It's probably obvious but I will ask anyway

Small Journal=LOW HP?

Large Journal=High HP?

 

No not exactly. I have no idea why GM decided to go with the large journal for the 327 in 1968, it was the very same time they introduced the 350. It had zero to do with horsepower.

In General, Large journals do work better for hard working engines, pulling loads etc but small journals work better for high RPM engines. High RPM heats the oil as it rotates between the Main bearing and the crankshaft.

The Ford 351W engines worked outstanding for truck engines but lousy for performance engines, the cause is the large jounal crankshaft used in the 351W. Much larger than the 302 Ford or the 350/327 Chevy. Sustained high RPM overheats the oil.

Nascar for many years now uses Honda rod bearing diameter in the small block GM engines making 850 + horsepower to keep the oil temp down and reduce friction. These are substantially smaller than OEM 327 journal size with far more stress on them.
Many 426 Hemi Drag racers convert the Hemi Crankshaft Journal size to the smaller 454 Chevy size to reduce friction. This takes a lot of work so I think these guys know it helps.

I bet if GM had a Re Do on the journal size they would have stayed smaller.

Counterpoint: Maybe they discovered the small journal crankshafts were breaking in Race Engines and they had to increase diameter to increase strength.
Someone in the forum may have the inside scoop on that one.

 

Glad i asked...i am buying a period correct 174 block and working doing a trade for another...both small journals.

I have a complete 305/tpi engine and a 700r tranny I'm trying to trade for both blocks...hope he goes for it...

 

Good luck, By the way, The GM 350 never had any issues with oil overheat in Corvettes that I am aware of so the slight increase in size to the large journal did not hurt them any.

 

The 327 main/rod journal size was increased in 1968 for several good reasons. It commonized the block main bore machining, commonized main/rod bearing shells, commonized big end rod bore macinining, not to mention crankshaft machining, all between the 327/350 engines.

It was made possible when the 327 went to the cast crank in '68 and had larger size raw journals cast in.

That was probably a very big cost savings to GM at the time due to a reduction in complexity.

 

As I recall, the switch to large journals roughly coincided with the switch from forged cranks to cast cranks. Sooooo, wild *** speculation on my part is that the larger journal diameter was needed to maintain acceptable rigidity and strength.

Just a hunch....

Jim

 

a SJ rod is not a weakness.....

 

In the history of the first generation Small Block, there were only TWO sizes of crank journals-----------------------EXCEPT the SB400.
From 55 to 67, ALLLLLLLLLLLLLLLL SBs (265-283-302-327) had the small journal crank sizes (2.0/2.3in). Again, EXCEPT the 1967 350 (YES, there was a large journal 350 in 1967-------------the SS350 Camaro).
Beginning with the 68 models, ALLLLLLLLLLLLLLL SBs got the large journal crank sizes (2.1/2.45in).
The SB400 was introduced for the 70 model (through 1980). It had the same size rod journal as all other 68-later SBs, but the mains were increased to 2.65in. As a result, this required a redesign of the bottom of the SB400 blocks.

Regarding journal size vs performance, there was NO relationship.
The highest performance small journal SB was the 64-65 fuel injected 375hp/327, and those engines had the small journals. Also, until 1964, ALLLLLLLLLLLLLLL cranks were forged, During the 64 model year, some 283s got a cast crank. As far as I know, ALL small journal 327 cranks were forged. So, if someone needs to build a small journal 327 from scratch, a good crank to use would be a lo-perf 2bl 327 crank that has probably never seen any excessive abuse.
And of course, there are MULTIPLE little/minor things that can be done to a small journal engine to increase its durability.

Tom Parsons

 

Those are several good reasons for "them", but not several good reasons for "us".

Smokey Yunick was one of those who had issues with early Pontiac large journal crankshafts. The larger surface area increases friction and therefore heating and led to early failure in high RPM applications. Larger journals are better for high torque-low RPM applications but not for high RPM-high horsepower applications.

Chevy converted the SBC to large journals in 1968 since they were already well on their way to stressing torque over horsepower by this time. They had done away with the true SHP mousemotors in 1965 in favor of the big, heavy, lumbering, truck engines known as the Mark IV series.

