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  1. #1
    Vern's Avatar
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    Exclamation Cam timing and compression testing, interesting results

    I have been playing with cam timing a bit and found some things I wanted to share. I also wanted input from other members on their cam settings vs compression readings, as well as some other things, so I decided to start a new thread just dedicated to cam timing.

    Some initial things … in my testing, I found that testing compression by removing only one spark plug at a time seemed to increase compression readings by 10 psi, so I stayed with that. So all my tests were with a cheap Harbor Freight compression tester, battery charged and on charger, engine cold, throttle held wide open, only one spark plug removed at a time. If you test with all plugs out, there may be a 10 psi difference in numbers, plus all compression testers might read slightly differently. Still, my numbers have always fallen into the range expected, and those reported by others.

    A little history on my ski … it has one of the somewhat rare 27 cams and has always ran well in stage 1, 2, and 3 setups. One winter I decided to upgrade to Ferrea valves, so I pulled the head, did the valves, added the ET 137 3mm wheel/spacer to my X charger, and put it all back together. I had the cam out of course … I think since then, I never got the cam re-installed and timed perfectly. The ski ran good, but I did not gain the performance I thought I would with the added boost … I now think it was because my cam timing was never right.

    When I reinstalled, I used the cam and crank locking tools, but found there was still a bit of ‘slop’ even when using the tools and lining up the 1503 marks. I was never confident I had it perfect, but tested compression, found 150 psi at full advance, and locked it down and ran it that way.

    I ran all season with my current cam timing, ski ran great. I tested compression and at full advance I got 150 psi, charger on, two plugs in. At full retard I got 110 psi, and idle was rough. Even tho it ran great all year at full advance, these numbers made me wonder still ...

    I took the sprocket off, moved the chain one tooth left (or moved the sprocket one tooth to the right, or one tooth clockwise), put it back together, tested compression again. NOW at full advance, I got 155 front, 175 mid, 175 rear (my motor has 265 hours on it, and front is a bit lower). NOW at full retard, I got 145 front, 155, 155. These numbers are significantly higher than I got on the previous tooth. Fired right up, ski sounded great idled smooth.

    I now think that at least on the 04-05 skis, 120 psi retarded, 150 advanced is possibly indicative of still being a tooth off, even tho the ski will run great at 135 psi plus. After moving the sprocket one tooth clockwise, everything else equal, my compression numbers jumped at least 20 psi in every slot setting, indicating to me that my timing previously was not correct/optimal.

    I had no idea my timing was not right, since the ski ran great all summer. What happened is a friend bought a ski very similar setup to mine (only significant difference was his Rude1 vs my ET137 + 3mm spacer wheel), and the pull of his ski shocked me ... I expected his to be faster, since he likely has .5 - 1 lb more boost, but it was WAY stronger than my ski. I have never been on a ski that pulled anywhere near that hard. I checked his compression ... he was at full retard on a 29 cam and we got 155 - 160 psi still. We went full advance and got 175 psi ... I had never seen numbers that high, and to me, it helped A LOT to explain why his ski ripped my arms off and was so fast. This is what caused me to go back and mess with my cam timing and I found out I could achieve very similar numbers. All tests were done with battery charger on, two spark plugs in, WOT, same compression tester for all tests.

    Its funny ... I had not really heard of compression testing above 150 psi previously. Thing is, what got my attention was how hard Jim's ski accelerated, AND it did not lay down on top, it pulled extremely hard all the way to 8600 rpms and his top speed was also very good. Taking into account the parts & mods on the boat, it still did not add up, so I checked his cam number and his compression, and found 155 - 160 psi and that was fully retarded, 175+ fully advanced. I had never seen that before and told Jim his ski was a freak, my assumption was he had unusually good piston/ring seal ... but I decided to play with my ski and found a cam setting that pretty much matched Jim's compression readings (except my ski has 150 more hours on it, and is slightly lower compression).

    I had read many times that the 04/05 cams run best when retarded to 140-145 psi ... thing is, Jim's ski was 175 psi advanced, and even fully retarded compression would never go lower than 155 psi. Is his cam set incorrectly? Unlikely since his ski absolutely rips all the way to top end.

    FWIW we also took the head off later and it has stock pistons, stock head gasket, head not decked, etc. His IC sprung a leak and we lost a head gasket, so we pulled the head off. So nothing unusual there ... and nothing unusual on mine either, and I also hit 175+ compression readings on mine. The only thing I changed on mine was to re-time it, going clockwise by one tooth, everything else stayed the same.


    So, that is my experience with cam timing, and it leaves me with some questions that I would love to get input from others on.
    1. What compression readings do you get at full retard and at full advance?
    2. How does your ski run versus other skis with similar setups?
    3. What year is your ski and what cam are you running?
    4. Have you ever had your cam or sprocket off?

    Things I wish I knew … how many degrees change is there per tooth change on the sprocket? How many degrees change from full advance to full retard on the slot on the sprocket? What I am wondering about is whether many cams are off one tooth, but the slot makes up the difference? But as others have reported, even at the same compression reading, the ski will run different when changed by one tooth ... this makes sense since the sprocket not only controls cam to crank physical timing of valve opening vs piston position, it also has a reluctor on the sprocket and a cam sensor (cps) that likely controls spark firing (and injector timing/firing maybe? I don't know), so there are potentially multiple variables that are controlled by sprocket position.

