Question(s) about 400ci, 402ci and 396ci differences - Team Camaro Tech
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post #1 of 33 (permalink) Old Sep 12th, 04, 06:35 PM Thread Starter
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I was talking to a neighbor up the street today who was having a huge automotive yard sale in order to finance his '62 Impala Conv. project and noticed an old engine on a stand in his garage.

I'm still deciding on and looking for something for my car. He said it's a '72 402 (he might have said Chevelle, which would be a 240hp version) and he doesn't really need it for anything. I might be interested (torque monster!) so I'm doing some research.

When someone says they have a 396\402, it's because they've had their 396 over-bored, correct?

I'm confused as to how a 400ci block can have the same bore and almost the exact same stroke as a 402ci, but be called a 400ci? A 400ci engine is actually a 401 if you compare the bore and stroke of the two.

Does that .010" less stroke of a 400ci engine equate to 2 less cubic inches than a 402ci? I don't know how to calculate the difference between a 3.75" and a 3.76" stroke and come out to 2 more cubic inches. There has to be something else I'm missing. I've played with displacement calculators and they DO show the 2ci difference. I guess less than a sixteenth of an inch in stroke distance really does ad up!

This engine will need complete machining. So when a .030 over-bore is done, it'll end up coming out as a 404ci (almost), correct?

Thanks.... sorry for the confusing questions.

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post #2 of 33 (permalink) Old Sep 12th, 04, 07:37 PM
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i think a 402 bored .030 is actually around 408 ci- which you could round up to 409 if you wanted to get some old school badges for the fenders or something.
Chev bored the 396 .030 over from the factory for some reason in 70 or so- probably because they couldn't have the big small block (400) be bigger than the small big block (396).
i remember a magazine that did a comparison of a 400 small block and 402 big block built as identical as possible- same bore and stroke, same compression, same lift and duration, same style intake, same vintage heads, etc. the big block made more power and a lot more torque than the small block, but weighed like 200 pounds more and cost more to build.
i think they even put the engines in the same car back to back, and the big block was quite a bit faster.
wish i could remember the magazine- i'm thinking Hot Rod about 8-10 years ago.

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post #3 of 33 (permalink) Old Sep 13th, 04, 05:16 AM
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396 and 402 ae basically the same except the bore size. As mentioned above Chevy sent them out from the factory that way (402). When you say 400 that is a small block.

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post #4 of 33 (permalink) Old Sep 13th, 04, 07:02 AM Thread Starter
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Alright... thanks for the clarifications guys.

Now, for a really stupid question that I'm sure has been asked by newbies a million times (or at least WANTED to ask).... bare with me!
What makes a SB a SB, and a BB a BB? I know the usual, easy-to-see differences. Hence the reason one's called small and the other is called big.

Bore and stroke..... small or big. Same bore\stroke.... one's big and one's small?? This is where I'm confused.

Is it strictly the physical block\head mass, or what? Why is a 4.125" bore and 3.75" stroke in a 400 Small Block not the same thing as the same size bore and almost the exact same stroke in a 402 Big Block? The pistons are the same bore, the stroke is almost identical. Is it physical crank shaft dimensions, rod dimensions, head mechanicals dimensions, that make a BB bigger? And why are big blocks\heads so much beefier and larger in dimensions than small blocks\heads?

I just don't know enough about the physical differences internally that make a BB a larger hunk of iron (or aluminum).

That age-old 400 small block, big block thing again. I'm just trying to wrap my head around all of this for my own benefit. It'll help me "build" or at least find the right engine for my project.

Thanks

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post #5 of 33 (permalink) Old Sep 13th, 04, 07:45 AM
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'Big Blocks' are - well "Bigger" - but, not necessarily "Larger"...

I get into lots of discussions with people I build engines for on what they should build for their car - be that a small block or big block.
I once heard Grumpy at a SEMA show explain that the 'cheapest & most reliable HP addition is Cubic Inches' - I have always agreed with that statement and followed his advise when going for more performance from an engine. I try to start with the largest displacement available for the given vehicle (or within the rules) and go from there - never leave extra cards on the table!
In the case of your project - I would have to know your budget, what driveline your car originally had (if it is to be part of the project) and what uses you intended for the vehicle.
If you have a Big Block car (or access to all the mounts and parts), a stout driveline (or the funds to install one) and are looking for a fun cruiser to 'WOW' the troops with I would say go with a Big Block.
If you have a Small Block car (or a 6 needing surgery) and a modest driveline budget and intend the car as a driver/cruiser then I would lean towards a Small Block with the mods to support your activity level. Small blocks can be reliably built that displace well over 400 CI - but I know of no law that says you have warn others by changing your 327 insignias

Hope some of this helps;

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post #6 of 33 (permalink) Old Sep 13th, 04, 09:31 AM
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Ok, correct me if I'm wrong...

