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Walt Fricke's Avatar
 
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Lift at the cam

I'm building a home brew "cam doctor" which can be used on an engine installed in an air cooled 911. It will require removing one intake rocker, and will enter lift data for both lobes every 5 (or so) degrees of cam rotation, which will be measured by a rotary encoder on the fan belt or directly on the crank pulley as the crank is rotated 720 degrees. A microprocessor should be programmable to send a "data" signal every 10 crank degrees to the two dial indicators It will measure lift at the cam lobe, not lift at the valve.

It requires two data output capable digital dial indicators (the cost of these has plummeted, and in intermediate encoder to produce CSV output to an Excel spread sheet is now also reasonable in cost, in contradistinction to the most common high end similar measuring devices). Sort of by mistake I purchased one with a half inch travel (the other two I already owned had an inch of travel).

I was thinking of max lift being about 12.5mm, but then started worrying I wouldn't have enough travel. Then I tried reassuring myself that lift normally is specified at the valve, where cam lobe lift is magnified by around 1.3-1.4X, so half an inch ought to be plenty.

Am I right about this?



The mockup is for the Mark 1 version, which easily accesses the exhaust lobe on its backside with no rocker in the way, but because of the attachment on the backside of the indicator prevented them from being placed on the same lobe pair - not a big deal as the spread sheet could compensate. But the intake it reached only shows a tiny sliver outside of the rocker. I thought a narrow indicator tip could use this, but on trying one saw that the angles involved would bend the tip (there is a reason lifters have a relatively large diameter in engines which use them, I guess). So plan B calls for pulling an intake rocker, which is not a big deal. And the dial indicators I am using come with a back without the attachment part.

But my question is about cam lift - the rest is background.
Old 12-24-2017, 02:14 PM
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Why measure the cam? Aren't all the specs in relation to the valve lift and crank degrees? How do you reconcile the cam's lift to the piston position or crank position?

Maybe I'm missing why you are just measuring the cam?
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Old 12-24-2017, 02:28 PM
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Gordon - you are. I need To determine if the cam is what it is supposed to be. That will be done from measuring known stock cams, and avoids wondering about how the cam timing was set and what the lash is, as well as the fact that the rocker ratios apparently vary some as the cam lobe rises and falls due to geometry effects.
Old 12-24-2017, 02:58 PM
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Put the DI tip on the back of the rocker foot to get lobe lift. Put it on the retainer to get valve lift. Putting it directly on the lobe will not replicate what is actually happening because of the geometry of the lobe/rocker foot vs. the lobe and the small DI tip.
Old 12-24-2017, 06:38 PM
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OK, I'll wait to see your results, how you compare two cams, and how you can determine the indexing (0 degree position) of each cam or lobe.
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Old 12-24-2017, 08:28 PM
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Gordon - I don't really care where the intake lobe is at engine TDC as long as the lobe centers are what they should be. The data should also show lift at overlap, and while that will be smaller than the spec for setting lash (on a stock cam), it ought to differentiate a stock cam from, say, the 5.2mm of my race motor cams.

I've got to mull over Boosted suggestion, but I'm not doing this to come up with a better performing cam.

And the question I'd like assurance on is whether I am correct that even the wildest cam out there for these engines doesn't have lift, at the cam lobe, over a half an inch.
Old 12-24-2017, 11:02 PM
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You may have to sacrifice a couple rockers and grind a flat spot on the back to rest the DI. The rocker foot has a large radius and the contact point between it and the lobe changes, whereas resting the DI directly on the lobe results in a single contact point which will most likely not be the same as what the rocker sees. Since your desired outcome is a graph of crank degrees vs. lobe lift you want to measure what the rocker actually sees.

And Merry Christmas, can you say that on here?
Old 12-25-2017, 05:15 AM
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Merry Christmas

I'll be interested to see how this works out. Post your graphs when you are done.
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Old 12-25-2017, 02:51 PM
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Boosted - you can say that to me - I had a merry one.

I did a bit of research using the search word "Cam Doctor." Turns out they are out of business, but Performance Trends has a helpful website touting their various software packages.

If you measure the cam with a normal indicator tip (being careful to have the shaft plumb with the cam centerline - I hadn't thought of that) you will come up with a profile, for a cam follower setup, which varies from what the cam follower sees. Some setups for bench testing simply insert a mock-up cam follower and measure from there, which is more or less what you suggest. For that kind of setup, the diameter of the follower matters also. However, these clever folks can adapt data taken with an ordinary tip to account for the follower effect. Which is helpful if what you really are after is what is going on at the valve, so you can design your own cam, or account for actual valve movement.

But my goal is simpler. Using a consistent method, measure some stock cams. Then measure some cams in race cars to see if the measurements come out the same. If they don't, the car doesn't have the stock cam and gets disqualified.

I'm not working on graphs, though. A cheater cam is apt to have steeper flanks and broader peaks, so the area under the lift curve is larger = better breathing (or so the owner hopes). Its total lift may well not be higher, so just measuring lift isn't going to account fully for what may have been done to a cam. But unless the graph is a very large scale - maybe even more than 100%, being sure things are different isn't going to be very definitive - the lines are apt to be quite close together. If duration is way off, or lobe centers are, or total lift, well those are the easy cases. A more sophisticated cheater cam won't show variation of those parameters.

The spread sheet will be tables, which is what is behind the graphs you see of cams. The sheet can display the differences between stock and measured, and a fatter cam - say a 964 cam in an SC - should stand out pretty clearly.

Since this has to be done at the track, it has to be quick.

