02RedHawk

iTrader: (17)

Curious....

As I sit here at work pondering the ±0.1mm tolerancing that GM is forcing me into, I started to wool-gather about my upcoming cam purchase. Ultimately, I got thinking about cam specs, and what is typically deemed as an "acceptable" tolerance for these specs.

Say you ordered a 224 / 224 .563" lift 114 LSA ideal cam.

You receive your hypothetical cam, and the cam-card says the cam spec'ed out to be: 223.6 / 223.4 .555" lift 115.1 LSA

Would you keep it? If it were me, I'd say 'no thanks' and return it for a more precise grind. Too far off from the ideal specs that I wanted.

What if it was 223.7 / 224.2 .561" 113.8 LSA ? Certainly this is much closer to the ideal specs that you wanted, and most would probably keep it without evening questioning it.

This brings up the question: Where are your personal tolerances set at? How close to ideal does your cam have to be before you'd consider it acceptable?

And likewise, what sort of tolerancing do the cam mfgrs hold themselves to when grinding cams? In all produced parts, there is such a thing as UCLs and LCLs to allow for slight variations. But, how big of upper / lower control limits do they use, and still have it cost-effective?

MyLS1Hauls

iTrader: (2)

I'd say +-<1deg duration and +-.5*LSA would be fine and would make no appreciable difference on a street/strip application. Lift within a few thou. You've got to remember, not all the lobes will be the same anyway.

2xLS1

iTrader: (25)

You are not going to get that info from a mfg's cam card. You will only get what it is supposed to be. You will only get that info if you buy the cam from a vendor who uses a cam doctor or similar equipment and checks what they sell and includes that data with the cam And they usually only check 1 intake and 1 exhaust lobe. I know Thunder Racing and TSP cam doctor what they sell.

SnakeKiller

iTrader: (7)

My new TRex was a couple degrees smaller on the intake (241) than I expected but was over 251 on the exhaust side. Good thing those new pistons have reliefs in them

Tranzor

I little variance is to be expected... that's why I'm glad that companies like TSP cam doctor the cam first and then send you the cam doctor sheet along with the cam spec sheet.

My TSP cam spec'd out to be 231.1/237.6 .598/.598 112 LSA... damn close to the cam card which was 231/237 .598/.595 112 LSA.

02RedHawk

iTrader: (17)

That was implied in my question.

Steve - Race Eng

iTrader: (1)

I do camshaft analysis and simulation testing of various grinds. I have a special high definition Cam Doctor and some expensive software to do the analysis with. I have also attended Harvey Crane's Camshaft School.

That said, I can tell you that a degree either side of spec is acceptable. Camshaft lobe lift usually will not be off very much if at all from the lobe spec. I generally desire my LSA to be within half a degree either side of what I ask for.

Even if the cam is ground exactly as shown on the cam card it can still be bad. There are more important considerations. With a hydraulic camshaft (flat or roller) the base circle run out needs to be less than .001" and the cam needs to be straight. I have sent back many a cam for regrinding that had .003" to .004" run out. Run out is critical because it will lead to noise in operation. It can also cause valve float depending on where the run out is in the lobe profile.

Another problem area is the quality of grind. Even if the master is a quality (well designed) profile, if the machine the cam is ground on has an out of balance grinding wheel the camshaft will be unacceptable. You can not see this defect by eye but it sticks out like a sore thumb when you plot the acceleration curve of the lobe via computer. You will also hear it in operation. Keep in mind that the camshaft is by far the most accurately made component in the engine. The lobes are ground to better than a hundred thousandth of an inch tolerance per degree of rotation.

The camshaft is the major component of course in the valve mechanism. However, the actual rocker arm geometry has much to do with the valve motion and actual duration at the valve. The engine sees only the timing at the valve. If you are fortunate to have a correctly made rocker arm (many brands are not right) you can correctly adjust the geometry by changing the rocker arm stand mounting height and adjusting the pushrod length for that mounting position. The roller tip should be exactly in the center of the valve stem at half lift - Ferry's has a nice gauge to measure this with. The sweep should be pretty much equal each side of that center position from valve opening to closing for best guide life. This is not possible to do with a lot of brands of rocker arms but you should get it as close as possible or switch brands! You can not use a hydraulic lifter to check this since the plunger will move. You need to fill a lifter with epoxy or some other method to keep the pushrod seat up against the clip during the checking. You can not use a solid roller because the wheel diameter is larger than on a hydraulic roller. Also the pushrod seat may not be in the same position. If the geometry is off the opening and closing points at .050" lift will be off as well. This will effect the operation of the engine much more than the camshaft being ground a degree off of spec. Note that each valve should stick up exactly the same amount in both cylinder heads. In other words, all the valves are the same length from seat to stem and all the valve seats are machined to the same depth in the head.

When satisfied the geometry is correct, check the point of max. intake valve lift with your solid mock up roller. If you have a 112 and want it straight up, intake valve should be at max. lift 112 degrees after TDC. Note the timing (opening and closing) at .050" lift at the valve for future reference. Check the exhaust while you are at it. If the lifter banks are off angle the point of max. lift will be off bank to bank. You may have to do some fudging to average the position from bank to bank. My software gives figures for valve timing at various lifts based on rocker arm ratio and correct rocker arm geometry. This process takes time but the results are worth the effort. That's but one of the reasons some of us charge a lot more to assemble an engine.

Steve Demirjian
Race Engine Development
Sponsor

CAT3

iTrader: (9)

Although I am no rocket (Cam) scientist. I would agree with Steve, veery educational reply btw. And, to think of the runout being off, or really anything else, I would think it would help prematurely wear the cam bearings, may be why so many LSx have not-so-nice cam bearings after only a few miles from factory.

Steve, do you custom grind cam's, or work with someone to do this? More questions on this topic if you dont mind me pm'ing you with them? Thanks.

Charlie

Steve - Race Eng

iTrader: (1)

Charlie,

I don't grind cams here but have them custom ground mainly by Isky but I work with Crower and sometimes Comp. depending on the application. Isky supplies cam data @ .050", .100", .200" and .300" lift which no other company does. This data helps me generate a cam profile matching one of their masters with more or less duration, lift, etc. with my camshaft analysis software. This profile is then input into my engine performance simulation software - Dynomation, to get an idea of how the engine will perform with that particular grind.

The cam bearing problem is most likely related to block distortion by the way.

You can send me a PM or e-mail me would be better so I can attach a file if necessary.

Steve