February 18, 2020 2 min read

WHAT’S BEEN HAPPENING IN THE INDUSTRY

There has been a lot of misinformation floating around on the interwebs about the weights of the internal components and how they add up and effect the new 2020 6.7 Powerstroke engine. SPE, being one of the longest running 6.7 Powerstroke aftermarket engine builders, would like to set it straight. SPE will share how the 6.7 powerstroke has and hasn't changed from Gen1 to Gen3 (2020) rotating assembly in terms of weights.


HOW IS BOB WEIGHT DETERMINED? 

Measuring internal components by themselves and adding them up in any order that you want is irrelevant to how they affect the crankshaft and balancing of the engine. Just because some components may be lighter than previous model years, this does not necessarily mean the effect on the rotating and reciprocating weight is less on the crank. What truly matters when factoring weight is what's actually hanging off of your crank. This is called bob weight. Individual component weight is irrelevant until it is all added together correctly to figure out its effect on the crank when spinning. Balancing the crankshaft is all based off of the bob weight which is added to the crank before the balancing process.


Bob weight spec refers to the mass located on a single crankshaft journal that includes both the reciprocating and rotating weight portions of the entire assembly. Rotating weight refers to the mass of the bottom half of the connecting rod that is attached to the crankshaft. The reciprocating weight is basically everything from the middle of the connecting rod upward, including the piston, small end of the rod, wrist pin, wrist pin clips, oil cling and rings. However, you don't just add all the values up to determine bob weight. The formula is 100 percent of the rotating weight plus 50 percent of the reciprocating weight.


Bob Weight= 100% of rotating weight + 50% reciprocating weight


WHAT DOES THE BOB WEIGHT MEAN FOR THE 6.7?

Below is the break down on each generation of the 6.7 Powerstroke in terms of weight that matters on the crankshaft. As you will see below there isn't much of a change between Gen2 and Gen3. They are within 6 grams. When weighing that many components that is actually an acceptable variance between cylinders. You could call them the same considering 6 grams x 8 cylinders = 48 grams which is equal to 0.1 pounds, roughly the weight of a pen or pencil.  You will also notice Gen1 is the lightest assembly out of all in terms of Bob weight. Keep in mind, oil cling is typically factored at 2g but for the sake of this we left it blank as it would remain the same in all applications shown.

 

6.7L Powerstroke Gen1 Reciprocating & Bob Weight

 

 

 6.7L Powerstroke Gen2 Reciprocating & Bob Weight

 

6.7L Powerstroke Gen3 Reciprocating & Bob Weight 

 


1 Response

Brad Helton
Brad Helton

April 19, 2020

Awesome Write up.

Leave a comment

Comments will be approved before showing up.

Subscribe