Hi-Octane

[smoothie]

The motor in my boat has 12:1 compression,the guy I bought the boat off of had a gas tank in the back of his pick up truck that he would take to the local airport and mix the airplane fuel with regular gas to run in the boat,Is there a good octane booster out there that I could pour in the tank if let's say Im at a marina at lake cumberland to fuel up.Or does anybody have a secret recipe they are willing to share?

[Ranman]

I've heard stories about aviation fuel, but don't know if that is a safe way to go. A good choice would be VP Red fuel. It's 105 octane and recommended in marine engines up to 12.5:1 compression. Don't know what it costs, but I pay about $5 a gallon for VP's C-12. The Red is probably a bit less, but most likely still expensive. You can check out http://www.vpracingfuels.com/main.html

There are a couple of guys on the board with high compression Donzis (Dennis I think), maybe they have a more economical suggestion. Good luck.

[Rootsy]

you'd be amazed at what kind of horsepower you can pick up by switching fuels or grades of fuel... just watch out cause you may see some rocket fuel and figure if some hot motor can run on it so can my motor.. and it may just be too much and it'll actually hurt your performance.. and possibly the engine... anyone with some really screaming stuff wants to squeeeeeze some extra juice out of it try some ERC... NHRA outlawed it... picked us up 1/10th and 1 1/2 mph and on a 90 degree day with the motor idling the heat of vaporization is so high for this fuel that the carburator had frost on it... its pretty wicked.. same goes for VP C-44 and 921 although it isn't quite as potent... just watch out though... this stuff isnt for mild or even slightly radical engines... and it can get pretty dang pricy... you can check it all out at vpracingfuels.com... another good choice for performance motors is some of the sunoco fuels... sunoco purple should be plenty for most anything out there...

[RickSE]

I have a SBC 400 with 11.5:1 compression that I've been running in a car for 15 years. This motor will run on 92-93 octane if I'm careful. Careful means backing off the timing, 4-6 deg advance and I have to shut the car off in gear or it will diesel.

I agree with Ranman, the VPRed is a great fuel and the octane is just about right for my application. I paid $5.00/gallon for it this Fall. With this fuel I can run the base timing up to 12 deg. and make some good power. Octane level is an important factor in making good power, too much octane and you will loose power, too little and you may loose an engine.

12:1 is pretty high. Mixing may help a little but your best bet would be to run straight race fuel, 105-110 octane. I would not mess with aviation fuel due to all the additives. Your other option is to change the heads, if their small chamber heads 62-64cc you can change to 76cc heads and have compression in the 10:1 range.

[Ranman]

RickS, Sounds like your right on the edge with the car motor. I know that retarding the timing and maybe even a colder plug would help. That scenario, however won't fly in a marine application though. There's just too much load on the engine. With 12:1 in a boat, you either need to run good fuel or lower the compression.

[GEOO]

Smoothie,

I have been doing some research on octane booster, (for my car). Most will add 2 to 7 points. ie. 92 octane boosted up to 92.2-92.7. Not too good. The best bet is to mix some race gas with high test, ie 10 gals 92 octane + 10 gals 100 octane= 96 octane. However, I came across several articles on the web about "Rocket Fuel". A paint thinner you can buy at Sherwin Williams called Toulene. I have read about this additive but have not tried it. Type in Toulene fuel additive or octane boost and do a web search. here's one site
http://members.aol.com/bluetalon2/octane.html GEOO

[RickSE]

There are a lot of web pages like this one that will help you determine the mixing ratio for the desired octane. Like Ranman said, you'll probably need at least 105 octane to really run right. Looks like a 1:1 ratio with 116 & 92 will give you 104.

http://www.gnttype.org/techarea/misc/Octanemix.html

Also, here is a good site for properties of various race fuels.

http://www.centuryperformance.com/racefuel.htm

[Greg]

I've had pilots tell me that one of the problems with running airplane fuel in an automotive/marine engine is, its not whats in it, but what's not in it.

[smoothie]

Thanks alot for the good info guys,looks like I need to do a some research and development.I think I really need to get in touch with the person that built the engine to see what I have inside the motor.

[BigGrizzly]

I have races engines with over 14:1 compression ratio and can tell you thid DON'T use ave gas. it has deicers in it. The race gas mix is a good way. Tolulene has been around for years and had been used into fuels before the feds got into it. When Chevron was the white gas it was used in it to up the octane rating. The only problem is mixing it the same all the time and it detereates some rubber hoses. The other problem is the concentration from paint store isn't always the same. It is also incorporated in some octane boosters. If it were my boat I would check the compression ratio and make sure it was really that high.

