airplane gas in a 435 ?

[Ray G.]

Hello;
Have read many of the posts on getting a 435 to run well.

But, not clear on whether gas available at local airports will help the 435 to run even better ?

Or, is it even a good to put airplane gas in a high performance engine ?

Since converting to full time vacuum advance, the start, run, and drive characteristics are good on my 435 with 93 octane pump gas.

Years ago my "Race Car" friends thought it helped their street racers, but I had my doubts and always used the highest Sunoco.

Thanks for your help.
Ray

[Tom P.]

Re: airplane gas in a 435 ?

Low lead 100 (LL100) avgas, which is about all that is available these days at the airports, seems to be an excellent fuel for the older high compression/high perf engines. I use it regularly in the SB400 that's in the 56.
It is NOT legal for an airport to put avgas in an automobile, but they can put it in a 5gal can for you.
When I go to the airport on Saturdays to work on the bomber, I often take two 5gal gas cans and have them filled (I can squeaze about 11gal between 2 cans), then take it home and pour it into the 56.
There are several others that do the same thing.

[Rich P.]

Re: airplane gas in a 435 ?

Ray,

typical av gas is 100LL or 100 octane low lead. In my experiance it helps with detonation, vapor lock, rough running at idle and more. It does not fully cure these problems on it's own but does help. I have seen the improvememts in 67-68-69 435's and L-89's 63-64-65 fuelies and 65 l-79's Those are just a few of previous customers cars I used it in and found improvemnts.

Rich

[Duke W.]

Re: airplane gas in a 435 ?

If you don't have any operational problems like detonatation or percolation, then your engine doesn't need it, and it won't make the it run any better.

Duke

[John G.]

Re: airplane gas in a 435 ?

My brother and I have run avgas in a '72 vega funny car with a 468 c.i for about 20 years...only on second engine. It does seem to like cooler weather tho.

John G

[Patrick T.]

Re: airplane gas in a 435 ?

In a nearby town where I live is an old gas station that's been there since 1959. It's located on the way to 75-80 Drag strip. They sell CAM2 110 octane leaded racing fuel @ $6.00 a gallon and they sell a LOT of it.

Since my '67 L-79 engine is completely stock, I put five gallons of CAM2 in my tank about once a month. The car starts easier when cold or hot, it runs cooler, no percolation problems and I can advance the timing wherever I want it with no pinging problems. Good stuff just like the old days but not 40 cents a gallon!

[Domenic T.]

Re: airplane gas in a 435 ?

I have used 100 avgas for years and find that it works best 50/50. It does make a difference. My car feels like it did in the good old daysand You can time the engine like it was supposed to be timed without it pinging. I had to leave my distributer loose enough to turn so that when I had to put pump gas back in I could open the door and reach it to retard it to stop the pinging.
I have tried it straight and find that it works better mixed. WHAT a difference in performance. Avgas also has enough lead to impregnate the valve seat if you are grinding the stock head as long as you use it at first start and run it for a couple hundred miles or so.
Avgas was designed for a slow running low compression engines of about 2500 RPM's and it burns slowly. So the story about burning valves in army jeeps is true if you keep your foot in it long enough. A WW2 army mechanic said the slow burning avgas is still burning as it passes thru the exhaust valve.
I have NEVER been able to time my high compression engines as to the book without it pinging unless I run a 50/50 mixture.

DOM

[Duke W.]

Re: airplane gas in a 435 ?

Myths! Those 2500 RPM aircraft engines may have strokes as long as 5" so they're running at 2000 FPM mean piston speed, which is a little over 3000 on a 427.

Most hydrocarbon blends that can be considered "gasoline" for spark ignition engines burn at about the same rate - about 300 FPS in a quiescent state, but the rate increases with turbulence.

For any decent combustion chamber, combustion is essentiallly complete an equal number of degrees ATC that the optimum spark is BTC.

