Camshaft Clearance Ramps

[Joe C.]

I have a Federal Mogul "Speed Pro" 30-30 reproduction camshaft installed in my L76 engine. I am trying to determine if, indeed, FM grinds these camshafts exactly the same as GM did.
I understand that the top of the clearance ramp is .017" above the base circle on the original GM cam. This would take up all valve clearance with a .0233" lash setting, based on an actual rocker ratio of 1.37:1.
Are these reliable numbers for the reproduction camshafts, as well? If there is any disparity between the two versions, then I would rather adjust a few thou too loose, than a few thou too tight! Can I safely use the .023" setting with this camshaft. How is this verified?

Joe

[Duke W.]

Re: Camshaft Clearance Ramps

It's reasonable to assume that the Speed-Pro 30-30 cam is ground to the same specs including production tolerance as the original. Otherwise, they would have to redesign the lobe and create a new set of specs in which case it would not longer be a reproduction.

If you want to verify the accuracy of your cam, take it to a shop that has a Cam Doctor" that will reveal lobe dimensional data and dynamic properties, which can be compared to OE spec. You would want to get the "ASCII file" that the Cam Doctor produces. The graphs and typical tabular data that are given to customers don't reveal detail to a low level, but the ASCII file is lift, velocity, acceleration, and jerk every crankshaft degree. The OEM design data is every camshaft degree, which is two crankshaft degrees.

The clearance ramps are constant velocity, so the top, above the base circle, is the point where you start to see positive jerk, which is .017" above the base circle for the OEM lobe data.

These ASCII files are about 20 pages of unlabeled data, so they can look like Greek to someone not accustomed to looking at huge tables of unlabeled data.

Duke

[Joe C.]

Re: Camshaft Clearance Ramps

Duke:

I don't need as much detail as provided in the American Standard Code file, besides, the cam is installed in the engine. But, since the clearance ramps are constant velocity (i.e.: zero acceleration, zero jerk), then even if there is some variation in tolerances between OE and Speed Pro, I am not clear on whether it should matter exactly where follower contact is made on the clearance ramps.
Assuming that there is zero variation between the OE 3849346 camshaft, and the Speed Pro reproduction, then .0235" follower clearance should result in contact at the top of the clearance ramp, in both cases (with no acceleration and no jerk). Anything more than .0235" tappet clearance will result in follower contact directly on the flank, with associated instantaneous acceleration and jerk, in both cases. So, I believe that you are recommending MAXIMUM clearances, as above, to insure gentle valve articulation.
The question is, how much LESS than .0235" do you consider prudent? Can you recommend a minimum tappet clearance setting which would cause the tappet to make contact at the very bottom of the clearance ramp? Wouldn't this increase lift and duration?

[Duke W.]

Re: Camshaft Clearance Ramps

I've never measured a meaningful difference between "hot" (idling) and cold clearance on my 327. This is to be expected since cast iron and steel have nearly the same thermal expansion coefficient.

At idle the valves are rather cool, however, when run hard the exhaust valve in particular will heat up and the stem will expand, closing down the clearance. Mechanical lifters must have some running clearance because if the valve stem expands enough to make it zero or slightly negative, it will hang opoen and rapidly burn. Even when running hard I don't think the stem expands more than a few thousands. Though the valve head may reach 1100 deg. F, the stem will run much cooler because 80 percent of the heat input to the valve is transferred to the cooling jacket through the va;ve seat, so the stem runs much cooler.

We want running clearance to be not greater than the top of the ramp and not less than zero, and since setting clearance cold will likely be the greatest the engine will see during operation, that's why my clearance recommendation is the top of the ramp times lash point rocker ratio, but if you want to set them a bit tighter at .022" I don't think that's going to cause any problems.

If anything you can run farther between lash checks, since the clearance tends to loosen slightly with normal mileage accumulation. Chevrolet recommended lash checks every 12K miles.

Duke

[Mike McKown]

I'm curious

why you don't want to run it at the factory specified .030? What do you hope to gain by closing up the clearance? Just bein' nosey.

I just bought one of these cams (Federal Mogul). Seems like the ramps checked .015, not .017. No big deal, I probably checked it wrong. I'm still settin' it on .030.

[Joe C.]

Re: Camshaft Clearance Ramps

OK Duke. That makes sense. If set to a MAXIMUM, 0.235", then that will prevent the lifter (follower) from slamming directly into the cam flank, and experiencing acceleration and jerk (changing acceleration, or in/deg/deg/deg). So, if I understand the cam dynamics correctly, then .0235" is the max allowable follower clearance (lash) that will enable the gentle , constant velocity clearance ramps to do their jobs.
And, if I understand you correctly, then the ideal (theoretical) contact point for the lifter (follower)is near the top of the clearance ramp (+/- ????)probably within .003, roughly speaking.

[Joe C.]

Re: I'm curious

Mike:

According to the measured data, and that provided on the actual cam profile drawing, then, the way that I understand it from Duke, is that if you set the lash at anything wider than .0235", then the clearance ramp will be "nullified", and the lifter will directly contact the flank of the lobe. This will cancel out the effect of the constant velocity (gentle action) clearance ramps, and subject the entire valvetrain to higher-than-designed-in wear.
I assume that these settings are NOT recommended for fuel injected(L84)engines, as per Jerry Bramlett's posts. I will leave it to others to expand on this. In theory, I must agree with Duke's logic. However, if you look back at previous posts on this thread, and related, you'll see that Chevrolet, in their "Infinite Wisdom" widened the lash as a concession to .030" for the fuelies in order to produce more idle vacuum.

