Fuel economy of 7.4L (454) powered 3/4 ton suburban

[oil pan 4]

Originally Posted by jakobnev

Remember to run the flame thrower exhausts on low.

I have set of 2nd gen Camaro headers on the way. I think they will fit.
The price was right. Might run side pipes, no mufflers or anything.
I just want to play with it a little before I pull it and drop in the diesel.

[oil pan 4]

Originally Posted by cosmick

a 6.2L diesel only makes a truly lame 240 ft/lbs TQ, no more than an equally lame 305 gasser. The 6.2 feels torquier to drive than the 305, but that's only because the torque comes on quicker and stronger off idle.
A 700R-4 does need to be "built" to survive a 6.2, but even stock 305s break the best of stock 700R-4s, however, the 700R-4 isn't inherently weak, most of the guts are as strong as TH400 guts.

I don't have a 6.2.
I have a 6.5L block, with 6.2L heads then that is twin turboed and intercooled.
Then while the diesel engine is out I am putting a waste gate on and I may put a the bigger new injector pump on along with new injectors. The injector pump has a lot of miles on it and I believe the injectors leak a little.

The Th400 has 3 major design flaws as far as I am concerned, it has no over drive and no torque converter lock up. There are after market options to put a locking torque converter on a TH400 but, but its still missing a gear.
Then the first gear in a TH400 is about 2.5:1 where a TH700 has a 3.1:1 first gear.

Cooling the TH700 wont be a problem. I have 3 transmission coolers and 1 factory installed transmission fluid heater. As long as the torque converter lockup is working the 3 cooler are tremendous over kill. When lockup is engaged not towing anything I think it actually needs the factory transmission heater.
I have only seen it get real hot with no converter lockup while towing another car on my trailer.
I weighed the 3/4 ton suburban, with a full 40 gallon fuel tank it weighed under 5600lb. So the weight difference between the half ton and 3/4 appears to be nearly nothing, or about as little as a full versus low tank of fuel.

I am having a transmission shop near Lubbock rebuild the transmission. They are going to put in a 30 spline 5 planetary gear set, high pressure high volume hydraulic pump and upgraded higher count clutch packs and most importantly a working 30 spline lockup converter.
Sad thing is they want almost the same amount of $ to build that bullet proof 700R4 as my local transmission shop wants to do a stock rebuild. I have been told my local shop is a rip off this appears to be true.

When towing something heavy my plan is to put the shifter in 3 making it like a TH400 since they share the same 3rd gear ratio of 1:1. And because my 700R4 is an 1984 the torque converter lockup will lockup in 3rd gear. Later 700R4 transmissions will not lockup in 3rd gear unless you modify them.
Plus I added a torque converter lockup control switch. I can turn the lockup off, run it like normal or turn it on when ever I want.

[oil pan 4]

Originally Posted by kir_kenix

Really interested what you can eek out of the 454 before doing the diesel swap. If you can pull down 12 or 13 on the highway that would be a major victory in my eyes...what with the weight, gearing, and lack of OD.

What year is the "new" suburban?

The half ton suburban was getting right about 21mpg towing my small trailer on the interstate at 70mph until it lost its torque converter lockup.

[oil pan 4]

Originally Posted by aardvarcus

Oil Pan,

What a project! What year of suburban, and 2wd/4wd?

In the short term, some 235/85R16 tires would provide some good “rubber gearing” to go along with your 3.42. The only other option I know would be an axle swap for a D60/D61 rear axle with a 3.07:1, but that may be more work than you want to want to for just a 11% reduction.

For the long term solution, are you set on keeping the automatic transmission? I had headache after headache on my 4l60 (which is basically just a 700R4) behind my 350 in my 1991 Chevy K2500, until I finally swapped it over to a NV4500 stick shift. That was without a doubt the single best fuel saving mod I have ever done on a vehicle.

If you want to keep it an auto, you could always try to find an Allison 1000 from a 2001-2007 Chevy 2500HD/3500 with either the 8.1L Vortec or 6.6 Duramax. Those are 5 or 6 speed (6 speed= double overdrive) depending on the year and built like a tank. I would think you could get one out of a wrecked 8.1 gasser reasonably cheap. They make stand alone TCMs to control them, as they are popular for cummins swaps.

I have dreams of a future ¾ ton diesel suburban project, so I will be watching with interest. Oh, and we want pictures!

Oh its 2wd. Its a C2500 (GMC). I found that with 4wd you spend more time working on it or around than actually using it.
Plus it seems like no one wants old 2wd suburbans so they are cheap.
It appears the only way to get a 2wd 3/4 ton suburban with full float axle, towing package with trailer brakes, 40 gallon fuel tank, class 4 hitch, heavy duty axle was to get one that had a 454 gas engine and TH400. Which is fine because I already have the engine I want.

My specification was to find a 2wd. Had to be 2wd, I did not want 4x4 at all.
With what I am going to tow with it I really should have a 3/4 ton and full float axle, I already am pushing the limit of what standard 15 inch tires on 5 lug wheels can carry. I found that late 90s and 2000s jeeps use 16 inch 5 on 5 wheels but then the 16 inch load range D tires exceed what the rest of the vehicle should be carrying.

I will be adjusting the rubber gearing. The half ton had 31 inch load range C tires. The 3/4 ton has 32 inch tall 10 ply tires on 16 inch wheels.

The 6 speed allison transmissions I can find are around $2200, that doesn't include freight shipping. Then the standalone controllers appear to be at least another $1500. Then I would need a custom drive shaft. That trans with a controller shipped to me I am looking at about $4000, plus whatever a custom drive shaft costs.
I can get the TH700R4 rebuilt and bullet proofed with a new torque converter for about $1400 to $1500.
If I could pick up a used 6 speed Allison locally for cheap, yeah I might do it.
The 5 speed Allison has the same top gear ratio as the 700R4, so that wont help fuel economy.

