The official 6bt conversion thread

[Zeke71250]

OK,

Having looked at fuel and torque curves for the 4BT, ~1800 is the most efficient place to run a 4BT.

Should you want to run more calcs or change the other variables, look here:

http://www.angelfire.com/fl/procrastination/rear.html

hth,

eric

Eric - yes - perfect - thank you very much for the set of figures. It should be fun plugging in numbers... and I can stop pestering folks here about it... Thanks!!!

[Zeke71250]

Thanks Dustin - and thank you for your patience.

[Zeke71250]

If you're pulling a load with a 4BT that you need 10 gears for you should have went with the 6BT.

My advice is get a reasonable 5 or 6 speed with the 4BT and upgrade the engine to suit.

Fortunately, I haven't bought the engine yet - so I'm not stuck either way.

So. I'm hearing one of two things - which is correct?
1.I should gear it so the engine is running at peak efficiency at the speeds I drive the most (basically 40, 50, and 60) and if the truck won't reach the top speed I want (75), I should tweak it so the RPM range will reach that speed... with the understanding that running at that speed will reduce mileage.

**OR**

2. I should gear it to run efficiently at my projected top speed (75) - and then bump it up to improve the acceleration in the lower gears (if I find it too slow)

[71PA_Highboy]

Gearing is the only variable. Tires make a difference too.

That said, I am looking at a ZF, (.74 final) with 33" tires and 4.11 gears.

That makes this whole deal a lot more affordable for me as I don't need to re-gear (~$350 per axle) and that $700 makes for more money I can put to use somewhere else.

Just my plan... Now if I could find the 4BT...

hth,

eric

[averagef250]

I bought one two days ago...In the breadtruck... It died from someone trying to run it on gas. I bought the whole Van for $900, I'm scrapping the body tomorrow for $1100 and I've got a rear end and 4 speed to sell as well. Not the easiest thing to tear one of these vans down though.

The engine will only turn 1/2 turn until it stops solid, with the pan off everything is tight and pretty so I'm betting on broken piston(s). Hopefully I can get away with 4 used pistons, new rings and some minor headwork. Haven't had the chance to pull the head yet though.

If you drive like normal people do gear it for 2000 RPM @ 65. That's 3.55 gears, a ZF5 and 31" tires. If you gear it differently your mileage and driveability suffers. It'll still work, but not as well.

You can easily put a spring in the pump to turn 3000 RPM. Cummins rates the 4BT for 3750 RPM. What you'll find though is that unless you play with the turbo you won't be doing much past 2500 under load no matter what the governor will let you.

The engine can only breath under load where the turbo's efficient.

I think you should go out and drive a 12 valve cummins dodge with a 5 speed so you can understand what the cummins powerband is like.

[Mooosman]

I don't know if it's the photo albums down again, or what, but the pictures in the early part of the thread don't work any more (says the file doesn't exist)????

Is there a way someone can post up the pics of the 4BT in the bumpside? I think the truck was red or orange? I could've sworn I saved them, but I can't find them anywhere on my computer!

Or, if anyone else saved them to hard drive, could you PM me?

Thanks!
Nick

[averagef250]

Nick,

This truck? I've got the other pics on my shop computer. What pics are you looking for? Engine compartment? I can take whatever you want.

[Mooosman]

That's the one!

I'll take any pics you have of it! Mostly looking for engine bay pics, from both sides, and any pics that may show the motor mounts and the oil pan clearance.

I've been on 4btswaps.com the last few days, and it's giving me bad ideas! I may think twice about selling my truck, and convert to an oilburner.

Shortbed, 2wd F100. Think I can knock out 30mpg?

Thanks
Nick

[averagef250]

I'd say 30 MPG is very realistic for your SWB 2wd. The 4x4 mounts would be completely different from your 2wd though.

From the measurements Ranchero50 took doing his 2wd 6.9 diesel swap I think the 4BT pan would clear fine and you'd have pretty easy time dropping a 4BT in.

[Texas69]

Dustin, You have a tough looking truck

[Mooosman]

Any luck with the pictures?

Nick

[averagef250]

Sorry! Here you go-

[averagef250]

More

[Mooosman]

Thank you sir!

Can I ask what you used for throttle linkage and pedal? Also, it must be just because of the angle of the picture, but it looks like the rear valve cover is like almost touching the firewall! Realistically, how close is it?

Thanks again

Nick

[averagef250]

I took the straight six gas pedal and bent the arm to come out on the driver side. It's hard to explain, but I had to make the pedal arm come out on the driver's side of the engine and line up with the injector pump. It was easy, just took some fiddling with the torch and vise.

There is a good 2.5 inches from the rear valve cover to the firewall. There is tons of room in every respect.