Cam Artical In Chevy HP Magazine

[Gene M.]

There is a good explanation in the April issue with explanations by Nolan Jamora and Chris Mays of Isky Racing Cams And COMP Cams. These true cam experts explain LSA and timing overlap effects on torque band and power. Some of which differs from ideas stated on this board.

It was very interesting that Jamora says tighter LSA works best on a street engine for a good combination of low end torque and top end power with good idle. Also Mays says the tighter LSA boost low end torque and narrow up the power band. Apparently wider LSA cuts down on bottom end torque. Just the opposite of what has been stated on this board.

After reading this article I went to COMP Cam web site and found the following information characteristics of closer LSA. The reverse is stated for wider lobe spacing.

Tighter LSA:
Moves Torque to Lower RPM
Increases Maximum Torque
Narrow Power band
Builds Higher Cylinder Pressure
Increase Chance of Engine Knock
Increase Effective Compression
Idle Vacuum is Reduced
Idle Quality Suffers
Open Valve-Overlap Increases
Closed Valve-Overlap Increases
Natural EGR Effect Increases
Decreases Piston-to-Valve Clearance

I just wanted to share this with the viewers of this board.
I do not endorse nor defy this info, just sharing.

[Duke W.]

Re: Cam Artical In Chevy HP Magazine

This is not how engineers design camshafts.

LSA is not a primary design parameter. Whether designing a racing engine or a road engine, the exhaust system has to be considered, which requires choosing between headers and manifolds and knowing exhaust system backpressure, which is usually zero for a racing engine, as low as two or three psi for a well designed legal road system, and up to 15 psi for a choked down system like the single catalyst systems of the seventies.

The other primary considerations are head flow, E/I head flow ratio, and the minumum 80 or 90 percent torque bandwidth target.

Now you have what is needed to optimize each of the four timing points, and out of that drops the LSA. It's a byproduct, not an design input.

Optimizing a design for headers and open or very low exhaust system backpressure will result in a narrower LSA than manifolds with the same exhaust backpressure, assuming equal duration. But different durations at the same LSA means the higher duration cam has more effective overlap, which further muddles the efficacy of LSA. Unless the lobes of two designs are identical, comparing LSAs is apples and oranges.

High overlap can harness the wave dynamics generated by well-designed headers to yield negative pressure exhaust scavenging near TDC, but exhaust system backpressure will negate some of this scavenging effect or even most/all if backpressure is high. Most OE-type manifolds can't effectively harness wave dynamics for overlap scavenging, so they don't like much overlap, regardless of exhaust backpressure.

One has to consider the complete engine configuration as a system including the complete induction and exhaust systems and have a specification for minimum low end torque and a "design speed", which is effectively the redline. For a 7500-rev racing engine torque below 3000-3500 may not be a consideration, but it is on a road engine, so the beginning of a racing engine's 80 or 90 percent torque bandwidth can be the lowest revs seen in the slowest corner of the track. For a road engine, the minumim acceptable starting point for 80 percent peak torque for most users is typically 2000. Any higher and the engine will feel very soggy in the lower half of the rev range.

Articles like this that focus on LSA have been showing up in hot rod magazines for decades, but that's not how experienced engine system engineers develop valve timing.

Look at the camshaft LSAs of modern Corvette engines. They are in the range of 119-121 degrees, which means they have very low overlap for their durations compared to aftermarket designs of similar duration. GM engineers didn't learn how to design camshafts by reading hot rod magazines.

Whether restoring a Corvette engine, souping it up, or designed an all out racing engine, don't get led down the primrose path by hot rod magazine articles' obsession with LSA.

Duke

[Bruce B.]

Re: Cam Artical In Chevy HP Magazine

Does the previous posting mean that Isky and Comp Cams engineers don't know what they are talking about?

[Duke W.]

Re: Cam Artical In Chevy HP Magazine

Most of the camshaft designers in the aftermarket industry are not engineers - no engineering degree or specialized training at schools that have advanced engine research programs like the U. of Wisconsin, U. of Michigan, and MIT, where one learns the physics of IC engines to a very low level of detail.

Also, they primarily design "racing cams", which give little thought to usable low end torque or idle qualilty.

That's why I had to design my own camshafts to improve 327/300 performance without altering the idle behavior and normal low speed driving characteristics, so they could pass a PV without question.

In that endeavor, my MSME from the U. of Wisconsin Engine Research Center (I was also accepted to Michigan and MIT, but chose Wisconsin.) and available PC-based engine simulation programs were great assets.

I outlined my basic system engineering approach in "A Tale of Two Camshafts" that appeared in the Fall, 2009 Corvette Restorer.

Duke

[Joe C.]

Re: Cam Artical In Chevy HP Magazine

Look at the camshaft LSAs of modern Corvette engines. They are in the range of 119-121 degrees, which means they have very low overlap for their durations compared to aftermarket designs of similar duration. GM engineers didn't learn how to design camshafts by reading hot rod magazines.