Randy

[smoothie]

Randy
I will check the compression this weekend,And if it really is 12:1 what would be the best thing I could do to get back to regular gas,I know for sure that it has alum.heads,cam,(dont have specs)solid lifters,pistons,torker manifold,4 bolt mains,holley 750 carb and nos system(used twice said he had a hard time steering the boat)its a chevy 454 bored .030 over.The previous owner also drag raced cars.He passed and I bought the boat from his 65 yr old sister so thats why i dont really know much about the mill.

[GEOO]

Smoothie,

What boat is this in? I think you will need to lower the comp. down to at least 10.5:1, If you have alum. heads, 10:1 with iron head, to run on 92-93 octane. Possibly opening up the combustion chamber on the heads or get new pistons. Don't use too thick of a gasket or play with a shorter rod, too much quench(space between top dead center on the flat spot of the piston and the head) won't run as well. ask Randy He'll explain it!! GEOO

[BigGrizzly]

Geoo is correct. Don't get sucked into the cam that can make high compression engines run on regular gas trick. I've learned alot about combustion design and its a magic all in its self. The rod thing is easier to explain. Too short of a rod in some cases a good thing but not in the american auto block boat configuration. two things happen 1) the rod angle gets too steep and puts more stress on the primary thrust side of the piston, 2)n the piston accelerates accross top dead center faster and causes excessive loads on therods and crank. Most companies offer longer rods to change these problems. Too long of a rod and the piston gets lazey accross top dead center and the engine accelerates slow. Years ago Honda road race team used the short rod thing to get better acceleration out of their engines, this caused a multitude of problems. They did accelerate well and with our budget durability didn't matter. My one engines use the longer rod when ever possible (my budget is small).

Randy

[Rick]

Aviation fuel is only 100 octane and low lead, thats not enough octane for a 12:1. Not to mention it is illegal for you and the airport. Put Av gas in an airplane and performance fuel in a performance motor. 105,109, and 114 octane are not uncommon with VP or Sunoco. Good luck!

[Tom Davis]

I ran a high compression 285hp outboard on nothing but av gas and amsoil. All oil injection was re-routed by merc racing lab. Done up right, this thing ran like a swiss watch and even idled. My understanding of av gas is that it is very clean fuel due to the requirements of airplane engines. It's hard to get towed back to the airport....
Just my .02
Donzi Tom

[smoothie]

Geo
The motor is in a 20ft cigarette.I called a speed shop today and they said to bring the heads in to see what they could do with them.Thanks again everybody for all the good info.

[Jamesbon]

Smoothie,
Sounds like a nice ride!! As far as I'm concerned, high compression rules!

On that note, will I be okay with 10.3:1 compression and Merlin iron heads in a 508 ci BBC? Will 93 octane pump gas & octane boost be okay? (assuming I'm somewhat conservative with timing...30-35 deg. total) Or should I consider mixing racing fuel? Also considering adding a knock sensor and exhaust temp. guage...

I know I'm on the borderline with the compression and would appreciate your input...

[BigGrizzly]

Nate, I never run my timming above 35 on any boat that I have had. Rember my Corsican, it has 10.5:1 compression ad only run 28 full advance. there are many different things in that motor I will not explain, but I can run 87 octane in it without trouble. I do normally run 93 if possible just because I want to. As you witnessed it runs just fine. I'm sure someone is going to say "you can get more power by advancing the timming", but before you do, I have worked with this engine for over 5 years and have eight different cams , 5 different carbs and three different exhausts. this combo works well for me. My Criterion runs 34 deg of timming and 9.5:1. point is you just got to try it and watch it. Piston dome or dish plays a big part in the timming. I also have an exhaust temp guage on Criterion.

Randy

[Jay Robertson]

I have seen aditives on a dyno. No hp increase in some cases a decrease.Some did control preignition. race fuel is always better.

[BigGrizzly]

Jay has point, If you have low octane gas, additives only help the pre ignition and then only to a point.I have also done the dyno test the other way. I used a low compression engine and raised the octane of the fuel and also lost power. The low octane gas burns faster and shorter. The result also made the exhaust dirty with CO and NOX. The more I advanced the timming the cleaner it got to a point and power dropped. Now we are talking small differences here.

Randy

[Klingon]

Aviation fuel mixed with regular gas will get you more octane numbers. Not only that but a little goes a long way. If you mix a gallon of unleaded super, lets say 92 oct, with a gal of 110 avgas, what you get is not the average of the two , 102, but about 106 oct. Aviation fuel is not the best choice for gas though, it is blended for a low rpm running, below 3000rpm. A better choice is a car racing fuel. You can usually find this stuff at your local hot rod shop or race track. I have found that all the legal over the counter octane additives don't really help, at the most maybe 1 octane number. Compression ratio is not the only factor in determining the octane you need. Cams come into play, fuel mix and altitude. Most of the newer cars run C/R 11 and above all on pump gas but they control the timing and mixture and in some cases the cam timing.
Alan