New cam design!

[Duke W.]

After diddling around with lobe design - basically trying to cut down the L-72/LT-1 inlet lobe to achieve the duration and dynamics I was looking for (It ain't as easy as I thought going in!) I bagged it and started "mixing and matching" OE SB lobes.

As OEM machined, "big port" SB heads (461, 462, 186, 492, etc.) have an E/I flow ratio of about 0.65, which needs a cam with longer exhaust duration in the form of a relatively early opening exhaust valve, and many OE cams feature this design architecture.

But when you pocket port the heads, the E/I ratio ends up at about 0.85 because pocket porting and the relatively short exhaust port combine to yield a much better improvement in flow effeciency on the exhaust side than on the inlet side, and this calls for a totally different valve timing scheme.

My goal is to increase torque bandwidth - almost as much bottom end grunt as the 300 HP engine with nearly the same top end power as a SHP engine. I used Engine Analyzer for most of the work along with lab and RW dyno tests from some engines that I have system engineered and have test data.

Right now the best combination is the L-46/82 inlet lobe, retarded six degrees combined with the 300 HP cam exhaust lobe advanced 4 degrees. Since the detail design data for these lobes is availalbe, it wouldn't be hard to have a custom ground cam manufactured assuming a blank is available that will support the "unconventional" points of max lift (POML) indexing for the lobes. (I use the term "POML" because the lobes are asymmetrical and the "centerlines" are not coincident with the POMLs.)

Basic specs are as follows (inlet/exhaust) and, of couse, it is a hydraulic lifter design:

Duration at .050" lifter rise, 224/202
POML: 120 ATDC/120 BTDC
LSA: 120
Gross lobe lift: .3067"/.2733"

This design looks decidedly "unconventional" compared to OE and especially typical aftermarket designs, but if you compare to modern LS-X cams, it's in good company.

Using the new LS7, which is the most extreme example, keep in mind that the designers intentionally crowded the exhaust port/valve to make as big an inlet port/valve as possible and ended up with a E/I ratio of about 0.65 (even with full CNC machining of both ports), which was a good decision because the "restrictive" exhaust port can be compensated with an early opening exhaust valve, and this is reflected in the timing and POML data.

The durations at .050" lifter rise are 211/230, inlet/exhaust, but the real revelation is that the POMLs are 124/117, which yields a LSA of 120.5, and almost no overlap. The inlet event is "extremely retarded" and the exhaust event is "well advanced".

The reason is that overlap and backpressue, even the low backpressure from a good low restriction exhaust system are mortal enemies! It's like mixing ammonia and bleach. The proof that this timing scheme is viable is reflected by the LS7's prodigious 80 percent torque bandwidth which extends from less than 2000 to about 7000.

This "new" SB cam I've cooked up represents the same philosophy albeit with the durations swapped to reflect the relative E/I ratio of vintage SB POCKET PORTED (not as OE machined) heads and the OE CNC ported LS7 heads.

Keep in mind that this valve timing is optimized for a SB with pocket ported head E/I flow ratio and WILL NOT perform well on a vintage SB with un-reworked heads. The "small" 1.94/1.5" valves work just fine with this cam on a 327 with the 2.02/1.6" valves only providing a marginal improvement. This valve timing will also produce similar results on longer stroke engines - 350s and 383s, again with pocket ported heads, but the 383, in particular will benefit from the larger valves.

This cam could yield a OE appearing 300 HP engine that makes nearly as much top end power as an SHP engine, and with proper "zero lashing" it should rev useably and cleanly to 6500. There will be a loss of low end torque, but only about half of what is lost with an OE SHP cam, and the idle quality will be just like a 300 HP engine - 500@18" and butter smooth.

If you select cams based on "how they sound at idle" go elsewhere and forget you ever read this, but if you have a 300 HP engine and want a "sleeper" with absolutely original appearance, this may be the hot setup.

Duke

[Duke W.]

Re: New cam design! PS

...forgot to mention that the retarded inlet event reduces the DCR to a mere 7:1 with an 11:1 static compression ratio, so even at 11:1 I believe it will operate detonation free on unleaded premium with the OE 300 HP or slightly more aggressive spark advance map.

