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Need a sanity check on cam card, doesn't all add up.

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Attached is a scan (pdf format) of the cam card that came with my cams. I've FINALLY gotten to the part in this degreeing process where I've got some measurements. I need 3 sanity checks please address all of them if you can.

My findings are:

My Intake cam is retarded 2 degrees and therefore I need +2 on the intake cam gear.

My Exhaust cam is advanced 6 degrees and therefore I need -6 on the exhaust cam gear.

Sanity Check #1: Does -6 on the exhaust cam sound outrageously far off? I just surprised it was that high.

Sanity Check #2: I believe there is a typo on the cam card. The cam card states the Exhaust closing event occurs at 27 degrees BTDC however I think it is supposed to read ATDC instead. The reasons I think this are (please check my math) given the advertised duration is 252 degrees and the open event occurs at 45 BBDC, when I add 252 degrees to 45 BBDC I end up at 27 ATDC. Additionally with my degree wheel I find the open event, the peak lift and closing event line up exactly with the cam card IF the cam card read with the closing event as ATDC (although in my case on the degree wheel they are all advanced 6 degrees). Also I've looked up cam cards for other cams and all the lessons I've learned from the videos at HPAcademy and in every other instance the exhaust closing event happens shortly after TDC. Is it possible that the cam card does in fact have a typo on it and I might be correct?

Sanity Check #3: I found the peak lift event timing lined up perfectly with the cam card for both intake and exhaust. However when I measured the value of the peak lift I got different numbers than what is advertised. In both cases my measured value was higher than the advertised value. For the intake I measured .455" which is only 2 thou higher than the advertised lift. On the exhaust I measured .434" which is 9 thou higher than the advertised lift. In particular that exhaust value seems like a lot more than advertised (though free extra lift I guess). Is it typical for the peak lift measurements to come in higher than advertised? Is this something they do to account for wear in?

Also attached is a photo of my setup to see how I'm taking the measurements.

Thank you so much for your input, this whole process is very stressful and high stakes. Need the sanity checks.

Attached Files

Yes, it'll be a typo, the exhaust does close after TDC, not before it.

From the image, it looks like the cam belt isn't as tight as I would expect - is it tensioned correctly? Normal practice is to set it, then turn the engine over by hand, gently, for two reasons - if there is a mis-timing on the sprockets as would happen with a tooth out one may feel the contact if there's V2P interference before damge can occur, and when initially setting the belt tension, the other sides may not have all the slack taken out.

With vernier pulleys and sprockets, sometimes the "0" vernier marking can be slightly off true. This isn't that important as you're going to be altering it anyway, as required, to get the correct alignment of the camshaft(s) to the crankshaft. I cannot comment on the timing you appear to have found, 'cept for the belt tension bit.

Good observation Gord. I’ve actually had a lot of stress with my timing belt setup. The one in the picture is a new Toda kevlar version in hopes for added safety. The odd thing is conventionally speaking I should not need it or the extra tensioner I added a while ago to help with the situation. You can see the extra tensioner in the new photo attached.

Since I first installed these cams which are supposed to be mild stage 1 the timing belt has always had an extreme loading and unloading as you turn the engine through it’s cycle. At parts in the cycle it gets very tight and then just past that suddenly snaps back to very loose. With the stock belt and without the tensioner I had to deviate from the factory tensioning instructions and pull up hard on the tensioner and even with that the long side of the belt was still flapping constantly and not just a little flapping. With the added tensioner the flapping is stopped (except for the first few seconds after a cold start). There is a fine line with the added tensioner of just right and too tight. Too tight I wreck my cam journals in the head. I’ve already got a little groove in the end journal from having it too tight for a little while.

it is a mystery why I have this challenge with the belt. The head milling is not outrageous and I’ve verified that by measuring my stock cam timing was only retarded by 2 degrees.

I have a suspicion that the exhaust cam has 2.2mm more peak lift than advertised (see my starter post to this thread) and that may be the cause of the extreme loading and unloading of the belt. I’m off to get a digital dial gauge now to make it easier to measure peak lift.

Is it common for peak lift to deviate from the advertised amounts even with the events occuring at precisely the advertised times? The intake cam seems to be only slightly larger peak lift. The exhaust cam looks like a lot more.


So I confirmed with a digital dial gauge, peak lift is much higher than advertised. Advertised is 10.80mm but what I’ve got is 11.12mm. Suddenly I’m more concearned about my v2v clearances. Attached is photo of measurement on new dial gauge. Skunk2 offers to replace the cam if I want, might be worth keeping, kind of a free upgrade if clearances work. I believe this explains my timing belt issues as the higher lift more sharply loads the timing belt and then it snaps free again just after the peak lift. Since the open/peak/close events all occur at adverised times this makes the ramp to peak much faster. Also Skunk2 finally confirmed they have a typo on their cam cards. Apparently not just my cam card. Looks like some serious quality control issues there.

Attached Files

I seem to have forgotten a point earlier, which was to recheck the tensioner after the turning by hand.

There are two/three ways manufacturers may specify lift. There is 'camshaft' which is from the base circle to the peak lift, this is what you can expect from a hydraulic camshaft but for a mechanical you need to take off the clearance to give 'net' and this is the actual lift of the camshaft and it can be further refined by whether it is direct acting (the lobe and follower act directly in line with the valve) or whether it is indirect acting (there is some form of rocker that the lobe operated and acts on the valve in turn) which introduces a multiplication ratio between their lifts - some engines may have aftermarket rockers available which will change the net valve lift at any point. For example, the original small block Chev' engine had a nominal ratio of 1.5 valve lift for 1.0 at the cam, but the aftermarket offered 1.55, 1.6, 1.65 and even a 1.7 ratio with different rocker designs.

As you noticed, the loads on the belt can seem rather substantial and there can be a lot of force applied to the lobes as the camshaft turns, both resisting and aiding their rotation - why I use an anti-scuff compound, or heavy oil, when checking.

Another factor that can cause variations in belt tension is runout in the pulleys, idlers or tensioners. With the camshaft lift, you will notice it varies multiple times per engine revolution, with the valve movements; with a cam' pulley (unlikely, that company has a good reputation) it will ease/tighten over two crank' revolutions; with the crank' pulley (or bent crank nose) it will be over one revolution; for idlers and tensioners, it will be in time with their rotations.

Yup, I follow the factory service manual for the timing belt tensioning, it just wasn’t enough without the additional manual tensioner to keep the long side of the belt from flapping violently. I did speak with Skunk2 today and went over the measurements with them and some pics etc... and they did verify that the cam profile is over spec. They sent it to their engineers and the engineers decided that it would not have a meaningful impact on reliability and no negative impact on power. The way they put it “in perspective” they said that even though it’s .3mm larger than spec they said “that’s the thickness of 4 human hairs”. True as that may be, those hairs add up haha, my timing bely sure notices it. After running these cams without degreeing them already for 1k miles I see no ill effects on the rocker arms. The timing belt, although flapping aside it looked fine, I changed to Toda for piece of mind and the added tensioner keeps it in check. The only mystery remaining is the valve seats given that the exhaust lobe is about half as big as their stage 2 cam (to put it “in perspective” 10 human hairs = stage 2 and I’m at 4 haha) and although the lobe is bigger the open/peak/close events are that of state 1 so the ramp rate is likely that of a stage 2 which might be closing the seats harder. I do have stiffer than than needed valve springs from Crower that would be suitable for a stage 2 or 3 even so that can probably handle it!

ALL belts, and chains, will "flap' in use - here's a rather extreme example https://www.facebook.com/NHRA/videos/10154091874991410

For 99.9999% of the time it isn't anything to worry about.

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