1994 Bluebird rearend?

[cadillackid]

with 4.78 gears and 11R tires.. you should be able to get 65 at 2600-2700 RPM.. in my DEV bus with that setup (MT643 trans still 1:1, it just doesnt slip).. I get 68-70 at 2800 (maxxed out)...


if your trans is not shifting into 4th then you would get about 52 at 2800-3000 RPM..

[NWOleman]

Sounds like maybe it isn't shifting into 4th then. Loaded question, but what would cause that? Linkage?

[BarnYardCamp]

what does the trans fluid look like? Does it smell "burned"?
Linkage probably isn't the culprit, if the linkage was messed up, then the other gears wouldn't get properly selected. IE, if the shifter was showing neutral, the tranny could be in gear and the engine wouldn't crank.

[cadillackid]

drive it and count the shifts... you should be able to feel them... also count the number "clicks" you feel when you shift into gear.. so when you start at 'R' and the bus moves in reverse the next click of the T handle should put you in 'N'.. should be able to free rev the engine without the bus moving (much).. then the next click of the Handle should be 'D' which is all 4 gears....



a test you can also do.. is shift the selector all the way down to '1'.. and go drive.. get the revs up pretty decent.. say 2200-2300 you should have no shifting.. let off the throttle and pull it up to '2' it should pop into 2nd gear.. youll really feel it.. now drive faster again in 2nd now you should have no shifting.. up to 2200-2300 let off the pedal and pull it up to '3' it should pop into 3rd... now drive up to the 2600-2700 RPM in 3rd and see your speed.. you should be able to make the 48-50 MPH mark (and I would use a GPS speedometer app on my phone and not trust the dashboard speedo)... then pull it up to 'D' with foot off the pedal and you should feel 4th...


that will tell you if you are getting all your gears in the trans..


the other possibility is that the tach is wrong and you arent revving nearly the RPMs you think you are...



lotds of factors here so we test out the ones we can eliminate.. like the trans not making all gears..

[NWOleman]

Thanks Cadillackid! That sounds like a solid plan to figure this out. Are you into caddy's by chance? I got a cool 70' coupe de Ville!

[musigenesis]

Originally Posted by Booyah45828

Well, here's the link for a simple mph calculator.

https://spicerparts.com/calculators/...rpm-calculator

So I wonder if I'm using this right. My R&P is 6.17, my tire height is 45" (I think - 22.5R10 means a 45" tire?), and I know my top speed is 67 mph at around 2500 RPM. With this calculator that means my transmission gear ratio in 5th is 0.81?

[skoontz]

Originally Posted by musigenesis

So I wonder if I'm using this right. My R&P is 6.17, my tire height is 45" (I think - 22.5R10 means a 45" tire?), and I know my top speed is 67 mph at around 2500 RPM. With this calculator that means my transmission gear ratio in 5th is 0.81?

I think you're using it right except for one detail. A 10r22.5 tire should be about about 40-41 inches tall. That would get a gear ratio of .71 to .74, which matches what I have seen for Allison transmissions.

[musigenesis]

Originally Posted by skoontz

I think you're using it right except for one detail. A 10r22.5 tire should be about about 40-41 inches tall. That would get a gear ratio of .71 to .74, which matches what I have seen for Allison transmissions.

I should go measure my tire. I figured R22.5 meant a radius of 22.5" and thus a diameter of 45".

[cadillackid]

22.5 is the rim diameter.. the tire sidewall varies in height..



im definitely into Caddies! i dont own any now but have had quite a few over the years.. LOVE the 70 deville!! I had a 76 eldorado ragtop.. and several late 70s Deville / Fleetwood series.. I used to buy the cars rather cheap.. fix em up and sell them.. and of course drive them.. Lincolns may have rode a bit smoother.. but the cadillacs had a lot more Get up and GO!

[sportyrick]

As I have said before sometimes you can find the ratio stamped on the pinion gear but you have to drop the driveshaft to see it.

[musigenesis]

Originally Posted by sportyrick

As I have said before sometimes you can find the ratio stamped on the pinion gear but you have to drop the driveshaft to see it.

I actually found mine stamped on a badly-rusted steel tag in an envelope in one of my storage compartments up front. It must have come off at some point in the past and some mechanic helpfully kept it. I was disappointed to see 0617 because I stupidly thought my spoiler meant my bus went really fast.

[musigenesis]

Originally Posted by skoontz

I think you're using it right except for one detail. A 10r22.5 tire should be about about 40-41 inches tall. That would get a gear ratio of .71 to .74, which matches what I have seen for Allison transmissions.

Aha, my rear tires are actually 39" high, so that gives me a transmission gear ratio of 0.73. That means a 4.1 R&P would give me 70 mph at 1800 RPM.

[Booyah45828]

4.78 puts your math even farther from what you're seeing. A lot of good things said, I feel you'd be going slower then 52 if you were only in 3rd gear though. Verifying the accuracy of the speedometer is a good idea. I get it that you have a tag in the bus stating 4.78, but we really need to verify it is 4.78, because if it is, you have problems that need addressed before you change axle ratios trying to go faster.

You can check your rear axle ratio and remove all doubt. You can also do it in a matter of minutes without raising a wheel or anything.

