[DeTomaso] Cam question

[Daniel C Jones]

> The consensus tells me the smaller the LSA, the lower the torque will
> be in the RPM's.  My goal is to get a more drivable mix and not really
> sure how much difference there is between 108, 110 & 112 LSA, can any
> one comment? Would I really notice?

First you have to make an apples-to-apples comparison.  For a camshaft
that means the same overlap.  Overlap is the period both the intake and
exhaust valves are off their seats.  Increasing overlap shifts the torque
curve up the RPM scale increasing higher RPM torque at the expense of lower
RPM torque.  Overlap is a function of both LSA and duration.  If you narrow
the LSA but do not decrease the duration, you've increased the duration so
it is not an apples-to-apples comparison.  In general, for a constant overlap,
a wider LSA flattens out the torque curve.  A narrower LSA concentrates the
power in a narrower RPM band which increases the area under the torque curve
between the shift points.  That said, each engine has an ideal LSA.  For
your engine that is 107 to 108 degrees.  In the "Be the Camshaft Expert"
article, Vizard presented the results of a LCA test:

http://www.bacomatic.org/gallery2/v/hidden/dan/dan-cars/album10/album15/LCA_comparison.jpg.html

 "Ideal LCA for that engine was 108 degrees. Narrowing to 105 degrees made
 similar power but had noticeably worse idle and low speed characteristics.
 Widening to 111 degrees lost power.

 At low speed, lots of overlap is bad as it hurts low end but overlap
 helps as the RPM increases. To a degree, you can offset overlap with
 static compression. In another article ("Compression Comprehension")
 about running up to 12:1 compression on pump gas, Vizard presented the
 results of a compression test:

 http://www.bacomatic.org/gallery2/v/hidden/dan/dan-cars/album10/album15/CR_vs_low_speed_output.jpg.html

 "When used in conjunction with a bigger cam, increased compression can
 work wonders for the entire curve. When a 265-degree cam (gray curve)
 was substituted for a 285-degree cam (blue curve), a substantial drop
 in low-speed output was seen. Raising the CR from 9:1 to 12:1 recovered
 almost all the lost low end and gave a further increase in top-end
 output"

> I spoke with Richard Iskydarian and he suggests a 112 LSA to increase
> vacuum to help the brake booster and is suggesting going with a smaller
> lift around .520, which is close to what I already have and my current
> cam has a 112 LSA and its a dog, but thats with open chamber low
> compression heads.

The extra compression will do increase the torque curve 15 to 20 ft-lbs
everywhere but the cam will behave basically the same.

> Richard also felt that the lift on the proposed cam was too high to cam
> durability and high rocker ratio.

There's no free lunch.  The more aggresive the flat tappet lobe, the shorter
it's life span which is why I chose a hydraulic roller for the 351C dyno engine.
As for the high rocker ratio, Bullet has lobe profiles for large rocker ratios.

Here's some advice from Dave Williams (I think I may have edited it a bit):

If you set up a cam on blocks or centers and use a lifter jig, or plot
it out on paper, it quickly becomes apparent that the range of actual
lobe profiles is sharply limited. 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. For rollers, the
diameter of the roller is your limiting factor. Eventually the pressure
line reaches the height of the axle, and spits the roller off to the side.

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. Note
that Comp recommends armor-faced lifters for flat tappet lobes that
require heavy spring rates.

For "street" cams, you have to assume the stock valvesprings are being
used, because a largish fraction of your customers *will* insist on
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. For solid rollers,
lifter failure happens first.

Power is cheap. Longevity is what costs the big money.

Crane and Isky like to talk about stepped lobes to handle harmonic
motion of the valvetrain bits, and other crap like that. It's just
marketing; on the dial indicator, it's all a smooth curve.

What's creepy is to mike a brand new cam, and find .005" difference in
lobe height from smallest to largest. Kind of makes me laugh at Crane,
which claims accuracy down to *millionths* when tossing cams into the
grinder...

Extended exhaust duration only adds overlap, with its attendant idle
and emissions problems. All-out racers use long exhaust durations for
scavenging, but that ruins your fuel economy on the street. It's primarily
a drag race trick to keep the power from dropping off quickly after power
peak. On the street, a symmetrical profile or even a short exhaust will
run just as well and with a lot more tractability. On the street, extended
exhaust profiles date back to the 15-psi-backpressure old-style catalytic
convertor days; they are obsolete.

Additional note from Gary Derian:

Overlap creates a lot of internal EGR during cruise which is great for fuel
economy if the spark is advanced enough. In the BMW world, intake and
exhaust cam profiles are either the same, or the intake gets the bigger cam.
Independent runner intake manifolding can withstand a lot more overlap without
trashing the low speed torque and tractability than a standard plenum carb
intake.

Dan Jones