t_vago's compound curve foam board pickup aeroshell

[t vago]

Originally Posted by dcb

Vago, have you considered a lower ratio rearend gear? It might be cost effective to do so with some shopping around.

I have considered a taller gear ratio, as well as a Laycock-type overdrive unit. I could go down to a 3.23 rear end with a taller gear, or I could effectively go to a 2.77 rear end with the Laycock unit.

Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck. Either option would push my engine speed well below 2000 RPM at cruise (1800 with the 3.23 diff, and 1560 with the Laycock unit), and any gain from running at lower engine speeds may likely be offset by the fact that the engine would be running well into the bad part of the torque curve. Nobody's been really able to produce a dyno chart for my engine (apart from the OEM propaganda), showing torque at speeds below 2000 RPM, so I can't really tell for certain.

[NeilBlanchard]

Will you be able to do tuft testing?

[Frank Lee]

Originally Posted by t vago

Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck. Either option would push my engine speed well below 2000 RPM at cruise (1800 with the 3.23 diff, and 1560 with the Laycock unit), and any gain from running at lower engine speeds may likely be offset by the fact that the engine would be running well into the bad part of the torque curve. Nobody's been really able to produce a dyno chart for my engine (apart from the OEM propaganda), showing torque at speeds below 2000 RPM, so I can't really tell for certain.

My F150 is actually geared too high (engine too slow). For one thing the a/t downshifts at the touch of a feather. For another it turns less than 1000 ft/mn piston speed at 55 mph. There seems to be something to this 1000-1200 ft/mn piston speed as being the zone for best fe. I think when one is lacking dyno and bsfc data, the piston speed thing will get one in the ballpark.

[dcb]

Originally Posted by t vago

Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck...

fwiw, the few bsfc charts I have looked at pretty much all indicate improved mpg with lower ratio than stock, the left hook on the hp curve heads north and bsfc improves. Not a guarantee of course.

You might look at the ratios in a manual trans too combined with the low auto rear end. If the junkyard/craigslist/ebay has a parts truck with a stick it can be done cheaply.

you might get a clue on bsfc map by plugging in a laptop/obd scanner and log the crap out of everything and try to sort it that way. It might be worth giving Smokey's Dyno a call too.

Just suggestions, nice job on the aero, sorry for the mini hijack.

[dcb]

re: piston speed, if your stroke is 3.4" then you would be at 1133 fpm @ 2000 RPM, but there may be more efficiency up and to the left of that point at partial throttle, really need to see the bsfc map to be sure (then factor in automatic trans weirdness)

[phunky.buddha]

Originally Posted by t vago

I have considered a taller gear ratio, as well as a Laycock-type overdrive unit. I could go down to a 3.23 rear end with a taller gear, or I could effectively go to a 2.77 rear end with the Laycock unit.

Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck. Either option would push my engine speed well below 2000 RPM at cruise (1800 with the 3.23 diff, and 1560 with the Laycock unit), and any gain from running at lower engine speeds may likely be offset by the fact that the engine would be running well into the bad part of the torque curve. Nobody's been really able to produce a dyno chart for my engine (apart from the OEM propaganda), showing torque at speeds below 2000 RPM, so I can't really tell for certain.

How big is your engine? My Frontier only has a 4.0L, and I get better FE running shifting at 1500rpm than I do shifting at 2000rpm when running around town- so at least in the city I'm sure you could benefit from a lower final drive ratio. I'm sure your engine is larger and has even more low end torque than my VQ40, especially since my engine is derived from the 350z and really made to rev. On the highway it may be a different story, but again- you HAVE to have more useable torque under 2k than I do- and I'm still pushing a 4500lb truck.

[t vago]

Originally Posted by NeilBlanchard

Will you be able to do tuft testing?

I think so. I just need to find a 3d person to operate the camera.

Originally Posted by dcb

re: piston speed, if your stroke is 3.4" then you would be at 1133 fpm @ 2000 RPM, but there may be more efficiency up and to the left of that point at partial throttle, really need to see the bsfc map to be sure (then factor in automatic trans weirdness)

My stroke is 3.63 inches, so that would equate to an average piston speed of about 1200 fpm. I probably could go down a bit and not sacrifice engine efficiency (well, not that much, anyway).

But you're right - this thread is about the aerocap, not gearing. Heh.

[jime57]

I think you are wise in taking a slow and deliberate approach to the gear change issue. I have a big old Cat in my motor home, and I have found that it definitely has a sweet spot around 1400 RPM. If I go lower or higher, I lose FE. The sweet spot corresponds to the minimum engine speed to keep the rig in top gear. Don't do anything until you can find a torque curve.

Originally Posted by t vago

I have considered a taller gear ratio, as well as a Laycock-type overdrive unit. I could go down to a 3.23 rear end with a taller gear, or I could effectively go to a 2.77 rear end with the Laycock unit.

Trouble is, I'm not sure if doing either of these things would actually gain any FE on my truck. Either option would push my engine speed well below 2000 RPM at cruise (1800 with the 3.23 diff, and 1560 with the Laycock unit), and any gain from running at lower engine speeds may likely be offset by the fact that the engine would be running well into the bad part of the torque curve. Nobody's been really able to produce a dyno chart for my engine (apart from the OEM propaganda), showing torque at speeds below 2000 RPM, so I can't really tell for certain.

[NeilBlanchard]

Hi,

Originally Posted by t vago

I think so. I just need to find a 3d person to operate the camera.

Another way to go is attach the camera to a boom -- this would have less aerodynamic interference and might be a lot less shaky, too.

[Varn]

My econoline gets it's best mileage at 1600 rpm (52-53mph). It goes down both below and above that rpm. I figure that the engine efficiency is balanced by the aero drag. If I were to get better aero with it . I might be able to have a higher speed that maximum efficiency occurs.