Thoughts on compression ratio and fuel octane???

[nomad]

I realize its a different fish but I'm setting up a Datsun engine for a Midget and it looks like my compression ratio will be somewhere around 9.25 to 1. Octane of gas around here is 89 at best. Think it will work??? What are folks running in hot BMC engine's? 87 down to 84 is common and I suppose we can expect it to continue down. If I stay close to home we are going to be getting blender pumps for picking your ethanol mix and that would allow higher octane. Have'nt decided whether to go this route or not.


Kurt.

 

[mrsprite]

Kurt,

I'm not exactly sure what my compression ration is with my A15 & GX head, but it's probably in the 9.0 to 1 range. I generally use premium, which for us in Colorado is 91 octane, but I have used regular 85 octane with no real ill effects. You should be OK with the 89, but maybe can't advance the timing as much as you could with higher octane.

 

[aeronca65t]

You have a few choices:

You can do trial-and-error method.
You can simply keep reducing your timing advance and then test drive the car. If you hear "pinging" when accelerting up a hill at moderate speeds in 4th gear (approx 3500 rpm), you need to back off timing more. If it sounds OK, you can advance timing until you hear pinging (and then back it off a degree or two).
That's one way.

Generally, engines do not ping at idle speeds. They ping at moderate speeds after timing has reached its full advance. So, if you want to have a better idea of what the real advance numbers, you can adjust your *total* timing advance.

To do this, you'll need a degree wheel on your front pully (or see note below) and you'll need to check your timing at the point where you're sure it has reached max advance. On most engines, this is around 5000 rpm (or less).
In other words, you rev your engine to about 5000 rpm and check your timing (it kind of a pain in the neck and you don't want to hold the engine at that speed for too long).
With your compression, I'd start with 25 degrees (BTDC) of total advance and see how that works.
You'll still need to do the trial-and-error process of listening for pinging, but at least you'll know what the actual numbers are and you can adjust based on those numbers.

Just for example, on my racing 1500, with 100 octane race gas, I run 30 degrees of total advance (again, checked at about 5000 rpm). Like most racers, I have no idea what my timing is at idle speed because I never bother to check it at low rpm. It really doesn't matter much as long as the car will start.


Note: if you don't have a degree wheel, or don't want to buy one, you can figure out where "0" degrees is on your front pully and then mark positions of 10 degrees, 20 degrees, 25 degrees and 30 degrees. If you're decent at geometry, this isn't really very hard. This should be enough marks to figure out where your total timing is.
One other note: if you read British publications, they call it "pinking" and not "pinging". Just an FYI to prevent any confusion.

G'luck!

 

[nomad]

Thanks for the comeback folks. I guess one can always retard the timing but as I recollect from a MGB I rebuilt in the 70's, there is'nt anything to be gained if compression is to high. With the MGB I went to a smaller dish in pistons just before premium gas became unavailable. I fought the comp ratio as long as I had that engine.