The IHC SV345 was one of the best medium duty truck gas engines ever built.
It was over built for strength, reliability, and ease of repair.
An SV engine weighs as much more than a GM big block as a big block is over a GM small block.
The engine has steel timing gears, full water jacket for very efficient cooling, hardened valve seats long before unleaded gas make it important, and over built bearing seats for the crank and main bearings.
Their one shortcoming was the oiling system for the top end had some drawbacks that were not a problem if the engine say daily use. When they sit around for a long time if the oil system is not pre-lubed before the engine is turned you run the distinct risk of spinning out the cam bearings.
I have seen them go 500,000 miles with nothing more than very basic care.
I have seen a lot of different transmissions behind them:
- Allison AT540 and MT640 automatic
- 5-speed with 5th direct
- 5-speed with 5th OD
- 4-speed with 4th direct
I have also seen 2-speed rear ends with all of the stick shifts except for the OD transmission.
Fuel mileage with a stick shift was in the 5-8 MPG range.
Fuel mileage with an automatic was in the 4-6 MPG range.
The highest speed rear gearing I have ever seen with one was 62 MPH, the lowest speed gearing I have ever seen with one was 47 MPH. The most common top speed gearing was 57 MPH. With the 57 MPH gearing it could cruise easily at 55 MPH along I-5 in WA state and never have to shift down for any hill, even with a full load of passengers and gear. The 62 MPH top speed bus would need to down shift on the bigger hills and the 47 MPH top speed bus would never need to down shift until you got near the top of a mountain pass.
As far as the best transimssion is concerned, it is all sort of a matter of what is best for you.
When I was 27 the stick shift would have been my choice.
Now that I am 57 the automatic has a lot of appeal.