Another thread reminded me of something I always wondered about, what’s the difference between a small block engine and a big block engine?
The motorheads I know usually ay something like ‘big blocks have more tourque’ or ‘big blocks don’t rev as high’. I am pretty sure it is not just a size thing because I seem to remember that a 400 CI motor is a small block, but a 396 CI motor is a big block.
So is there a specific design difference between small blocks and big blocks or is it just a marketing term?
There is a definite difference in the physical size of the engine. While some small block engines may actually have a higher displacement than certain big blocks, that is the exception rather than the rule. The 400 Chevy small block has an extreme crank throw and very thin cylinder walls. 400 cubic inches is pretty much the upper limit for the old small blocks, though I’ve heard of some 427 inch one-offs. These would be very susceptible to cracks, especially between the bores.
You could have a “big block” with relatively small cylinder bores. It could be a “big block”, capable of cylinder displacement of 450+ cubic inches, but it may be bored out to just 396.
On the other hand, you could have a small block that is a 350 modified to a total cylinder displacement of 400.
The 350 is really stretched to the max, with thin walls between cylinders, and it still is a small block, really stretched out. The actual block is small. The block the bore are in is just small.
The 396 is a big block, with plenty of room to grow - the big engine blokc might allow expansion to 500 cubic inches of cylinder displacement.
(Small and big are relative terms referring to the size of the block, not the size of the cylinders - the size of the cylinders determines the cubic inch displacement)
I know nothing about engines but this link has some info that may be of interest. I didn’t realize that big block engines were only manufactured for a few years (1969-1971). Too bad…I remember some of those cars and they were impressive (also ate gas like there was no tomorrow…probably a big reason the auto companies stopped making them).
I also noted in the other thread questions on the interchangeability of parts between engines and the link above gives some info on that as well (at least for the following engines: 429 / 429CJ (COBRA JET) / 429SCJ (SUPER COBRA JET) / 429 BOSS / 460).
Your link refers only to a few specific versions of Ford big blocks. The 460 was in production up until about 1993 or so, IIRC, but it was only available in trucks after 1977 or so. Ditto for Chrysler’s 440. Chevy finally killed the 454 last year for trucks, but now has an even bigger engine available. GM cars with big blocks were available up unitl about 1977 or so as well. As far as the origins, I think the Chevy 409 (circa 1962 or so) was the first big block.
I may be off a year or two on some of these, but it was definitely a far longer era than 1969-1971.
Actually, displacement is the volume swept by the piston, so it is expressed by the formula (bore*0.5)[sup]2[/sup]stroke8 which is the formula for the volume of a cylinder multiplied by 8 since we’re talking about 8 cylinder engines here.
Big blocks are basically just physically larger versions which allow for larger bores and longer strokes, hence higher displacement. Torque, and by derivation, horsepower are greater because of the higher displacement. Redline is more a function of the design of the valve train. Racing big blocks are just as capable of 8000-9000 rpm redlines as their smaller cousins.
I’ve often heard that as a general rule, sub-400 cubic inches is a small-block, and greater than 400 ci is a big block. However, I think it really depends on the specific engine, and “big block” is mainly just a descriptive term. I think it really depends on which specific engine it is, and its just agreed upon that “oh, the chevy 350 is a smallblock v8”.
It’s more than just a size difference. There are mechanical differences between a small-block and a big-block. Unfortunattly, I don’t remember them. The Ford engine known has a 351 Cleveland is a big block, but the Ford engine known as a 351 Windsor is a small block. Same displacement, but the engines are internally different.
Not entirely. Displacement is three-dimensional, and the bore accounts for just two dimensions (circular in shape); the stroke–the vertical space through which the pistons move–is the third dimension. Two engines with the same bore will have different sizes (displacements) if one has a longer stroke and the other a shorter stroke.
If you start off with a small engine block, you can bore out the cylinders to increase displacement (to an extent). I’m under the impression that you’re a lot more limited in terms of what you can do with the stroke. If that is true, then a small-block engine with a comparatively large displacement probably has a higher bore-to-stroke ratio than a big-block engine with the same displacement. In general, I believe engines with a high bore-to-stroke ratio do their stuff at higher RPM. (The camshafts and other valve train elements with which they are equipped will probably be tuned to the engine’s advantages in that respect).
As far as the Ford Cleveland and Windsor blocks, both are considered small blocks. The Windsor is just a wee bit smaller. The 390/427/428/429/460 is the Ford big block family. These blocks are quite different from the Cleveland.
Big Block refers to the size of the engine block itself, not the size of the cyclinders (the size of the cylinders factors in determining Cubic inch displacement and/or Litres) Theoretically, you could make a 302 CI/4.9 litre engine out of a big block. Yes, piston travel has final say…like what’s the point of a loooong cylinder if the piston travels in a small part of it. The point is to help people realize that “big block” is not really a displacement issue…not a cyclinder issue…not a piston travel issue.
TORQUE: Ultimatley, a big block could be the torque king because you could use the big iron block to create a very loooooong stroke in the motor - because the piston travels up and down a relatively looong distance versus other motors, it would be a long stroke. It is this long travel that creates bunches of low end torque. You have room to pull this feat off in a big block - of course.
Most motors today are “oversqaure” - this is, the piston travels less than the distance of the side to side bore. The bored cylinder whole might be 2.5 inches wide, but the piston might only travel 2 inches down and 2 up (short stroke means high end power, less torquey)
Long strokes mean good torque - and a big block has room to create a long stroke.
Big blocks “rev lower” because big blocks tend to be longer stroking engines: engines that make a ton of torque at low RPM. They don’t rev well because of several factors:
Long stroking motor’s don’t rev well, they lose power in higher RPMS and can’t get the revs up (phsyicists, get in 'ere with explanations…friction?)
Valve train: Off the shelf valvetrain tends to be pushrod technology…true camshafts are more precise and have less friction, so can generally rev higher and deliver precise air and fuel at higher RPM, creating even more RPM. Old big blocks with their primitive pushrods (frequently, even when upgraded) can’t get air/fuel delivered like a good camshaft, and the exhaust cannot be scavenged as fast, and this slows down the piston. The ol’ piston ain’t gonna rev to high when it runs into backpressure.
I wrote up a bit about the fundamentals of horsepower, torque, and their relationships here. I also wrote a bit about how to increase power, the impacts of valve timing, etc.
I also wrote up a bit about proper sizing of bore, stroke, and crank throw in a chart about halfway down the page here too.
OK, now we are getting somewhere, I did mean bore and stroke BTW.
The tendency of big blocks to have a longer stroke (because the room is available) accounts for the increased torque and lower redline than a small block with similar displacement since it will have a shorter stroke and larger bore. Are there any other basic design differences?
Ohhh, I get to answer a question in my own thread. In a long stroke motor the piston has to move further with every revolution, and therefore has to move faster. So a motor with a longer stroke would have a higher piston speed than a motor with a shorter stroke at the same RPM .
I strongly recommend you look at the two links I posted above. There’s a bit about piston speed/crank throw/stroke/bore relationships, and their impacts.
352/390/428 (FE) and 429/460 are the Ford big block families. The Cleveland/Modified Cleveland shares a few parts and design features with the 429/460 but is still considered a small block, since it came out as a 351 when big blocks had >400ci displacements.
Smallblocks have small blocks; big blocks have big blocks. (H. Ross Perot) “It’s just that simple!”