I have a M45 chassis mounting a LeRoi 210 Compressor, as I recall the vehicle
weight is 16,700 lbs. Powerplant is OA331 gasser.
Power fades incredibly at road speed on any kind of hill, although on flat
level ground it will run 45-50 without a lot of trouble.
My 530C fire truck on M45A2 chassis has a multifuel LDT-465 engine, weighs a
couple thousand pounds more, and sails up the hills that suffocate the gasser.
My M292A2 expansible van has a cross-sectional area presented to the wind
that is similar to your M109, and it is undoubtedly the slowest and hardest
working multifuel deuce I own.
Therefore, I suspect that your 109 may have some minor malady of the engine,
but more likely is suffering from the weight of the van box as well as wind
resistance.
HOWEVER, I believe if it were mine I would replace the transmission and not
the engine. My thinking is as follows:
1. I am lazy, and the transmission swap requires NO modifications to the rest
of the truck (unless you consider swapping the shift pattern data plate a
mod!).
2. You are unlikely to subject the truck to the extreme conditions it was
designed for.
3. Transmissions are cheaper than engines as a rule.
The transmission to use is the overdrive transmission out of an M35A1 or
M35A2.
It looks exactly like the direct drive transmission that came in your 109,
and bolts up the same way. The visible difference is that the position of
the shift lever in 4 & 5 are reversed.
I do not believe this swap will help you with performance on hills, but it
should give some increased road speed on flat ground.
Check your engine over carefully, the OA331 when properly running is smooth,
and so quite you can stand in front of truck with hood up and engine running
and converse in normal tones.
Hope this helps,
David Doyle
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