396,427,454 High Performance camshaft question?

[Warren F.]

In reading Colvin's Corvette by the Numbers book, I've been intrigued by the camshaft profiles for the L78, L72, L71 & LS6. Can someone explain to me why the lift and duration were changed slightly in these years and what advantage might have been achieved or not by this.

In the L78, the lift was different for intake and exhaust respectively, .497 & .503, however the duration remained the same for intake and exhaust 348/348.

In the L72, the lift was the same for intake and exhaust, although higher at .520 & .520, however the duration remain the same, but lower for the intake and exhaust 336/336.

In the L71 & LS6, the lift was still .520 for both intake and exhaust, but now the duration was split for intake and exhaust, 316 & 302, respectively.

Of course on the L78,L72 & L71 we have about 11.0 to 1 compression, what does the 9.0 to 1 comp. ratio do to the LS6 in this circumstance? All the car magazines of the "era" say that the old '143' solid lifter cam was used in all these engines, but there are different part #'s as well as casting #'s, must be something going on here.

[Everett Ogilvie]

Clem, Duke?

This also interests me. I am by no means a cam expert, but my experience with long duration cams is that they can be pretty radical for street applications (lots of overlap can make for rough idle, low vacuum etc.). One of my old hot rods had a grind with long duration and only medium lift (as compared to other grinds for the same application), and this cam was a high rpm beast. It did not make much low end and did not start to "smooth out" until about 3400 rpm and made good power to 6500. Maybe Chevrolet decided to reduce the duration but increase the lift a bit for the L72 based on the setup of the L78? This possibly would make a similar power band but with better idle characteristics and better vacuum? I am sure Duke and Clem can help out here - Clem, what kinds of grinds did you use for racing vs. street grind? Duke - one of these days Dan Wilson and I will set up the dial indicator as promised and characterize our L78 and L72 cams so we can come up with optimal valve clearances. For now, using 022 exhaust and 026 intake works very well on both engines.

[Everett Ogilvie]

OOPS - Typo...

- clearances should read 022 intake, 026 exhaust.

[Warren F.]

Still puzzling ?

I guess, I'm more interested in knowing if these cam spec differences are responsible for a greater increase in horsepower, or if increase in cubic inches is a greater factor?

If the L72 grind was used in the L78 engine would this have gained more power, conversely would the L78 have gained more horsepower using the L72 specs? This thought comes about because of the articles written about the 450 hp factor in '66.

Also, since the L71 cam has an even different spec, would this cam bring a better horsepower increase over the other two grinds?

Would the LS6, being a larger displacement over the other two engines, be even more powerful, with the split duration cam, had it been the compression of the other two engines, L72 & L78.

Since the '71 LS6 uses the ZL1 heads, which were improved over the closed chamber design L88 heads, in several minor areas, and were touted in many articles to produce between 30 to 40 horsepower over the earlier versions. Would this engine have been more powerful than the others, with an 11.0 to 1 comp. ratio?

I keep looking at the engine specs, in various publications.

L78 396 cid, 425 hp @ 6400 rpm's

L72 427 cid, 425 hp @ 6400 rpm's

L71 427 cid, 435 hp @ 5800 rpm's

LS6 454 cid, 425 hp @ 5600 rpm's

and of course the '70 version

LS6 454 cid, 450 hp @ 5600 rpm's

We all know about the L88 and ZL1 and their ratings, being well below it's true capabilities. Obviously, the L71 & LS6 have ratings below what the engines should be capable of turning with the same quality internals as the L78 & L72, which are rated closer to the redline on the tach.

[Clem Z.]

Re: 396,427,454 High Performance camshaft question

my spec sheet list 2 different part # for BBC solid lifter cams. part# 3863143 intake 54/102,dur 336, exhaust 102/54, dur 336,lift for both .5197. part# 3904366 intake 44/92, dur 316, exhaust 86/36 dur 302,lift both .520, valve lash both cams .024/.028 but it says they are both the same cam. the part # that GM now lists is 3904362 for this cam. that number is close to 3904366. my spec list only goes to 1969. if you are going to run less than 11:1 CR i would go for a aftermarket cam that allows more cylinder pressure buildup.

[Clem Z.]

the difference between the street and race cams

is the over lap to allow the open exhaust system to help pull fuel into the cylinder. if you use a cam designed for open headers like the L-88 cam for the street you have a poor running street engine because you are getting a lot of the intake charge deluted with the exhaust gas still in the cylinder.

[Everett Ogilvie]

Re: Still puzzling ?

Warren - if you are referring to my two articles about the 450 HP L72, the 450 and 425 ratings had nothing to do with the cam. The HP rating was reduced from 450 to 425 mostly for political reasons coming from Detroit. You have a typo: you stated the L72 was rated 425 at 6400 - it was rated 425 at 5600, which is the key to the reduced rating. When GM management dictated that the engine rating was to be reduced, they simply reported out the HP at a much lower rpm (5600 vs. 6500).

