What is the FE sweet spot for...

[bowtieguy]

does anyone know the FE sweet spot(hwy) for GM 3.8 v6(fwd) cars? my 95 olds apparently is not scan gauge compatible. will be testing my wife's car(same motor) for this soon, but since she drives very little hwy and her car is rwd, there prolly is no info benefit in it(at least for my car).

keep in mind that i'm NOT willing to drive 55mph on the 70mph FL turnpike. just want to know if 75 is more efficient than 70 or if 65 is MUCH more efficient than 70,etc.

any help would be appreciated! rh77 and anyone else that has owned/owns or rented one of these vehicles thanks in advance for your info.

[2TonJellyBean]

I used to rent Park Aves a lot around that time for work up north. I'd go 60 - 75 mph and get get around 28 MPG US. No idea where the sweet spot was, but it's an impressive powertrain for economy.

[omgwtfbyobbq]

Test it. Find a gas station and two a few round trip runs on the highway w/o traffic and the cruise set on 65/70/75.

[rh77]

It really depends on the drag coefficient. The best FE seems to have been just above TC lockup. These cars like low RPM cruise. From the 3.1 in the Century or Beretta to the 3.8 in the Park Ave, Grand Prix, and LaCrosse, the best FE came from an average cruise speed around 50.

A good compromise might be 60 mph, but you'd have to test it out. Depends on the car...

RH77

[thisisntjared]

the best way to know would be with a scan gauge, unfortunately i think that the sweet spot on an american v6 is pretty low on the rpm range :( i know they probably throw that top gear out pretty far so i think that rh77 has a pretty close estimate.

the slower the better. and i bet you will get significantly better fuel economy going 65 instead of 75.

i understand you want someone who speaks from experience, but my advice to you is don't go to slow, try to keep it at the speed recommendation(limit) of 70 :p

[bowtieguy]

thank you all.

50mph does sound about right according to rpm(well below 2k). interesting tho, 70mph is ABOVE 2k(maybe 2200) w/cruise control, while without, it is right at 2k.

[rh77]

interesting tho, 70mph is ABOVE 2k(maybe 2200) w/cruise control, while without, it is right at 2k.

TC lock and unlock is pretty touchy on GM 4-speeds -- can you tell if the TC is locked at 70 with the CC on? That may be the issue. TomO mentioned this fix (http://www.gassavers.org/showpost.php?p=73781&postcount=9) in another post -- firms-up the shift and helps prevent damage.

RH77

[bowtieguy]

TC lock and unlock is pretty touchy on GM 4-speeds -- can you tell if the TC is locked at 70 with the CC on? That may be the issue. TomO mentioned this fix (http://www.gassavers.org/showpost.php?p=73781&postcount=9) in another post -- firms-up the shift and helps prevent damage.

RH77

very good link! i can feel my shifts, but not sure how to tell if TC is locked or not. how can i know? BTW, using amsoil AT fluid, so temp problems are not likely.

[2TonJellyBean]

You can tell by comparing the tach to speedo ratio...

[Pete]

Bowtieguy,
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

For the 3.8 GM V6 this is between 1740 to 2100 RPM.

Cheers , Pete.

[kamesama980]

once you get to a speed, feather the gas just a little. if the rpms move (wihtout the speed changing) the TC is not locked up. if you can feather the gas and the rpms don't move, the TC is locked up.

The above formula seems to work for my car as well as where it takes the least throttle input to maintain speed.

[bowtieguy]

thanks both of you. after experimenting, i've found 4th gear TC lockup is ~50 mph. this apparently is my sweet spot.

[8307c4]

Obviously slower is better, I would try constant gas 65, hopefully it won't deviate past 55-75 that way.

[theholycow]

Bowtieguy,
As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s). Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

For the 3.8 GM V6 this is between 1740 to 2100 RPM.

Cheers , Pete.

Old bump...

I've had Pete's post about piston speed in my sig for a long time but never actually did the math. Here's the math for my 2008 VW Rabbit:
2 * 3.6 inch stroke * rpm / 720 = piston speed (optimally 16.4 to 19.8 ft/s)
simplifies to this very convenient number:
7.2 * rpm / 720 = piston speed
which results in numbers like this:
7.2 * 1640 / 720 = 16.4

So, 1640 to 1980 RPM would be the most efficient piston speed. That's a measly 38 to 46 mph in my "high" gear (not really very high at all, but the highest I've got).

I guess that means I should experiment with normal cruising instead of engine-on P&G at that speed, though I usually EOC at that speed (which would have to be better than even the most efficient piston speed).

Also, I wonder if it means I should downshift to keep in that range when cruising at slower than that. There's a heavily enforced 30mph road on my way to work where I have been cruising at 30mph in 5th (~1300rpm), but maybe I should try 4th and see how that affects me. I could actually watch the fuel rate meter for this purpose. I think I'll try that, I may have never tried it at 30mph.

Okay, so my 2002 GMC 5.3 V8's stroke is 3.62204724 inches...almost exactly the same. Now if I could only convince the transmission to shift earlier under heavier throttle...

[theholycow]

Results:
I don't think the calculation works for me. I didn't use it quite as intended but my test should have confirmed it if it's true.

My VW is in its ideal piston speed sweet spot as figured above at 38-46 mph in 5th, or closer to 30mph in 4th. I have some portions of my commute where I hold a steady 30mph, and tried 4th and 5th in them while watching fuel rate. Fuel rate jumped almost 50% when I downshifted to 4th (after stabilizing my speed again) to bring it into the "ideal" RPM, while on level land.

There's one place where I hold a steady 30mph uphill, and in 5th I'm at WOT to hold that speed without accelerating. On that hill, 4th with partial throttle to hold the speed read about the same fuel rate as 5th @ WOT.

I guess the ideal piston speed sweet spot is not significant enough to overcome other FE issues such as gearing and aerodynamic drag...