The jist of the problem is most likely the lobe intensity. Isky claims that the Comp XE cams violate the 47.5% rule. The 47.5% rule applies to flat tappet cams for chevy sb's with 1.5 rockers but the concept is still the same for other configurations where the designs are "on the edge" or "over the edge" for lobe intensity. For 1.5 ratio SBC's, the duration at .50 must exceed 47.5% of the total valve lift or your asking valve train problems. For example, take a Comp Cams Magnum 280H, with 230 duration and, 480 lift...230/.480 = 47.9% which exceeds 47.5% therefore would not pose a threat to components. We do not regularly hear about the older, safer HE and Magnum designs rounding off lobes anywhere near as often as the XE cam designs. Unfortunately, some of the Comp Cams XE dual pattern lobes break this 47.5% rule on the intake side so they are likely to be problematic. The design has "steeper" ramps that are too quick for durability and reliability according to other cam manuacturers. They will wipe lobes in a heart beat especialy if you have not followed the proper break-in procedure. Other designs are more forgiving during break-in and less likely to fail.
-Mark.

If this is correct, then the new Lunati Voodoo line is trash also???
The voodoo (219/227 @.50) 262/268 .468/.489 would equal 46.7% and 46.4% ---they do state "roller rockers highly recommended"

Opinions?

 

Interesting! Did Ed come up with this? I had a couple of discussions with him a few years ago. Interesting guy!

The ratio for the 30-30 cam is 239/455 = 52.5% and the LT-1 cam inlet lobe is 231/435 = 53.1%. (The LT-1 cam exhaust lobe is the 30-30 lobe, but is advanced four degrees.) The L-79 cam is 221/447 = 49.4%. The 300 HP cam is 194/390 = 49.7% on the inlet side and 202/410 = 49.3% on the exhaust side.

Since the LT-1 inlet cam lobe is the same as the L-72 cam lobe (both inlet and exhaust are the same), the acceptable number for big blocks must be a little smaller, eventhough they have the cam lifter heel. For the L-72 I get 231/496 = 46.5%. This lobe is relatively milder on the SB because of the lower rocker ratio. The Duntov cam is the mildest of all the vintage SB cams: 220/382 = 57.6%. "Soft action" was Duntov's design philosophy - keep lift the same while increasing duration. The resulting milder dynamics allowed higher limiting speed with the standard production valvetrain components, including springs. Ed implied to me that Duntov got this idea from Ed Winfield. Could be. Winfield was an intuitive genius. Maybe Winfield even came up with this "rule".

I have a complete set of engineering data for these lobes - displacement, velocity, acceleration, and jerk, and they are relatively mild. In fact, the mechanical lifter cam dynamics are milder than the hydraulic cams, and this is born out by the above calculations using this rule of thumb.

Pushing the limit of valvetrain dynamics doesn't yield signficantly better torque curves, but it sure sends valvetrain reliability to hell in a handbasket! That's why I like OE cams. In addition to not having excessive overlap like most aftermarket cams of similar duration, they don't tear up the valvetrain.

For reference my Cosworth Vega is 218/355 = 61.4%. I can just crack open the valves (barely) with both thumbs. The valvetrain limiting speed is 9000, and I've never heard of one of these engines wearing out a cam lobe. They must be good for a million miles!

Duke

 

Interesting! I have been following the "nightmare Posts" of the cam failures of the XE series. I am planning a cam swap in next few weeks. After reading all the posts I am leaning more to the older Comp Cams grinds or Crane's cams. I have a SB and I think the old L-79 cam is a safe bet and would improve performance on my "base" 350 in my 72.
Mark

 

The Comp Cams EX268H is a popular grind, but there have been reports of wiped lobes: 224/477 = 46.9 % inlet, 230/480 = 47.7%, exhaust

XE262H: 218/462 = 47.2%, 224/469 = 47.8%
XE274H: 230/487 = 47.2%, 236/490 = 48.2%

Hmmmmm!

(Data from my circa 2000 Comp Cams catalog.)

Duke

 

I'm going to watch this thread since I just ordered a Lunati Voodoo 213/219, .454/.468 cam. That's 46.7%.

 

I just recieved my lunati voodoo 262/268 with specs listed in the first post.

I'll be using roller rockers as recommended and believe this would allow exceeding the 47.5% ramp standard - anyone know how to factor different variables into this equation?

What is the standard for full roller?

