Internally balanced vs Externally Balanced

[v80z]

First Thanks for all the help in the past.

Now I want to put a sfi rated balancer on my 1970 4 bolt main. The machinist said I would have to get the whole rotating assembly rebalanced if I used an aftermarket balancer.

When I built my racing L28 I took all the rotating pieces and had them balanced. The Machinist would only concern himself with the rod pistons and crank. He said the race clutch and euro dampener were already balanced that all we need to do was the rods crank and pistons. Everything seemed to spin well up to 7850 rpm.

 

I have read that a 400cid sbc is externally balanced and others are internally balanced. Does externally mean balanced with the balancer and flywheel/clutch? Does internally mean just the pistons and rods? Is a 1970 sbc 350 cid intaernally or externally balanced? Bottom Line. Can I use an aftermarket balancer(fluidampener) without rebalancing the whole assembly. My suspicion is that if the flywheel is balanced and the aftermarket balancer is balanced that it would be no problem.

 

Anyway All facts and opinions are welcome!!!

 

 

Thanks

[Jolane]

Yes, externally balanced includes harmonic balancer and flywheel. The SBC 400 is externally balanced. The 350 is internally balanced. I believe you can get a new crank that is internally balanced to build a 400 with.

The bottom line. If you have a 350 that has been internally balanced, then you should be just fine getting a flywheel and harmonic balancer which are internally balanced also. They need to be neutral balanced.

Joshua

[v80z]

Excellent!!! Thanks!! It agrees with all the other posts I have researched here.

[jt1]

I would be a little suspicious of that machinest in the future. If the rotating assembly (crank, rods, and pistons) are internally balanced, you can use any neutral balanced damper or flywheel you want.

 

John

[Guest zfan]

That is correct. If it is internally balanced then you can use any nuetral balancer and flex plate. I went internally balanced on my 383 with all forged internals.

 

If anyone is looking for a Fluid damper SFI 7.75" balancer for an externally balanced 383 assembly. I have one used that costs 325 bucks that I will sell for 100 bucks and I also have the 168 tooth flexplate TCI brand and SFI rated external balance that I paid 80 bucks for and will sell for 45 bucks. Buyer pays shipping.

 

Mike

[Pyro]

There is a weight stuck on the balancer and flywheel of stock 400 sbc and 454 bbc. The external weights (out of the oil pan) are required to bring the engine into balance so these engines are externally balanced.

 

All 350's are internally balanced (no extra weights on the balancer or flywheel).

 

In strick engine balancing, the entire rotating assembly needs to be balanced together (crank, flywheel, and balancer), However, this is overkill in 99% of the cases.

[wheelman]

All 350s are not internally balanced.

The LT1 and late model SBCs with the 1 piece rear main seal are externally balanced.

 

Wheelman

[Mikelly]

YUp, Wheelman gets a gold star... that is correct!

 

Mike

[Guest Blazer406]

All 350s are not internally balanced.

The LT1 and late model SBCs with the 1 piece rear main seal are externally balanced.

 

Wheelman

 

 

I'm glad to know that..... I could have (at some point in the future) potentially screwed up if I had to build a late model engine.......

 

Thanks for the tip.

[wheelman]

I had to research the subject when building my LT1.

I decided to use a T5 rather than a T56 so the standard LT1 flywheel and clutch wouldn't work. Turns out the flywheel from a late model 305 with a 1 piece rear main is only 16 pounds, bolts right up and has an integrated counter weight. Best part is they are pretty cheap, although for a real performance application (high RPM) I'd use an aftermarket unit that is stronger.

 

Wheelman

[grumpyvette]

let me try to explain in non-tech terms

 

the differance between the INTERNALL BALLANCED CRANKS and EXTERNAL BALLANCED CRANKS is the stress levels the crank and block are put thru,

let me try and explain the differances (both methods work, but the internal ballance has far less stress) and keep in mind that the EXTERNAL ballance cranks normally use a 8" dia. damper which can cause clearance problems on a frame that designed to clear a 6.0-thru 7"" internal ballance damper dia.

 

on either method the rods and pistons,are matched as to weight first to the lowest common weight to the lightest of the 8 in each set

 

on an EXTERNALLY balanced crank the entire crank, rods pistons, are then ballanced as a unit by adding or subtracting weight from the damper and flexplate/flywheel mounted at the ends of the crank.

 

on the internally ballanced crank, the crank itself is ballanced first, so it is equally in weight radially from the crankshaft center line then the damper and flexplate are mounted and they are match ballanced to the cranks ballance

 

why does it make a differance , you ask?

 

well if your externally ballanced the crank is constantly flexing the crank from one end to the othrer while the ends are trying to slow or speed up to absorb or release energy from the piston/rods thrust on the crank due to cylinder pressures

on an internally ballanced crank that thrust force is basically working only against the individually crank throw

again, Im sure your thinking , whats differance

 

well it might help you get a better grip on the concept if you have a strand of UNCOOKED spageti (PASTA) I cant spell) and use a black majic marker to draw a line on one edge only

 

place a single strand between your index finger and and your thumb on each end of the strand and roll it rapidly back and forth, thats similar to INTERNAL BALLANCE STRESS, notice the line stays strait

 

now do the same thing but roll the pasta from only one end while holding the other end to provide resistance, notice the line spirals first in one then the other dirrection,

 

this same flexing happends to the cranks under hundreds of pounds of cylinder pressures exerted over the four cylinder pressure points and millions of times at up to 60 times a second, , now youll have a nearly impossiable time breaking the pasta using the internally ballanced roll test because the pastas under almost no stress, but add resistance to the EXTERNAL test and the pasta SNAPS EASILY

 

keep in mind stress is cumulative

 

 

the internally ballanced crank has the individual cylinder exert major force over one throw, externall cranks absorb the stress over the full crank length,

 

 

 

now add to tha the fact that many cheaper externall ballanced cranks are comonly cast steel with smaller counter weights and many of the more expensive internally ballanced cranks are comonly FORGED STEEL with larger counter weights making ballincing easier,and youll find the differance in resistance to flex is a huge differance

 

 

if you ever wondered what those holes drilled in the counter weights were there for, it was done to remove weight from the counter weights