Leaded race gas - issues using it?
 

The disadvantages that I know of using leaded race gas are spark plugs need changed more often and WB O2 sensors do not last as long and need calibrated more. ....don't have to worry about cat converter issues on race car.

Are there other issues using it?

Reason I'm asking is I can get 110 leaded for about the same price as unleaded 100 so if I mix premium 93 at 50/50 with the 110 I get 101 fuel for a lower cost.

Gasoline blending is a non-linear process, so a 50/50 mixture will not necessary have an octane rating that is the average of 110 and 93. Lead susceptibility is especially non-linear. The actual blending behavior depends on the composition of the fuels. This is not to say that the octane wouldn't be slightly higher than what you're estimating, but it could be lower. Does your engine have knock sensors?

Aside from sensor wear and tear (and ignoring cats since you don't have them), I've never heard of other negative effects to your car from it.

A couple other things to consider:
- Is this for your racecar? High compression aircooled motor often like as much octane as you can give them to avoid detonation damage. Blending with pump gas may compromise this.
- Pump gas is known to not be friendly to fuel systems (notably fuel cell foam), especially on cars that sit most of the time like racecars.

Also worth mentioning - tetraethyl lead is a neurotoxin. Just be careful handling leaded gasoline. Personally I wear rubber gloves.

You could probably get 100LL at the airport cheaper than race gas. "Low Lead" is a misnomer, as the lead content is still twice the amount ever used in pump gasoline.

But first things first. Nascar eliminated the use of lead in race gasoline years ago (2007?) after blood tests on crew members revealed high levels of lead. Let that sink in for a minute.

I have been using 110 leaded race fuel for almost 10 years with zero issues.

But, if all you need is 100 octane then buy the 100 octane unleaded.

What are the specs of your air-cooled engine? Compression? Displacement? HP?

What octane unleaded fuel should be used with a 2.8 with 9:5.1 compression single plug and Mod S cams. I was told by Mr. Parr at PMO not to run Ethanol base fuel with PMOs

Interesting idea on the 100LL. I live about 5mins from a local small airport. Just called them - $5.95/gal. Still not sure they'll sell to me, need to call back Monday.

Are there power advantages going from 100 to 110? 3.8 at 11.5 CR

I was previously running mix of 30% 100 / 70% 93 pump, so around 95 octane in the end.

Don't run aviation fuel......it's formulated for aviation conditions.

A 3.8L running 11.5:1 probably needs 110.....that's what I would use.

AvGas is more consistent, batch to batch than most race gas. And no, it's not formulated for "aviation conditions." Ask the people who make it. One thing about it is there is ZERO ethanol/methanol in it, as alcohol can absorb moisture and cause corrosion and degradation of rubber components. This lack of oxygenation may cause a slight decrease in power if you normally use E10 or higher gasoline. R+M number for 100LL is 96. If your octane requirement is higher, then you'll obviously need the appropriate race gas.

You are aware that until the 90's, almost all racing gasoline other than 76 and Sunoco started from base stocks of AvGas, right? VP's Motorsport 98L is a current example.

Like when switching any kind of fuel, verify A/F mixture and adjust accordingly.

And of course, there are no road taxes paid on aviation fuel, so it's technically illegal to use on street cars, much like red diesel fuel. I have never had a problem buying 100LL from the airport, but bring approved containers, don't drive your car up to the pump.

100ll Avgas ! what I found the hard way!!

Let's be real:

https://www.chevron.com/-/media/chevron/operations/documents/aviation-tech-review.pdf

Page 44:

"There are two main specifications that cover avgas: ASTM D 910 and DEF STAN 91-90."

Page 67:

"Today, avgas is a highly refined product specifically manufactured to meet the demanding performance requirements of aircraft engines."

A technical specialist at Sunoco doesn't think you should run it....but maybe he wants to sell his fuel instead:

https://classicmotorsports.com/articles/fuel-facts-why-not-avgas-your-car/

From your link he makes it sound like AvGas isn't such a bad deal after all!

Like I said, VP uses it, Trick did, and I know for a fact that ERC did (haven't bought from them for a few years). It was the starting base stock for many racing fuels. To each their own, and run whatever you want, but it does work just fine.

If avgas and race fuel are so interchangeable, how come the airplane side of the world is having such a hard time coming up with unleaded avgas? Why? Because airplanes have special requirements and specifications that the avgas must meet.

Can you run avgas in some race engines? Sure. People done it for decades. Is it a great idea to give general advice that it is fine to run avgas? No, it is not. Might the OPs engine run on avgas? Maybe. But, any airport caught selling avgas for car use (on or off road) would be in big trouble.

Back in the late 80s I raced a built Yamaha 350 Banchee at Glamis sand dunes for years on straight Avgas that I would drive right up to El monte air strip with my Trick gas cans and fill up. Never had a issue with motor. Just don't be behind me or your eyes would burn from my exhaust fumes. Good ol days!

Avgas is formulated to have a lower vapor pressure to reduce potential vapor lock issues at altitude. This is one reason aircraft engines and cars using avgas are more difficult to start at low temperatures.

Most places it is illegal to sell avgas or any untaxed fuel to a road-going vehicle. Everyone involved is subject to big fines. So most airports are loathe to sell it other than putting it in airplanes.

At the small airport I used to instruct at we would occasionally sell it to racers we knew only, not to just anyone that walked in. And only if we trusted them not to “pass the word around.”

...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.

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The tried and true method for adjusting ignition advance is to advance until the power stops increasing. Continuing to increase advance after that point just increases the chance of detonation with no benefit. From your graph, it looks like you're there. Higher octane fuel will only reduce the chance of detonation but won't increase power output.

Too much spark advance and the engine fights itself by having to compress combustion pressure against a still rising piston.

Usually you start backing off timing after torque peak.....

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

...yes, what I was really wondering is what stownsen914 said.

I thought that with the lower octane fuel you were running into the start of detonation causing torque to not increase and not the fact that you've gone past the point where the fuel is igniting and maximum pressure is at the optimal point to get the most torque (leverage) on the crankshaft.

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.

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 $.

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

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

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.

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.

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

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

"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.

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.


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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)?

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.