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Fuel Usage for Weight Calculation


King Matt1548534716

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I'm trying to establish my pre-race weight. Can anybody give me some examples of fuel usage in pounds during a typical 20 or 30 minute session for a Mustang? I'm thinking it will be anywhere from 5-10 gallons at about 6 pounds per gallon. Is that high or low? Also, how close are most of you running to the post-race minimum weight?

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Matt, 5 gallons is a good estimate, though I thought it's closer to 8 pounds per.

 

Don't forget to figure that you need to be higher though. You don't want to go out and run 4 caution laps before the race is red flagged. Next thing you know, you've only burned 2 gallons, and you don't want to come off under.

 

I think the safest way is to know when your car will run out of gas on the track (i.e 5/8 tank on a 3rd gen). THAT's where you should get the weight reading - ensuring you won't be under.

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...or take the approach that I took at weighing...

 

Fill it to the top and at least you're starting with a known amount and not the amount that the GM gauge says... BTW - gas is about 8lbs/gallon.

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Accrding to the US Dept. of Weights and Measures the weight of one gallon of:

 

water - 8.37 pounds

ethanol - 6.59 pounds

gas - 5.8 to 6.5 poounds

 

yep, different blends have different weights with gas...my flight manual says 5.67 pounds per gallon...

 

but, hey, when you carry an extra 50 pounds around the middle like me it doesn't really matter (another good reason to race a 4th gen and the higher weight combined) Richard "this scale must be broken" Pryor

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Well, now that have established that gasoline does in fact weigh about 6 pounds per gallon, can anybody with a Mustang tell me approximately how much of it they are using in a sprint race? I'd also be interested to hear how much of a post-race weight margin anybody is running.

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Sorry. Maybe it's just me but it seems like this log in thing is getting worse. I swear I just logged in before posting.

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Since I have an f-body, I'm not sure what your consumption should be. Since I have the stock tank I have to start each session full to avoid starvation towards the end of the sessions. Besides, on a front-heavy car, a little extra weight in the back isn't necessarily a bad thing.

 

Weight and power tolerances. FWIW - I plan to run a minimum 3% margin for both items. On the hp, this is a little tight. According to DynoJet themselves, you can expect a 3% variance dyno-to-dyno. I have to believe scales would be in this neighborhood, too.

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Keith,

 

3 percent of the 3150-pound limit for a Mustang is 95 pounds! I'm sure the scales are far more accurate than that. I'm thinking the tolerance should be more in the range of about 0.5 percent, or about 15 pounds.

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Yeah I agree that the scales would be much more accurate that than, but I have seen a variance of over 100 lbs for my car last year between these three scales with no mods:

 

1) The 4-pad system I used last week for my CMC efforts (I kinda trust this)

2) The nice, indoor, permenant scale system at Mid Ohio (used by CART - I trust this one)

3) The 2-Pad system we used at the first NASA Beaver Run race last year (I don't know if I trust this one)

 

The variance I saw was beyond fuel loads. I'd really hate to come off the track, get weighed, then get DQ-ed. At the Beaver Run race, I seem to remember someone being very annoyed at their weight. I can't remember if they were DQ-ed or not.

 

It's not a just a matter of repeatability of the device - it's also the variance between the scale systems that worries me (including how level the scale system really is). I take the safe route on this. 100 lbs isn't got to make/break my lap times. Last weekend, someone was DQ-ed in the Texas region race for being 40lbs under. I don't claim to know the specifics but I find it very hard to believe that this guy started the race knowing he was under... ...and 40lbs!

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The closer you get to the fire, the easier it is to get burned.

 

Seriously, if I spent as much time working on my driving that has been used to weigh differing octane grades of fuel, I'd have at least a half second improvement.

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Last weekend, someone was DQ-ed in the Texas region race for being 40lbs under. I don't claim to know the specifics but I find it very hard to believe that this guy started the race knowing he was under... ...and 40lbs!

 

IIRC, he hadn't weighed the car at all that weekend. I'll let him fill in the details.

I did benifit from it though, by being bumped to third place.

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No doubt there could be variations between equipment, but I should hope that we can count on being able to weigh in at the beginning of the weekend, make adjustments based on that baseline, and have confidence that there won't be a 100-pound margin of error with that same equipment. Obviously if you try to cut your weight too close to the limit, you might get burned.

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I agree that there shouldn't be that much variance, but there could be that much variance and that is what I am hedging my bets against...

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Put the car on the scales with a full tank of fuel before the race. 5 gallons of fuel used per 20-30min sprint race is a good estimate for a Mustang. Figuring 7lbs. per gallon, if you're not 35 lbs. over minimum, then add ballast.

 

At MSR, I was 25 lbs. over min. weight in post-race impound using this method.

 

I always like to start the race with a full tank because it is a more favorable location for 'ballast'.

 

--Neil

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