8996chevy

iTrader: (1)

I've read so much on this topic and am still confused. Should I buy an Internal
or external balanced crank? I've got a 4 bolt main, 1 pc. rear main seal roller
block that I'm going to make into a 383 stroker. External= Weight in Dampner,
Weight on Flywheel. Internal= No weight on dampner, No weight on flywheel.
Right?

On page 165 of the 2011 GM performance catalog it says,

"NOTE: IMPORTANT! All Chevy Small-Block and Big-Block engines with
one-piece crankshaft seal require an externally balanced flywheel or
flexplate."

Which should i buy???

toomany

When they went to the one piece rear seal on the small blocks...they lost the counterweight that's was at that end. Which is why one piece seal flywheels/flex plates need to be indexed when out on. When you buy an aftermarket crank, the manufacurer should provide the info as to what kind of ballance to use. My guess is it will be an external ballance unit for the flywheel. For the ballancer I'm not sure.

sofakingdom

iTrader: (1)

I have written the answer to this so many times, that some people are likely to roll their eyes and go "not again...."

ALL stock Chevy small blocks are internally balanced, EXCEPT the 400. The 400 is the ONLY stock SBC that is externally balanced.

The terms "internal" and "external" balance DO NOT refer to which side of the rear main seal the balance weight is on, or what it's part of, or even whether it's part of the crank or not. This means, the 305s and 350s with 1-pc rear main seal, with a small "bat-wing" weight welded to the flex plate or on the flywheel, ARE NOT "external" balance, regardless of what they look like to the naked eye.

Consider what "balance" means. Start with the simplest possible case, a 1-cyl engine.

Imagine you took a piece of water pipe and bent a "U" in the middle of it and used it as a crank. Odds are, it would vibe, no?

So what would you do? It would be a fairly simple matter to mount it on bearings, suspend it in space, attach the various moving parts to it, and attach another weight to it, on the side opposite the "U". Let's assume that for whatever reason you had to locate the weight at some distance away from the "U". You could still adjust its mass and how far it was from the crank CL, to make it exactly counterbalance the reciprocating parts; and once you had done this, it would be "statically" balanced. But it would still vibe when moving, no? After all, at any given instant, the reciprocating parts would be trying to fly off one way, and the counterbalance would be trying to fly off the other way. It would be like "balancing" a wheel with a pound of weight at a point on the inside rim opposite a pound of weight opposite it on the outside rim. Sure, it would be balanced "statically", i.e. sitting still; but it would vibrate horribly, because it is not balanced "dynamically" (in motion). So what would you do?

The answer should be obvious: you'd split the weight in half, and put half on either side of the reciprocating parts, right? Not too tough to figure out. Now, at any speed you spin the thing at, there will be no tendency to make the crankshaft wobble. It would be "dynamically" balanced.

Now let's say you want to build the next more complex engine, with 2 cylinders. You could put 2 "U" bends in your water pipe, and the whole thing would be inherently balanced, statically; but you'd be right back to the same spot as the 1-cyl motor with 1 big CW, right? So what would you do? Right: you'd add a weight to each end of the crank, opposite the throw that it's next to, and you'd size the weight so that it exactly balanced its neighboring throw, eh? Still, not too complex to understand.

OK, let's move to a crank with 4 throws. Assume that it has a main bearing at each end, that being the most logical thing you could do. You could easily do the same thing to balance it that you did in the 2-cyl motor, a weight for for each throw. But there's a problem. As you start to spin it faster and faster, you have all this stuff all slinging back and forth at very high speeds, putting enourmous side loads on the crank, tending to bend it. It won't spin very fast at all before something breaks. So in order to make the engine more robust and able to spin to a higher RPM without coming apart, you add more bearings in the middle; ideally, one between each throw, for a total of 5 mains. Sound like anything you've ever seen before?

But wait: what would you do about the balance weights between each throw? There's a bearing there now.

Right, of course you would: you'd split them up on either side of the bearing, which would keep each one associated with the throw that it was responsible for dynamically balancing, and inherently minimizing the side loading on the crank.

Butt weight: what would you do at the "front" and "rear" of the engine? After all, at the "rear", you have whatever the engine is powering, hooked up to it; and in the front, maybe something too, like pulleys and belts to power low-power accessories that might be necessary. So, back at the rear, you might have this ginormous "flange" kind of thing, that would have to be big and massive to carry the engine's full power without breaking; wouldn't it be possible to integrate that rear-most CW into that? Of course it would be. Go look at a 305 or 350 2-pc RMS crank, and observe the shape of the flywheel flange, and wonder: why is it that funky shape instead of round? And let it dawn on you, that THAT'S the rearmost CW. Note that it it EXTERNAL to the seal, and even to the block itself. Interesting.

Now let's assume that you want to make this whole thing as compact as possible. After all, people want it to fit under their hood and such as that. So think for a minute about the relationship of stroke, piston height, and rod length. When a piston is at TDC, life is good, the piston is completely out of the way of the crank and everything else. But what about at BDC? The piston moves down, as the crank throw spins around toward the "back" side of the crank. Butt weight: something else is spinning around at the same time, no? The CW, right? Where is it at any given moment? Right: it's EXACTLY OPPOSITE the throw, right? Meaning, when the piston is at BDC, and the throw is pointed directly opposite the piston, the CW is now pointed DIRECTLY TOWARD the piston, right? Hmmmmm.... maybe there's a potential problem here.

