Help with engine build

[e30davie]

Well there you go. Of course there are 3 different types of b25.. So how do we know the op has the 8.8:1 one? or does the US only get the 8.8:1 one?

[Lugnuts]

why the m20b25 is only 8.8:1 but the m20b23 is 9.5:1? (according to wikipedia...)

http://www.e30zone.net/e30zonewiki/index.php/M20#Pistons

Some versions of the M20B25 are 9.7:1.

[e30davie]

Whilst not entirely on topic, but does anyone have any good ideas as to why the m20b25 is only 8.8:1 but the m20b23 is 9.5:1? (according to wikipedia...)

Considering the b25 is supposed to be the sportier one?

[Lugnuts]

If you use the flat pistons and 885 head, you end up with a horrible 8.4:1

And a knock limited engine with lack-lustre performance. Go ahead, ask me how I know.

[ForcedFirebird]

Compression ring .017-.018", .019-.020" for the second.

[ak-]

I'd argue you won't need ARP mains or ARP con rod bolts if it's going to be the daily with occasional WOT fun.

Can someone comment on ring gap for m20 turbo's when using stock rings? Lets say 350-400hp.

[i_build]

I definitely don't have the budget to do it "properly" was thinking if I got over 4 years of 30 miles a day driving with a stock b25 and my tune I should be able to get at least that much with fresh rings gapped for forced induction new bearings and the b25 head with b25 rotating assembly. Didn't realize compression would drop that much with the b27 Pistons. Just don't have the funds for a new crank and Pistons. Best I can do is rings, bearings, arp fasteners, arp head studs and gaskets. Will measure the bores to make sure they are in spec and hone. Already checked block and head surfaces. Was hoping to ask about rings and gaps.

[digger]

Thanks. I'm doing a bunch of reading. It seems like it might be the way I want to go as I am reading that it will lower my compression and that's what I'm looking for with my turbo setup.

Like said people think they need lower compression but it's not really the case. It's done more for convenience to achieve the 2.7 with the better head. Most don't have the desire to do things "properly " because it costs more and takes longer hence the budget stroker term for the 325i head on a 325e short block

[ForcedFirebird]

The m20b25 is already "low" compression at 8.8:1. If you can use the 885 head and domed pistons, go for it, it always makes more power than 885 head and flat pistons. If you use the flat pistons and 885 head, you end up with a horrible 8.4:1

[i_build]

Thanks. I'm doing a bunch of reading. It seems like it might be the way I want to go as I am reading that it will lower my compression and that's what I'm looking for with my turbo setup.

[digger]

Can the m20 b25 head be used with the 325 block and rotating assembly? The top of the Pistons are fairly flat on the m20 b27 versus the Pistons on the b25.

yes it can, it is done quite commonly

[i_build]

Can the m20 b25 head be used with the 325 block and rotating assembly? The top of the Pistons are fairly flat on the m20 b27 versus the Pistons on the b25.

[SkiFree]

. Any suggestions for things I can do during the build that will give me the most engine life?

Beyond new half-over Mahle pistons, a fresh bore, and the other standard stuff? I'd consider a rebuild oil pump, have the machinist balance the rotating assembly, replace rockers/cam, etc.

Hope that helps.

[i_build]

Trying to figure out what the best options are. Was running the stock 325is m20b25 until I lost compression in 2 cylinders. After pulling the head I saw that one of the cylinder bores had a slight groove worn where it appears a ring gave way. Found an 86 325 engine with good compression. It's my first rebuild and from what I have read so far I'm thinking that I should be able to use the 86 block and my b25's head and rotating assembly. My goal is to put together a reliable daily driver. My stock engine and boost setup had lasted over 4 years including regular oil changes and an occasional race. Any suggestions for things I can do during the build that will give me the most engine life?