M20 Build Options - Please Advise

[Jon_E30_vert]

Hello All - new to the forum but long time BMW enthusiast. Have owned E46 M3, E92 M3, F82 M4, F85 X5M, E30 325i and enjoyed all of them. I have a stock 1993 325i convertible with approximately 95k miles on it. I did a complete refresh when I purchased it a year ago, new suspension down to all bushings, new brakes, refreshed and flushed all fluids, etc.

I just picked up a spare complete block with an “e” 885 head (so I was told). I want to build a new engine for a swap next year, I am leaning towards 2.8 stroker with forced induction. I have tried to read through many (I mean many) of your posts on this forum and it seems like there is some great advice and experience here, so I am hopeful I will receive some help on deciding what to build.
My initial goal of the build is to be 400+ whp and my budget is flexible, what I mean is I want to do it right but done want to spend $8k+ on the build as I was quoted from an engine builder. This car is my daily driver in the summer and I want it to continue to be an excellent driving car (idle through full power). Hope that makes sense. I am not interested in engine swap away from the M20.

What would you suggest is the “best” build? Stroke size? Piston? Crank? Turbo? Head configuration?

I have looked through many of your posts and builds, but could not find a direct answer to my questions.

Thank you all in advance for your help!

[JehTehsus]

I don't think there is any practical way to meet your power goals on an m20 without going turbo.

What are you looking for in terms of reliability? You say 400+ whp initially? I am going for a turbo m20 build that will see a bit of track time and a tiny bit of street time, with an aim for around 500whp, that I am fine to fully tear down and change bearings, inspect etc every year or two if needed.

I am into it for well over 8k. That said, you can certainly build a 350whp turbo m20 without going too overboard.

[Jon_E30_vert]

Thanks for the feedback JehTehsus.

I do not mind going to a forced induction (turbo) motor.

[ForcedFirebird]

A stock m20b25 can easily make 400hp with a turbo. Just get the head to stay down with studs.

[Gus]

Curious to know - why is 400hp a goal? Personally for what you describe (vert, street, someone else doing the engine build) - I'd go for a 240hp 2.8 and call it a day. Sounds great, plenty of power for the street, close enough to stock the other parts of the car don't need doing, way less hassle...still M20. or 2.9 if you want to do custom pistons. If you use the car currently, try keep it on the road and enjoy it. 400hp sounds like a lot of heart break to play out over the next 5 years. I can only tell you what I would do in the same situation so do whatever you can afford and makes you happy.

[Jon_E30_vert]

Curious to know - why is 400hp a goal? Personally for what you describe (vert, street, someone else doing the engine build) - I'd go for a 240hp 2.8 and call it a day. Sounds great, plenty of power for the street, close enough to stock the other parts of the car don't need doing, way less hassle...still M20. or 2.9 if you want to do custom pistons. If you use the car currently, try keep it on the road and enjoy it. 400hp sounds like a lot of heart break to play out over the next 5 years. I can only tell you what I would do in the same situation so do whatever you can afford and makes you happy.

Thank you for the feedback Gus, I do appreciate it.

What is the direction I need to take to get to 240hp 2.8l? Stock pistons with M52 crank? (is that right?) What about the head and cam? Headers? Intake? I am going back and forth about to move to forced induction or keep it naturally aspirated. I like the idea of staying NA because most of the car can remain (controls, rear diff, etc. - I have read in some builds where people get into control issues and I'm not sure I am ready to take that hurdle on) I have a spare engine, so I can do this build over time while the current engine is still running okay, I think it needs some head work.

BTW - I would like to do the engine build, minus the machining since I do not have that capability in my garage.

Again, thank you for your feedback, I do appreciate it.

[Jon_E30_vert]

A stock m20b25 can easily make 400hp with a turbo. Just get the head to stay down with studs.

Thank you for the response FF, it seems like you have a lot of knowledge about these engine builds. As I said to Gus above, I am back and forth on NA vs FI.

The goal of 400whp was just the fun factor, but maybe Gus is right and I just need to build a nice NA for the car. Again, I have gone back and forth about this in my mine many times. I enjoy the car now, which from an engine perspective is stock other than a Stromung exhaust and free flowing exhaust system with stock headers and a SSSQuid chip.

So to reset my goals (maybe)
1) Build the engine myself - I am an automotive engineer with background in cars, building and fixing cars etc....but I have never built an engine from 'scratch'
2) Increase performance (I was thinking 300+hp and stated a goal of 400 because using FI that seemed reasonable and well to be honest fun)
a) Gus' comment made me rethink it a bit though. I do like drive-ability and want it to be drive-able as a daily
3) Minimize the investment, mostly due to being my first engine build and I am trying to approach with some caution.

