M20b31 build questions

[CanadianBen325IS]

Hello everyone!

First off, I'd like to thank everyone as, while I have not been a member myself, I have always been able to find fantastic information here for any troubles I have with my car. Unfortunately, I've hit a rather large snag now, and I would like to ask for more personal help.

I have a 1987 325IS with around 500,000 kms on it, and we believe it has finally given up on us. As a result, my father and I are intent on rebuilding the motor, but if we're doing this, we may as well do the motor I've always looked at: a 3.1L stroker. This is where I need help though, as I know that this has been done before, but most threads I find are a few years old, and I'm sure options on the market have changed. I have always heard to start with a 325e block and just reuse my 325i head to get the 2.7 stroker out the gate, but by boring it to 3.1, I was hoping for recommendations on which additional parts to use (ie pistons, cams etc). The car is my summer daily and is autocrossed as well just for context.

Also, I know that there will be the comment to swap the motor for an m30, m50 or a s52, but I must admit, I love the idea of keeping the original motor and making it so much more while keeping as much of a "stock" look as possible. Not that it's a bad idea, just not what I'm wanting right now.

Thanks in advance for all your help!

[jeffnhiscars]

Others will weigh in I'm sure but I'd say that the options have been quite thoroughly fleshed out and that there isn't much to improve upon from the tried and proven internal upgrades you've read about.

[digger]

You can't bore it to 3.1L using the eta crank. The I block or e Block will net the same capacity.

You must use the 89.6mm stroke crankshaft and bore it to 86 for just over 3.1L or 85 if you want a bit under 3.1L

[downforce22]

You can't bore it to 3.1L using the eta crank. The I block or e Block will net the same capacity.

You must use the 89.6mm stroke crankshaft and bore it to 86 for just over 3.1L or 85 if you want a bit under 3.1L

nothing more than what has been stated by the one an only digger

/thread

[CanadianBen325IS]

You can't bore it to 3.1L using the eta crank. The I block or e Block will net the same capacity.

You must use the 89.6mm stroke crankshaft and bore it to 86 for just over 3.1L or 85 if you want a bit under 3.1L

Thank you guys for your responses! So just to make sure I have this right Digger, I can keep the i block and bore it to the 3.1, then the Ireland Engineering stroker kit with larger injectors and an ECU remap, is that correct? I must admit, this will be our first full engine build, so we are novices in this regard, so I apologize if any of my questions seem fairly basic. :)

[digger]

Yes m20b25 (I) block then bored and stroked.

I recommended s52 rods or molnar aftermarket for NA engines as they are compact minimising interfences.

You also need a sleeve for the front oil seal and slightly modified ( slimmed down centre section) intermediate shaft

[CanadianBen325IS]

Thanks Digger! Your help is greatly appreciated! Since it looks like you have this motor as well, what's your opinions on the feel of it? By any chance have you had it on a dyno?

[digger]

If you approach it from looking at a street build to get lowend and midrange gains it's great.

If you expect it to inherently make a lot more topend hp you will be disappointed as that's not how it works.

If you want to make a lot more power then you need bigger valves, cylinder head port work, different camshaft, delete standard intake manifold and aftermarket ecu and the costs skyrocket

[downforce22]

If you approach it from looking at a street build to get lowend and midrange gains it's great.

If you expect it to inherently make a lot more topend hp you will be disappointed as that's not how it works.

If you want to make a lot more power then you need bigger valves, cylinder head port work, different camshaft, delete standard intake manifold and aftermarket ecu and the costs skyrocket

Great low end torque on mine, power falls off as you rev above 5000 rpm with stock 885 head. I'm running a turbo so it helps a bit on the top end.

[CanadianBen325IS]

That sounds like exactly what I'm looking for, so I'm even more excited to start this build! I've always considered low end more enjoyable anyways, so that's perfect. Have either of you noticed any quirks or things to keep an eye out with these motors as 3.1s? Both during the build and after living with it would be appreciated!

[digger]

Don't use generic flat top style Pistons. Use an OE replica and 4032 alloy for the street. I'd ditch motronic and get a aftermarket ECU and a proper tune

[SkiFree]

I'd suggest looking into the 2.9L kit. Once you go with the 3.1L along with the OE-dish you start getting a piston design that isn't going to be super reliable. If you guys are ok with going 30k-40k and needing a rebuild, then go for it.


