Bumped the timing and turned down the fuel today...

[greg_a_morton]

hey david85 what kind of fuel mileage are you getting out of your truck with the OD and 3.08 gears? I am shooting for more mileage at hwy speeds in my truck. I am running a hx30w on my 4bt for low end power and it runs hot at higher rpms with my 4.10s. I am running a gear vendors but gearing is just not there. With my 72 frame it is running a dana 70 and there is no aftermarket gears for it because the gears have a different offset than newer. I have been looking or waiting for a dually or cab chassis sterling rearend. I have looked for 3.08s with no luck but have found a few 3.55rears. There is a aftermarket 3.31 gearset. I am trying to figure out what will help rather than hurt mileage. My engine should be around 400lbs torque and 150hp so I am thinking maybe fuel mileage could be comparable with yours. I am getting 20mpg or so now at 55mph with pump turned up a bit and a 35hp fuel pin(Blows the smoke). I hope to help that with my hx35w after rebuild. Sorry about changing topic a bit!

 

[david85]

In general my truck has delivered 20 MPG @ 70 MPH and 24 MPG @ 55 MPH. Best single day average at 70 MPH was 22.5, and best single day average for 55 MPH was 24.8.

Your truck is auto or stick? If its auto, then going to higher gears (numerically lower) won't gain you much because of torque converter slip that happens below stall speed. My E4OD can lock the torque converter and that allows me to cruise well below the stall speed and roughly 1200 RPM. If you can get your RPMs down on the highway it will probably help, not sure about city driving.

In my case, the tall gears do nothing for fuel economy in stop and go driving situation. However the cummins might not mind the tall gears as much even in those situations.

 

[greg_a_morton]

I am running the th475 auto, heavy duty turbo 400. The gear vendors has a .78 od. I don't have a tach in truck yet so I am going by a rpm calculator and at 55 I am at 1850 or so. It shows 2350 at 70 and it seems like engine really is working. My engine has the factory 2500rpm spring and am going to change to a 3100rpm spring one day. Most of the guys at 4btswaps.com seem to say that 1700-2000rpm is best at cruise speed . I will get it figured out over time but it is expensive to play with different combos these days!

 

[david85]

My guess is you could probably go down as far as 1600 theoretical gearing before the stall speed of the torque converter starts to negate your gears. So there is certainly room to improve things from 2300. Thats not a bad towing RPM for an IDI, but too high for a cummins at cruise.

Thats quite the combo you have in that truck by the way. Cummins engine, chevy transmission, in a mercury truck.

 

[greg_a_morton]

If I had the money I would buy the adapter to go to a ford or dodge 6 speed. These parts are bringing mint because of conversions. Heck I would be very happy with a np435 or t19 with my gear vendors. I would still need rearend change though. I have talked with many people with no real answer but my truck when I stop I shift to neutral because of vibration and truck seems to want to still go. It is the converter that was in tranny when I got the combo out of a bread van. I have thought that I need a new converter to solve this problem but no one seems to have no answer to this.

 

[david85]

So the truck is sort of trying to bump forward when in gear as the engine idles? (instead of a gentle steady pressure)

If so, that could be your idle being set a little too high for that torque converter.

4 cyl engines are by nature rough idling machines and produce harder torque spikes than a 6 or 8 cyl engine so its never going to idle as smooth as other diesels with more holes. In general any passenger car that has has a 4 popper is 2L displacement or less or has silent shafts (or both in some cases). Thats strictly to smooth out the engine and serves no other purpose. The 4BT breaks both those design rules by a wide margin so its not going to run smooth at lower RPM. Its also another reason, you will never see something like a 4BT in a luxury vehicle.

What you could try is lowering the idle slightly. Also might want to find out what the stall speed on your current torque converter is and see what sort of gap there is between the recommended curb idle for the cummins and the stall speed of the converter. Its a long shot but you could try researching the vehicle the transmission came out of an see what sort of data is out there on the original setup.

It is normal for autos to pull forward (or reverse if in "R") when in gear but if you are having to really fight to keep the truck from pulling forward, there might be a setup issue with the engine, transmission, or both.

 

[greg_a_morton]

I know the engine and tranny where out of a late 80s p30 chevy bread van. The motor has been rebuilt and the tranny also cause there is a tag on it. I idled the engine up to around 1000rpm because of the vibrations at lower rpm. I put hydro boost on truck off a 1990 superduty and before then the truck would not hold even with power brakes and disks up front. I have changed motor mounts twice trying to get the vibes down also. The mounts I have now helped a bunch but when in gear it still has major vibes when stopped in gear. My front end is all chrome at is shakes at the people in front of me!

 

[david85]

Huh, Well the only 4bt swap I ever saw in the flesh was a chevy 1500 (1991 ish) but he was running a stick shift transmission. Maybe I'll ask about it next time I bump into him at the cardlock. I remember him saying he loved the engine but noted that it was "a little shakey" in his words.

1000 RPM seems a little high but I'm not 100% sure what the 4bt calls for (some brief googling returns 600 RPM as the factory spec). Although if you were to swap from 4.10s to 3.08s, it would cut down on the rear axle torque by about 25%. That would help reduce the pull you are getting at idle, but it wouldn't really fix the problem. Its hard for me to really say for sure if this is normal or not. Are you running the engine mounts that came with the cummins?

 

[greg_a_morton]

yea I am running the van mounts now. I have not tried to reset the idle since I put the new mounts in. I may try and see what happens with that next weekend. When I had my pump redone the tech set the idle at 950-1000rpm. The new idle helped with tons of the crazy vibes! Hey thanks David for talking today. Maybe we can talk when I do the 7.3idi in the 68100 mercury that I am going to build. I may need some help with the aftermarket overdrive controller. I love To burn oil!

 

[david85]

No problem, I love a good diesel chat. Repowers are when things get really interesting. I did a ranger repower once but it was from another ranger so it wasn't as big a jump as you did here. Can't wait to see that F100.