[JehTehsus]

If you do go the FI route, as noted it can hit 400HP, which if you look around most people will agree is a nutso power level in a car with the weight and setup of an E30. Based on the research I have done this does not need much else except for head studs and some new gaskets (I would certainly refresh most of the rest of the engine while in there, like timing belt, all gaskets, etc.), a new engine computer (there are plug and play megasquirt options that would run this, I believe in the $5-700USD range) and almost certainly a different clutch. I would personally do this if I was looking for a relatively inexpensive but powerful M20 build, it definitely yields the most bang for your buck.

Stroking and machining work add up quickly. Doing a bottom end rebuild in your garage (plus external machining work) will also be expensive and take a while, and may not buy you much of anything if rebuilding with stock internals (except peace of mind). Getting above 200WHP on a NA M20 seems to be an expensive endeavor. I am not even sure what mine will make pre-boost, but I will be happy with 200 at the wheels, and extremely happy with 220, and this is with oversized valves, porting, and new pistons and internals (2.9L stroker). If I switched to ITBs I would probably see an easy 220 at the wheels, again, based on just my casual research. Strictly speaking my motor is also setup to run E85, but I am not sure there will be any point in doing so NA with my compression ration (~9.8:1).

[vWBIRDWv]

if you want to make over 200Hp NA on a m20 ... You better be ready to sell your soul ;)

[vWBIRDWv]

It is the cheapest to go FI to get the power you want... just dont expect ultimate drive-ability..for about 3,000$ you will have 400 HP it will cost more than that to make 200NA.... you decide

[digger]

Normally you choose FI or NA and build to suit but since you aren't after anything extreme I'd get some better 24V rods, a set of forged slugs and m52 crank as foundation for the build that works for both NA or FI. Don't go lazy on the CR maybe 9.5:1 is a good balance that will work with either config.

[ForcedFirebird]

I'd get some better 24V rods.

Aside from weight, how are 24v rods "better"?

[McGyver]

Aside from weight, how are 24v rods "better"?

Donor engine had +12 valves. Pair that with some silicone coolant hoses and you're looking at a solid +5 hp to the wheels.

[AWDBOB]

Meh on forced induction. Turbo cars are never "done". It's a constant fiddle, which is fine, and can be rewarding. My vote goes to N/A for a simple car you can enjoy with relatively low hassle and good power. If you do go FI, you don't need 400hp in an E30. Some head studs and an MLS gasket on an m20b25 with an efficient snail goes a long way. Tune well and you don't need a built bottom end for that.

Alternatively, m20 stroker motors get pricey, very fast. To make power from an m20 the "stroker configuration" is only part of the equation, as a ton of your power will come from the upper half. And you can EASILY have $3K+ in the cylinder head alone, just to make 200-220whp. Not to mention the bottom end.

But regarding the bottom half, an N/A m20 build has many "tiers", before you even get into FI.

-M52b28 crank with 130mm rods and "i" pistons is a "budget" 2.8L, but requires machine work to the counterweights and/or piston skirts, plus crank snout adapter, plus slight block deck, aka $$ in machine work and extra stuff, but using cheap used parts. Still 84mm bore


-M20b27 crank with 130mm rods and "i" pistons is a similar compression 2.7L and doesn't involve any of the machining that the above does outside of decking the block 2mm, and uses cheap-er used parts. Still 84mm bore


-M52b28 crank with 135mm rods and the 2.9L IE "ultimate street stroker" Mahle pistons works well, but then introduces high piston costs among other things


-But then s52b32/m54b30 crank works without modifying counterweights, so if you're already going to spend $1200 on pistons, it makes sense to use an s52 crank instead for more stroke and use that $1200 and get custom slugs made to fit your exact application for the same price.


So, either go big or go small. A healthy 2.7L with the above formula is proven to make a healthy 180/180 by ForcedFirebird, and most of the other random configurations above that cost more only make ~20hp/tq more and cost 6x as much.


The stock intake manifold sucks, so to extract solid power out of the above options, ITBs should be on your list, which is another large sunk cost. The bigger the motor, the more important the head work and supporting mods will be.

MY VOTE:

Build a simple 2.7L per the above, but assemble with ARPs and an MLS gasket. Get your car running really well on Megasquirt N/A, and once you're tired of it, go turbo.

[digger]

Aside from weight, how are 24v rods "better"?

better clearance with a stroker which matters if more than 84 stroke, stronger atleast M50NV rods are, even S52 rods are probably stronger out of plane, granted either would work for 400 but id future proof the bottom end so even some decent but still el cheapo aftermarket rods for like $600 e.g. Molnar if theres any chance of wanting more down the line