Towing with a 99 GMC Suburban 7.4L

OP...hope you don't mind this kind of stuff...research for my rebuild and of great enjoyment...

A 1,000 HP 4L80E with a 3 year warranty is around $3K-$4K and in the same ball park as a standard 4L80E rebuild

Gear Vendors is around $3K plus install. Begets an 8 speed 4L80E

Currently have 4.1's (10.17 first and 3.08 OD 4th) and noodling 4.88's to have a 12.1 first gear and a 2.85 double OD 8th gear.Compare that to the 4L60E that everyone says has a better first gear over a 4L80E...

A for sure thing on my have to have list for the rebuild




Click For Full-Size Image.

IIRC these numbers, but should be close enough for discussion...

Our Vette with a L88 (big block 427) we ran on track days back in the late 60's had (great discussion and disagreements) 550 Ft/Lbs and 530 HP at around 5K RPMs. To run at that level required 103 Octane. Sold out of that partnership, as knew I'd kill myself in it. Bought into that partnership when it had a small block 327 (such a sweet setup and one of my best engine builds) and they talked me into funding the L88...but...that body was NOT built for that kind of engine...first time nailed it wanting "to go this'a way" and "it went that'a way"...said it was going to kill me one day if did't get out of that partnership...

My L21 (big block 7.4L) is around 410 Ft/Lbs (1,800 RPMs and up) and 290 HP. Red line is dashed from around 4,300 RPM and solid at 4,800 RPM. Power drops off at around 3,800 RPMs so not much to gain above that except for HP

The L18 (8.1 big block) is around 450 Ft/Lbs and 330 HP.

Now with +168K miles of extreme usage (worst was towing a 8.5K + trailer from sea level up to Lake Tahoe..often and WOT keeping up with the others with newer TVs) and just plain fun boy racer used as a car...could smoke the rear's anytime AFTER figuring out that the computer denied power mode when it got over 200*F.

Today, it no longer has the juice to smoke'm, but does run strong in reference to just car boy racer stuff. Towing (sold the boat and just borrow trailers from church members) really shows that it has lost a few pony's...and the why noodling a crate (dropping that, but will continue to search as a just in case) or rebuilding. Target is L88 levels or more on 87 to 9X Octane (cam, valve train, tune, etc) and still pass Calif SMOG...maybe a turbo installed in back for altitude towing...throttle lag isn't a problem there...and a maybe brake launch when testosterone kicks in...

To the OP, there are lots of other stuff forgotten and will noodle to post. Fun stuff for me...

On my second steering box. First had to be tightened often till it started to leak. Since you say 'farm' am noting this...as used to off road much more and used the front tires to move stones out'a the way...

There is an adjusting set screw on top of the steering box and locked in with a jam nut. Careful, as you can ruin the box by over tightening. I'll dig up the old thread posted on HOW2, but it has Photobucket pics...which won't work and have to find original pic's to post

BenK wrote:
NOTE that the newer TVs, not just GM but most all, has "Torque Management" and this era Suburban does NOT, so it runs with that pack well, or at least mine does...to even taking them when "The Boy Racer" kicks in when challenged by another Boy Racer...

The 8.1 is drive by wire and loaded with torque mgt to keep you from shredding the tires when you step on the gas.....

Agreed! But only to flip it. I have no use for a big block Sub that gets half the mileage and half the power of the truck. Would flip it in a heartbeat for 2-3x as much.
But I don’t think the OP would be getting a bro deal on it if he was going to flip it. That would p!ss off his buddy!

Golly, if I lived near there I'd buy that thing in a second. In fact, if you don't want it, I may drive down there. 454 is a great motor, overheating and A/C are very easy to fix. Towed with an '88 Burb 454 for years with double slide 30' Arctic Fox. Because the '88's were lower compression and small valves, I had to add headers and aluminum radiator (when we lived in Las Vegas). Up here in the Pacific NW, 454's are really hard to find. As for 9,000 lbs, my '94 K-1500 Burb with 350 and 3.73's is rated to tow 10,000 lbs.

Thanks for all the good info.
Benk, that was quite the thorough write up.
I ended up buying the Suburban because it’s such a good deal and it will be a good work truck for the “ranch”.
I won’t be living full time there as I have a place on the beach in Ventura, but I will be able to ride my motorcycle up the hill and still have a vehicle there to get around with and visit the hardware store.
I will have to tow my TH just to see how it does but will use the Ram for towing.
I’m sure I can get my money back if I decide to sell it.

DaveF-250SD wrote:
I think it would be a good vehicle to have around your property to haul things around to save wear and possible damage to your new truck. If it runs hot and slow with a 5,000 lb. trailer, something is seriously wrong with it. A friend of mine has a class C motorhome with a 454 that probably weighs 10K empty, and he never has any issues with overheating, even in the middle of summer. My own '78 GMC 1 ton SRW 454 pickup (have to update my signature) runs very cool under all conditions, empty and pulling a car on a trailer, just under 7K pulled weight, and can do 65 up any hill or mountain pass I encounter while towing.

Agreed. It shouldn’t be overheating, but make sure it’s something easy to fix and not a head gasket or something

Lines on this SUV are still beautiful...clean.

It will do it, albeit since older, a bit slower up the steeps

Waiting for mine to die or wear to the point just have to rebuild... :B

Will keep mine for a long while. Hate payments, can afford to buy one out right, but their prices are insane to me. It is a truck to me, not a car. Know that trucks are designed with margins for work, not like cars...

