Suggestions for piston upgrades

[GarbageCanE30]

Hey everyone, after my last post and measuring with a micrometer, my old pistons aren't worth putting back in the car but my b25 block can easily be remedied with a hone or a bore job. The hone is only about 60 bucks less then boring it to a 2.7 which brings me to my first question... Would you recommend just getting it bored? This motor will be used with a turbo. Next, what pistons would you recommend for a 2.5/2.7 build? It seems like buying OEM pistons new is about as cost effective as getting a set of wisecos or something similar. Let me know what youre thinking or if ya got any pistons for sale haha. Thanks

[ForcedFirebird]

2.7 comes from the stroke, not bore. New OE pistons are pretty reliable, we use them often in builds here at work.

[McGyver]

2.7 comes from the stroke, not bore. New OE pistons are pretty reliable, we use them often in builds here at work.

How would you deal with using a 81mm crank to get to 2.7L and an 885 head to retain good flow, cam, and dual valve springs? Also, OP intends on going FI using a Dinan Turbo kit with MegaSquirt2.

- 130mm eta rods with seta pistons would be ideal, but seta pistons are impossible to find (from my searches)
- 130mm eta rods with eta pistons yields low compression and poor off-boost performance
- 130mm eta rods with eta pistons and a decked head or block restores compression with minor loss of displacement, but requires messing with the cam timing
- 130mm eta rods with i pistons and a decked head or block restores compression with minor loss of displacement, but requires messing with the cam timing
- 130mm eta rods with custom forged pistons

[ForcedFirebird]

How would you deal with using a 81mm crank to get to 2.7L and an 885 head to retain good flow, cam, and dual valve springs? Also, OP intends on going FI using a Dinan Turbo kit with MegaSquirt2.

- 130mm eta rods with seta pistons would be ideal, but seta pistons are impossible to find (from my searches)
- 130mm eta rods with eta pistons yields low compression and poor off-boost performance
- 130mm eta rods with eta pistons and a decked head or block restores compression with minor loss of displacement, but requires messing with the cam timing
- 130mm eta rods with i pistons and a decked head or block restores compression with minor loss of displacement, but requires messing with the cam timing
- 130mm eta rods with custom forged pistons

81mm crank, 130mm rods, stock compression-height b25 pistons, 2mm off the block deck. They do quite well...


https://www.r3vlimited.com/board/for...52#post7038352

If you scroll through that thread, I have several dyno plots for various strokers, turbo, ITB's etc. Several were built at work.

[McGyver]

81mm crank, 130mm rods, stock compression-height b25 pistons, 2mm off the block deck. They do quite well...

Thanks for posting that, I've definitely read through it at least once before (have it bookmarked in my Turbo folder).

New custom forged pistons are $900-$1100 ish for Ross, CP, JE, etc.

New 84.48 mm b25 pistons run about $550 and a nuke adjustable cam is $190, so $740. (I get that the adjustable cam isn't required, but I assume the engine would benefit from it.) I assume decking the block would be at least $150, taking the total to $890.

Why use stock pistons and deck the block instead of custom forged on the 130mm rod when forged should only be $200 more?

[ForcedFirebird]

6 one, half a dozen the other. Stock pistons have shown they hold, some have gotten close to 500whp with them. Why go through the extra expense, as well as having all that piston to bore clearance? All motors technically can benefit from adj cam gear, so really you are down to comparing just the pistons. Typically you have the block decked in a true rebuild anyways - won't be much, if at all more to shave it .040" instead if .004.

Shaving cost would depend on location etc. We charge $100.

[digger]

Why use stock pistons and deck the block instead of custom forged on the 130mm rod when forged should only be $200 more?

and if you get custom pistons why would you use a 81 mm stroke crank when bigger ones are $100-300 more?

[E30 Wagen]

Why use 130mm rods when he can use the 135mm he already has for a better rod ratio. Get a 81mm crank and some off the shelf low compression pistons from VAC or somewhere. No skimming the head or block. Simple.

[McGyver]

6 one, half a dozen the other. Stock pistons have shown they hold, some have gotten close to 500whp with them. Why go through the extra expense, as well as having all that piston to bore clearance? All motors technically can benefit from adj cam gear, so really you are down to comparing just the pistons. Typically you have the block decked in a true rebuild anyways - won't be much, if at all more to shave it .040" instead if .004.

Fair enough, i appreciate all the info. Per your dyno charts, the engine should make 170-180 whp/wtq at around 8.8:1 CR. Sounds fun, even without boost. dvallis' dyno chart gives an indication of what to expect with boost on a stock piston. Cost savings and less piston-to-bore clearance, while still being safe for the power goals, seams like a no-brainer.

