fmcokc

Is anyone here running a s1 supermiata and if so what kind of fuel consumption numbers are you seeing? gallons/hour?

ryansmoneypit

iTrader: (6)

I'm curious for other reasons. Totally unrelated, I drove a honda prelude in a champ race this weekend. H22a and we were at 8gal. Hr....VIR north course.

Savington

iTrader: (31)

~10gal/hr

ryansmoneypit

iTrader: (6)

Holy crap. That would be a pit stop every 1:15.

fmcokc

How about k20 and k24’s? What kind of rates are they seeing. New rules changes in WRL has me thinking and wondering.

Padlock

You are essentially just asking which engine has the best BSFC (fuel used per horsepower per hour) in the rpm range that you will be operating in... there are a lot more factors that meet the eye to give an accurate answer for your particular track and setup... I'd just assume for simplicity that at any given HP with the same intake configuration (n/a, turbo, or SC) that the fuel demands will be close to the same.

Under WOT, rough numbers to approximate (in lb/hp/hr) could be something like the below:
NA ~0.45
SC ~0.60
Turbo ~0.55

fmcokc

I understand all of that, what I am seeking is people's real world fuel consumption observations in a NA or NB chassis Miata. Knowing whether they are running aero or not and trans and diff would also be helpful.

Example: We run a BP4W with NA 156whp, 5 sp trans, 4.3 diff in a 1999 NB with tunneled splitter and NLR rear wing . 2112 lbs full of fuel. We are seeing 6.7 gallons per hour of consumption on average at one of our typical 1.8-2.4 mile tracks such as Hallett, Barber or Hastings.

I am just trying to calculate the potential benefit of stepping up to a higher class with more power/fuel to see if the lower lap times outweighs the longer stint times of where we are now.

Savington

iTrader: (31)

Aero doesn't matter. Weight doesn't matter either. You might turn faster laps with those things, but the engine consumes the same quantity of fuel no matter what. The only things that matter for fuel consumption are how much average power you make (affected by peak power, overall powerband, trans+diff gearing, track config, and driver skill) and how efficiently you can make that power (engine efficiency, induction type, and air/fuel ratio at WOT).

Midtenn

iTrader: (15)

You also have to rethink your driving strategy with the introduction of more torque. Many of the big HP/torque cars run lower RPM most of the time to save fuel. They only use the power when the need it. I find it super annoying as a driver in a car having to do 9-10/10ths to catch them and then they just pull away with power when you catch them.

z31maniac

I'll accept that I'm not the smartest guy, and possibly I missed the point you are making, but I'm curious how aero and weight don't impact the amount of HP (fuel) needed to propel a car forward.

ryansmoneypit

iTrader: (6)

because at a given power level, the fuel requirement is the same, based on a 100% engine load. its the 'ole what weighs more, a pound of feathers or a pound of rocks.

Padlock

Keep in mind Savington is referring to a FUEL per HOUR basis. He is not referring to FUEL per UNIT OF DISTANCE which is affected by aero and weight, which is probably where you are getting confused.

codrus

Think about a car cruising on the freeway: it's going 70 mph, milage is driven by how much power it takes to maintain that speed. More weight and more drag require more power to maintain that speed, and it takes more fuel to make that power.

Now think about a car on the track in a sprint race: The goal here isn't to maintain a given speed, it's to go as fast as possible. The throttle is wide open every chance you get, and things like aero and weight only have a relatively small impact on what percentage of the lap is at WOT. So milage is driven by the amount of fuel the engine consumes at max power, which is primarily just related to how much power that is, with a smaller component on how efficiently it makes that power (BSFC, turbo == richer for the same power, thus more fuel).

As Savington says, aero and weight improvements will let the car achieve a faster lap time at the same fuel milage, but the flip side of this is that you can also use those improvements to achieve the same lap time with less power and thus less fuel required.

--Ian

andym

Also an important note, e85 uses a lot more fuel for the same power. So moving away from e85 and back to 91 or 93 octane would get more time between refueling.

Savington

iTrader: (31)

They do, but only when you care about how far it goes. In this case, we're talking about time, not distance. A heavy car with crappy aero on a slow track might only go 75 miles in an hour, while a light car with good aero on a fast track might go 100 or 105 miles in the same hour. If it's the same engine in both cars, though, the two cars burn the same quantity of fuel over that distance.

The extreme example is a generator. It burns fuel and generates power and doesn't go anywhere at all.

fmcokc

That’s all fine if the time on throttle per lap is the same for both cars. With a big aero car there is typically more time on throttle due to slower straight speeds, deeper braking zones, more opportunity to utilize the power in sweepers, etc vs the no aero car. It can be a significant difference over the course of 2 hours.

These are not generators and everything affects the parameters in which they operate and consume fuel.

Savington

iTrader: (31)

You're overthinking it.

If you want to account for aero or weight, you should also account for the major differences in on-throttle duty cycle between slow and fast tracks, since that's dramatically larger than any difference an airdam/splitter/wing is going to make. You would also want to consider the capabilities of various drivers, including a driver's ability to consistently run 1-2sec under prime pace by shortshifting (using less power as a result), or the capability of a bad driver to overslow through corners and then use more throttle (and thus more fuel) to go the same pace (or likely slower) than a good driver.

Tires alone add an enigma's worth of variability to this dataset. The difference in corner speed between something like an RS4 and a Hoosier R7 is greater than the difference between a non-aero vs. aero car on the same tire. The health of those tires should also be taken into consideration, since an R7 that someone put 2 cycles on 12mos ago is slower than a fresh RS4 (or maybe it isn't, depending on how it was stored).

Track config and tires alone are going to alter fuel consumption far, far more than any difference a moderate aero package is going to make. Best guess, the difference between a slow track and a fast track alone will make a ~20% difference in consumption, all else held equal. Or maybe it won't, since what we really need to look at is TPS% over time, and even that wouldn't be truly accurate - you really need the power to the ground at all MAP/RPM combos, and then you could begin to calculate fuel consumption. At that point, an actual flowmeter is probably easier to implement. Faster tracks generally require more power, but that won't always be true for every track.

This is all, of course, ridiculous. You're never going to gather data here at a level which allows you to accurately correct for any of this. At best you will get a range of data which will incorporate all of these variables indiscriminately. Sorting out what variables affect the exact consumption within that range is a fool's errand IMO.

What's written between the lines of my posts is that you can safely ignore all of the small variables and just focus on the big ones:

1. What kind of engine is it
2. How much power does it make
3. Does it have a turbo/supercharger

These three are going to have the largest effect on fuel consumption and they are very easy variables to account for when gathering data at an anecdotal level.

fmcokc

1

emilio700

iTrader: (3)

You asked a question based on some false assumptions and an incomplete understanding of fuel economy or more specifically, gallons per hour (as aircraft and marine). You got a very detailed answer from a two time Thunderhill 25 hr winning driver and crew chief who has several hundred hours enduro experience and is one of the worlds leading experts on all things NA/NB. Any you reply with "If you do not want to or cannot give them then don’t say anything."

Take a deep breath, toss aside your preconceived notions, tuck your ego away and google "Brake Specific Fuel Consumption".

dc2696

iTrader: (2)

This^ Trying to get our fuel consumption dialed in at a new track that is WOT for significantly longer than any track you've ever had the car at before can be surprising.

Random meaningless data point:

Our 2.4 ecotec, driving it at appropriate tractor engine speeds at Thunderhill 5 mile was managing 6gal/hr running ~3:27 laps over the course of 14hr's