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Reply to "Questions regarding George's 275/285 Custom Street Cam Profile"

quote:

Originally posted by Rocky:

... I am wondering how to determine the redline of this cams ...



An engine will rev as high as the induction, the exhaust, the ignition, the valve train allows it .... or until something breaks. Smiler An engine's rpm isn't necessarily the limiting factor ... it may be how fast the engine accelerates, and therefore how fast the valve train accelerates. The old time sports car guys remember that dual over-head cam valve train was universally fitted to in-line 6 sports car engines of the 1950s because they had very light crankshafts (no counter weights) and they revved as fast as hell. A V8 is not as demanding of an application as an in-line 6.

The factory M code engine had a low lift cam, a small carburetor, and a single point distributor. It didn't rev too high or too fast, in spite of the 4V heads. Uncorking the engine with a 780 carb, headers, a CJ cam & dual point ignition made a big difference, the engine revved faster and higher. But it still had limitations ... i.e. the push rods and the valve springs. The Cobra Jet engine only had 277 pounds valve spring force over the nose, and the factory push rods were only good to about 6000 rpm in spite of the low valve spring force.

I have a friend named Bob, the 351 Cobra Jet in his 1972 Mach 1 Mustang had 351 Boss valve springs, stiffer push rods, & dual point ignition. It revved to 7000 rpm easily ... even with the 4300D carburetor and factory Cobra Jet cam. There's an instance when an engine equipped with the Cobra Jet cam proved it could rev. Should I have told Bob it wasn't supposed to do that?

Any hydraulic tappet valve train will eventually be limited by tappet collapse. If the engine revs to 7000 rpm its doing good, don't expect more. Tappet collapse is the reason why, when making a hydraulic cam motor rev higher, its better to lighten the valves than to use more spring force.

quote:

Originally posted by Rocky:

... But now I am seeing reports that cams / engines are "all done" at 5x00 RPM ...



Most "hot rod" engines built following modern guidelines open the exhaust valves too late, due to using cams with 108 or 110 LSA. See my reply to Jack DeRyke (i.e. Bosswrench) on page 4 of this thread. Street engines (muffler equipped) gotta open the exhaust valve early to prevent the torque curve from falling-off like a brick at upper rpm. Aftermarket cams are designed to make the buyers happy. They have narrow LSA, narrow power bands, and lopey idle. They make an engine sound "cool" at a stop light, they burn rubber real well, and they flatten-out between 5000 to 6000 rpm. As far as the cam grinders are concerned, that's what the public wants. And they're right. Most North American kids, the young guns who buy all the parts sold in magazines, relate to drag racing. My oldest son is one of them. Every performance parts catalog lists dozens of street/strip cams. When is the last time you saw a "sports car" cam?

Notice in this instance 110° LSA does not equate to higher rpm and more power. Just a narrow power band.

Such lore and mysticism has sprung up around the lobe separation number. Grinders don't grind cams with more than 110° LSA because nobody will buy them. The number was never meant to be that important. In order to grind a low overlap cam with 110° LSA it is necessary to use very short duration lobes, like about 265° to 270° duration. Combine short duration lobes with the small cross-section ports of the average cylinder heads and the result is an engine with a very low rpm power band. A tractor motor. And since the exhaust valve is opening very late, well the torque curve doesn't just flatten out at high rpm, it drops-off like a brick. Combine that cam with an engine having decent static compression, like 10:1, and it will ping on premium gasoline because the intake valve closes so early that DYNAMIC compression is too high. How many of those complaints have you heard over and over? But tell a guy his engine will perform better with a cam having 114° LSA and he'll think you don't know what you're talking about. He'll think you're crazy, and old, and behind the times. I know people think this about me.

The other issue causing engine revs to flatten out early is the newest high-ramp-rate lobes, i.e. the Comp Cams X-treme Energy lobes and the Lunati VooDoo lobes. They tax the valve train and cause valve float & lifter collapse to set in earlier, or they cause other sorts of valve train issues such as spring surge.

The 351 Cobra Jet having a cam with 270° intake duration, and the 351 Boss having a cam with 290° intake duration, both make peak horsepower around 5800 to 6000 rpm. There's another instance, like the CJ engine in Bob's Mach 1, where intake duration had no bearing on rpm and horsepower. This is because the 4V heads are tuned for 6000 rpm. The power band is not dependent upon camshaft intake duration like it is in other engines. If you have an engine, equipped with 4V heads, that flattens out before 6000 rpm, it has a problem. Floating valves, collapsed tappets, valves sticking in their guides, carburetor secondaries that don't open, malfunctioning ignition, exhaust valves opening too late, somebody's pounded a potato up the tail pipe ... but the problem is not a short duration cam.

