Higher octane fuel made less power...

[swampbuggy]

Look at this from a COMMON SENSE Angle....as an engines static compression is increased AND the dynamic compression (or what you wish to call it) also increases (varying somewhat with camshaft selection) and running cylinder pressures are higher, we know that one has to increase the motor octane number (the MOST important number) to accommodate the higher pressures. It stands to reason that the opposite would be true if you lower cylinder pressures. ??? Mark H.

[F-BIRD'88]

Ya I get it it's a 2bbl motor... Test it with a 4 bbl on it.
If the timing requirement is now less, look at the cam and headers. And the 2 bbl carb too.

[digger]

The elephant in the room is the ignition fault. That's where I'd be looking. There may still be ignition issues with the 93 fuel that are mild enough to not be detected but causing it to run sub optimally. I'm not sure why the different fuel was causing misfires, were the plugs inspected ? Maybe some deposits were causing issues

[prairiehotrodder]

I'd jet it for most power rather than looking for a certain AFR ratio. You could be lean and thats why you lost power. Don't trust an AFR gage. Give it the jetting that makes the most power, then maybe look at the gage to see what is reads for future tuning. I've learned this lesson allready.
Brian

[JoePorting]

I was thinking along the lines of what digger was talking about. Engine probably has a weak ignition system. May have run good on pump gas, but the race gas may have caused it to miss with the weak spark.

[CharlieB53]

I have to agree with jetting and ignition checking.

Just changing fuel requires the whole tune to be redone.

Verify spark intensity, jet for power, readjust timing. Perhaps even a change in spark plug. It all can be affected by different fuel.

You really didn't give the better fuel a decent chance.

Once dialed in then try mixing the fuels 50/50 and do it all again.

Go with whatever combination works best for you. I would almost bet it will be a mixture, perhaps 25% better fuel to 75% pump. If you add compression then add higher % of the better fuel.

[Newold1]

Until both dyno runs of the engine were able to pull the same top rpm measuring points the comparisons or drawing conclusions about fuel type are useless.

Something was obviously wrong with the engine setup on the race fuel test and the fact that the engine could not be brought successfully to near or the same peak rpms of the pump gas pull on the dyno means that something else is wrong here besides just fuel octane numbers and fuel used. I suspect a plug or ignition failure that was causing a misfire that was holding rpms way down on the race gas pull.

How old was the race gas in the drum and did anyone do a specific gravity test on the fuels?

[JRice]

Until both dyno runs of the engine were able to pull the same top rpm measuring points the comparisons or drawing conclusions about fuel type are useless.

Something was obviously wrong with the engine setup on the race fuel test and the fact that the engine could not be brought successfully to near or the same peak rpms of the pump gas pull on the dyno means that something else is wrong here besides just fuel octane numbers and fuel used. I suspect a plug or ignition failure that was causing a misfire that was holding rpms way down on the race gas pull.

How old was the race gas in the drum and did anyone do a specific gravity test on the fuels?

The drum of fuel was opened 2 days before we did this Dyno session. Didn’t check specific gravity on it.
We plan to try again with a 50/50 mix and a see about a hotter heat range plug.
Ignition system has to stock. MSD HEI that’s it.

[Tuner]

We ran as high as 42* in our 9.5cr 18* engines. And that was with a pretty efficient chamber and piston combo.

Was that with a 1-11/16" 830 carb or a 1-7/16" 390?

[Tuner]

Fuel octane has absolutely NO effect on flame speed. Period, end of story.

Two fuels with the same octane could produce different power levels, depending on the
composition of the fuels. Conversely, two fuels with different octanes could produce the same power.

Octane numbers should never be used to compare fuels. The fuel distillation curves are much better
predictors of fuel performance. Octane numbers are only intended to be used to predict the onset of
detonation. They do not prevent or predict pre-ignition, misfires, combustion temperatures or performance.

Flame speed is primarily determined by inlet air temperature and chamber turbulence produced by squish effects.
Once an engine is built, squish velocity becomes a function of piston speed.

Freshness of fuel, race or pump gas, is essential. Stale fuel does not reflect the product that was built by
the refinery. A new fuel drum is not an indicator of fuel freshness. Only a Certificate of Analysis can ensure
the fuel is what the refinery says it is.

Expect fuel variations, especially with pump gas.

Well said ^^^ tell it like it is.

[Warp Speed]

We ran as high as 42* in our 9.5cr 18* engines. And that was with a pretty efficient chamber and piston combo.

Was that with a 1-11/16" 830 carb or a 1-7/16" 390?

390

[In-Tech]

It seems some forget that a limited app like this has less cylinder pressure once the carb goes into vacuum, ya know, less air. You can add a LOT of timing at the upper rpm due to the cylinder pressure loss, just be careful what you do on restarts and off the corner rpm. I've seen as much as a 70 hp gain running 47 degrees at 8000rpm but the only way to keep it off the preignition/detonation wall after a few laps was really high octane, plus the dyno also showed quite a big loss of power/torque under 6500rpm. Of course we are not going to get into the high EGT's that happen in high vacuum. Timing is a big band aid, then a pneumatic on the mag to increase timing after 6500(in our app) fixed it and back to 36 degrees we went with the 10 degree bump.

[novadude]

Fuel octane has absolutely NO effect on flame speed. Period, end of story.

Two fuels with the same octane could produce different power levels, depending on the
composition of the fuels. Conversely, two fuels with different octanes could produce the same power.

Octane numbers should never be used to compare fuels. The fuel distillation curves are much better
predictors of fuel performance. Octane numbers are only intended to be used to predict the onset of
detonation. They do not prevent or predict pre-ignition, misfires, combustion temperatures or performance.

Flame speed is primarily determined by inlet air temperature and chamber turbulence produced by squish effects.
Once an engine is built, squish velocity becomes a function of piston speed.

Freshness of fuel, race or pump gas, is essential. Stale fuel does not reflect the product that was built by
the refinery. A new fuel drum is not an indicator of fuel freshness. Only a Certificate of Analysis can ensure
the fuel is what the refinery says it is.

Expect fuel variations, especially with pump gas.

Well said ^^^ tell it like it is.

X2. The "higher octane burns slower" myth needs to die.

[Frankshaft]

Op, I don't think you can conclude anything from your test. You need to figure out why it was breaking up and Missi g on the race fuel. That was obviously causing the power loss, not the fuel. The other thing, you either have a super good drivetrain that eats little power, or the Dyno is a bit optimistic. A "good" b mod makes 420 real hp. Some shops have sheets that say 460+, but, those engines get pulled by the 420hp ones. . Just an observation. As long as it's being used as it was intended, to measure a change, that's fine. But if it's being used for marketing purposes, then

[Tuner]

We ran as high as 42* in our 9.5cr 18* engines. And that was with a pretty efficient chamber and piston combo.

Was that with a 1-11/16" 830 carb or a 1-7/16" 390?

390

Thanks. How much intake vacuum? How much timing with the larger carb?

Several years ago (mid-late '90s) I handled some Roush Ford Motorsports distributors with Motorcraft FoMoCo advance mechanism (same geometry as car/truck production units) that had high RPM heavy secondary spring advance curves that might have been to compensate for the high intake vacuum.