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One thing that most people overlook is the pushrod length.
Milling the heads, block, using a thicker or thinner gasket, new
valves or even just a valve job can change the required length.
Aftermarket heads with longer valves will also change the required
length. Any time any part of the valvetrain is changed, the
geometry should be checked. Changing the pushrod length is the
easiest way to change the rocker to valve tip contact point, in
most cases the pushrod length can be adjusted to compensate for
best power and reliability. Is some rare cases, the rocker stud
might have to be relocated, like when 1.7 big-block rockers are
used on a small-block Chevy or when a longer valve is used in a
stock head (this is usually only a problem when the valve is
+.200" or more).
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A rockers pivot point is lower than the pushrod cup and the
valve contact points for a good reason, so the wiping action
across the valve tip is the greatest at the lowest spring
pressures, this helps reduce guide wear. When higher than stock
lifts or any of the modifications listed above are done, the
pushrod might be too long. This would cause more side loads at
peak lift, which is where spring forces are the greatest. Imagine
a line from the center of the pivot point to the valve tip contact
point. As that line passes 90° of the valve stem, the wiping
action across the valve tip is minimal. So it makes sense to put
that point where the spring forces are highest. For best valve
guide life that point should be at about 2/3 of peak lift. If the
rocker pivot point is in the correct place, the tip should be in
the center of that valve at that point also. To figure the best
length, you must use an adjustable pushrod and adjust it until you
get what you need, you can then order a set of pushrods the same
size as you adjustable one, you should do this with a solid lifter
to avoid any leak down and remember add any hydraulic pre load to
the length. Manley offers an inexpensive pushrod length checkers
for most popular engines, these work fine if you know how to use
them, and most people don't. For an example, the tool to check
length on a small-block Chevy is set to check correct length with
0.600" lift at the valve. If your valve lift is not
0.600" you will need to do some math, or you can just use it
as a starting point to adjust your adjustable pushrod. Manley will
provide you with the information needed for your application. On a
small-block Chevy, any lift more than 0.600" gets multiplied
times 0.22 and that is subtracted from the pushrod length. So if
the valve is lifted 0.650", you will need to multiply 0.22
with 0.050 and the pushrod will need to be 0.011" shorter
than measured. If the valve is lifted less than 0.600", you
take a different approach. Subtract your lift from 0.600 and
divide it by 3. Then take a feeler gauge of that thickness and put
on the valve tip when checking length. If your valve lift is
0.500" the difference would be 0.100", divide it by
three to get 0.033", now insert a feeler gauge of that size
between the checker and the valve, and the measured length will be
correct. Once you adjust the pushrod to the right length, you can
then measure it and order the correct length pushrods.
The pictures shows the Manley pushrod length checker for
small-block Chevy engines. The engine here has a cam with
0.510" lift. In order to get the proper length, I need to
subtract 0.510 from 0.600 to get 0.090". 0.090 divided by 3
is 0.030. One last thing to consider on a hydraulic cam is that
when you pre load the lifters, the plunger will change the pushrod
position. A 3/8" rocker stud has 24 threads per inch. You
generally pre load the lifters about 1/2 turn, which will put the
pushrod about 0.021" lower. This needs to be considered as
well. So I need a 0.021" as well as a 0.030" shim on the
valve tip to make my measurement. I simply put 0.051" feeler
gauge on the tip of the valve between under the checker. Now I
just need to adjust the checker pushrod to just touch the checker,
lock it, and pull it out to measure it. In my case, with the
hydraulic roller, the length cam out to 7.345" long. It so
happens that 7.350" is a popular size, and that's pretty darn
close, so that's what's going in. Pushrods generally come in
0.050" increments. It's better to go a bit long that short,
so it's better to round up unless you are less than 0.015"
short.
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Setting the correct pushrod length will get the most mileage
out of the valve guides, but when radical cams are used they might
not be the best for power output. What many engine builders don't
realize is that the rocker ratio is not constant. The rocker ratio
changes through the lift cycle due to the geometry, this gives a
chance to change the lift curve slightly. On most engines, the
peak piston velocity will fall in between 73 and 78° after TDC,
so the higher you can get the intake valve lifted at that point
the easier it will be to fill the cylinder. Once you get the
pushrod length correct, try adjusting the rod a little longer and
shorter. See where the point is that you get maximum lift at 75°
ATDC. On most engines this should be pretty close to the correct
length. You will also find that when geometry is optimized for
performance, the point of maximum rocker ratio will be close to
TDC. After you find the best lift at 75° ATDC go back and recheck
your rocker to valve contact point. The geometry for best power
will usually be close to what was measured. Small details like
this could mean 10 hp on a 500hp race engine, so it pays to check.
I have heard a more than once that the proper length pushrod will
give the highest peak lift, this is simply not true. You want as
much lift as you can get at peak piston speed, which is the hight
airflow demand. With todays technology, we just can't get the
valves open fast enough. The point of peak airflow demand comes
somewhere between 73 and 78° after TDC, but we can't get peak
valve lift until more than 100° after TDC. What I'm getting at is
that if the piston speed peaks at 75° ATDC and peak valve lift is
at 108° ATDC, extra lift will help more at 75° ATDC than it will
at 108° ATDC.
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