LSOHOLIC

iTrader: (3)

What does everyone feel is the max rpm (mechanical limitation) of a stock bottom '04+ (better wrist pin design) bottom end ??

I'm going to assume rod bolts are on the list of items to be upgraded. But aside from that...all stock rotating components.

Imo, the weak links could be any of the following....
~rod cap
~wrist pin
~wrist pin boss (piston failure)
~reluctor (signal issue)

Here are some basics to help analyze.....
Conrod journal = 2.0995"
Crank main journal = 2.5589
Max piston speed crank angle = 74.64*
Rod ratio = 1.6836
Crank journal overlap = 0.52
2004 and later piston weight = 425 grams
2004 and later conrod & wrist pin weight w/clip = 640 grams

I included weights for G calc's if anyone was interested.....

Here are some max piston speed numbers to evaluate.....
7250rpm = 7173 piston speed
7500rpm = 7420 piston speed
7750rpm = 7668 piston speed
8000rpm = 7915 piston speed
8250rpm = 8162 piston speed
8500rpm = 8410 piston speed
8750rpm = 8657piston speed
9000rpm = 8905 piston speed


So, in summary the valve train is not considered in this theoretical discussion.



Thanks.............



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LSOHOLIC

iTrader: (3)

Also any thoughts, input or constructive criticism is always welcome....

Thanks.....

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Blown06

iTrader: (32)

Is this discussion gear toward the 3.622" cranks only...........

What are the piston speeds with a 4.8 crankshaft?

C5_Pete

iTrader: (8)

Purely a mechanical limit?

Assuming oiling and valve train is good? Plenty of airflow in and out? So a pure mechanical limit on how much force the stock pieces can withstand?

I think most would agree that the crank is the strongest link. My bet is the rods would fail before you pulled a pin out of a piston.

LSOHOLIC

iTrader: (3)

Yes...both good points, oil and valve train are both big concerns. But yes, purely mechanical speaking....

The powdered rod is probably the "weakest" mechanical component in the rotating mass, as you pointed out.

I've seen a bunch of trashed rods on boosted motors. Has there been any failures in a N/A application ?? Specifically when rod bolts have been addressed ??

This is purely speculation, but on a clean tune I would have to think ECU/ reluctor signal issues would break down before a mechanical failure.


I spin my relatively high...and would like to make some mods (air & valve train) in an attempt to spin it higher.

But....I have to admit...I'm not a fan of driving through my own oil !!


Thanks....


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DietCoke

iTrader: (18)

I'd say 8k+, I think the valvetrain will give before the bottom end. I spun my stock ls3 to 7600 without much issue for about a year and a half before I toasted some rings on nitrous.

LSOHOLIC

iTrader: (3)

76* is the crank angle where max piston speed is with the 3.78" bore and 3.27" stroke. This is a 4.8L

7500rpm = 6649fpm
8000rpm = 7092fpm


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LSOHOLIC

iTrader: (3)

Thats nice to know....thank you.


So, I'm looking to design the head and valvetrain around a 7800rpm max rpm. Looks like I'll be fine....

LMAO....


Thanks...


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panic

Piston acceleration is a better predictor of inertial load on the rod bolts, piston pins, etc.
Z ~ the square of the RPM, and slightly affected by rod ratio (n = higher number > lower inertia).
Where:
Z = piston accel in f/s/s
N = RPM
S = stroke in inches
n = rod to stroke ratio

Z = (N^2 * S * (1 + 1/2n))/2189

Acceptable numbers range from about 100,000 for stock pistons to over 150,000 for forged with thin rings.