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Old 02-24-2012, 03:27 PM   #1
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Default Fuel strategy for endurance racing

We do a lot of chump/lemons racing in our 94 Miata. At lemons we have a much more relaxed attitude to the racing, but Chumpcar is a different story. They allow a maximum of a 2 hour stint for each drive to be on the track and to be competitive we need to move closer to there from our current hour and a half stints. We are trying to figure out the best way to increase our fuel efficiency without sucking the fun out of it.

Currently we are throwing around a couple ideas. One would be to run with an SM throttle body plate and another is to install a shift light to shift at 6700 instead of winding it all the way out. Is our logic flawed in thinking that the smaller intake of air will result in better fuel economy for the drop in power? A programmable ECU seems like it would do wonders to give us both better fuel economy and more power, but sadly that option is off the table.

The main thing is that we need to use the entire capacity of the 12.7 gallon fuel tank. If we can put 11.5 gallons in consistently, that will help immensely as it will be about 1.5 gallons more than what we were putting in before. A programmable fuel gauge will help this as the damn OEM one shows empty with about 2-2.5 gallons left.

We came in second at our last lemons race with just fast pits and mostly penalty free drivers. Mechanical reliability was great too. Again, we were having to pit every hour and a half though.
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Old 02-24-2012, 04:04 PM   #2
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We do a lot of chump/lemons racing in our 94 Miata. At lemons we have a much more relaxed attitude to the racing, but Chumpcar is a different story. They allow a maximum of a 2 hour stint for each drive to be on the track and to be competitive we need to move closer to there from our current hour and a half stints. We are trying to figure out the best way to increase our fuel efficiency without sucking the fun out of it.

Currently we are throwing around a couple ideas. One would be to run with an SM throttle body plate and another is to install a shift light to shift at 6700 instead of winding it all the way out. Is our logic flawed in thinking that the smaller intake of air will result in better fuel economy for the drop in power? A programmable ECU seems like it would do wonders to give us both better fuel economy and more power, but sadly that option is off the table.

The main thing is that we need to use the entire capacity of the 12.7 gallon fuel tank. If we can put 11.5 gallons in consistently, that will help immensely as it will be about 1.5 gallons more than what we were putting in before. A programmable fuel gauge will help this as the damn OEM one shows empty with about 2-2.5 gallons left.

We came in second at our last lemons race with just fast pits and mostly penalty free drivers. Mechanical reliability was great too. Again, we were having to pit every hour and a half though.
Why don't you just not drive %100 all the time, It is a 24 hour race with manditory 5 minute fueling pit stops. We usually make it 1:55 minutes on a tank of fuel, and especially at tws, you will feel the car hiccup in the stooges, that will tell you that the car has about a lap of fuel left. Any other questions feel free to PM me, But of course, I have some secrets that I just cant tell you
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Old 02-24-2012, 04:28 PM   #3
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How about just setting up a throttle stop and fine tuning it over time? Much easier to eliminate a "soft-footing it" variable, especially when racing and someone is pulling away and you mentally want to chase them down. You are still being able to use WOT.

You can't use any ECU mods at all? Can you use a variable rate fuel pressure regulator to manually reduce fuel pressure at the rail which could allow you to lean it out a fraction of a point in AFR?
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Old 02-24-2012, 07:24 PM   #4
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What year is this rharris? If its the early trapdoor maf, you can crack her open and tighten the spring a click or two. it will lean the whole fueal table out, and you can actually pick up a few hp. although I am willing to bet you already knew this from sm.
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Old 02-24-2012, 08:45 PM   #5
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Why don't you just not drive %100 all the time, It is a 24 hour race with manditory 5 minute fueling pit stops. We usually make it 1:55 minutes on a tank of fuel, and especially at tws, you will feel the car hiccup in the stooges, that will tell you that the car has about a lap of fuel left. Any other questions feel free to PM me, But of course, I have some secrets that I just cant tell you
The guys on this team are good sprint drivers. Telling them to go easy will be like telling the fat guy he needs to work out. They know it and they may try it, but will give up because they think it sucks.

