RPO codes question

I have a 1990 2x4 Silverado that I'm close to finishing restoration, and it looks great. I'm trying find out the factory codes for the rear axel, I have all the codes in the glove box, but two things kinda throw me for a curve. My truck has a code 'GMC & GU6 the latter being a ratio of 3:42. Now the truck had/has a heavy duty towing hitch, but my RPO codes don't show a towing package (I can only asume it was a dealer installed item, but you would think this setup would also have a Posi-Traction rear end, but I can't find any mention of it.

Finally what should I look for in buying a rebuilt Posi rear end ? There are a lot out there, 28 spilne 30 spline and so on. The truch TBI and I bought a "Throttle Body Spacer to go under the injector throttle body. What do you think about the spaces? They clain to ad 25 horse power and 30 LBS touque, which I would love to have a little better off idle pickup. And do you folk think that besides the frame installed towing hitch, ( it is mounted with 6 x 3/8 inch bolts, three on each side / Thank you !

Towing package does not automatically mean posi.

It sounds like you have a 3.42 open diff, if there sno mention of posi, and hasn't been swapped in at a later date.

Nice looking truck!

RPO code GMC is the plant it was made at "GMC : PLANT CODE PONTIAC, MI, USA"

I have a 90 Suburban (old square body still) with factory tow package, it comes with 3:73 gears, oil cooler and trans cooler along with four core radiator. But even back then I don't think the hitch was included?

For a posi if you really need one get it for the splines on your axle, 28, 30 etc...

I've only ever seen one dyno test on TBI spacer and it showed no gains? Although years ago the CFM tech spacer did show small gains in a stock motor, it is the one that opens both bores.

I a sucker for any item that claims to give you small gains in torque !

Leaf blower!!!
http://www.youtube.com/watch?v=UQ7YWT6rX0o

You can find substantial gains using the stock manifold by cutting the divider down about 1.25" to 1.5". You'll have to weld the coolant and EGR passages that are opened up during that operation but the difference is very noticeable.

You might be able to get a nice listing of what's in your truck using compnine.com. I found a detailed listing for my '99 Suburban using the free pages.

Thank you ! That worked out nicely ! Much better than a carfax in many ways !

Thank you. Do you think "Throttle Body Spacers" are worthless ? I respect your opinion, but so many folks have bought them and are using them, without any mention of the thing being a waste ? They are simple enough so I would assume your right, ( If GM thought for a minute a spacer would increase HP and toque, they'd made the manifold taller. Having said that, I am astounded that the have such a fan base. I have a C5 Corvette and belong to a club and the members there swear by them. I often wonder if they are the 'Sugar Pill' as they all speak of such and such increased performance. Me, I put a K&N filter in mine and it doesn't do much other than giving me the sense the filter is working much much better.

Did your 99 Suburban come from the factory with the "Tow Package" ? Mine look exactly as it douse in the 1990 sale brochure, so I'm wondering it was installed back at the dealership as the wiring loom & bumper mounted pig tale outlet works perfect and the truck is a PA Class II towing hitch, so I can pull 8.900 Lbs. When I redid the restoration we had to remove the weight class sticker and sent me a new one. It has to be right of the state inspection stickers, but not too close so the inspection stickers have to be changed every year and the weight class sticker stays on forever. In the pics below you can see the two inch square receiver where the ball and hitch ( I bought a 2 inch hitch with a 2inch drop as the was the size of the frame mounted hitch.) and the sealed wiring outlet works perfect, lights and electric brakes , Etc. Thank you for your kind assetment of my 22 year old pickup. My grand father always said "If it's worth keeping, it's worth taking care of!"

As for the hitch my Camaro had what I assume to be a dealer installed one on it. Digging through the spare tire well one day I found the installation instructions for it that had a date of 3/88 and the car was made in 7/88.

As for the posi I'd stay away from the gov lock units and clutch type posi's. Go with a Auburn or Detroit locker as their a lot stronger and don't wear out. If your truck has a 8.5" rear I know some of the 8.5's came with the Auburn unit, I used to have one sitting in the garage till I sold it to a guy with a turbo Buick. Also don't buy a "rebuilt" rear from someone unless you know they were capable of doing the work. Pinion depth and back lash have to be properly set and takes special tools/know how.

Hay thanks dude85,
I was planing on having a real good and long toothed transmission shop, (They just rebuilt my 700/R4 in my truck. The front clutch pack died, but what the hell, it had 109K on it.) But I talked with the owner of the shop/company. They have been in business since the sixties and he told me Yes, he could swap out the standard 3:42 (I think it is a 12 bolt rear) I'm a disabled veteran and it's not a possiblty for me to do on my own. So I'm glad to have someone I trust to do it. But you've brought up a very good question, Where to find one.

