New to me 87 k5, a few questions

[Philbrockett87]

New member here, had a few questions for anyone who might know, my 1987 blazer has had the stock 700r4 replaced with a th350. Would going back to the 4 speed net me some mpg? I'm currently getting about 10. Truck has a fairly stock 350, headers aluminum intake and a 600cfm edlbrock 4 barrel. 3.08 gears and 33 bf Goodrich all terrains. I'd ideally like to get around 15 highway. but I realize this thing is shaped like a brick and isn't exactly designed for mileage. Any help would be appreciated! Thanks.

 

[Philbrockett87]

I've also installed a hei dizzy and set timing 10* advance at idle and 32* at 2000 rpm before the vacuum kicks in.

 

[diesel4me]

Personally I think 15 mpg isn't possible no matter what transmission you had,with a carbed small block of "old school" tech..12-13 mph is about the best I ever got with similar trucks like yours in excellent tune and condition..even with a 2 barrel carbed 307 I had in one...

The overdrive might add 2-3 MPG ,but with 3:08's it'll be lugging a lot and might even drop your MPG...
I think the TH350 is a stronger transmission than a stock 700R4 too,for what it'll cost you to build a 700R4 that'll hold up and get in installed,it'll take a long time to break even ,if you gain any MPG at all...you are probably better just leaving it as-is...

 

[Philbrockett87]

Thanks for the reply, that's kinda what I was figuring. I spied one at the local junkyard, for some time and 200 bucks, I could have nabbed it. I'll just wait till this trans dies and maybe I'll upgrade. I was worried about the gear ratio as well. O/d and 3.73 gears would probably be a better match. I just don't like turning 2200 rpm at 60.

 

[diesel4me]

Actually 2200 isn't that bad for a relatively stock engine,thats about where the peak torque and HP is...bigger tires would slow it down a bit...

I had a '72 K5 with a TH350,stock 350 except for a mild crane cam,3:08 gears,and when I first got it,the former owner had installed a small rochester 2 bbl and a 283 intake manifold on it..
It got about 13 mph on highway runs,and 11 or so around town..

I lived 65 miles from my job,so I decided to hopefully "improve" my MPG by putting a 4 barrel & Q-jet on it...it didn't gain 1 mpg,and if anything,it killed some of the low end torque it had,I used to be able to lug it around a corner in 3rd gear and step on the gas with the 2 bbl,and it would pull strong,but with the Q-jet,it wanted to lurch and ping unless I downshifted..

I tried swapping a Holley aluminum intake (dual plane) with a Carter AFB carb,and it ran somewhat stronger,but didn't get any better MPG,and still didn't seem as peppy as it was with the 2bbl around town ...

I decided to convert it to an SM465 after a year or so,when the TH350 started delaying going into forward gears in cold weather,or in the morning after sitting overnight...

I hoped for a MPG increase,but if anything,I think it lost a few,it was too high geared,I had to use 3rd gear up to about 45 mph,or it'd just lay an egg going uphill...to pass on the highway,I could pop it in 3rd and wind it up to 75 mph before the secondaries kicked in...4th was like overdrive...I put stock sized 235/75-15's back on it instead of the 12 x 16.5's I had on it,that seemed to help a lot,but the mpg didn't change enough to make any noticeable difference..

I had a 1974 C-10 long bed with a 454 and TH400,3:08 gears,that got 15 mpg with a Holley "Economaster" 450 cfm 4 bbl carb on it though...sometimes the bigger engines "loaf" along fine with highway gears and the torque makes up for the high gearing...I used it to commute to work daily at 65-70 mph for 6 months...was a lot cheaper on gas than my '69 GTO's 400 /TH400 was...that thing got like 6-10 mph,if you babied it!..but it had 4:10's too..