Mark the tire where it contacts on the pavement, and mark a spot on the driveshaft, preferably near the rear axle. With a helper(either driving or them outside) drive the bus 1 tire revolution forward, so that your mark is back on the bottom. While driving forward, have either you or the helper stand on the side of the bus and count the number of turns the driveshaft made. The number of turns it makes should coincide to your ratio. 4 3/4 turns means your tag is right, anything outside of that says your tag is wrong.

But you/we need to know if the ratio is what you say it is, before you go about changing gears or if you need to repair/swap/upgrade transmissions. Swapping to a 4.10 gear on an at545 that's on it's last leg is a recipe for disaster.

[Booyah45828]

Originally Posted by musigenesis

Aha, my rear tires are actually 39" high, so that gives me a transmission gear ratio of 0.73. That means a 4.1 R&P would give me 70 mph at 1800 RPM.

Your ratio would be a fixed number, either .71 or .74 depending on the transmission, as it's determined by gear tooth count.

Your tire height can vary, and is actually hard to physically measure because load will change it. Height really isn't the number you're after anyways, they're just using it to calculate the circumference. Most of the time I just use an estimate based off of tire size, like 10r22.5 being 40.4 inches tall, 11r22.5 being 41.4, etc.

That height will vary between tire manufacturers and even specific models. It will also vary due to wear.

Here's a helpful image that I reference from time to time dealing with metric vs standard tire sizes https://www.toyotires.ca/sites/defau...ison_Pg_25.pdf

[BamaBus]

I brought a 92 version home last year(?) and it wouldn't go very fast. I thought I had the throttle pedal pressed as far as it would go but then scooted forward and really pressed on it to gain another 8-10 mph. Not sure what the rpms were. 4.78 tag on rear housing with the 545 trans. Previously posted advise will be more beneficial.

[Booyah45828]

Originally Posted by musigenesis

Aha, my rear tires are actually 39" high, so that gives me a transmission gear ratio of 0.73. That means a 4.1 R&P would give me 70 mph at 1800 RPM.

That 4.10 sounds great and all, but you better have a big torque engine driving it, or you'll likely only be in OD when lightly loaded flat cruising. Apply any throttle at 1800 and the trans will likely downshift out of OD in a hurry. Stick with something that puts you around 1800@ 60, and then live with a little more rpm then you want @ 70. It will make a much better driving rig.

[musigenesis]

Originally Posted by Booyah45828

That 4.10 sounds great and all, but you better have a big torque engine driving it, or you'll likely only be in OD when lightly loaded flat cruising. Apply any throttle at 1800 and the trans will likely downshift out of OD in a hurry. Stick with something that puts you around 1800@ 60, and then live with a little more rpm then you want @ 70. It will make a much better driving rig.

I actually seriously doubt that I will ever change it. It would probably cost too much for me as a non-mechanic, and for the local (and hilly) driving I do these days 6.17 is really great. If I ever end up traveling long distances in it after my parents pass, I may re-gear for that. I'll take your advice on it if I do.

Is the DT466 not considered a high-torque engine?

[cadillackid]

Originally Posted by musigenesis

I actually seriously doubt that I will ever change it. It would probably cost too much for me as a non-mechanic, and for the local (and hilly) driving I do these days 6.17 is really great. If I ever end up traveling long distances in it after my parents pass, I may re-gear for that. I'll take your advice on it if I do.

Is the DT466 not considered a high-torque engine?

if you have the high torque version it can be a real powerful engine.. most school busses have the 195 or maybe 215 HP low torque version..


the high torque versions run different pistons and turbos...



with a 6.17 your driveshaft speed is just higher than I like for long periods.. if the driveshaft assembly is properly phased and in good balance it shouldnt hurt too much..

[LargeMargeInBaja]

Originally Posted by NWOleman

...1994 Bluebird with a front...5.9 and AT545 with 4.88...

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I think you are up against 'windage', the frontal area fighting the increasing air-pressure as the vehicle accelerates.
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And I think you are entering 'test-pilot' territory, attempting to side-step the engineers at the 1994 Bluebird factory.
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And I think you would be facing a 'full-throttle' situation as you attempt to take advantage of the potential higher speed.
Full-throttle stresses all the components... including the cooling system and U-joints.
Say 'sayonara' to mpg, you are 'chucking quarters down the carb' as we said during my hot-rod days.
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An aside:
Our rig has your big sibling -- Cummins 505ci/8.3-liter (litre) -- with substantially more torque than the little guy.
With our Allison 3060, we are comfortable loafing-along at 52mph/1,825rpm in the slow-lane.
And 'yes', we realize this frosts the patooties of the rushers.
Accordingly, we migrate to rural rustic two-lanes at the earliest off-ramp.
For us, 35mph is more fun.
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An another aside:
Those rough logger tracks to an unnamed mountain lake?
Those unmarked desert trails to some isolated Baja beach?
Our 6.88 axle-ratio seems ideal for 'crushing our enemies, hearing the laminations of the scattered survivors'.
Kidding.
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What is our rig?
Here is our introduction with plenty of portraits, plus our reasons for our decisions:
https://vanlivingforum.com/threads/e...8/#post-576110