Smallblocks had a 4" bore spacing

Bigblocks had a 4.5" bore spacing.

Bigblocks use splayed valve heads, smallblocks do not (Without special hardware! Yes its out there..) Bigblock heads will have the valves visibly angled from one another ( like this \ / ) where smallblock valves are parallel (Like this | | )

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post #7 of 33 (permalink) Old Sep 13th, 04, 10:47 AM Thread Starter
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BusDriver....

That bore spacing between the two is getting closer to what I'm looking for. Is there a reason the spacing is .5" more in a BB?
WHY is a big block bigger? Larger water jackets\oil galleys?
Why the bore spacing difference if the bore and stroke are the same?

**I know, I know... SHUT UP JOE!!**

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post #8 of 33 (permalink) Old Sep 13th, 04, 11:08 AM
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The 400 SB has the center two bores on each side "siamesed", without a water jacket in between. So, I would say yep on the "larger" water jackets for the same bore (anything is bigger than none).

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post #9 of 33 (permalink) Old Sep 13th, 04, 02:17 PM
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Right... BB & SB are completely different engine families...like comparing an olds to a pontiac, both came in a 350 and a 455, but not a single part will interchange.

All small block V8's share the same basic dimensions. Main bearing and bore spacing, deack height, bolt patterns, etc. you could scatter all the parts from a 302, 327, 350, & 400 in a room, and bolt 4 engines together without regard to where the parts came from. (no, they wouldn'tall work) All small blocks were produced on the same or similar foundries and production tooling.

Exactly the same is true of the big block family, but they are different from small blocks.





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post #10 of 33 (permalink) Old Sep 13th, 04, 03:16 PM
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WHY is a big block bigger?

O o Which letter is bigger? Why?

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post #11 of 33 (permalink) Old Sep 13th, 04, 05:27 PM
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onovakind67 I'm not falling for that optical illusion. You want me to say the first one is bigger, but it's really the second one!

Joe there was a 400 cubic inch limitation on GM intemediate passenger cars that was lifted for the 1970 model year. You could get larger displacement engines ordering COPO cars, but not RPO in those bodies. Anyway in 1970 they then punched the 396 out to 402 cubic inches. Why they continued to call it a 396 was probably a marketing decision since they already had everyone thinking 396 from the previous years performance passenger cars. There is probably more to this than I've offered here, but from what I've read and understand they couldn't offer RPO engines larger than 400 cid in those bodies until 1970.

The reason the BB is bigger is someone at Chevrolet wanted to go faster than the outdated fifties design SB would let them. Although Chevrolet didn't participate in racing officially at the time. They already had a big block in the 348 and 409 engines, the new style heads however is what really got things going, welcom the mystery motor. It was for sale a couple years ago at Yunicks shop sitting there on a pallet.

[ 09-16-2004, 06:09 PM: Message edited by: SY1 ]
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post #12 of 33 (permalink) Old Sep 13th, 04, 06:22 PM Thread Starter
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OK, see if I have this right then....

A BB engine is simply just bigger in ALL respects: the cranks main journals, rod journals, rods, cam journals, combustion chamber, valvetrain, internal cavities for water and oil, etc. etc.
EVERYTHING is just larger dimensionally.

An engines "cubic inches" are determined by the combustion chamber volume at BDC. So if a 400ci SB and a 402ci BB engine have the same bore and basically the same stroke, the things that make the 402 a Big Block are all of the physical sizes of all of it's parts? It's simply "bigger"?
That's it?!

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post #13 of 33 (permalink) Old Sep 14th, 04, 01:25 AM
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just take a look at a big lock in a car, then find a similar car with a small block in it.. you will then understand what makes a big block a big block and a small block a small block.
don't even ask what makes a Mopar big block a wedge or a Hemi...

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post #14 of 33 (permalink) Old Sep 14th, 04, 03:16 AM Thread Starter
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Well, see Derrik, that's where I'm coming from. I was just looking deeper than physical size. It's more than plainly obvious when there's a BB under the hood...
I was just wondering why it would be bigger when all else is the same (bore\stroke).

I've had my fill. Thanks for the replies all.

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post #15 of 33 (permalink) Old Sep 14th, 04, 04:08 PM
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Right.

The larger bore spacing and higher deck height allow for a much larger maximum bore and stroke than the small block design.

They made a 396 because of the 400 cube corporate limitation discussed above, in a block that could easily go to 500 cubes.





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