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Author Topic: Building an M20 stroker  (Read 60060 times)
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« on: November 09, 2009, 12:45:37 AM »

M20 Engine types:

M20B20/23 - gray market only E21/E30/E28 520i                                           (Note: Smallest ports, no distributor takeoff on head on E21 models, flat head and pistons)
M20B27 Eta - early E30/E28 528i                                                                (Note: medium ports, 4 bearing cam, high compression, flat head and pistons)
M20B27 Super Eta - late E30 (after '87)/E28 528i                                          (Note: medium ports, 7 bearing cam, low compression, flat head and pistons)
M20B25 - All US E30 325i/is + E34 525i 89-91                                               (Note: Largest intake ports, scalloped head and domed pistons)

Do This:

For easy, decent power, get an Eta or S-Eta block and use the I head/ECU. This is a super easy swap. The compression on this setup is around 9:1 or so on the S-Eta block while the Eta block gets about 9.5:1. If you use I pistons on Eta rods the compression is really too low. (Unless you turbo!) You can skim the block to raise compression (2mm is good). If you have access to a machine shop, this is a good setup for this block/head combo. Don't skim the head. However, you (might?) have to cut down the piston skirts because or they will hit the crank. This is the best option if done properly and carefully because the I pistons match the I head.

Putting Eta/S-Eta guts (crank, rods, pistons) in an I block is the same as putting an I head on an Eta/S-Eta short block. The B25 and B27 blocks vary little. The above applies for this approach.

Another good setup is a ported early head on the early eta block. The early head is easily ported to match the I head's intake ports, and this combination yields almost 11:1 compression. The only bummer with this is that the I head/piston design is more efficient than flat pistons and head. However, early heads are hard to get. As in, you really can't in the States.

One more option is to weld/fill the scalloped portion in the I head to match the Eta/S-Eta head. This also brings up compression and doesn't create clearance issues. I would recommend this over anything else, as all the work is done on the head, and building this stroker can be done in-car (if you have an Eta/S-Eta).  It's also easier to weld and polish the aluminum head than it is to skim the block.

Estimate about 185-195 crank HP for Eta blocks and 180-190 crank HP for S-Eta blocks with just a head/motronic swap. An M30 AFM mod (AFM mod, injectors, fuel pressure increase), exhaust, and chip can bring this into the 200 range. Cams can put you well into 200-220 with all the aforementioned mods.

Don't Do This:

All eta rods are 5 mm shorter than I rods. Basically this means using an Eta/S-Eta crank to increase throw and then using I rods and pistons to increase TDC piston position another 5mm can get tricky. Compression is good, but you really should have deep valve reliefs cut in the I piston tops. Putting blu-tac on the pistons when the engine is together and rotating the crank manually may show ok valve/piston clearance, but a chipped M20 can rev 7100+. Rod stretching at this RPM might not work so well with tight clearances, especially with any more duration/lift than stock. A thick head gasket can offset this (plus using stock redline; no fun), and longer rods make great revving engines, but to be safe, just don't use I rods.

DO NOT use I rods on any Eta crank with any Eta pistons. You'll hit the valves, period. Basically, the I rods are long and the pistons are short. The Eta pistons are long and rods are short. Don't combine long and long.


You don't need to bore your block. Anyone who says that is an idiot or using custom pistons.

The 2.7 crank can handle the revs. If you're not dry-humping your chipped stroker on the 7150 redline all day, you're fine. The Eta valvetrain is the weak spot in these engines.

Last Notes:

When Alpina made their strokers they used M21 cranks. They are forged steel instead of cast iron and are stronger. Unfortunately, the M21 block is a diesel, and they are hard to find. The only way to get one in the States is to pull it out of a Lincoln MarkVII diesel. BMW sold M21s to Lincoln for this weird car, but good luck, there were less than 500 made. I've only seen one for sale, ever, in the area. Diesel cranks take a hard beating, so they should always be checked for cracks. (If you get one.) Alpina never had this problem, they got new parts.

Unfortunately there is no perfect "OEM frankenstein" setup; you just have to do what's best for your budget and availability. A fun setup using all OEM parts would be the 524td crank/exhaust manifold, Eta block/rods and I pistons/head.

If you want to go crazy, M50 cranks can be modded to fit, and as a result so can S50/S52 cranks. M3/328i rods can also be used, they are much lighter. At this point, you're almost better buying custom rods/pistons or maybe swapping the whole M50/S50/S52. Or just send the damn thing down to Metric Mechanic and saying: "Build me a 3200 rally engine."

J Gongaware,
No reproducing without permission.

I think this covers anything. If I (or someone else) finds any falsities, I'll edit this post.

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