///// How many extra MPGs can you get driving at the best BSFC of your engine ? /////

[Andyinchville1]

Hi All,

I have a 1990 F250 4x4 that I tow with (approx 9000 lbs). It has the 7.3 IDI engine (NOT Powerstroke so not alot of power but durable and cheaper to fix BOTH plusses in my book ).

Anyways unfortunately the auto trans is having issues.

With that, I plan on going to a manual tranny for better pulling power and possibly fuel economy (technically I should not say better pulling power since the factory rates the manual tranny version at less to capacity than the auto tranny BUT that is only since the factory 5 speed is not a granny low version so getting a heavy load rolling requires a bit of slipping of the clutch (not good) BUT once under way most people with the 5 speed like it more than the 4 speed auto because it has 1 more gear to keep the engine pulling better.

Anyways, since I have to change the tranny eventually anyways, I was thinking it may be better to use a big truck transmission (10 , 13 or go for broke 18 speed trans) to have lower first gear starts and more gears to keep the engine in it's "happy" BSFC zone.

My question is .... how many extra MPG's can one hope to get by keeping the engine in it's best BSFC rather than just working it haphazardly because of limited gearing ? ( I do realize there would be a weight penalty / possible frictional losses with more gears BUT I think operating the engine in a better BSFC range would or should more than offset that ..... hopefully!

Anyways ... what are your thoughts?

Thanks!

[Piotrsko]

Have a 00 7.3. Never use #1 unless I am towing heavy up a steep hill offroad. If you came across a different ratio rear, say 2.56:1 then more gears gets you operation in the sweet spot between 1500 and 2000 rpm for different road speeds

[jcp123]

Can’t comment on BSFC stats since I don’t have them for that engine, but I will comment on getting a “big boy” transmission. I drove rigs with predominantly 10-speeds and they were strictly adequate behind the detroit diesels in the Freightliners we used - I always thought a 13 speed would be ideal for not making shift patterns too painful, but would be enough to cover some dead spots in the gearing. I always felt too wound up in 7th, but borderline lugging and no power in 8th at 35mph, for instance, ditto at 45 with 8th and 9th. The gears were technically close enough to work but any speed limits on the 5’s were in reality between gears, so a 13-speed would have been nice, and when they gave me a minty-fresh power unit with a 12-speed automated manual behind a basically identical DD15 power plant (550hp, 1650lb-ft, redline around 2000rpm) from my previous truck, I found it covered the power better than a 10-speed as well. But lacking a turbo I don’t know how to account for gearing on the IDI engine.

I low-key want an old heavy duty pickup with a smallish Detroit two stroke, though. A one-ton with a 3-53t and ten speed could get a LOT done.

[Ecky]

The thing about BSFC, is that it's both load and RPM based. Load is quite transient, and typically an engine is sized large enough that, during cruising, you will always be either at a lower RPM or lower load than provides peak BSFC. Because BSFC islands are wider than they are tall, the best you can do is to have tall enough gearing to get as near full load as possible while cruising.

[Ecky]

In terms of how many extra MPG... in my Insight project, I initially had an unusable 6th gear. After fixing the issue (incorrect bearing size), it added around 5-6mpg under cruise, compared with 5th.

[aerohead]

Originally Posted by Andyinchville1

Hi All,

I have a 1990 F250 4x4 that I tow with (approx 9000 lbs). It has the 7.3 IDI engine (NOT Powerstroke so not alot of power but durable and cheaper to fix BOTH plusses in my book ).

Anyways unfortunately the auto trans is having issues.

With that, I plan on going to a manual tranny for better pulling power and possibly fuel economy (technically I should not say better pulling power since the factory rates the manual tranny version at less to capacity than the auto tranny BUT that is only since the factory 5 speed is not a granny low version so getting a heavy load rolling requires a bit of slipping of the clutch (not good) BUT once under way most people with the 5 speed like it more than the 4 speed auto because it has 1 more gear to keep the engine pulling better.

Anyways, since I have to change the tranny eventually anyways, I was thinking it may be better to use a big truck transmission (10 , 13 or go for broke 18 speed trans) to have lower first gear starts and more gears to keep the engine in it's "happy" BSFC zone.

My question is .... how many extra MPG's can one hope to get by keeping the engine in it's best BSFC rather than just working it haphazardly because of limited gearing ? ( I do realize there would be a weight penalty / possible frictional losses with more gears BUT I think operating the engine in a better BSFC range would or should more than offset that ..... hopefully!

Anyways ... what are your thoughts?

Thanks!

A 12-speed has the same efficiency as a CVT, so going to anything beyond 12 gears would probably just be unnecessary transient losses.
The 'load' is going to determine the gear you run in, whether with 'acceleration drag', 'climbing drag', 'curve drag', 'headwind drag', 'quartering-wind drag', 'passing drag', 'traffic congestion drag,' 'wet-road drag', 'winter temperature losses, etc..
If, in all the years of towing, you experience one scenario more than any other for all the miles you drive, perhaps you could target that condition as the one you'd try to optimize for.
If you tow at high speed, then aerodynamic drag reduction will pay the greatest dividend than any other technology available.

[rmay635703]

I would be moving 120mph so it’s doubtful I would gain fuel economy