[Duke W.]

Re: Camshaft Clearance Ramps

As the running clearance increases from .023", the valves will be opened and seated at greater than ramp velocity, and at the opening point the lobe will be in an regime of increasing acceleration and jerk, so the valve train will see greater loading and the rate of seat recession will be higher.

As long as you set the cold clearance at .023, I don't think any operating condition will see any signficantly greater operating clearance, but the exhaust clearance will close up a few thou on hard running, which is okay, because there is plenty of ramp to maintain more than enough clearance to keep the valve from hanging open. Since, on this pushrod design, clearance usually opens up a bit with mileage accumulation, setting them a thou or two tighter than .023" will allow you to at least double the adjustment interval, and I don't think there will be any ill-effect.

Most OEMs now quote (cold) clearance over a range of about .004" or more. On a DOHC engine, clearance will close up as the valves and seats slowly wear and recess, but if you set them at the high end of the range, it will usually take tens of thousands of miles for them to close up even a thou or two.

For example, the inlet/exhaust cold clearances on my MR2 (2.2L 5SFE engine) are .007-.011"/.011-.015", and the recommended checking interval is 60K miles. Over the first 30K I checked them three times and reshimmed to achieve .014" on the exhaust and .009" on the inlet. I used close to the max on the exhaust since they are likely to receed faster than the inlets, and if I never check them again, they'll probably be okay for as long as I'm around.

The cold clearance spec on my Cosworth Vega is .014" both sides (same lobe) and from my own measurements, I know that the clearance expands .002" between room temp and operating temp. Also, a Cam Doctor analysis of the lobe shows that the top of the ramp is .016", so the .014" cold clearance is just right, and it will slowly tighten with use, which, again, at the rate I am accumulating miles means I can just about forget about ever checking them again, but I know I will.

It's too bad Chevrolet did not give us a range of operating clearanace rather than specifying just one value.

Duke

[Clem Z.]

the orignal GM specs were .025 for this cam *NM*

[Joe C.]

Re: Camshaft Clearance Ramps

Clem:

I remember you saying that last week in another post. It sounds like GM changed the setting to .030", possibly because the fuellies needed more idle vacuum. Jerry B. says that he cannot adjust a fuel injection unit unless the lash is set wide enough to provide decent idle vacuum. Sez he likes to see about 12-13 inches. All well and good. I have no vacuum issues, and I am very, very happy with the way the engine performs with 8-9" idle vac. I'm gonna set mine at .023" after a fair amount of evidence points out the advantages of doing so.
Thanks for your interest, Clem.

Joe

[Joe C.]

OOOPS, I sent it to myself

Clem:

I remember you saying that last week in another post. It sounds like GM changed the setting to .030", possibly because the fuellies needed more idle vacuum. Jerry B. says that he cannot adjust a fuel injection unit unless the lash is set wide enough to provide decent idle vacuum. Sez he likes to see about 12-13 inches. All well and good. I have no vacuum issues, and I am very, very happy with the way the engine performs with 8-9" idle vac. I'm gonna set mine at .023" after a fair amount of evidence points out the advantages of doing so.
Thanks for your interest, Clem.

Joe

[Duke W.]

Re: the orignal GM specs were .025 for this cam

The clearance spec on the engineering drawing is .025". How did they get that? By multiplying .017" - the top of the clearance ramp - by 1.5, but the lash point rocker ratio is not 1.5, but 1.37:1 according to my measurements and 1.44:1 at max lift with about about a .300" lobe. Thus 1.37 x .017" = .023".

The .030" clearance may well have been to get the FI engines to have acceptable idle quality. Idle quality was ALWAYS a problem with FI - more so than carbureted versions with the same long block. In any event, that loose clearance certainly doesn't do the valvetrain any good.

I know of at least one 327/365 owner who has tightened his cold lash to .023", and he didn't report any change of operating characteristics from my previous recommendation of .026", which was based on Bill Clupper's dial indicator reading of the lobe that indicated the top of the clearanace ramp is .020" above the base circle.

The engineering drawing is at least an order of magnitude more accurate than a dial indicator test, and the drawing data and dynamic analysis clearly show that the top of the lobe is about .016 -.017" above the base circle for the 30-30 lobe.

The LT-1 inlet/L-72 lobe top of the clearance ramp is about .012" (LT-1 exhaust side is the same lobe as the 30-30), and the Duntov cam has clearance ramp heights of .008"/.012", inlet/exhaust.

Duke

[Duke W.]

PS - FI

For FI with the 30-30 cam I would cold lash to .023" and then increase the idle speed to whatever it takes to achieve acceptable idle quality and stability. That will probably be in the range of 1000-1200 RPM.

For the 302 and 327/365 30-30 cam engines with carburetors, the minimum idle speed commensurate with (what I consider to be) acceptable idle quality/stability is about 900.

Duke

[Michael H.]

Re: Camshaft Clearance Ramps

Joe,

Just wondered what the results of the new lash settings were. Did you try .023?

[Joe C.]

Re: Camshaft Clearance Ramps

Mike:

I had set them last week to .025". The car ran just fine. Nice rumpity-rump. Caught rubber in all 4 gears with the 3.70:1 axle. Idle vac was 8 - 9" @ 700. No more detonation.

I'm gonna' remove the Posilocks today, reset to .023", check timing and adjust if needed (I don't think so), and set the idle speed to 800 - 900 as needed. I don't anticipate any drivability difference between .023" and .025", but if there is any, I'll post it.

Joe