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Old 05-06-2022, 08:53 AM   #1
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Engine: 454ci Big Block Chevrolet
Rear end gear change in a 1993 Bluebird Mini bird

I'm starting this thread to document my rear end modification for anyone who might be interested in doing this.


Background:
  1. My bus is 26 feet long on the P series GM chassis. It weighs loaded about 13k to 14k pounds.
  2. Factory put a 350ci small block. I have a 454 in it now, and I'm about to make additional engine modifications.
  3. Rear end is a Dana 80 with disc brakes, 37 spline axles and a 5.13:1 factory gear ratio.
My goals are to reduce highway rpm for better fuel mileage and engine longevity. I'm currently getting 4.6mpg, which I think I can at minimum, double to 8mpg, with a goal of 10mpg. Note: If you do a gear change, you should take into account your engine's rpm range and power output range.

With my 4L80E transmission, which has a 1:1 ratio in 3rd gear, and a 0.75:1 ratio in 4th gear (0.75 crankshaft turns to 1 turn of the transmission output shaft), 33 inch diameter tires, 5.13:1 rear end gears, I turn just around 2,400 rpm at 60mph on the highway in 4th gear.

I'm putting a 4.10:1 rear end gear ratio, which will yield just around 1900rpm at 60mph, and just around 2,000 rpm at 65mph in 4th gear. Downshifting to third will put the engine in the upper 2000 rpms range, which allows enough torque remaining to go up hills without running out of power. This is only because of the heads and camshaft change that I'm doing to the big block right after the rear end gear, so that they work in unison. The biggest change is making sure the power band is down low as possible, avoiding the need to spin up the engine to make any meaningful power. The 454 has a 4" stroke from the factory, which allows it to produce very meaningful torque numbers with some engine modifications.

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Old 05-06-2022, 09:15 AM   #2
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Current status

I removed the rear axle by disconnecting the brake lines and draftshaft after jacking up the rear of the bus with a heavy duty bottle jack and sturdy jack stands. It was so heavy getting that thing out. I used a floor jack to lower the entire axle and slide it out from under the bus. Next, I used an engine hoist to get it into the back of my pickup truck. It's not at a speciality shop that only works on rear ends.

The above paragraph makes it sound easy, however it was nothing of the sort. I have a patch of grass next to my driveway that made hauling the axle over that.....interesting. Disconnecting the axle wasn't too bad as long as you have a an impact. It was moving it that was really difficult.

While I'm at it, I've also removed the leaf springs and I've brought them to a suspension shop to have them re-arched if needed and inspected. I've got a lean on one side, so they're going to determine if one spring arch height is lower than the other, which could be responsible for the lean, or if I have a weight imbalance. I may wind up adding an additional leaf to one spring to raise it up about 2 inches.
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Old 05-06-2022, 09:57 AM   #3
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Some pictures

Here are some pictures of the progress so far.
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Old 05-06-2022, 02:38 PM   #4
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I'll follow along.

Shouldn't be too difficult to source parts. The dana 80, 4l80, 454 is fairly common and parts should still be available. You probably have a weight imbalance, so long as none of the springs are found to be broken. Springs needing re-arched is kind of a lost art and isn't something that's done anymore.

Are you regearing the axle yourself, or having someone do it for you? I'm not sure if you'll achieve your 8-10 mpg goals though. It'll be an improvement, but I'd be surprised if it doubled. I'm guessing about 6-7 is realistic.
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Old 05-06-2022, 05:44 PM   #5
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Thanks for replying

I am having the springs checked for cracks and I am definitely having them re-arched at a shop around here that still does it. I'm going to determine if it's a weight imbalance or if one spring is damaged and sagging.


As for the gear change, I'm leaving that up to somebody else. It's not cheap though. I've been meaning to do this for a while. I'm going to have to solder the GM DRAC circuit board jumpers to adjust the pulse width to fix the speedometer also.