Are hypereutectic 11:1 pistons available? ...don't want any of that SHP loose clearanced forged piston slap to marr the quiet operation of a 300 HP engine, do we?

Duke

[Verle R.]

Re: New cam design! PS

Duke,

You might check with Comp Cams to see if they have a blank that will support the proposed cam. I suspect a number of people would be interested in the cam if it could be produced at a reasonable (competitive) cost.

Verle

[James W.]

Re: New cam design! PS

Duke,

Sign me up! I take it I will have to have this cam ground myself?

James West
327/300 owner

[Duke W.]

Re: New cam design! PS

Yes, but since this camshaft consists of two lobes of known design, any cam manufacturer that produces 300 HP and L-46/82 camshaft clones has lobe masters and can probably grind it on a suitable blank.

One way would be a "group program" such as has been done with Crower Sportsman rods, but as with any new design there is risk and a chance that actual performance may not meet projections to one degree or another. The only way to find out for sure is build and test.

This might be a good project for an enterprising soul who is looking to rebuild a 327/300 or a base 350 or build a 383 and is looking for broad torque bandwidth and doesn't demand a low end torque-killing lumpy idle. And since the useable rev range will extend to about 6500 (on a 327) a rod upgrade, especially for pre-'66 327s is highly advised. On longer stroke engines the power peaks sooner, and the useable revs should extend to about 6200 on a 383 with peak power at about 5700 with 2.02/1.6" valves. A higher than OE flowing inlet manifold doesn't change peak power revs or maximum useable revs, but makes more power in the upper rev range without a loss of low end torque, especially on a 383. Something like an Edelbrock Performer and a Quadrajet or other carb that flows about the same 750 CFM @ 1.5" Hg. would be a good match for a 383.

My current EA simulations on a 327 with 1.94/1.5" valve pocket ported 461/462 heads and true 11:1 compression indicate that low end torque splits the difference between the LT-1 cam and 300 HP cam. The midrange is about the same as the LT-1 cam, and top end power is actually a little higher than the LT-1 cam with 2.02/1.6" valves, but the difference is not great, so I call it about the same.

Also, I don't think this cam with exhibit the typical SHP "hole in the torque curve" (due to all the overlap) that creates the surge of power at 3500-4000 because torque is fairly flat up to about 3000. Torque rise should be much more linear from the bottom end to the torque peak. SOTP it should feel something like a 300 HP cam up to 3500-4000, and like an SHP cam above this range - the best of both worlds!

Again, I want to emphasis that the timing of this cam is engineered specifically for OE heads that receive a typical pocket porting/port matching/multiangle valve job that yields an E/I flow ratio of about 0.85, and true CR of no less than 10.5:1. It is also optimized for OE exhaust manifolds and the OE exhaust system. All my simulation work on EA is done using "SAE net" air density, a clutch fan, and, very importantly, a model of the OE Corvette exhaust system that I believe accurately represents flow capacity and backpressure.

It will not perform well with production OE machined heads or low compression. I have not analyzed its performance with any aftermarket heads, but if they can be reworked to yield an E/I flow ratio of about 0.85 it should perform very well, and greater absolute flows than pocket ported OE heads will increase peak power and rev range, which could take peak revs up beyond the valvegear limiting speed on a short stroke 327, but not a 383.

Duke

[Kenny Hancock]

Re: New cam design!

Very interesting. How do you think this cam might respond to 1.6 rocker arms in a 383? On a slightly related subject, how would the stock L-46/82 cam perform in a 327? Is the L-46/82 in a 350 similar to the L-79 in a 327? Thnaks for the interesting analysis and reading.

[Duke W.]

Re: New cam design!

I'm not a big fan of aftermarket "high ratio" rocker arms. Engine Analyzer simulations indicate a few percent more top end power for rockers with 0.1 greater ratio, but can also take as much away at the low end.

Also, no one knows what the actual ratios are for the various aftermarket offerings, but I can guarantee you that, like OE rockers THEY ARE NOT CONSTANT, and the "advertised" ratio may not be real!