 

That first started in very late '64 for the 1965 model Chevy II V-8. That change also incorporated a new engine block that was scalloped along the panrail to accept larger crankcounterweights.

As far as commonization goes that I mentioned above, I should have also included commonizing machining and bearings between the 307/327/350, not just 327/350.

 

Really?

By the time Detroit started stressing "torque", the engines were so pitifully low on power (however you want to define it), they would barely get out of their own smoke.

They cut the power in half and then double the bearing area to handle it?

 

Ah............I see.
Thanks for that brilliant and insightful clarification.

 

Don Nickleson and Eddie Shartman was using the 427 Cammer and Connie Kaltta was Chrysler powered ........Ford gave all three of them their 460 ci engines and none of them could get those large journal engines to perform even using 100% loads and adding hydrazine.........Connie went back to his Chrysler.......Don and Ed became pro stockers .....and large journals are best for trucks and heavy equipment, not for fast cars

 

Anytime! See ya' on the flip side!

 

What is the plan for the build?

 

Informative thread, thanks

 

Hey Professor
Were there any small journal 327 2 bbls, I never heard or read of any ?
I don't think there were any in light duty (pick up) trucks either.
If there were any in trucks it would be hard to find any that weren't abused.
Although I don't know about the concern about "excessive" abuse - you're getting the crank ground anyway ?!
They'd just be well seasoned
Jerry

 

ALLLLLLLLLLLL 327 engines were 2bolt only.
ALLLLLLLLLLLLLLLLL small journal engines were 2 bolt only.
The very first year model for a 4bolt small block was 1969 (NO, none of the 67 or 68 big journal small blocks were 4bolt)
ALL 69 302 engines (they were Z28, hi-rpm engines) were 4bolt.
Beginning with the 69 models, various 350 engines (including many truck 350 engines) got 4bolt blocks.
NOT ALL truck 4bolt 350 engines got forged cranks (it is commonly believed that if an engine had 4bolt mains, it also had a forged crank. WRONG).
For a rebuild, a crank MAY NOT always get ground. I have torn down a few engines, and discovered a crank would be good to go with only polishing. Low perf engines, that have been well maintained, and had regular oil changes will oftern have a near perfect crank.
A lot of people will swear all day long on Sunday that SB400 engines had 2bolts in 70-72 and 4bolts in 73-80. NOT SO! ALLLLL 70-71-72 SB400s were ONLY 4bolt engines. ALLLLLL 73-80 SB400s had 2bolt mains. NOW, there MAY be an exception to this. This is NOT solidly documented. SOME, maybe SOME, VERY EARLY 73 models got the very tail end of 4bolt SB400s from the 72 models. But we're talking about an EXTREMELY SMALL quantity of very early 73 cars that MAY have received a 4bolt SB400. So, for the record, 70-72 400s were all 4bolt and 73-80 400s were 2bolt only. That is NOT from me, that is from official GM documentation. And I'm in agreement with that, because all the 70-72 400s I've torn down/rebuilt were 4bolt blocks and all the 73-80 400s were 2bolt.
That would be similar (thinking) to a very early 63 FI Corvette receiving a left over 62 FI unit-------------------DIDN'T HAPPEN!
I emphasize this 2-4bolt SB400 issue because there have been MANY people swear that 4bolt 400s were installed AFTER the 72 model run---------------------------but none of 'um have provided any proof/documentation (I suspect those people can't tell a 72 from a 73 Chevy).

Tom Parsons

 

Tommy , Tommy !

I know it's the end of probably a long work week, or maybe you are into that Black stuff already.

But "We" were talking 2 bbls as in carburators not as in 2 bolt mains.

I was going to give you the benefit of the doubt - I don't know if there was such a thing as a SJ 307, but maybe that is what you might of been thinking of when you refered to 2 bbl motors with sj 327 cranks.

Anyway I'm sure if anyone will know - it will be you

Jerry

 

Nice, but you didn't answer QIK59's question.