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First off the three "C" are still in affect for first time engine builders, They all have a tendency to over board on all three "C's" Cam, Carburetion and Compression. Most of our experience has been in Off-Road racing and play buggy's. So all you drag racers will need more help. We started off with what we thought was closes and what every one else was running. As we gained experience we gradually went to milder and milder cams and started winning more and more races. We raced on week ends, tore the motor down on Tuesday, rebuilt with some changes on Wednesday and Dyno tested on Thursday and road tested and loaded up on Friday so we could race again on the week end. This was when I was younger and the wife was more understanding. We never found a split Duration cam's we really liked, but we did have great luck with using different ratio rockers on regular cams. We ran 1.1, 1.25 and 1.4 on The Intakes and normally 1.1 or 1.0 on the exhaust. valves. This was very easy to experiment with but not all motors responded the same. We often had several motors for each class, built up slightly different. One motor would might gain top end with the 1.25 or 1.4 rockers on the intakes and produce more low end with the 1.0 on the exhaust. Another motor almost identical built, but using a different cam might respond to the more lift on the intakes but loose low end power with the 1.0 on the exhaust valves. Like I said the rockers are easy to switch so we did a lot of experimenting. Most cams are ground on 107, 108 degree lobe centers and 109/115 degree lobe centers and split duration cams. 112 degree lobe centers on some turbo cams. Most builders tend to forget that the longer the valve stays open the less distance (time) the piston has to produce power. See the cam card below, and note that the when the intake valve closes and the exhaust valve opens.  Typical cam card on left gives cam identification number, cam lift and duration plus the valves setting. Also when both the intake valve and exhaust valve open and closes. Also at what lift the these figures are figured at. Engle cams are checked at .050 lift at lifter and other brands might be checked at the valve or at different lift. So keep the cam card and know how the your cam figures are figured. Follow the cam manufacture recommendations on break in. Just
remember that the first few minutes are the most important. This is
when the cam and lifters become burnished together. Always install
new lifters with your new cam. Always check valve to piston clearance
(about .100" minimum) and valve springs for coil binding
(about .090" at maximum lift) One cam company clams 98% of
all cam failure is due to excessive valve spring pressure or too
many RPM during the break in period. When running dual springs
they recommend a 30 minute break in period with only the outer
spring installed, then install the inner springs. |
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