M20 stroker questions?

[whodwho]

From an S50, 89.6 is S52\M54B30

[Hooffenstein HD]

From an S50, 89.6 is S52\M54B30

Doesn't the S50 have much larger big ends? Never mind. I realise you meant USDM spec, 3.0 "S50".

When using cranks with such a long throw, like the 85.8 and 89.6mm, is longevity of the engine greatly effected by decreasing room for rod length and piston compression height? I've read people saying their budget 2.8's with 130mm rods don't last very long due to the poor rod ratio but with the 84mm stroke, there is still room to use longer rods and have a decent comp height piston.

[SkiFree]

In part yes. Especially on the 89.6mm the piston skirt drops out of the bore a decent amount and then gets rammed back upwards into the bore. I tend to see more skirt wear with the guys running the 135mm rods and 89.6 crank than any other combination (other factors being the same).

I have had to have the wrist pin encroach the oil ring with the 85.8 crank / 135mm rods when I spec for a healthy ring package, especially if the guy wants to leave enough meat on top to opt for the offset dish.

Now, to be fair if people are willing to compromise on relative longevity (lets say 60k service life on a non-race-spec 2618-alloy street NA 3.0L/3.1L setup) then they will find some extra ponies. Whether or not it is worth it is subjective to your goals.

[Kotty30]

So if I use the eta crank my best bet is to use the eta rods as well?


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[Kotty30]

Thanks for all the info everyone!


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[SkiFree]

Well no .... Aside from giving you a theoretical increase in torque the longer rod length necessitates a shorter piston. A shorter piston means less reciprocating mass which means a faster rev speed (just like lightening your crank or flywheel).

If you were already planning on using the 81mm stroke crank and going with custom pistons then there is no decent reason not to run the 135mm rods.

[Kotty30]

Well no .... Aside from giving you a theoretical increase in torque the longer rod length necessitates a shorter piston. A shorter piston means less reciprocating mass which means a faster rev speed (just like lightening your crank or flywheel).

If you were already planning on using the 81mm stroke crank and going with custom pistons then there is no decent reason not to run the 135mm rods.

So with the 81mm crank, 135mm rods and custom Pistons is that still good for a turbo down the road or is that increasing my compression too much? If not what's my compression looking like at this point?


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[SkiFree]

You will spec what compression you want when ordering the pistons.

[digger]

Doesn't the S50 have much larger big ends? Never mind. I realise you meant USDM spec, 3.0 "S50".

When using cranks with such a long throw, like the 85.8 and 89.6mm, is longevity of the engine greatly effected by decreasing room for rod length and piston compression height? I've read people saying their budget 2.8's with 130mm rods don't last very long due to the poor rod ratio but with the 84mm stroke, there is still room to use longer rods and have a decent comp height piston.

the reduction in compression height is not an issue, you will need to use an oil ring rail on the big crank but this is not uncommon and not an issue. the important thing is the pistons are machined with appropriate shape to account for operating conditions, rings are installed properly and the block bores properly sized, honed with torque plate to correct finish etc

the 84mm stroke 130mm rod pulls the pin down the bore further than the 89.6/135 crank but slightly less angularity. the 89.6mm still has 1/4" of bore below the bottom of the pin at BDC. you usually worry when the pin starts coming out the bore and i would get a skirt coating

here is a 84 stroke 135 rod (89.6 stroker will come down 2.8mm further)



here is a m54b30 (89.6/135)



dont expect to run as many miles as a stock engine before rebuild but thats a given

[Kotty30]

Does all go together pretty smoothly? Am I gunna need to make any fab? Is there any common problems I may run into?


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[digger]

81mm fits without issues with normal rods, bigger cranks need a sleeve for oil seal, and different crank bolt length and may require clearancing in the block for the crank counterweights. the 89.6mm crank may require clearancing of the aux shaft and bottom of bore so the rod has clearance, but depends on the rods