Philes' Forum by Vic Lucariello

Hello, Bimmerphiles. Things have been pretty busy around here with preparations for the March Lime Rock School, the completion of my `02s roll cage, and, unfortunately, a lot of stuff at work. I've been unable to put together much of a column for you folks, so instead of skipping this month, I decided to rerun the portion of my April, 1988 column that dealt with engine displacement. Here goes:

"The cars look like some future transport system gone disastrously amok: the enormous rear tires, slouching in on themselves like the flaccid carapace of extragallactic crawlers. The motors rip the air and sear the eyes with toxic stink from nitro-methanol. The noise is a physical assault--hard, sharp and fast like a sonic band saw." --- David Barry on Top Fuel Dragsters and Funny Cars (essentially fuel dragsters fitted with more streamlined, coupe bodies) in AUTOWEEK, February 22, 1988.

As a former enthusiast of and occasional participant in drag racing, I found Mr. Barry's description particularly appealing. I still have the Hot Rod I began building (hot rods are never "finished") in 1966 when I was er....very, very, young! It's a 1960 Mercury Comet which has been fitted with a 4.3 Liter (260 cubic inch) displacement Ford V-8 engine built to Cobra specs. While the original engine was an anemic 2.4 L, 144 cubic inch in-line six-cylinder rated at 85 HP, the V-8 pumps out about 200 HP more! As the old saying goes, "There ain't no substitute for cubic inches!". What exactly are these cubic inches and liters referring to, anyway, you may ask (as we deftly segue into this month's Philes' Forum)....

Hello Bimmerphiles! Last year we introduced this column as a means of exchanging technical information among members. If you're having a problem with your Bimmer and/or would like to share a solution or technical tidbit with our membership, this forum is for you!

Reciprocating (i.e. piston, as opposed to rotary and turbine) internal combustion engines are commonly described by their piston displacements expressed in liters, cubic centimeters, or cubic inches (more on the Metric-English conversion below). Piston displacement can be described as the volume of air which would be displaced as a piston slowly moved from its position closest to the crankshaft (called Bottom-Dead-Center, or, BDC), to its position farthest from the crankshaft (called Top-Dead-Center, or, TDC). Mathematically, piston displacement can be expressed as the area of the piston top (0.785 X Cylinder Bore Diameter X Cylinder Bore Diameter) times the distance the piston moves in its bore (this distance, measured from BDC to TDC, is called the "stroke"). Since in a multi-cylinder engine, the pistons are nominally of equal size, total piston displacement is simply the displacement of any piston times the number of pistons.

DISPLACEMENT = # CYL X .785 X BORE X BORE X STROKE

Let,s apply our trick formula to the six-cylinder engine used in the new [E30] 325i models and see what happens. (Ha! I,ll bet you thought I'd use a 2002 for an example!) The stock bore and stroke of a 325i are, respectively, 84.0 mm (8.4 cm) and 75.0 mm (7.5 cm).

DISPLACEMENT = 6 X .785 X 8.4 X 8.4 X 7.5

which equals 2494 cubic centimeters (cc) displacement

Lo and behold, this agrees with the published value! Since a liter can be expressed as 1000 cc, our 2494 cc 325i can also be called a 2.5 Liter machine.

Want to express the value in cubic inches? Simple. Either use bore and stroke measurements in inches (1 inch = 2.54 cm 25.4mm) in the formula, or divide the displacement in cc by 16.39. Either way, our 325i example yields 152 cubic inches displacement, significantly smaller than my Hot Rod's engine, which was itself considered to be miniscule during the muscle car era, when engines of 426 cubic inches (6982 cc) and 454 cubic inches (7441cc) were commonplace.

When I wrote the foregoing in 1988, I neglected to bring to your attention the difference in rated horsepower between my old 2.4 Liter six from the Comet and the 2.5 Liter E30 six. The 2.5 is rated at 168 HP [SAE Net] while the 2.4 put out only 85 HP [SAE Gross]. I estimate that the SAE Net rating of the 2.4 would be about 80. Quite a difference for two engines of similar displacement, especially when you consider that the 2.4 was built prior to any emissions regulations!

Do I still have the Hot Rod? What do you think? Is Ross Karlin a good Driver? OOPS.....wrong analogy!

Returning to the present, Chapter Treasurer Hank Farber had a question about electrical relays. Hank wants to know where he can find a 12 volt relay with contacts that are rated at 30 amps (This part is easy, most new bimmers are full of them). In addition Hank wants a relay with a mounting tab on its case. The answer is Radio Shack Part No. 275-226, which costs about $5.

While I'm on the subject of relays, you'll recall that in a recent column I talked about E30 [`84 - `91 3-Series] starting problems attributable to the main relay. I suggested that you carry a spare main relay, because it is somewhat special and that no other relay on the car can be used in its place. Well, I neglected to tell you the part number! The BMW Part Number is XXXXXXX, and the Bosch Part Number is YYYYYYYYYY. Bosch list price is about $13.

That's all for now, Bimmerphiles, I gotta get back to that roll cage!

Anyone wishing to contribute to Philes' Forum should contact me via the info on the masthead. I'm interested in tech tips, repair/maintenance questions and/or tips, dealer horror stories, product evaluations, etc. Please call before 8 PM, and if you leave a message, PLEASE INCLUDE YOUR WORK PHONE NUMBER. Also, please be patient, I try to return every phone call, but it sometimes takes awhile. The best way to contact me is via e-mail.

Copyright 1997 - V. M. Lucariello, PE