Cam quiz!

[Duke W.]

What do the LT-1 cam and SHP mechanical lifter big block cam(s) have in common? Yes, they both have mechanical lifters. That's not the answer I'm looking for. It's much more revealing and revelational.

Hint: The LT-1 cam uses the same exhaust cam lobe as the 30-30 cam, but it's phased four degrees earlier.

Duke

[George Poynter]

Re: Cam quiz!

They to come with a 4 degree advance designed into the cam from previously offered factory mechanical camshaft???

[Clem Z.]

Re: Cam quiz!

both have .485 exhaust lift

[Duke W.]

Re: Cam quiz!

Lobe phasing is different for all, but that's not what I was getting at. Here's another hint. Think "why reinvent the wheel".

Gross lobe lifts above the base circle, inlet/exhaust.

30-30: .32336/.32336
LT-1: .30572/.32336
L-72: .30572/.30572

Duke

[Chas Kingston]

[Duke W.]

Re: Cam quiz!

I have the engineering drawings, which tabulate the lobe lift every camshaft degree (two crankshaft degrees) to one one-hundred thousanth of an inch from the peak of the lobe to the base circle on both the opening and closing flanks.

Obviously, the production tolerance cannot be held to this degree of accuracy,but the important parameter to compare is the "design eccentricity" of the lobes as stated in the tables.

Duke

[Joe R.]

Re: Cam quiz!

Duh,

Was that the answer to the quiz? You have been keeping us on the edge of our seats all day.

Maybe the answer is: It opens and closes the valves.

Let me know if I'm correct. The suspense is killing me!

Regards,

JR

[George Poynter]

Re: Cam quiz!

Sounds like the camshaft engineer liked those numbers! Since the rocker arm ratio between the BB and the SB is only the start of the differences in the engines what do you feel was the point of using the lift number all the way out to the fifth decimal point?

[Duke W.]

Re: Cam quiz!

A mechanical lifter cam usually starts with what is known as a constant velocity ramp. For Chevrolet SHP mechanical lifter cams it is typically in the range of .0003 -.0004 inches per cam degree. At the top of the ramp, where we assume that running lash has been taken up, acceleration begins, but it cannot jump instantly to the peak allowable value without shockloading the valve train, so at the top of the ramp acceleration typically starts out at .00001 inch-squared per cam degree and from there will increase to a maximum of .00030-.00040. Over the top of the lobe, valve action is controlled by the spring (at peak engine speed), so maximum decceleration is about half the accleration that can be achieved early and late in the cycle when valve motion is controlled by the cam.

Back in the sixties this was probably all worked out by hand using empirically derived peak accleration and jerk (the rate of change of acceleration) profiles that the valvetrain could take based on past experience.

As you might know, Zora designed his famous "Duntov cam" by stretching out the lash points and using milder acceleration so the all the other OE components could be used while keeping the valve train stable and reliable for another 1000 revs to 6500. The behavior of the Duntov cam was probably the "baseline" for the following designs, but use of the Optron showed its deficiencies, particularly valve bounce at closing, which is probably why all the later design lobes are asymmetrical.

No cam can be manufactured to ,00001" tolerace. Actual tolerance is probably about +/-.0005", and this can sometimes be seen on a Cam Pro Plus analysis of a lobe in the form of some bumps and dips in what should theoretically be smooth lift, velocity, acceleration, and jerk curves. Any slight high spots will probably be worn off early in the cam's life, so if it is within manufacturing tolerance it should provide very close to the design action and lead a satisfactory service life.

Nowadays most "computer designed cams" are so called because of simple programs that work out all the arithmetic, but the designer must have an indepth understanding of the valvetrain dynamics to design a good lobe that will live and not destroy the valvetrain. This primarily means understanding the maximum acceleration and jerk that the valvetrain can take at any point on the lobe at maximum operating speed. The OEMs have sophisticated analysis programs that take into account pushrod and rocker stiffness to predict dynamic behavior including unwanted resonances and can accurately predict the peak acceleration and jerk profiles that the valvetrain can handle. Or, if a cam design causes some unwanted harmonics the offending components can be stiffened as necessary.

This is the real state-of-the-art in cam design, and is why the modern LS-X engines can have lifts in the .500"-.550" range with very conservative durations.