I have 4.1's and suggest you consider it if it doesn't have it now

Flush EVERYTHING with synthetic. Mine has been on synthetic's since new and MolyB (2 micron) additive in everything except except for the 4L80E and Diff (anything with clutches will be ruined with such a low co-efficient friction modifier). Make sure power steering (also is the power brake source) and MC (DOT4, non-silicone)

Change out the engine thermostat to a 180* and I've drilled a 3/32" dia hole in the flange...as the tiny <1/64" dia OEM hole is too small and temp over shots when really, really hot)

Change out the fan clutch to a 'SEVERE' duty. OEM is 'standard' and some have found to have a 'HD'. HD and severe duty fan clutch of that era couples up to 90% of shaft RPMs. Standard best is around 50%-60% shaft RPMs are coupled. Also, since so much is coupled...the bearings are beefier and SEVERE duty has the biggest bearings. It will ROAR when it kicks in and surprised me the first time. Can see the dash temp gauge needle drop like a rock when it kicks in

Know other TV's computers have backed off via their torque management and mine still provides 'Power Mode' when they start to slow...of course naturally aspired...forced fed has that advantage and if they have a good cooling system with lots of margin... Also, mine handles...more so compared to newer which are so tall because the fashion statement crowd thinks taller is more capable...my track is +10.5" wider than stock to boot, so cornering is something different...

Make sure the radiator has the 'short' neck and is in good order. The 'short' neck has the 7 fin ATF, in tank, radiator (liquid to liquid).

Depending where it has been parked/kept/etc...leaf & debris build up between the AC and radiator partially blocks air flow. Creates mystery heating/cooling issues. I blow mine out with 100 PSI air every so often

Duct tape the intake holes on the engine bay side of the air ducting to the filter box. I've made my own ram, cold air intake that is in parallel to the OEM intake routing.

Keeping the intake charge important to always have power mode available.

Change out the air filter with the OEM optional "HD" and just drops into the OEM filter box. It is about 2" larger in dia.

Read diff is the EXACT same as the 1 ton dually of that year...except for the cylinder and MC (dia's are larger and DO NOT lose the short rod between the MC and booster...almost impossible to replace and is a junk yard thing) and change them to that 1 ton dually

Check if you have the 3.5" wide rear shoes...those are the same as the 1 ton dually...mine came that way and why parts folks always ask how wide your rear shoes are....no matter what

Manually adjust the rear shoes often, as the proportioning valve is off....not sending enough to the rears. Decided to NOT touch that and just manually adjust them

Pulling is common and is due to the rears not in the braking game...in a pinch...apply the E-Brake pedal a bit to move the shoes into the game

I like Praise Dyno, but the GM Parts "Dura Stop" rear shoes almost as good to provide "Nose Bleed Braking"

Super Tune the fronts...as the slides stick...I've made a special grease gun with a hypo needle to poke in high temp brake grease (filled with MOLYB)...and make sure to change out the slide O-Rings

Spark Plugs...have modified with by 'cutting back' the ground electrode

SS WIRE ignition RF suppressed wires...I like copper over Platinum plugs and change them around 50K miles or so

Copper inserts on the dist cap::Gater Back serpentine belt

Change the Serpentine tensioner...they typically last in the 60K miles or so for me

NOTE that the newer TVs, not just GM but most all, has "Torque Management" and this era Suburban does NOT, so it runs with that pack well, or at least mine does...to even taking them when "The Boy Racer" kicks in when challenged by another Boy Racer...

Know that there are most things forgotten and will noodle that for a while...

Plans for the day that, gosh darn, just have to rebuild...

• A 'built' engine that will pass 1996 SMOG, I live in SF Bayarea
• 1,000 HP 4L80E, but only going to use less than half depending on what the engine builder can do and still pass SMOG...so it will last a looooong time...these 1K HP unites are warrantied for 3 years, so going to last
• A gear spliter OD with a 0.75 OD ratio...gear spliter, so it will insert itself in-between each gear to make 8 gears...times 2 in LOW
• New diff ratio in the 4.8 or maybe 5.x for a final double OD to the axles in the 2.x range

All of the Internet searches finds +500 HP 454's, but are for carberators...no good ones found to date for a Vortec...yet...or will bite the bullet and have one built. Engine builder passed on and am looking right now

Also, if it has the F60 option (check the glove box label listing all options it left the factory with) and you will have a 'Snow Plow' prep package that begets 1 ton front torsion springs. Since it comes with the 6,000 RGAWR as the 1 ton dually of the same year...tongue'n cheek refer mine as a K3500 Suburban...

If me...I'd buy it and have two Sub's and make the choice of which one to be the parts one...




mysub mysuburban

wnjj wrote:

blt2ski wrote:
Scott, I came back from Duvall at 40-45k gcw in dump truck. Mind you it is a 175/330 non turbo 7.3 V8! Fortunately, 7 forward gears. In 1st which is 10.08-1, 2800 rpm at switch back motor starts slowing down to 2100 as I straighten back up, motor speeds up.....thought I was going to have to back down with 5 yrs of soil in truck, 12k equipment trailer behind me!
Needed clean shorts after that.
Never as of yet have I stalled that rig on a hill! My auto trans 454s, stalled them out all the time. One was a 4ad, into 4lo, no big. Rwd dually, burned up transmissions every 30k like clockwork, with 5k fluid fluid change.....talking COMPLETE fluid change including fluid into the TC!

Marty

Maybe not practical on a steep slope, but could you unload the equipment and drive it up and/or help the truck with it?

I probably could have removed the bobcat and mini trackhoe. That would be 9000 lbs between them. That would have put me at a weight I could have kept going 1 to 2 gears taller.
But as mentioned, you can have all the HP/torque in the motor you want, but if it can not get to the ground in a practical manner, you're screwed! I've always had better luck towing with a properly setup drivetrain in a lower HP rig like this Navistar, than a higher power, but improper, usually transmission setup rig.

Marty