what's better and why? I know the differences, I want to know the advantages/disadvantages. thanks

 

im in the same boat

 

the only thing i can think of being a non engine builder is the use of stock int.bal flexplate/flywheel and balancers.

 

Different pros/cos depending on the cubes, how many CI's?

 

Well im thinking somewhere around 489-509

 

I'm not sure, but I think it has to do with size. The part of the crankshaft opposite the throws are called counterweights. Those weights have to equal the weight of the piston/rod assembly on the throw, but they must also fit inside the block and not hit anything. Internal balancing is best because each counterweight balances one piston/rod set. I'm talking about a V type engine here. An inline engine can be designed to use 2 counterweights to balance one piston/rod set. The problem occurs when when the weight of the piston/rod assembly is heavier than the maximum weight that the counterweight can attain and still fit in the space available. When that happens, then the extra balancing is done by extra weight on the flywheel and/or vibration dampner. I think internally balanced is the preferred method, and external balance is only done when internal balance is impossible. You want to go internally balanced if possible because you balnace the crank along it's full length. As an aside slightly off topic but related, if you are building a small block 350 with forged parts that are significantly lighter than the stock pistons and rods, you can save some money and time by using a 305 crankshaft instead of a 350 crank.

 

I think the biggest downer to externally balancing a motor is rotating mass and the fact that you have to use a the whole set balanced together. Damper/crank/flex or flywheel.

If your going to be doing rpm less weight is better. I use light weight hollow drilled cranks with the smallest counter weights. The only external I ever owned was a piece of junk 393 stroker small block. The crank had massive heavy metal slugs welded into the crank and of course non SFI heavy damper. I was so dissapointed in it I sold it at a loss and went with a quality stroker motor.

 

There is absolutely no diff internal or ext. Two ways to get the same thing. balanced is balanced.Big inch motors are internally bal cause they have big cranks to start with and can bal with mallory metal.Just simpler on them.

 

The rotating weight will be the same...the only difference is that external will have some of the weight on the Dampener and flywheel, the other will have it on the first and last counterweight on the crank usually. *IF* you have a pretty heavy crank, and relatively light pistons/rods, it's pretty easy to internal balance stuff. Just drill the counterweights. But if the crank is very light, there just isn't much mass to play with. So you either have to add some back on the ends, or drill counterweights and press in mallory metal. Some shops try to get by with just pressing in slugs, but the best way is to weld them to make sure they stay put. Usually ugly, but that's how it's done. Welds could be prettied up, but most balance guys just say you are grinding off some of the weight he just went to all that trouble to get put on there because he needs it to balance it all! I've even seen some very successful drag high rpm motors balanced by welding a giant *wing* to the rear of crank at flywheel flange to allow balancing. Ugly but functional. You can buy bolt on balance weights to go on flywheel/crank flange that you can grind down as needed. It's done all the time.

So to me, it really doesn't matter for most stuff. You are either going to add it on the outside of the #1 and #5 main cap, or on the inside of it by drilling crank and adding Mallory metal. It's all about the same thing when you're done.

*IF* you can do it internally, sure go for it..it's easier. But if you have to externally do it..go for that too. Spend money on something that makes you go faster. Even old Boss 429 in NASCAR were external balanced and did very well at mega rpms.

GM did the 400's and 454's externally because it was cheaper than coming up with a whole new setup for crank making. Worked fine. These days the aftermarket cranks are often being made with enough counterweight to allow internal stuff easily. GM did go internal on th enew 572, but again, they were making a whole 'nuther crank setup anyway, no reason not to design it in already. Uh.oh...we better not get into who actually makes all that stuff for the 572 huh?

JIM

 

I beg to differ on no difference. I have never seen or heard of light weight externally balanced crank shafts. Then You can not buy light weight externally balanced dampers. Then what's the use of buying a light weight flywheel and having to weld on weight tabs to balance it.


Light weight cranks don't have heavy mallory metal slugs welded in. They have balancing holes drilled to loose even more weight.

Just look at the figures. I have a stroker SBC crank just under 40 lbs. I use the small SFI 6 1/4 inch damper. SFI flex plate and 9.5 inch TC. My external balanced stroker crank was 54 pounds and had heavy metal slugs welded in it. It had a 8 inch heavy damper that was twice the weight of my little 6 1/4 The matching flex plate wasn't SFI rated and had welded tabs.

Conservitively the rotating assembly is 20 pounds more which is equal to untold 1000's of pounds of inertia when it's spinning 7000+ rpm

 

It generally costs more to have a rotating assembly internally balanced(strokers) than externally. Good for fooling people if they are questioning engine size.

 

George, I think the difference is that your lightweight 40 lb crank is also a purpose built piece where the weight was added in the correct places. Usually those cranks are designed with a particular rod in mind as well as total bobweight. Most other cranks, even stroker ones are made off common forgings and have heavy couterweights to allow for lots of variations. They can be made into lots of different stuff during machining. So to make up for it all it's easy to just add some weight on the ends. I would imagine that 54lb crank also had much heavier reciprocating assy too?

For sure, if you can easily balance it internally, it's great and the best way. Getting rotating weight down by 20lbs is huge. It's all fun stuff when you think about it. The total weight of crank,flywheel and balancer are all rotating weight. It's interesting to see some folks that go to great lengths to reduce crank weight and then add a heavy flywheel on it. Once it's running it's all the same really...same with balancers, other than the flywheel effect of weight being further out from crank center. Sorta like a 10" 30# flywheel will act different than a 12" 30" flywheel.



Jeez...this is fun stuff!


JIM

 

If your building an engine from scratch, it`s best to have an internal balanced assembly. It will avoid some of the problems already mentioned. Most racing engines are this way. Actually I would say all, but there always be a loner out there somewhere.

 

Personally, I was thinking off issues like the flywheel, bellhousing, starter, and if it is a stick I know I ran into stack up height issues, but I was dealing with a Small Block.

I have an 406 internally balanced front crank using a stock SBC damper, and in the rear it is externally balanced using a 400CI balance plate with all clutch, flywheel etc, nuetrally balanced.

I had to drill the block to match the starter holes, machine metal from the back of the flywheel etc, just a lot of stuff I had not plannned on.

If the rotatting assembly is internally balanced, I think you have more options, less hassles.

But I am pretty new to all of the "specifics" and this is how I got into some of the issues that came up, being a newbie!


Larry

 

George. What im saying is either your assembly is balanced or it isnt,You can achieve it either way.Obviously a lighter crank will have a diff aproach to balancing versus a heavier assembly.Either way your assembly method of ext or int wont have a hp advantage. The goal is the same balanced to your assembly.Lighter can be better but which way you achieve it will only be determined with what you start with.