81 k5 engine swap?

[good ol boy]

I own a 1981 k5 blazer with a 305 and 350 turbo tran. the tran. needs to be replaced and i would like to replace the 305 as it is very underpowered.
i would like to put a 350 and a 400 turbo. will be driving offroad very often as i have a 3 mile long 4x4 driveway. would like to be able to pull small trailers for work and still have fun offroad.
my question is what kind of gas millage can get with the 350 and 400 and what can i do to make it as econmical as possible.
I have 31's on it right now and have gotten as low as 2mpg.

 

[dyeager535]

Just as an observation of my own, I doubt the 305 is as underpowered as you think it is. It's no powerhouse, but the 305 trucks (at least early ones like yours) almost universally got HORRIBLE gears, normally 3.08. They work fine when you hold it in each gear and stand on the pedal, but some 3.42's or 3.73's will make a very noticeable difference. Those setups were designed solely for economy, not off-road performance, or even freeway on-ramp performance lol.

I, personally, wouldn't even consider an engine swap UNTIL I had swapped out the axles for some 3.42's or 3.73's. Then if still too underpowered, consider swapping it out. Heck, a friend and I put a 305 from a Monte SS in his '83 C30 about 10 years back, and he has trailered some significant loads with it. Motor is still in that truck, and hasn't had a lick of problems.

Yes, a 454 would tow better, but the free motor, coupled with a TH400 and 4.10's or 4.56's (not sure what it has for gears), allows that motor to keep the truck moving. As poor as the 305's are for power, they seem to hold up pretty well.

 

[good ol boy]

the k5 has 4.10 in it now the engine has 139000 miles on it but mostly offroad on deer lease by previous owner

 

[dyeager535]

I'm surprised you feel it's underpowered with those gears! But you have them, so that's whats important.

A 350 will give you some decent power over the 305, so you are probably on the right track.

If you've got the money, I'd probably consider a GOOD 700R4 over the TH400. The TH400 is going to cost you some power, which is only going to further hurt economy. With those gears and those tires, I think a 700R4 would hold up well for your usage. That lower first gear of the 700 will also help. Might even try doing that first, then going with the motor if still needed.

Got headers? I wouldn't put anything onto/into that 305 that couldn't be used on a 350 though.

 

[82355]

Why do you want to replace the TH350 with a TH400? That will not improve economy.

Martin

 

[good ol boy]

I was told that the 350 and the 400 were the same a exept the 400 had overdrive.
with what you guys are saying maybe my 305 is having mechanical probs. i have to engage 4x4 low to get up a big hill especialy with pulling a trailer. the truck has a custom dual exaust with two flowmasters.

 

[dyeager535]

No, 400 is a (the) heavy duty three speed. Good trans, but no OD and it uses more power to turn than the TH350. The 700R4 is essentially a TH350 with OD. It's more complex than that, but close enough in this case. You want an '87+ one, but has to have a TV cable. GM changed the name to 4L60 later on, then went to the 4L60E. Can't use the 4L60E since it requires a computer to operate correctly. There are always exceptions, but unless looking to spend a ton of money, that's enough info.

A big hill with a heavy trailer, I can see a 305 struggling. But depends HOW heavy, and how steep the hill is.

 

[good ol boy]

single axle 18' trailer will be pulling from a 600 lbs mower to up to 4000 lbs of mulch. hill well not a mountain i live in a very hilly area at most 70-80% grade

 

[PhredzK512]

5.0 to 5.7

I own a 1981 k5 blazer with a 305 and 350 turbo tran. the tran. needs to be replaced and i would like to replace the 305 as it is very underpowered.
i would like to put a 350 and a 400 turbo. will be driving offroad very often as i have a 3 mile long 4x4 driveway. would like to be able to pull small trailers for work and still have fun offroad.
my question is what kind of gas millage can get with the 350 and 400 and what can i do to make it as econmical as possible.
I have 31's on it right now and have gotten as low as 2mpg.

In my '87 K5 Custom Deluxe, I've decided to have my tired 305 taken out and have a strong running 350 put in,just decided that today. So how has the swap been for you? My tranny is the stock SM465. I am wondering how my gears are going to hold out once the 350 is in. I have a 350 in my other '87 Silverado K5 and it has the 700R4 tranny and I like the power.

 

[snowdriver]

single axle 18' trailer will be pulling from a 600 lbs mower to up to 4000 lbs of mulch. hill well not a mountain i live in a very hilly area at most 70-80% grade

70-80% grade really??? am i reading thisright,you have a rise of 70-80 feet for every 1oo feet traveled,i am trying to figure out degree of incline???

 

[snowdriver]

wow,did i evere get off topic

 

[82355]

In my '87 K5 Custom Deluxe, I've decided to have my tired 305 taken out and have a strong running 350 put in,just decided that today. So how has the swap been for you? My tranny is the stock SM465. I am wondering how my gears are going to hold out once the 350 is in. I have a 350 in my other '87 Silverado K5 and it has the 700R4 tranny and I like the power.

Are you keeping the TBI? I would. That SM465 will stand up to whatever you put in front of it. It is THE strongest transmission available in these pickups, bar none.

Martin