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Final Motor Specs


August 25, 2018


As described in the last post, the motor build is mostly completed. There are a few weird odds-and-ends yet to be done, like a drill and tap for a bolt to secure warm-up regulator to the block and such – nothing major. But I realized that the final build specifications were never discussed fully. So here’s a chart and explanation of my build, comparing it to stock BMW and the Alpina build.

Virtually every decision-point was vetted. Twenty-eight hundred cc displacement was chosen because I wanted to use a turbo-diesel crank (it’s a very stout crack, able to withstand the high pressures of the diesel motor) and I always kinda liked the e30 c2 2.7, based on the same crank and big bore. The pistons were an easy decision, given the crank and desire to get at least 2700cc. I didn’t feel the need for a much bigger bore and knew that several folks had successfully used a 86 mm bore on the stock 325i M20 block.

The rods were a compromise. Because the pistons were custom, I could use virtually any BMW rod that fit the crank. If I used stock BMW rods, I would spend money buying them, testing and refreshing them, and lightening and balancing them. If I went with proven, USA forged rods like Carrillo or such I’d spend a fortune. For the same price as used rods plus machine shop work, however, I could get forged Chinese rods that were finished in the USA by a reputable shop. That is just what I did, with 135mm rods from Molnar Technologies. Tom Molnar was the Metallurgist and engineer for Oliver Rods and Cranks for over 20 years; he was the founder of K-1 Technologies and recently started his own shop. I was confident with that background that my compromise rods were built by someone with the necessary know-how.

The Schrick 272 was also a compromise camshaft, but for different reasons. Retaining the Bosch K-Jet fuel injection was the limiting factor. From what I’ve been told, the flap in the air-fuel meter is sensitive to pressure changes in the intake plenum and a higher-duration cam will create too much pressure change for the K-Jet fuel injection. Normally, with the increased displacement, I would build in more cam – and while the Schrick 272 is a “bigger” cam than the stock Alpina 268, it is not monumentally so. Obviously, it's only 4 degrees more duration than the Alpina cam and 12 more than BMW. In the faux A4S I built, for example, I had 2.2 liters of displacement (S14 crank in stock M10 block) and a Schrick 304. The 304 has significantly more duration than the 264 degree stock M10 cam and is usually thought to be too much cam for the street, with high horsepower but little bottom-end power. But the increased displacement made up for that and the motor was actually very docile and flexible – lots of horsepower but the increased cubic inches (or centimeters, as the case may be) gave it lots of low-end torque. If I could have done that here, I would have. But no such luck and the 272 was the most duration I could comfortably get while retaining the stock(ish) K-Jet fuel injection.

The M20B25 block was chosen simply for convenience – there are a ton of them available and but for the drill-and-tap for warm-up regulator, there are no practical differences. The 885 head is said to flow best of the available M20 heads, so that choice was simple too. The head and intake plenum were mildly ported and matched.

I guestimate the motor to put out about 200 horsepower and 185 foot-pounds of torque. Both numbers represent a gain of 30 horsepower and foot-pounds. The horsepower is an educated guess, starting at the Alpina 170 mark for the 2.3 liter C1 motor and then comparing to the Alpina B6 2.8, which has a tad less displacement (2788 v. 2823). The B6 motor puts out 200 or 218, depending on fuel injection and cam. The Pierburg version of the B6, with an airflow meter similar to the K-Jet and a 260 degree cam, puts out 200. The Bosch L-Jet version with a 280 degree cam, puts out 218. Mine is probably somewhere in between but 200 should be a safe number, given the displacement, cam, compression ratio and fuel injection. (Note that Alpina used considerably less cam duration for fuel injection similar to mine.)

Early B6 (Pierburg) above; late B6 (L-Jet) below.

For torque I used the B6 2.8 as a reference point as well. The early B6 motor put out 182 and the later 195. Mine should be in the middle as well but 185 is a safe number.

If these numbers are right, I will be getting B6 performance essentially with weight savings of 57 pounds – all at the nose. (The M20 weight is 258 lbs while the M30 is 315 pounds.) That’s not bad!

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