Bullet lists lobes by the lifter diameter they are designed to. See:
The "Mechanical Flat Tappets" are designed for 0.842" diameter SBC lifters
which are smaller than SBF but can be used with larger SBF lifters. Lobes
designed for 0.875" diameter lifters are for SBF (including 351C) and the
0.904" diameter are for Mopars. One thing to be aware of is the Bullet
F285/355 and F295/358 lobes are listed under the smaller base circle SBC
lobes. When those lobes are ground on a larger diameter 351C core, they
will grow in duration (and overlap) over what is listed in the Bullet Master
catalog (and the cam specs above).
Also, Bullet does not break out cam lobes by application (street, oval track
or drag race). All lobes are listed from mild street to drag race-only. A
measure of a lobe's aggressiveness is how close the seat and advertised
durations are. The the Bullet F285/355 and F295/358 lobes are close together
so are quite aggressive. More aggressive lobes require stiffer springs and
may experience faster wear. The specs on the Bullet cam suggested elsewhere
250/260 deg @ 0.050
285/295 deg seat
0.585"/0.590" net lift (w/0.029" lash)
60.0 degrees overlap
As a point-of-reference, a friend runs a solid flat tappet cam in a 393C
powered Pantera with specs of:
239/244 deg @ 0.050
268/273 deg seat
0.576"/0.559" net lift (w/0.029" lash)
54.5 degrees overlap
While Mike's cam is smaller, the lobes are similarly aggressive. Even though
Mike's cam has a tighter LSA, the overlap is lower due to the shorter duration.
With CHI-4V heads and one of our ported Blue Thunder intake manifolds, the
engine made 411 rear wheel horsepower (over 500 HP at the crank) on a Dynojet
through Pantera GTS headers and short mufflers with tail-pipe inserts. BTW,
my Dynomation simulation predicted 417 RWHP at 6400 RPM but Mike forgot to
remove the tail pipe inserts on the Dynojet run so we think the prediction
was even closer. I ran the Bullet cam through Mike's 393C simulation and it
was down 45 ft-lbs of torque at 3000 RPM an didn't catch up until 6000 RPM.
I then re-ran the simulations assuming a 10:1 351C with 750 CFM carb with
similar results but the cross-over where the Bullet cam made a higher power
shifted up to 6200-6300 RPM. Remember, the Bullet cam will be larger (more
duration) when ground on a larger base circle Cleveland core so the cross-over
point will likely be even higher. To be clear, I wouldn't recommend Mike's
cam for your daily driver application. I'm just presenting it as a point of
reference. For an application like yours, I'd be looking at something with
an LSA of 110 to 111 degrees. When you get the LSA right, the area under the
curve will improve.
Mike's cam used Comp Cams lobes and the Comp catalog mentions that cams using
those lobes "should have provisions for increased oiling", along with other
cautions typical of race lobes. Mike assembled the engine himself but the
builder who specified the cam said to use lifters with EDM holes, beehive valve
springs and oil with high ZDDP content oil if he wanted the cam to live on the
street. With a short duration cam, getting a spring that can control the
valvetrain can be a problem. Avoiding periods of prolonged idling is also
prudent. In an Engine Builder magazine article, Vizard suggests similar
In addition to lifters with EDM holes, Vizard suggests grooving the lifter
bores to provide additional oiling the the cam lobes and lifter faces.
Whether lobes like these are suitable for a daily driver is debatable.
FWIW, Mike had to pull the heads due to an unrelated valve guide issue when
the engine had less than 3000 miles on the rebuild and the wear appeared
normal (didn't lose any lobes on break-in).
Vizard's intensity rule-of-thumb in the article referenced above apply to
hydraulic lifters. To bypass the clearance ramps, solid liter intensity is
the difference between duration at 0.020" and 0.050". I don't really have
a handle on acceptable (from a wear standpoint) solid flat tappet intensity.
Does anyone have long term experience with an aggressive lobe solid flat
tappet cam on the street? Everything I have experience with is in a street
toy with low miles. I've got a couple of engines going together soon
(Rover/Buick aluminum V8's which have a smaller base circle and lifter)
where we've got to make the hydraulic roller versus solid flat tappet
decision so any input is appreciated.
P.S. Some thoughts on flat tappet cam wear from an engine builder I know.
"For flat tappet cams, the diameter of the lifter or the length and
curvature of the finger follower defines the limits of possible profiles.
Once your pressure line gets to the edge of the follower, there's no more
to be had... That's geometry. It does make a difference; that's why flat
tappet racing lobes are specified by lifter diameter, and why rollers go
to inverted flanks.
Then you need ramps to take up the slack on opening, and to keep from
pounding things on closing. That further restricts allowable profiles.
Then you come to the big real-world part - wear. For a flat tappet,
wear is pretty much a function of load at the pressure line. That's why
Detroit cams are all so wimpy. They had to last 75,000 or more miles
without wearing to the point where they had to be replaced. For
hydraulic rollers, oil viscosity and pressure are your main limits,
where leakdown starts to change the valve motion significantly from the
lobe motion. And remember, that's assuming a 10 year old car that still
has the oil it left the factory with, in a worst-case service condition,
like pulling a camping trailer in New Mexico in the summer.
For a "performance" cam, longevity is sacrificed first. You can load
the cam more heavily, and shorten the ramps, and pick up lots of "free"
power by kicking the valves open faster and closing them sooner.
Competition Cams is infamous for this; some of their more aggressive
street cams seldom saw more than 20,000 miles before rounding off lobes
or hollowing lifter bases. They did
perform as advertised.
For "street" cams, you have to assume the stock valves springs are being
used, because a largish fraction of your customers will
running them. Getting more area under the curve with stock springs is a
damned good trick; you can whack it off the seat, but you have to stop
lifting early and carefully to keep the valvetrain together at max lift,
and you have to set the valve down carefully to keep it from bouncing.
Drag cams are quite specialized now, which is why you see the lobes
separated from circle track stuff. Longevity on a cutting-edge drag cam
is often less than a dozen runs for a flat tappet."