If fact, designing a cam for a pushrod engine is a much more difficult design task because of the relative lack of stiffness of the pushrod and rocker arm. By comparison, designing a cam for a direct acting DOHC engine is a no-brainer.

Duke

[Duke W.]

THE ANSWER!!!

Okay, I guess I better post it before this thread falls off the page. Repeating the question:

"What do the LT-1 cam and SHP mechanical lifter big block cam(s) have in common?"

The answer is that that the LT-1 inlet lobe is the exact same design as the L-72 cam, which shares the same lobe for both inlet and exhaust.

The lobes are not identical because the L-72 lobe has a larger base circle, but the eccentricity design is identical from the beginning of the opening clearance ramp to the end of the closing clearance ramp.

So the LT-1 cam is a combination of the L-72 lobe for the inlet and the 30-30 lobe for the exhaust. All three cams (LT-1, L-72, and 30-30) have differing lobe phasing because they were tailored for the relative exhaust/inlet flow of each engine, and the SB has is somewhat exhaust flow restricted.

Also of note is that the LT-1 inlet/L-72 lobe is "more asymmetric" than the 30-30 lobe - probably in deference to the BBs heavier valve train. Because the lobes are assymetric the "centerlines" are not coincident with the points of maximum lift. This makes comparisons of "centerlines" and "lobe center angles" meaningless as a way to compare with typical aftermarket cams, which usually have symmetric lobes. That's one reason why the OE cams are misunderstood by the average enthusiast who puts great faith in aftermarket "specs" such as duration at .050", centerlines, and lobe center angle. If you dumb down cam design to that level, you'll never understand it.

So the bottom line appears to be that the LT-1 cam was designed by using the existing and proven L-72 lobe on the inlet and the 30-30 lobe on the exhaust with the centerlines juggled to compensate for the SBs relatively restricted exhaust flow.

This brings up an interesting case. The LT-1 exhaust lobe is the longer duration 30-30 lobe was phased very early (4 degrees earlier than the 30-30 exhaust lobe) to account for the SBs restricted exhaust port. Pocket porting improves the exhaust flow relatively more than the inlet, which brings the ratio of exhaust/inlet flow closer to the 0.75 "ideal" that would indicate close to equal durations with near symmetric phasing, so I am looking into a design I have coined the "Super Duntov" using the LT-1 inlet lobe on the exhaust side on an SB with pocket ported heads. What's left is to work out the phasing.

Duke

[bruce11495]

Re: THE ANSWER!!!

Duke....When does production start????

[Duke W.]

Re: THE ANSWER!!!

Well, this may be just be an academic exercise, but I will complete the analysis to work out the lobe phasing. The cam could be ground on the blank used by Crane or Federal Mogul to make the LT-1 cam clone that they both currently produce.

At that point it's a matter of someone approaching them with a proposal to grind the cam. They already have the lobe profiles, so its just a matter of cutting the blank to create and LT-1/L-72 lobe for both inlet and exhaust at the specifed phasing.

Duke

[bruce11495]

Re: THE ANSWER!!!

Shouldn't be that difficult or expensive, I would think. But are you going to be the "test mule"?

[Charles M.]

Re: THE ANSWER!!!

Duke, Chuck Miller here. If you remember from our past conversations I'm planning a rebuild of my 327, expecting to use the LT-1 cam. Due to the cost (around here) of porting my iron 461 heads I'm thinking about saving them and buying TrickFlow alum heads. If you get your Super cam off the ground in the next few weeks or months and are looking for a test engine that is other than stock I would be interested in helping. Let me know.

[Duke W.]

Re: THE ANSWER!!!

Thanks. The basic difference would be indexing the LT-1 inlet lobe for
the exhaust so as to open the exhaust valve later while closing it at
the same point, so the reduced duration would all come off the front
end.

I've been through this with the Cosworth Vega and concluded that it
wasn't worth the effort of modifying the lobe by regrinding the exhaust
cam, and I may come to the same conclusion on the LT-1 cam.

It hurts the torque curve to open the exhaust valve too late due to
increased pumping loss. Opening the valve too early costs a little on
the low end, but it's not dramatic. Basic rule of thumb is to error on
the side of opening the valve too early, which is the case of the LT-1
cam with pocket ported heads.

Duke