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1991 burb expectations

V

vdubnut

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south jordan, ut
hey all,

been a member for a while but just upgraded my membership....

A few Questions for you all....

1. I have a 91 burb that i bought about 3 yrs ago for our family camping trips and various projects around the house. it was a 1500 and i bought another salvage burb and took the axles, driveshaft, brakes, leaf springs, etc., off and swapped them onto mine. What im wondering is what to expect performance wise from the burb. I bought a 3000 lb. 20 ft travel camper that i have been pulling with it....but i feel like i have to push it really hard just to travel at a reasonble pace on the freeway.. if i hit any headwind or any slight grade, my speed goes way down and im pushing it even harder. Am i expecting too much from the old 350tbi or do i have a worn out motor. the truck has a 350 700r4 trans, with a crane rv cam, jacobs headers, no cat, msd cap rotor and wires....4:10 gears, stock 3/4 ton springs, 33x12.5x16.5 tires...engine work done by PO.

2. torque converter lock...is it typical for the torque converter to unlock with the slightest increase in the throttle? it seems like it should stay locked until the demand is greater. in fact my old 84 burb would downshift into 3rd and stay locked, only come unlocked when it was almost floored....

I really love the truck...we've always had one in the family since i was a baby so they're in my blood....i love the look of the 91 and it runs really well, but towing my trailer gets stressful since i have to push it so hard all the time...i can maintain 55 in 3rd gear, but 4th at 65 is tough to keep.

thoughts?

thanks
jamie.
 
IMO its lack of power...but thats why im asking the question....what should i expect? i got 6.5 mpg from nephi to cedar city, ut this past weekend and i wasnt trying to go 80, just trying to maintain around 65.
 
700R4's arent the best for towing to start with. You should have enough gearing to compensate for the bigger tires.

Just for comparision, I've got two Burbs that see towing duty. The 88 is 2wd and a mildly upgraded 350tbi, 10b with 3.73's and stock diameter, but wider tires. I'm running a Ranger OD and 465 trans. Had a 32' TT that weighed 6k lbs loaded. Never had an issue towing even at 65 with headwind, averaged 8mpg.

The 90 is a 4wd, completely stock engine and dual exhaust, 10b axles with 3.73's and 33" tires. Same as the 88, it has a 465 trans, but no OD. Pulling the same trailer, I can run down the road at 55 with no issues (has power to go faster, but the driveshaft vibrates too much), averaging roughly 6-8mpg.

So in short, you shouldnt be having issues towing a 20' TT with your setup, but I suspect the engine might have an issue with timing, thus robbing some of your power. The TC will kick in/out but TCI makes a conversion kit to manually lock the TC so it doesnt jump around.
 
so, on your 88 with "mild" upgrades and the 465 w/OD....what kind of final drive did you have? if i understand correctly, i should have a .070 4th (OD) .... i figured that with the 4:10 gears, that the tire size shouldnt be causing a problem as the tire diameter isnt too far off from stock on a 2500. 8mpg would be fine with me if i felt good cruisin at 65... what kind of upgrades does that motor have?

I will look into the timing and see if its off. I set it at zero degrees and let the comp do the rest correct?

on the TC...is there any way to adjust the setting of when it locks/unlocks? just seems to unlock to easily....is this common or do you think there is a malfunciton somewhere? I'd like to keep it automatic if possible.
 
Well I also own a 91 suburban however it's the 2500 4wd and I gotta say you need gears. Knowing the vehicle you have and the pain of towing anything with weight in these heavy things stepping upto 4.56 gears and you'll be in tow heaven. The whole idea is that with 4.10's you are back at roughly stock ratio's for stock tires and axle ranges. That's not good enough for these big bus things.
*removed stuff cause you have headers*

Stepping upto 4.56 gears you will have plenty of towing power without the "seeking" trans. I went ahead and also did a tbi spacer kit (30 bucks on ebay I think) which may or may not have helped but it made me feel better. Another thing I noticed quite a difference with while pulling was switching to electric fans. The big argument on here is that the alt is taking the same power as the clutch fan however this doesn't apply the same while towing. When towing up a hill it needs to kick on more often (clutch) causing you to lose power further. Where as the electric fans simply stay on low typically with me until I am nearly cresting the hill (huge power improvement.

So in short. Start with the fans (If you want) then onto gears. Once you do the gears you will have all the towing you could hope for out of a stock 350. Hope this helps.

Josh
 
The OD is 28%, so 0.72. I dont know if the TC can be adjusted, just that there are kits that allow you to manually control it.

As for the mods to the engine; the long block is a new "K" code (350tbi) meant to directly replace the stock engine. From there I built up the top end with an Edelbrock Performer intake, Holley 670cfm throttle body and 80lb/h injectors. I found out later the injectors are too big for a 350 and would actually benefit going a tad smaller. I'm running Heddman shorty headers with 2.5" true dual exhaust. The dizzy is a stock replacement, but I upgraded the coil to a MSD (or Mallory, cant remember) 52k volt versus the 45k volt stock. I cant remember what brand the plug wires are, but they are larger than stock (8mm vs 6mm). The plugs are Bosch Platinum +2's, but I'm thinking of jumping up to the +4's to compliment the added coil voltage. Replaced the V-belt with a serpentine and installed electric fans. Offset that by dual alternators though (I have alot of kid entertaining stuff in this truck).

