Pistons choices for s50 crank m20?

[88Sedan]

I've read every thread on m20 stroker builds as I will be building one for my street e30 to replace its' semi-tired m20b25. Those that have splurged for custom pistons seem to agree if you're going that far you should try and maximize engine displacement within reasonable spending limits.

I would like to use an s50 crank to avoid having crank counterweights machined like is required on most m52b28 cranks. I am aware the intermediate shaft may need to be turned down a bit and that the bottom of the block bores may need rod-clearancing. Since this crank is used less frequently in strokers I have some questions.

-Would I be sacrificing anything with regards to engine longevity/oil ring control by using 135mm rods? Will the very short piston/wrist pin height present issues for a street motor?

-Anything else to consider with short height pistons, in terms of drawbacks? I would only want to use 130mm rods if either they save me significant money on ordering pistons, or the taller pistons used would work better in the long run on a street motor.

My goals are essentially an awesome street engine with stock or close to stock redline but with tons of torque. I would really like to get a lot of miles out of this engine, and I don't need the motor to make peak power at 7k rpm...I'd be happy with peak around 5500-6500 rpm.

Essentially the build I want is to use s5x 135mm rods, s50 crank, custom pistons, 272 camshaft, and some form of standalone engine management. I want to do this on a budget as much as possible. I would have block bored over either .5mm or 1mm depending on block condition when I source an m20 block for this build.

I know 2.9L's wont make much power over a 2.7L with a stock 885 head at 6000rpm, but it really is torque that I'm after with this build. I would consider a larger cam, but being in a smog state eliminates that possibility for me.

[AWDBOB]

S52 crank is lighter than S50, if you're set on doing custom pistons, I'd go that route. I didn't have to clearance anything on my 3.1L regarding counterweights using the S52 crank. S50 counterweights are a few mm smaller.

I am not convinced rod to stroke ratio is as big of a deal as people make it out to be, regarding side loading, skirt wear, or oil control on a short skirt piston, but idk how to quantify that outside of my own experience with a 3.1L M20 using 135mm rods.

Shorter rod is worse for dwell and makes the angle worse for side loading the piston (and shorter rod may not clear the bottom of the bore?)

I used top end performance for my custom JE pistons and had them done in a 4032 alloy so I could gap them tighter than a 2618 piston.

You can do 85mm bore and get away with a stock gasket as stock gasket measures over 85mm. 86mm bore requires an 86mm gasket which is 5x the price.

Hot take: If you're limited by smog/cam/budget I'd just build a simple 2.7 using factory 84.5mm mahle pistons. They're $100 a hole vs $250 a hole (or even less if you use the Turkish yenmax M20b25 pistons). 2.7 crank/rods are practically free. If budget isn't much of an issue, and you plan on keeping the car for a while, custom pistons/s52 crank is the way to go if you want all the torques.

[88Sedan]

Thanks, I do want to keep this car for a long time. A close friend has a 2.7L/cam in his stripped e30 and while I like the cam profile of the 272, I still think the lower powerband could be better especially for street driving. My car is full interior, heavier being a sedan, plus I added braces and bashguard weight. That is interesting that the s52 crank didn't interfere with the intermediate shaft at all, also which rods did you use? Point me in the direction of your build information, I'd love to look at the fine print if its available.

[AWDBOB]

Thanks, I do want to keep this car for a long time. A close friend has a 2.7L/cam in his stripped e30 and while I like the cam profile of the 272, I still think the lower powerband could be better especially for street driving. My car is full interior, heavier being a sedan, plus I added braces and bashguard weight. That is interesting that the s52 crank didn't interfere with the intermediate shaft at all, also which rods did you use? Point me in the direction of your build information, I'd love to look at the fine print if its available.

The build is jumbled throughout this thread which is a mess, and starts around here: https://www.r3vlimited.com/board/for...1-lives/page43

M54b30 Crank/Rods
10.5:1 4032 JE Pistons
Ported 731 head with +2mm valves
284/280 IE Cam (non regrind)

What kind of 2.7L does your buddy have? We did a few comparisons between a friends 3L M20, my old 3.1L M20, and my 9.4:1 2.7L M20 on the same dyno.

The 3L wasn't much different at all than my 2.7L, and felt very similar on the street, but had flat top pistons which don't work well with an 885. My old 3.1L made a ton more torque than the 3L and 2.7L.

That post is here:

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/378740-forcedfirebird-s-m20-dyno-thread?p=10100642#post10100642

I made the mistake of using a ported 731 head with a very small combustion chamber on the 3.1L, which resulted in the custom pistons being made weird. I assume that's why the HP numbers were less than anticipated.

If I had to do it over again I would've used an 885 head and had pistons made with the dome profile to match the 885. The torque curve on that car felt great.

