m20 stroker (using m51d25-e36 325td)

[harry02]

Hello group ,

Im new to r3vlimited, if i post in the wrong section please admin, feel free to redirect me ; already try to post yesterday but seems it didn't appear on this section.
Im Harry , driving 1973 bmw 02 , m20b25 swap ; my engine need a rebuilt so instead of just rebuild it , i will stroke it using m51d25 crank (84.2mm) as it is available here, M engine crank is almost impossible to find in this island.

I plan to use m20b20 130mm rods , and machine down the stock b25 piston to 14mm as said in other website, my question are :

1/ In another website, they said i need to deck down the block to 1.1mm; if my calculation is right i will have 11:1 CR , perfect but with 1.1m down my timing will be retarded , does 1.1m affect that much the engine , performance ?
2/ Is 280° camshaft good enough for 2.8L ?

Thank you

[digger]

firstly note that the m51d25 is a 82.8mm stroke not 84.2mm, once i realised this your numbers started to make sense

how big are the counterweights on that crank? i hope they arent too big being a diesel as it will make it harder to fit

nominally to achieve the same stack height as m202b5

135+34.2+(75/2) = 206.7

your stack height will be

130+34.2+(82.8/2) = 205.6

206.7-205.6 = 1.1 mm off the block

but best to measure the clearance between piston and head and optimize that for 1.00-1.10 mm clearance which might require slightly more than 1.1mm off the block

you wont have 11:1 CR it will be less than 10:1 closer to 9.5

ideally you should use an adjustable timing gear as the cam timing will be out a little

what 280 cam exactly? and "enough" for what? a 280 cam has potential to make quite a bit more power than a stock m20b25 cam

[harry02]

yes m51d25 is 82.8mm , sorry i mistype it. I dont know much about the counterweight size, what i know is the crank is "straight fit" to m20, just need to machine the piston skirt to -14mm, and oil spacer.

Please note that i have m20b25 eurospecs , 9.X CR , long piston engine.

The ''little out"" is it a problem if i want to drive the car for some months before i change the cam and the adjustable timing gear with it.
What i mean by enough is enough for 2.8L engine , trying to push 200Rwhp ?

My other plan mode are : m20b20 camshaft, M30 TB, AFM , Injectors , headwork and BBTB 6 branch , straight pipe. I will roll with this for a while then i will change the cam to 285/285 catcam.

[digger]

yes m51d25 is 82.2mm , sorry i mistype it.Please note that i have m20b25 eurospecs , 9.X CR , long piston engine.

The ''little out"" is it a problem if i want to drive the car for some months before i change the cam and the adjustable timing gear with it.
What i mean by enough is enough for 2.8L engine , trying to push 200Rwhp ?

My other plan mode are : m20b20 camshaft, M30 TB, AFM , Injectors , headwork and BBTB 6 branch , straight pipe. I will roll with this for a while then i will change the cam to 285/285 catcam.

ok so about 10.3:1 off top of my head would be the CR.

it will be perfectly fine to run the small cam without an adjustable gear if its retarded a bit there is no problem. the m20b20 cam seems to be advanced anyway so will compensate a bit

i dont think you will make 200whp maybe ~190+/- unless you do some cylinder head work or RHD intake kit

[harry02]

Thank you Digger ,

10.3:1 CR with -1.1mm on the block ? also do you think i should skimmed the head also, like 0.5mm on the block 0.5mm on the head , using stock 0.172mm headgasket.
Yes i will do a cylinder head work , RHD ITBs is out of my budget for the moment.

[digger]

those are approximate nominal numbers. you work with what parts you have and measure

i would make the head flat (cleans up), not take too much off though. a little bit of compression isnt worth much unless you are in a competitive form of racing where a % or so might matter

[harry02]

ok will do that , unfortunatly s/m52 crank is not readily available here, and there is no much m51d25 crank well documented.

[digger]

It's possibly to buy overseas, sometimes the shipping is not too bad even for heavy things.

[harry02]

Sorry what is the 34.2 number on your calculation ?

[digger]

Piston compression height (Distance from centre of the pin to the "top" of piston)

[harry02]

Talk with the guy in sweden who did the exact same stroker, he didnt mill the block.
Did i miss something ?
Intake close ?
Piston clearance ?
Top ring height ?

[Blach178]

i dont think you will make 200whp maybe ~190+/- unless you do some cylinder head work or RHD intake kit

sorry for interrupting.

Ive had the same crank in my engine aswell with low compression 8.8:1 pistons and there were no clearance issues what so ever. but well i didnt skim the block or head. so maybe thats why :P

but well now i got the new setup with det m52 84mm crank and the 9.4:1 prefacelift pistons in it block skimmed 0.5mm. Ported head, 284/272 cam, RHD ITB, bigger injectors and MSPNP2.
well im only getting 229/271 BHP :)
so my question what am I doing wrong ?
alot is talking about 200whp but im only getting 192WHP ?

[harry02]

Here also my thinking (maybe stupid ) if with 84mm they advise to 0.5mm "clean up " the block , for sure with 82.8mm crank you must skimmed it for 10 + compression ratio.

Using Digger calculation :

135+34.2+(75/2) = 206.7
130+34.2+(84/2)= 206.2
130+34.2+(82.8/2) = 205.6

m52 84mm crank : 206.7-206.2 = 0.5mm
m51 82.8mm crank : 206.7-205.6 = 1.1 mm

Best is to do it and physically check all this, but i need to take a decision before buying parts :)

Blach178 : If you , with RHD ITB you have this number , for sure i will have around 35/40hp less

[digger]

sorry for interrupting.

Ive had the same crank in my engine aswell with low compression 8.8:1 pistons and there were no clearance issues what so ever. but well i didnt skim the block or head. so maybe thats why :P

but well now i got the new setup with det m52 84mm crank and the 9.4:1 prefacelift pistons in it block skimmed 0.5mm. Ported head, 284/272 cam, RHD ITB, bigger injectors and MSPNP2.
well im only getting 229/271 BHP :)
so my question what am I doing wrong ?
alot is talking about 200whp but im only getting 192WHP ?

yeah i would have expected a bit more but it depends on the brand of dyno. nothing stands out as being off with the setup

[digger]

Talk with the guy in sweden who did the exact same stroker, he didnt mill the block.
Did i miss something ?
Intake close ?
Piston clearance ?
Top ring height ?

yes, somewhere you need to account for the piston postilion in relation to top of block (deck clearance). if the piston is 1mm below the top of block or 1mm above you need to input this

head is closer to 40cc not 42 from what ive measured. the gasket bore is 85.5mm, stroke 82.8mm, inlet close 58, top ring land 7mm, not sure the piston dome cc for those pistons