I currently have a stock 323i engine wise, besides the "Johnson box" 02 sensor & catalytic converter from when it was imported to the US back in the 80s. It's currently very run down and it finally deserves a proper rebuild.
I have in my garage a stock low milage 2.7 ETA bottom end with crank, rods, pistons, etc.
My goal is to build an N/A inline 6 engine that is smooth running, likes to rev, and gets at least 200ish crank horsepower. I like the idea of building my original engine, or at least the top end. In my opinion it is a less boring option than a 24V swap.
Also, it'd be nice if it passes the sniffer test here in California. Probably not the visual, seeing as I want headers and aftermarket cats, but not so hot that it's impossible to pass the sniffer.
To accomplish this, I was thinking M50 or S50 rods, custoshowbox speed test
m pistons, high compression with a medium-mildish cam. Then port my 731 head and rebuild it with 325 valves and valve springs.
To complement this, new injectors and a motronic 1.3 conversion, b25 throttle body, possibly with an MAF conversion.
Does anyone have any thoughts on this? You think it's a little overboard? Any critical pieces I'm missing?
Thanks in advance for any advice or comments.
M20 Engine build - advice appreciated!
Moderator: martauto
IMHO don't do a 24v conversion. A 323i chromie deserves much better than that
Interesting sounding build, but not sure which way you are thinking of going? Are you planning to use the 2.7 eta bottom end 731 head? This sort of thing has been looked at before, have a look back through the past posts. Search for the user on here "Reggid" and then search his posts for "731" it will throw up some good discussions like this one
viewtopic.php?f=2&t=113922&hilit=731
He usually puts into discussions like this but not here that often
Problem is getting 200bhp with a 731 head is going to be difficult especially using the B23 inlet, a few who tried something similar only managed about 180 but a nice-driving 180. Alpina only got 210 with the 885 head using all the low-hanging-fruit gains available.
A flat top 2.7 engine with 135mm rods, custom flat top pistons and heavily worked 731 head sounds interesting but it would struggle to keep up with a similar engine with 885 head and B25 inlet with a lot less build effort. However you would keep the looks of the old B23 with 30 more bhp and a lot more torque.
For pistons talk to Steve Nelson at TopEnd, for an M20 he knows what can be done
https://www.racetep.com
Unless your 731 is ported exceptionally well its not going to make much if at all more power than a stock b25. The stock 731 head is better than the 200 ETA head but it is still fairly rubbish once you start throwing some stroke into it it simply chokes too easy in stock form.
If you know someone with proven porting skills and results with the 731 head its an entirely different matter as the ultimate potential between 885 and 731 are essentially the same its just how much effort and skill required to get there. The 885 out of the box is closer to the mark IMO for a 200bhp combination
It does cost decent $$$ with a 731 to go with the needed bigger valves, possibly larger seats, the amount of grinding and the usual refresh items and so on adds up, but that's what it takes to make decent power and make it worth the effort.
There are some 885 build options that may be "simpler" in many respects, but unless that's a realistic option for you no point outlining those.
Also the 323i is a nice revvy engine that you could build on those strengths if that's the character you like
If you know someone with proven porting skills and results with the 731 head its an entirely different matter as the ultimate potential between 885 and 731 are essentially the same its just how much effort and skill required to get there. The 885 out of the box is closer to the mark IMO for a 200bhp combination
It does cost decent $$$ with a 731 to go with the needed bigger valves, possibly larger seats, the amount of grinding and the usual refresh items and so on adds up, but that's what it takes to make decent power and make it worth the effort.
There are some 885 build options that may be "simpler" in many respects, but unless that's a realistic option for you no point outlining those.
Also the 323i is a nice revvy engine that you could build on those strengths if that's the character you like
E30 325is with M20B31
I am currently doing an M20b275 stroker. I took a Super Eta bottom end and removed the original pistons and installed 9.4 compression euro domed pistons. I rebuilt a 885 head which was already matched to the ETA bottom end because it was a Super eta. Now if I didn't have the SETA it wouldn't have mattered as the bottom end of an ETA is the same ( pistons only differ - head also differs) I bored my block .50 over since my piston walls were scored. I decided to planning the block and head to increase my compression. But I won't get into tolerances etc.
As the others have discussed and given their input. You would ideally be better off getting a set of pistons to match a 885 head since the machining required to rebuild a 731 will for sure outweigh the costs of just finding a 885. it has the bigger valves - more vol to the combustion chamber - 7 oil journals and a higher rev cam. there isn't much to do to that head unless you want to port match it to the intake. Take all the b25 325 top end components as mentioned inlet (intake) TB, DME and wiring harness - if needed. also the injectors. optional is headers as well as a tune..But you might get away with just a performance chip.
Since the super ETA motor was only built for one year will be hard to source those 8.5:1 comp pistons if you so desired. There are many options. the m20 is a great motor and contrary to what people might say about their weakness - it begs to be asked if they know how to take care of one. They are robust and reliable.
Another thing to consider is your budget. it's better to put money where it really matters then just throw money at the build.
You can do anything with enough money.
As the others have discussed and given their input. You would ideally be better off getting a set of pistons to match a 885 head since the machining required to rebuild a 731 will for sure outweigh the costs of just finding a 885. it has the bigger valves - more vol to the combustion chamber - 7 oil journals and a higher rev cam. there isn't much to do to that head unless you want to port match it to the intake. Take all the b25 325 top end components as mentioned inlet (intake) TB, DME and wiring harness - if needed. also the injectors. optional is headers as well as a tune..But you might get away with just a performance chip.
Since the super ETA motor was only built for one year will be hard to source those 8.5:1 comp pistons if you so desired. There are many options. the m20 is a great motor and contrary to what people might say about their weakness - it begs to be asked if they know how to take care of one. They are robust and reliable.
Another thing to consider is your budget. it's better to put money where it really matters then just throw money at the build.
You can do anything with enough money.