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Discussion Starter · #1 ·
. I was having a little 2500 rpm ping problem in my jet so I decided to play with some good gas, 111 mixed about 50/50 with 91. I forget to bring the jet kit to the lake. I had it jetted for a 3500 ft. lake in Nebraska so I figured that I wouldn't have worry about being lean at 4800 feet. Well anyhow my question is with all conditions being the same what would be the difference? My boat ran lean and I had to time my way out of it since I didn't bring any jets. I will be running a heavier advance spring next weekend wich should clear up my lower rpm ping but I just wanted to play with some good gas. Well anyhow my question is with all conditions being the same what would be the difference in jetting between 91 and race gas??
 

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I think you are adding way too much race gas for you application unless your comp ratio is fairly high. I have a pump gas 565, and mix mine approx. 4 1/2 gal of 91 to 1 1/4 gal of 110, which results in about a rating of 95.2. Works fine for my deal, but my engine was built to run on 91. I do it for peace of mind more than to cover a problem.

If your jetted safely, I don't think the change in base altitude would cause a problem. As the corrected altitude goes up you will be running richer.
 

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I don't know chit, I mixed 110 and 87 octane with limited success, this is a total busby question. But I think jetting wise it depends on if the race fuel is oxygenated or not.
 

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Discussion Starter · #4 ·
I think you are adding way too much race gas for you application unless your comp ratio is fairly high. I have a pump gas 565, and mix mine approx. 4 1/2 gal of 91 to 1 1/4 gal of 110, which results in about a rating of 95.2. Works fine for my deal, but my engine was built to run on 91. I do it for peace of mind more than to cover a problem.

If your jetted safely, I don't think the change in base altitude would cause a problem. As the corrected altitude goes up you will be running richer.
It was actually 9 at 91 to 4 at 111 so I don't know why I posted 50/50. My compression was 11.0 and I have run fine at 900 feet with 93 octane. I was just surprised about it running lean. I also put a new dist cap on it the same morning and I think it changed my timing slightly. I was just curious as to why good gas would run leaner with the same jetting. I am having a tough time finding my jeting by plug color since I switched to aluminum heads. I am stuck going by rpm for jetting, for now, and nailed it perfectly last weekend at lower altitude. I was also up one jet size from what I normally run at Pueblo so I was surprised by it running lean.
 

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race fuel

Several years ago I run a 50/50 mix of 114 and av gas in a 13 to 1 mill. an old man told me that its bettet to run one or the other. He told me they had different specific gravities ( one a little heaver than the other ) and would not mix well togather. Also told me that they would burn at a different rate so presice tunning would not be as accurate. Myself I dont know, But I started running 114 only and was suprised at the difference in power. If someone with some understanding of fuel could elaborate on this I would sure dig it.
 

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Amber Racing Services
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Jetting has everything to do with the specific gravity of the fuel.

The specific gravity relates to how heavy the fuel is ... kind of like viscosity in oil.

If you remember that a heavier fuel will act as if it is thicker ... you'll be ok. The heavier the fuel, the larger the jets you'll need to move the same volume of a lighter/thinner fuel.

As far as mixing race gas w/ pump gas ... you should be fine if you're mixing a 91 and 110 if you have a safe jetting now. That should take care of your pinging.

Just make sure you adjust your float levels & check your plug readings to make sure you're not too lean.
 
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Years ago I was running a 9.5 to 1 lake boat on 87 pump gas. Ran great. One day I was out of gas and the guys I was running with were using Av gas. I put 5 gallon of it in mine and it ran like crap. Idle sounded like a fueler and it would only run on plan if I had my foot in it. Got back on the trailer and when I pumped it out, I only got about 3.5 gallon back. I only drove it about a half mile. Anyone know why it didn't work in my engine?

Tim
 

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Amber Racing Services
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Years ago I was running a 9.5 to 1 lake boat on 87 pump gas. Ran great. One day I was out of gas and the guys I was running with were using Av gas. I put 5 gallon of it in mine and it ran like crap. Idle sounded like a fueler and it would only run on plan if I had my foot in it. Got back on the trailer and when I pumped it out, I only got about 3.5 gallon back. I only drove it about a half mile. Anyone know why it didn't work in my engine?

Tim
Well Tim,

I'm not too sure & I'm not sure anyone else would be either. Sounds kind of like a riddle. LOL

There are just too many variables, but it should have ran ok with that compression.

All I can see from what you've posted is that you insatlled 5 gallons and only drove 1/2 mile, in which time you used 1.5 gallons.

Something is wrong there.

Av Gas specific gravity varies from .670 - .720, 91/92 octane pump gas has a specific gravity of .730 - .760. So pump gas is a heavier fuel and therefore has more mass. Would require a larger jet to move the same volume.

Would this be enough of a difference to matter? For a perfectly adjusted engine, probably not, but if it were a bit off to begin with it could be problematic.

Not saying that your floats/jets were off, but if they were a bit off to begin with, using a different fuel with a different specific gravity could make it run very poorly.

Maybe that helps?
 
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Thanks Busby. This was many years ago. Always wondered why it did that. The other guys ran fine on it. What I've learned about fuels and what you're saying, I guess that I was probably jetted rich and when I put in the AV gas with its lower specific gravity it let the fuel pour through the jets and went way rich. Could that be what happned?

Tim
 

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Jetting has everything to do with the specific gravity of the fuel.

The specific gravity relates to how heavy the fuel is ... kind of like viscosity in oil.

If you remember that a heavier fuel will act as if it is thicker ... you'll be ok. The heavier the fuel, the larger the jets you'll need to move the same volume of a lighter/thinner fuel.

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Stupid question here.......

Does this at all relate to how the fuel is made ? BSFC numbers are generated by fuel weight ( lbs per hr) Doesn't a heavier fuel flow more weight through a jet than a light fuel say; as an example ( just numbers) .740 SG vs .718 ? I thought I remembered jetting up when we ran a lighter fuel back in the day we ran a VP vs a Trick :)bulb Maybe not.

S CP :D
 
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