133mm rods instead of 135mm in an m50b28 turbo build?

[Simpan304]

Hello there! I'm going to rebuild my m50b25 to 2.8 for my turbo build. The setup I'm have been planning to go with is an std m50b25 block, std m52b28 crankshaft and pistons. I was planing to use 135mm forged rods at first but then i found some 133mm forged rods for decreasing the compression. I have an std coopering headasket and arp head bolts so my question is would i get something good out of chosing 133mm instead 135mm and by that lowering my compression? My aim is around 500-600 hp.

I know I can go ful forged engine and that with original pistons may be pushing the last bit at so much hp but now I'm wondering over the rods

http://www.ebay.co.uk/itm/324330603563

Best regards.

[Liquidity]

Where did you get the idea that 2mm shorter rods are for lowering compression? Dude. That’s .080”. Your compression ratio is gonna be like 7.5:1. Shorter rods are for pistons with a taller compression height, not for lowering compression ratio. Always run the longest rods you can because they reduce side-loading of the piston against the cylinder wall. Short rods generate drag and consumes power. You control your c:r with pistons, head gasket thickness, and head combustion chamber volume.

[Eric93se]

Check out this site. HG has minimal effect.

https://www.torquecars.com/tuning/lo...sion-ratio.php

[Simpan304]

Where did you get the idea that 2mm shorter rods are for lowering compression? Dude. That’s .080”. Your compression ratio is gonna be like 7.5:1. Shorter rods are for pistons with a taller compression height, not for lowering compression ratio. Always run the longest rods you can because they reduce side-loading of the piston against the cylinder wall. Short rods generate drag and consumes power. You control your c:r with pistons, head gasket thickness, and head combustion chamber volume.

that's why I asked haha ;-) why do that kind of rods exist if they are such a bad idea to use, just read in the ad that you lowered the compression for turbo build with them that's why I got curious and wanted answer, and was curious why and what to go with when using the other 2.8l stuff, but guess I got my answer and going with 135mm then...?

Check out this site. HG has minimal effect.


https://www.torquecars.com/tuning/lo...sion-ratio.php

Thanks!

[Simpan304]

Does anyone know what the compression ratio would be with m50b25 block, b28 crank and pistons, 135mm rod? Then also with the same setup but with 133mm rods? Both with coopering headasket std measurements.

[gdavid]

Does anyone know what the compression ratio would be with m50b25 block, b28 crank and pistons, 135mm rod? Then also with the same setup but with 133mm rods? Both with coopering headasket std measurements.

The answer to your first question is 10.5:1, you effectively have an iron block m52b28 (like we got in the US). The position of the crank position sensor is the only difference between the US m50 and m52 blocks. A somewhat custom build is to put the internals of a m54b30 into a m50 or m52. It is straightforward.

I don't know the answer to your second question.

[importbanana]

Does anyone know what the compression ratio would be with m50b25 block, b28 crank and pistons, 135mm rod? Then also with the same setup but with 133mm rods? Both with coopering headasket std measurements.

does rod length influence compression ratio? I'm unsure actually. Technically it would, because the piston travel stays the same, but the room above the piston will be bigger, therefore lowering the compression ratio. I think there are better ways to lower the compression ratio.

[pbonsalb]

I don’t see why shorter rods would not work. Most people use a thicker headgasket or spacer, but the effect should be the same. That said, the most popular compression ratio for pump gas turbo M/S52 is 8.5 to 9.0:1. A double thickness headgasket or standard plus a shim equal to one gives 9.0:1. Stock thickness is about 0.070 or around 1mm and double about 0.140 or around 2mm. I would be concerned that 2mm shorter rods plus a standard gasket would drop compression too far. 1 mm shorter seems like it would be about right. I like the idea but have not yet read of anyone trying the rods. There was some discussion in the FI subforum last year about these. I think someguy2800 mentioned them but am not certain.

[carwiz008]

Does anyone know what the compression ratio would be with m50b25 block, b28 crank and pistons, 135mm rod? Then also with the same setup but with 133mm rods? Both with coopering headasket std measurements.

135mm rods = 10.0:1
133mm rods = 8.4:1

Head vol = 34cc
Displacement = 466cc
Gasket = 11cc ( .079 cooper ring)
Clearance = 6.5cc standard or 17.6cc with 133mm rods.

(H+G+C+D) / (H+G+C)

The answer to your first question is 10.5:1, you effectively have an iron block m52b28 (like we got in the US)

BMW rates the m52b28 as 10.2:1. The slightly thicker cut-ring gasket drops .2 points.