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Thoughts on engines, transmissions, and gear ratios.

E

esquire

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I'm the proud new owner of a 1987 K5 that has the 305 and TH350 combo. Axle ratio is 3.08 I believe. Obviously it's a bit of a dog when climbing hills and trying to maintain speed.

Purpose of the truck is to daily drive when the urge strikes, use it for a hunting truck, and just general cruising. I want better than average off-road capability, but I won't be doing Moab in it any time soon. Thinking I'll go with 33" tires, MAYBE 35", but that's a big maybe.

That being said, I'm looking to do an engine swap, and have considered the LS route, but I'm not entirely sold on it. I have an angle on an 80s-vintage 454 and TH400 sitting in a motorhome that I think I can get my hands on pretty reasonably. If I decide to go this route, should I just keep the 3.08 gears or should I try to source an automatic that has an overdrive and re-gear the axles or swap in new ones? I'm a bit concerned that a 454 turning the RPMS at 70 mph with no overdrive would be really, really hard on gas.

What thoughts do you guys have?
 
esquire said:
I'm the proud new owner of a 1987 K5 that has the 305 and TH350 combo. Axle ratio is 3.08 I believe. Obviously it's a bit of a dog when climbing hills and trying to maintain speed.

Purpose of the truck is to daily drive when the urge strikes, use it for a hunting truck, and just general cruising. I want better than average off-road capability, but I won't be doing Moab in it any time soon. Thinking I'll go with 33" tires, MAYBE 35", but that's a big maybe.

That being said, I'm looking to do an engine swap, and have considered the LS route, but I'm not entirely sold on it. I have an angle on an 80s-vintage 454 and TH400 sitting in a motorhome that I think I can get my hands on pretty reasonably. If I decide to go this route, should I just keep the 3.08 gears or should I try to source an automatic that has an overdrive and re-gear the axles or swap in new ones? I'm a bit concerned that a 454 turning the RPMS at 70 mph with no overdrive would be really, really hard on gas.

What thoughts do you guys have?
Click to expand...
BBC = hard on gas
 
Personally, I find that 33's with a 2" lift makes it really easy to hop in the truck and go and still has decent capabilities for a variety of places. But I don't play in mud, so that could be a factor for you. I had 33's on my '70 K10 for years, with 4" of lift, 454, 700R4, NP205 with 4:56 gears. I took it up in the mountains and even to Moab once. Daily drove it a good amount, too. It felt more spirited on the street than it does with the 35's now, and my old knee noticed the difference getting into it. Still fun to drive, but it took a little bit of the pep away. I could get 11 mpg around town with the smaller tires, it probably lost a little by stepping up.

I had 33's on my '90 Jimmy for a while. (Tired TBI 350) I took it to Moab once like that, and enjoyed it more when I swapped the 3:73 gears for 4:10s while driving the highway and offroad. That was with a 14B semi-floating 6 lug rear axle. Nice axle IMO.
Then I put it on one ton axles, 4:56 gears with 37" tires, with a good 350, 700R4 and doubler. It goes places that I am amazed by, however I rarely drive in into town since I have other trucks which a more daily friendly. It goes down the highway fine, but I feel that it could use a slightly lower ratio on the interstate as it pulls easier at 82 mph than at 75 mph. It gets the rpm up to where the 350 is happier. Fuel economy isn't a consideration on that truck, but I don't feel like it's decent.

I would get rid of the 10 bolt rear axle. Unless you dump a ton into it to go 30 spline axles and aftermarket gears and carrier. Most guys would consider that crazy, but a 6 lug 14BSF isn't a bolt-in job, and 3/4 ton axle swap means wheels and tires need swapped and the rear differential hangs low with 33 inch tires.
 
I think for a daily driver, an LS makes all the sense in the world. I mean you already have TBI, if you like that, then LS will be the same (turn the key and drive, every day, any condition or angle) except more power and even more refined. I doubt you'll find many that regret an LS swap.

Are you sure you don't already have the 700R4? Pretty sure a US K5 couldn't get a 350 that vintage.

No question 3.08s have to go. But you'll have to think about what you expect...trans and tire size really matter when selecting gearing. I think personally with an auto overdrive and 33's, 4.10s make sense. I run 33s and 3.42s with no OD (2400rpm @ 70), same but with OD and 4.10s would turn 2000RPM, which would be great cruising, and offer pretty respectable low speed RPM. Honestly, 4.56 with OD might be an option to consider even with that "small" of a tire.
 
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Yes, positive its a TH350. Pan shape and driving it give it away. There's no overdrive.

