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Old 04-11-2010, 04:25 PM
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BBC inherently more powerful than sbc of same ci?

Anyone have an opinion as to whether a, say 400 cube small block could be more powerful than a 396/402 big block? Assuming similar mods of course- just wondering if a big block naturally makes more hp/tq for some special reason. Only thing I can think of would be the heads/valves that would be the major advantage.
I just bought a 1 ton truck with a strong 454 ($500!) and I'm considering swapping it into my Chevelle wagon. Got me thinking about my above question...

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Old 04-11-2010, 05:19 PM
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with everything being the same, air flow/velocity in the heads, cam size etc i would have to belive that the 400SBC (4.125" bore 3.75" stroke) would make more power just due to the fact is has a larger bore than the 396 (4.094" bore 3.76" stroke) but the numbers would be very minimal and as for the 402 (4.124" bore 3.76" stroke) i can't see there being any real difference as the bore is only .001" larger.

the way i have been taught is that the larger the bore the easier it is to make hp, the larger the stroke the easier it is to make low end tq

that is why the 305 (3.736" bore 3.48" stroke) never did so well, it's hard to make power with the smaller bore, but the 305 does make decent tq for a smaller motor

but take the chevy 302 or the ford 302 both had 4.000" bore and 3.00" stroke they both have been known to make decent power, mainly the ford 302 but that is because the produced it for years and the chevy 302 was only produced from 67'-69'.

think of it like sucking throught a big straw that is 1/4" wide by 4" long, then sucking throught a straw that was 1/2" wide by 2" long, both have the same amount of area inside the straw but yet the larger diameter straw can pull in more air and at a faster rate
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Old 04-11-2010, 05:38 PM
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Like 87z said, if ones going to make more power than the other it's going to have to do more with the bore to stroke ratio and how the top end exploits it.

If you're comparing 400sbc to 402bbc you're probably better off with the SBC if you're pouring money into it just for the weight reduction.
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Old 04-11-2010, 07:12 PM
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I'm a Small Block guy but the BBC wins hands down due to the head design, canted intake valves move away from the bore as the valve opens reducing shrouding around the edge of the valve. Built for max power the BBC will always make more.

The .035" difference in bore size is of little consequence in this case.
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Old 04-11-2010, 07:27 PM
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The .035" difference in bore size is of little consequence in this case.[/QUOTE]


i agree on both statments, i was going off of everything being the same which i know that it can't be, but scientificly if it could the larger bore will make more horsepower, but like you said .035" will make next to no difference.

it's like those guys that want to bore a motor out .060" for the extra X cubes but yet the motor only needs a .030 to be cleaned up.
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Old 04-11-2010, 08:25 PM
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A BBC cylinder head design wins over the SBC head's design. Even if the port volumes are the same.
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Old 04-11-2010, 08:52 PM
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Displacement is displacement. That is not to say that a 400 SBC will always make the same output as a 402 BBC, but from a theoretical standpoint, cubes are what determine potential of the total combination.

The rest is purely up for debate. BBC heads flow better, but typically you need much larger port cross section to get that flow which has drawbacks. BBC heads also typically have larger chambers which means you need domes to get the same compression as a SBC. Domes can have drawbacks, as can large ports.

Bore vs. Stroke is also more or less an argument that died years ago. Compare for instance a Buick 455 (huge bore, short stroke) and an Olds 455 (small bore, long stroke). With the same cam, head flow properties, and other things the same, they will both share the same torque peak and HP peak. The old thought was that long stroke makes more low end torque since the longer crank throws put more leverage on it, but that is not the case. Given the same 455ci displacement, the buick has less leverage on the crank, but a greater surface area of piston to push it. The Olds has more leverage at the crank, but less surface area to push it.

In the end, pick the one you want. I tend to choose large bore/short stroke engines like the buick 455. Why? Most of my builds are low-rpm torque builds for heavy cars, but every once in a while I use one of my engines in an application that I want to rev. In that case, its easy to make a buick rev with its short stroke, but trying to make an olds rev costs big money and reliability. For this same reason, I never use a 305 on the street for performance. The tiny bores limit valve size which limits breathing at higher RPMs. But, in a marine application where they rarely go over 5000 RPM a 305 is a perfectly viable engine.