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How about just setting up a throttle stop and fine tuning it over time? Much easier to eliminate a "soft-footing it" variable, especially when racing and someone is pulling away and you mentally want to chase them down. You are still being able to use WOT.

You can't use any ECU mods at all? Can you use a variable rate fuel pressure regulator to manually reduce fuel pressure at the rail which could allow you to lean it out a fraction of a point in AFR?
That may be a viable solution that I was trying to get at with the restrictor plate.

They see the car as a $500 car as is, so no performance mods are allowed at all without penalty. Some people get away with it on various cars, but our will stay 2 legit 2 quit.

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What year is this rharris? If its the early trapdoor maf, you can crack her open and tighten the spring a click or two. it will lean the whole fueal table out, and you can actually pick up a few hp. although I am willing to bet you already knew this from sm.
The car is 94, so we have a maf. I know plenty of the 1.6 guys do it and most never get caught in SM.
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Old 02-24-2012, 09:55 PM   #6
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The car is 94, so we have a maf. I know plenty of the 1.6 guys do it and most never get caught in SM.
Its actually legal in sm. I know its a new rule change, but it is in fact legal.
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Old 02-24-2012, 11:30 PM   #7
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Did not know that. I know people where getting in trouble that hadn't actually touched it.
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Old 02-25-2012, 01:48 PM   #8
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Yeah, its in the GCR now.

Edit: although I cant seem to find it currently. I recall reading it in some form way or shape of gcr

Last edited by Gryff; 02-25-2012 at 01:58 PM.
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Old 02-25-2012, 02:00 PM   #9
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We have put 3 cars through chump racing. 2 1.6's and 1 1.8. Have always managed to squeeze 2 hours out of each stint. Of course full course cautions and lots of traffic typically help a good bit too. We agreed to a 6500 rpm limit for the majority of the race for our last event in October and it seemed to help a good bit. Most of our drivers would make their last two laps of a stint with the car coughing on fumes however.
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Old 02-25-2012, 07:57 PM   #10
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Is our logic flawed in thinking that the smaller intake of air will result in better fuel economy for the drop in power?
Flawed. No different than part-throttle.

Can you run 100 octane and run it leaner with an AFM signal modifier?
Or reduce fuel pressure by 10 psi or so?
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Old 02-25-2012, 08:17 PM   #11
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Flawed. No different than part-throttle.

Can you run 100 octane and run it leaner with an AFM signal modifier?
Or reduce fuel pressure by 10 psi or so?
I second the notion of flawed. Just from an engineering standpoint, you are going to want as much excess air as possible to ensure a more complete burn of the fuel. Less air = lower efficiency, IIRC.
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Old 02-25-2012, 09:19 PM   #12
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Flawed. No different than part-throttle.

Can you run 100 octane and run it leaner with an AFM signal modifier?
Or reduce fuel pressure by 10 psi or so?
I am not saying that it will lean it out, but trying to limit that air coming in to effectively keep it at partial throttle to lower fuel consuption. I was hoping it would be about the same and the result would be less fuel used. My question though is how much.

We could run a higher octain and do that but the cost for 100 is around $9 a gallon at the track. Using around 175 gallons would get crazt expensive.
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Old 02-25-2012, 10:23 PM   #13
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How about a low level warning light, or a better readout instead of a needle for the fuel gauge? Most senders are simply a variable resistor, so if you can get a repeatable resistance value at low and high levels, it should give you a little more resolution on how full your tank is. A super cheap (aka lemons legal) way to do it would be to wire in a harbor freight multimeter ($4) and use the resistance measurement as your gauge. I know it's not as easy as looking at a needle, but it should give you a better idea of how much fuel is really left. Just a thought.

The other option would be to wire in a dummy light as a low fuel warning when a certain resistance is reached, but off the top of my head I can't think of a super cheap way to do that...
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Old 02-25-2012, 10:42 PM   #14
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I am not saying that it will lean it out, but trying to limit that air coming in to effectively keep it at partial throttle to lower fuel consuption. I was hoping it would be about the same and the result would be less fuel used. My question though is how much.

