History of the Chevrolet/GM BBC big block Chevy

[pdq67]

wyrmrider,

Try the old '56, 265" head that had the sparkplug trough design, the old -306 head..

I have a pair just for the heck of it is all... Suckers have 60 cc chambers and 118 to 120 cc intake ports so they are small by tiny to say the least..

It is a way early double quench design that I figure went on to become the old REAL fuelie head, the -461 and "x" and the later the 305 HO, -601 head and others like it.

pdq67

[Truckedup]

The heads suffered because there was no flow bench work at the time other than what was in the carb div at AC Rochester. With a good flow bench the short comings of the heads could have been improved had time and a budget been allocated.

Ras, why do you say this? Although aftermarket head porters generally, or universally, did not do flow benching, the port contours of OEM heads appeared to be the result of flow benching. This is more apparent when you look at the cross sections of early SBCs. A prominent feature of the powerpack and the camel hump heads was a venturi neck that Mondello and others would promptly grind away. There also was the finding that SBC heads flowed more with the valve in than with it out, an uncommon result that had to be the result of flow contouring, not free-hand design. People like Harry Weslake had been flow benching for years before 1963.
The chambers were bad in that they were too-deep bathtubs, but that was a near universal design style apparently based on the theory of wedge chamber combustion.

Smokey Yunick said GM didn't use flow benches for head design in the 1960's.Supposedly only Yunick and CR Axtell had proper flow benches at that time.Of course,Smokey said all kinds of things

[ZIGGY]

Smokey also said, at the end of an article on his build of an early 396 bored to 427, that "you can go lookin' for Fords but you don't qualify as a hemi hunter yet".

[rfoll]

That would be the Tornado.

[140Air]

The heads suffered because there was no flow bench work at the time other than what was in the carb div at AC Rochester. With a good flow bench the short comings of the heads could have been improved had time and a budget been allocated.

Ras, why do you say this? Although aftermarket head porters generally, or universally, did not do flow benching, the port contours of OEM heads appeared to be the result of flow benching. This is more apparent when you look at the cross sections of early SBCs. A prominent feature of the powerpack and the camel hump heads was a venturi neck that Mondello and others would promptly grind away. There also was the finding that SBC heads flowed more with the valve in than with it out, an uncommon result that had to be the result of flow contouring, not free-hand design. People like Harry Weslake had been flow benching for years before 1963.
The chambers were bad in that they were too-deep bathtubs, but that was a near universal design style apparently based on the theory of wedge chamber combustion.

Smokey Yunick said GM didn't use flow benches for head design in the 1960's.Supposedly only Yunick and CR Axtell had proper flow benches at that time.Of course,Smokey said all kinds of things

This comports with what RAS was saying. What puzzles me is that the cross sections look pretty well informed to be mere guess work. In the late 60s (66-67) Pontiac released updates to their heads where they showed small changes in the port contour. It's hard to imagine they were doing guesswork in iron and only verifying changes on the dyno. They HAD to be using flow benches.
When you look at some port cross sections from racing engines of much earlier days you would see "straight shots" from port mouth to valve. Later they started contouring the short side a lot better. GM engineers should have been up to date on this, especially guys involved in racing like Duntov for example. Doing this kind of subtle work by guess is unlikely. But, I wasn't there, so...

[Truckedup]

GM was the world's largest manufacturing company from the 1930's into the 70's. They made more vehicles than the other makers combined... But I think Mopar and Ford did more experimentation ....Duntov always butted heads with GM design improvements,his Vette stuff was always a compromise in GM's favor.

[140Air]

GM was the world's largest manufacturing company from the 1930's into the 70's. They made more vehicles than the other makers combined... But I think Mopar and Ford did more experimentation ....Duntov always butted heads with GM design improvements,his Vette stuff was always a compromise in GM's favor.

This is probably right about Ford and Mopar. But, another piece of evidence for flow porting by GM is the change in some heads and manifolds during the '60s. For example if you look at the '62 through '65 SBC high rise manifold, it's a "log" design that could have been free hand designed (it is basically the same as the Z-11 log manifolds). In fact, log manifolds were historically (and are still) first approximation layouts to get substantially equal distribution between cylinders. Similarly, the standard dual plane (180 degree) manifold design used for all single carb installations from the beginning (at least from the 40s) was a log manifold based on the tri-Y principle without flow contouring. Then the 1967 Z-28 high rise is fully contoured. You usually cannot be sure a manifold like that will give good results without flow benching (in fact, wet flow benching). It is still considered an excellent manifold. Also, contemporary aftermarket manifolds by Edelbrock and Weiand appeared to be flow contoured. IIRC, Offy manifolds from that period did NOT look to be flow contoured.

[Truckedup]

It's possible GM was working off data gathered by non factory tuners......There was a back door Skunk Works working off funds "stolen" under the noses of stuffy GM executives

[numboltz]

"Smokey also said, at the end of an article on his build of an early 396 bored to 427, that "you can go lookin' for Fords but you don't qualify as a hemi hunter yet""

Scary old age. I don't remember the article, but I sure remembered Smokey's wisecrack from it.

In 1966-8 when the 396s first really showed up here in the frozen north about all that was available was the 325 to 360HP low perfs. Buddy was deeply involved in setting up a few for drag racing. One customer wanted a high perf version but no luck, so they got what they could afford from GM. Tried the HP intake on the round port heads and it worked well. A bit later he did an L-88 clone, right after the engine came out. When the cam was pulled out of the GM parts cam tube it had Iskenderian 550 Superleggera stamped on it.

