Inliners International Forums General Discussion Engines Appropriate Octane

Ian67's thread spins off a great subject. What octane fuel is appropriate for these modified engines (but not blown/injected)? Is it as simple as a table that has compression ratios with 87, 89, 93 for US fuel? How much does the CAM have to do with it?

thanks!

The cam has everything to do with it. The cam decides how much compression pressure remains in the chamber when the fuel mixture is ignited. Compression pressure for a given engine with unique build details is what dictates what octane you need to run.
There are lots of variables, making it impossible to say what octane you need to run for which cam and (static) compression ratio. You can get pretty useful ballpark figures, but the bottom line is, run as LOW octane as your engine can work reliably with, without detonating etc. Too high octane is not really "bad" for the engine per se, just a waste of money. Higher octane also doesn't burn/ignite as easily (Duhh, that's it's main design feature).

Even altitude makes a difference. Shape of combustion chambers, piston tops, deck height......

If you are not sure, start with the highest octane pump gas you can get, see how it runs. If no problems, go down to mid grade and see what happens. Any detonation, pinging, dieseling, stumbling on acceleration etc.... go back to high octane and see if those symptoms disappear. If mid grade behaved well, try the local dish water (lowest octane). See above diagnosis, repeat.

Agree.

I will add that when you do get some spark knock this can be used as a tuning aid to help you dial in ignition lead.

Just as an aside: That was one of the clues about my wiped cam. I had to go with premium gas because regular started causing a knock. The #6 exhaust cam was shot. Probably got a TON of compression in that cylinder.

Just one of many little things that should have clued me in earlier about the cam.

I'm having a hard time connecting a bad cam and a TON of compression. Why would more compression cause a wiped lobe? Doesn't the exhaust lobe start to open the valve when the piston is near BDC, when cylinder pressure has fallen off significantly from its peak?

The exhaust valve never really opened up. Compression built up with each stroke compounding it upon what was already in there.

Nix that. I couldn't have pulled in more than atmosphere during intake.

regardless it pinged like crazy until I changed the cam or went with a higher octane.

Intake closing too early from worn intake cam lobe will trap more fresh mixture at slow speeds, potentially causing spark knock. This may be what gbauer observed.

I don't know the stock cam specs but intake closing for a stock cam might be 20 or 30 degrees after BDC on compression stroke. A worn cam lobe and/or mis adjusted rockers could move this figure closer to BDC and trap more charge.

Intake closing point has more effect on how the engine runs than any other valve event.

Yes, the exhaust lobe was the worst but I'll bet the other lobes were worn too.

They were all pretty shot. I'm actually surprised it ran as well as it did! Guess this noob isn't so bad when it comes to tuning up an old car...

I figure if I could make her run that well I can really make it run well now.

anything above 10.0.1 you'll want to run 92. Even at 9.5.1 I'd be running 92.

I'd been running premium in the ill conceived notion that it has less ethanol. But everything except gas tank gaskets upgraded, so it's not an issue anymore.

I'll have about 9.25:1 with the flat tops, so barring trial and error differencs, it sounds like mid grade minimum, maybe premium. The guys who wok with the old Chevy engines keep on the virtue of stock. Regular gas is one reason. But to them i say MORE POWER! :-)

But the more aggressive CAM (same as gbauer) will affect it too, but don't understand affect yet.

Thanks,
Mark

There are many things to consider for knock sensitivity.
Static compression
Cam
Ignition timing
Quench
To name a few.
My 65 wagon does not rattle. I have burned premium but mostly use midgrade. Have burned 87 with no ill rattles. Has 214 degrees duration and 9.5 compression. And a talled rear geared stick shift. Less duration and it would be a rattler.

89 would be OK, but I even run premium most of the time in my 305 V6 which has 7.5.1 compression. lol Even though 87 would be fine for it. Course I don't know your gas prices where you are. Premium is just over 3 bucks a gallon here.

For a reference point, my stroked 250 (296 inch displacement) is about 9.5:1 static CR. Nice tight squish/quench about 0.030 inch clearance. Camshaft is a too short Comp 252 at about 205 duration. I run 87 octane no ethanol. It will rattle if I intentionally lug it down but most driving it is fine.

I need a longer duration cam. The engine is out now for just that.

I have stock compression and have no pinging at all with my cam running 87.

You're not all that much higher than me so you might be on that edge where you can sneak by on 87.