safe octane for 13 to 1?

[mike the snake]

I have a post in the racing section, and it was suggested I ask this question here in the engine forum as well, so here goes;

Engine is a 3.5 (3.2 with 100mm barrels) 13 to 1 compression, GE80 cams, 50mm PMO's, and lots more. Timing and red-line are not too agressive for now.

The PO said he used a pump gas/race gas mix, but couldn't remember the octane of the race gas.

So what's a safe octane for my engine? The track near me has 116 leaded, 114, and lower.

My buddy who races 16 to 1 1100hp drag cars said i didn't need the 116, and I think I can run lower as well, but how low?

I want to be safe, if it's best I just run the highest octane, then I'll do that.

Is there an average octane for certain compression ratios?

[kenikh]

13:1? Wow, that is super duper high, even on race gas and GE80 cams, especially for big 100mm pistons. You will definitely need to use C12, C14, e85 or e100 and likely an EMS w/ knock control.

Your dragster buddy may be right for dragsters, but totally wrong for an air cooled 911 motor.

As far as your octane to compression question, I wish I had the math to do it, but a refernce point is that @15:1 CR, you'd need 137 octane. All of the 2 liter racers use C16 for their 13:1 cars, so you mgiht be able to interpolate a calculation from those numbers. Remember, your pistons are 1.5 cm larger in diameter than the 2 liter racers, so you will have a higher propensity to knock due to increased flame travel distances.

[mike the snake]

Thanks.
The engine was previously run with in this configuration so I know it all works, and I have duplicated everything as it was last run.

The PO made it sound like he got away with mixing pump and race gas.

I installed an XDi ignition(s) which has a knock sensor ability. I will look into what it entails to install knock sensors.

I'll buy the 116 leaded race gas until I can get any concrete info.

[Steve@Rennsport]

Originally Posted by mike the snake

I want to be safe, if it's best I just run the highest octane, then I'll do that.

You didn't say whether you have twin-ignition or not, but I would tell you that an air-cooled race engine at 13:1 needs 112 octane fuel to survive, especially in your climate.

[mike the snake]

Yes it's a twin plug.

Thanks very much.

[ONQRACIng]

We run a 3.7 twin plug, 13:1 ish (a bit higher), Elgin cams (proprietary grinds) JE 100mm's, GT Plus 104 Unleaded, so far, no issues,besides availability here on the the east coast..

We run Sunoco Standard leaded inour 2.9, and plan on a switch to the same GT plus by weeks end..

[mpeastend]

Mine is a 3.8 (3.74 actually) w/twin plugs @ 12.5:1 CR I'm using 110 (Sunoco Purple aka standard). BTW, no engine management, just 50 PMO's.

[mike the snake]

I picked up 5 gallons of 112 leaded today. I'm going to run 112, tune from there, and I think I'll be fine.
The car won't be driven under too hard of a load to start as my first events will be HPDE and drivers ed.

Thanks for the help