331 vs. 347 Stroker

[302 LX Eric]

Chris - As I was ordering my 331 kit, I was screwing around with numbers trying to figure out how to calculate the volume of a cylinder when I remembered the following formula for volume (regardless of block type )

Formula for volume of a cylinder = Pi x Diameter x Height

Your calculation is pretty close say for a '306'

Yours = 4.030^2 x 3.00 x .7853982 x 8 = 306.133767

Mine = 4.060 x 3.00 x Pi x 8 = 306.1167882

E

[SlowGT]

Thanks for the extra info.

[Conman]

Originally posted by 302 LX Eric
Chris - As I was ordering my 331 kit, I was screwing around with numbers trying to figure out how to calculate the volume of a cylinder when I remembered the following formula for volume (regardless of block type )

Formula for volume of a cylinder = Pi x Diameter x Height

Your calculation is pretty close say for a '306'

Yours = 4.030^2 x 3.00 x .7853982 x 8 = 306.133767

Mine = 4.060 x 3.00 x Pi x 8 = 306.1167882

E

you gotta be careful with that formula the volume of a cylinder is not Pi x diameter x height

the correct formula is Pi x radius squared x height
for a 4" bore the radius is 2" so when you square that you get 4 again so it works out in this case but it will not work out if you had any other bore
I don't know if you figured it out or not but that is why in the "yours" formula you used 4.030 and in the "mine" formula you had to use 4.060 (4.030/2 and then squared) and this is not exact.
if you do it exactly you get the same answer to 4 decimal places

[PKRWUD]

I kinda figured there hyad to be a reason I hadn't heard of that before. I wanted to know what kind of block you had because you say that your block is 30 over at 4.06", and that is only true if your block came stock at 4.030". 4.060" is 60 over.

Take care,
~Chris

[93GTDIN]

not to mention that most cylinders are tapered downward. I dont know if that effects the usable cylinder volume or not, but its true.

[Conman]

How much taper is there in the walls?
if it only tapers by a few thousanths it would be a negligable volume to worry about
when you are talking about over 300 cubes half of a cubic inch doesn't make any difference

[ultraflo]

Originally posted by 93GTDIN
not to mention that most cylinders are tapered downward. I dont know if that effects the usable cylinder volume or not, but its true.

...please explain

[Mopar1]

Would a 351 be a better choice than either. just wondering, was a possible thought in my future

[93GTDIN]

The diameter at the top of the cylinder is bigger than that of the bottom. Each cylinder in your engine block is not a perfect cylinder. They are tapered. The difference may be negligable when computing volume, but they are definitely tapered.

[Oldschoolpony]

4.0 bore is stock. Correct me if Im wrong, but if you're rebuilding a stocl motor, arent you going to bore the block? at least 10 over?? Generally, almost every motor Ive seen rebuilt has been at least 10 over if not 30. Also, CHP states that their 347 kit is a 100,000 mile capable engine. But everyone had their opinion

[302 LX Eric]

Conman - thanks for setting me straight with the proper formula. Like I said, I was just screwing around with my calculator trying to figure out the formula and now I see why my formula worked (2 squared = 4, which is very close to my 4.030 bore).

Chris - sorry if I confused you. Now we both have the proper formula.

The diameter at the top of the cylinder is bigger than that of the bottom. Each cylinder in your engine block is not a perfect cylinder. They are tapered. The difference may be negligable when
computing volume, but they are definitely tapered.

This I didn't know! If it's true, then wouldn't the pistons be 'squeezed' at the bottom of the stroke - especially on a 331 or 347 where the stroke is farther down in the hole? And wouldn't this squeezing effect creat lots of extra heat, and friction causing premature wear on the piston rings/cylinder walls?

E

[SlowGT]

Why is ring gap so important if the cylinder is tapered???


Just a thought that came to mind. I've never heard of the tapered cylinder theory before.

[ultraflo]

...we're only talking a couple thousandths of an inch, but yes, he is correct about the taper... most pistons have a small amount of taper from top to bottom, again, only a couple thousandths...

