Help with Calculations

[Massimo]

Hi All

Trying to find what formula I should use to calculate the force apply on a connecting rod on the exhaust stroke, as I believe that is the stroke that applies the most force to the conrod.

my goal is.

I have just picked up a stroke crank for the M42, my plan is to run to 7000rpm and whan to know what conrod I should use
either a M44 or M42 rod.

My preferance is the M42 rod as it is lighter, but I am not sure if it can handle the increased stroke and rpm.

here are the details.

Stock 6500rpm
Stroked 7000rpm

M42 Stock Stroke 81mm
M47 Stroker Crank 88mm

M42 Piston weight 360g
Custom Piston 306g


I think that is all I need to calculate the stretching force applied to the rod.

[Dj Buttchug]

m42 rods are forged. id use those over m44 rods any day

[Massimo]

m42 rods are forged. id use those over m44 rods any day

M44 are forged to from what I have read

[gte175x]

Shouldn't the load be calculated by max cylinder pressure vs. crank angle?

[Massimo]

Not sure hence why I am asking. Read somewhere that the exhaust stroke applies the most force because there is no counter force on the piston when it reaches TDC

[Dj Buttchug]

M44 are forged to from what I have read

coulda sworn all m44 was cast internals. hmmm

[Massimo]

look forged to me they have the parting line down the side. m44 are the second from the left.

Also I must have got mixed up the M44 rods are lighter then the M42 rods

[LJ851]

look forged to me they have the parting line down the side. m44 are the second from the left.

Also I must have got mixed up the M44 rods are lighter then the M42 rods

You can't see the parting line on the rods to tell if they are cast or forged. Take the same picture but turn the rods 90 degrees to show the parting line.

[varg]

You can't see the parting line on the rods to tell if they are cast or forged. Take the same picture but turn the rods 90 degrees to show the parting line.

Found this on someone's photobucket

[nando]

m42 rods are forged. id use those over m44 rods any day

all rods are forged, pretty much.

Personally I wouldn't worry about a 500rpm bump. I'd try to balance the pieces though, and use the lightest parts possible.

[LJ851]

Those rods look forged to me with the wide part lines.


In my opinion what you should be concerned with is piston speed. That longer crank is going to raise your piston speed for a given rpm.

There are general piston speed limits that should be considered. Unless those rods are known to be weak, i wouldn't worry about them and use the lightest ones as nando mentioned.

[audi90kid]

Please don't mind if I nerd out a bit.
There are two things you are looking at:
1. The force if the explosion on the piston putting the rod in compression.
2. The axial force around the crank due it its rotation.
3. The resistance of the piston drag at the different rpm speeds you listed.

Unless you really know what your doing this isn't going to be a simple hand calculation. Usually most parts when they are new have a safety factor of at least 1.5 meaning that they can handle 1.5 times there expected load before failure. Plus even if you were to spend all the time to develop the equations you still would not be able to find out the material properties needed to execute the calculations. Each manufacturer has their own set of material specs they shoot for in the range the BMW specs out.

Generally with building a motor go with the strongest components you can afford. With that being said the forged option will most always be stronger than the cast.


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[TobyB]

Yeah, the weak link in rods is usually the bolts on the exhaust stroke. That's when they let go.

I found with the M10 that it was easiest (and not so expensive) to go with an aftermarket
rod which had nice fasteners. The OE fasteners are good- up to their limits, and BMW's not
great about sharing. The aftermarket rod guys can tell you in a heartbeat what you
can spin with their rods and usually ARP bolts.

yes, it's money up front, but compared to the cost of a rod coming apart, it's almost free.

t
running Pauter rods in the M10...