Thread: Comp Ratio check

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Originally Posted by grifo7

The effective cr calculator that techinspector mentioned is great! Worked for the Lunati cam figures that I have in the car at the moment. They are quoted at .050" - IO: 1 BTDC, IC: 37 ABDC, EO: 49 BBDC, EC: -3 ATDC.
BUT... the Comp Cams one (CB XM 284HR) I plan to install is quoted at .006" only. Figures are - IO: 32 BTDC, IC: 72 ABTD, EO: 79 BBDC, EC: 31 ATDC. These specs are for cam installed at 110 deg centreline. Duration at .050" is 230/236, advertised 284/290, LC is 112 deg.
Is there an approximate way to convert these figs to .050" ones, or do I have to contact Comp Cams themselves?

Since we have no way of knowing how quick or how slow the closing ramp is, you'll have to get the info from Comp. Most cams today are ground asymetrically (opening ramp is different from closing ramp). Assuming a closing point of around 40 degrees ABDC, the DCR would be 8.161:1 with that cam.

Have you done a compression check with the current cam in place? Remove all the spark plugs and wire the throttle primaries open. Disconnect the coil. Run each cylinder through at least 4 to 5 cycles so you get a good reading. Then post your findings on here please. I'm curious to see the correlation between DCR and cranking compression. Thanks.