The "marketed" ratings were mostly that - the actual power curves for the L72 indicate it made over 430 HP at 5600 (and 462 at 6500) but they reduced the marketing rating to 425 because apparently at that moment in time GM policy was not to label any engine above 425 HP. This changed the following year when they labeled the L71 at 435 at 5800 (which is really identical to what the L72 actually produces).

[Duke W.]

Re: 396,427,454 High Performance camshaft question

Through the ages, Chevrolet camshaft specifications have been a mystery because the methods of measuring "duration" and other parameters changed, and this "data context" was never specified, so the published camshaft data is essentially meaningless and useless.

Timing points and durations have to be given "in context" by specifying either lifter of valve lift. The Duntov cam duration of 287 is the "lash point duration", which is the point that the lobe flank transitions from a constant velocity ramp to increasing velocity by adding acceleration.

The original "30-30" cam duration of 346 degrees was taken at some lift point above the base circle, but included a lot of the clearance ramps. Despite having a copy of Bill Clupper's lift-crank angle diagram, I have never been able to figure out how Chevrolet came up with the original 30-30 specs. Same with the LT-1 cam, for which I took a lift-crank angle diagram.

Long after the release of the old Chevrolet SHP mechanical lifter camshafts
SAE developed the J604d standard that specifies "duration" as measured at .006" valve lift. On a pushrod engine the rocker ratio comes into play here, and it is NOT 1.5:1 for a SB or 1.7:1 for a BB. On an SB it starts out at about 1.37:1 at the lash point and increases to 1.44 at maximum lift. A BB will show similar variation, but no one that I am aware of has ever taken and published the data.

Most aftermarket cam companies use SAE J604d to determine "advertised duration", but they use the "marketing number" rocker ratios, not the actual rocker ratios, so timing on the engine will be different, and clearance specs based on the "advertised rocker ratio" will be too loose.

Someone needs to determine the actual BB rocker ratio throughout the range of valve lift and take some lift-crank angle diagrams of the SHP cams. The ideal time to do this is when the engine is being disassembled or assembled, and the most critical part of the profile is from the lash point to about .0200" lift at both ends of the cam, so you need good data granularity in these regions. Other than determining max lift, the shape of the profile over the top is not that important.

My suspicion is that all the BB SHP cams have the same basic lobe. The specs are different either because of differences in measurment context, or there may have been small changes to the profile - probably to aid valvetrain durability with little if any effect on the torque curve.

My research into SB cams and rocker arm geometry has allowed me to essentially "reverse engineer" them, understand their design philosophy, determine very accurate lash specs (which are different than what GM published) and develop accurate cam files for DD2000 and other simulation program that yield excellent simulation results. The same needs to be done with BB cams, but until someone measures the rocker ratio behavior and comes up with lift crank angle diagrams the BB cams will remain a mystery.

I have a number of "cam doctor" and "cam pro" files of various cam lobes, however, the data is never sufficiently granular in the area of the lash point to gain anything of value in terms of analysing the cams or developing accurate cam files for simulations. You get pretty pictures and lots of data, most of which is useless. The best way to get the necessary data on the cam is to use a good old fashioned degree wheel and two dial indicators - one on the valve retainer and another on the rocker arm pushrod socket. With the lash set at zero on the base circle you get both lifter lift and valve lift from the beginning of the opening ramp to the end of the closing ramp, which will yield a complete lift-crank angle diagram, and the ratio of pushrod movement to valve movement allows the computation of the rocker ratio behavior throughout the engine lift range.

That's all there is to it. Who with some BB hardware available is going to volunteer?

Duke

[Everett Ogilvie]

Cubic Inches

Regarding your question about cam specs vs. cubic inches - if the L78 and L72 had identical cams the L72 would still produce considerably more power. HP varies directly as a function of rpm and torque (both are in the numerator of the equation). Torque is a direct function of cubic inches and the 427 makes more torque at every rpm than the 396 (L72 makes 460 lb-ft at 4000, L78 makes 415 lb-ft at 4000). Both engines will rev to the same 6500 redline, at which point the L72 makes more torque, thus more HP. A factor to consider when looking at an engine's high rpm and HP capability is the bore/stroke ratio. The 396 and 427 share the same stroke, but the larger bore of the 427 makes it more over-square which is the fundamental reason the 427 makes more power than the 396.

[Joe L.]

Re: 396,427,454 High Performance camshaft question

Warren-----

Duke's information essentially strikes at the core of the issue---- the "conventions" used to describe camshaft specifications changed and/or were not consistent. So, you can't necessarily compare camshafts by using published specification data, even if the data comes from the same source (in this case, GM).

