In a message dated 96-09-01 18:59:05 EDT, you write:
>
>I am installing a new cam (270 duration from British Parts in Oregon. We'll
>see.) on my Spit 1500 (75 Federal model). The new sprockets had no timing
>marks, soooo.......I followed the insturctions in the Autopress book on how
>to set it up: #1 Piston at TDC. #1 and #2 lobes at "point of balance" (one
>lobe coming off equally to the other lobe coming on).
>
>Question is this: will this technique be accurate for this non-stock cam?
>
>
Brian-
Sounds like you have the makings of a" tres" cool street ride...
Given the fact that you are installing an aftermarket cam, you need to do a
little more "homework". Did you get a spec sheet with your cam ? If not, call
the company and get a complete spec sheet for the cam. This sheet should
include opening and closing numbers as well as duration, lift, and
clearances. The way you are trying to set your cam is called a split overlap
method. In theory this will work ok, however, many cam grinders will put what
is termed an asymetric grind on the lobes, so that the area under the lift
curve is shifted one way or another (earlier or later). This type of cam
grind can not be setup this way. In addition, when a cam is reground for
different specs, the lobe centers may be changed (they usually vary from
about 100 degrees to 112 degrees on most cams) so that setting the cam up
using the highest lift at some degree setting, will not always work either.
In addition, using the ole "eyeball" method can often result in errors which
may not look like much, but in reality, account for all the additional
expected horsepower from the new cam. The only way to really set up any cam
is with a degree wheel. You paid the extra money for the cam, you might as
well set it up right.
So once you have a degree wheel (most any speed shop, about $10) attach it to
the crank, find top dead center, mark the degree wheel at zero, put a dial
indicator on the push rod/lifter, and turn the crank til you get to the
grinders spec (some thing like 20 thou lift at 30 degrees before top dead
center for the open point). Only turn the crank in the same direction it
normally rotates. Check both the opening and closing points. You will
probably have to change the cam gear around a few times to get it right. The
specs from the grinder never quite match up with what you can get. Try
checking the specs on another cylinder if you seem to be off by a set amount.
Try to get as close to both the opening and closing as you can, but if it
comes down to a decision over a degree or so, get the specs so that the
opening is the closest to the specs you have (wear retards the timing).
Don't forget to adjust your push rod length in order to keep decent geometry.
This will keep your guides and valve stems alive much longer.
You will want to shorten your pushrods until the rocker arm reaches a point
where the rocker is approx parallel with the head at 1/2 of the cam lift.
The Car Curmudgeon
Nick in Nor Cal
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