Hot Rod magazine, August 2005 issue, letter in their "Bench Racing" section, page 22. Guy named Bill Howell says he was a senior devlopment engineer for the BBC's, Mk II & IV. Talked mostly about the 396, which was developed because of rumors NASCAR was going to limit displacement to that number.

1st plan was to use the same bore as the 427 they also had planned, which would be 4.250", I assume (he didn't say). With what he called a street solid cam and rectangular port heads, two test engines both made 460 HP at 6200 RPMs.

In their infinate wisdom, Chevy decided to make the crank stroke common between the 427 and 396, rather than the bore. That produced the odd-ball 4.096" bore of the 396. With that configuration, same cam & heads, they made 445 HP at 6200 RPMs. He attributed the loss to what he called "motoring friction", which he claimed they measured. I don't recall hearing that term before, or any real discussion of friction with regards to long vs. short stroke - although piston speed is sometimes discussed.

He said the short stroke 396's were superior at all RPMs (they started taking data at 1200).

In his parting shot, he said, "The current NASCAR engine are short stroke and big bore for a reason, even though it may be difficult to quantify it."

 

That really makes no sense though. Because the shorter stroke 396 actually has more surface area swept by the piston, 74.98in^2 vs. 74.65in^2 for the real longer stroke 396.

So i think that argument goes out the window. It may have to do with more side loading friction and other angularity effects of the longer stroke but from a purely mathematical sense that is pure bunk.

FWIW i read that article also.

 

I would hope that an engineer would use proper terminology, but about all I can come up with is piston speed (friction becomes more of a factor as speed increases) and rotating mass acceleration (higher piston speed means more momentum, both to accelerate the piston and slow it down on each stroke).

For all the talk about valve shrouding, he never mentioned it. The original article he referenced did talk about that.

 

Well its really a much more complicated issue than what people make it out to be.

Sorry but i've been really tired lately, i don't know what it is but i can't think at all. I also never claimed to be an expert on the issue. It would take many years of specialized schooling to even break the surface of such a complex issue.

But your momentum issue also has some holes. The larger/heavier piston required by the larger bore engine would also have interial and momental effects.

I don't buy any of this short stroke engine stuff though, some may call me skeptical. It has do more with piston speeds more than anything else. But today due to metallurical technolgy is not a big an issue today as in earlier years.

I don't know, its too complicated of an issue to even comprehend right now.

 

Heavier, but slower.

 

Momentum is 1/2 mass times the velocity squared. So, velocity is an exponential factor, while mass is linear.

It's why braking distance from 60 MPH is more than twice the braking distance from 30 MPH.

I will agree that it is more complicated than typically made out to be. In reality, the main reason there are more 383's out there than 377's is there are more good 350 blocks than there are good 400 blocks (for the same money, for sure). And, a scored 400 crank can be turned down to 350 main size, but a 350 crank can't be turned up to 400 main size.

 

That's kinetic energy, not momentum. Momentum is the product of mass and velocity.

 

There are the bearing spacers to put medium journal cranks into the large journal blocks, but I think you're right, the limiting factor is the relative availability of blocks.

 

Dang, guess I'm the one with the tired brain.

Okay, the energy required to ...

 

Hmm... I'm almost bored enough to go play physicist on the whiteboard for a while.

 

Motoring friction is pretty much exactly what it means. This is measured by actually spinning the engine with a DC (electric) dynamometer, which has both motoring and generating capabilities. Basically the dynamometer is spinning the engine with fuel and spark shut off. A load cell transducer records how much torque is required to "motor" a given engine at various rpm's, and the torque readings are interpreted as internal friction.

I also read the article and found it to be very interesting. I would think that piston side loads would increase somewhat under the force of combustion on the power stroke though, which wouldn't show up on a motoring friction test.

 

I think the small bore of the 396 affected the power the most. On the hi- perf 396's the large valve rectanglular heads required the 396's bores to be notched to clear the big intake valve. 427's and 454's do not need this notch on the top of the block. the intake valves are definatly more shrouded by the small boer on the 396. Especially with the closed chambered style BB head of the day (1963 to 1970)

The developement work for the Nascar "mystery motor" he's refering to was done around '63 I believe.

Open chamber heads just make way more power on any BB.

454's with their long 4" stroke make even more power than 427's. So his therory does not hold water.

Anybody who has done flow bench work on BB heads and many others will tell you that a bigger bore will get you more flow with the same cylinder head and valve size. the bigger the valve and the smaller the boer the more critical.