RE: Gear Ratios & Economical IH Engine RPMs

[Jim Grammer <jimg@domain.elided>]

OK, this has taken a while. I wanted to look closely at an IH brochure I 
have from '69 called 'towing trailers with the Travelall'. I figgered who 
better to tell us where the IH motors should be running under load than 
sales engineering?.

>What is the minimum RPM that you can run an IH V-8 at freeway speeds and
>still have it effectively work? I ask this in terms of seeking better
>fuel economy.

I have always thought that optimum cruising RPM's for any motor should be 
near or around the torque peak. Below it, and roll on acceleration and hill 
climbing suffer, especially with larger tires. Far enough below it, and the 
power needed for acceleration is gonna wipe out fuel economy gains from the 
lower rev's. Witness my Scout 80 with 31's. Put 235/75's on it and it gets 
better gas mileage by a long shot despite the increased revs. This all 
assumes your rig isn't hugely overpowered and just loafing along no matter 
what the load.

Here's engine #'s



Interesting to note how 'out of line' the 345 is in terms of peak torque 
RPM. I'd be interested in any engine development details about why this is 
so. Were they trying for the low torque peak, or was it a by-product of 
some other part of the design?

The IH brochure is clearly oriented towards towing or otherwise loaded 
operation. It breaks out equpment recommendations into specific categories 
based on trailer weight(light, medium, heavy, extra heavy), usage(vacation, 
seasonal, year 'round), and service(average and extreme). Recommendations 
are specific to chassis model, 4x4 or not, engine, tire size/load rating 
and axle gearing. I figured some engine rpm's at 60mph based on their 
spec's, and got a range from 105% of the torque peak(304,light trailer, 
vacation, average service) to about 116% of the torque peak(392, extra 
heavy trailer, year 'round, extreme service). It makes sense that you'd 
want more 'reserve' rpm with a heavier rig to avoid falling into the 
'downshift hole' as quick.

Here's some more #'s



Or, for the bigger tire crowd



And yes, I ran max speeds for all the engine/axle combos of these 2 tire 
sizes. I won't waste the space here, lets just say that the 266 is faster 
than the 392, if you can wind it to redline and you don't over-rev the 392. 
And no, 116mph ain't gonna set records.

Looking at the numbers, I'd say 3.07 is as high an axle ratio as I'd go if 
you want any semblance of acceleration with all the motors except the 345. 
Since it's torque peak rpm is down almost 17% from the 392 it looks like 
you could run something as low as 2.56 if you didn't need to pull anything. 
Sounds like 3.54 with a 27% overdrive might be perfect. That'd give you 
1720rpm at 60mph with 31's in o/d.

Jim