The GM #3863143 and GM #3904362 were IDENTICAL cams except for a few features:

1) The CASTING numbers were different (3863144 and 3904366, respectively)

2) The GM #3863143 had a groove in the rear journal for the "early" oiling configuration; the GM #3904362 had no groove for the later design oiling configuration.

If the groove was the only difference, then why did the camshafts have different casting numbers? Well, because the groove was a feature of the camshaft core (casting) and NOT just the finished camshaft machining operation (although the groove was FINISHED as part of machining).

[Duke W.]

Re: Cubic Inches

To add to this discussion. Consider identically configured engines other than stroke. Examples would be four-inch bore SBs of 377, 350, 327, and 302 CID. All things being equal except stroke, all these displacements would produce about the SAME peak power at the SAME mean piston speed. Say that mean piston speed is 3200 ft/minute, so the respective power peaks would be about 5100, 5500, 5900, and 6400.

Of course, the 377 would be the best street engine because it's greater average torque means more AVERAGE power across the rev range, but with ideal gearing in a given chassis the 302 would have the same top speed, but take longer to get there.

Also, assuming you establish a redline about 10 percent above the power peak, the 302 may suffer valvetrain problems without stiffer springs (and other premium valvetrain components which will add to cost), which could reduce valvetrain longevity, but the 377 can easily handle 5500 with generic production vavletrain components.

Duke

[Warren F.]

Re: Cubic Inches

Everett:

Thanks! I did know the right rpm's for the L72, didn't catch it before posting, actually pulled it from Lugvigsen's Corvette: America's Star Spangled Sports Car statistics page in rear of book.

Duke, Clem; Thank you for the explanation, a bit technical for me, but I believe some of the info will 'sink in'. Warren

[Warren F.]

Re: 396,427,454 High Performance camshaft question

Joe:

Thanks, again!

As usual, you are a god send! Reading your post, I am intrigued with your ZL1 project. I took one of my other big block sharks and made it a '70 LS6 car, once I learned that Chevrolet "failed" to emission certify it, in time for the model year 1970.

I am thinking of building another '71 LS6 engine, factory stock, but with one deviation, exchanging the pistons for a higher compression ratio. I would be very interested in feeling the results!

[Clem Z.]

more info

it turns out that #3904366 is the casting # for cam # 3904362 no wonder the # was close

[mike cobine]

May not be completely true in practice

Of course, the 377 would be the best street engine because it's greater average torque means more AVERAGE power across the rev range, but with ideal gearing in a given chassis the 302 would have the same top speed, but take longer to get there.

Also, assuming you establish a redline about 10 percent above the power peak, the 302 may suffer valvetrain problems without stiffer springs (and other premium valvetrain components which will add to cost), which could reduce valvetrain longevity, but the 377 can easily handle 5500 with generic production vavletrain components.



I agree the 377 would be the better street engine (high torque just works better on the street), but the 302 could make more power than the 377 due to piston speed.

In the 377 the piston speed is greater than the 302 at any given RPM, so therefore the 302 can rev higher than the 377 before each explodes.

Power is a function of RPM so with a 302, you could reach higher RPM and make more power.

For example, using the FAA formula for horsepower based on cylinder pressure, bore, stroke, and RPM, we have (note: I don't remember actual cylinder pressures and just used 200 for the example):

200 psi, 4.00 bore, 3.00 stroke, 8 cyl
7000 rpm 533.1 hp 426.5 hp with loss

200 psi, 4.00 bore, 3.75 stroke, 8 cyl
6500 rpm 618.8 hp 495.0 hp with loss

For most 377s built with Chevy parts 6500 is about it, but I've known a few to take the 302 to 8500 in crate form and so:

200 psi, 4.00 bore, 3.00 stroke, 8 cyl
8500 rpm 647.4 hp 517.9 hp with loss

Even my BIL built a 302 from used Chevy parts (typical 327 block/283 crank) right out of high school in '75 and turned 7500 toting an old '67 Impala so a 7040 RPM redline is not realistic with a 302. And he couldn't afford anything exotic. He couldn't even afford new parts.

As to getting to the same top end but slower, my 302 actually turned faster lap times at Sebring than my 427, but some of that can be attributed to lighter weight in the nose for better handling. But a 427 isn't the same bore. When running it in my essentially B/prod chassis against 350s in their B/Prod chassis, those with same rear accelerated faster but had lower top ends (past them in Big Bend) and those with higher rears would match my Top End in Big Bend, but lose the drag race out of the Hairpin.

But for someone a bit more famous, Grumpy Jenkins ran the 327 with the shorter stroke in his Vegas instead of the 350 and went faster. There would be insignificant weight differences in a 327 and 350.