



Thread Tools  Display Modes 
10232001, 04:33 AM  #1 
Sooby Guru
Member#: 1914
Join Date: Jul 2000
Chapter/Region:
SWIC
Location: 2013 335i, 2011 G37 coupe
Vehicle:2000 2.5RS w/ EJ22T swap and N20. gone. : ( 
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
* Registered users of the site do not see these ads.

10232001, 06:18 AM  #2 
Scooby Specialist
Member#: 10313
Join Date: Sep 2001
Chapter/Region:
SCIC
Location: Mission Viejo, CA
Vehicle:2002 WRX wagon silver 
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 
10242001, 01:56 AM  #3  
Scooby Newbie
Member#: 8935
Join Date: Aug 2001
Location: Albuquerque, NM
Vehicle:2001 Impreza 2.5RS Aspen White 
Quote:
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. 

10242001, 02:09 AM  #4  
Sooby Guru
Member#: 1914
Join Date: Jul 2000
Chapter/Region:
SWIC
Location: 2013 335i, 2011 G37 coupe
Vehicle:2000 2.5RS w/ EJ22T swap and N20. gone. : ( 
Quote:
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; 10242001 at 02:14 AM. 

10242001, 08:17 AM  #5 
Scooby Newbie
Member#: 2293
Join Date: Sep 2000
Chapter/Region:
South East
Location: WF NC, USA
Vehicle:2002 GOLF TDI Silver 
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.) 
10242001, 01:54 PM  #6 
Scooby Newbie
Member#: 3057
Join Date: Nov 2000
Location: Arlington, MA

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 
10242001, 02:17 PM  #7 
Scooby Specialist
Member#: 4181
Join Date: Feb 2001
Location: keeping santa cruz weird
Vehicle:2003 Lancer Evo 8 It's Blue, ok? 
[rant] even more proof that metric is better....

10242001, 02:20 PM  #8 
Dynamic Uno
Member#: 1
Join Date: Jun 1999
Chapter/Region:
BAIC
Location: Alameda, CA
Vehicle:2011 Impreza WRX STi Black 
Of course metric is better. There is a reason only 2 countries in the World refuse to adopt it as a primary system.

10242001, 02:22 PM  #9 
Scooby Newbie
Member#: 5283
Join Date: Mar 2001
Location: somewhere probably sucking on mentos
Vehicle:02 wrx wagon black 
thanks, i have a headache now.

10242001, 05:31 PM  #10 
Scooby Newbie
Member#: 3083
Join Date: Nov 2000
Location: CA
Vehicle:1997 Miata MEdition Green 
Bunch of squares

10242001, 06:00 PM  #11  
Scooby Specialist
Member#: 7453
Join Date: Jun 2001
Chapter/Region:
TXIC
Location: Grapevine, TX
Vehicle:2005 Legacy GT Wagon 2006 LJ Rubicon 
Quote:
Quote:
Drakos 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
What's Wrong With My Tranny???? Help  Pinz0311  Transmission (AT/MT) & Driveline  12  03232008 05:25 PM 
Help  What's wrong with my rotors?  alfcanada  Brakes, Steering & Suspension  3  01012007 04:01 AM 
What's wrong with my computer?!?! Help!  Guru Imakuni  OffTopic  13  08292004 10:33 PM 
Help! What's wrong with my car  gnildronde  Normally Aspirated Powertrain  6  08022002 07:47 PM 
Help! What's wrong with my subframe?  chandy  Brakes, Steering & Suspension  5  02092002 01:17 AM 