• If you log in, the ads disappear in the forum and gallery. If you need help logging in or getting registered, send request to: webmaster@southernairboat.com

AV Gas in an Automotive boat


Well-known member
What would running AV gas in a car motor do (harm or help). My father-in-law said that they used to run av gas in his boat that had a mopar 383 driect drive on it. Would it give more power or really accomplish nothing but cost more? I run a DD 500 Caddy.

I think this issue is a lot more complicated than a theoretical opinion. I would like to see Felber or thunder weigh in on this. They can probably tell us how a chevy behaves. Combustion chamber and port design is a complex issue and it is where the HP is.

I was a little surprised by the article too.

octane slows combustion, to slow down detonation. Running higher octane fuel will not improve performance, it will actually in a "perfect world" (perfect timing curve and jetting with low octane/vs/ same engine add high octane fuel) decrease performance. In high performance auto engines they recomend using the lowest octane you can run without detonation. Using avgas will not hurt your engine only your wallet.
What Jim said is true and airboat is a good dyno, if you wanted to try both ways it would be a good test. Just make sure the test conditions are the same, same temp and humidity and water conditions.
Have fun
The octane needed by your engine depends on many factors. Of course compression ratio is the predominant factor, yet it is only one factor. If your motor has low enough compression, you wont detonate(nock and ping) even when the motor is extremely poorly built. When any cylinder starts to detonate that cylinder quits contributing any more power and actually starts to reduce its output! If you have a decent motor with 9.0CR or less AV gas(100LL) and racing gas wont help you. The octane you choose(need) should be based on your engine combination. Piston dome/dish shape,combustion chamber design,spark plug location,quench,air swirl, tumble, and fuel atomization are some of the things that dictate what octane will be needed. Detonation is the one thing that you are trying to avoid by using higher octanes. All the factors above influence when and where detonation occurs. when you build a motor you should engineer it to suppress detonation by optimizing as many factors as possible. A well built motor will have as high as possible compression with as low as detonation possible. Compression makes power/detonation kills everything!! In short :shock: too much octane wont help much, too little will destroy your pistons!!! Thunder put in a thread that its amazing how much power you can make if you purpose build a motor correctly. Thats so very true because you maximize the positives and minimize the negatives. A good web site on this subject is http://WWW.KB-Silvolite.com, they go into depth about detonation suppression. Happy Engineering :D
Wow I would rather debate creation or evolution. The fact is its cylinder pressure not compression is the main issue. I can build a 13 to 1 motor that will run on pump gas and will never detonate and I could build a 9 to 1 motor that will detonate worse than a 13 to 1 motor. Cylinder pressure and hot spots are the main causes detonation. The best way to dictate this pressure is by a camshaft. When cylinder pressures become to high the combustion of the fuel becomes to violent and fast so detonation occurs. Because the fuel charge is exploding instead of burning. Here is where octane comes in YOU WILL MAKE THE MAXUMIM HORSEPOWER WHEN YOU RUN THE LOWEST OCTANE POSSIBLE WITHOUT THE MOTOR GOING INTO DETONATION. Another way to look at it is the lower the octane the more volatile the fuel is. The higher the octane the slower the fuel burns, you will create the most horsepower by burning the fuel as fast as possible with out going into detonation. When you build a high horsepower motor with big compression you need the higher octane fuel to slow down the combustion so you’re motor doesn’t detonate. Running a higher octane fuel in a motor that doesn’t develop enough cylinder pressure will always cause you to make less horsepower. So if your caddy is not detonating on pump gas keep running pump gas because going to AV gas will cause you to make less HP
thanks for all the info. I didn't believe that running av gas would help with power like he said. I think mine runs pretty good on the cheapest pump gas available. I don't have any problems with run on or detonation. On the other hand my uncle has a 472 caddy DD that is built the way the guys at the CAD Company recommended. I think he put $10K in it rebuilding it.(at least that is his story) All new parts except the block and cast heads. (ordered from cad company of course) He has to run a minimum of 50/50 bled of pump gas and racing fuel to get the engine to shut off. This is what he believes anyway. (and it works) He isn't one to listen to recommendations when I told him to put a fast idle solenoid on the carb to help he just laughed like I was crazy. I think his cam is incorrect and he needs to go back to stock but again... he don't want to believe that his $10K engine is a pig. So he is a firm believer that you need higher octane on a boat. Its fun when someone don't want to listen to you because you are a kid. My opinion is they guys a cad company built him an engine that would be great on a reduction boat but is a horrible DD engine.
You are dead on with the solenoid IDEA. That would end his running on problem because a Holley has to come off it’s idle circuit to swining a prop while at idle the throttle blades are so far open the motor keeps drawing fuel when you shut it off so it will keep running on. The best way to prove your point is when he runs it hold the throttle at idle. Then back off the idle screw while maintaining the same Idle RPM THEN CLOSE THE THROTTLE WHILE HE TURNS OFF THE IGNITION THE MOTOR WILL IMMEDIATELY SHUT OFF AFTER THIS HAPPENS HE MAY TAKE YOUR ADVICE.