Reply to "106 Lobe Separation Angle"

> It's a hydraulic roller cam.
> Int Exh
> Avg Duration: 276 290
> Dur @ .050: 220 230
> Valve Lift: .591 .588
> Lobe Sep: 106

Are you sure that 106 is the LSA and not ICL (Intake Centerline). Most of
Comps street grinds are on a 110 LSA but are installed 4 degrees advanced on
a 106 ICL. Do you have the cam card? If it really is a 106 LSA, that's too
narrow for a 351C with the sort of duration and specs you are talking about.

> I want a "HIGH REVVING" 6,500+ Cleveland that will idle nicely & PULL throughout the powerband!!!

A 106 LSA with 220/230 degrees @ 0.050" is not going to idle well or make
very good power. The 10 degree intake/exhaust split only makes things worse.
I went too narrow on the LSA for the 351C dyno mule. That cam was:

228/232 degrees @ 0.050" lift (280/284 @ 0.006"), 0.588"/0.588" lift,
107 LSA, 68 degrees overlap, installed in the engine on a 104 intake centerline.

We picked the cam specs before I had access to Dynomation (the tool I now use
to design cams). I used Vizard's cam spec rules-of-thumb and came up up with
a compromise cam since we were testing a bunch of different heads (iron 2V and 4V,
CHI 3V, Pro Comp, A3 and C302B high ports, etc.). However, I made a mistake and
subtracted 2 degrees from the LSA for canted valves when I should have added
2 degrees so the LSA was off. Later I re-designed the cam using the Dynomation
simulation program. I validated the simulation model using dyno data gathered
from several 351C, 393C and 408C engines on Dave McLain's dyno. The new cam came
in with specs:

228/234 degrees @ 0.050" lift (282/288 @ 0.006"), 0.620"/0.580" lift,
111 LSA, 63 degrees overlap, installed in the engine on a 106 intake
centerline.

If I had added 2 degrees to 109 instead of subtracting, I would have also
gotten 111 degrees using Vizard's simplified rules-of-thumb. Compared to
the original cam, the simulation suggests it's worth about 20 HP at 5800 RPM,
being better everyhwhere in the RPM range. Vizard's testing suggest that
109 degrees would yield similar performance but have a rougher idle. Narrow
it further and you'll start to give up power and have worse manners.

Here's a right up I did a while back on Vizard's rules of thumb. These are
simplifications of what his cam design software does but yield a good starting
point and check on my Dynomation simulations.

In "Be the Camshaft Expert" (July 2006 issue of Popular Hot Rodding) David Vizard
presents some rules of thumb for selecting cam specs based upon what he's learned
in developing a cam selection program. The program was reportedly 18 years in the
making and uses data gathered from several thousand cam tests. The basic idea is
that. for a given engine, there is an ideal lobe separation angle that works best
over a relatively wide range. This ideal LSA is primarily a function of cubic inch
displacement (CID) per cylinder per inch of valve diameter, as well as compression
ratio and valve inclination (canted or inline). Once ideal LCA is known, you pick
the desired overlap via application:

1. Street towing 10 to 40 degrees
2. Regular street 30 to 60 degrees
3. Street Performance 50 to 75 degrees
4. Street/Strip 70 to 90 degrees
5. Amateur Race 85 to 100 degrees
6. Professional Race 95 to 115 degrees

Overlap is the period when both intake and exhaust valve are open and serves
to set the RPM range over which the cam will be best suited. More overlap
means a rougher idle and poorer low end response due to reversion of the
exhaust charge into the intake plenum as well as loss of "effective"
compression ratio (compression is literally blown out the exhaust port at
low RPM). Where you fall in the overlap range is a function of valve size
per cubic inch. Big valves on a 302 use the low end, a 350 with typical size
valves use the mid-point, big inch small block or big block, use the right
hand side. Cam overlap sets the RPM range in which an engine will best operate
and the total overlap is a function of the cam duration and lobe center angle
(LCA). If you know the ideal LCA and the RPM range you wish to operate in,
the duration falls out of the equation.

In the article, Vizard presents graphs of cubic inch displacement (CID) per
cylinder per inch of valve diameter versus ideal LCA. The graphs are for inline
valve heads with compression ratios between 9:1 and 11:1. If canted valve heads
are used, the ideal LCA is adjusted by adding 2 degrees to minimize the reversion
(canted valve heads are more susceptible to reversion than typical inline valve
heads). The optimal LCA is also adjusted for compression ratios outside the assumed
range. Once LCA and overlap are known, duration falls out of the equation. Maximum
lift is a function of the intake valve diameter. Large valve heads will continue
to gain flow for relatively large lifts so the maximum lift for a Cleveland is
usually set by other considerations (seat and valve guide lift, piston-to-valve
clearance, etc.).

In the article, Vizard presented the results of a test demonstrating the
importance of picking the right LCA:

http://www.bacomatic.org/galle..._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. Another LCA test was performed on a
302 Small-Block Ford and repeated on a stroked (347 CID) version of the same
engine:

http://www.popularhotrodding.c...basics/photo_12.html

"When used in the 302, the 276/280 roller hydraulic cam on a 110 LCA proved
optimal, as even a 1 degree change either way produced worse results. Using
a SCAT stroker kit, this engine was stretched to 347 inches and re-tested
with the original 110 LCA cam. The stroker kit really helped both power and
torque. When the 110 LCA was replaced with a more appropriate 108 LCA cam,
the output made a further jump to the tune of some 20 lb-ft and 20 hp. The
108 cam in the 347 gave as much in terms of idle and vacuum as did the 110
LCA in the 302. Dozens of tests such as this show conclusively that the
overlap and LCA--not the duration--are the first steps toward generating a
cam spec."

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. Another point raised in the article is that, for most V8's with
reasonable heads, the ability to raise low speed torque with compression
increase holds to around 285 to 290 degrees (at lash point) of cam duration.
After that, drop off is faster than an increase in compression can recover.
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/galle...peed_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 wrote a little computer program based upon the article. The required
inputs for the simplified program are:

bore diameter (in inches)
crankshaft stroke (in inches)
intake valve diameter (in inches)
static compression ratio
canted or inline valve heads
desired overlap (picked from the ranges listed above)

Vizard's cam selection program is not available to the general public but I
know for a fact it takes much more into consideration than the simplified
rules presented above. A friend has run his program and it uses actual head
flow data, port size (length and cross-sectional areas), valve diameters,
rocker ratios, desired idle vacuum, compression, bore, stroke, fuel octane,
thermostat temperature, rod length and more. Basically, it attempts to feed
the "air pump" in the most efficient manner, given the parameters entered.
What it doesn't do is tell you what ramp rates you need. The recommendation
is to use the most aggressive ramp you can tolerate for your application.
However, more aggressive lobe shapes are noisier, wear more quickly and are
harder on valvetrain parts so that neds to be taken into consideration as
well.

The simplified program assumes you'll use the same lobe profile for intake
and exhaust. There's also an implicit assumption that the exhaust flow is
reasonable compared to the intake. Vizard also suggests the rocker ratio
on exhaust is best kept 0.1 to 0.2 of a ratio lower than the intake ratio.
The exhaust is under higher pressure and blows down early in the lift cycle
plus the exhaust is less sensitive to valve acceleration than the intake but
is sensitive to duration. I noticed in the Engine Masters Competition, most
of the entries were using shorter rocker ratios. I tested this on the dyno
mule 351C and we did indeed pick up average power (mostly in the mid-range).
If an existing cam's LCA is too wide, higher ratio rockers may increase output.
For rocker ratios between 1.5 to 1.9, each 0.1 ratio increase on the intake,
the LCA needs to be spread by 0.75 to 1. Be aware there are cases where the
ideal lobe center may be too narrow for acceptable street manners. For instance,
when stroking an engine (keeping all other variables constant), Vizard suggests
tightening the LCA up by a degree for something like every 16 cubic inches
increase in displacement. That will lead to very tight lobe centers which may
not be acceptable for certain applications. For instance, an engine equipped
with mass-air EFI (which will measure reversion flow as if it were induction
flow) might not be too happy with 104 degree lobe centers. Vizard's full-up
program allows you to specify idle vacuum to get around problems like that
but the simplified program doesn't have that sort of flexibility.

Another thing to be aware of is that narrow lobe separation angles require
an efficient exhaust with minimal back-pressure. If you have a bunch of
back-pressure (from things like restrictive mufflers or headers that turn
down sharply at the exhaust port exit) it will hurt a narrow lobe separation
angle engine more than a wide one. A quote from the article drives home this
point home:

"Let's make one thing clear here: Big (but not excessive) overlap is a prime
key to big power numbers, but only if your exhaust system sucks. Literally.
If you have ever heard that an engine needs a little backpressure, you might
want to ask yourself why an engine would want an exhaust system that literally
pushes exhaust back into the combustion chamber rather than sucking it out.
The simple answer is, it doesn't. If a big-overlap, big-cammed engine has an
exhaust system with any measurable backpressure, the price paid is a big drop
in output."

If your exhaust system is restrictive, it may be wise to err on the side of
a wider LCA (better yet fix the exhaust).

Since large valved heads tend to increase flow through high lift levels,
the lift recommended by Vizard's rules may be excessive for heads equipped
with large valves. The recommendation is to use a lift consistent with your
reliability goal (higher lift wears valves, guides and seats more quickly).

More info on Vizard's cam rules can be found at:

http://www.popularhotrodding.c...ft_basics/index.html

Dan Jones