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Fb 12a Track Mpg, fuel starvation

Old 04-13-12, 02:50 PM
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Fb 12a Track Mpg, fuel starvation

1. I hope its not too crazy of a question, but what is the average mpg people are seeing on their fully stock Fb 12a on the track? Using it at Road America.

Car FULLY gutted with cage and no glass except windshield.

looking for a range since I know there are a lot of factors.

2. Also, do you guys get fuel starvation on this car on street tires? First time I'm taking a carb'd motor out on track so don't really have knowledge if the tank/bowls/pump give enough fuel around fast corners.

If yes, what's the minimum amount of fuel required in the tank to prevent it?
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Old 04-16-12, 12:09 PM
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Saw this one a bit late but here's my observation. The car is a 1985 that is prepped for SCCA IT7/ITA/STU and NASA PTE classes. We have also run this car in the Chumpcar series in their "EC" class. We run a stock 12A with a PB&J carb and Racing Beat headers with a custom exhaust. Nothing special about the fuel delivery system either. (Need to work on this part though!)

Ran one SCCA weekend before a Chumpcar race a few years ago just to get a handle on fuel mileage for the 24 hour Chumpcar race. Much to my surprise the car returned a credible 9 MPG for that particular weekend! I did not baby it at all winning my class both days and top 5 finishes overall in the run group.

For the Chumpcar race we set an arbitrary 6800 rpm rev limit to get even better consumption. Our recorded mileage for a total of 13 hours of the event (till we blew an o-ring seal!) was right at 10 MPG. Now take into consideration this was for an engine that gets all new tune up stuff every race weekend along with driveline fluid and all filter changes. We also did a tune up on it during one of the extended 15 minute breaks at the 8 hour mark of the event.

No problems with fuel starvation as we have managed to run it empty during one HPDE just to see how it would react to low fuel and sloshing around.

Quick description on level of car prep: Camber plates up front, big ST swaybar up front and no rear bar, Koni DA's on a custom coil overs up front with Koni SA's in the rear, stock diff (welded!), along with all the cage/safety stuff per the regs. Hopefully this helps a bit. I wouldn't be surprised to see a not so well tuned carb/engine combo to get less than 7 MPG in a track environment. Maybe way less!
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Old 04-16-12, 02:36 PM
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Awesome reply, thanks!

Have you measured a/f ratios by any chance and what's usually the ideal numbers out on track? I'm probably going to put a wideband on the car.
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Old 04-17-12, 07:09 AM
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Never done that. However..............................

Every time we pull the plugs they come out clean as can be with very light brown coloring and no indications of an over or under rich condition. I am not sure what size jets the carb has but it really seems to work and has been flawless to date. And one other point, this is all on regular unleaded pump gas 87 octane.
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Old 04-23-12, 09:34 AM
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I really dont know the milage.. but in a 45 minute stint at Sebring or Daytona I use about 5 gallons of fuel...

As far as starvation goes.. yes I used to get it pretty bad at the top of 3rd of halfway through 4th. This is with all the floats set properly etc.

My solution from the builder who was teaching me tricks from classes like Sparkplug challenge RX3s, when I started.. (Gregg OBrien) was to run a 1/2 inch fuel line right up to a regulator before the carb. Then run 6 or 7 pounds of fuel pressure.. enough so that you dont overpressure things and flood.
Also a holly blue fuel pump, or similar that is higher volume. Im currently running a mallory Comp 140 fuel pump.. and have been for about 6 years or so.

The higher volume pump, with the bumped up pressure, will have a tendency to alleviate all but the most severe starvation issues we have. THe larger lines allow the higher volume of fuel.
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Old 04-23-12, 05:28 PM
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Thanks for the info guys. We finish the weekend at Road America and looks like about 9mpg was the magic number. Car is completely stock aside from an unknown header. We did have a oil pressure issue, so revs were kept under 6.5k.

But did not have any fuel starvation issues anywhere on track.
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Old 04-23-12, 05:29 PM
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Originally Posted by SPiN Racing View Post
I really dont know the milage.. but in a 45 minute stint at Sebring or Daytona I use about 5 gallons of fuel...

As far as starvation goes.. yes I used to get it pretty bad at the top of 3rd of halfway through 4th. This is with all the floats set properly etc.

My solution from the builder who was teaching me tricks from classes like Sparkplug challenge RX3s, when I started.. (Gregg OBrien) was to run a 1/2 inch fuel line right up to a regulator before the carb. Then run 6 or 7 pounds of fuel pressure.. enough so that you dont overpressure things and flood.
Also a holly blue fuel pump, or similar that is higher volume. Im currently running a mallory Comp 140 fuel pump.. and have been for about 6 years or so.

The higher volume pump, with the bumped up pressure, will have a tendency to alleviate all but the most severe starvation issues we have. THe larger lines allow the higher volume of fuel.
Interesting trick, definitely will keep in mind if we build a thirstier engine.
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Old 04-27-12, 08:05 PM
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Are you running a Nikki carb? Can't recall off hand. If so, you'll be looking for something around 3psi for fuel pressure.

If you start bogging on left hand turns, slightly reduce fuel pressure until it stops.

Starvation at top end of 3rd or 4th gear, the first place to look is the fuel filter. Even when brand new, they just barely flow enough to keep the carb happy. Even a 5% loss in flow rate will leave you gasping at the top of 4th gear (yes, that is a total guestimate).

Here's what I did to handle the filter bottle neck, and there are some other good ideas that others contributed in this thread: https://www.rx7club.com/1st-generation-specific-1979-1985-18/dual-fuel-filter-mod-528445/




.
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