ISX 2350 485

[schISM]

(02-05-2017 )gearhead Wrote:  There is alot more to it than just size. The ar ratio of the housing. In cm2250 the turbo is alot smaller overall to make it easy to spoll. One of the main reasons for the vgt is a way to control backpressure for the egr valve. Cummins made a motor in Australia cm2250 only has scr system no dpf and no egr. It has a non vgt hx60 just like the pre egr cm570. I really don't think cummins was thinking about the vgt for the sole purpose of fuel mileage. If that was the case the the cm2250 from Australia would also have vgt. I can't speak for them this is just my opinion. But the truth about the vgt being a way to control back pressure is definitely true. They needed a way to close exhaust off some to make the exhaust go through the egr valve.

I know, I know this is gonna flare up the "fixed" vs. "variable" vane turbo squabble again but I'm gonna say Cummins wasn't thinking about fuel mileage at all with the VGT, only a means to regulate the pressure differential of drive/compressor pressures in order to introduce egr into the intake in the amounts they were looking for.
Not only the Australian ISXe5 but also the new European QSM 12 liter and the smaller 3.8 through 9 liter tier V emissions engines introduced this past year don't have egr. Cummins could have used the VGT turbo on these engines but chose not to.
I'd say that's a pretty good testament to the efficiency that can be had by a fixed vane turbo that maintains a correct a/f ratio without relying on a pre-plotted table that might be just a little off in certain situations. Just my .02 cents.