M20 stroker questions?

[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

[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]

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

[SkiFree]

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

[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]

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]

Thanks for all the info everyone!


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[Kotty30]

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


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[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.

[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.

[whodwho]

From an S50, 89.6 is S52\M54B30

[Hooffenstein HD]

What is the 86mm crank from? I thought 2.8 was 84mm then it went up to 89.6mm for the 3.0?

[digger]

it should be possible to use a 86mm crank without an oil ring rail with custom pistons and 135mm rod. without a good cylinder head you wont make anymore power just more bottom end and midrange

[mr2peak]

This is what I discovered in my research, YMMV.

If you use an ETA crank, you need to use 130mm (eta) rods. Moving up to a 2.8 with the M52 crank, stock 2.5i pistons are used with 130mm rods, bumping the pistons spec up proximately 1 point. If you use 135mm rods, pistons must be custom which adds a lot to the cost. If you're going custom pistons, you might as well go for an S50 crank. At this point, 135mm rods start taking away space for the ring lands, leading to excess oil consumption. Moving to a 3.2L crank, the piston starts to come very very far down into the block, and the sleeves move past the ends of the bores. This leads to excess piston skirt wear, which leads to bore wear, which leads right back to oil consumption. You also have to clearance a few places in the block when using a crank larger than the M52.

Many people end up parting out pieced-together stroker kits and going with an off-the-shelf kit for these reasons; those kits have been engineered to overcome some of those obstacles.

An M52 2.8L is hard to beat. It skips a lot of these issues, goes together with stock parts, bumps up the compression ratio and maintains an ok rod/stroke ratio. New stock pistons are a good idea, and maybe a hone, but every M20 I've looked inside of still had the factory crosshatching without any visible signs of bore wear.

In summary: If you want to BUILD a stroker, 2.8l is the best bang per buck option. Still not cheap.

[digger]

yeah, it makes sense to look for a set of 24V rods that are 135mm long though as they are lighter, stronger and more compact