Duke

This is true most likely because the LS engine series heads flow so well that they have little need for impulse tuning. Also probably because their design enables them to fill more quickly in the early part of the intake stroke. Likely that negative effects of low RPM drivability issues and exhaust gas dilution due to high effective overlap more than cancels any minimal power gains developed as a result of the paltry ram tuning effects of high RPM inlet and exhaust pulse tuning.

All of the LT engines through 1996 had LSA's between 112 and 115.5. The early LS1 motor used a LSA of 111 in 1997 which later evolved to 121 for the LS7 and 117.5 for the LS6

Old designed heads respond much more favorably to pulse tuning.

[Joe C.]

[Joe C.]

Reprint from David Vizard: How to Build Max Perf SBC

More from Vizard. Click on "view" then zoom to 200 per cent:

[Clem Z.]

Re: Cam Artical In Chevy HP Magazine

the LS head design are so good companies cast blocks that use these style heads while the block is the old style chevy small block to bolt directly into older chevy cars. http://www.hotrod.com/techarticles/e...ine/index.html

[Gene M.]

Re: Cam Artical In Chevy HP Magazine

Does the previous posting mean that Isky and Comp Cams engineers don't know what they are talking about?

[Bruce B.]

Re: Cam Artical In Chevy HP Magazine

Gene,
I agree with you 100%.
Bruce B

[Duke W.]

Re: Cam Artical In Chevy HP Magazine

For all you nascent cam designers out there - design a cam for a 327/300 with no visible external modifications that doesn't affect idle behavior or low end torque, but makes equal or better top end power than a Flint-built L-79. This will require dyno testing to verify your design.

Now send it through Flight judging and a PV and see if it can earn a Duntov award.

Once the Duntov award is achieved, write an article for The Corvette Restorer explaining, in detail, how you designed it and all the test results.

That's what I did.

...better get cracking!

Duke

[Gene M.]

Re: Cam Artical In Chevy HP Magazine

For all you nascent cam designers out there - design a cam for a 327/300 with no visible external modifications that doesn't affect idle behavior or low end torque, but makes equal or better top end power than a Flint-built L-79. This will require dyno testing to verify your design.

Now send it through Flight judging and a PV and see if it can earn a Duntov award.

Once the Duntov award is achieved, write an article for The Corvette Restorer explaining, in detail, how you designed it and all the test results.

That's what I did.

...better get cracking!

Duke

I don't think there are any "Camshaft designers" on this web site. I'm under the impression most NCRS guys use a camshaft of original design supplied by on of the major suppliers. But I do see that there are quite a few that are interested in cam specifications and discussion.

But for those that want more zip I suggest that Edelbrock has done a lot of the work for us. They offer cam and lifter kit #2102 that is very strong in an otherwise stock 250/300 hp 327. And I do mean stock, exhaust and everything. Bottom end is stronger, even from a stop, mid range way better as well as top end. It is very indistinguishable at idle from the 250/300 HP stock chevy cam. And it is made in USA. I don't plan of setting it on a dyno since I don't have nor drive a dyno. A seat on the pants indication is all I need.

I'm happy for you that you are content with your creation. But I don't see the need to do that.

[William F.]

Re: Cam Artical In Chevy HP Magazine

Duke,
Maybe I need to reread your article "Tale of Two Camshafts", But I thought they didn't give you quite the results you thought you would achieve.

[Duke W.]

Re: Cam Artical In Chevy HP Magazine

The first design (Special 300 HP camshaft) produced better low end torque than EA predicted with as much top end power as a Flint-built L-79, but that was less than EA predicted. (And don't forget that this L-79 top end power was achieved without affecting the 327/300's idle behavior, which was a design constraint. Some loss of low end torque was expected and acceptable, but it didn't happen.)

Good results in an absolute sense, but disappointing in a relative sense because my top end power expectation was higher.

The second design (McCagh Special camshaft) tested in the ballpark of EA's prediction for both stump-pullling low end torque with high top end power - higher that the Special 300 HP camshaft (after converting McRae's SAE corrected RWHP to SAE gross on a lab dyno), indicating that the McCagh Special may be a superior design, but Mike's engine was a different configuration - similar displacement, but less head flow and a longer stroke, so the question remains of what the McCagh Special will really do in a true 327/300 on a Dynojet chassis dyno, and one is in work.

A lot of people appear to have misinterpreted the article, thinking the whole project was a failure. That's hardly the case, but I didn't try to gild the lily about the first design not providing the top end power I expected (even though it made better than expected low end torque), which was better than a Fllint-built L-79.

Now, you should reread the article. In the process of writing and editing I read it dozens of times, and I know what it says, but the final chapter still awaits test data for the '64 327/300 with the McCagh Special camshaft, which should be forthcoming in the next few months.

I have the head flow data, and it's right were I want it, especially on the exhaust side, which may have been an issue on John McRae's engine.

Duke