There is indeed. However large the CW has to be to counterbalance the reciprocating parts, it has to FIT in between the crank and the bottom of the piston when the piston is at BDC. Not hard to see. There is a lower limit on the rod length, for any given stroks and size of CW. If the rod were any shorter, the bottom of the piston would hit the CW. Furthermore, the longer the stroke, the larger the CW; so as you increase the stroke, you get a DOUBLE WHAMMY on the rod length.

So now you've worked your way up through the ranks of engine designers, and you've been appointed the task of producing SBCs with ever more CID. You look back through the notes of the engineers that worked on the 350 and discover that for a 9.000" tall rotating assembly and 5.7" rods, the longest stroke you can use that still allows the wrist pin to be far enough down in the piston to allow the ring package to stay the same as it always has been (and wasn't really adequate even then), is 3.48". But you now have to build a SBC with a longer stroke than that, to get as many CID as possible. furthermore, you can't compromise the ring package; the wrist pin has to stay where it is. What do you do?

What the designers of the 400 did, was to shorten the rod. Remember, the wrist pin height from the top of the piston, plus the rod length, plus half the stroke, has to equal the height of the block; 9.000" in the SBC. You can't move the wrist pin up, you have to increase the stroke, so what can you do? Only choice left is, shorten the rod.

But that causes a different problem. Remember how the CW has to fit below the bottom of the piston at BDC? Well here you go increasing the stroke, AND shortening the rod; what do you do about the CW? You can no longer fit an adequate amount of CW to properly balance the motor. In fact, you even have to "flat cut" the top of the EXISTING CW diameter, so that when the piston descends all the way, the CW is SMALLER across the center of the top. You can spot a 400 crank in a pile of SBC cranks from 100 yards away by this feature: ALL OTHER SBC cranks have ROUND CWs, but 400 cranks have a compound oval shape cut into them.

Butt weight again: now we just lost the ability to dynamically balance the motor. What now?

Well, you just have to live with it. The solution in the case of the 400 was to accept a small amount of dynamic imbalance, and to put the extra CW that SHOULD have been mounted along the central portion of the crank - INTERNAL to the crank - out at the ends of the crank, beyond the region where the pistons are - EXTERNAL to the crank. This is the origin of those terms. They refer to whether the weights are located between the throws, where they belong, or instead have to be located beyond the volume occupied by pistons. Note that it would be POSSIBLE to have an "externally balanced" engine with ALL of the CWs INSIDE the oil seals; but that would require the engine as a whole to be larger and, on top of that, DIFFERENT from the others). Clearly unacceptable. And similarly, it is possible to have an "internally balanced" engine with some of the weight OUTSIDE of the seals, or even mounted on a different piece; because the INTERNAL CWs, the ones between the throws, are ADEQUATE. The physically "external" weights are technically still internal.

So a 400 has inadequate internal CWs, and this is made up for by adding external weights. All other SBCs have adequate internal CWs, and require no extra external weight.

[takes a deep breath and lets fingers rest]

This brings us around to 1-pc rear main seal motors. Recall we talked about the big funky pork-chop flange on the 305 and 350, that makes up the rearmost internal balance weight... how are you going to slip a 1-pc lip seal over that? Right: you can't. So what do you do? Easy: you move this internal weight to the flywheel. Did that suddenly make this internal weight "external"? No. Is the engine's balance accomplished any differently? No. Are the engine's internal CWs suddenly inadequate to internally balance it? No. Therefore: IS THE ENGINE "EXTERNALLY" BALANCED? NO.

But there is SO MUCH confusion about this in the land of hot-rodder mytholgy and lore and legend, that it's basically impossible to cut through the fog. Even the crank mfrs have given up on being technically accurate, and are calling 1-pc RMS 305s and 350s "externally balanced", even though they're clearly NOT. They are still INTERNALLY balanced, same as they always have been; the rearmost weight is just mounted where it looks to the untrained ignorant hot-rodder's eye as if it's "external".



The point of all this?


Avoid those words altogether. Don't even utter them, pay no attention to them, ignore them if you see them; because it's IMPOSSIBLE TO KNOW what the person using them is saying. Are they being CORRECT? Or did they just decide to give up and "go with the flow" of all the Friday night McDonalds parking lot monkey-spank? Who knows?

Instead of that, use the word "stock". Real simple. Stay away from "internal" and "external", and instead, stick with "stock"; and EVERYBODY will automatically be talking about the same thing, with no confusion.

Therefore, when buying a 383 crank and ESPECIALLY one for a 1-pc RMS motor, pay no attention to "internal" or "external". If the catalog doesn't say, call and ask if it's stock: that is, can you bolt up a stock 305/350 damper and flywheel to it. That's what you want. If the answer is yes, it's easy. If the answer is no, don't buy it.

Note also, that it is IMPOSSIBLE to build an internally balanced crank for a 400 stroke, using stock 400 (5.565") rods, without using the $$$special$$$ heavy baancing metal. This applies to the 400 itself, and to ANY OTHER SBC using that stroke (or longer), such as a 383. You will need pistons that use a non-stock ring package, in order to use a 5.7" or longer rod, if you want to have the long stroke and stock (internal) balance. This is what I would STRONGLY recommend. You can also tell about the balance, in the aftermarket stuff, by what rods are allowed; if it's compatible with stock 400 rods, YOU DON'T WANT IT; but if it requires 5.7" or longer rods, then there's hope.

Now my fingers are REAL tired. But I hope maybe the confusion is a bit lessened.

vetteoz

iTrader: (13)

Excellent writeup , have had the same argument across other forums for many years

Don't bet on it , some are set in their ways

oscarfromla

iTrader: (9)

Sticky?