Of course, at some point I may teach myself how to use the graph feature of Excel - that might make seeing lift at overlap easier to spot. But you'd need some kind of conversion program to account for the fact that the distances from the fulcrum (the rocker shaft center) change both as the point on the rocker face changes (we all know how far the rocker wear area extends, and while that can't be a 1:1 change in fulcrum distance, it is a change. And from working with front suspensions, we know how even the elephant foot contact to fulcrum distance changes as well, even if we've never quantified it. For our usual tasks of rebuilding engines, none of that matters - we just want unworn parts installed in the usual way. And I don't want to get into that - head flow is tricky enough, and using detailed cam specs - as in every degree or even less - to predict how cam A compares to cam B sounds like it is difficult to do home brew style.

But no one has confirmed my lift at the cam question. I've got a spare set of SC cams so I could just measure, but I'm insulating and redoing my garage, and the stuff I've had to move around completely obscures where I think these are, and I need to purchase a second indicator.
Old 12-25-2017, 09:23 PM
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This is what I suspected your goal was, checking for "cheater cams". I will be interested in finding out how you can go about this. Seems to me finding the center of the lobe consistently will be a challenge. I still don't see how you will be accurately indexing them either, but I guess you have it figured out.
I don't believe there are any cams with more than .5" lift except maybe some custom exotics perhaps. Web Race is .520" and DC 100 is .530" both measured at the valve.
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Old 12-25-2017, 09:49 PM
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Walt,

Sounds like a worthy undertaking from what I've heard about how things have gotten in E class. I assume you'd only be checking cams for stock classes on aircooled 911 engines. If that's the case, you should be fine with .5" max valve lift. Elgin has a pretty comprehensive web page showing data for all their cam profiles + most or all of the stock profiles. They show cam lift and valve lift, and also mention that they assume a rocker ratio of 1.45 for regular stock rockers. The page is a little hard to read, but there's lots of data there. I scanned quickly, and it looks like the only cams with more than .5" valve lift are the aftermarket grinds.

http://www.elgincams.com/c-por2.html

Scott

Last edited by stownsen914; 12-26-2017 at 06:49 AM..
Old 12-26-2017, 06:46 AM
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Thanks, Scott - that's what I remembered. Though I may purchase a 1" range indicator. Having had one apart, I'm pretty sure you don't get better resolution (at this price level) for 1/2" than 1". For the range of interest, the internal slider moves the same distance, and my suspicion is that the PC board, which has a grid which in some clever way reads a parallel grid on the slider (it is not a potentiometer), is the same too. To have 2X better resolution one or both would have to have twice as many traces in the same space.

With a different holder it should work on the twin cam 4 cylinder cars (944S, S2, and 968). With a long extension on the indicator it should work like a champ on a 914! Been a while since I've seen one of those in a stock class, though.

Walt
Old 12-26-2017, 10:44 PM
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Cam profile measurement......

Walt,

If I understood your post, you wanted a procedure to identify a certain cam that is installed in a race motor. Then all you have to do is tabulate two axes. Degree (0°~720°) versus cam profile (mm) and tabulate the data in Excel to give you a chart. This will be good for a certain cam power configuration using your above set up.

In summary, you could identify a certain cam from “control cam” using a dial indicator and a degree wheel. I believe Steve Stomski has an electronic degree tool I saw in Hershey.

Tony
Old 12-27-2017, 07:46 AM
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BlueDial, Bluetooth Digital Dial Indicator
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Old 12-27-2017, 09:19 AM
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I put the indicator tip on the top surface of the valve spring retainer.
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Old 12-27-2017, 09:42 AM
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Patrick - the M word in dial indicators means "out of my price range." Using blue tooth - without even looking at the cost - is going to up the ante beyond my considering it.

Mike - I don't want to measure valve lift. That would probably work, but there would be the variability due to rocker face wear, reground rockers, and perhaps other variables which I don't want to deal with. All I care about is "does the measured cam profile follow exactly the stock cam profile." Don't need to know lift. I can probably multiply the results by 1.45 to get a reasonable approximation of what sort of lift a particular cam is set at at overlap as a kind of reality check. If that was out of spec, then overlap could be measured the usual way pretty quickly to confirm something not allowed is going on. I've checked overlap at an event, but it seems that the 964 cam can be set within the SC overlap range, so checking cam timing that way isn't going to do the compliance check.

Tony - I want to avoid fussing with a degree wheel at the race track - how are you going to attach it to the crank pulley? Magnets, maybe, and be sure the 17mm pulley nut centers it? Back that bolt out and use it to hold the wheel?

But the idea here is to automate it, because entering 360 or so readings into a spread sheet by keystrokes is pretty tedious and time consuming.
Old 12-28-2017, 12:26 AM
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Walt, if you measure mine you could just attach it to the turbo or the nitrous bottle.

Arthur
Old 12-28-2017, 09:09 AM
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Blue Dial.......

Walt,

Have you checked the Blue Dial indicators attached by Pat Keefe? Those are very promising measuring tools for your project.

Tony
Old 12-29-2017, 07:31 PM
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Walt,
In the field, as a first check, It would be much faster to just check cam timing (Cylinder #1 Intake valve lift at TDC).
If it was any higher than say 1.6mm it probably isn't running stock cams.
Then you could go into further inspections to confirm a non-stock cam?
Lyn
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Old 01-04-2018, 11:43 AM
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Walt, why not use a metal template on the cam lobe? With a few pieces of metal on a key ring you could cover all the "stock" class cars."
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