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WinRice WinRice is offline
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Join Date: Jun 2005
Location: Colorado
Posts: 702
Originally Posted by 911nut View Post
Cams cannot affect compression since they are closed during compression.
Not true, you have two types of compression, geometric which is the ratio of the the volume of the cylinder at BDC vs the volume at TDC. Then you have effective compression which is when the intake valve closes just past BDC and only the remaining volume gets compressed.

For example, the stock 930 cam specs are:
Intake 214 degrees @ 0.040" with a 110 degree centerline
Exhaust 202 degrees @ 0.040" with a 106 degree centerline

Starting at valve overlap at TDC the intake valve would close at 110 + 214/2 = 217 degrees, if BDC is 180 degrees from TDC, that puts the intake valve closing 37 degrees after BDC, into the compression stroke, or 143 degrees before TDC. You will not be getting a full stroke of compression.

If you use a 964 cam with intake duration of 246, lobe center of 113, the intake valve closes at 113 + 246/2 = 236 degrees, or 56 degrees after BDC, 124 degrees before TDC, leaving even less volume to compress.

The 964 cam will have less effective cylinder pressure than the stock turbo cam.
1986 911 Turbo
3.3L, K27HFS, Tial 46mm, TurboKraft Intercooler, 964 Cams, Monty Muffler, MS3Pro Evo, M&W Ignition, Zietronix WBO2 Data Logger, Wevo shifter, coupler and motor mounts.
Old 01-30-2010, 07:31 PM
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