1972 307 cam selection... HELP!

[MikeB]

Originally Posted by BigBird05

Doesn't any body that grew up in the sixties remember what we called the 305 cid engines. Unless I was doing a restoration I would never put a dime into a 305. For the money you are talking about putting into your engine you could get a really good 350.

But you'd still be stuck with a carburetor!

And we're talking about doing it on a 307, which is a totally different animal than the small bore 305.

[68Stepbed]

Also note that the 307 block can be safely bored +.125 over. That's a 327!

[MikeB]

Originally Posted by 68Stepbed

Also note that the 307 block can be safely bored +.125 over. That's a 327!

Are you sure about that? It assumes 327 or 350 blocks were used for the 307. Or that 307 blocks had thicker cylinder walls than all other blocks.

At Mortec.com I found 8 casting numbers for 307 blocks. But not a single one of them is used for 327 or 350.

I guess a 307 block with zero core shift could take a +.125 bore, but who knows how much flex there might be in the cylinder walls?

I'd do a max overbore of .060" on a 307, just like I did with the 283 back in the day to get 292 cubes. That was back when the 301 was popular, but a machinist convinced me to stick with +.060". I doubt the power difference between 292 and 301 was even measurable. And heck, a 292 would probably last longer than a 301 with thin cylinder walls.

[68Stepbed]

Originally Posted by MikeB

Are you sure about that? It assumes 327 or 350 blocks were used for the 307. Or that 307 blocks had thicker cylinder walls than all other blocks.

At Mortec.com I found 8 casting numbers for 307 blocks. But not a single one of them is used for 327 or 350.

I guess a 307 block with zero core shift could take a +.125 bore, but who knows how much flex there might be in the cylinder walls?

I'd do a max overbore of .060" on a 307, just like I did with the 283 back in the day to get 292 cubes. That was back when the 301 was popular, but a machinist convinced me to stick with +.060". I doubt the power difference between 292 and 301 was even measurable. And heck, a 292 would probably last longer than a 301 with thin cylinder walls.

Many people don't like it when I tell them this, but the 307 was a culmination of GM getting rid of stock. With the popularity of the SCCA Trans Am series, and the 5.0L engine size limit, GM used the 327 block (4.00" bore) and used the steel 283 crank (3.00" stroke) to make a 302. That left a multitude of 283 blocks (3.875" bore) and 327 cranks (3.25" stroke). They were put together, and with a set of wheezy heads, they were thrown into base and low option modeled cars and trucks.

The early 283 blocks (pre-62) could actually be safely bored +.155. The later blocks could only be taken to +.125

The biggest difference in 327 & 350 blocks were bearing sizes. The 350 cranks had larger main journals, which is why people don't destroke 350's with 327 cranks. It can be done, but with custom 3.25" cranks with the larger 350 journals.

[MikeB]

Originally Posted by 68Stepbed

That left a multitude of 283 blocks (3.875" bore) and 327 cranks (3.25" stroke). They were put together, and with a set of wheezy heads, they were thrown into base and low option modeled cars and trucks.

I don't know if the 307 was built to get rid of parts, or simply to make more torque than a base 283 in the new longer, wider, heavier cars.

All 283 blocks were small journal, and all 307 blocks were medium journal, right? I guess the raw block castings could have been machined for larger main bores. But the real question is how would GM have changed the casting number in a leftover block that was cast for a 283?

Also, the 307 crank was a different animal. It had lighter counterweights than the 327 crank. Again, it could have been the same raw casting but just machined differently. It would be interesting to talk to a Chevy powertrain engineer from back in the late 60s.

I think you overestimate the number of 302 engines produced. I mean, they were available only in the Camaro Z28, and only for 3 years. Whereas 283 and 327 engines were available in virtually any car or truck. I'd bet there were easily 50 times more 327s sold than 302s during those years. When I was a kid slobbering over hot new Chevys that I couldn't afford, I would see dozens of 327 emblems every day, even in 4-door sedans. Z-28 302 emblems? MAYBE a couple per month.


***I love your stepside!

[BigBird05]

GM only keep on hand 2 or 3 days of block castings. They never stock pile them.

[gvw5400]

Spiced Ice Pirate,
Don't over think your build!!! Of course you can do any of the
many ideas offered here by everyone offering their thoughts to help.
We are all good at spending your money! You might consider taking a simpler
cost effective approach, install 3.73 as others have recommended or (4.10 gears with Bigger Tires) free up the exhaust side of the 307, ( might be good enough now).
Give it a great tune on ignition and carb.
Unless you really want a huge project, $$$ and the 307 is basically sound now,
just do the gears.
Have fun driving your truck !!, get it on the road and have Fun !!!!
All The Best,
Dirk

[MikeB]

Originally Posted by gvw5400

Spiced Ice Pirate,

Give it a great tune on ignition and carb.
Unless you really want a huge project, $$$ and the 307 is basically sound now,
just do the gears.

Dirk

Have to jump in again and say my 69 C10 had a totally stock 307 with single exhaust. I installed duals (only 2" pipes) with generic turbo mufflers, and installed a used Q-jet manifold with a 450 cfm 4bbl Holley Economaster carb. What a huge difference that made!

