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E46 M3 (2001-2006) Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006.


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Old Tue, Nov-03-2015, 02:32:28 AM   #31
bimmerdriver
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Originally Posted by digger View Post
It doesn’t have the same peak speed. It has the same average (MEAN) speed

Longer Rod:
The piston dwells longer at TDC,
lower peak acceleration at TDC
Lower peak velocity (occurs approx. 70* ATDC) .
Around BDC is where the long rod “catches” up for lack of better term, you might say it slows down quicker

ill plot graphs later on to show im not making this up
While you're at it, can you confirm if the peak piston velocity is always less than the angular velocity of the crankshaft? I think so, but just wondering. I used to have a good paper about this, but can't find it.
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Old Tue, Nov-03-2015, 02:41:18 AM   #32
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Originally Posted by Drewster View Post
The key point of the derivations is that rod length changes *where* peak acceleration occurs - you cannot plug in 90* to your sine/ cosine equations and solve (as some on the web try)

However, MAX acceleration should still work out to be something like
(RPM^2 * stroke / 2189) * 1.333

In other words, yes, rod length will change the velocity curve, but after derivation the maxima should be close to the above.
peak acc occurs at TDC always, at BDC the peak (there are two) are just either side due to camel hump and will be less than that at TDC in practice. with an infinite rod length peak acc at BDC = peak acc at TDC

Last edited by digger; Tue, Nov-03-2015 at 02:45:35 AM.
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Old Tue, Nov-03-2015, 02:44:29 AM   #33
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Originally Posted by bimmerdriver View Post
While you're at it, can you confirm if the peak piston velocity is always less than the angular velocity of the crankshaft? I think so, but just wondering. I used to have a good paper about this, but can't find it.
that question doesnt make sense. can you explain what you mean further?
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Old Tue, Nov-03-2015, 03:08:24 AM   #34
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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that question doesnt make sense. can you explain what you mean further?
The angular velocity of the crankshaft in ft/s is 2*Pi*Stroke*RPM/60 where the stroke is in ft. It's an approximation that the peak piston speed is equal to the angular velocity for a "very long" connecting rod, but I'm wondering if it's possible to show that the calculated value is always less than or equal the angular velocity. It's been a long time since Physics 100. Since you said you could graph the values, I was wondering if you would check.
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Old Tue, Nov-03-2015, 03:12:00 AM   #35
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Originally Posted by bimmerdriver View Post
The angular velocity of the crankshaft in ft/s is 2*Pi*Stroke*RPM/60 where the stroke is in ft. It's an approximation that the peak piston speed is equal to the angular velocity for a "very long" connecting rod, but I'm wondering if it's possible to show that the calculated value is always less than or equal the angular velocity. It's been a long time since Physics 100. Since you said you could graph the values, I was wondering if you would check.
angular velocity of the crank is nothing more than 2*pi*rpm/60 (in radians/s). its directly proportional to rpm. i think you are thinking of something else besides angular velocity of the crankshaft.

edit: i think you mean the surface/tangential speed of the crank pin Centre which is angular velocity x 0.5 * stroke (i.e throw). ill take a look later, i think it will be true

Last edited by digger; Tue, Nov-03-2015 at 03:21:38 AM.
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Old Tue, Nov-03-2015, 04:25:02 AM   #36
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Originally Posted by digger View Post
angular velocity of the crank is nothing more than 2*pi*rpm/60 (in radians/s). its directly proportional to rpm. i think you are thinking of something else besides angular velocity of the crankshaft.

edit: i think you mean the surface/tangential speed of the crank pin Centre which is angular velocity x 0.5 * stroke (i.e throw). ill take a look later, i think it will be true
Yes, that's what I meant. (It's been a long day.)
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Old Tue, Nov-03-2015, 05:52:19 AM   #37
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Default Re: REM (ISF) Microfinished S54 Crankshaft

That is very cool. This is going to be an awesome motor build.
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Old Tue, Nov-03-2015, 08:29:58 AM   #38
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Yes, that's what I meant. (It's been a long day.)
So lets take a stock S54 stock 139mm rod at 8000rpm vs no change other than 150mm rod. i chose 150mm so the differences can be seen

