Planning an engine build, new injector and exhaust ?'s page 2

[Reichart12]

Ok so as the title says, Im planning an engine build into my 1987 325e. The car does not need it but its something Ive always wanted to do. Still saving up and researching so I don't expect this swap to start until about January. Some background... I feel like I have the know how to do this and with the help of my dad and some friends it shouldn't be a problem. This car is my daily driver so it will stay streetable and reliable. But I would like to take it to a track day or autocross next year.

My options are 1. e bottom I top. 2. all I. 3. (distant thought) m30.
So I'd like to stay with the m20 and stay N/A. Originally I was thinking the e/I combo would be best but now that I realize I would build the entire engine out of the car then put it in, would the combo be better than rebuilding a b25?

My budget right now would be around 5,000 and that would include getting parts, machine work (not done by me), and maybe a transmission (definitely new clutch and fly). I want the engine to have internal work done to increase the displacement for either option. So I guess I'm starting to ramble on. Before I start planning my parts needed list my main question would have to be...

An e block with I head as a 2.8/2.9 or a rebuilt b25 with parts to increase that displacement, which would make more sense practically? Is the e/I best when adding on to a b27 in the car already or is it even worth it when an entirely fresh engine is being swapped?

Any guidance or assistance would be greatly appreciated. Thanks

[digger]

forget about the E/I combo anything that doesn't have decent compression and matched piston / chamber is not worth rebuilding for a NA performance rebuild

IMO get another cheap b25 and do a 2.8L with M52 crank, B25 pistons, ETA rods, 885 head. take your time and swap in when finished.

alternatively an eta crank, b25 piston, ETA rod and decked block and adjustable cam gear which is similar to the 2.8L but a few less cubes and slightly lower compression but all goes to together a bit easier as you dont need a crank sleeve of get the counterweights/ piston skirts machined.

anything more requires custom pistons which might be out of your budget

[Reichart12]

Thanks digger! That really helps knock some sense into me to stay away from the combo. Definitely leaning to the b25 2.8L. Now to read up some more on that.

[digger]

Thanks digger! That really helps knock some sense into me to stay away from the combo. Definitely leaning to the b25 2.8L. Now to read up some more on that.

i should add the E/I combo works great if starting with an eta and just slapping on an i head on since anything is better than an eta but its hardly better then a b25 so doesn't make sense to spend alot on a missmatched rebuild as the BFYB isnt there IMO

[Reichart12]

Yeah I think originally I was all about the combo because Ive had an E for the past 2 years. But now that I would build the engine on a stand, youre right it doesn't make sense.

Ive been trying to look it up, do you know of any threads or info that talk about how much of the skirts would need to be machined?

[redlightpete]

The US 2.8 cranks seem to have larger counterweights, and would require too much modification of the skirts. You'll need to get the crank machined down. Measure your actual crank and pistons, but mine required cutting .20" off the radius of the crank. You should probably rebalance it afterward. Using the 2.7 crank avoids this, but you need to deck the block, as mentioned earlier.

[digger]

about 5mm machined from the crank iirc, must use short skirt pistons

[Reichart12]

Ok thanks, Im thinking its going to be best to get all the parts together first then take them to a shop to be fitted and machined as a whole. For the pistons, the short skirts ones are straight across the bottom as opposed to rounded, or can it only be told by measuring?

[digger]

im pretty sure the shortest skirt ones are not flat but radiused/profiled to clear the web of crank but with the radius skirts there appears to be a long and short version. its the skirt length directly below the piston boss that matters most, as opposed to the overall length at the thrust and non thrust sides

[16vCorey]

I just finished building one, and you need the short skirt pistons, but they still need to have the skirts cut down. The counterweights of the crank need to be trimmed down .25"-ish, the block decked .5mm, and you have to use eta rods. After that, it all fits like a glove.

PS - The reason you have to use the short skirt piston instead of just machining the skirt off of a long skirt piston is because of the way the part of the piston directly under the wrist pin is cast. The long skirt piston has a flat spot cast into it there that rubs the crank, and I was scared to take too much off of the crank. If you cut it down too far it won't balance.

[Reichart12]

I'm getting pretty good on everything that is needed. By any chance do you have a photo of the pistons you used or a parts number? I think shopping for the piston is the only thing that's an uncertainty as of now, but I still have a while before this is going together.

Thanks again for the help.

[whodwho]

If you are coming from an 2.7 eta motor you will probably be happy with just going with a fresh b25 and that is ~50hp additional gain and factory designed and reasonable to build.

The additional gain from a b25 to a 2.8 is going to be maybe ~20ish hp(can be more depending on $ invested) and will take more $ in additional parts and machining to make that happen.

If you are willing to invest the additional money and effort the 2.8 is a good way to go.

[16vCorey]

There are a few different piston designs for the M20B25. This pic shows an aftermarket, eta piston, "i" short skirt, and "i" long skirt. There's actually another type of "i" long skirt available. You want the third one from the left, and you still have to trim it down some.


This pic shows why you can't use the other ones and just trim them down. The long skirts have this raised spot (circled in red) that will hit the counter weights on the crank.

[Reichart12]

Awesome, that second picture is what I needed. That really shows why it wont work. Thanks a bunch. Im glad I can learn something, having just graduated college I have no school to look forward to this fall so its nice to be learning about engines (yes I liked school, yes I know that's weird).

I like driving my e, I don't think its a bad engine having never driven the i though. With updated suspension parts its fast enough on twisty roads without the chance of speeding too fast on straight roads lol. I want to do a stroker swap to be able to say Ive done it. I could do m50 or whatever but that lacks the challenge and stigma I think this has.

[16vCorey]

I'm glad to share the info, so someone doesn't have to learn it the hard way! It kind of sucked to spend the time to machine the pistons down, only to have them rub and have to scrap them!