In this month's (May '09) Car Craft, Jeff Smith answers a SBF build question and tells the guy not to use 1.7 rockers with an aggressive ComCams XE274H because
"this would have a peak horsepower point of 6,700 . . . you will need dual valve springs with lightweight retainers."
Can anyone give us a general equation for determining when rocker ratio plus cam lobe intensity becomes detrimental? For instance, it is to be assumed a Ford FE with 1.72 rockers wouldn't work well with the same lobe which would max out a same displacement big block Mopar with 1.5 rockers and vice versa.
thnx, jack vines
Jack Vines
Studebaker-Packard V8 Limited
Obsolete Engineering
Jack , I am seeing this phenominum showing up a lot lately, if I have this figured out correctly the high lift 28* 1.5 type lobe ramp is too aggressive to control two things , one is valve loft with a 1.7 rocker and the second is pushrod/polevault deflection on the opening side of the ramp , hence the need today for 5/16 x .125 wall pushrods or heavier/stiffer and hollow valves or titanium valves with titanium retainers and locks to control the valve train harmonics. Seat pressures today are just out of control, 10 years ago WE RAN 36* intensity cams with a hell of lot lower spring pressure and a lot higher RPM's , now mayby our power level was down a tad but the other end of the equasion is the cup cars that can not go 500 miles. The power level went up with the closer intensity but durability dropped dramaticaly. Myself I am looking for someone with 32* lobes so I can sorta get the best of both worlds.
Mike Jones are you listening ? 246@.050 single pattern what do you have with a 1.5 rocker sbc. or 246x248x106 solid ? John at Baprace@aol.com
How does it make sense the higher rocker ratios should not be used on the hi intensity lobes when the cam companies use that same lobe for engines with 1.7 or higher ratios? BBC and BBF comes to mind.
Not to mention the valve train is much heavier in a big block Chevy than a small block. I do agree the added ratio will need more spring to maintain stability at the same rpm regardless of valvetrain weight.
What I run in a certain Pro-Stocker has the following intake specs:
310 at .020, 281 at .050 207.51 at .200, .5753" lobe lift, and a 1.9:1 rocker arm for a gross valve lift of 1.093".
It has been on the Spintron and holds 10500 without valve float for over 90 seconds. The other cams previously used would go into destructive valve loft at 10,200, when they reached that RPM. The operators believie that at 10,500 they had reached the limit of the electric motor on the Spintron to spin against that valve spring load.
That cam has a 29.47 degree Major Intensity, 27.49 on the opening side, 31.46 on the closing side.
My 31 degree Major Intensity cams are 28 on the opening side, 34 on the closing, and we use them with 1.8 and 1.9 rockers all the time, even with .515" lobe lift. There have been no complaints about valve spring life with these ramps.
The Major Intensity number, the amount of duration from .020" to .050", is a reasonably good prediction of how a cam might perform. However, there is a big difference between symetrical and unsymetrical cams, in that unsymetrical cams have 2 very different Major Intensities.
The opening side predicts reversion control and intake port velocities, the closing side predicts valve train life and RPM.
i've had great luck with them. excellent durability. i prefer the xtl or sxtl lobes. i've run the xtl with 1.7 rockers no problem.
i dont think you can look at just major intensity. i think its all in the way the cam is designed as a whole to prevent unwanted valvetrain motions/harmonics.
Carroll wrote:I'm not sure but I think the XE274H is a hdy. cam. The problem with alot of rocker ratio might be with the hdy. lifter not being able to keep up.
Bingo.
That's what jeff was talking about.
The hydraulic lifter falls off sooner with the higher rocker ratio on those type of lobes..
UDharold, I think you are responding with a roller lobe cam spec, I am talking flat tappet and all I am saying is cams are wearing out like crazy because we need so much "spring pressure to valve weight ratio" to control the valve train action, I think we are headed in a direction that needs a cam and lifter change as quickly as a race car needs new tires, I'm not saying that the high intensity cams do not make some more power I am saying that the return per dollar spent is getting smaller to the point of cost versus longevity tradeoff is not balanced anymore, now if you opt for a harder core and tool steel lifters, it does last a little longer but for an average weekend racer it's not cost effective. 99% of racers are not all out maximum effort engines, they need some sort of balance between cost and longivity. He11 we are almost running these engines on pure zinc and direct lube lifters just to make them live.
Baprace wrote:
Mike Jones are you listening ? 246@.050 single pattern what do you have with a 1.5 rocker sbc. or 246x248x106 solid ? John at Baprace@aol.com
Here's a couple solid lobes in the area you're talking about.
Here are a few of my current lobes, and a few of my 29-year old lobes:
Current:
272 at .020, 243 at .050, 155 at .200, .34883" lobe lift
278 at .020, 249 at .050, 161 at .200, .36350" lobe lift
281 at .020, 253 at .050, 165 at .200, .36670" lobe lift
288 at .020, 259 at .050, 171 at .200, .36830" lobe lift
All have the same Major Intensity, the same positive and negative acceleration rates, and the same maximum velocity, staying over .017" from the edge of the lifter.
Old, from April 1980:
276 at .020, 243 at .050, 153 at .200, .34500" lobe lift
280 at .020, 247 at .050, 159 at .200, .35000" lobe lift
288 at .020, 255 at .050, 166 at .200, .36000" lobe lift---Master F1
292 at .020, 259 at .050, 170 at .200, .36770" lobe lift---Master F4
Ditto on THEIR Maqjor Intensity, positive and negative acceleration rates, and maximum velocity, still staying around .017" from the edge.
All these April 1980 lobes have gone over 100,000 miles on the street in various SBC and BBC engines.
They used VT-1992 mech lifters, with NO oil feed to the cam lobes.
The difference is strictly in the current oil used.
Of course, a bad design is still a bad design, and will cause trouble.
Find a cam company you trust, and stay with them.
Old School wrote: ↑Sat Mar 07, 2009 8:27 am
How does it make sense the higher rocker ratios should not be used on the hi intensity lobes when the cam companies use that same lobe for engines with 1.7 or higher ratios? BBC and BBF comes to mind.
Not to mention the valve train is much heavier in a big block Chevy than a small block. I do agree the added ratio will need more spring to maintain stability at the same rpm regardless of valvetrain weight.
You may look at the lobe specs and think they are the same lobes, but a lot of the lobes with the same spec are altered for lifter diameter and cam core/base circle size. That may have been a general assumption.
Adger Smith
Adger Smith Performance Engines
903 794 7223 shop
903 824 4924 cell
adgersperf@aol.com e-mail
PackardV8 wrote: ↑Fri Mar 06, 2009 3:21 pm
Greetings, Speedtalkers,
In this month's (May '09) Car Craft, Jeff Smith answers a SBF build question and tells the guy not to use 1.7 rockers with an aggressive ComCams XE274H because
"this would have a peak horsepower point of 6,700 . . . you will need dual valve springs with lightweight retainers."
Can anyone give us a general equation for determining when rocker ratio plus cam lobe intensity becomes detrimental? For instance, it is to be assumed a Ford FE with 1.72 rockers wouldn't work well with the same lobe which would max out a same displacement big block Mopar with 1.5 rockers and vice versa.
thnx, jack vines
Sounds like he must have been talking from an experience with this particular camshaft combination.
Experience teaches that "why" isn't necessary to know; knowing the actuality matters most.
Of course, he does not seem to be considering valve train weight either, which will make a big difference.