 

Roller tip rockers theoretically offer better valve guide life, since there should be less side force on the guide, but they have no real affect on valve train dynamics other than mass. If they are heavier than OE rockers, they will increase valve train loading.

If your rocker geometry is okay valve guide life is pretty good, and my simulations only show a 1-3 percent gain in top end power, less at lower revs. (The Engine Analyser program allows you to pick any rocker ratio for any cam lobe.) IMO roller tip rockers are a waste of money, especially when you consider the valvetrain reliability issues.

If the ratio is higher than OE they will increase valve train loading. You have to use the increased valve lift in the formula.

If a cam yields 47.5 % with OE 1.5:1 rockers, 1.6 rockers will be:

1.5/1.6(47.5) = 44.5%

Not good! You're going the wrong way. If you take a cam that is already pushing valvetrain dynamics to the ragged edge and increase rocker ratio, you are looking for trouble.

Duke

 

this is interesting

 

I took a look at some cam ads and catologs and sure enough, only the newest cams "violate" this "rule". The latest cams from Comp, Lunati, and Crane all seem to have higher lift to duration ratios than their earlier cams. I also noticed that this "rule" only seems to apply to sb Chevies. Anyone know why?

 

Valve lift is partially determined by rocker ratio, which may be different than 1.5:1 for other engines. The design of the lubrication system may also be a factor. Take a look at my earlier post that discussed how the L-72 cam violates the "47.5% rule" even though it has the same lobe width and lifter heel diameter as SBs, and the L-72 lobe is used on the inlet side of the LT-1, which doesn't violate the "rule". The BB has a 1.7:1 rocker ratio, and its valvetrain is generally not considered to be as reliable as the SB valvetrain.

Duke

 

"The BB has a 1.7:1 rocker ratio, and its valvetrain is generally not considered to be as reliable as the SB valvetrain."

I'm just curious as to which big blocks and according to who? I have never heard/read this other than maybe the valve rotators some older big blocks used.
Other than that older b.b. use larger 7/16 studs and bigger short/long pushrods- same lifters, same style rocker arms as a small block except ratio as you point out.
Newer Mark V/VI b.b. use non adjustable with 3/8 bolt which can be converted to older style rather easily. Same roller lifters as s.b.
I suppose if you don't know what you're doing you can make one unreliable

As for ramp rates obviously there has to be a limit how steep a flat tappet cam lobe can be before the lifter literaly digs into the side of the lobe. Thats where roller lifters solve problems with steep ramps.
I would think with any agressive cam that break in is critical.
I can't imagine that Comp and Lunati havn't done hours and hours of testing before putting these cams out but who knows if mabe they have stepped over the line in terms of reliabilty for the sake of horse power claims. Perhaps Shubecks radious lifter would work well with these cams. They're pretty neat except for the price.

 

Anectodal evidence over the last 40 years. It may be colored from the fact that early BBs suffered a lot of valve spring failures.

Duke

 

have you guys fooled around with drilled lifters to provide some oil to the lobes... besides just splash oiling? Drilled through the face to the oil passage way, robbing some of the oil from the pushrod feed to the rocker, to pressure oil the cam lobes?

 

Any problems with XRs? Is there maybe some other ratio? I put a 269 in my motor that i'm putting in and it has 1.6 RRs too.

 

How does increasing the rocker ratio violate that rule? Going from 1.5 to 1.6 is only going to put more of a load on the lifter, not change any angles. That is what they are talking about, right? How steep an angle can be before the lifter edge digs into the cam? I would guess that the 47.5 number is just a reference for 1.5 rockers only.

 

A value or rule for a 1.6 ratio would be 44.5%, right?

 

Go back and look at the formula. It's base on VALVE LIFT, not lobe lift.

Duke

 

Maybe I'm missing something. Isn't it about lobe lift or the cams' profile? I just don't understand how a different ratio rocker can change the rule...All the rocker will change is the amt of pressure on the cam.

 

Yes, you are missing something. You need to CAREFULLY read the entire thread.

Duke

 

So, what you are saying is that if I install 1.4 ratio rocker arms on my motor that has a Comp Cams XE 268, I'll end up with a 50.3% value (which is considered "safe"). Hmmm, so at this point the steepness of the ramps are OK now, considering how they were much too steep with the 1.5's having the 46.9 value.

"The Comp Cams EX268H is a popular grind, but there have been reports of wiped lobes: 224/477 = 46.9 % inlet, 230/480 = 47.7%, exhaust"