Ignition Timing

[BDR561]

OK guys...

So my car is running (and running great!!!), Here's my question...

My motor has approx 12.5 C/R, I am currently running a 50/50 blend of C113 and 93 premium (netting a little over 103) with initial timing at 14 and full advance of 30. The car runs great with this combination...

I have a MSD digital 6 that has the ability to retard the ignition with a flip of a switch (usually for nitrous), but I was wondering if I could use this feature and retard the timing in the event that I would like to run just 93. The retard can be adjusted I believe from 0 up to a -20 from what is currently dialed in.

Thoughts?

[Bob In Ct]

Hey BDR:
I'm way out of my league here, but might it have been better to run 9.5CR and use pump gas and more timing (ie 36 degrees)?

Bob

[RedBarchetta]

I used to run 11:1 on the street with a '70 Z-28 and I had to retard timing to run 92 octane pump gas without pinging. Also used water injection for a while. At 12.5:1 I personally wouldn't be confident in running 93 pump gas even with a heavily retarded timing profile. But that's JMHO.

-Dean

[blykins]

Cam intake duration has a lot to do with what you can get away with. However, with 12.5:1 SCR, I don't think you can make it on race gas. You can play with DCR to a certain point, but after that, efficiency starts playing a key role. At some point, the engine is gonna be efficient, and you're gonna see a true 12.5:1 SCR.

If you're sick of mixing gas, there may be a few others things you can try to get the ratio down.

[bobcowan]

If I read that right, you 14* of intial, and 30* of advance, for a total of 44* of ignition advance. That's a lot of advance. Most SBF's peak out at around 34-36* total. More than that costs you power. Less than 30* and you become very inefficient, and lose power.

I would set initial at 10*, advance at 24*, and continue to mix fuel.

You built an engine with 12.5 SCR, and you live at sea level. With that much cylinder pressure you need high octane fuel. There's really no way around that. It might be cheaper to buy premium pump gas and add toluene to raise the octane.

[BDR561]

I have a total of 30 at the moment... Don't mind mixing, but just wondering what my options where. I did not build the motor, I happened to come accross it for less than 50% of the parts price, all new and test ran. Figured worst case scenario I could change the heads out and lower the C/R that way if I need to and still be WAY ahead of the game $$ wise.

The car seems real happy at the moment, running a little rich, but that's OK for the moment while I break it in over the weekend.

Is there a way to figure the actual C/R, compression gauge??