Why the overbore from 396 to 402

copo9566aa · #1 10-23-2003, 02:18 AM

My opinion
GM started the 400 Chevy SB in 70 and for the 396 Chevy BB oups problem GM overbore the 396 to 402 BB good no problem [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]
Just my opinion [img]/ubbthreads/images/graemlins/smile.gif[/img]

COPOL89 · #2 10-23-2003, 02:26 AM

[img]/ubbthreads/images/graemlins/biggthumpup.gif[/img] [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

Mr. T · #3 10-23-2003, 02:31 AM

I know some guys who have 69 SS 396 Camaro's and Elcamino's. They told me the reason for the overbore from a 396 to a 402 was to compensate for the powerloss due to the A/C conditioner option. [img]/ubbthreads/images/graemlins/headbang.gif[/img]

55chevy · #4 10-23-2003, 02:44 AM

[ QUOTE ]
They told me the reason for the overbore from a 396 to a 402 was to compensate for the powerloss due to the A/C conditioner option.

[/ QUOTE ]

Um... no.. The 396 was increased to 402 cubic inches for 1970. It's called Growth. It also had to do with GM lifting the 400 cubic inch limit on intermediate cars. And like I said, it's called growth.. There was a cubic inch war going on at that time. 427 grew to 454... 396 grew to 402...350 grew to 400... etc.. And I also have heard the story that GM stayed with the SS396 emblems because of success.. Why mess with what works? But who's going to argue with having 402 cubic inches compared to 396?? I'll take six more if I can get em. [img]/ubbthreads/images/graemlins/grin.gif[/img]

JoeC · #5 10-23-2003, 02:59 AM

Engines over 400 cu in were allowed higher emission spec. So they avoided putting some emission equip on some BB engines by making engine slightly bigger. 402 cost them less to produce then 396. They end up making a 400 SB turbo fire and a 400 BB turbo jet that was really a 402 and a 396 that was really a 402. They were crazy not stupid. [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

**COPO** · #6 10-23-2003, 03:05 AM

I would agree w/Joe C. One reason: EMMISSIONS

Fhakya · #7 10-23-2003, 08:59 AM

I would tend to believe JoeC's explantation as most plausible thus far. When I 1st read the post, I was formulating a guess that the overbore did 2 possible things (both emissions related) but purely speculation: 1. Larger bore unshrouds the valves and contributes to more efficient combustion, thusly lower emissions? 2. Larger bore helps to create a larger combustion chamber for lower compression? (this 2nd one seems flawed though considering the year implemented...and probably a few other factors.)

Just guessing,
-Mark

COPOL89 · #8 10-23-2003, 01:02 PM

Joe: The emission delete makes sense, In the engineering department one of there "mottos" was parts not installed have no warranty problems. Also weren't they having problems with the smog pump on some hipo engines over heating the exhaust systems? I could be wrong about the smog pump over heat thing it could have been an isolated thing at the dealership i worked at the time.(or maybe a loophole in the law to remove it) [img]/ubbthreads/images/graemlins/dunno.gif[/img]

resto4u · #9 10-23-2003, 01:44 PM

Gm has done some weird things. They put 400 emblems on trucks that had the BB 402, go figure. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

**Kurt S** · #10 11-05-2003, 05:52 AM

JoeC got it right. It was emission rules related.

The GM literature (parts books, manuals, etc) got real confusing, calling the BB a 400 alot of the time.

I assume the new block handled the .030 overbore.
Interestingly, the late 69 blocks did not use the 70 3 digit code format, but the late 69 axles did.

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