The mysteries behind engine balancing?

-
dibbons

dibbons

Well-Known Member
Joined
Nov 6, 2009
Messages
5,727
Reaction score
3,794
Location
La Paz, B.C.S., Mexico
Back in the 70's the my 273 engine balance included a small weight welded to my clutch pressure plate by who ever did the work in Salinas, CA. About a year later, when I sent the pressure plate back to the manufacturer for a rebuild, I was told my warranty would be voided if I ever had anyone weld anything on the pressure plate again. What I know about engine balancing is each piston, pin, and rod assembly should weight the same as the others. What I don't understand, is how the same crankshaft with the same counterweights can be used in different motors. For example, the 273 and 318 motors use the same crankshaft, but don't the pistons differ in weight for those two motors? So how can the same counterweight on the crankshaft compensate for that difference? The other mystery is what else is done to balance our motors, besides equaling the piston/rod weights? Is the crankshaft spun like balancing a tire? And what was the purpose of that small weight added to the pressure plate? Was the clutch assembly being balanced separately from the motor? Obviously the balance job was effective, taking into account a best ET of 12.72 at 108 mph by a 273 with a 600 CFM carburetor. Thank you for any insight.
 

Attachments

  • 1965 Valiant Effort.jpg
    51.4 KB · Views: 327
:happy1: I am interested in this as well. From what I have learned about balancing Harley engines. The balance factor whether 50, 55,60% depends on the rpm range an engine is designed to run in. The automotive end never seems to discuss this. "one size fits all" just doesn't seem right.
 
I'm not too sure 273/ 318 cranks ARE the same, even though they will physically interchange
 
Back in 1973-74 my stock 1965 273 crank (forged) was scored after a bearing failure. I exchanged it at the local machine shop for a 318 crank (forged). That convinces me they are interchangeable and I believe casting numbers for some years between the two motors are the same.
 
67Dart273 said:
I'm not too sure 273/ 318 cranks ARE the same, even though they will physically interchange
Click to expand...

How do you mean? Pertaining to balance issues? Im running my 273 crank in the same year 318, no issues at all, only 150 miles on it so far but problems usually show up right away, IME. MT:burnout:
 
67Dart273 said:
I'm not too sure 273/ 318 cranks ARE the same, even though they will physically interchange
Click to expand...

X2 One would think they would be balanced differently solely on the piston weights between the two engines being different. Reciprocating weight has a bigger impact on balance than rotating weight.
 
GeorgeH said:
X2 One would think they would be balanced differently solely on the piston weights between the two engines being different.
Click to expand...

yes but all the pistons in the engine would weigh the same. or close. so it seems it would stay in balance. MT
 

Attachments

  • casting.jpg
    24.2 KB · Views: 286
Consult balancing equipment manufacturer regarding balance factors? Wish there was a little more in depth at that point.
 
If you notice the 318-3 steel truck crank, that was recommended by Direct Connection, it didn't have a 5/8 hole drilled in the front & rear rod journals.

From what I was told, these holes were added to offset the weight of the 340 connecting rod on the steel 340 crankshaft.

With the exception of the 360, the 273,318 & 340 are the same size.
I could see using the 318 crank in place of the 273 & visa versa.
The balance would be close.

Ed Hamburger used to sell a blueprinted 318-3 crank kit years ago.
I used one in a 340 that I had balanced, with no concerns.
Their selling point was , it was stronger due to not having the lightning hole drilled thru the rod journal .
 
mikedevore said:
If you notice the 318-3 steel truck crank, that was recommended by Direct Connection, it didn't have a 5/8 hole drilled in the front & rear rod journals.

From what I was told, these holes were added to offset the weight of the 340 connecting rod on the steel 340 crankshaft.

With the exception of the 360, the 273,318 & 340 are the same size.
I could see using the 318 crank in place of the 273 & visa versa.
The balance would be close.

Ed Hamburger used to sell a blueprinted 318-3 crank kit years ago.
I used one in a 340 that I had balanced, with no concerns.
Their selling point was , it was stronger due to not having the lightning hole drilled thru the rod journal .
Click to expand...

Mike DeVorce,

You are a 'wise man'.

One serving of Barbara Eden {1967 Model} for you.

https://sp.yimg.com/ib/th?id=JN.348kmevRfOKSQxa/CrPxpw&pid=15.1&P=0
 
The balance "might be close" but factory balancing might not BE "that close." One case is the 70 440-6. They used heavier rods, and that engine was externally balanced. I don't know, offhand, what the weight difference is, and the downside is, those engines had a history of spitting out the no6 rod just like mine did.
 
