Why the overbore from 396 to 402

[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

[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

[COPOL89]

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]

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]

[copo9566aa]

Another strange move for GM in 70
GM never put the new 400 SB in a Camaro for what ???
emission or warranty problems or bigger problem with this
new block for 1970 ???
GM like more overbore the 396 to 402 [img]/ubbthreads/images/graemlins/dunno.gif[/img]
Strange move [img]/ubbthreads/images/graemlins/biggthumpup.gif[/img]

[COPOL89]

Its funny you mention the truck emblems, My 72 Cheyenne is a 402 with the 400 badges. [img]/ubbthreads/images/graemlins/[/img]

[Pantera]

I will have to check it out later but I have a 72 super Cheyenne that has a 402 emblem on it. [img]/ubbthreads/images/graemlins/dunno.gif[/img]

[JTH74]

I think that Jerry McNeish said in his book that some of the late 69 SS396 Camaros were actually 402's, I will have to go back and read that chapter again just to make sure, I remember him saying that the block casting #'s changed.

[Chevy454]

On the same note, some head changes were made along the way in '69 as well. I found this out by accident, as I bought a set of 840s that had never had valves or seats put in them, but were made for the 1.88 exhuast valve, and tapered plug. On the shelf we had a set of 840s that originally had the 1.72 with flat seat plug...the interesting part is they were cast in the SAME WEEK (in May sometime). So, I had the last of the old style with specs of the new style, just by luck. I called MacNeish and Colvin and they said that was correct, that they did a changeover in May sometime to get ready for the upcoming year, and my 840s were kind of a mix between the old 840s and the new 291s. Some cars ended up with 291 heads on them, and it's probably safe to say some got "funky" 840s like the ones I have.

Some wierd stuff took place back in the day... [img]/ubbthreads/images/graemlins/smirk.gif[/img]