Better highway gas mileage at high speed

[imzjustplayin]

Originally Posted by StorminMatt

Actually, required power theoretically increases by the square of speed. This is, of course, used by SO many advocates of lower speed limits to make it seem like slower is always better. BUT, there are other factors to take into account. One of them is drag coefficient. This is often treated like it is some sort of constant. But it is NOT. Drag coefficient can basically do ANYTHING as you speed up - it can increase, stay constant, or even decrease. Pretty much ANY engineering text has some sort of a plot of drag coefficients for various geometries vs Reynold's Number (a dimensionless quantity which takes into account the fluid, size of the object, and speed). And there is virtually NOTHING predictable about the curves. Sometimes, a sharp drop can be seen at some point. Because of this, it is VERY possible that a given car might experience LOWER aerodynamic drag at higher speeds rather than the higher drag that everybody thinks is the general rule.

Of course, there is also the factor that someone mentioned about engine efficiency. Some engines just don't like low revs. For instance, when I tried to use a CRX HF transmission on a DPFI Civic DX, I was rewarded for my efforts with a car that was REALLY good at ridding me of all that pesky gas that was in the tank. I would guess that this whole 'taller is better' thing all goes back to the old days when all engines had only two valves per cylinder and a 6500RPM redline was considered sky-high. However, nothing these days has two valves per cylinder other than GM V8s.

I guess this all means that you have to experiment to find out what speed works best when it comes to mileage. Because if you just stick firmly to the 'slower is better' mantra that is just SO common in fuel economy circles, you could find yourself burning MORE gas in order to take MORE time to get where you are going. And that certainly doesn't do you any good, does it?

You say DPFI civic DX, why so vague? How about you be more specific about the car you're talking about. Also while I have read that the civic HF has taller gears than the VX, I've also read that the VX has taller gears than the HF so I have no idea who to believe. Generally it has been observed that a taller gear ratio results in better fuel economy. I think what happened with you and again I have no idea what civic you're talking about, but I think you simply reved much higher than before because of the taller gearing and or you drove around in WOT which caused the ECU to dump a bunch of fuel, again I have no idea what civic you're talking about.

Be more specific about the "civic DX" since for all I know you could've put it into a civic DX of the same era.

[GasSavers_RoadWarrior]

Something just jumped out and poked me in the eye, don't know why I didn't see it before when I was playing with these numbers, see last half of this post...

http://www.gassavers.org/showpost.ph...3&postcount=47

Now it makes sense why a lot of people with minivans are saying that mileage is better at 65 than 55. I feel something on mine around there, I thought it was the motor getting more fuel or something, but it appears I'm transitioning flow regimes and getting a drop in drag.

There's a funny thing here... if your vehicle is very unaerodynamic, then you maybe get a jump in drag when flow transitions, so 55 mph is your speed. If it's mildly aerodynamic, you transition somewhere in the 60-65mph range... if it's very aerodynamic, you transition around 70-75mph, as in all those Corvette owners saying mileage is best at around there. Of course it climbs again with the square of the speed still, but it appears that because of the effective Cd change, you get a notch in the envelope.

Dammit, now I know what that is, I'm not staying at 55.

[ShadowWorks]

Having an engine running at its most balance speed is important as well, blue printed engines seem to give better FE, there is always a perfect balanced engine speed, knowing that may help your FE as well, less energy is wasted shaking the hell out of 400 pound of engine!

[GasSavers_RoadWarrior]

Aha, coast (CIG) tested at 110-100Kph and 100-90kph.. 110-100 = about 17 seconds 100-90 about 12 seconds. I knew there was a "sweet spot" there but didn't know if it was the ECU increasing fuelling, the throttle cam radius tightening, or what.... but it appears to be the aero... trouble is trying to keep it at 105ish, merest twitch of the foot has it racing away up to 115ish.

[91CavGT]

Very interesting results!! I'll have to try that test at differant speeds in Wagonstein to see what the results are.

[StorminMatt]

Originally Posted by *************

You say DPFI civic DX, why so vague? How about you be more specific about the car you're talking about. Also while I have read that the civic HF has taller gears than the VX, I've also read that the VX has taller gears than the HF so I have no idea who to believe. Generally it has been observed that a taller gear ratio results in better fuel economy. I think what happened with you and again I have no idea what civic you're talking about, but I think you simply reved much higher than before because of the taller gearing and or you drove around in WOT which caused the ECU to dump a bunch of fuel, again I have no idea what civic you're talking about.

Be more specific about the "civic DX" since for all I know you could've put it into a civic DX of the same era.

DPFI = dual point fuel injection. This could only be a 1988-1991 Civic DX or STD. In my case, the car was actually a STD with a DX motor and ECU. And no, it had nothing to do with revving the motor during shifts. We are talking about freeway mileage here. And the car just didn't like that HF transmission. It got something pathetic like 27-28MPG on a drive to Portland from Sacramento earlier this year. That's down a couple from what the car used to get with a 1988-1991 SI transmission. And my B18C5 CRX would have done MUCH better than that - and been able to climb the passes better.

As for gearing between the VX and HF transmissions, they are about the same. And differences in gearing are pretty minor.

[imzjustplayin]

Originally Posted by StorminMatt

DPFI = dual point fuel injection. This could only be a 1988-1991 Civic DX or STD. In my case, the car was actually a STD with a DX motor and ECU. And no, it had nothing to do with revving the motor during shifts. We are talking about freeway mileage here. And the car just didn't like that HF transmission. It got something pathetic like 27-28MPG on a drive to Portland from Sacramento earlier this year. That's down a couple from what the car used to get with a 1988-1991 SI transmission. And my B18C5 CRX would have done MUCH better than that - and been able to climb the passes better.

As for gearing between the VX and HF transmissions, they are about the same. And differences in gearing are pretty minor.

How many HP + torque is that engine? I bet the problem was that the car was too heavy and underpowered for a transmission such as the HF.

[StorminMatt]

Originally Posted by *************

How many HP + torque is that engine? I bet the problem was that the car was too heavy and underpowered for a transmission such as the HF.

The engine has 92HP and 89ft-lb torque. SO it's not a powerhouse. But it has more HP than the HF motor (62HP, 90ft-lb torque). And the car only weighs 1933 pounds. This, by the way, it LIGHTER than a 1990-1991 CRX HF (although it is certainly less aerodynamic).

[91CavGT]

I did some coasting testing yesterday. At 70 mph, coasting down 5 mph to 65 mph it took 10 seconds. At 60 mph coasting down to 55 mph it took 11 seconds. So it seems like the aerodynamics are a bit worse at the higher speed, but not by much. I want to do this again, but test at 75 mph and at 65 mph so I can get a better idea of how the aerodynamics are at multiple differant speeds.

[theholycow]

Measure distance in the coastdown test, not time. You measured 1/10 time shorter coast for the faster speed, but you were going 1/7 faster, which I think means you coasted a longer distance (which is what matters for FE).