"New" engine/trans for my truck (250/SM420)

[66Submarine]

I finally swapped the old greasy 283/SM420 out of my truck for my nicer 250/SM420 I had. Still need to work a couple little things out, but so far so good.

I'm expecting much better mileage out of this little six than the tired 283; should be interesting to see. (The 283 normally seemed to get something like 13-14MPG FWIW.)

A few pictures of the swap:

[TJ's Chevy]

It'll be a tad slower on acceleration, but you'll get some better low end. I read all yer other threads on this...quite the temperature adventure! LOL!

[Fastest32]

What did you do as far as the Bell Housing? I have the same 250/sm420 set up and would eventually like to put a v8 in it. I didn't know what all I would have to change.

[66Submarine]

No change is needed; the bellhousing and everything is the same between the two. I just unbolted the old 283/SM420 and slipped the new 250/SM420 in place.

Also, a small driving update. Mileage seems pretty much the same more or less (haven't messed with the carb jetting--did add timing), although more aggressive driving conditions have made me seem to average slightly less than 13MPG. (I need lighter shoes! And maybe some OD gearing going 70MPH...)

I pulled a trailer with it yesterday (16' car trailer + Mercury Grand Marquis) back from Rome, GA and it did pretty good pulling hills in the mountainous terrain; almost no downshifting--normally maintained 50-55MPH up hills. I also didn't have excessive tongue weight this time, so the truck itself was super stable and handled very well. The 292 should be real nice!

[MikeN]

I only ever got around 13-14 MPG with the 230ci and 3-speed in my 64 C10. They're just big heavy trucks with no aerodynamics.

Interesting tidbit...I have a 69 Chevelle I drag race, and tow behind my modern Ford truck on an open trailer. I've also towed my 64 C10 on occasion on the same trailer. The Chevelle and the C10 weigh nearly the same, but mileage on the Ford when towing is a good 2 MPG less when pulling the C10, compared to the Chevelle, because it sits up so much higher and has that near vertical windshield.

[66Submarine]

Interesting. FWIW, my other 250/3 speed half ton got about 18MPG when my father was driving it, and his truck also seems to get that. Not expecting to get the same with the one ton (taller, heavier, more rotating weight, etc), but I'm hoping to get a little better with some future tweaking/mods.

[T2vigil]

You could also try a HEI distributer and a "Soft" vacuum advance. I was getting about 19.3 MPG @ 55 cruising. Those changes made a lot of difference in the driveability.

[66Submarine]

The 250 has one of those HEI things that replace the points. Seems to work pretty well, although I don't find points/HEI to be a night-and-day thing like some do, anyway.

I did add some timing in; what do you mean by "soft" vac advance? All in at a lower vacuum?

I think the truck itself and the driving conditions are part (or all) of the problem. The driveline is all much heavier than a half ton (much more rotating weight), and I'm generally driving fairly fast, or driving around town. It's not really "in-town", but it's enough that you encounter a fair number of turns/stoplights. I'm not going easy on it, anyway. It's not much (or any) worse than people I know with late model 1/2 tons under similar conditions, so I probably shouldn't complain too much. Still working on it, though--I think there are improvements to be made.

[TJ's Chevy]

I believe softer advance springs make it snappier and advances the spark a little earlier. You must have a pertronix in your 250? What made a day an night difference on my 292 was putting a 60K coil on! LOL!

[66Submarine]

Yeah, but he said vac advance. The 250 does have one of those Pertronix things in it; the guy before me put it in there. I also have an actual GM HEI dist that came with my 292; maybe I'll throw that in there for kicks.

I really need to check the timing and see what I have all-in and where that is; looking at the spec in my book I can probably add in a good bit if I don't get pinging when lugging it. I might mess with that some today.

[66Submarine]

Huh; I'd have thought "soft" would mean all in at a lower vacuum. Odd.

[T2vigil]

The soft vacuum advance allows for a delay in advance. This helps the motor gain rpm before maximum vacuum is reached. Using this will help with better village and the stumble you sometimes get at full throttle acceleration.

[TJ's Chevy]

Here ya go 66Submarine! Try revin yer ol 250 as high as I rev my ol 292! LOL!
https://www.youtube.com/watch?v=YoiM0nIA_7c

[66Submarine]

I've had the little 250 up over 4,500RPM, but no video evidence of it (yet). Doesn't seem to really do much over 4,000 or so, anyway (just runs out of breath), and I'd like to see more oil pressure when I'm going totally stupid like that.

I do have more self-incriminating truck abuse evidence from the 283 days, though! (Excuse the poor shifting , etc.) Didn't really wind it up that much, but still more than I probably should have!

0-45 with worn out 283 + SM420! - YouTube

[TJ's Chevy]

Sounds like you went over 4k! LOL! Nice vid.

[66Submarine]

Glad you liked it. I'll try to catch it on film if I try to auto-disassemble the 250 again.