[RAS]

I know it's hard for enthusiasts to imagine but, politics and the good ol boy network were very pervasive at the big 3 back in the 60s. You were invited to work on certain things. It was a culture. If you were a fit there were no limits. The Z11 was the last BB development with a racing approval from corporate. Although the chamber in cylinder design was a known negative by then the cylinder heads were killer. No chamber, and those raised ports were the road map to todays best. The engine made power, just didn't rev as well as the competition. Pontiac also had a raised port with the SD and a decent valve angle. Pontiac did more head port mods than Chevy did. Someone over there was on a mission. Did quite well at Daytona in 61-62. Chevy did not do flow bench work back then. Rochester Div. did but I don't know to what degree. Compare the size of the Pontiac head ports with the new BB Chevy at Daytona in 63. Chevy was twice as big. Faster, barely. No one knew what this engine really wanted. Like more cam lift. They just didn't have the components to make it all work. These were factory built engines not something from some skunk works with one off parts. The word stock still had meaning. Modifications yes, but no substitutes. By 66-67 things really changed. It was about then, Bill Jenkins, Jim Hall/McClaren, Ray Fox-Johnson-Yates got involved. Not to mention, Smokey. If just parts were allowed to be shipped out it might have been different. The racing ban was serious.

[Dan Timberlake]

Dan, I just sold my last new 502 block to a Marina that is rebuilding a customers blown engine. The entire rear main is breaking out of the block. Cracks everywhere. Now we all know boaters have 2 speeds, no wake and full speed. Tough customers. The BBC having a rear bearing with thrust puts a lot of stress in one location. The crank harmonics usually take out the no2 main. Note the Chrysler and Ford Big Blocks have center thrust bearings and do not have this issue,ever. Every Mk5 502 I know of that was built in the early to mid 90s killed this bearing, mine included. I think these blocks were run on the MK4 tooling production line. The later Mk6 blocks were better but keep in mind that these were not intended to race at 8000 rpm either. The BBC was designed to run at 6000 rpm all day long in 1963. The engine had a very heavy reciprocating mass and valve train. Go 50 rpm to much and it flew apart. The best explanation on BBC thrust bearing issues comes from Jon Kaase. You might find it on the WWW. I still consider the BBC the pump all others will be measured by for a long time to come.

Hi RAS,

I spent some time Googling Kaase but only get meaningful FORD hits.

I guess I'm struggling thinking using the rear main for thrust was a problem for the bearings toward the front of the engine then SBC would have it too.

[RAS]

The thrust bearing deal is about crank deflection. The fact that the engine is externally balanced also doesn't help the front of the crank. I'm not an engineer on this topic. This only shows up in extreme usage. There is a lot of forgiveness in the BBC. Back in 1973-74 I forget, Dick Bourgeois who ran the Doug's Headers Chevrolet powered Funny Car had a crank in the car that had hundreds of runs on it. He wanted to replace it? I think it was a HTC crank. Anyway, Dick bought a new forged crank at the Chevy dealer for about $100. We measured the location of the Mallory metal plugs in the old crank, drilled holes in the new factory crank, transferred the Mallory metal and then Romeo Palamides welded it up. Done. Ran great. Never had a bearing problem and made about 1500HP all day long. 6.60s at 220. Stock block, heads and crank on 88% nitro. Internally balanced. More or less!
Anyone that wants to believe that flow benches existed at GM back then please go ahead and do so. It's fine with me. Keep in mind, the original rectangular port 990 head flow in the bottom 1/3 of the port is as dead as it could be. Clay it up and there is next to no change. 33% of the port flows less than 10% of the total port. A Flow Bench would have been nice.
I just read in a Drag Illustrated ad from RM that the new SR20 flows only 45HP and 20 lb.ft. of torque less than the latest Profiler Raptor 2 head on the same size motor. Slick Rick has one insane good version the BBC head here. SR is the man. Hope someone puts a solid or billet version on a nostalgia fuel car.

[rally]

Never underestimate stock hi per Chevy BB parts.

[140Air]

I posted this on another thread and several suggested it needed it's own heading to help with searches. Here it is:

FWIW, I got this first-hand from Francis Preve, the Chevrolet engine historian. By the time it was in production, it was obvious the 348"-409"-427" W-series head/block interface was a technological dead-end and Chevrolet Engineering studied several options for their next big block.

The chamber-in-block (sometimes called chamber-in-piston), angled deck design was used by Mercedes for their 300SL slant 6 engine. Ford with the MEL engine and GM with the W engine adapted this idea. I think there were some benefits re valve unshrouding from the design, but whatever problems are associated with the design I think there are combustion difficulties. I saw a technical paper by GM that showed the combustion peak, instead of being a straight spike had a slight tilt to it at the top, apparently from a slowed pressure rise just at the peak. I thought this was strange, still do. Two problems are obvious with this type of chamber. The first is what everybody notes, a greatly increased crevice space above the piston's top ring, especially with the high compression piston dome. Added to that, the head to block gasket crevice occurs in the cool, acute corner arching over the top of the wedge. I think the combination results in a big increase in unburned gasses during combustion. All-in-all, a bad chamber design.