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Discussion Starter · #1 ·
What is the lowest RPM a 4BT can run without stalling? I ask this becasue I'm trying to figure out a good gear ratio so i can get the most out of my fuel.

Pantherman
 

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someone here had a good post with a fuel curve chart - i can't remember who it was though. hopefully they'll read this and re-direct!
 

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Does anyone know what the timing specs are for the CPL 1963 vs. CPL 858? I'm looking at the two curves and the BTA CPL 1963 has max torque, min fuel consumption at 1500-1700 rpm's. The BT CPL 858 max torque, min fuel is at about 1700- 1800. I'm working on my gearing. I'm using an air-air intercooler so if I just bump the timing to the CPL 1963 spec I should basically have the 1963 curve right? Except I've got an HY 35 turbo. If I go with 3.08 with 36" tires I'll be turning 1650 @ 70 mph which puts the engine right in the sweet spot for the CPL 1963 spec.
 

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Thanks Mike!

Wrench - from looking at those graphs and talking to a local Diesel FI shop the general concensus is as long as you gear correctly for a 1700-1800 rpm cruise speed, you'll be getting the best economy.
 

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Discussion Starter · #6 ·
Mikel, is there a link for those? They're a bit small at that size and my eyes can't see all the info clearly.



Pantherman
 

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Sorry, I saved them into my hard drive a while ago and I can't remember where they came from. Possibly from this site or TDR.

Try saving them to your computer and opening the files with a program that allows you to zoom in. I can read even the smallest text in those two documents.
 

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just a quick hijack. i have decided on using 3 to 1 diff gears in my car. with a 31 inch tyre, this gives approx 2100rpm at 65. this will be the working setup ( towing a car trailer. ) one gearbox i was fairly keen on using is out of a nissan cabstar. this will give 5 gears, with a .73:1 overdrive. the only drama i have is that it will run the engine at 1650-ish rpm at 70 ! seems a bit slow. the cabstar box is as strong as buggery, and easy to convert to floor shift. i can also buy a sbf bellhousing for it, and a good one is $700. only drama i can see is the o/d being too big. any suggestions ?
 

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I put 3.08 gears and 31" tires (677 revs/mile per BFG for 31X10.5) into my calculator and for a 0.73 OD ratio I get 1776 rpm @70 mph, 1649 @65. Assuming 4th is direct that would give 2433 @70 and 2259 @ 65. I use the BFG tire data for revs/mile for a given tire size, that is more accurate then just calculating tire revs/mile based on the unloaded tire diameter because it takes into consideration the tire squat with the vehicle weight on it.
 
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