Next question, any suggestions on picking up a set of stock m20 over-sized pistons? AutohausAZ has Mahle second overbore for $90.69/piston. Just get any Mahle piston from any vendor, or should I be looking for something in particular?

and if you get custom pistons why would you use a 81 mm stroke crank when bigger ones are $100-300 more?

I kind of got backed into that decision. My #4 connecting rod bearing ate itself. I impulsively bought a seta engine for the crank and rods and hoped the pistons would still be good. I'm pretty sure the pistons are going to be toast, so now I want to make the best of my 81mm crank.

[digger]

Why use 130mm rods when he can use the 135mm he already has for a better rod ratio. Get a 81mm crank and some off the shelf low compression pistons from VAC or somewhere. No skimming the head or block. Simple.

you wouldn't use 130 mm rods with custom pistons. i would also use OE pistons and 130 mm rods over the VAC forged random domes made from 2618. forged pistons need to have the correct dome and be from a material suited to the application.

[GarbageCanE30]

6 one, half a dozen the other. Stock pistons have shown they hold, some have gotten close to 500whp with them. Why go through the extra expense, as well as having all that piston to bore clearance? All motors technically can benefit from adj cam gear, so really you are down to comparing just the pistons. Typically you have the block decked in a true rebuild anyways - won't be much, if at all more to shave it .040" instead if .004.

Shaving cost would depend on location etc. We charge $100.

This was certainly a lot to take in, as I'm still just getting in to the fine ins and outs of engine building, but I thank all of you guys and everyone on r3v for that matter for being the most helpful community I've ever experienced.

With that being said firebird, man I wish I was local to you as I'd probably just have you sort this out and pay you handsomely for it. Unfortunately, there aren't any master m20 builders around me and the machine shops alone are few and far between.

I've found a "built" complete motor with ARP studs installed and a freshly rebuilt head for sale for around 600 bucks somewhat local to me, and am hoping I can take that route to save myself quite a few bucks and a lot of time.

The head job alone, assuming I don't have any bent valves, and with how this has been going thus far, is extremely unlikely, will be close to 400 bucks alone. With the pistons coming into play, that brings me near if not well over $1000 depending on if I end up going OEM or a forged aftermarket variant. I still have yet to consider the ARP headstuds in that price and whatever else pops up along the way.

With that being said, unfortunately and unsurprisingly this project has already ended up being a nightmare due to a dishonest previous owner who managed to get one over on me on a car he had listed as a "9/10." The biggest thing for me despite speaking of price earlier, is time. I'm not sure how long I will have access to the garage my car is currently in pieces in, and time is of the essence unfortunately. With all of these set backs, I need to save time where I can, but really want to do this build right and build an absolutely killer m20. I can't really spend the time hunting down healthy seta pistons as they seem to be a highly contested piece of equipment on the market nowadays.

If you were in my position, what would you do? Obviously buying a complete "turbo ready" motor would be the most ideal situation for me, but honestly I don't really trust anything I haven't seen with my own eyes. The idea of basically starting from scratch and essentially having a zero mile motor is a super attractive one to me, but I need to weigh that with the time it will take me to get the parts I need to do it right.

[ForcedFirebird]

We need to stop confusing the community and just slap a real m20 compression graph as a "sticky”. 81mm crank with stock compression height, 2mm off block = 9.4:1 m20b27. Not to be confused with m20b25 "885" head on a stock b27 rotating assy

[ForcedFirebird]

GarbageCanE30 please feel free to email: John@m20guru.com.

[LowR3V'in]

We need to stop confusing the community and just slap a real m20 compression graph as a "sticky”. 81mm crank with stock compression height, 2mm off block = 9.4:1 m20b27. Not to be confused with m20b25 "885" head on a stock b27 rotating assy

this is with everything. cuts down on people going through thread after thread of garbage.
straight bullet point facts.

[McGyver]

We need to stop confusing the community and just slap a real m20 compression graph as a "sticky”. 81mm crank with stock compression height, 2mm off block = 9.4:1 m20b27. Not to be confused with m20b25 "885" head on a stock b27 rotating assy

It would be awesome if you or Digger could put that together, especially if it detailed and had part numbers for the different piston/conrod/crank combinations. I've been trying to go by the info on the e30zone wiki, which has this chart:


Semi-related, are these irregular groves from kissing the exhaust valve?




Any word general words of advice on using these pistons?