Driving ProStars with Different Engines

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October 13, 2015 by Minuteman Trucks

With Almost Identical Engine Ratings, Does Bigger Displacement Really Make a Difference?


October 2015,
By Tom Berg

The old Studebaker proving grounds in northern Indiana are now under the ownership of Navistar Inc., and an event for trade press reporters proved to be an interesting opportunity to compare some of the different engines offered in International trucks.

The facility south of New Carlisle is far more spacious than the company’s old tech center and track in Fort Wayne, and reporters engaged in a multi-vehicle ride and drive that covered many of the on- and off-road testing paths.

A 3-mile paved and banked oval allows steady 65- to 70-mph cruising by heavy tractor-trailers without using public roads, something Navistar engineers never had before. There are also several miles of dirt trails and oddly paved surfaces that shake and rattle trucks nearly to pieces.

Navistar bought the facility from Robert Bosch, the German component maker that had acquired it in the 1990s from Bendix Corp., which picked it up after Studebaker-Packard folded in the 1960s.

The ride and drive included 13 International trucks and an IC school bus. Among those was a pair of ProStar ES sleeper-cab tractors designed for over-the-road service. Both trailers weighed about the same so we could make reasonable comparisons.

Both ProStars had Eaton Advantage 10-speed automated manual transmissions, but the engines differed: One had a Navistar N13 diesel and the other a Cummins ISX15, both rated at 450 hp with multiple torque output: 1,550/1,700 for the 12.4-liter N13 and 1,550/1,750 for the 14.9-liter ISX15. On paper, there was only 50 lb-ft of difference between the two engines, so one might think performance would be almost identical. But the Cummins had a larger displacement, by 2.5 liters or 153 cubic inches. Should that matter?

My seat-of-the-pants judgment was yes. Once up to highway cruising speed on the long oval track, the Cummins simply felt a bit stronger. When I nudged the accelerator to ask for more speed, the ISX seemed to respond with more authority than the N13, though the Navistar diesel’s reaction was entirely acceptable. If I had driven only an N13-450-powered truck, I’d have been pleased; but once exposed to the ISX15, I preferred it.

Was that merely perception prompted by my knowing that the Cummins was bigger? Did the extra 50 lb-ft matter? It would be interesting to do timed acceleration testing with these two rigs, from 50 to 60 mph, then from 50 to 70 mph, and see if there really is a quantifiable difference. Now there’s a project where that long track would come in handy.


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