From what I've been able to find/figure, GM had some surplus TH350s and a handful of Blazers got them behind the 305 in that year. As a matter of fact, it has been hell finding parts for them at the parts houses because many of their computers don't show the TH350 as an option for that year. I don't think the 305/350 combo was swapped into it by a prior owner, judging by the shoddiness of other repairs that have been made under the hood. I don't think anybody who owned it before me had any serious mechanical ability or the drive to repair things the right way. For instance, the truck has true dual exhaust, but both exhaust pipes were connected to the manifolds with baling wire. I kid you not. I don't even understand how they did it. For those reasons, I really doubt there's been an engine swap.
 
You could not get a TH350 in a 1987 Blazer, period. It has been swapped in. Very common. Have any pictures of the transfer case adapter?

I would go with the 454 if it is not a daily driver.

Martin
 
How far is your daily drive? If it isn't that far, then the 454 doesn't hit the pocket book as bad and a big block gets about the same mileage loaded, unloaded, uphill downhill, etc. My cousin has a 454 in his service truck and he gets 8 to 10 but he also doesn't have a lack of power either. The 454 TH400 will be dirt simple while the LS will have a computer and everything that goes with it. I think your weak link is that 10 bolt. Me personally, get the 454, go with 3.73 gears and 35's to help with mileage and get a 12 bolt. And just eat the mileage. I have a 400 putting out 415 to 425hp and am running 3.73 gears and 35's. I am heavy, but I can get 12mpg running the fire roads on the west side.

FWIW
Joel
norcal
 
82355 said:
You could not get a TH350 in a 1987 Blazer, period. It has been swapped in. Very common. Have any pictures of the transfer case adapter?

I would go with the 454 if it is not a daily driver.

Martin
Click to expand...

Well, I'll have to give it to you. After I read your reply, I ran the RPO codes found on the sticker in the glove box and sure enough, it came with a 700R4 from the factory. MD8 RPO code.

I had read some conflicting sources online about whether the TH350 was used in these trucks and I had come to the conclusion that GM had some surplus TH350s that they stuck in the bare bones trim package trucks (this one is a Custom Deluxe, no power anything). It makes sense now why we've had so much issue at parts stores.

Thanks for setting me straight!!!
 
82355 said:
A truck 12 bolt is not an upgrade over a truck 10 bolt.

Martin
Click to expand...
How do you figure? Not quite sure I buy the fact that the pinion diameter is the same on both axles, or the axle diameter, or the ring gear size. Might not be much of a difference, but there is a difference. You go slapping the torque of a 454 into the package and I guarantee there's going to be a difference.
 
Joel Wilson said:
How do you figure? Not quite sure I buy the fact that the pinion diameter is the same on both axles, or the axle diameter, or the ring gear size. Might not be much of a difference, but there is a difference. You go slapping the torque of a 454 into the package and I guarantee there's going to be a difference.
Click to expand...
I say this as someone who has a big block K10 with a 12B rear axle. I don't remember the specifics of each axle as far as pinion and bearing dimensions, but I don't honestly see the 12B being a large enough improvement over the 10B to consider it an upgrade. The ring gear may be larger, however I put them on a similar level. I have seen plenty of worn or completely used up examples of each axle. From destroyed pinion bearings in a 12, to a pinion missing teeth in a 10. Junk differentials in both versions. Both versions will kill wheel bearings and axles bearing surfaces.
Have I killed mine yet? No, but it doesn't have a single OEM part inside it anymore and I know that it could still die.
If I remember correctly, isn't the outer pinion bearing smaller in a 12B? Just slightly, but I could be wrong.
I believe that one reason the 12B lasted longer for some is because they typically didn't have to put up with the 3:06 first gear of a 700R4 and possibly the 2.62 low range of a 208. Just a thought.
 
The 8.5-inch will withstand some abuse; it shares the same pinion shaft diameter as the more expensive 12-bolts. Ring is only 1/4" smaller diameter. Late model 10 bolts have the 30 spline axle, that the 12 bolt had.
 
3.08 gears are fine if you have the sm465 or nv4500. Both have lower first gears to make up for the lack of axle gearing.

With an auto and a SBC 305/350. 3.73 axle is the minimum. 3.73 will be fine with 30-33" tires for general purpose/Daily use. Allow for good MPG, easy hwy cruising, and enough to get up hills.

Go with 4.10 or 4.56 if you need more off-road/hill climbing ability or decide on 35"+ tires


As for engine choices. If you want simple, cheap and reliable, a TBI 350 will last forever. If you already have TBI it will be straight forward upgrade. If your state allows, a crate 350 with aftermarket EFI is also a very simple swap. This will get you plenty of HP/Torque without going BBC route
 
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