0* and 360* = TDC
180* = BDC



longer rod dwells more at TDC and less at BDC





longer rod has lower peak piston speed





longer rod has lower acceleration at TDC but slighty higher around BDC



longer rod reduces peak rod angularity from 18.1 to 16.9*

Peak piston speed is the same as the crankpin speed for very long rod.


moral of the story is a minor change in rod length doesn't produce vastly different piston motion.
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Old Tue, Nov-03-2015, 12:28:17 PM   #39
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Default Re: REM (ISF) Microfinished S54 Crankshaft

Alright, well since you guys are in physics mode maybe you can answer this question for me:

The machine shop wants every last component that bolts to crankshaft to complete dynamic balancing. I'd like to go with the twin plate clutch from Max PSI, though I really can't squeeze it out of the budget until spring. That being said, if the shop is balancing the crank using the stock DM flywheel, clutch & bolt weights, how severely will I throw off the final balance when I eventually go with the new clutch?
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Old Tue, Nov-03-2015, 01:29:11 PM   #40
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Default Re: REM (ISF) Microfinished S54 Crankshaft

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Alright, well since you guys are in physics mode maybe you can answer this question for me:

The machine shop wants every last component that bolts to crankshaft to complete dynamic balancing. I'd like to go with the twin plate clutch from Max PSI, though I really can't squeeze it out of the budget until spring. That being said, if the shop is balancing the crank using the stock DM flywheel, clutch & bolt weights, how severely will I throw off the final balance when I eventually go with the new clutch?
First off, kudos on the shop wanting every component on there for balancing! I'd think that's a good sign.

Personally, I would wait until you have everything on there. As I said in another thread (to much debate), an inline 6 is a very balanced configuration. *However* the problem comes in when you've got a crank that's reaaally long, and you put a varying load on both ends.. and you run it up to 8,000 rpm

Unlike other engines that aren't balanced internally, there shouldn't be anything you need to bolt onto the crank to *balance* it externally, but you do need a damper to keep those oscillations in check. Looking at the graph here, you want to avoid the big, scary sine wave https://en.wikipedia.org/wiki/Fileamping_1.svg from this article https://en.wikipedia.org/wiki/Damping. Since the crankshaft is a constantly moving object under constant stress and strain, you really want to avoid those oscillations. Hence, the thing bolted on the accessory side is a *damper*, not a harmonic *balancer*.. although multiple people vehemently disagreed with that last bit, so I'll leave that grain of salt there. You don't read about snapped cranks too much with an OEM damper in good condition, but I've read about/ heard people who "upgraded" the damper and lost a crankshaft/ engine... I guess that's my really verbose way of saying to make sure the stock damper is used and to not to screw with the damper as long as its in good shape.

Even though the engine is an ideally balanced configuration... you still have the issue of some stuff going up and down while other stuff goes round and round. A normal part of balancing is using precise bob weights for rotating weight and reciprocating weight.. which would be thrown off by going back and reducing the rotating weight later. Something I'm curious about and you might ask them is "overbalancing". Essentially you use a little more than the normal 50% reciprocating weight to make sure the engine is balanced at high RPM.. but not necessarily outside of the desired operating range. If you're going to be racing it and spending the vast majority of the time at 7900 RPM rather than 900, it's supposed to help reduce the higher RPM oscillation/ vibration and save wear on components.

Point being - using a lightweight assembly at one end rather than what the engine was set up for could theoretically make things behave very differently than the stock mass. If you're spending the money to do it right, wait until you can let them do everything right.

Last edited by Drewster; Tue, Nov-03-2015 at 01:52:40 PM.
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Discussing REM (ISF) Microfinished S54 Crankshaft in the E46 M3 (2001-2006) Forum - Engine: S54 - Max Hp: 333 hp at 7,900 rpm / 262 lb/ft at 4,900 rpm
Total Produced: 45,000+ - Years Produced: 2001 to 2006. at BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)