What is the ratio of the rear differential on a 78 300D?

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What is the ratio of the rear differential on a 78 300D?

Postby inframanii » Thu Apr 16, 2009 12:41 pm

What is the ratio of the rear differential on a US 1978 300D Non turbo? I have swapped a 300D motor into a Dodge Dakota with 3.90 gears. I am running 3300 to 3400 rpms at 70 mph. Is this too high? What is the cruising rpm and the max rpm of this motor? I greatly appreciate all information.

Have an excellent day! :D

IR Man
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Postby David » Fri Apr 17, 2009 12:58 am

3400 @ 70 Sounds a lot slower than my 300D revs.
I think I'd be pretty happy with that.
They are a pretty low powered engine. I don't think they have that much torque to pull high gearing so need plenty of revs instead.
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Postby BMW Fan » Fri Apr 17, 2009 10:25 pm

W123 / 300D RATIO 1:3.46 for the NA version


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78 300D rpm

Postby inframanii » Sun Apr 19, 2009 7:50 pm

BMW Fan and David,

Thank you for the information!! Would either of you know what the cruising rpm and max rpm would be for a 78 300D non-turbo motor?

Thanks again, :D

IR Man
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Postby BMW Fan » Mon Apr 20, 2009 5:53 am

No, sorry I don't.
I only have two 300 SD and see 3000rpm at 115 km/ hr

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Postby WD8CDH » Mon Apr 20, 2009 8:44 am

What is your tire size on the Dakota?
Ron Schroeder
WD8CDH
'85 MB 300DT 2 Tank
Since '81 former WVO conversions:
'83 240D
'80 Audi 4000D
'83 Isuzu Pup
'86 Golf
'76 Honda Civic with Kubota engine
Prior to the cars, several generators
Kubota Tractor
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Postby inframanii » Tue Apr 21, 2009 2:09 pm

WD8CDH,

The tires on the Dakota are 30x9.50R15. They measure 29.25" high. I think I know where you are going but having a hard time remembering how to figure it out! Any help is very much appreciated!!

Have an excellent day!

IR Man
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Postby BMW Fan » Tue Apr 21, 2009 5:10 pm

This RPM calculator may help you

http://www.ringpinion.com/Calc_RPM.aspx

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Postby WD8CDH » Wed Apr 22, 2009 5:57 am

30x9.50 tires with a 3.90 rear end will give you the same rpm and road speed as 195/70R14 tires with a 3.30 rear end.
Ron Schroeder
WD8CDH
'85 MB 300DT 2 Tank
Since '81 former WVO conversions:
'83 240D
'80 Audi 4000D
'83 Isuzu Pup
'86 Golf
'76 Honda Civic with Kubota engine
Prior to the cars, several generators
Kubota Tractor
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Postby 240Volvo » Wed Apr 22, 2009 8:29 am

How is the Dakota to drive with the new engine in comparison with the gasser? How much trouble was it to swap? What did you do for vacuum, etc.?

Sorry for the OT, but it is very interesting.
1984 Volvo 240 diesel with a single tank Elsbett conversion: electric fuel filter heater, FPHE, glow plugs, and injectors. Also injector line heaters and block heater, running 20%kero/80%WVO winter blend.
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Postby WD8CDH » Wed Apr 22, 2009 8:44 am

What was the tire size on the US 1978 300D Non turbo?
Ron Schroeder
WD8CDH
'85 MB 300DT 2 Tank
Since '81 former WVO conversions:
'83 240D
'80 Audi 4000D
'83 Isuzu Pup
'86 Golf
'76 Honda Civic with Kubota engine
Prior to the cars, several generators
Kubota Tractor
WD8CDH
 
Posts: 103
Joined: Tue May 27, 2008 11:44 am
Location: NY

Postby BMW Fan » Wed Apr 22, 2009 11:24 am

195 / 70 / 14

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Postby WD8CDH » Wed Apr 22, 2009 11:27 am

Thanks, my guess was correct for calculating the "equivalent" rear end ratio.
Ron Schroeder
WD8CDH
'85 MB 300DT 2 Tank
Since '81 former WVO conversions:
'83 240D
'80 Audi 4000D
'83 Isuzu Pup
'86 Golf
'76 Honda Civic with Kubota engine
Prior to the cars, several generators
Kubota Tractor
WD8CDH
 
Posts: 103
Joined: Tue May 27, 2008 11:44 am
Location: NY

Postby inframanii » Wed Apr 22, 2009 12:55 pm

WD8CDH and BMW Fan,

Thanks for all the information!! Now I will be able to sleep at nite and not worry about destroying my motor. :D :D :D

Thanks much,

IR Man :D
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Postby inframanii » Wed Apr 22, 2009 2:04 pm

240Volvo,

I don't know if this is the place to discuss this but I will give you a quick overview. The Dakota is a 97 ext cab 2wd (first year of the style change). The Dakota didn't have a motor at the start of the project. So I can't give you the actually comparison but I just purchased the same truck in the older style with the same engine. Older Dakota, 3.9l rubber at take off vs newer Dakota diesel, VW rabbit take off). In all fairness, the first motor wasn't all that great!! It was suppose to be a low mileage motor (92K) but had 220 psi for compression. I was unable to do a compression check when I bought it, but it didn't smoke and ran fine till 30F. Also the kick down for the tranny wasn't working. After 30 mph, it was respectable especially when getting 26-27 mpg at 70mph. I can tell you more in a couple of days when I get the newer motor in. It was about a solid 6 week project. I had to figure out the wiring for the truck (especially for the dash), changed out the heater core ( tear down to bare firewall) after I had finished the wiring for a second time. Cut the rain tray to remove valve cover, made new motor mounts, made a modified drive shaft (MB on one end Dodge on the other), modified the exhaust, modified shifting linkage, installed sound proofing and more. This 6 weeks time, also includes installing A home made WVO system ( hopefully robust enough to handle northern MN winter temps). I hope to fire it up in the next couple of days. If there is interest, I have taken a lot of pictures and kept some notes. I feel the project would go faster if you learned from my many mistakes and you have a good knowledge of MB engines. I didn't but learned, many, many thanks to Omar :P
If you want more info, I probably should start a new post.

Have an excellent day!

IR Man
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