John McCauley Wrote:
<<<< I have just installed a performance camshaft in my new engine, and it
hasno guts. I have been told it is the cam timing, so I puchased a
degreewheel. I have installed it on my car and it is set to top dead
center. Ican't make any sense of how to use it beyond that.
My cam specs are:
Duration at .050 229 deg.
Intake opens 8 closes 41
Exhaust opens 41 closes 8
Cam rise .295 L/C 108 deg. >>>>
John, you have a fairly hot cam there. For duration, the key words are
"at .050." This signafies that the measured duration is the total time
that the valve is open at least .050" and that is a lot less time than
total duration. The .050 duration is a much more accurate measure of a
cam's performance potential than total duration. I would guess that 229 at
.050 translates to about 285 total duration. Also the lift is measured at
the cam so valve lift would be .295 X 1.5 - .018 or .425 which is fairly
agressive for an LBC cam. If you used high ratio rockers (1.65 vs the
stock 1.5) you would see .469 which would be beyond what the valve springs
can probably take.
The lobe centers at 108 (degrees between the center of the intake lobe and
the center of the exhaust lobe) would be fine for a smaller cam but on a
cam this size would really kill low end torque. A wider lobe center of 110
or 112 would reduce overlap and help low end torque at the expanse of
useable stroke, compression, and ultimate power. Modern emission
compliant performance cams usually have the lobe centers farther apart
because overlap kills emissions control. This is one reason why new
performance engines can run such high static compression ratios on standard
gas- they lose a lot of the compression stroke because the cam stays open
longer.
Summit Racing suggests that you should stay under 220 (if my memory serves
me correctly) duration at .050 for a street engine which would be around
275 total duration.
The cam literature should suggest a point for cam timing. This can be at a
certain opening lift for the intake (look for words to the effect of "set
valve clearance to .030 and time just as intake starts to open XX before
TDC") or they will give the position for the center line of the intake
lobe. If the center line dimension is given, find it by figuring out the
timing at .010" lift down from maximum before and after the center. The
lobe center line will be half way between these numbers.
It's your car but I think that running this cam in a stock engine would be
a poor choice. It needs high static compression and a modified head to
really run and, even then, it might be tough to live with on the street.
Sorry.
Regards,
Bill Eastman
61 MGA and a lot of time working on American Iron
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