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Re: Those are Excellant numbers for small head!

April 26 2012 at 10:09 AM
Dan Jones  (Login 74Pantera)
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Response to Those are Excellant numbers for small head!

> Dan I'm impressed by those numbers for sure.

For heads that only had 220 CFM intake flow (but 200 CFM exhaust), the results
were pretty good. The first 408C we did with iron 4V heads that had 322/200
CFM flow and the results were not that much better. I put that down to a
better optimized cam for the 393C. I modeled the engine and optimized the
cam in Dynomation for the 393C. I didn't have that capability when we did
the first 408C.

> What was cam size on that pantera and did it offer good street manners also?

288/296 (234/238) degrees duration, 0.593"/0.562", 109 LSA
overlap = 74 degrees overlap, 2.85 ramp rate (mild)

Using these Bullet hydraulic roller lobes:

Intake: HR288/343 288 234 150 .3430 .593 CRA
Exhaust: HR296/325 296 238 148 .3250 .562 CRA

The Bullet lobe catalog lists lobes by lifter type, lifter diameter, nose
shape, lobe symmetry, and RPM or Torque lobe shape (ramp rates) and groups
the lobes by a 3 letter code:

The first letter is either "C" for a conventional shaped nose
on the lobe, or "D" for a dwell nose. Dwell lobes are often
used when there is a lift rule, such as NHRA stock classes.

The second letter is either "R" (RPM) for lobes suited for higher
RPM applications or motors with numerically high rocker ratios,
or "T" (Torque) for lobes suited for lower RPM or motors with
numerically low rocker ratios.

The third letter is either "S" for symmetrical lobes (opening
and closing ramps the same), or "A" for asymmetrical lobes
(opening and closing ramps different). Asymmetrical lobes usually
have a slower closing rate to help prevent valve bounce.

so the lobes I chose were CRA which mean a conventional nose, RPM/high rocker
ratio and asymmetric. In the 393C, this cam is well behaved on the street.
The owner is in his late 60's so the cam is a little milder than I normally
do for a Pantera owner. He had his engine rebuilt twice before bringing it
to us and both times the flat tappet cam ate a lobe so he wanted to switch
to a hydraulic roller.

Dynomation doesn't know if the heads are canted valve or not. Vizard had
made a suggestion that the lobe separation angle should be altered by 2
degrees (based upon his BBC testing). However, I misunderstood and narrowed
the LSA by 2 degrees when Vizard suggested it be widened by 2 degrees.
Turns out this recommendation is due to the fact that canted valve heads
are more susceptible to reversion. Vizard's suggestion is that if idle
quality is important, you can add 2 degree to the lobe separation to
minimize the effect of reversion.

Also, Dynomation doesn't know if the heads have been flow tested with an
exhaust stub (which can add 20 to 25 CFM to the exhaust flow) or without.
We flow test without a stub while most cylinder head manufacturer data is
with a stub. This tends to add exhaust duration to the optimized cam specs.

We were using a relatively mild single spring so went conservative on the
the ramp rate. We've since tested some springs that will allow a more
aggressive hydraulic roller in a 351C to rev higher. If I were to design
that cam today, I'd do it on a 111 or 112 LSA, shorten the exhaust duration
and be looking for lobes with ramp rates around 3.2. For that particular
engine combination, the cam would look more like:

285/288 (235/238) degrees duration, 0.593"/0.562", 111 LSA
overlap = 64.5 degrees overlap, 3.17 ramp rate

Dynomation predicts that cam will have the same peak power as the earlier
design with better power and fuel consumption below 5000 RPM.

Dan Jones

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