I've never put this truck on the dyno, so I cant tell you how much improvement over stock all my changes made. I can tell you it isnt a high horsepower race engine. I was just trying to point out that a SBC should be more than enough to tow what you are towing. My 90 is running over 200k on the engine, and just recently I've noticed it dogging a little on tough pulls.
 
thanks mtn man...

i do have headers on the engine already...and no cat, so i have a pretty low restriction exhaust. i've thought about doing gears but that can get pricey...

im going to check the timing and maybe do a compression test to see where things are....the engine runs smoothly and doesnt smoke, but also doesnt seem to have much power...

interesting idea with "timing" the fans so you're not losing power until the top of the hill...

thanks!
 
i agree cabledawg....im not trying to set any speed records but do feel like something is lacking. sounds like some good upgrades in your truck. i'll do some checking (timing, compression) and see what i can find. my truck "only" has 120k...should be just broken in right!
 
It was alot of money, but the old engine had alot of bad stuff and some stuff got broken during the swap (oops!) so I had to replace it anyways, why not upgrade at the same time?
 
Really it's just that the fans don't kick on until the end of a long pull not that anything is being done. Even when the fans kick onto high their is still quite a bit more power then stock clutch fan kicking in. I'm sure it's a difference and I notice the power change but it's much much better than clutch especially on hills where the clutch fan typically kicks in anyway.

After driving this thing for years an pulling various trailers and quads and such I gotta say the only thing left for me is to bump up the gearing. Nothing else will quite give a 350tbi in a heavy ass suburban the power it needs. Unless of course you step up to a 383 but that's another story. Do the gears and you'll be very pleased.
 
one further question

one other thing i have been wondering about....

I was talking to a guy at a local chevy parts counter about this and he said that one of the first things to do was to "upgrade" the computer with one from the dealer that had all the programming updates. Theoretically would make a big difference in how the truck runs, idles, etc.

Is there anything to this or was he just trying to sell me another computer? Currently it appears that i have some reconditioned or repaired ECU from NAPA, and although i feel the truck is underpowered, it starts right up and runs fairly well.

The guy was asking about the specific ECU model numbers so he could get the "latest" programmed one for my burb.

Thoughts?

thanks
jamie.
 
I doubt it. I have no idea how you'd even know if there were updates to the programming that came out later. As I understand it, pretty common nowadays for OBD2 vehicles to get "flashed" by the dealer to get something in the programming updated, but on older stuff, I've never heard of any sort of service bulletin recommending vehicles come in to get a new chip.

ECU isn't likely to be different, the only thing that would have changed would be the programming, although sometimes the replacements have some upgraded "hard parts" that might have been prone to failure. I remember seeing someone talk about returned ECM's who worked with them, saying something like 99% of them weren't bad. Should tell you something about the likelihood they are a problem.

Converter lock/unlock is controlled via the ECM, so pretty unlikely it's not working right. Remember, the converter clutch is NOT designed to hold up to any power, as it's a single clutch plate, unlike every gear which is typically 3+ clutches. This is similar to OD, in "early" GM transmissions OD is a "weak" gear, partially due to the fact that the surface area of the clutches was pretty small.

The logic to TCC operation is probably that the more load on the motor (towing, uphill, higher speeds, etc) the less that TCC should be engaged. Forcing it to hold up to more load invariably leads to failure...check out the Grand National guys that have tested the 200-4R's with/without TCC in the 1/4 mile. Not exactly the same, but the reason is...too much power to a piece not made for it.
 
Yeah, but towing a distance without the lockup on would cause a lot of heat in the tranny, thus dramatically shortening the lifespan.
 
GM had to figure all of that out before they put it together, if keeping it locked up worked, they would have done it from the factory.

I know that running without lockup generates heat, but towing in general creates heat. For all I know there is a temp switch in the valve body that feeds the ECM or something internal to the trans to try and keep it cool.

I suspect it's a mixture of the two. Long life and better economy when unloaded (which is where MOST light trucks spend their lives) while sacrificing some longevity when loaded.

Later on weren't trans coolers or additional radiator fans on the trans cooling side standard based on GVW?
 
I had coolers on both (they were originally 700's, but swapped to 465's) and a temp gauge on the 88. The thing would heat real quick with light towing and city traffic driving, but onthe open road it did OK.

From my research onthe 700's, the stock cooler should be upgraded (i.e. a bigger cooler) and either electric fans to get more air through or a better fan/shroud in using the mech fan setup. I never got far enough to actually use that advice, so you might want to do some reading of your own.
 
I agree with all your points....I do have a trans cooler in front of the a/c condenser...it measures about 7" square, maybe a little bigger. I also have an engine oil cooler up there...I don't believe either are stock...

I think the computer is sensing throttle position and thinking heavy load/power and unlocking the TC

So if I had additional power at lighter throttle it wouldn't disengage so easily/early
I also have worried about trans temp...vs the locked up load...

All part of a tangled web I suppose....
 
You'd really need to hook up a trans temp gauge to see whats going on.

Contrary to popular belief, the 700 doesn't HAVE to run lockup to survive, you just need the cooling beefed up to handle the additional heat. But you'd have no way of knowing how effective cooling is without a trans temp gauge.

I've got an idea kicking around in my head to hook up an engine temp gauge to the oil pan, no reason you couldn't do the same for the trans, IF you could find the right sender.
 
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