OE SB rockers start out near 1.37:1 at low valve lift and achieve about 1.44:1 at max lift with about a 0.3" lobe, and 1.44:1 is what I use in EA simulations (It does not allow for a variable rocker ratio, but 1.44:1 will yield more accurate results the the "theoretical design ratio" of 1.5:1.)

Rockers are cheap to build, easy for the user to install, and easily hyped because only a handful of people in the world have actually measured and studied rocker ratio behavior and its effect on engine output. For the typical $150 that the cheapest set costs (and some are much more expensive) I don't think they're worth much. Plus higher average and peak ratios will increase valvetrain dynamic loads, which will cut valvetrain limiting speed. You can compensate with higher rate springs, but then you may raise some durabililty issues.

I like the OE valvetrain and 3911068 OE valvesprings for all street applications and even prefer all OE valvetrain components for a vintage racing 327FI with a 30-30 cam retarded 4 degrees, which yields (from EA) over 450 SAE gross HP at 7000 with headers and open exhaust and true 11.25:1 CR, which is achieveable with the OE SHP pistons if you're willing to deck the block to achieve minimum quench clearance. Install some short skirt 12:1 pistons and it will come close to 500 from the combination of higher CR and lower internal engine friction.

The OE valvetrain is CHEAP and BULLETPROOF, and IMO it's not an area that will yield cost effective performance improvements or yield significant improvements without compromising durability and longevity unless you are getting into a serious racing engine that needs to be engineered with a power band that extends well beyond 7200 to make competitive power, and now you're talking BIG bucks, and such an engine would be TOTALLY unsuitable for street use.

The L-46/82 cam is best analyzed by comparing it to the L-79 cam. The basic specs (inlet/exhaust) - duration at .050" lifter rise, POMLs, and LSA are as follows (Lifts are within a few thou, so I have ignored them.):

L-46/82 cam - 224/224, 114/114, 114

L-79 cam - 222/222, 110/118, 114

So basically the L-46/82 cam is the L-79 cam with two degrees more duration retarded four degrees. This is okay for the longer stroke 350 (or 383), but if installed in a 327 I recommend advancing it four degrees to achieve the same POMLs as the L-79 cam in order to avoid loosing too much low end torque, and I doubt if there would be a meaningful difference in the torque/power curves within the accuracy of dyno testing. (EA actually shows about the same top end power but a little less low end torque with the L-46/82 cam advanced 4 degrees relative to the L-79 cam in a 327.)

It's not at all clear to me why GM didn't use the L-79 lobe for the L-46/82 cam. The L-46/82 lobes (very slight difference side to side; L-79 uses the same lobe on both sides) have very similar dynamics, and nothing significant jumps out from a comparison of the lift, velocity, acceleration, and jerk profiles. The L-46/82 exhaust lobe shows a little more aggressive average negative acceleration over the top - maybe because the exhaust valve is slightly lighter - but this is splitting hairs at a very low level of engineering detail. I expect the designer took the L-79 lobe and made some incremental improvements based on better and more detailed dynamic analysis than was done during the L-79 cam design process four years earlier.

Bottom line: The L-79 cam is best for a short stroke SHP engine - 283, 302, or 327, and the L-46/82 cam is best for a longer stroke engine - 350 or 383, but it's okay to use the L-46/82 cam in a 327 if it's advance 4 degrees from the OE indexing.

Duke

[Clem Z.]

increasing the rocker arm ratio

can cause premature valve float

[Kenny Hancock]

Re: New cam design!

Thanks for the response Duke. As a matter of interest, do shaft mounted rocker systems offer more accurate / consistent ratios than stock type?

[Duke W.]

Re: New cam design!

Shaft mounted rockers also exhibit variable ratio throughout the lift cycle, but behave slightly differently.

The instant center of a shaft mounted rocker remains fixed during the cycle, but the effective lever arm lengths change. The effective lever arms also change on a ball mounted rocker and the instant center also changes slightly.

One is not necessarily better than the other from a geometry standpoint. They just have slightly different behavior.

Determining shaft rocker ratio behavior can be calculated from all the angles with the valve closed and open. Calculating ball mounted rocker ratio is tougher because of the changing instant center, but either one can be measured on the engine fairly easily with a couple of dial indicators.

Duke