with. As a result, there’s a big move toward large
diameter, ellipsoidal or tapered pushrods. Pushrod
column strength is the key.
You can pretty much rate the overall performance
of different cams this way. The best is a solid
roller cam, followed by solid flat tappet, hydraulic
roller, then hydraulic flat tappet. How can a solid
flat tappet cam outpower a hydraulic roller? It’s
the weight of the lifter. A hydraulic roller lifter is a
rather hefty component; add the weight of the oil
and you can see why a solid flat tappet cam with a
lighter lifter can outperform a hydraulic roller cam.
Loft is something else to consider. Loft is where
the engine actually sees more lift than the cam
provides. It’s like controlled valve float. It occurs
in many flat tappet NHRA Stock Eliminator engine
combinations. Loft can increase both lift and
duration. In practice, it is possible to really “hit”
a flat tappet cam to achieve loft, particularly a
solid cam. It’s also possible to achieve loft with
a hydraulic if you use short travel (small preload)
hydraulic race lifters. Bottom line, loft is good for
power if it can be controlled.
Lifter-to-bore tolerances and overall geometry
are critical, particularly when you consider many
of the engines using flat tappet lifters have been
around for decades. It’s not uncommon to sleeve a
set of lifter bores to fix wear, past block damage,
and geometry issues. When lifter bore sleeves are
installed, the machine shop sets the block up in a
fixture, which in turn determines the angle of the
reaming tool. Once the bores have been aligned
and opened oversize, sleeves (most often bronze)
are pressed into place. The sleeves are then
trimmed and beveled before they’re finish honed
to size. This “blueprints” the bores, providing
proper lifter-to-bore clearance along with correct
geometry.
I should point out that if the geometry is spot
on, and the bores and lifter-to-bore clearance are
good, then all the bores need is a quick touch with
a lifter-honing tool. The idea is to create a smooth,
crosshatched finish. Each machine shop will have
their own concepts of how smooth the crosshatch
will be, so keep that in mind.
Finally, oil is key when working with a flat tappet
combination. As the motor vehicle manufacturers
moved away from flat tappet cams to rollers, the
major oil companies altered the additive packages
in motor oil. One additive that went away was a
compound called Zinc Dialkyl Dithio Phosphate,
or ZDDP for short. Zinc-based additives have
excellent anti-wear/anti-oxidant properties, which
is critical to flat tappet cams due to the lifter-to-
lobe contact.
There was an issue with ZDDP: the phosphorous
found in the additive doesn’t agree with catalytic
converters. Coupled with the OEMs’ move to roller
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