Suburban k20 motor
 

I have a 1985 k20 suburban. It has a small block 350. I had put 2 motors into it since I've owned it both of the motors had catastrophic failure. It has a th400 transmission with 4.10 gears and 305/70/16 tires approximately 33x12.5. The mechanic told me not to go on the freeway. That the rpm's could have been what blew the motors. I put a tach in it and at 45mph the motor is turning 2000 rpms. He suggested that I put in a newer 4 speed transmission in it. He was telling me to put a 700r4 in it. I did some research and the 4L80E seems like a better transmission. And the transfer case would have the same 32 splines.

My question is what range of rpm's is safe. I know if I can keep It low that is better. Is 2500 rpms too high if I want to go on a 2 hour road trip? 50mph 2200+ rpms I feel like that is ok. Right now there is about 1000 miles on the motor. Was told to take it easy for 2000 miles and no pulling my trailer.

Re: Suburban k20 motor
 

All depends on how heavy your foot is and how fast you need to go. If your doing 70 with 4.10’s and a three speed you’re probably well over 3k rpm.

Ideally you want to be cruising around 1800-2200 rpm. 2500-2800 is probably fine too.

Re: Suburban k20 motor
 

4l80e is a heavier duty trans but it needs a computer controller + some electronic sensors. IIRC it is also enough longer that you will need two new drive shafts.

700R4 does not need the computer and can be bought with improvements that negate the original weaknesses. Don't remember on the length.

My C10 with 350/TH350 + 3.42 gears = about 2800 rpm @ 70. Probably a bit more due to slippage. Current motor has over 100K miles on it.

Re: Suburban k20 motor
 

Your only at 3000 rom at 70 in the truck. Which is not bad, and I have had engines last over 100,000 with no issues spinning north of 2500 all the time. A couple I was able to keep track of pasted to nearly 300,000.

My guess is that the K20 Burb is very heavy, and can really take a toll on motors in general. I have seen crate engine wore out at 100,000 because of the weight. Plus you have more drag with the tires.

What kind of crate or rebuild engines were these?

Re: Suburban k20 motor
 

A lot of engines have catastrophic failures just by being low on oil. If the oil isn't draining back into the pan fast enough (engine sludge) at higher rpm's it could be the same effect. I don't like running my engines at 3k rpm for extended periods. My truck rolls around 70 at 2500 rpm or so.

Re: Suburban k20 motor
 

I'd find another mechanic. He has head where the sun don't shine. The stock 350 should spin up to 5k intermittently with no issues. 33 inch tire with 4.10s is fine unless your driving 90 everywhere. Last thing you want to do is put a 700 in that heavy suburban. 4l80 is perfect.

Re: Suburban k20 motor
 

I've driven mine at 3000-3200 rpm for up to 10 hours straight many times. No problems yet (other then with my ears from being loud) I've put about 60k miles on my 350. Overdrive would be great but not absolutely necessary.

K20 Carbed Vortec 350, Sm465, np205, 14bff 4.10, 31" tires