Rears are a lot like Trucks and Corvettes. It's damn near impossible to find one thats not beat-up or damaged in some way. buy I am going to look for one locally. They seem to cost somewhere between $ 400.00 and $600.00 Rebuilt. And I doubt about any new ones around that I could afford. You don'r have a lot of cash hanging around living on a SS & veterans persion account. But I'll look and I'm sure I'll find one somewhere. And NO, I'd never buy one from 'Greigslist or Ebay' unless they come from a brick and morter business and come with a Good warranty. Thank you for your help. My truck is finally coming together nicely.

Buy the spacer. Use the spacer. If it makes you feel happy, then the money isn't wasted. If you want something that makes a real difference there are other options. I paid $15 for an intake, spent a couple of hours using a cutoff wheel, carbide burrs, and a die grinder, then paid a guy with a TIG welder $35 for welding.

Attachment 3380

Now lessee... what does that new spacer cost??


Throttle body Corvettes? Not unless they're 82 or 84 models. The Vette engines have a different intake, and the spacer solves a different problem on those engines. TBI truck and car intakes have a tiny plenum and intake air is heated both by the heat riser and the coolant circulated from rear of manifold to front. Remove the heat and open the plenum halves up so they both breathe together and the intake works better.

Does your service manual say the towing capacity is 8900 lbs? Mine is 5000 lbs with 3.42 gears. If I had 3.73 gears it would be 6000 lbs. Truck options include:
Z82 TRAILERING PACKAGE(SPECIAL)(REESE TYPE HITCH) PROVISION FOR HD TRAILER EQUIP(Z82)
VR4 TRAILER HITCH(WEIGHT DISTRIBUTING PLATFORM) EQLZR HITCH(VR4)
GU6 AXLE,REAR,3.42 RATIO 3.42R(GU6)
G80 AXLE,REAR,POSITRACTION,LIMITED SLIP POSITRACTION L/SLIP R/AXL(G80)
KC4 COOLING SYSTEM,ENGINE OIL ENG OIL CLG(KC4)
KG9 GENERATOR,140 AMPERE 140 AMP(KG9)
KNP COOLING SYSTEM,TRANSMISSION,HEAVY DUTY HVY DUTY TRANS CLG SYS(KNP)
C6A GVW RATING(7300 LBS) 7300 LB GVWR(C6A)

Z82 is typically considered the "towing package" option. KNP and KC4 can be found without the towing package but imo if you don't have them and you plan to tow, get something equivalent. Even with heavy duty trans cooling on mine I've purchased a massive trans cooler to install.

Auburn style is probably the least expensive option that's worth the price. I've repaired or replaced many of the governor style units over the years.

That's what I do with stock intakes or conversions, here's one for an 345 IH engine, intake opened up for the 5.7L TBI and adpater made the same.
http://www.gearhead-efi.com/Fuel-Inj...1&d=1328417373



109k on a rear axle and your on a budget? Just do an inspection, it probably has another 109k left.

Okay guys, you got my interest here. Just a couple of quick questions before I go get an intake at the JY. 1project, what are you welding up? I need the EGR to work for calif. emissions testing. Does it have an effect on how the truck runs in cold weather, because it does get somewhat cold here. (single digits aren't uncommon in the winter)

The EGR delivery ports are in the throttle bores. Notice the small, rectangle opening just below the throttle body mounting surface on the rear half of the manifold?
Attachment 3381

When you open the intake manifold you'll probably bridge this port area and open it up to both bores creating a disturbance to the incoming fuel / air mix. Between the EGR port and the square opening in the divider is a coolant passage which will also be breached when you open the manifold. The coolant passage will need to be sealed and welding seems the most reliable route. A guy with a little time and a TIG could fill any opening created between the EGR passages and use porting / grinding tools to smooth it to match the new shape of the throttle bores. For simplicity, we chose to fill it all.

Substantially changing stock manifold configuration almost always affects how an engine runs. That's usually the point. ;) By blocking coolant flow under the TB you're reducing intake heat which in turn reduces vaporization and tends to aggravate puddling and fuel / air separation. In the case of the manifold above the vehicle owner spent time tuning it so it could be started and run with minimal warmup time. The engine was a 305 configured with "cop car" injectors, 1.6 rockers, and full LT1 exhaust. The car would pass Massachusetts emissions testing which, at the time, was modeled after CA's program. It would also get 20+ MPG in a Caprice. Single digits may not be the norm in that area of MA but it's definitely possible to see those temps.

FWIW the manifold in the picture above has larger than stock throttle bores and might be a very good starting point if you're going to open the plenum divider.
http://www.ebay.com/itm/Chevy-350-38...39cd0b&vxp=mtr

Here's a neat manifold I've never seen before.
http://www.ebay.com/itm/HOLLEY-PRO-J...a200ca&vxp=mtr