Now a taper after about 50,000 mi. would be considered wear , but again, there is a taper in the cylinders to begin with (freshly machined)

...not enough taper to 'squeeze' the piston, though

When you figure a ring gap around .020 +/- a few thousandths per application, etc. then you've all kinds of room to work with the taper in the block...

.002 is something that would not be easily discernable with the 'naked' eye... a dial bore gauge comes into the equation then, which I just happen to have one and a freshly machined 400 small block (my dad's) in the garage that I'm going to play with here in a bit to see what I come up with...

...ya learn something new everyday

[302 LX Eric]

ultraflo - keep us posted on what you find out with your Dad's 400 engine. I, for one, am curious to see what you find.

E

[Chevyguy]

Ok, I'll throw a bone in here.

My grasp of the 347 vs 331 issue falls into the rod vs stroke ratio.

The 347 with the 3.4 stroke and a 5.09 rod = 1.497 I figure I am off somewhat on the stock ford rod length but follow me here.

The Stock 302 3.0 stroke 5.09 rod = 1.696

3.4 stroke and 5.4 rod = 1.588

3.25 stroke and 5.09 rod = 1.566

3.25 stroke and 5.4 rod = 1.6615

A Chevy 350 3.48 stroke 5.7 rod = 1.638

327 3.25 stroke 5.7 rod = 1.754

Chevy 302/283 3.0 stroke 5.7 rod 1.90 !!!

Chevy SB 400 3.75 stroke 5.56 rod 1.483

3.75 stroke 5.7 rod = 1.52

3.75 stroke 6.0 rod = 1.60


Now you figure, what the hell is he talking about

Where the reliability issue comes to play is the side loading of the piston. Having a low rod to stroke ratio pulls the piston toward the side of the cyllinder at the bottom of the stroke. This makes for a less revving motor and can cause excess wear on the piston. On the other hand, the Chevy 302 with the big 2.02 valves and a 1.9 rod to stroke ratio will rev to the moon.

The stock Chevy 400 was thought to be a big POS for years until they started to put 5.7 or 6.0" rods in them. Note how low those rod/stroke ratios are.

I imagine the first 347 kits used the stock rods and had some problems with piston wear, with the 5.4" rods they should be ok.

The 3.25 crank and stock rods is almost as good as a 5.4 rod 347, and 3.25 crank and 5.4 rods is pretty good

[ultraflo]

One more bone for the pile

I set my 306 up with a 5.4 rod for a 1.8:1 rod:stroke ratio

Purrrrrs right to 7000 (more like 'screams' to 7k)

...and my lazy bones haven't made it to the garage yet (soon) to see what I can come up with for the cylinders on my old man's POS 400 small block (which he has 6.0" C&A rods and custom Ross pistons for) ....it's also been converted to 4-bolt mains via the Milodon kit (three center caps anyhow)

The CHP 347 kit is the only one I'd consider if I were to run a 347... otherwise go 331. It's all about the components...

Rod:stroke ratio is a highly debated topic, but I personally adhere to and believe in it.

[bigblockcoupe]

just do some research on the individual kits and decide which sounds most like what your looking for.

[302 LX Eric]

How about this ratio in my 331:

5.315 rod with 3.25 stroke = 1.635

Is this a good ratio for a soon to be blower motor?

E

[todd95GT]

quote from my builder when I posed the 331 vs. 347 question to him: "...the tolerances are right, the machine work is right, the parts are right, build the bigger motor and will be just fine for ya, Todd." I have an Eagle forged 4340 crank, forged 4340 H-beam rods, and Wiseco pistons. I trust this guy as he is not only a personal friend, but has made such accomplishments as building a 2500 horsepower 612 c.i. Merlin block-based Mopar motor (with a 10-71), a 1400 horsepower Toyota Supra T.T. motor, and a 800 horsepower 2.2 liter (that's right, 2.2L)turbo motor. He also does machine work for some heavyweight Nascar and NHRA builders.

[SlowGT]

"...the tolerances are right, the machine work is right, the parts are right, build the bigger motor and will be just fine for ya,

That's exactly what mine told me, too. Then he continued to tell me about all the results of sloppy machine work and cheap stroker kits. And finished with "...there's no substitute for cubic inches". Jim Wohlford's motors came from this very same engine builder. I figure I'll put my money on his advice anytime.