In short, when trying to meet your torque, hp, and streetability goals, use displacement as your guide. If you want 600 hp and use it on the street you need big cubes (or a diesel)

So... if you are trying to choose between a 400 SBC and a 396/402 BBC, the choice is pretty clear to me... SBC. They are lighter, smaller, cheaper. To offset the additional weight of the BBC you would need to make about 35-45 more HP from the BBC to keep the same acceleration. Since they both have the same displacement, chances are you will have to reduce low end torque and streetability to make the BBC feel the same... which means potentially higher stall converters, lower rear gears, etc.

Cubes, head flow, flow velocity, and bore diameter are all in a dance. I had a 73 Impala station wagon. I wanted 400 hp with cross-country reliability. That meant a 60-over 454. I got my 400 hp with tons of low end torque to get the heavy car and 2.73 gears moving.

If you're talking about "everything else being the same", cubes are the determining factor of where the torque peaks, which in turn is the determining factor of where the HP peaks.
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Old 04-11-2010, 08:59 PM
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A while ago, there was a test done by one of the car rags, where a 400 SBC was compared to a 400-range BBC. The port volumes were equalized, as was the cam and CR.

The engines were installed into a Chevelle, same gears and TH400 tranny.

The BBC out ET'ed the SBC by a significant margin.

Take it for what it's worth.
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Old 04-29-2010, 10:51 PM
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Quote:
Originally Posted by cobalt327
A while ago, there was a test done by one of the car rags, where a 400 SBC was compared to a 400-range BBC. The port volumes were equalized, as was the cam and CR.

The engines were installed into a Chevelle, same gears and TH400 tranny.

The BBC out ET'ed the SBC by a significant margin.

Take it for what it's worth.
I ran across a mention of the test I referred to above- it was by CHP, and is linked to in the article, HERE, in the second paragraph "Rat vs. Mouse", but the f-ing link didn't work for me.

Bottom line, both were as identical as can be realistically made to be considering their innate differences. Both were 408 cid, the BBC ruled in this case.
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Old 04-29-2010, 11:50 PM
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Yep, I saw that article also, the same cubic inch size and port volume BBC was something like a half second or maybe a little more faster in the same car in the 1/4 mile. All because of the canted valve head design.

Some of the baddest SBC heads have always been near copies of this design.... The Dart Splayed Valve(looks just like a mini version of a BBC Rectangle Port head), the Dart/Buick SBC race head(quite similar), and now the SB2 and and current R07 Nascar stuff.

Link didn't work for me either, but if I dug into my archives in the basement I have the issue somewhere.
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Old 04-30-2010, 12:23 AM
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I stand corrected. Thanks for keeping up with the thread.
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Old 04-30-2010, 11:25 AM
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Quote:
Originally Posted by cobalt327
I ran across a mention of the test I referred to above- it was by CHP, and is linked to in the article, HERE, in the second paragraph "Rat vs. Mouse", but the f-ing link didn't work for me.

Bottom line, both were as identical as can be realistically made to be considering their innate differences. Both were 408 cid, the BBC ruled in this case.
Your link works for me, the article has both engines at 408 inches the BBC at 465 lb/ft and 464 horses and the SBC at 443 and 432 respectively. They made the engines as identical as possible given the parts available.

I rather think the SBC 2.2 design with its mirror ports and slightly splayed valves is a hat tip toward the BBC layout being a better top end breather. Afterall, the SBC Mystery Motor was made to go after the Chrysler Hemi which the W block (348, 409, 427) just couldn't do. Even Ford after High Risers and Tunnel Ports on the 427 had to throw in the towel on the FE and design a whole new series engine in the 385 family of 429/460 to keep up with the top end breathing of other peoples canted valve engines.

Bogie
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