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7.3L IDI Diesels (Not Power Strokes) Technical discussion of topics related to vehicles powered by the 7.3 Liter In-Direct Injection Navistar engines.

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Old 02-05-2008, 10:57 PM   #1 (permalink)
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Max/optimum RPM?

Hello all, first post from a diesel newbie. Great site, I've already learned a lot!!!

My tach (when it works) shows about 2900 RPM at 60 mph. The tach shows redline about 3500 RPM.

How fast should I be cruising with this?
Is there an optimum RPM for a diesel on the highway?
Any warnings for me?

Thanks,
Dave
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Old 02-06-2008, 12:49 AM   #2 (permalink)
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Someone may come around with some tach info for you (to get it working all of the time). Optimum RPM is only limited by your pocketbook (gas mileage). The engine has a govener so you can't overrev it. That C6 is going to hurt your mileage (no overdrive).
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Old 02-06-2008, 06:33 AM   #3 (permalink)
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What size tires and what rear end gear ratio do you have?

RPM is goverened at 3300 rpm. With an auto trans,265/75's, and 3.55 rear end i run about 1500-1600 doing 60. 75 gets me about 2100. This is with an e4od, your numbers will be higher with a C6.
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Old 02-07-2008, 09:15 AM   #4 (permalink)
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What size tires and what rear end gear ratio do you have?
Tires are 21585R16 commercial (former U-Haul). Unknown rear end at this time.
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Old 02-07-2008, 05:48 PM   #5 (permalink)
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2900 at 60 is screaming. I'd try to fit the biggest tires possible, and look into gear reduction. Althought the motor is governed at 3300 (3800 for mine), it is still hard on it. Your truck will probably get half the mileage as it would at around 2200 or so.
Diesels make their torque much lower in RPMs than gas motors- ours start coming on at 1600 and the curve is completely falling off at anything past 2700RPM or so.
Vehicles usually get their best fuel economy when towing at or around their torque peak. I'm not sure, but I'd venture to say for our trucks that magic RPM would be around 2200.
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Old 02-07-2008, 09:14 PM   #6 (permalink)
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Torque peak on these IDI's is 1400.. But I have been running there and that is 50mph and my mileage seems to drop a little for some reason. I know that rule works good on my dodge cummins though, max tq is at 1600 and it revs so high that I get 21mpg on the highway doing 70 and 23mpg in the city. Revs way too high. Can't figure out why the ford likes higher rpm's though.
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Old 02-07-2008, 09:19 PM   #7 (permalink)
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I would like to cruise at 2k but I would get run over, 55 mph with 4:10's and 255's. Towing up a hill I run it at 2,700-2,800. I need to keep it 2k and above for the flats, towing 5k.
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Old 02-07-2008, 11:10 PM   #8 (permalink)
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My 90 7.3 idi E4OD 4.10 rear 235/85/16 OEM tires. At 2200 65mph I get 17.5 mpg. With a C6 and a 4.10 you are gonna run 500 rpm higher, and with those small tires you have even higher.
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Old 02-08-2008, 11:08 AM   #9 (permalink)
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My 90 7.3 idi E4OD 4.10 rear 235/85/16 OEM tires. At 2200 65mph I get 17.5 mpg. With a C6 and a 4.10 you are gonna run 500 rpm higher, and with those small tires you have even higher.
My ford being lugged through town and kept around 55mph on the ride home gets 12mpg. I am sick of getting such low numbers so that is why I am done with this engine and a cummins is going in. Ticks me off seeing my dodge never get below 20 yet the ford being 1000lbs lighter can't get anywhere near that. I think my TC has a lot to do with it though too, I had the trans rebuilt and ever since it seems like they used a 4 banger TC because it pretty much just free revs up to above 2000 and barely gets anywhere. I put in the lockup switch so now I just wait till I hear it hit second and then I lock it up so I might get somewhere. Converting it to a 5spd one of these days.
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Old 02-08-2008, 09:07 PM   #10 (permalink)
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You don't have a TC out of a 4 banger, they just most likely installed an aftermarket one made for a gasser, with a higher stall speed. That's judging by the sound of it, anyways. If it takes higher RPMs to move the truck, it has a higher stall.
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