I've finally got a tune that I think I can live with.

But then again, why would my MAF car with the 6E mask be getting higher mileage (a couple of mpg according to the computer) at higher rpms and higher speeds? I'm not running highway spark, but then again this wouldn't matter because all things are equal in the two tests anyway.

Does this indicate there's more to be had from a performance standpoint? What parameters should I be looking at?

All insight appreciated!

 

Is it calculating MPG from airflow or PW?
It may be an inaccuracy in the injector offset causing the discrepancy (just a MPG calculation problem - not needing fixing for engine to run better). I think it would have to be too large to cause this problem (but my brain is going in circles on this one).

Is spark different between the two points? Could try adding a little at the less economic point to see if it picks up. If MPG picks up, then you know it's closer to MBT (torque has picked up, bsfc has dropped, basically defining Minimum Timing for Best Torque).

 

Either the tune isn't the same at that load/rpm, bsfc curve is lowest at that rpm, or you need to pump up the tire pressure. Having a low tire pressure will hurt your mileage at low speeds until centrifugal forces ballon the tire out to a better shape. Aero can do this too where at higher speeds it's lifting off the tire lowering the tires rolling resistance... it's a balance but I'm leaning towards the tune and/or bsfc curve.

 

An engine is it's most efficient at max torque. Take a car that's designed to run at 55-65, and raise the peak torque RPM, and you'll find all the time. The exceptions, being some of the ultra low RPM strategies, of the 6 Speed cars, or extremely low rear axle geared cars. I've heard of some incredible MPG numbers on an 04 vette, at high speed.

The best MPG I've gotten was at 90 MPH, on a 90dF day, with what had to be 98% humidty.. And it was also a matter of almost 3 MPG.
BTW, my car is just slightly more aero (actually might be worse), then a Kenworth.

 

I am with Grumpy. I have worse aero than a Kenworth, but a very tall gear and OD. I turned about 1,800 doing 75 with 3.08s. I used to get maybe 5 mpg worse doing 60-65 than 75-80.

Well, I guess I am also going to have to find where my best speed for MPG with 3.73s is.

 

An engine isn't it's most efficient (fuel wise) at max torque.
This would be true if there were no friction and no pumping losses. The friction from all the parts and the pumping is 3 ways. You've got the pull/push of intake/exhaust, then there is the windage in the crank case and lastly the power lost at changing the piston/rod direction. When you add those into the equation almost always the peak mechanical efficiency is well below peak torque. A typical sbc uses only 10-15hp friction and less than 5hp pumping at 2000rpm where as at a typical peak torque those numbers will be more around 25-30hp friction and like 15hp pumping at 4000rpm. So aero excluded, an engine will be more *fuel* efficient at a lower rpm because the friction and pumping goes up less with load and more with RPM.
For max highway mileage I'd get something that puts my motor right around 2000rpm. It's fact, look at the Lt1's that got the best od in GM's history and they certainly are NOT setup to make peak torque down that low. Once they add rear gears or better yet, do a milage test in 5th gear as apposed to 6th... their mileage suffers. I did the test myself with a friend but he had a 93z which didn't have the .5od, it was still noticable and he commuted 30 miles each way all highway from his house to OSU which was just off the highway. He never speeded (he was a panzy) and cruised at the limit no matter what getting 30mpg in 6th and a "crappy" 26mpg in 5th. Car was completely stock.
Yeah, G-bodies everywhere weren't known for their aero package as much as NASCAR tried...
I'm assuming this is with the turbo car. 3 things stick out in my mind for why you would get better mileage at that speed than lower and the most obvious is the humidity. High humidity = lower density = lower drag, same goes for the temperature. Hotter air is less dense than cooler... and then the turbo factor. I bet it has something to do with dictating the bsfc curve at part throttle. I'm seeing something along the lines of the turbo being "too big" for econo but obviously not "too big" to be hurting drivability. The bigger turbo probably needs the higher air flow (rpm and load from higher speeds) to get into it's efficiency range.
Wow, that was a long reply. I wish I had more data on turbo engines . That's going to be my next ride. I'm shopping for a VW tdi. Can't beat the fuel efficiency of a diesel + turbo + direct inject + small car factor.

 

Cam timing will have an effect.
Little bigger cam might not like running at 1750. Might want 2k.
Easy way to check is to see if your map goes down with slight increase in rpm.

Hair splitting...
Air density going down with Humidity going up???
The O2 content will go down.
But I'd think that the lbs/ft^3 aka density would not decrease with the humidity going up. The baro is a different story. With a lower baro usually accompaning higher humidity.

 

Ya, whatever.

*I'm seeing*, are you going for laughter?. BTW, I was at over 30 MPG, so that instantly shoots down the *too big for econo*. 30+ MPG in a car that was weighting right at 4,000lbs., including passenger, and luggage, IMO, ain't too bad.

The turbo was hardly more then idling along. Getting into this turbo's lower area of efficiency means over 12 PSI.

BTW, I've also did a mileage check at 75 MPH, and 90/98, and there was a pronounced difference in MPG, so the 90/98 was a *constant*. I just stated the operating conditions, so as to be inclusive of some of the details, not that they were the cause of the mileage increase.

 

Ain't that the truth....

If you want to get into the chemistry of it, then one might look at both sides of the chemical equation, ie during compression, and after the reaction begins, in reference to what H2O does. Humidity is just a low volume water injection set-up, and the NACA papers (as will Ricardo) will supply some clues about in-cylinder cooling in that reguard.

 

Thanks to all for the replies.
The major difference is the addition of a slightly bigger cam (ZZ4) and, I'll admit, the car is much happier at those speeds than it was with the stock cam.

I'll try a little more timing where it's less efficient although would an indicator of too much timing be a bit of a "bucking" at speed? I'm experiencing some of this although I'm ignoring it because the throttle response is very good.
**I don't know the answer to the first question Reds**

 

SD calcs mileage on PW. Not sure if MAF does.......

Bucking can be timing or fuel.

 

If your into a lean surge, then, IMO, you need to rethink the balance of fuel and timing you have going.

Don't ignore anything, that's the only way an engine can tell you it's not happy!.

I'd take some complete notes of where you're currently at, and then dither the timing and fuel, and take notes of exactly what happens, and look for trends from that info..

 

I'd take some complete notes of where you're currently at, and then dither the timing and fuel, and take notes of exactly what happens, and look for trends from that info..

I'm well on my way