Pistons for m20 Stroker

[digger]

im using 135mm rod and 46.9mm throw but 20mm pin to fit

[digger]

the calculation is relatively straight forward

CL = rod length - throw - CW - Pin/2 - throat

[bmwmaster81]

Thank you for calculation digger. Seems to be tighter than I expected but schould be enough.
The squish on oem m20 is also 1.2mm without Problems. I will check if the tolerances are right an when everything appears equal between the different cylinders I think it schould rotate free also unter high rpm.


Because of the long stroke I also wouldn't rev more than 6500 rpm.

[digger]

by my calc

pin is 22 mm
throat is 5.5 mm
rod 133 mm
CW R70.5 mm
throw 44.8 mm

then the clearance is 1.2 mm to the pin boss.

[2mAn]

doesn't IE has more or they are out?

Im sure they have more, but the price is different when bought new from IE VS in the r3v classifieds. I just waited too long because I really am nowhere near close to actually building a 2.8L M20

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[ForcedFirebird]

The s54 has notches in the bottom of the cylinder bores in the block, and notches on the piston skirts to accommodate bob weights. The biggest trick in a short rod:stroke engine is to keep as close cylinder/piston boundary as possible, maximize the piston skirt. Side loads are high and rotational clearances get tighter. TDC dwell has become near non-existent. Now we are talking about stuffing that in the shorter m20 block with more primitive valve train.

[zaq123]

so I just found out who bought the 85mm Mahle pistons I was trying to buy :hitler:

doesn't IE has more or they are out?

[bmwmaster81]

I think there should be enough clearance to the clyinder walls. I had aleready a lot of combos in the m20 block and most of them with oem pistons and rods. They are much beafier and you get interference probs much earlier.

The Bore is little bigger, the rods are smaller on the relevant edge and the piston skirts are much shorter to leave room to the counterweights.

The crank, rods and pistons will be fully ballanced so if there is a problem i can let the shop knife edge the counterweights to get clearance.

@digger
i don't get custom pistons because i think the MS pistons are an good allround package.
I also would be happy to combine them with the 84mm crank, so if the 3051ccm plan doesn't work I will build the 2860ccm engine.

I have both cranks here, so when I get the pistons i will put them with the 84mm crank in the block (which has to be bored first) and see the tolerances especially sqish and true CR. If there is enough room, I will order the 133mm rods.
Have already a good ported head with race seats and rhd ITBs, and also going standalone. The Target would be about 240 bhp I think.

With the 11 to 11.5 cr im not sure if the engine will be knock limited already?
Also I don't want to throw a to big cam in it because its for street use.

[2mAn]

so I just found out who bought the 85mm Mahle pistons I was trying to buy :hitler:

[digger]

Depending on rod should be able to clear bottom of cylinder especially with overbore , at worst slight chamfer required

The 89.6 crank has smaller counterweights radius by about 2mm so it depends how close the piston Is to the smaller crank and how much margin designed in , my gut tells me the piston and piston Pin boss will just clear crank Don't have envelope or pen to do a 2min calculation . If not can be fixed anyway.

With all these potential stuffing around it's a wonder you don't just get custom set to of Pistons to suit

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[zaq123]

Actually not.
133/89.6 will be 0.2 mm higher in the cylinder than 130/84 at BDC.

We are not talkimg about height. His crank will move rod futher left/right at 90' crank than 84km crank. This wi'll move rod closer to the bottom edge of the cylinder where the interference usually occurs

Sent from my VS985 4G using Tapatalk

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