283 heads on 350

[cppursell]

I have a '67 1/2 ton chevy truck with a 70's 350 engine, with #3973487 heads (they match letter "K" on this guide: http://fastnuf.com/Headguide.html). I love it, but I hate the mileage: 11 mpg. My goal is to get better fuel economy. My cousin Jim has experimented with several small block engines and says we can put heads from a 283 engine onto mine. He has successfully done so on a 350 of his, and has been driving fine with no pinging. He reports a compression of 11:1. My cousin also has a 307 in one of his trucks with 283 heads, resulting in a 10:1 compression; according to him, that truck gets a minimum of 15 mpg. (those heads match letter "B" on above guide).

Would installing 283 heads onto my 350 be a good idea? What other tips do you have for me to maximize my fuel economy? Thanks for your advice!

P.S. Other info that might be handy: Turbo 350 Transmission, 3.54 posi rear, engine timing is set at 7.

['72customdeluxe]

having 7 degrees initial timing can't be helping your cause. Bump it up to atleast 12

[cppursell]

up to 12? Okay, I'll give it a try. I had it set there 'cause an old mechanic buddy of mine said to set it between 6 and 8.

[mclairmo]

2 barrel carb, HEI, vacuum advance, taller tires, taller gears (numerically lower), lose weight.

['72customdeluxe]

Originally Posted by cppursell

up to 12? Okay, I'll give it a try. I had it set there 'cause an old mechanic buddy of mine said to set it between 6 and 8.

the gm timing settings were for emissions purposes I believe. That's not anywhere near ideal

[Dingfodgy]

Besides what others have said-with the 3.54 diff, I would seriously consider swapping over to a 700-R4 or 200-4r overdrive transmission. This will drop your highway RPM's about 30%. Converting to EFI would get you some gains, but can be cost prohibitive.

[cppursell]

Okay, some good advice. However I'm not ready to change gears or transmissions, and my tires are newer. I don't have money to change to fuel injection.

What about my head swap idea?

[gkesseru]

I still remember from a college thermodynamics course (40 years ago) that in an ideal engine, efficiency is a direct ratio of compression. This seems to say that MPG would go up with compression ratio increase. It would be interesting to hear real world feedback from you grizzly old guys whether this actually happens.

[C-10 simplex]

i'm eager to hear this as well as i have been thinking of this very concept lately---raising compression to increase mpg.

1) Do you already have the 283 heads? If not, to confuse the issue further consider the L98 corvette heads; Being that they are 58cc's (i think?), this would also help to raise compression in addition to also being aluminum--less weight which is probably the best way to get better mpg. Still sold as new at GMPP i believe.


2) i have shyed away from the higher compression route for a while now mainly due to concerns about durabilty/longevity compared to a lower compression engine.
However, after more thinking which do you think would last longer?

a) a higher compression 350 w/ say 10:1 that was driven "normally," spending most of it's time from 2500rpm and below..

b) a lower compression 350, say a crate 350 which has about 8:1 to 8.5:1 or even an engine w/ lower compression---say 7.5:1 which has the pee run out of it and frequently raced at stoplights/track at upwards of 4500-5000+ rpm frequently?

i say "a" would last longer due to my belief that high rpm is far more stressful to an engine than higher cylinder pressure.

[junkyardjohn]

I'VE BEEN DRIVING & WORKING ON THESE OLD TRUCKS SINCE 1985. I'VE HAD INLINES, SMALL BLOCKS & BIG BLOCKS, 4 SPEEDS, 3 ON THE TREES & AUTO-MATICS. MY GAS MILLIAGE VARIED FROM 8 TO 12 MPG. HIGHER COMPRESSION MOTORS REQUIRE HIGHER OCTAIN FUEL TO RUN. IF YOU GET OVER 10 1/2 - 1 COMPRESSION YOU EITHER HAVE TO MIX YOUR OWN FUEL OR RUN RACING GAS OR AVIATION FUEL. (& I HAVE) IT GETS KINDA EXPENSIVE. NO ONE EVER UNDER ESTIMATES THEIR MILLIAGE OR HORSEPOWER. DON'T FORGET.. YOUR TRYING TO PUSH A BRICK SHAPED, 4000 POUND VEHICAL DOWN THE HI-WAY AT 70 MPH. I'LL PROBABLY GET SLAMMED FOR THIS AS SEVERAL PEOPLE CLAIM TO GET 18 20 OR EVEN 23MPG ON A SOMEWHAT STOCK SETUP. I CALL B.S. & FIGURE THESE PEOPLE WERE JUST NOT VERY GOOD AT MATH. IF YOU ABSOLUTELY NEED GAS MILLIAGE.. BUY A VOLKSWAGEN DIESEL. I HERE THEY GET 50 MPGs.

[71meangreenc10]

Originally Posted by junkyardjohn

SEVERAL PEOPLE CLAIM TO GET 18 20 OR EVEN 23MPG ON A SOMEWHAT STOCK SETUP. I CALL B.S. & FIGURE THESE PEOPLE WERE JUST NOT VERY GOOD AT MATH. IF YOU ABSOLUTELY NEED GAS MILLIAGE.. BUY A VOLKSWAGEN DIESEL. I HERE THEY GET 50 MPGs.

Have to agree with that statement.......


Smitty

['72customdeluxe]

Instead of experimenting and tearing it doWn just play with the timing. For around $180 you can get a wideband o2 sensor kit which will let you know your exact air/fuel ratios and what you need to do to tune.