As I recall, fuel mileage around the suburbs, with some highway, went from 10-11 mpg to 12-13 mpg. And the engine felt so much better when accelerating and at highway speeds, like it went from asthma to breathing freely. Back in 1990 it was the best $250 I ever spent.

[68Stepbed]

Originally Posted by MikeB

I don't know if the 307 was built to get rid of parts, or simply to make more torque than a base 283 in the new longer, wider, heavier cars.

All 283 blocks were small journal, and all 307 blocks were medium journal, right? I guess the raw block castings could have been machined for larger main bores. But the real question is how would GM have changed the casting number in a leftover block that was cast for a 283?

Also, the 307 crank was a different animal. It had lighter counterweights than the 327 crank. Again, it could have been the same raw casting but just machined differently. It would be interesting to talk to a Chevy powertrain engineer from back in the late 60s.

I think you overestimate the number of 302 engines produced. I mean, they were available only in the Camaro Z28, and only for 3 years. Whereas 283 and 327 engines were available in virtually any car or truck. I'd bet there were easily 50 times more 327s sold than 302s during those years. When I was a kid slobbering over hot new Chevys that I couldn't afford, I would see dozens of 327 emblems every day, even in 4-door sedans. Z-28 302 emblems? MAYBE a couple per month.


***I love your stepside!

At a certain point, they ditched the 3.00" 283 crank, and most likely just went into production to make the 307 because it was cheap, and they already had the machines setup to cast and machine them, so they continued producing them.

The pre-62 283s were small journal, but the 62-68 283s were medium journal, just like the 327s.

The 307 crank was just balanced different for the 307 piston weight. Smaller piston= less weight

As for production numbers of the 302, SCCA requirements said the engine had to be in a "production" car and meet a certain sales requirement. They only made the minimal requirements in the Z/28 Camaro. However, race teams had quite a few spare engines laying around as they experimented different induction systems, ignition systems, etc. to gain an edge on the competition, along with replacing blown engines before the next race.

As for sales, the 327 was the mainstay for a few years, and since most hot rodders of the day were more about drag racing than road racing, the all new "350 SS" Camaro, then the 396 equipped Camaros caught all the limelight. The Z/28 versions weren't popular with drag racers. The little 302 made impressive horsepower, but torque was limited since it was a high winding road race engine designed to turn 8000rpm(race version at least) on long straights at speed.

Thank you!

[MikeB]

Originally Posted by 68Stepbed

The pre-62 283s were small journal, but the 62-68 283s were medium journal, just like the 327s.

Actually, medium journal cranks were not used in the 327 until 1968. And I think all 283s were small journal. Google it. (For those who aren't sure what we're talking about, I say "medium journal" because only the 400 had what is referred to as "large journals".)

Originally Posted by 68Stepbed

The 307 crank was just balanced different for the 307 piston weight. Smaller piston= less weight

Correct. Several years ago I built a 327 using a 350 block and 307 crank. The shop that balanced it had to add Mallory metal to a few counterweights.

Originally Posted by 68Stepbed

The little 302 made impressive horsepower, but torque was limited since it was a high winding road race engine designed to turn 8000rpm (race version at least) on long straights at speed.

Exactly. A buddy of mine had a late 60s Z28 back in the day, and said it was a rocket ship, but not great as a daily driver. The race versions used dual quads on a cross ram manifold and reportedly made over 500hp(!) on an engine dyno when it was tested by Smokey Yunick.
https://www.jalopyjournal.com/forum/...8#post-6908697
There's another story out there saying Holman-Moody got similar numbers.

[68Stepbed]

Found the article!! It was Super Chevy magazine, not Car Craft.

http://www.superchevy.com/how-to/eng...ne-performance

[MikeB]

Interesting article from Super Chevy, but they were wrong about GM producing a 307 with a small journal crank. Quite unusual for them.

Here's an excerpt from Hot Rod Magazine. It's the second paragraph.
https://www.hotrod.com/articles/ccrp...-engine-specs/

And here's a Wikipedia article on small blocks. Read the first and last paragraphs under the section entitled "3.875 in. bore family (1955–1973)".
https://en.wikipedia.org/wiki/Chevro...l-block_engine

Finally, go halfway down the page here:
http://chevellestuff.net/qd/crank/crank.htm

Bottom line is I think GM needed a new block for the 307 with more "cast-in" clearance for rods (due to .25" longer stroke). And why cast a brand new block with small journals, when they were moving to medium journal cranks on all other small blocks (302, 327, 350)?

I did find a list of crank castings at Mortec.com that shows only three castings used for the 307, and they were also used for 327s. So, they were the same raw castings but machined differently for balancing with different piston weights.

3911001....307,327...cast....medium journal...3.25" stroke
3911011....307,327...cast....medium journal...3.25" stroke
3941174....307,327...cast....medium journal...3.25" stroke

A couple pictures of my 307 crank rebalanced for 327 (+.030") pistons and SCAT rods: