camshafts

[Clem Z.]

if duke leaves you wanting to know more about cams go to, http://www.popularhotrodding.com/tec...ics/index.html here are explainations with pictures. like they say "a picture worth a 1000 words"

[Duke W.]

Re: camshafts

Vizard has done some good work over the years, but he fell off the deep end on this one and totally muddled the issue.

Point number one: Lobe separation angle is NOT a design parameter. It's a manufacturing parameter.

A designer works with actual timing points: Inlet valve open-inlet valve close/exhaust valve open- exhaust vavle close.

I recently designed a cam that is targeted at the 327/300 and later base 350 CID engines. A primary criterion was to maintain OE
idle characteristics and quality, so I started out with the same IVO and EVC as the 929 cam to maintain the same effective overlap,
and I also used the same EVO as a starting point.

I used the Engine Analyzer simulation program and SAE NET conditions. Over the past several years I've system engineered
several configurations with EA and have validated my SAE net system models with actual test data from these projects. SAE net by
its nature must include exhaust system backpressure and front end accessories. Big power can be made on a lab dyno to SAE
gross conditions with open exhaust and no accessories, but when you put it in a car, the loss can be HUGE, especially with a high
overlap cam that combines with exhaust backpressure to just KILL the torque curve.

The first parameter I worked with was the IVC point because it, of the four timing points, has the most dramatic effect on the torque
curve. As the IVC point is delayed the torque curve is shifted up the rev scale, so I delayed it until I achieved the maximum start point(revs) of the 80 percent torque bandwidth I had specified as a design criterion.

Since this cam is designed specifically for the exhaust/inlet flow ratio of pocket ported heads, which is VERY DIFFERENT than OE
machined heads with the ratio being much higher, I then delayed the exhaust opening point to beef up the low end torque, which
cost very little at the top end. Because pocket porting increases the flow efficiency of the exhaust port more than the inlet port, you
can delay exhaust valve opening, and the design ended up with LESS exhaust than inlet duration, which is the opposite of most SB
cams.

So now I had a basic design, and for the first time I looked at the centerlines and lobe separation angle, which are very different than
any OE or aftermarket cam, but durations and LSA are very similar to the LS7 when you factor in the relative port flow efficiencies of
the two engines, so apparently I arrived at the same solution as GM Powertrain.

Since cams are ground on mechanical machines that use a scaled up "master lobe" to create the contour, I decided to use existing
lobes that are closest in duration to what I had developed because creating a new master is very expensive, and it saved me the
trouble of having to actually design the lobe, which must include a through analysis of the dynamics. There were a couple of OE
lobes that were fairly close, but the exhaust and inlet are from different cams. I have all the dimension data, have done dynamic
analyses of both and they look good. Plus they are proven designs!

So I setup these lobes at the indicated centerlines from my original analysis, and then juggled them a bit from there to yield the best
torque bandwidth from 1500 to 6500 revs, and at this point I had a viable design, but I NEVER, EVER paid any attention to LSA
until it was finished.

Being as how both of these lobes are produced by cam vendors (and all have lobes that are very close in terms of basic specs) I
approached a vendor who sells repros of both cams. The question was whether or not these lobes (late phased inlet, early phased
exhaust, wide LSA) could be ground on the standard blank (It looks like all aftermarket SB cams are ground on essentially the same
blank, but I don't know the source.). For existing master lobes, grinders just need the master lobe number, inlet centerline, and LSA.
Alterntively, specifying both centerlines is usually satisfactory, since LSA is just the arithmetic average of the two centerlines.

So far the answer appears to be yes, but I am still working some details.

If you want to design your own cam, you will be well served to follow the above methodology. Forget about LSA until it comes time to
throw your design over the wall to the manufacturing guys.

Duke

[John D.]

Re: camshafts

Clem, Guess Duke told you for sure. I promise not to tell local folks if you buy next lunch. They think u still Smokey Clem.

[Clem Z.]

i think duke misunderstood the reason for the

the article. it just pointed out how the different cam terminalogy applied to the manufacture of camshafts and how it effects cam preformance with some graphs to help you understand. it also gave some real life tests vs theory.

[Duke W.]

Re: i think duke misunderstood the reason for the

But the real life tests were SAE gross on a lab dyno with open exhaust. That's not the real life of a street engine. A muffled street engine, even with a very good exhaust system is still going to generate several psi of backpressure at the top end, which completely changes the equation. Overlap and exhaust backpressure work hand in hand to kill torque bandwidth.

As long as guys continue to focus on top end power readings off a lab dyno with open exhaust, they'll never be able to configure a good broad torque bandwidth street engine.

Duke