Hey PRO - Mechanical cam questions

[Tim_K]

I'm wondering about why mechanical cams seem to need so much lash. To be specific: I'm considering building a new /6 for my '71 Valiant. The stock cam specs say to use 0.010" and 0.020" lash. OK. The MP catalog says to use 0.028" and 0.032" on their 268 and 276 and 284 mechanical cams. Why is this? Why can't you use 0.010" and 0.020" on the bigger cams? As long as the valves are fully seated, which would be ANY amount of lash, why so much clearance, which would be quite noisy and irritating?
Also - when mechanical cams are made, and certain lift and lash figures are set, does the lash come out of the lift? What I mean to say is - If you have a 0.460" lift cam with 0.028" lash, is it really only opening the valve 0.432", or is the lobe made to give 0.488" lift, and the 0.028" lash brings it back to the 0.460" lift figure?

[OldReliable]

A lot of real good questions. The answer to the last part of your question is yes. Lift is given for most cams at 0 lash also referred to as "gross" valve lift. But then I think you had already figured it out. Until recently most performance cams used a lot of lash. Companies like Ultra Dyne and Lunati now make tight lash cams. For instance I have a Lunati for a 426 Max Wedge that requires only .012"lash. If you crank the lash down on a cam with a large lash you will upset the duration figures. It was generally thought in the past that wilder cams needed the large lash and clearance ramps to get the lifter moving but since the advant of computer design that is beginning to change. Hope this helps . If you want more I'm at "big-d@sympatico.ca"

------------------
Don
94 V10 Ram(001),74 440 Charger,63 Max Wedge

[PRO]

Great questions Tim!! your making me clean out a few cobwebs!! 1st off lash gives the lifter a "run" at the valve spring pressure since theres no give in the lifter like a hydraulic.The easiest way to explain it is try to push a locked door open while standing right next to it,now take two steps back and get a run at it,a little lash not only provides a bit of momentum it also assures that the lifter will rotate on the lobe,the more aggressive the cam spec the more lash required remember you also have higher spring rates,also the higher the rpm capability the higher the chances of pushrod/lifter/rocker failure,consequently more lash will lessen the chances of it.Remember the lifters are literally jumping around on the lobes at high rpms if you run less lash your actually trying to over contain them and the results will be disastrous.As far as lift,no the advertised lift is actual,now dont go measure your lift with stock rockers and get confused as stock rockers have an actual ratio of 1.44-1.46,but the advertised lift is based on true 1.5 rockers.Usually the base circle of the cam is less to make up for lash specs.Hope this clarifies things a bit...PRO..

[DartGT66]

About the lift; the announced lift is always gross lift and you have to subtract the lash to get the comparable 'real' lift. They only measure the lobe height and times it by 1.5. You can see it from the cam cards, so the advertised lift is not net lift, it's gross at least in the cams I have had. By the way, two years ago a friend of mine adjusted the 451's valves, he asked what the lash was and I told it's .020 and he did the job. We run it for about two months, a couple of races and so on, and when I checked the lash they were very tight. I asked the friend how did he adjust them, and it seemed like he had done everything like it should be done. I had to back the adjusting screws pretty much in every valve when I realized what was wrong; he had adjusted the valves with a mm gauge, and the lash was actually 0.20 mm, which is about .008". Anyway, the car run just fine with the tighter lash too.

[This message has been edited by DartGT66 (edited February 09, 2000).]

[Tim_K]

Ok, here's the next part of the question:
If the lift is rated at its 'gross' number at zero lash, and therefore has less lift when the recommended lash is added, wouldn't that also go for duration? If a 268/0.460" /6 Purple Shaft has 268 degrees at zero lash, wouldn't it be more like 260-262 with the 0.028" and 0.032" lash numbers set? I don't know why the manufacturer would measure lift and duration in different ways on the same cam.
I'm surprised I've never heard this before:
Instead of using a caliper to measure across a lobe at the open and closed parts, why not just set it up in a lathe (carefully), spray a little WD-40 on the lobes, and use a plunger-type micrometer to measure it up? Shouldn't be too hard to get a degree wheel in there, too.

[DartGT66]

Yes, the degrees vary too. The advertised degrees are usually given with a certain 'lash'that varies between the cam manufacturers. And if you vary the lash, or even rocker ratio, the degrees will change too. I have never measured that, but it should not be too hard to do.

[PRO]

Hey Tim and Dart do you have a mech cam you can measure ? I just measure a 273 mech cam installed in the motor with correct lash it reads .399 at the valve,advertised lift is .399,this is with aftermarket ductile iron 1.5 rockers,so I disagree that advertised lift is greater than actual lift,and besides ALL cam manufacturers have always given actual #s for lift and duration,this is what they sell and do for a living so to not be "precise" here would ruin their credability wouldnt it?Surely a cam manufacturer can subtract lash from gross lift and give you actual lift(dah)I have another mech cam in a big block I'll check next week sometime also but I'm sure it will prove the same point..PRO..

[OldReliable]

The better cam manufacturers have right on their cam ads "Valve lift at zero lash" Isky and Racer Brown to name a few. 96/97 Isky top of page 161 says "valve lift with zero lash"
It takes about 14 more degrees duration at .050" lift and .020" to .030"lift (whatever the lash is) for a solid cam to equal a hydraulic of the same specs. The 14 degrees duration makes up for the clearance ramps at the front a back side of the lobes (about 7 degrees each) that are used to accelerate the lifter before it makes contact with the push rod.Does that mean that a hydraulic cam can make more power for the same specs? Yes ,it does provided of course that the RPM range is not to the point of lifter pump up. Although it used to be a big problem years and years ago many hydraulic cams can now reach 7000 rpm without trouble although that is about the pratical limit.
big-d@sympatico.ca. if you have any questions I will try to answer them.

------------------
Don
94 V10 Ram(001),74 440 Charger,63 Max Wedge

[Tim_K]

Something I forgot to mention before about spinning lifters - I don't think they need all that much of a start to get going. Here's why I say this. Whenever I'm adjusting valves on my /6 (doesn't need it often) I have the engine idling with the valve cover off, lash set to the stock cam's 0.010" and 0.020", and I can see every pushrod twirling like crazy. No problem getting those lifters to spin. It's hard to tell, but I'd say they're making a full turn every 2 up and down cycles, but maybe even faster than that. Now this is on a high mile (187,000) engine with stock cam and 129,000 mile valve springs, so maybe things are loosened up a little, but how much different can it be on a H.P. cam and springs?

[ChristianCuda]

Hey Pro while we are on this subject. Which would you suggest be used a hydraulic or Mechanical or just jump to a roller for something say over .533 lift total. or at the cam lift of .355. I realize with a .355 hydraulic you can get to .568 with 1.6 Rockers. But with the same ratio rockers which do you think would be better.

Christian


------------------
68 'Cuda 383
Working on adding EFI