Anytime you change the combination inside an engine it should be balanced. Will it blow up if you don't? Could be. A quote from 69 CUDA 440

Stock Piston Weight (273/318/340/360)
Stock 'Cast' Piston Weight Measurements

* 273 (2-Barrel).........530 Grams
* 273 'Commando'......569 Grams
* 318......................592 Grams
* 340......................719 Grams
* 360......................584 Grams

* Speed Pro 'Forged' Replacement Piston for a 340 (723 Grams)
 
toolmanmike said:
Anytime you change the combination inside an engine it should be balanced. Will it blow up if you don't? Could be. A quote from 69 CUDA 440

Stock Piston Weight (273/318/340/360)
Stock 'Cast' Piston Weight Measurements

* 273 (2-Barrel).........530 Grams
* 273 'Commando'......569 Grams
* 318......................592 Grams
* 340......................719 Grams
* 360......................584 Grams

* Speed Pro 'Forged' Replacement Piston for a 340 (723 Grams)
Click to expand...

Thanks T-M-R,

For the 'Street/Strip' Engines.

No big deal between the 273 '2-Barrel', 273 'Commando' and 318 'LA Engine'.

That 'Forged Crankshaft' won't know the difference between the Pistons and
Connecting-Rods.
 
Balancing means different things to different people. In terms of my stuff, it measn the lower end is internally balanced to a tolerance of less than a gram. That means the small ends of the rods, pistons, rings, pins, and locks all weight exactly the same. The big ends of the rods all weight the same. And the bobweight is precisely offset by the weighting of the counterweights. (and yes - it's balanced just like a tire once the bobweights are attached to the crank. It's spun in a saddle with sensors to note the location and amount of imbalance.) The factory took the path of 69 Cuda... "close enough will get us by".
In terms of the OP, the shop years ago included the flywheel, disc, and pressure plate on the crank when it was fitted with the bobweights during the balancing work - and they added weight there. That's one of the issues with external balancing - which is what that old shop did. Internal puts the counterweight within the block's main saddles which makes it less stressful on the block and parts. External makes it smooth but adds stress to the block and crank. In your case it also voids the warrany on any parts they might have to modify...lol
 
The 273 2 bbl and 273 4 bbl (hipo) had different piston pin weights to compensate for the 39 gram difference in the 2bbl and 4bbl pistons. The extra 24 grams going from the 273 4 bbl piston to the 318 piston was also compensated by lowering the pin weight.

Engine: Rod+pin+piston=total
273 2 bbl: 726+220+530=1476 gr
273 4 bbl: 726+184+569=1479 gr
318: 726+154+592=1472 gr

So the later 273 and early 318 cranks could be the same. Not sure if the later 318's changed weights or not.

The 340 total weight shot up to 1631 grams, so not the same crank, even though it will fit in place of the 318 or 273 and vice versa.
 
That piston pin explanation makes a lot of sense. Just out of curiosity, is that documented anywhere in Chrysler manuals or is that just from personal experience of working on these small blocks?
 
dibbons said:
That piston pin explanation makes a lot of sense. Just out of curiosity, is that documented anywhere in Chrysler manuals or is that just from personal experience of working on these small blocks?
Click to expand...
I was working up a new crank/rod/piston combo last week and did a lot of digging on the internet. Had to run down the pin info on 2 or 3 different sites to get all the numbers.

Seems like the 318 went to a heavier rod in '73 or thereabouts but don't quote me on that; I'll be looking at that again so may be able to report back later.

BTW, I like the lettering on your car in the pix; that was classic for the day!
 
For nm9stheham or anyone else interested, here is a picture of my old street legal 273 CID bracket car "Valiant Effort" today (I sold it in 1980):
 

Attachments

  • 431534_489285264434585_1957937747_n.jpg
    50.7 KB · Views: 115
nm9stheham said:
The 273 2 bbl and 273 4 bbl (hipo) had different piston pin weights to compensate for the 39 gram difference in the 2bbl and 4bbl pistons. The extra 24 grams going from the 273 4 bbl piston to the 318 piston was also compensated by lowering the pin weight.

Engine: Rod+pin+piston=total
273 2 bbl: 726+220+530=1476 gr
273 4 bbl: 726+184+569=1479 gr
318: 726+154+592=1472 gr

So the later 273 and early 318 cranks could be the same. Not sure if the later 318's changed weights or not.

The 340 total weight shot up to 1631 grams, so not the same crank, even though it will fit in place of the 318 or 273 and vice versa.
Click to expand...

Nicely stated. Excellent post for information.
 
You must log in or register to reply here.
-

Back
Top