So if the optimum spark for peak torque at a given speed is 38 degrees BTC, combustion is complete at about 38 ATC, and this tends to remain constant because of increasing turbulence as revs increase, so "clock time" for complete combustion is reduced.

Glowing exhaust does not mean that combustion is still occuring. It simply means that some combustion byproducts, especially free carbon, are hot enought to radiate in the visible spectrum.

Combustion does take longer in flatheads because they have such large, poor combustion chamber geometry. Modern quench chambers burn the mixture quickly, which is why they require less spark advance to achieve best torque/power. The less the required spark advance, the closer combustion comes the themodynamic ideal of combustion at constant volume.

Duke

[Domenic T.]

Re: airplane gas in a 435 ?

Myths! Those 2500 RPM aircraft engines may have strokes as long as 5" so they're running at 2000 FPM mean piston speed, which is a little over 3000 on a 427.

Most hydrocarbon blends that can be considered "gasoline" for spark ignition engines burn at about the same rate - about 300 FPS in a quiescent state, but the rate increases with turbulence.

For any decent combustion chamber, combustion is essentiallly complete an equal number of degrees ATC that the optimum spark is BTC.

So if the optimum spark for peak torque at a given speed is 38 degrees BTC, combustion is complete at about 38 ATC, and this tends to remain constant because of increasing turbulence as revs increase, so "clock time" for complete combustion is reduced.

Glowing exhaust does not mean that combustion is still occuring. It simply means that some combustion byproducts, especially free carbon, are hot enought to radiate in the visible spectrum.

Combustion does take longer in flatheads because they have such large, poor combustion chamber geometry. Modern quench chambers burn the mixture quickly, which is why they require less spark advance to achieve best torque/power. The less the required spark advance, the closer combustion comes the themodynamic ideal of combustion at constant volume.

Duke

I hear what your saying but as a active certified aircraft mechanic we learned that 80 octane burns much faster than 100 and that's why we had to re-stamp the dataplates on some engines with new timing instructions to prevent detonation which can't be heard from the cockpit. Also we had to change valve & seat angles from 30% to 45% on them due to the octane changes when 80/87 was being phased out so the valves would not burn using the 100 low lead.
The compression ratios are low, about 6 or 7 to one.
The large radials turn at 1500 RPM's with those long strokes as you were pointing out.
Another advantage of aviation fuel is that it does not varnish. It's way better than STA BILL or the anti varnish additives. I use it in my lawn mower and other engines that will not be used regulary.
It would be interesting to dyno avgas vs. pump gas (with timing change)and put it in black & white. Avgas makes it possible for me to break the tires loose by just pushing the pedal where I cannot do that with my car timed to use our highest octane here in CA of 91.
The pump gas today was not concidered to be a good grade of regular in the early days. A 68 ford manual reads " use a good grade regular of 94 or more octane."
Try it youl like the way it runs I guarentee it.

DOM

[Jack C.]

Re: airplane gas in a 435 ?

Dom, I been using it in my 427/400 for over 7 years and I swear by it. My local airport looks the other way when I pull up to the pump. I know my configuration isn't as robust as a 435, but to me it's worth it.

[Duke W.]

Re: airplane gas in a 435 ?

The pump gas today was not concidered to be a good grade of regular in the early days. A 68 ford manual reads " use a good grade regular of 94 or more octane."
Try it youl like the way it runs I guarentee it.

DOM

You're comparing apples and oranges. The mogas octane specified back in the sixties was based on RON. Modern fuels are specified as PON. Then there is MON and Aviation Method.

All these have been explained in the past on this Board along with how to convert from one to another and how to compute the octane of a blend.

I've recommended blending 100LL avgas with current pump gas if the engine detonates or has a severe percolation problem. Unless either of the two exist, it's not worth the hassle.

Some vintage engines may require less than ideal spark advance to avoid detonation. In that case, blending in enough 100LL to quell detonation can yield more power.

If timing has to be retarded about 2-3 degree from optimum, power is reduced about one percent.