The Stovebolt Site Forums The Shop Area The HiPo Shop 305 heads on 350

I've done some checking on my 64 Suburban and found I've got a 350 block (3970010) with 3.5" stroke & 305(354434) heads,topped with a 2 barrel carb. Why would this combo be done? Good or bad, runs like a sewing machine & is pulling 21.5" vac at 600rpms. I'm not going to race a 5500 lb. sled but would like to get better mileage, it gets about 13mpg now. Any suggestions?

Redneck engineering... That's a cheap way of boosting compression.

Is it possible this was a circle track engine? They must use a 2bbl carb and 305 heads on a 350 is pretty common at the track as they must also run production heads.

To improve your mileage we need more info. What transmission do you have? What rear end? What size tires?

Some folks are of the opinion that 305 heads with their smaller combustion chamber will raise the compression ratio (true) and increase performance (generally false.)

There's another thread in this forum where it's been discussed at great length. But the bottom line is that 305 heads are designed for a motor with a much smaller bore than the 350 and as a result have smaller valves and create a weird combustion chamber shape.

I agree with 4.3 about needing more info before making suggestions about fuel mileage...but just as a general suggestion, you might consider some 350 heads...882s or 883s are common and not all that bad, for smogger heads. With 1.94 or 2.02 intakes they flow decently. And if you go with a 4-barrel carb that has small primaries and large (relatively) secondaries, you can get pretty decent mileage if you keep your foot outta the carb and don't open up the secondaries.

I run a Carter 625 AFB (basically the same thing as an Edelbrock 1406 600 CFM electric choke)on my 292 and very rarely open it up. I get about 15 mpg with 4.10 gears. I'm ok with that.

Probably the best thing anyone can do to improve mileage is a higher (lower numerical) rear gear. But performance can suffer. I have an 80 El Camino with a mild 400 smallblock that had 2.73 gears in it and it got 15-18 mpg. But it was a dog off the line. When that rearend grenaded I put a 3.73 posi into it. It's a whole lot better off the line (burnouts are a lotta fun) but the mileage dropped to single digits...and since it has a 1:1 top gear (TH350 trans) it winds too high to be used comfortably (for me, anyway) on the freeway. So I have an overdrive (TH200-4R) that I'll be putting in it. That otta bring it back to the 15-18 range.

Good luck with your Burb.

I have changed rearend gears from 4.11 to 3.08, I have a 3 Speed w/ OD, 15" wheels with 27.5" tires. I also have a GM aluminim intake & Q-jet carb that I am trying to find best combo of primary metering rods & jets for, lots of info on maximizing performance,1/4 mile times,but this is no racer.

I had a "freind" one time that brought a set of heads to me for his 76 Chevy 350. I put them one without paying close enough attention to them. Until I tried to set the timing and make it idle right. Tinkered with that truck for over a year before finally decided something was wrong. Pulled the heads back off, one was a 350 head, one was a 305 head. If you set the timing where it would half-way run, it wouldn't start without grunting at you. If you set the timing where it would start when it was warm, it didn't have any power.

The factory did use 305 heads on some 350's, but they had recessed piston tops and I guess they did that for emission purposes.

Watch using 305 heads, if they came from a TPI
car (Camaro/Firebird) the intake manifold bolt
pattern is different. GM did that to prevent a
350 TPI manifold from being used on a 305.
The 4 inner bolt on a TPI 305 head are at a 45
degree angle to the gasket surface, instaid of
paralell like a 350.

longbox are you sure about that?

I have a 305 TPI intake on a set of 882 heads, it fits without a problem.

Early TPI intakes fit pre 87 style head bolt patterns.

I worked on a 305 in shop class 15 years ago and beleive it may have been from an '88 or '89 Firebird. we compared the manifold and heads to
the other sbc parts, they were different. There
was even a 350 of the same vintage and they were not the same. Maybe it was a running change? Of
course the engine was trash anyway, the front 4
rod throws were .140" out of round and someone stole the tpi.

I worked on a 305 in shop class 15 years ago and beleive it may have been from an '88 or '89 Firebird. we compared the manifold and heads to
the other sbc parts, they were different. There
was even a 350 of the same vintage and they were not the same. Maybe it was a running change?

We used to build our race car engines like that. A 350 with 305 heads and a 2bbl. Rules stated we had to use OEM parts and a 2bbl. It upped the compression ratio and they ran fine. As far as Longboxes comment- I am not sure about newer Pontiac motors, but I know the older ones (70's) were not the same as Chevy despite being the same displacement. (ie. a Pontiac 400 and a Chevy 400 are not the same)

It was an old school trick. Putting 305 heads on a 350 does give more bottom end torque with higher compression and smaller airflow passages. It should also improve fuel milage. Where the combination suffers is in top-end performance, usually you don't get much above 4,000 rpm. But you also have to look at how much you want to run above 4,000. In a driver it isn't much.

My recommendation is to leave the heads, but put on the q-jet. Properly tuned, it will get you better milage than the 2 bbl.

people do it becuase its cheap some people get offers from friends or just find a good deal on some cheap head

if some one needs heads to keep there vehical going most times they take what fits and inexpencive