Why the overbore from 396 to 402

[70-SS/RS-L78]

Can anyone explain GMís decision to overbore the 396 032 in 1970 to make
it a 402? And did they use the same casting or did they thicken/widen the cylinder
walls to compensate for the larger bore? [img]/ubbthreads/images/graemlins/dunno.gif[/img]

Thanks
Mark

[COPOL89]

Thats a real good question , I also have wondered the real reason for the over bore. I have herard many reasons but i think most were personal opinions but one that stuck in my mind had something to do with the onset of tighter emissions. I did read in an artical onetime that when the over bore was introduced the 396 engine ID was to remain because they didn't want to change the legandary SS396 name plate because of sales. Does anyone else out there have any clad reasons for this? And didn't they change the casting from the 272 to the 854 when the over bore was introduced? I also remember the confusion at the parts counter because it was refered to a 400 mark 4 and they also introduced the 400 small block later on. [img]/ubbthreads/images/graemlins/confused.gif[/img]

[copo9566aa]

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]

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

[Mr. T]

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]

[ 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]

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]

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

[toner]

I seem to remember car insurance cost going higher if the motor was over 400 cubic inch and that the 396 was always closer to 402. [img]/ubbthreads/images/graemlins/confused.gif[/img] So GM said that 402s were 396 to keep below the 400 cubic inch insurance benchmark. But maybe what I heard was just an urban legend. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

toner
p.s. is this pick on charley week? [img]/ubbthreads/images/graemlins/grin.gif[/img]

[Fhakya]

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