A word about cams -- Prompted by the "crappy gas mileage" comment.
All cams close the intake valve somewhat after bottom dead center as the
piston is on it's way back up on the compression stroke. Late intake
valve closing is done to increase cylinder filling at higher rpm.
Since compression cannot begin until the intake valve is closed, the
dynamic effective compression ratio is less than the static (rated)
compression ratio. With a short duration (mild) cam, the effective point
of intake valve closing might be around 45 degrees after bottom dead
center. For a static compression ratio of 7.9 to 1, this would yield an
effective compression ratio of around 7.2 to 1.
If the cam duration is increased, the intake valve is closed later on
it's way toward TDC. A typical "hotter" cam would close the intake valve
at maybe 65 degrees after bottom dead center. The static compression
ratio would still be 7.9 to 1, but the dynamic compression ratio would
now be 6.3 to 1 instead of 7.2 to 1 as with the milder cam. This CR
decrease would reduce low & mid range torque & fuel mileage by about 14%.
To get back to the original 7.2 to 1 dynamic ratio that the milder cam
had, it would require raising the static compression ratio to 9.1 to 1.
This would regain the low rpm torque & fuel mileage losses.
Notes:
If the compression ratio is not increased, this low & mid rpm torque
loss will eventually be compensated for as rpm increases & intake gas
inertia overcomes the rising piston on the compression stroke, causing
cylinder filling to be more complete, & the dynamic compression ratio
becomes nearly the same as the static compression ratio. But only at
higher rpm.
With the CR increase, torque at low & mid rpm is maintained at the
original level & will show a substantial gain at higher rpm if intake &
exhaust flows are compatible with the cam characteristics.
The intake & exhaust systems must be designed to handle the increased
flows at higher rpm. Everything HAS to work together. Remember the old
saying, cams, carbs, compression, & exhaust.
Here is a good web site to compare cam timing, static, & dynamic
compression ratios.
http://kb-silvolite.com/calc.php?action=comp
The exact results will depend on the stroke length, bore diameter, &
connecting rod length of your particular engine.
Regards,
Dave Russell
john close wrote:
I'd like to put in a good word for Elgin Cams. In my BJ8 engine, I have
their 264 grind. Duration 265, Overlap 50. Lift at Cam 0.264". Nice
performance, quiet, crappy gas mileage, as to be expected. - JRC
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