Leaded race gas - issues using it?

[dannobee]

Originally Posted by stownsen914

Any chance that additional octane would allow the engine to produce more power with additional timing? The timing numbers in the graph seem conservative.

At some point adding more ignition timing will cause the power to decrease, even without detonating. (Other engine damage may occur though, like hammered rod bearings and broken pistons). Higher octane fuel won't help that. Dyno testing will help you find that plateau. If he added a few degrees and saw no increase (or even a decrease), it's a good indicator that that's all she's got.

[JoeMag]

Thanks... This will be interesting to see on the dyno with pure race gas (i'll probably try 110). The graphs are what I did on the dyno shortly after initial tune (after a small mod) to see what sort of headroom was there. Went back to baseline.

At 7.2k that's a 5hp increase with the +2 degrees to baseline. ...question for me will be if the same with 110, why spend the extra $.

[stownsen914]

Will be interesting to see if the extra octane helps with that, if you don't mind sharing after you get to the dyno

[Mr Beau]

Originally Posted by JoeMag

...ok, so time for a little data. Below is my ignition advance curve and below it is dyno curves of baseline,+2 degrees on baseline and +3 degrees on baseline. Fuel is 70% pump 93 with 30% 100 mixed in. ...so +2 seems close to mbt

My question is with 110, roughly about how much additional advance would a motor support over fuel I'm running now? I know there are tons of variables to get the real numbers however I do not have experience in the differences in a motor tuned with different octane level fuels, so just looking for ballparks.

If you've reached MBT and it's not knocking, then there's no more power to be had from additional ignition timing/octane. What are you hoping to obtain?

Somewhat related: https://youtu.be/m-Sj8WkWH6E

[3rd_gear_Ted]

In race cars, leaded fuel helps transfer the heat from the valves to the seats. The combustion burn is quicker too. All together your race motor will run cooler @ WOT with 110 leaded.

[Walt Fricke]

The combustion burn is quicker? Everything I have read says that the higher the octane, the slower the burn. Which is why you can advance farther without knocking/pre-ignition. So running more octane than your engine needs is counter productive generally, unless what you gain by advancing the spark or increasing the CR outweighs the lower power the slower burn produces. It is the higher squeeze, I think, which is why it makes sense in a race motor to use a more compatible fuel. Of course, getting the max combustion pressure at the right place in the crank rotation is going to help.

When I ran 100 LL Av gas in my 2.3, 2.5 and 2.7 10.5 or so twin plug race motors (because I was scared of pre-ignition), it worked fine (but wasn't dynoed) until others convinced me with my two plugs and largish cams I could run pump premium, so I did. That was before the days of mandatory alcohol in fuels most places.

Some airport fuel stations had no issue with filling the 55 gallon fuel barrel in my closed trailer with a race car inside. One told me no and hell no.

Av gas is certainly well controlled as to its specs. One vendor proudly showed me the extensive test sheet they get from testing every delivery.

A guy would have to be nuts to run an engine which needed this kind of octane in a street driven car. There he would, indeed, not be paying his share of the cost of highway maintenance. You would not suppose that a rational bureaucrat would care much about exclusive off road use. But not all are inclined to think of the purpose of a rule when applying it.

[winders]

https://www.sunocoracefuels.com/tech-article/beyond-octane

With race fuels, burn speed is not directly related to octane….

[Mr Beau]

"Everything I have read says that the higher the octane, the slower the burn. Which is why you can advance farther without knocking/pre-ignition."

I'm not sure that's true. Let's say the spark plug has initiated combustion (so we're talking about detonation, not pre-ignition).

Things start burning, raising the heat and pressure in the combustion chamber. The stuff furthest away from the spark plug is starting the feel the heat/pressure, but the flame has yet to reach it. Low octane fuel is likely to have spontaneous combustion (i.e. detonation) occur in this unburned region, whereas high octane is more resistant.

If you add ignition timing, the heat/pressure in the unburned areas rises, which might result in more spontaneous combustion if the octane is too low.

So you can run more ignition timing if you have higher octane, but this doesn't mean that higher octane needs more ignition timing.

[JoeMag]

Here's the data running 110 (vs the 95 equiv (70% 93, 30% 100)... This time did a baseline, and then a run at -2 deg over-all trim to see the affect (power down across board). Then did +2, +3, +4 over-all trim. Put +3 and +4 on different graph (graph was getting too busy). Gradual increase in power -2 to +3 across board as advance increased, then little down +4.

Ran my knock monitoring, however, screwed up saving the audio files so could not re-review. Saw a little knock at +3 and +4, however, wish I could’ve re-reviewed the files and fiddled with listening parameters and filters. No widespread knock.

Since this time I saw an increase in power at +3deg and did not in the 95oct runs, would I be correct in that I was getting knock in the 95oct runs limiting the power increase in +3deg? I did not have my knock-monitor on when I did the 95oct runs, so may have even seen it in the +2deg.

Pulled plug and seemed fine, no spots on porcelain. The electrodes and outer ring looked a little chewed, however, looked at a new plug and they’re pretty beat up on a new one.

There’s no question what kind of gas you’re running with exhaust tips this white.

[stownsen914]

Thanks for sharing that data. It makes sense to me that you'd be able to run more timing wtih the 110 than with 95 octane with a motor like yours. Out of curiosity, what are your AFRs (target in case you don't have actual)?

[JoeMag]

They were in the mid 12's for the above runs. ..in the end will track tune in to around 13-13.2 at max torque and then around 12.8 at max hp.