As for the fuel mileage, the current big block I think is overcammed for the application and I've got too low of vacuum for the fuel density type fuel injection with a MAP sensor to work correctly. It's way too rich right now, so fixing that with a proper camshaft and Vortec head with higher static compression I think will yeild a more dramatic result in mileage, I hope. I am going to report back on here what I find out.
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Old 05-11-2022, 09:57 AM   #6
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OK, so the work on the differential is supposed to be done this week sometime. I'm waiting on the shop with the springs too.
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Old 05-11-2022, 11:21 AM   #7
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P30's suspensions have a hard life. They're also famous for having a list to one side.
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Old 05-11-2022, 03:19 PM   #8
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Quote:
Originally Posted by craigify View Post
As for the gear change, I'm leaving that up to somebody else. It's not cheap though.
How much is it costing you? I've been considering doing this to my bus since I currently have a 6.17, which means my ideal cruising speed is 47 mph even with my 5-speed transmission. I've seen estimates from $1500 to $2000 - which doesn't even seem that bad given some other things I've had to pay for.
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Old 05-11-2022, 05:51 PM   #9
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I have changed the gear ratios in the "new Crown" (actually paid a mechanic to do it.). I changed from a 4.10 gear ratio to 3.42. You have an overdrive transmission so the higher ratio probably works better for you. I am glad I had it done though. I haven't driven the bus far with the new ratio, but discovered that 70 mph comes real easy. I am curious how yours will work out. I think you are doing the correct thing by having a professional do the work inside the differential. Mine was expensive being I have tandem axles and heavy duty differentials (think 18 wheeler parts).
I am hoping for 8 mpg and would be ecstatic at 10 mpg.
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Old 05-12-2022, 09:30 AM   #10
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Quote:
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P30's suspensions have a hard life. They're also famous for having a list to one side.
Perhaps. This chassis is pretty strong, I think. I've weighed it almost fully loaded and post-conversion on an agricultural scale and came in around 13,500 lbs. That's well within specs.

Well, this suspension is all around leaf springs. No coil springs and no air bags. The lean was on the side where the wheelchair lift was in the rear, which may have contributed to the sag problem. When building it, we considered the weight distribution the best we would, putting different things on each side. I'm not sure how well that worked out since it's not like we were scientific about it, but I know that it would be easy to just built it however without thinking of weight at all, which I'm guessing some do, especially when they turn them into food trucks.

I'm confident these springs were original or at least were an old replacement, since they had a GM part number stamped right on them. Plus they had what looked like a manufacture date of 93.

In the front, I have already replaced the springs with new Dayton springs and Gas-a-just shocks.
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Old 05-12-2022, 09:39 AM   #11
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Quote:
Originally Posted by musigenesis View Post
How much is it costing you? I've been considering doing this to my bus since I currently have a 6.17, which means my ideal cruising speed is 47 mph even with my 5-speed transmission. I've seen estimates from $1500 to $2000 - which doesn't even seem that bad given some other things I've had to pay for.
Total cost, including gears, bearings and whatever else goes in there, gear oil and labor turns out to be just around $2100.00. It was more than I expected. I was thinking around $1500.00 or so, and I was honestly hoping for less considering I brought them the damn axle.

Considering inflation and the world we now live in, I suppose it's probably right. I just don't like it when it's much higher than my own estimate lol.

I really didn't want to learn how to set up those gears and risk any problems from improper tooth contact and whatever else you'd need to worry about. I was able to keep the same carrier too, which is great, considering that would increase the price by a couple of hundered just for that.
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Old 05-12-2022, 09:56 AM   #12
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Quote:
Originally Posted by flattracker View Post
I have changed the gear ratios in the "new Crown" (actually paid a mechanic to do it.). I changed from a 4.10 gear ratio to 3.42. You have an overdrive transmission so the higher ratio probably works better for you. I am glad I had it done though. I haven't driven the bus far with the new ratio, but discovered that 70 mph comes real easy. I am curious how yours will work out. I think you are doing the correct thing by having a professional do the work inside the differential. Mine was expensive being I have tandem axles and heavy duty differentials (think 18 wheeler parts).
I am hoping for 8 mpg and would be ecstatic at 10 mpg.
I'm glad to hear it works well for you. This also means I'm probably going down the right path! I have to admit, I spent a long time going back and fourth in my mind between 4.10 and 4.30 or so, until I just decided to go with 4.10.

I wanted to make sure the internals of the differential are set up properly, which is definitely why I brought it to a speciality shop!

Those crowns are really cool looking.
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Old 05-12-2022, 09:56 AM   #13
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Quote:
Originally Posted by flattracker View Post
I have changed the gear ratios in the "new Crown" (actually paid a mechanic to do it.). I changed from a 4.10 gear ratio to 3.42. You have an overdrive transmission so the higher ratio probably works better for you. I am glad I had it done though. I haven't driven the bus far with the new ratio, but discovered that 70 mph comes real easy. I am curious how yours will work out. I think you are doing the correct thing by having a professional do the work inside the differential. Mine was expensive being I have tandem axles and heavy duty differentials (think 18 wheeler parts).
I am hoping for 8 mpg and would be ecstatic at 10 mpg.
I have a 3.42 in my shorty and it easily does 10mpg. Good luck!
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Old 05-12-2022, 12:29 PM   #14
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If you have a good running 454, and live in a state where you can do a few mods 3:42-3:73 would be great. If you can get that motor to around the 1hp:cid 3:42’s would not be a problem.
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Old 05-12-2022, 07:13 PM   #15
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Engine mods

Well I have an engine thread but it doesn't seem to have gotten any traction like this one has.