[88Sedan]

My friends 2.7L is an eta crank, eta rods and he ordered custom pistons through IE, .5mm overbore and 272 cam. Its pretty fun in a stripped car with no sunroof. My car probably is over 150lbs heavier than his and I want more torque available at lower rpms.

Are the m54b30 crank counterweights similar/same size as s52?

[AWDBOB]

My friends 2.7L is an eta crank, eta rods and he ordered custom pistons through IE, .5mm overbore and 272 cam. Its pretty fun in a stripped car with no sunroof. My car probably is over 150lbs heavier than his and I want more torque available at lower rpms.

Are the m54b30 crank counterweights similar/same size as s52?

Same size

[digger]

IMO on a stock head a m20b28 crank is the maximum that still results in a balanced setup that works with the stock intake manifold. The longer stroke pulls too hard on the stock ports and so you lose rpm range due to power falling off the cliff. If you have more usable rpm then you can run a shorter diff (if you don't do a lot of highway miles) and you're better off performance wise as the diff ratio gives extra torque e.g. 3.91/3.73. i ran a 3.91 for ages and it is fine

you shouldn't have any issues with counterweights as with custom pistons this can be specified, and most would normally clear.

135 mm rod is the way to go, OE from a M5x are good if they check out

Do IE still sell the Mahle MS slugs?

[88Sedan]

Idk which supplier IE used for my buddies' build, but I know he ordered them there and they were made to order for him and they are forged.

I am aware that anything beyond 84mm stroke means the top end won't breathe well enough to make more top end HP than a 2.8L at similar rpms. I'm really after torque and I believe this is the best way to achieve it.

According to the data in the forced firebird thread (I think that one anyway) the stock 'i' intake is actually oversized for the stock displacement so I assumed larger displacement would actually better match the C.S.A. of the stock intake. In terms of reading data that is available I have really tried to do my 'due diligence' to choose the right components for my build. That is in addition to people's direct experience.

Numerous people that have custom piston strokers say they wish they went for a little more displacement since they crossed that cost-barrier. Smog really limits camshaft choice for me, and cam suppliers and bimmerheads have told me they would not go past a 272 for any kind of smogged street motor because any more overlap really upsets the HC content of the emissions and makes the motors tune (in terms of tailpipe tests) much more difficult.

Having driven my buddies e30 with the 2.7L/272 cam I can say it could still use more low end torque but it pulls nicely mid to top end. Even if a 2.9/3.0L lost a bit up top, but was more torquey everywhere else I think I would be very happy.

And as far as the 84mm cranks go, I am sure I saw at least one example where the counterweights interfered directly with one side of the block. Not wanting to try and convince a machinist to lathe some mm off the CW's nor pay for it is why I'm avoiding those cranks. I also live in an area that is like a machinist-desert, the less I require of one the better off I'll be in the budget/time department.

[AWDBOB]

Do IE still sell the Mahle MS slugs?

they stopped making them for a while- you should do a run of them for use with S52 crank

Edit: looks like they’re back in stock

[AWDBOB]

Idk which supplier IE used for my buddies' build, but I know he ordered them there and they were made to order for him and they are forged.

IE typically uses Ross, and depending on when he got them, they could’ve been in their flat top stage where the dome profile wasn’t matched to the chamber of an 885, and thus were knock prone and inefficient. Hopefully that’s not the case but figured I’d mention it

[88Sedan]

Well if the general consensus is that I'm only sacrificing some rev-ability and not sacrificing longevity or oil control then I'm gonna plan on either s50/s52 crank build with s5x 135mm rods.

I already have a 3.73 LSD and tbh I think its a bit high strung already. After this build which won't commence for a while I want to trade someone for a 3.25-3.5ish lsd diff that isn't the viscous style. They are very hard to find, I know.

[digger]

Well if the general consensus is that I'm only sacrificing some rev-ability and not sacrificing longevity or oil control then I'm gonna plan on either s50/s52 crank build with s5x 135mm rods.

I already have a 3.73 LSD and tbh I think its a bit high strung already. After this build which won't commence for a while I want to trade someone for a 3.25-3.5ish lsd diff that isn't the viscous style. They are very hard to find, I know.

the shorter stroke will last longer and have better oil control all else equal, however, it is a factory configuration so provided the pistons are good quality suitable for the street, the machine work done correctly and the engine run in/tuned properly and maintained properly it will still outlast your needs.

[88Sedan]

Has any consensus been reached on which type of aluminum is best for street motor pistons? Many of the threads seem to show people having different experiences and opinions, and most folks havent purchased any more than 1 set so they cannot compare directly.

[digger]

cast are the best and 4032 have the closest properties to that.