M20 2.7L Eta -> 3.0L Eta stroker idea

[ZeKahr]

Had an idea for a project I plan on doing a couple of years down the line after I get my 325e 4-speed auto mechanically sorted. The idea behind project is completely from how a 99% of the E30 community modifies their M20s, which is with an emphasis maximizing top-end power. Instead, I will be aiming to increase ONLY low-end torque as I use the car as a classic cruiser (that has better driving characteristics compared to anything else from its time) as opposed to something that I drive hard (which is why I picked the 325e with an automatic). I want the meat of my powerband to be at 3000 RPM and less since that is where the engine spends most of its time in regular driving (a logically-driven decision) as opposed to 5000-7000 RPM (a emotionally-driven decision)

The goals of this project are to:

• improve the part-throttle drive-ability of my car all-around with the increase in torque while keeping the 4-speed auto
  • Why not just swap over to a manual for better part-throttle driveability? Because I live in the USA, and as expressed in this Curbside Classic article about the 528e (which has the same engine), "BMW felt that a four was out of the question for the US. Automatics, AC, and stop-light grunt were all part of that reality". Here in America, the spirit is all about displacement, low-end torque, and laziness; forget about the screams of a tiny high-revving engine and the PITA of having to shift your own gears.
• reduce accelerating and cruising RPMs as low as possible with the torque boost, which translates to less internal friction, which translates to longer engine life and better fuel effeciency
• maximize the two main selling factors of the Eta: fuel economy and low-end torque

My plan to achieve these goals:

• Maximize the stroke of the M20B27 without increasing the bore to increase torque all-around (from this wiki, it seems that I will be limited to a max displacement of 3.0L)
• Maintain the Eta head, intake system, and exhaust system to maintain low-end torque (from my understanding, these parts are what allow the Eta to have a torque peak 1000 RPM lower than the 325i)

The parts I plan on using for this project are:

• M54 3.0L/S54 crank
• 325i pistons (may switch to aftermarket non-interference pistons which would have the added benefit of not grenading my engine should my timing belt snap, but will have to research into these options)
• 2.79 final drive and associated speed sensor (I believe that my car has a 2.93 being a 1986 model)

I am estimating that the following changes will boost low-end torque to around 195 lb*ft (I am basing this figures off of the fact that the M20 makes around 65 lb*ft/L). Here is my estimated rear-wheel torque comparisons (excluding drivetrain losses and the transmission gearing, which is constant)

• stock 2.7L Eta w/2.93: 170 lb*ft * 2.93 = 498.1 lb*ft rw torque
• 3.0L Eta w/2.93: 195 lb*ft * 2.93 = 544.05 lb*ft rw torque

I am unsure what increasing the displacement while maintaining the restricted head, intake, and exhaust system will do to the overall torque curve, would need someone with more engine knowledge and/or engine-building experience to give me a accurate answer.

Want to hear the community's thoughts on this idea.

[digger]

IMO the most important things for bottom end torque are
- cubes
- compression and mixture motion
- conservative cam timing
- counter reversion

that's the recipe from a high level

you will exceed 195 ft/lb with ease

[ForcedFirebird]

Our shop race car has 180whp/180wtq with a b25 stroked to 81mm (still a 2.7, but 885 head/domed "i" pistons, 9.4:1 static comp). I bet if we swapped the eta manifold, TQ would go up considerably, with slight (if at all) sacrifice of HP. A few years ago we did an eta rotating assy, using 885 head and eta intake manifold ported to match the "i" TB, and it put down 170wtq, but only 145whp - when we swapped it to the "i" intake, the TQ dropped to 155tq, but HP went up to 150.

The head will limit the power, but is designed for TQ. The 731 head with 885 valves is a much better option IMO, but not so easy to find here in the states.

As Digger said, trapped mass is where it's at for high TQ at lower RPM. The eta cam has very little overlap, plus more compression than the b25, then adding the long skinny runners, you have every good part of the recipe.

Interesting project, very curious of the results...

[jeffnhiscars]

........The 731 head with 885 valves is a much better option IMO, but not so easy to find here in the states

having just done this with a 272, I couldn't agree more. Great bang for the buck

[digger]

how long is the eta runners? and size at head and entry?

[redlightpete]

If you use the 89.6mm stroke crank, 325i pistons, and eta 130mm rods, the pistons will be sticking out of the block 2.3 mm more than normal. This would require a very thick head gasket, shorter rods, or custom pistons.

[ForcedFirebird]

If you use the 89.6mm stroke crank, 325i pistons, and eta 130mm rods, the pistons will be sticking out of the block 2.3 mm more than normal. This would require a very thick head gasket, shorter rods, or custom pistons.

Good catch...and the 325i pistons won't agree with an eta 200 head casting. Not sure why I assumed an entire m54 rotating assembly, which would work with the 200 head, but would need valve reliefs machined in.

[digger]

should be able to use OE slugs and rods from a 24V engine

or just turn stock b25 into flat tops with 2.3 mm off, or a lesser amount off the top and thicker MLS or combination thereof. you just dont want to have to recut the pockets because you cut them away too much. youd also need to clearance bottom of pistons. stock rods with that crank is not a good idea as the intermediate shaft and bottom of bores need more clearancing than other options

so it comes back to aftermarket slugs, funny that

[ZeKahr]

If you use the 89.6mm stroke crank, 325i pistons, and eta 130mm rods, the pistons will be sticking out of the block 2.3 mm more than normal. This would require a very thick head gasket, shorter rods, or custom pistons.

Ah didn't realize this. I need to do more research on which components will work for this stroker build. I just looked at the list of stroker permutations on E30Zone and went with the 3.0L one, thinking it would be a bolt-on setup. Would anyone here be able to provide a stroker permutation that's as "plug-and-play" as possible and doesn't require fabrication or custom parts? I'm probably going to have a shop do this kind of work if I ever get to it (right I'm running around rebuilding my car) and don't want to throw too much money into this kind of project

[digger]

IMO your best bet considering what you are looking for is to pillage from a m54b30.

You’ll want the following gizzards

• 89.6 mm crank
• 135 mm rods
• Stock 84 mm bore, 28.3 mm CH pistons.

With this setup the pistons are 1.9mm higher than on stock eta (208.1 mm vs 206.2 mm)

Mods/notes

• Off the shelf oil seal sleeve required (required for all 84, 85.8, 89.6 stroke cranks etc)
• Either thicker MLS head gasket or stock 2.05mm HG and machine approx 1mm from pistons to get the piston at the correct position in relation to the head
• Clearance the block and intermediate shaft internally (possibly)
• With an eta cam you probably won’t need any reliefs cut into the pistons

Any machine shop can do these mods there isn’t anything major.