Stroker and swap advice

[Rdannyr]

Need some opinion.
so I have a 91 325i with a g260 that I just re-sealed and swapped in but my headgasket just went out on my m20 so I'm going 24v. Now here's where I'm undecided. I have a whole e36 323i with a m52b25 and I believe a getrag 5 speed which neither seem ideal. I also have a complete m52b28 that is my first choice to put in. It was pulled from a junk yard and I have no idea if there is anything wrong with it. This didn't matter much to me because it was cheap and it was getting stroked anyways. For the stroke parts I purchased a complete m54b30 long block and planned on using the whole rotating assembly. Unfortunately I just finished the tear down of the m54 and piston #6 was damaged and the rod was bent.
My questions are

Can I still use the crank other Pistons and exhaust cam as they all still looks good? And can I buy just 1 good piston/rod set to replace the bad one?

Can I use the the m52b25 and stroke that because it only has about 70k miles and I know it's good. Will it be the same power output with m50 manifold, exhaust, cams, tune? It's basically the same engine I believe just choked up. Or should I use the b28?

Which transmission would be preferred the g260 or the e36 getrag tranny?

Also lastly any recommendations for cheap machine shops in SoCal/sgv?

I've done a lot of research but there seems to be a lot of different opinions on different forums. Any input helps thanks guys!


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[CMBusch]

Not sure how much I'll be able to help, but I'll give it a shot.

First things first, you wanna stick with the G260. The G250 will break with abuse from even a M50. There of plenty of aftermarket options for using the G260 with a 24V swap.

Regarding the engines, do you plan to do an entire rebuild?
IIRC the M52B25 and M52B28 are mostly the same except for stroke and pistons.
Couldn't you use the crank and cams out of the M54 in either M52 block with the stock 135mm rods? Then you'd only need to buy just one piston. Either way, It's all dependent on whether or not the M54 crank is trashed.

[NufnSus]

Can I still use the crank other Pistons and exhaust cam as they all still looks good? And can I buy just 1 good piston/rod set to replace the bad one?

Can I use the the m52b25 and stroke that because it only has about 70k miles and I know it's good. Will it be the same power output with m50 manifold, exhaust, cams, tune? It's basically the same engine I believe just choked up. Or should I use the b28?

Which transmission would be preferred the g260 or the e36 getrag tranny?

If you're stroking the M52 using the M54B30 crank and pistons, you can re-use the M54 rods, as well as the M52B28 rods - both are 135mm c/c length.
I don't know the rod length of M52B25 engines, but would assume they are longer (or the piston tops thicker), since the bore of both blocks is identical @ 84mm

You can buy just one piston and rod, assuming all other rods are straight, and all other pistons are undamaged, inc. the ring seats.

However, the M54B30 is notorious for its issues relating to harmonic vibrations caused by the long stroke setup of the B30 crank; 84mm bore vs 89.6mm stroke
Therefore, the M54B30 rotating assembly should be balanced before install, ESPECIALLY if a different (i.e. not from the original assembly) piston is added to the mix.

M52B25 isn't a 'choked up' version of the M52B28, it's a smaller displacement engine (2.5l vs 2.8l) with a short stroke configuration. IMO, the square stroke config of the B28 is much better suited to the application, which I assume is fun road car/ AutoX.

Definitely use the G260 over the G250

[pazi88]

However, the M54B30 is notorious for its issues relating to harmonic vibrations caused by the long stroke setup of the B30 crank; 84mm bore vs 89.6mm stroke
Therefore, the M54B30 rotating assembly should be balanced before install, ESPECIALLY if a different (i.e. not from the original assembly) piston is added to the mix.

The harmonic vibrations are nothing that can be fixed with balancing. It's about how long the stroke compared to the connecting rod length. Only thing that can reduce the vibrations, is to use longer for example 140mm connecting rod with custom pistons.

Also the crank for example is balanced by itself. Not with other parts. So you can freely swap parts without worrying about balancing. Of course all the pistons need to be same weight (from same engine).

Here is something about stroke/rod ratio: http://www.superstreetonline.com/how...-stroke-ratio/

[NufnSus]

The harmonic vibrations are nothing that can be fixed with balancing. It's about how long the stroke compared to the connecting rod length. Only thing that can reduce the vibrations, is to use longer for example 140mm connecting rod with custom pistons.

Also the crank for example is balanced by itself. Not with other parts. So you can freely swap parts without worrying about balancing. Of course all the pistons need to be same weight (from same engine).

My point is that installing unbalanced components onto a crank that is renowned for causing harmonic vibration issues due to...

...the long stroke setup of the B30 crank; 84mm bore vs 89.6mm stroke...

...is an inherently bad idea.
Of course, a balanced assembly won't eliminate the issue all together, but it certainly shouldn't make it any worse, and after all, balancing is 101 of engine rebuilding.