I don't want to do an NV4500 swap because my engines internals were balanced by a machine shop in Virginia beach. Swapping from a flex plate to fly wheel would mess with my engine balance.

[oil pan 4]

Originally Posted by cosmick

3.21:1 gears exist, but can't be had new anymore, for the 10.5" axle. .

I was wondering if 3.21 is a type-o, but looks like I was looking for the wrong ratio. I found parts referencing that they work with "3.21, 3.42, 3.73 and 4.10" gear carriers.
Kind of like trying to find 4.11 gears, when they never made them, but always had 4.10 gears.
So thank you, I am use to 10 bolt stuff which has 3.23 gears.
I will look for 3.21 gears if I cant find any I will go with 3.42 gears.
I don't have to have new gears, I could go for a used set.

[cosmick]

Originally Posted by oil pan 4

When towing something heavy my plan is to put the shifter in 3 making it like a TH400 since they share the same 3rd gear ratio of 1:1. And because my 700R4 is an 1984 the torque converter lockup will lockup in 3rd gear. Later 700R4 transmissions will not lockup in 3rd gear unless you modify them.
Plus I added a torque converter lockup control switch. I can turn the lockup off, run it like normal or turn it on when ever I want.

All 700R-4s can lock up in 2nd, 3rd, and 4th, if you wire the pressure switches correctly. Forget the manual switch, just wire in a vacuum switch for engage / WOT disengage, and a brake switch so you don't stall your engine or hurt your lockup in a panic stop, when you'll forget a manual unlock.
Any torque converter the average guy can easily afford isn't built to hold WOT while locked up. There are $900 race converters that can, but neither a mild 454 nor any diesel can run a 4400-stall converter.
Too many guys with manual switches are either too bold or too cautious about when to use lockup.
The biggest problem with lockup in 2 / 3 is getting it to unlock for shifts, plus seldom are you cruising at an RPM less than the converter's stall speed except in OD.
Stock stalls start around 1400 for the 700, and around 1200 for the TH350 / TH400, but as I referenced earlier, a GM 1200 will slip more at cruise than a good aftermarket 2000.

[cosmick]

Originally Posted by oil pan 4

The 6 speed allison transmissions I can find are around $2200, that doesn't include freight shipping. Then the standalone controllers appear to be at least another $1500. Then I would need a custom drive shaft. That trans with a controller shipped to me I am looking at about $4000, plus whatever a custom drive shaft costs.
I can get the TH700R4 rebuilt and bullet proofed with a new torque converter for about $1400 to $1500.
If I could pick up a used 6 speed Allison locally for cheap, yeah I might do it.
The 5 speed Allison has the same top gear ratio as the 700R4, so that wont help fuel economy.

I don't want to do an NV4500 swap because my engines internals were balanced by a machine shop in Virginia beach. Swapping from a flex plate to fly wheel would mess with my engine balance.

Having balanced more than a few engines, I can say that if it was done correctly, your last statement is a false assumption.
You might also consider the 4L80E, with a '92-'93 diesel GM controller from a salvage yard. This is the cheap way to control any electronic OD 4-speed auto. $20 for the box, $85 for the reprogramming.

[cosmick]

Originally Posted by oil pan 4

I was wondering if 3.21 is a type-o, but looks like I was looking for the wrong ratio. I found parts referencing that they work with "3.21, 3.42, 3.73 and 4.10" gear carriers.
Kind of like trying to find 4.11 gears, when they never made them, but always had 4.10 gears.
So thank you, I am use to 10 bolt stuff which has 3.23 gears.
I will look for 3.21 gears if I cant find any I will go with 3.42 gears.
I don't have to have new gears, I could go for a used set.

Actually, in my salvage yard Saturdays, I have found 4.09:1, 4.10:1, and 4.11:1 gears in GM 10.5s. Some genuine GM, some aftermarket, but all exist. And really, it doesn't matter that greatly which you end up with, even if you have a 4X4 with a front axle 0.02:1 off.
The 3.21:1s are most likely found in C20 C6Ps, but sometimes in C30s. They only came behind 454s, they were mid-'70s to mid-'80s, and were used behind SM465s and TH400s.
This info comes from GM's heritage center.
I've been seeking a set of 3.21s myself, no luck in the last year. The tooth count is 45:14

[oil pan 4]

I only added the manual switch after my torque converter lockup turned to dust for trouble shooting.
I wont run the lockup in 2nd gear. I didn't think it was a good idea.

I drove this 7.4L powered suburban as is for 55 miles and it averaged 6.5mpg. I cant say for sure that's its driving around town fuel economy, but the trend is pretty clear. I filled it up and its not leaving the yard again until the edelbrock carb goes on there for tuning or unless I have my very large trailer on it. I also picked up a used edelbrock oval port BBC intake manifold, since I don't want to use a spread bore to square bore adaptor.

[cosmick]

What's your initial timing set at? Computer-controlled advance, or vacuum + centrifugal? Does it have a QJ with sealed idle mixture screws, or are the screws accessible? If accessible, have you adjusted for best idle vacuum? You want as much initial as it'll take without becoming hard to start hot or cold. No less than 6 degrees initial, even if computer-controlled. Probably no more than 14 degrees if non-computer.
If vacuum advance, use manifold vacuum. If computer controlled rather than vac / centrif, then there may be some wire to unplug. Typically tan with black stripe.
The QJ has more potential for better MPG than the Edelbrock carb, while the engine is stock, you just need to find and fix the leaks, both fuel and vacuum.
Once you convert to the Edelbrock parts, then get a non-computer HEI.