Okay to run original ETA pistons with 885 (i) head if I'm going to turbo?

[jc1000]

Hello! I have a 1987 325 with the M20B27 ETA. I am looking to make more power and have decided to try the stroker option plus turbo. What I am having a hard time deciding is whether it is worth swapping out the pistons. I would likely go with 325i pistons if I can find them (and have to change the rods) or possibly custom ones. This will be my first engine build, so I would like to avoid doing so if possible, but am also willing to prioritize it if need be. I am pretty new to the scene and have relatively low expectations and would also like to be as "economical" as possible. Thanks for your thoughts.

[It's Soda Not Pop]

https://strictlyeta.net/technical/328i/

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/243337-eta-to-i-head-swap-questions

Read through this and should be able to give you some insight.

[digger]

Add the b25 topend and induction and then boost it

[jc1000]

https://strictlyeta.net/technical/328i/

https://www.r3vlimited.com/board/forum/e30-technical-forums/engine-drivetrain/m20/243337-eta-to-i-head-swap-questions

Read through this and should be able to give you some insight.

Thanks. Yes, in that first source (strictlyeta), he doesn't seem to like the idea of keeping the ETA pistons. But I am hearing that a lot have done it and had good results.

[It's Soda Not Pop]

I think it's because the head doesn't quite match up with pistons. It will work. ForcedFirebird knows best about this.

[jc1000]

I think it's because the head doesn't quite match up with pistons. It will work. ForcedFirebird knows best about this.

ForcedFirebird would love to hear your thoughts!

[digger]

It will work fine, even though not the most ideal boost fixes everything.

[hasa]

Why not boost whole engine as it is? Is this a some oddball high compression engine which can't take 1bar/14psi unopened.

[digger]

An eta with 1b is still only like 240hp lol

[ForcedFirebird]

If you are starting with an eta car, adding the 885 is an upgrade. If you are starting with a b25, then it's best to keep it like that. Only in 1988 SETA, the 325e uses the same dome shape as the b25 pistons (that fit the 885 head properly), except with 8.5:1 static compression (b25 is 8.8:1). Don't get confused between the two.

I have posted many dyno plots in the m20 dyno thread. When putting an 885 head on a eta bottom end, you get around 135-140whp. A healthy b25 makes 155, rebuilds are 160-ish.

By using the b25 pistons, eta rods/crank and shaving the block 2mm puts you around 180whp. So, that in itself should show you the importance of piston shape in the m20. The 2.7i (885 head on flat pistons) and budget stroker both displace the same, but the 2.7i will have about 8.3:1 compression, the budget stroker is closer to 9.3:1. By using the proper shaped pistons, that 1:1 compression bump gives you ~40whp more.

[jc1000]

If you are starting with an eta car, adding the 885 is an upgrade. If you are starting with a b25, then it's best to keep it like that. Only in 1988 SETA, the 325e uses the same dome shape as the b25 pistons (that fit the 885 head properly), except with 8.5:1 static compression (b25 is 8.8:1). Don't get confused between the two.

I have posted many dyno plots in the m20 dyno thread. When putting an 885 head on a eta bottom end, you get around 135-140whp. A healthy b25 makes 155, rebuilds are 160-ish.

By using the b25 pistons, eta rods/crank and shaving the block 2mm puts you around 180whp. So, that in itself should show you the importance of piston shape in the m20. The 2.7i (885 head on flat pistons) and budget stroker both displace the same, but the 2.7i will have about 8.3:1 compression, the budget stroker is closer to 9.3:1. By using the proper shaped pistons, that 1:1 compression bump gives you ~40whp more.

Can you clarify the budget stroker for me? Why shave the block 2mm? We are talking about the ETA block, right? I thought the B25 pistons on the ETA rods/crank already stick out too much? Sorry, I realize these are newbie questions.

[hasa]

When considering that this is his first build, I'd minimise all risks and start from easy configuration.

99% of the success is made by selecting right turbo for the configuration, low boost, adequate fueling components (injectors & pump) and tune engine properly with good quality aftermarket ecu.

All the buzz about squish areas, piston shapes, piston brands, rod ratios etc is much less important and can developed later when you learn by doing. Otherwise you'll try way too complex setup and get lost in the details.

[mikey.antonakakis]

I’m currently using the eta pistons with 885 head. I live at high altitude so the deficiencies are more noticeable, especially with turbo. So I’m going to build a not-budget stroker, lol. And I wish I had used the “i” pistons from the start.

That said, I 100% agree with hasa in this case, and although I want a different setup now, that’s exactly why I went with that in the first place. I had to draw the line in project budget and scope to focus on other aspects, like engine management.

[jc1000]

I’m currently using the eta pistons with 885 head. I live at high altitude so the deficiencies are more noticeable, especially with turbo. So I’m going to build a not-budget stroker, lol. And I wish I had used the “i” pistons from the start.

That said, I 100% agree with hasa in this case, and although I want a different setup now, that’s exactly why I went with that in the first place. I had to draw the line in project budget and scope to focus on other aspects, like engine management.

mikey.antonakakis If I am able to source some i pistons, what would be the most straightfoward setup for using them? You mention you wished you had used the i pistons from the start. I am also at altitude, and don't want to go through all the effort to build an engine just to find it falling short.

[digger]

I second the keep it simple. Put the m20b25 head, topend gubbins and induction and boost it. Let sleeping dogs in the bottom end lie provided the comp test and leak down don’t show its munted. The 40whp gain FF refers to is along way from an apples to apples comparison. If this was a NA engine it would be a totally different approach.
also if building a stroker bottom end for boost a 2.7 with factory rods and slugs is not the best choice and savings are not enough to justify these budget items IMO