Building a 2.8

BUILDING A 2.8 LITRE M20 ENGINE © Argos 2009

Really, this is pretty much the same as building a 2.7. I first built a 2.8 back in 2001 - 2.8 M52 engines had only been out for around 5-6 years then but there were a few ruined Nikasil short engines about. The 2.7 525e is now becoming rare - they stopped making them at the end of 1987, most have done big miles and many are just knackered now and need a crank regrind. The 2.8 M52 was fitted to the E36 328i and E39 528i, both of which are at the scrapyard stage now so an old 2.8 engine with suspected head gasket issues is literally £50 worth of scrap.

The M52 is a later alloy block version of the M50 which itself was only ever a 24v version of the M20. The block dimensions are the same and the 2.8 crank drops straight in. Later ones from the end of 1998 have Double Vanos and these cranks have a TDC trigger wheel screwed to the back as well as bigger counterweights meaning they are hard work to fit - it involved grinding out the block to provide clearance. But do check because some later cranks fit without a problem once the TDC trigger wheel has been removed and binned. But to keep things simple, just use the early single Vanos crank.

The beauty of the 2.8 crank is that it has an 84mm stroke - with standard 325i pistons and 130mm 2 litre, 2.3 litre and 525e M20 ‘short’ rods - allows the pistons to come to the top of the block. No machining of the block face is required although a 5 thou clean up skim is a very good idea.

Now, because the M52 had a timing chain and not a belt, the oil seal ran directly on the crank pulley hub and not the crank. This means you need to make up a spacer to slide onto the crank nose for the oil seal to run on. Fortunately, the M52 (and M50) crank pulley hub is perfect for the job - simply remove the damper (six bolts) and get a machine shop to mill it down to the dimensions shown. This will be the same tightish sliding fit on the crank, and once the crank bolt is tightened fully, it’s not going anywhere. Get the machine shop to mill a nice smooth chamfer on the front of the spacer so the crank oil seal slides over nicely. Use standard 325i crank bearings and rod bolts.

Other pistons?

Only the 325i pistons will work, NOTHING else!

Early 9.75:1 CR pistons from the pre 9/87 steel bumper 325i will need the bottom of the skirts taking off - I found that 14mm did the job but at the machine shop, assemble a piston and rod to the crank in the block and check. Later 8.8:1 CR pistons SHOULD fit, but either Mahle or KS (Karl Schmidt) pistons will need a light shave. A very good machine shop with incredibly sensitive and accurate scales will be able to balance all six pistons for weight, but really, if they are all shortened exactly the same they will be okay.

Compression ratios?

Use the 8.8:1 pistons and the compression ratio will be a very acceptable 9.75:1 That’s a very decent compression and won’t cause any detonation.

Use the 9.75:1 pistons and the compression ratio will be a rather high 10.8:1. That’s going to require super unleaded, but it will go like a rocket on 99 octane. Using the BMW thicker head gasket will drop this to about 10.3:1. Either way the engine will be very responsive but it won’t like standard 95 octane fuel much.

Camshaft

The standard 325i cam will seriously limit power. You will need something in the region of a 270-276 degree camshaft so that the engine can fill its lungs. With the standard cam it will just lack top end power.

Other stuff

Like the 2.7, a 2.8 will need a good custom made ECU chip tailor made with the aid of a clever bloke with a dyno and an emulator. Standard 325i injectors are just about good enough. If you find they aren’t quite flowing enough, think about using the next injector up, the 0280 150 714 from the E34 530i and 535i. In most cases, the 325i injector does the job though. The standard 325i radiator will be fine as long as it’s in perfect (i.e as new) condition. The standard air flow meter is also fine. Fitting a bigger one from a 535i doesn’t work.

Best exhaust

The standard BMW factory exhaust gives great power as long as it is a genuine BMW system made by either Boysen or Eberspacher. Some after-market systems give more power, many do not. The later cast iron manifold is much maligned - you might get 5 bhp from a tubular manifold but spend the money on a camshaft and a remap first.

How much power?

A good high compression 2.8 with a 270 degree cam, a proper ECU remap and 99 octane fuel will do around 220 bhp. Oil Seal Spacer Specs (for Fabrication)