your 'seat to seat' intake closing spec when doing this calc...not your @ .050 spec.

I used two different online calculators and they did not specify if the INT/CLOSE number was to be at .050 which is on my cam card, or at the seat to seat which I do not show on my cam card. I assumed that it was at the .050.

This is one that I used.

http://www.wallaceracing.com/dynamic-cr.php