I'm keeping the GM TBI system that I've already had reprogrammed. I'm putting vortec heads with 100cc chambers and a different chamber shape on the mark iv block. I think I've got quench down to about .040, but I won't know exactly until I measure actual piston depth.

Static compression, based on calculation, should be right about 9:1 with what I think are factory flat top pistons. I've got a cam from Bullet that's of the higher lift and not so long duration variety. Lobe separation angle is something like 112 degrees. I'm after a power band that comes in around 1,400 or so up to 4,000 rpm. Already have tri-y long tube headers and flowmaster 60s on there. See pic for cam specs.
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Old 05-12-2022, 09:06 PM   #16
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That is not enough cam IMO. But I’m nobody. Talk to Brian Harris at harris custom fuel. I would think stock you are more like 8.2-8.5 compression. If you were 9.0 to 9.5 to one with vortec’s and headers, the edelbrock TBI intake and a cam with a lift around .5 and a duration around .250 at fifty you would be in the ball park. Lunati makes some cool split duration/ hybrid cams that will give a smooth Idle . I run the entry level comp thumpr on customer’s tbi cars with good sucess.
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Old 05-13-2022, 09:06 AM   #17
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Seems small to me too.

454 would need a little more duration IMO then that, but you'll hit your power band easily with that guy. Not sure if you'll need that duration/lift split either if you're running tri-y long tubes and vortec heads. Truthfully, I'd think you'd want more intake split over the exhaust if you're using a factory intake manifold/tbi. IMO your over-exhausted so to say, especially with the power band you're after.

I assume being a bullet cam that someone spec'd that for you?
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Old 05-13-2022, 09:45 AM   #18
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I didn't say stock is 9:1 static compression. I said that I'm building this 454 with a target static compression ratio of right around there in order to continue to run lower octane fuel and most importantly, to be more efficient with the fuel that's being used to make more power without jamming in more air and more fuel. Stock compression for 454s with big open chamber heads is really low even with flat top pistons, which I assume you know. I'd be surprised if one would even reach 8.0:1.

Second, adding duration is going to keep moving the power band up. If I spin past 3,500 RPM I'm going to be pissed off, and it's probably only going to be climbing up a really steep hill or mountain on a state highway or US highway. So, help me understand your focus on duration here.
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Old 05-13-2022, 09:48 AM   #19
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Seems small to me too.

454 would need a little more duration IMO then that, but you'll hit your power band easily with that guy. Not sure if you'll need that duration/lift split either if you're running tri-y long tubes and vortec heads. Truthfully, I'd think you'd want more intake split over the exhaust if you're using a factory intake manifold/tbi. IMO your over-exhausted so to say, especially with the power band you're after.

I assume being a bullet cam that someone spec'd that for you?
Well, my gut reaction was the duration numbers were too low also, but I called Bullet cams and spoke to somebody there for 30 or 45 minutes and we went over my application. This is the result of what we talked about it, right from the cam manufacturer. I'm kind of anti authority, but sometimes I force myself to trust somebody else -- and in this case, the cam manufacturer.


Yes, I'm using a factory style cast iron intake manifold because it has the EGR port on it



Yes, I agree I probably have a bit too much into that exhaust then what's needed for my rpm range. Maybe the vortec exhaust port size will even out with the try-Y headers and increased exhaust duration on the cam spec? Port is a little smaller. Who knows.
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Old 05-13-2022, 10:24 AM   #20
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I think a lot of companies run light on intake duration to compensate for TBI slow reaction time. Possibly fearing high cylinder and exhaust temps? More duration will only help you if you get to that 9:1, 9.5:1. most BBC’s that get built around here have more lift and duration, use some manner of vortec head, and are around 9.5:1. They end up making peak torque at 3800 and peak horsepower around 5400ish. Generally they end up making .84-.90 hp per cubic inch at the flywheel based on calculations from RWHP on the neighbor’s dynojet. We’ve never really built anything that doesn't run on 91 octane. Either way good luck, and call Brian Harris if you need TBI tuning help.
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