Indeed, there's no issue with swapping a set of balanced rods and/or pistons with another set of balanced rods and/or pistons onto a balanced crank. Maybe my post wasn't quite clear on that.
However, my point to the OP was that simply adding a new/ used piston to an existing set WITHOUT balancing them first is exactly how you get that crank singing.

*edit - looks like you edited your post pazi; seems like we were on the same page after all :up:

[Rdannyr]

I see thanks. I probably purchase another parts block then. I think I’m going to just throw in the b25 as is for now so I can take my time building the stroker from the b28 block properly. Thanks guys


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[digger]

The harmonic vibrations are nothing that can be fixed with balancing. It's about how long the stroke compared to the connecting rod length. Only thing that can reduce the vibrations, is to use longer for example 140mm connecting rod with custom pistons.

Also the crank for example is balanced by itself. Not with other parts. So you can freely swap parts without worrying about balancing. Of course all the pistons need to be same weight (from same engine).

Here is something about stroke/rod ratio: http://www.superstreetonline.com/how...-stroke-ratio/

its really little to nothing to do with rod length as that is a 2nd order effect, you could use longer rods and youd still have the same issue as its really the increased stroke thats driving it. its just that long strokes pretty much always have lower rod stroke ratios due to a fixed deck height that is a constraint on most engines. this confuses the internet and people think this is the cause rather than the effect

the lower ratio does increase side loading but more friction comes from the longer stroke (higher piston speed) than additional angle.

With long stroke the masses and pressure pulses act further out from centreline and torsional stiffness is lower so it gets excited more at lower rpm which happens to be within the operating rpm of the engine. A shorter stroke engine never really sees the problem rpm as its out of steam by the time it gets to the problem rpm.

the main issue with long stroke cranks seems to be the oil pump nut but this is exacerbated by lack of tensioner and why even with uber $$$ ATI parts some still have issues.

i would go long stroke all day unless you were going to see sustained high rpm at track upon which there is no advantage for stroke anyway

[pazi88]

And somehow s50b32 and s54 have even longer stroke and they don't vibrate oil pump nut loose and rev even higher rpm. (Those have 139 mm connecting rods ;) )

[digger]

And somehow s50b32 and s54 have even longer stroke and they don't vibrate oil pump nut loose and rev even higher rpm. (Those have 139 mm connecting rods ;) )

rod stroke ratio on s54 is basically the same

139/91 = 1.527
135/89.6 = 1.507

if you think that difference is the reason you should actually do some calculations and see how little the the rod length alters the displacement/velocity/acceleration vs crank angle and also how little it affects the rod angularity

they have a tensioner for the oil pump drive

http://www.realoem.com/bmw/enUS/showparts?id=BG92-EUR-06-1996-E36-BMW-M3&diagId=11_6119

http://www.realoem.com/bmw/enUS/showparts?id=BG93-USA-02-1997-E36-BMW-M3&diagId=11_6017

http://www.realoem.com/bmw/enUS/showparts?id=BL93-USA-01-2003-E46-BMW-M3&diagId=11_1579

plus they probably have a damper on the front of the engine tuned for the higher rpm. the way the pump drive comes of the snout means it better integrated

they also have a stiffer crank due to the larger main and rod bearing overlap

there are probably other change that were designed specifically for the higher rpm application

[digger]

lets look at some rod angles firstly

S52B32
Stroke = 89.6 mm
Rod Length = 135mm
RR = 1.507
Peak Rod Angle = tan^-1(stroke/2/rod length) = 18.36 degrees

S54B32
Stroke = 91.0 mm
Rod Length = 139mm
RR = 1.527
Peak Rod Angle = tan^-1(stroke/2/rod length) = 18.13 degrees

That’s a difference of 0.23 degrees

The S52b32 would need a rod length of 136.9mm to achieve the same angle

So let’s look at what happens to displacement velocity and acceleration with the 135mm rod and 136.9mm rod at 7000rpm

Displacement


Velocity


Difference in peak value 34.62m/s for shortest vs 34.57m/s for longer rod. So a 0.15% change

Acceleration



At TDC it is 3,268 g vs 3,257 g which is 0.3%
At BDC it is 1,640 g vs 1,651 g which is 0.7%

You can barely distinguish between the lines the changes are insignificant

Now let’s compare the 89.6mm stroke vs 89.6mm stroke but a 10% increase in rod length, so 135mm vs 148.5mm.




still the changes are pretty small but atleast you can see them and you really arent able to use a rod 148.5mm long in an engine without drastic measures

Now let’s compare the 89.6mm stroke vs 98.56mm stroke 10% more but let’s keep rod ratio the same





It should be pretty clear that the stroke has much more influence than the connecting rod length on the rod angularity, piston displacement, piston velocity and piston acceleration. Any minor changes in rod length are a second order effect.

Also the the s54b32 isnt better at high rpm because it uses a rod a few mm longer...