We could run a higher octain and do that but the cost for 100 is around $9 a gallon at the track. Using around 175 gallons would get crazt expensive.
Unfortunately, limiting air is more likely to decrease mpg as opposed to increasing it. You are going to be injecting the same amount of fuel with less air, and just combusting that air less completely. I do not anything about throttle limiting per say, but I do know that choking your engine of air is not going to do it, or at least not what you would like.

I hope you find a solution, quite an interesting debacle!
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Old 02-25-2012, 11:17 PM   #15
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Old 02-25-2012, 11:39 PM   #16
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How about a low level warning light, or a better readout instead of a needle for the fuel gauge? Most senders are simply a variable resistor, so if you can get a repeatable resistance value at low and high levels, it should give you a little more resolution on how full your tank is. A super cheap (aka lemons legal) way to do it would be to wire in a harbor freight multimeter ($4) and use the resistance measurement as your gauge. I know it's not as easy as looking at a needle, but it should give you a better idea of how much fuel is really left. Just a thought.

The other option would be to wire in a dummy light as a low fuel warning when a certain resistance is reached, but off the top of my head I can't think of a super cheap way to do that...
What's the point? The car is going to stumble during high load corners and it will be real obvious you need to pit. Tone down your last few laps to avoid fuel slosh and starving. No need for a warning light. Assuming typical Chumpcar drama you will have some full course caution laps likely during everyone's stint to conserve fuel.

Limit your rpm use to 6500 for most of the race and you should be able to make it 2 hours for each driver. We've never had issues. It will be close and the car should literally come into the pits sucking rust from the bottom of your tank. Drive wisely let the 6 and 8 cyl cars duke it out and hit the pits early for fuel. The pack will "Darwin" itself out a few hours in and then it's time to start pedaling when the track clears up.

I assume you have a com system? Your crew chief should be calling the shots on the pace you are setting and be giving you an idea of timing while you relay fuel situation back.
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Old 02-25-2012, 11:39 PM   #17
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in LeMon, is it a 5 minutes mandatory as in Chump ?
is there a limit (like NASA) on how much gas you can put in per stop ?

better mpg is not the goal, it is about getting better mpg at same power level..
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Old 02-26-2012, 01:39 AM   #18
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Unfortunately, limiting air is more likely to decrease mpg as opposed to increasing it. You are going to be injecting the same amount of fuel with less air, and just combusting that air less completely. I do not anything about throttle limiting per say, but I do know that choking your engine of air is not going to do it, or at least not what you would like.

I hope you find a solution, quite an interesting debacle!
I would really fail to see how it would be different from partial throttle and save fuel on a car with a MAF and not a MAP sensor. The ECU only injects what the MAF tells it is getting in air and if the air is restricted, then it will be injecting less.


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in LeMon, is it a 5 minutes mandatory as in Chump ?
is there a limit (like NASA) on how much gas you can put in per stop ?
LeMons is as fast as you can get in and out of the pits. Chump requires a full 5 minutes if doing anything with fuel.

You can get as much fuel in as the tank can hold. LeMons allows much larger fuel cells where Chump says they must be within 2 gallons of OEM. With the OEM tank being 12.7 gallons, that really only leaves 12 gallon cells. Plus a fuel cell system with FIA/SFI certs costs around $900 when all is said and done.


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better mpg is not the goal, it is about getting better mpg at same power level..
I agree that's the way it should be and would love to have that as an option, but it is looking like I may not be able to have my cake and eat it too on this one due to the limitations. Since the OEM ecu is worthless for tuning anything other than a higher redline, all that I would be left with to maintain the same power would be reducing drag.
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Old 02-26-2012, 01:44 AM   #19
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Lean it out with 1.6 injectors? I have never tried to do this, but plenty of people do the opposite to make a 1.6 run rich.
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Old 02-26-2012, 01:48 AM   #20
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I thought about getting a smaller injector and trying it out, but I am worried that going to the 1.6 in a 1.8 may prove to be too lean. I could try it out and hook up my LC1 to see what the numbers look like and if they are acceptable.
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