Quote:
Originally Posted by Nevadan
The one major concern I had is the gear ratios being to low for a diesel but I'm doing it anyway. It drives 70mph at around 3200 rpms so it will be fine for it's intended use, a ski-mobile, mostly winter driving on mountain roads around Reno.
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I was going to ask what you were planning to do about gearing, but then I didn't because I think you're right, the gasser gearing will probably be fine as is. These old IDI motors like to rev and there are seemingly no downsides to letting them do it. In the example of the Volvos, the diesel models use exactly the same transmission and rear axle gearing from the factory as most of the equivalent gas models. In fact the diesels are actually geared shorter than some of the NA gassers. The engines are suited by design for fairly high engine speeds, small lightweight pistons, direct acting OHC valvetrain, etc, it was clearly part of the plan for them. Especially true for the original design "short stroke" family like the D24/T and your "D20T" (the 1.5/1.6 4cyl, 2.0 5cyl, 2.4 6cyl). The raised deck generation IDI and TDI motors (1.9/2.0 4cyl and 2.4/2.5 5cyl) with longer strokes are maybe a little less optimized for it.
Anyway if the gas engine is turning 3200 at 70 that seems like it would be just fine with the diesel. My '83 760 D24T with stock size tires and stock gearing turns about 3400 rpm in overdrive at 85mph, which is still too slow to keep up with highway traffic here. But it seems happy to do those revs or more all day, in fact it seems to be capable of quite a bit more, although with the 1983-only 85mph speedo I am not too tempted to explore.
Really the ability to rev smoothly and provide a wide power band is one of the neat features of the old IDI turbo engines, and makes them fun to use for hard driving. The later direct injection engines are much more limited on the top end, although their low end response is better. They seem to pretty much run out of breath even before 4000 rpm.