One very common problem is a retro-fit hydraulic roller cam using the Ford factory 5.0 hydraulic roller tappets. Those tappets are not designed to endure the side thrust forces of the Cleveland valve train, they distort, and they collapse at relatively low rpm.

quote:

Originally posted by PanteraDoug:

... It strikes me that the timing events on this proposed cam are very similar to the stock CJ cam, just with roller lifters.

The characteristics then should be very similar to a stock CJ cam, i.e., all done by about 6000rpm ...

...and I think that maybe that is why George is calling it a "Roller CJ Cam"?



Factory Cleveland street cams

1971 M Code (4V, 118.5° LSA)
EVO = 81° BBDC
IVO = 18° BTDC
Overlap = 37°
EVC = 19° ATDC
IVC = 70° ABDC


1971 Q Code (Cobra Jet, 117° LSA)
EVO = 82° BBDC
IVO = 18° BTDC
Overlap = 46°
EVC = 28° ATDC
IVC = 72° ABDC


1971 R code (351 Boss, solid tappet, 116° LSA)
EVO = 86° BBDC
IVO = 34° BTDC
Overlap = 58°
EVC = 24° ATDC
IVC = 76° ABDC

1970 D1ZZ-BX cam (505 cam, 114° LSA)
EVO = 84° BBDC
IVO = 36° BTDC
Overlap = 62°
EVC = 26° ATDC
IVC = 74° ABDC

SVO A341 cam (112° LSA)
EVO = 77° BBDC
IVO = 28° BTDC
Overlap = 61°
EVC = 33° ATDC
IVC = 72° ABDC

Look at all these street cams, notice they all but one open the exhaust valve (EVO) by at least 81° BBDC. The A341 cam opens the exhaust valve 4 degrees later than the others because it was ground "straight-up". The cam was known to flatten out early, it works better if it is advanced by 3 or 4 degrees. All the cams close the intake valve (IVC) at about 70° ABDC.

This valve event timing is not peculiar to the Cobra Jet cam. Or even to the 351C! I can show you cams in the current Crane Cams catalog, ground for the 302/5.0 engine which have the same timing events. In fact, I can show you a similar cam ground for the 289, in an early 1960s Ford performance parts catalog. The M code engine, which revved lower, and the R code engine which revved higher, share these timing events. Truth be known, these are just well performing, wide power band, valve timing events.

My cams are not "all done" by 6000 rpm, not in the least. Ask Chuck, his engine has one installed. It wants to rev, doesn't it Chuck? The original 351 Cobra Jet engine was limited by valve springs & push rods. The Autolite 4300D carbs didn't always work perfectly either. But with a good carb, good springs, good push rods it revved!

Low rpm performance and drivability of 351C engines diminishes quickly as overlap exceeds 62° due to their large valves; this is especially true of engines equipped with factory 4V heads. Street performance, low rpm pep, drivability and manifold vacuum all improve as overlap decreases. The exhaust valve opening (EVO) should occur at 80° BBDC or earlier to extend the engine’s upper rpm performance and to allow the engine to perform better in conjunction with muffled exhaust systems. The intake valve closing (IVC) should occur at about 70° ABDC to avoid reversion and preserve dynamic compression. Staying within those 3 parameters requires camshafts having lobe centerline separation (i.e. LSA) no less than 112°. Camshafts having a 10° difference between intake duration and exhaust duration can be timed to center the overlap event on top dead center while meeting the criteria of the two valve events (EVO & IVC). Centering the overlap event on top dead center aids low rpm performance and drivability because piston motion is almost nil from 30° BTDC to 30° ATDC (known as the “dwell” period); piston motion provides the energy making intake and exhaust gases flow where we don’t want them to.

Valve event timing, duration, nor anything else had any bearing regarding why I chose to call the cam a Cobra Jet cam. My motivations were far more sinister. You see, I like the name Cobra Jet, it sounded better than "M code" cam, which in reality my cam more closely duplicates the valve event timing of the M code cam.

Make good decisions.
Last edited by George P
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