    I would really like to hear from others on their compression numbers since I don’t recall hearing much over 150 psi reported before. Since Jim runs a 29 cam and I run a 27 cam, his ski has 100 hours, mine has 265 hours, there is no way that our two skis are special or unique … so that makes me wonder if quite a few skis have cam timing that is slightly off?

    I will try to get back to the barn on Monday and take some pictures too.
    Last edited by Vern; 11-28-2010 at 10:07 PM. Reason: spacing


  2. #2
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    so FWIW on my 08 px with a 29 cam full retard i get no lower then 150psi. full advance is 165+. at first i locked cam and crank and didnt like the way the 1503 marks lined up so i "moved" the sprocket..... hahaha according to the work you've done who knows where it ended up but i know it runs like a banshee and full retard like i said is no lower then 150 psi

  3. #3
    Vern's Avatar
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    Nice numbers Mike, that sounds like Jim and my setups. Do you happen to know the same numbers with your stock 51 cam before you swapped? Big change from your stock cam?

  4. #4
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    wish i did i had a LOT of issues going on at one time!! when i finally got the 29 to what i would consider "right" i know it runs better then the 51 in the stock from the factory set up.... all that info is pretty useless though considering the detailed work you've done is documented....

    what i learned much like you is that, locked cam, locked crank with the tools is still not right! when i hear people say to get cam timing right all you have to do is lock them both and run it i cringe. i myself am not "technical" enough to explain why that doesn't work but just let it slide.... hopefully with your/this write up people can learn a thing or TEN

  5. #5
    Vern's Avatar
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    Totally agree ... from what I know now, cam locked/crank locked gets you close, but does not mean it is optimal. I wish our weather was better, I would like to water test, but hopefully we can get enough input like yours to reinforce what is optimal cam timing. Have you run it at full advance vs full retard and noticed differences in how it ran?

  6. #6
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    yea sure have. full advance will pull from idle to near 8000rpm far better then full retard but in my case (riva ecu injectors) i felt like it fell on its face after that. seat of the pants i could feel it lay the nose down.
    same prop and everything else @ full retard, it wouldn't spin the prop out of the hole (noticeably different, less power down low) but made up for the lack of power down low with a better top end. it would pull to the limiter and sit on it.

    im in a slightly different zone then most of this board as i'm getting into the cc racing so where i prefer timing and where i left it is more then likely different then others. i do have a 4224 cam on the way so ill continue to watch this and report back when i can!

  7. #7
    Good to see some experimentation on this, but let me offer some suggestions & cautions:

    1. You generally cannot make the inference that "more compression is better" when talking about cam timings. Cam profiles and timings are generally a compromise, a balance between lo and hi end performance. You can skew it one way or the other, and create unintended consequences to performance.
    2. It is tough to ask for others to report compression numbers and make it a meaningful comparison. (especially when you are using a HF gauge, LOL...) There are too many variables at play here.
    3. The cam sensor has no bearing on ignition timing; that is the function of the crank sensor. The cam sensor simply allows the ECU to know which cylinder to fire.
    4. The cam gear has 38 teeth, which means each tooth is 9.47 degrees of cam rotation, or 18.94 degrees of crank rotation. The slots give about half a tooth of adjustment either way, or a full tooth total. That is a lot of change in a precision engine like these. Moving a whole tooth is really out of whack.
    5. Remember these are interference engines. You can get dangerously close to killing the engine by advancing or retarding too much. For example, moving the timing off one tooth might allow it to run stronger at a certain RPM, but a slight amount of valve float might cause the valves to hit the piston at high RPM.

  8. #8

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    see dood, you saved me alot of typing. especially number 2 and 5.

    Vern, if you can, try and get hold of a snap-on gauge. I think you will be surprised at what you find. I know of a few cases where guys lost valves by missing the cam by one tooth.

    the best way I know of checking timing is to remove the cone from your pump and use a allen to turn the engine. while turning the engine ccw, have someone drop the locking tool in the cam. if you miss it, go cw around a quarter of a turn and try again. don't try to lock the cam while turning the engine cw. you will relieve tension on the left side of the chain and not get a accurate setting. now look into the hole where the crank locking tool goes and you shoud see a full circle. if not, loosen the chain sprocket and adjust. when you think you got it pretty close, rotate the engine over 2 times and lock the cam again. look into the hole again and verify that you see a full circle.
    you will get different opinions on whether advancing or retarding is better. you can decide what is better for your setup but using the crank hole while the cam is locked will give you the most accurate setup. if you want to advance or retard, the circle will start looking like a crescent shape.(if this doesn't make sense, try it and you'll see what I mean). than rotate the engine ccw ,lock the cam and make sure the shape in the crank hole looks the same like where you originally set it up.

    btw, this is best done with the plugs out and rocker off.




    I can be picturing this wrong but based on your comment

    I took the sprocket off, moved the chain one tooth left (or moved the sprocket one tooth to the right, or one tooth clockwise),

    that should retard the cam timing.


    chain strectch can also make setting up difficult.

    a stock x ski will run better with the cam retarded.(just a couple of degrees)

  9. #9
    Hi Vern,
    Very interesting thread.
    How do you get a full retard and a full advance when cranking?

  10. #10
    Moderator beerdart's Avatar
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    Some cam info posted in another thread by PWCGUY
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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