[Duncan and son]

Originally Posted by 71meangreenc10

Have to agree with that statement.......


Smitty

x3

[klownkar]

the 350 in my 70 chevy with a performer cam and manifold, and a 600 carter afb with a manual choke got right at 15 mpg consistently on the highway. It was bored .030 over and had mild compression. I think that that is about as good as you will bet unless you go with a 700-r4. I have to agree with junkyard, anybody claiming more than that with a carb is blowing smoke and doesn't know the math.

[cppursell]

Originally Posted by '72customdeluxe

Instead of experimenting and tearing it doWn just play with the timing. For around $180 you can get a wideband o2 sensor kit which will let you know your exact air/fuel ratios and what you need to do to tune.

I think I can start by improving my timing. I've only set the initial timing, but I think I'll follow this tutorial to get my engine tuned better: http://outintheshop.com/faq/Lars%20timing%20.pdf

As for replacing the heads, I'm still considering it 'cause of my cousin's 350 which runs, and without pinging, on regular fuel. I won't get to the project until July if I decide to give it a try.

[El Jay]

Quote:
Originally Posted by junkyardjohn View Post
SEVERAL PEOPLE CLAIM TO GET 18 20 OR EVEN 23MPG ON A SOMEWHAT STOCK SETUP. I CALL B.S. & FIGURE THESE PEOPLE WERE JUST NOT VERY GOOD AT MATH. IF YOU ABSOLUTELY NEED GAS MILLIAGE.. BUY A VOLKSWAGEN DIESEL. I HERE THEY GET 50 MPGs.
Have to agree with that statement.......


Smitty

Originally Posted by Duncan and son

x3

Make it x4.

I have gotten into the 14 mpg range with my Chevy (350/350, 4.11s, & corrected speedo); but that was on a long trip, all highway driving; driving with the tach never breaking the 3K mark, & never opening the secondaries.
I was driving for "economy" (if you can do such a thing in 1 of these trucks).

[panhandler62]

I used to have a 72 Camaro with a 350. When I got it it was set up with large chambered angle plug heads and a 2bbl carb. It got around 17mpg.

I swapped the manifold for a Torker II & a 600 cfm Holley. It got aroung 15mpg.

I cracked one of the stock heads and put on a pair of small chambered 283 heads. It got around 15mpg.

I swapped the 283 heads for a pair of camel humps with 2.05/1.94 valves and mild pocket porting and lost the 600cfm for an all mechanical 780cfm Holley and added a set of Hookers -- it got around 13mpg but ran like a bat out of hades.

Bottom line is I never got back to the milage it was getting when I bought it stock.

[68C15]

fuel economy is a direct descendant of volumetric efficiency (VE).

VE is achieved many ways but it all comes down to better flow in and out while getting the most use of the air-fuel mix (AF) while in the cylinder.
granted you buddy may have gotten better MPG with the 283 heads and no pinging but you need to look at the whole picture. what cam/lifter set did he have? some will bleed off compression at lower speeds. I had a NICE 355 in an 80 K20 with a crower 00210 cam which bled off the static 11:1 until it hit about 2300. along with a grocery list of other one-off parts and TONS of tuning it netted 17 MPG @ 70 MPH.

IMHO the best way to increase VE is with forced induction (blower etc.).

[kgt]

beating a dead horse by tring to some mpgs ouuta these trucks....heads will make no difference in mpgs not near enough to warrant the cost, buy a small car and enjoy the truck for sunny days,,

[cppursell]

It seems installing a 200r4 would be a good option in the future, but a higher priority right now is a paint job .
I just set my timing by following the tutorial I mentioned above, and my new "curb idle spec" is 18! That's a lot more than 7... I will road test it soon, but I look forward to seeing what kind of mileage difference I will get from that adjustment.

I might still experiment with the 283 heads in the later summer, but for now, are there any other enhancements/upgrades that I can use on my engine to get more performance/economy? panhandler62 mentions using special spark plugs, and '72customdeluxe mentions an 02 sensor kit, is that really worth the money for tuning the engine better?

[cppursell]

One note: Which is easier to swap with my T350 transmission, the 700R4 or the 200R4?

[68C15]

if your TH350 is the 9" version then a 700R4 will swap nicely except for having to move the crossmember about 1 1/4"

there are 3 different 350 tailshaft housings. 6", 9", and 12".

[panhandler62]

Not special plugs... It came with heads like these:



No real performance or milaege difference, just a different look. Tune her up properly and don't expect technology to beat physics. It takes a lot of power to move a 2 ton shoe box.

I have heard of trucks with modern engines and 4 or 5 speed modern transmissions getting very high teens reliably, but that's so much money that you should look at the improved milage as a side benefit because you'll never recoup that cost in saved gas.

[68C15]

Tune her up properly and don't expect technology to beat physics.
very nicely put.

BTW, how dare you post a pic of a 460 furd here?! especially one with fuel injection! LOL

[cppursell]

My engine seems to be running smoother after tuning it, except when I was road testing it, when I was accelerating on the freeway it makes a sort of higher pitched rattling noise, only in 3rd gear with the secondary barrels engaged. Any idea on what that is? I'll check tomorrow if retarding the timing a bit helps with that.

Thanks for everyone's advice so far. Yes, I realize these trucks are heavy boxes, but I love them so much! I wish I had a vaporizer carburetor and that gas at the pump would vaporize, then I'd be getting 100mpg everyday!!