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Old 10-23-2001, 04:33 AM   #1
Eric SS
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Arrow Pleasae help. What's wrong with my math?

This is Subaru related because I'm trying ot figure out how Subaru came up with their calculations for the EJ25 motor. The reason I'm doing this is to decide how much overbore I should go (not going into the details here )and for some reason, my math just isn;t working out with theirs.

According to Subaru, the bore on the EJ25 is 99.5mm, The Stroke is 79.0mm, and the displacment is 2,457cc's which is 2.457 Litres.

So, this is how my math comes out. please let me know what I am doing wrong:

99.500mm=3.917"
79.000mm=3.110"

To get the engines displacement, I do the equation Bore x stroke x Pi x # of cylinders. With this I get:

3.917" x 3.110" x 3.141 x 4 = 153.053 cubic inches

1 cubic inch = 16.387cc's

Therefor:

153.053 ci's x 16.387 cc's = 2508.079 cc's

And: 2508.079 Litres = 2.508 litres!

Where is my math wrong? Thank you.

Eric
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Old 10-23-2001, 06:18 AM   #2
Mark Avery
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Converting from metric to inches then back to metric is not necessary.

The bore measurement is the diameter the cylinder, but to get the volume we need the area. So, take half of the bore measurement, that's the radius, square it, then multiply by Pi. Also, since the answer you want is in cubic centimeters, let's convert the bore and stroke measurements to centimeters at the start.

9.95 / 2 = 4.975 x 4.975 = 24.750625 x 3.14159 = 77.756315994

That's the area in square centimeters of one cylinder bore. Now for the displacement of a cylinder, multiply that by the stroke. Then for the total displacement, multiply by the number of cylinders.

77.756315994 x 7.9 = 614.27489635 X 4 = 2457.0995854

or 2.457 litres.

-Mark
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Old 10-24-2001, 01:56 AM   #3
Daveish
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Quote:
To get the engines displacement, I do the equation Bore x stroke x Pi x # of cylinders. With this I get:

3.917" x 3.110" x 3.141 x 4 = 153.053 cubic inches

1 cubic inch = 16.387cc's
Jes for reference, look @ your units... you have:

inches/cylinders x inches x (no unit) x cylinders...

that gives you inches squared -- which isn't the cubic inches you're using in your conversion back to cc's.
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Old 10-24-2001, 02:09 AM   #4
Eric SS
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Quote:
Originally posted by Daveish


Jes for reference, look @ your units... you have:

inches/cylinders x inches x (no unit) x cylinders...

that gives you inches squared -- which isn't the cubic inches you're using in your conversion back to cc's.
Using Pi gived me my cubic inches
The actual way to measure Displacment is:
CID = Number of cylinders x 0.7854" x bore" x bore" x stroke

THAT gives you your cubic inches, However, the firstway I did it is a simplified way of doing it and nets the same result

Last edited by Eric SS; 10-24-2001 at 02:14 AM.
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Old 10-24-2001, 08:17 AM   #5
red
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An easy way of douing it is Bore x Bore x .7854 x stroke x # of cylinders. All units should be in cm.
(i.e. 9.95 x 9.95 x .7854 x 7.9 x 4 = 2457.1074066cc or 2.4571074066L)
I do this all the time. The same technique is also used for english.
( i.e. 3.917 x 3.917 x .7854 x 3.11 x 4 = 149.905794456264 cu.in.)
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Old 10-24-2001, 01:54 PM   #6
mhj
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Yup, too many roundoff errors.

Staying in metric, using:

Pi=3.14159
Bore=9.95 cm
Stroke= 7.90 cm

and the formulas:

Area = Pi*(Bore/2)^2
Volume= Area*Stroke
Displacement=Volume*4

I get Displacement=2457.0995854025 which rounds to 2457.1cc
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Old 10-24-2001, 02:17 PM   #7
ChrisW
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[rant] even more proof that metric is better....
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Old 10-24-2001, 02:20 PM   #8
Imprezer
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Of course metric is better. There is a reason only 2 countries in the World refuse to adopt it as a primary system.
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Old 10-24-2001, 02:22 PM   #9
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thanks, i have a headache now.
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Old 10-24-2001, 05:31 PM   #10
H1
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Bunch of squares
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Old 10-24-2001, 06:00 PM   #11
driggity
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Quote:
Originally posted by Imprezinator


Using Pi gived me my cubic inches
The actual way to measure Displacment is:
CID = Number of cylinders x 0.7854" x bore" x bore" x stroke

THAT gives you your cubic inches, However, the firstway I did it is a simplified way of doing it and nets the same result
Uhm the firstway you did it is not a simplified version of this. The first thing you did was

Quote:
Originally posted by Imprezinator

Bore x stroke x Pi x # of cylinders
The only reason this came out somewhat close is because .7854 * bore happens to be somewhat close to pi: 3.08 vs 3.141 (in the case of the 2.5 L engine). I'll agree that metric is better but you can convert back and forth with no problems if you use good conversions and don't go rounding off willy-nilly. Oh, and since pi doesn't have units it can't convert from in^2 to in^3

Drakos
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