List,
I'm trying to decide on rocker ratio, a cam, and valve lift. I could use
some help.
David Vizard makes a good case that it's better to get as much of the
total valve lift as possible from high ratio roller rockers, and then
get whatever is left from the cam.
Basically his arguments are that it's good to minimize the cam lobe
height, and the high ratio rockers give the valve springs a mechanical
advantage -- a valve spring of a given strength will exert a greater
force on the cam followers using a 1.65:1 rocker than it will with a
1.46:1 rocker.
Steve Gruenwald at Integral cams says that the cam grid should take into
account the rocker ratio, and physical aspects of the rocker. That is,
if a cam is ground assuming a 1.65:1 rocker ratio, and you use it with a
1.46:1 rockers, the cam will not be operating as well as it should.
Conversely if the cam is ground assuming 1.46:1 rockers and you use it
with 1.65:1 rockers, you may end up breaking things. See his comments
below.
Vizard also states that the theoretical maximum value valve lift is not
achievable -- it is limited by other aspects of the valve train. I
assume things like spring binding. I guess this means to get as much
valve lift as possible. You can't get too much until the springs get
excessive or place too much force on the cam followers.
My impression is that for a street engine TR250 I should get a short
duration cam -- maybe 270 to 280 duration and a high valve lift. What
would be a good maximum valve lift I should shoot for without going nuts
about it? This is a sunny day 2500 mile/year street engine not a race
engine. I think I should get as much lift as possible without doing
something stupid that would seriously compromise reliability. On the
other hand it will only be driven 2500 miles per year -- not every day.
Probably a few times a week, 30 weeks out of the year. I will have cam
bearings.
All of this leads me to think that I should start by selecting the
highest ratio rockers I can find and then get the rest of the valve lift
from the cam. Does this make sense, and are there any disadvantages of
the 1.65:1 rockers (assuming I get an appropriate cam to go with them).
The highest I know of are the 1.65:1, but they are needle bearing
rockers and Vizard and others recommend bushed rockers rather than
needle bearing rockers. I know lots of people use needle bearings, so
I'm not saying they are unacceptable or that something is wrong with
them. Does anyone know where I can get 1.65:1 bushed rockers? Ted
Schumacher sells 1.55:1 both bushed and needle bearing for the same
price but does not recommend needle bearings. It's also interesting that
APT (Vizard) only sells 1.65:1. I don't know if they are needle or
bushed. They are expensive $699.00. I bet bushed and I will check.
I assume I will get a hardened shaft even with the bushed rockers.
I guess I've rambled on a bit. The bottom line is....
Is there any problem with 1.65:1 rockers assuming I get the "right" cam.
What is the most valve lift I should use assuming a short duration high
lift cam. Maybe this is impossible to answer w/o knowing the flow
characteristics of the head? I guess I would assume port matching, three
angle valve grind, cut back valves per Kastner's book, 30 degree valve
angle, and "mild" head work -- sharp angles removed. Not going nuts on
the head.
Comments? Thoughts? Other ideas? Am I making this too complicated? If
you get the drift I'm confused by all this, you are right.
Thanks,
Don Malling
>From Steve Gruenwald:
The resulting valve lift may be different with the 1.55:1 rockers. Alot
depends on their mass and their moment of inertia. That is why I need
to
look at a sample part. There are a lot of factors in cam design and
sometimes a higher rocker ratio lets you move around different things
(i.e.
velocity, acceleration, and jerk) inside the profile so that you can get
the
same endurance but with more valve lift. When I design the lobe for a
certain rocker ratio, I have to take into account the acceleration of
the
profile which effects the loads on the parts as well as the harmonic
signature which effects the valve spring. Most cam lobes are designed
for a
certain rocker ratio. It is dangerous to start putting more rocker arm
on
any cam lobe because you can get premature cam wear, valve float at a
low
rpm, and possibly even break parts. Some cam manufacturers design their
cams for the highest ratio that is likey and then it is safe to run a
lower
ratio than that. (Hence, where the range comes from.) The only problem
with this is that if you end up using the lower ratio, you have a cam
lobe
that is too conservative. That is why I designed our lobes for a
specific
rocker arm ratio (the stock one). At this point, I am not sure if it
is
worth the cost of the rocker arms for .025" more valve lift (a likely
number). We would have to do back-to-back dyno tests with both sets of
rockers on two different cams with the same duration to see what the
difference was.
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