treyZ28

The only info i can find right now is for little engines (2.0 4 cyl) coming in between around 2,500 and 3,000 rpm.

The reason I ask is that my truck gets better gas milage at 85mph than 70mph.

70mph = 1920rpm
85mph = ~2250rpm


(85^2 - 70^2)/(70^2) = 0.475
meaning almost 50% more drag? I'm not sure if that is right, but if cross sectional area, viscosity, CD etc are all constant- it seems like it should be within reason. Intuition tells me i'm missing something because those numbers sound very high. Anyway, its certainly much higher at 85mph than 70mph.

what is going on here?

Adnectere

I'll venture a guess via piston speed. Here goes:

Every motor has a particular area of rpm it likes to operate in for fuel efficiency. Every motor's rpm is different because of its internal dimensions, bore & stroke. When the stroke changes, the piston speed changes. So here goes from Taylor's #1 book p 474 (I know most of you guys have it ) "For spark-ignition engines...performance maps nearly always show best fuel economy in the range 1000-1400 ft/min." So if your motor is a Vortec 6000, it has bore x stroke of 4 x 3.62 in. That means that your best fuel economy (if Taylor's generalization holds) would be from 1000-1400 ft/min or 1657-2320 rpms.

I presume that your specific motor holds more to the upper range of those rpms for fuel economy. My motor does the same thing, it gets better fuel when you go faster, probably in the 75-85 range and I've even run it at 60 to see, and it doesn't do better but worse. My bet is the piston speed and the induction, etc. mean that your motor has best bsfc at an rpm point that matches up to your car at 85. Of course, your speed is high at 85 because the truck has tall gears to match with that torque curve on a 6 Liter.

BTW I got specs for a new Honda Civic 1.8L that's 3.19x3.44 so it's peak efficiency for bsfc would be 1744-2441. But his gears are going to be short b/c of the lack of torque and the shape of his curve for a 4 cylinder. The short gears means the rpm will be run up higher for a given speed, so that may be where you're seeing the higher numbers for the smaller motors. It's just a little bit shyer on the torque part than your truck

treyZ28

Thanks for the info! my books are all at school. I really wish I brought them with me, pretty bored right now.

Although, I cant say i'm shocked. 1600-2300rpm. I guessed that. There must be a Huuuge drop in bsfc from 1900 to 2200 rpm to compensate for all the extra drag and friction

Adnectere

There might be a big drop. If you look at a bsfc curve, it's highest at really high rpms and also down low at idle. It kind of swoops down, stabilizes, then starts swooping back up. It might be that part of the curve where it reaches that bottom part of the swoop. How scientific, using adjectives like 'the swoopy curve'.

Anyways, I think your numbers are right. If you double the speed, you quadruple the drag. So going from 70->140 would give it 4x. Here's how I ran some #'s: (((140-70)/70) + 1)^2. So that's ((70/70)+1)^2 then ((1)+1)^2 then (2)^2 = 4; (((85-70)/70+1)^2 = ((15/70)+1)^2=(1.21)^2=1.46.
If you think about it, going halfway from 70->140 would be giving you double drag instead of 4x; so that would be 105 mph; it kind of makes sense:
70 mph = 1 fake unit of drag
85 = 1.46 fake unit of drag
105 = 2 fake unit of drag
So it kind of progresses logically.

--Not getting any Nobel prizes in aero math anytime soon

There may be a big drop in bsfc after the curve stabilizes past idle. Your motor is a truck motor so it's nice to have a ton of torque down low, maybe the motor is tuned for great torque just off-idle. After all, people towing thousands of pounds don't smash the gas, you ease it off the line. Once you're past that off-idle, it may lean out a bit. 2250 rpm is pretty far from a 900ish idle. Maybe Chevy knows that when you tow at speed, you'll be about that rpm so they give you a nice bsfc so that when you're towing your mileage doesn't totally get trashed. Also, not sure how the rotating parts handle the rpms. Maybe oils and fluids like to be treated at higher-than-near-idle rpms. Lots of drag in the tranny, axles, etc. The moon and tides may have something to do with it too (Since I'm throwing out every option on the planet, we might as well go for some off-planet)

treyZ28

the only other thing I can think of is that the last leg of the trip (where I was speeding and got the best gas milage) was also where there were the biggest hills.

This may seem like more confusing piece of evidence until you take into account that the engine shuts off fuel after 4 seconds of zero throttle position and rpm above idle.

Maybe i would be coasting for 3-4 seconds on the small downhills and coasting for 6-8 seconds on the long downhills.

P Mack

iTrader: (6)

Your math is way wrong. Going from 70 to 105 wouldn't give you 2x drag. It would give you 2.25x drag. Foing from 70 to 99 would give you 2x drag (70 times square root of 2). I have no idea where you got that formula, but it's wrong.

P Mack

iTrader: (6)

Trey, your math is right, I got 47.4% difference. Assuming constant Cd may not be accurate though, maybe at that speed the air transitions to go smoothly over the tailgate or something weird like that.