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Old 27 Sep 2010, 08:23 (Ref:2765300)   #1
Burfs
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Complete novice question - sorry!

Hi,

I have raced my Lenham for the last few months without knowing how much fuel to fill it with. I have been running on 3/4's full at all circuits, knowing full well that I am probably carrying too much. I measured my tank over the weekend and found that is is 20 litres when full. My question is, what average MPG (a range would be better than nothing!) do you think I am getting so that I can adjust the fuel load? I have a Peter May 1298 A series with twin 1 1/2 inch SU's. The car weighs about 780kg. As I said, any idea is better than me guessing at the moment...!

Thanks in advance for any help,

Ian.
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Old 27 Sep 2010, 09:14 (Ref:2765326)   #2
John Turner
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Ian, no need to apologise; seems a perfectly reasonable question. Hope you get some equally reasonable replies. With hindsight, I guess that if you had recorded roughly how much fuel you had been carrying at each event, how long the race was and how much you had left at the end you would have come up with a rough estimate, although it would probably vary a bit at each circuit depending on the circuit's characteristics. It may be the way to go for next season if your racing season has already ended, if nothing better appears here!
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Old 27 Sep 2010, 09:26 (Ref:2765339)   #3
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I find that it can vary, however if you know the vloume of fuel at the start of practice, count the number of laps you do then measure it afterwards, you can determine the consumption. Its what I do and I know many others do so too.

Bare in mind you aren't going to lose three tenths to the next bloke due to fuel weight.
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Old 27 Sep 2010, 11:04 (Ref:2765385)   #4
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Fairly difficult to hazard a guess - it can vary dramatically - our Porsche probably does 8 - 10 mpg at racing speeds, tho we usually aim to have up to 1/4 of a tank left at the end of the race - as this can show as empty at various parts of a circuit and I prefer to have one less thing to worry about...

I'd surmise that you'd probably be getting around 15 mpg at a worst case...but as said its really just down to doing an accurate assessment by for one race putting in a known amount (on the generous side) then pumping out what remains at the end of the race and measuring it.....and of course not worrying too much about it as an extra gallon or so isn't going to kill your lap times
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Old 27 Sep 2010, 11:34 (Ref:2765406)   #5
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Another way is to estimate using your known engine BHP and running time.

For 100 BHP you will use about 31 litres of fuel per hour, running at peak revs with a wide open throttle - you won't use any more than that unless the air/fuel ratio is very wrong.

So you should be able to work out the maximum you would need if you are going to a new circuit - a 15 minute race with a 100 BHP engine would need 7.75 litres maximum for the race, plus a bit more for getting from the paddock onto the grid, the slowing down lap and for getting back to the paddock.

Once in the ballpark you can measure how much you have left in the tank after you are back in the paddock to find out the true consumption. Then make a note for the next time you are at the same circuit.
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Old 27 Sep 2010, 11:45 (Ref:2765421)   #6
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Burfs should be qualifying in the top 10 on the grid
Thanks all. I have plenty of ideas now, and I should have really made better notes at the circuits I visited this year! There was so much to learn and take in during the first year that something like fuel usage took a back seat..
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Old 27 Sep 2010, 11:55 (Ref:2765428)   #7
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Colin McKay should be qualifying in the top 10 on the gridColin McKay should be qualifying in the top 10 on the grid
Some guys are quite obsessive about weight and I know one chap who drains the fuel after every qual/race and then puts a known measure back in, all to save the risk of carrying an extra kilo or two which in a touring car seems a bit of overkill. My own car varies a fair bit from race to race, eg at Spa it uses less per mile than at Cadwell where you're doing a lot more braking and accelerating.Not that I've ever measured it, all done by intuition and never been caught out except for the time the car wouldn't start when I went to unload it after a meeting, perfectly judged
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Old 27 Sep 2010, 17:45 (Ref:2765625)   #8
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Should be no need to guess Ian. These days when you can look at all your lap times and how long you raced when you get home.Fill the tank before the race or qualafing then refill afterwards that will tell you how much you have used then look at your distance covered in race plus a bit for warm up lap and then its job done.
I will say this when i took my racing a bit more serios than i do now in the early 80,s i had an engine builder build my engines and we would split the prize money. He was obsessed with working out the fuel we would need.needless to say the first race i was stolling away in the lead untill last lap and we ran out.
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Old 27 Sep 2010, 20:08 (Ref:2765708)   #9
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skentellytubby should be qualifying in the top 10 on the grid
Ian's suggestion is what I do...now..

You can be too obsessive about it. As its my first season I thought there was a really scientific way of doing it and thought 'I'll be reeeally clever and professional, like!"

My car is a bit overweight like for like so I have looked for an edge, rather than listening to advice given to me (that being what Ian has suggested above!). Consequently I have 'calculated' my fuel load based on 'known' fuel consumption and had to be towed in twice (on my in laps luckily, once at the end of a race and once at the end of qually)....this season alone!?!!

Given up on being too clever. We are talking old cars. No common rails in sight! The variability is huge track to track so base it on what you've actually used (handy if you've had a test day before, otherwise you just have to make sure you've got enough for qually and use that as the measurement for the race)

Race notes as soon as you get home are very important for fuel usage (as long as you don't soup your engine up in the close season, then its start again time!) and Mike (my co-driver and esteemed MG man) has drummed this into me.

Anyway, from now on I'll avoid pies, forget the weight of the extra fuel and make sure I've got "one or two extra litres"!

Last edited by skentellytubby; 27 Sep 2010 at 20:15.
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Old 27 Sep 2010, 20:16 (Ref:2765712)   #10
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With our 1380 FISC cars we used to run about 3/4 full for a 30 minute race and never suffered surge. That was using the larger "1500" Midget tank mind you, which measures over 6 gallons so we were putting in 4 gallons-ish. Which tank are you running?

Of course, we never had the grip to suffer from surge too badly.

100 km at Nordchleife was touch and go, we brimmed to the top of the filler neck ;-)

I reckon you're OK at 3/4 full, especially if you're only running the small tank.
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Old 28 Sep 2010, 11:57 (Ref:2766009)   #11
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Old 29 Sep 2010, 22:23 (Ref:2766900)   #12
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Nick JM should be qualifying in the top 10 on the grid
Agree with Delta... in my wealth of 5 race experience. Just don't runout on the warm down as it could be embarrassing, eh Skells?
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Old 30 Sep 2010, 16:09 (Ref:2767256)   #13
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gilduffy should be qualifying in the top 10 on the grid
Fuel Consumption

We work on 35 litres per hour on a 1460 modsport Midget with around 130 / 140 BHP 540 kilo and big slicks. Lots left if it's wet!
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Old 30 Sep 2010, 18:41 (Ref:2767355)   #14
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Agree with Delta... in my wealth of 5 race experience. Just don't runout on the warm down as it could be embarrassing, eh Skells?

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Old 30 Sep 2010, 18:45 (Ref:2767357)   #15
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Agree with Delta... in my wealth of 5 race experience. Just don't runout on the warm down as it could be embarrassing, eh Skells?
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