Here is the engine checklist of the 2.9L system I developed when working at IE.

https://adamsautosport.com/information/tutorials/bmw-m20-ultimate-street-stroker-engine-checklist/

Also, here is an "intro" so-to-speak for cylinder head first-timers. It's geared toward the M10 engine, but the principles are the same.

https://adamsautosport.com/bmw-m10-cylinder-head-rebuild-introduction/

[digger]

i can give my personal experience, mine has done close to 100k now and been running for almost 12 years. for 11 of those it was a daily driver. i'm using a set of MM rally pistons made by wiseco from 4032 alloy. i am using a 138 mm rod but the extra few mm does very little to changing the rod angularity that people seem to pay more attention to than they should as its a factory configuration. i would use a 135 mm rod with 89.6 mm.

Compression is 180-185psi with a decent cam (would be closer to 200 with more mild cam) and the blowby meter on the dyno barely registers.
i never have to top up the oil due to any kind of burning and the plugs have no signs of oil. the only time i had an oil issue was when i accidentally partially crimped off the valve cover breather and that the ports a little oily.

Also when setting up the cold start on my standalone i had it way way too rich for a few years so its had less than easy life.

Don't get me wrong there is alot i would improve with the pistons i have and how it was setup but if youre needing a rebuild after 30-40k something was incorrectly machined or the wrong parts were used or spec'd for the application. race pistons for the street what can go wrong....

these days you can run:
- OE dome for proper combustion ,
- 4032 low expansion better wearing alloy
- non-rail for the oil ring on a 89.6 mm stroke engine.
- if youre worried about skirt wear there are numerous coatings.

youre not going to get anywhere near stock mileage as many do quite alot more than 200k+ on stock bottom end but you will get plenty of mileage done properly.

its like everything engineering its all in the details

[SkiFree]

Coming from my experience of speccing hundreds of M20 stroker kits and getting feed-back for a variety of clients/machine-shops.... while it's all in the details, to extrapolate one expert's experience as the baseline for an entire market demographic is a recipe for a bad time. Especially when that expert does not offer the engine-formula in kit-form for sale. This would constitute as questionable data at best.

I greatly respect your opinion, but it's one thing to say you've done it, and another telling someone who has never built an engine before that he can do it as well (and expect similar results).

The 2.9L kit I've listed is largely an off-the-shelf setup that balances "safe" features with performance. The "safe" features address where I've found issues to arise (again, pooling from an aggregate of machine shops. Not using one very-experienced builder as the baseline).

I'd love to see a direct product listing to the piston featuring all the specs you describe. Frankly, it sounds awesome. If this is indeed where the market is, then there should be a direct link, not a "custom"-spec piston.

[digger]

Coming from my experience of speccing hundreds of M20 stroker kits and getting feed-back for a variety of clients/machine-shops.... while it's all in the details, to extrapolate one expert's experience as the baseline for an entire market demographic is a recipe for a bad time. Especially when that expert does not offer the engine-formula in kit-form for sale. This would constitute as questionable data at best.

I greatly respect your opinion, but it's one thing to say you've done it, and another telling someone who has never built an engine before that he can do it as well (and expect similar results).

The 2.9L kit I've listed is largely an off-the-shelf setup that balances "safe" features with performance. The "safe" features address where I've found issues to arise (again, pooling from an aggregate of machine shops. Not using one very-experienced builder as the baseline).

I'd love to see a direct product listing to the piston featuring all the specs you describe. Frankly, it sounds awesome. If this is indeed where the market is, then there should be a direct link, not a "custom"-spec piston.

One data point proves it can be done though . the Pistons I have are nothing special they were developed along time ago. I'm sure the supplier of mine would have many similar examples of engines achieving good mileage. I'm not saying that the engines you are talking about did not require a rebuild but you've posted the reasons before in the thread on the 2.9 piston development . I'm guessing most of the 3.1L kits have the same underlying design so eould behave similarly.

It's a custom piston 4032 Made by JE with modern total seal conventional rings and slightly smaller pin. It would probably go for roughly the same price as the 2.9 depending which boxes you tick. Whether it's a off the shelf or custom is largely irrelevant This is more about what's possible.

Almost all places will still try and flog you a 2618 race piston so if people haven't done research then they will end up with something with large compromises if it's a street engine.