308 Cam Belt tension | FerrariChat

308 Cam Belt tension

Discussion in '308/328' started by Andyinfrance, Jun 18, 2022.

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  1. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    Can any of the knowledgable crowd here confirm the tension ( frequency in kHz ) for cam belts ?
    I’ve heard that tensioning belts is a lot more precise with the frequency tuning method rather than the displacement pressure method or clamping the tensioner after turning the engine over twice, I still can’t get my head around that method anyway ! I’m going to try a free app to help with this sound frequency, if anyone is interested I’ll post here later to let people know how it goes


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  2. thorn

    thorn F1 Rookie
    Silver Subscribed

    Aug 7, 2012
    3,324
    Tallahassee, FL
    You lock the cam belts based on the position of the tensioner, not a frequency.
     
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  3. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    I’ve heard that now garages use a frequency counter to tension belts so I was wondering what the correct frequency would be for the corresponding correct tension


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  4. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    Mike 996
    Don't overthink this; garages claiming they use a frequency counter are more about trying to convince people to spend (more) money to solve a non-existent problem. The Ferrari 3x8 owners manual covers belt tensioning and it works perfectly. ;)
     
  5. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    2,191
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    Lawrence A. Coppari
    As has been said already, follow the 3x8 manual. I've owned my '87 for nearly 35 years and have changed the cam belts every 5-6 years exactly the way the manual says to do it. Just make sure your tensioners freely move so they can put proper tension on the slack span of the belt. Tensioning is done with a cold engine.
     
  6. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    From reading some of the old threads here there seems to be a lots of variation on the spring tensioners themselves so I can’t see the method in the service manual being very reliable after time, there appears to be variations in spring lengths, condition of the springs, corrosion, obviously relaxing of tension with age etc, so surely the tension can be set with a sonic frequency app ( there’s a YouTube video using a free app that seems to make sense )


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  7. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    2,191
    Kingsport, TN
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    Lawrence A. Coppari
    I would be surprised if the tension between the driving pulley and the intake cam pulley remained constant throughout two cam rotations. The portion of the belt that feeds the exhaust pulley is on the slack span of the belt so it must be kept wrapped around the exhaust pulley to ensure an exhaust cog is not jumped. So where do you measure the frequency? Between drive cog and intake cam pulley or between drive cog and tensioner? Who publishes the correct values? Moreover, will belts by different manufactures produce the same frequencies?
     
  8. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    I believe measurements are taken along the longest section when the belt is “pinged” and I think that the rotations beforehand are necessary to balance out the tension in the belt and ensure that the belt is correctly positioned in its normal running place, has anyone done any measurements for the frequency if using a sonic meter ?


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  9. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    Although I think sonic testing is a "solution for which there is no problem," the most important thing is that if you think sonic testing is necessary for belt tensioning, then do it!! Otherwise you will always be thinking, "I wish I would have done the sonic testing." Being comfortable with your choices is the most important thing. ;)

    We all do things based on our personal comfort level, whether its timing belt replacement frequency, what oil to use, or what kind of tires to install! :)
     
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  10. derekw

    derekw Formula 3

    Sep 7, 2010
    1,521
    London, UK
    Full Name:
    Derek W
    Andy, the belts are more comfortable with ambiguity than you are. Just follow the procedure and lock them at the max sprung extension and you’ll be fine. A little more or less won’t make a huge difference as, given your apparent attention to detail, you’ll change them before they become critical. If you don’t sleep well I have a couple of sets of round tooth GT2/3 pulley sets left.
     
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  11. Nino1964

    Nino1964 Formula Junior
    Silver Subscribed

    Aug 14, 2015
    302
    North Georgia
    Andy the WSM tension procedure works just fine.
     
  12. johnk...

    johnk... F1 World Champ
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    Jun 11, 2004
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    John Kreskovsky
    Tensioning by frequency makes sense only when you have a floating tensioner, like on a F355. On a 3x8 the tensioner is fixed thus belt tension will vary as the engine rotates from position to position. So if you set the frequency and then tighten the tensioner you will find a different frequency when the engine is rotated some amount.

    The reality is that it just isn't that critical.
     
  13. bl10

    bl10 Formula Junior

    Jun 8, 2011
    441
    Chatsworth, CA
    Full Name:
    Barry Leavengood
    The belt tension is not really critical so long as its not really tight. If to tight the drive pully bearings take a beating. I always use the factory method and have never had a problem. I switched a while back to Derek Whites round tooth belts and adjustable aluminum pulleys as I didn't trust the plastic Ferrari pulleys (I had a fence separate) and Derek's pulleys made degreeing the cams much easier. I changed the stock belts ever five years just because. Don't overthink this stuff. The engine is straight forward and really bullet proof.
     
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  14. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    2,191
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    If you use the electronic device and 'ping' the span between drive and intake, how do you set the slack side? With the spring-loaded tensioner?
     
  15. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    I think the idea is that after you’ve turned the engine over a couple of times the tension evens out throughout the belt, so in theory there shouldn’t be a slack side ?


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  16. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    When the engine runs, the tension must be greatest between the drive and the intake cam. If the tension in the belt between the drive and intake cam is the same as the tension between the drive and tension roller, no work is being done implying that it takes no torque to run the cams. Think of a bicycle chain.
     
  17. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
    Agreed but the measurement should be taken when the engine is static but tension is spread evenly on the belts ?


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  18. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Not so. Tension between driver and intake cam differs from tension between driver and tensioner depending on where the cam lobes happen to be on the little hockey pucks when the engine is not running. If you do not lock the cam pulleys and remove the belt, odds are that one or both cams will move on you. The fact that either wants to move changes the tension in one or the other span. When you use the tensioner to set the tension, you watch it as the engine rotates. You lock it when the tensioner is at maximum extension. Changes in tension cause the tensioner to extend and retract.

    To keep cams stationary when belt is removed, I use two little pieces of hardwood the shape of trapezoids, each with a corresponding hole drilled through it (altitude of trapezoid), and a bolt passing through both. One goes above while the other goes below the cam pulleys. When you tighten the wing nut on the bolt, the cams don't move because the trapezoids get wedged between the pulleys locking them in place.
     
  19. Andyinfrance

    Andyinfrance Karting

    Dec 29, 2019
    86
    France
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    Something I fabricated quickly to lock the cams, very diy but it works, turned it round to use on the other side and different bracket made



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  20. mike996

    mike996 F1 Veteran

    Jun 14, 2008
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    Mike 996
    FWIW, I know lots of folks feel the need to lock the cams to replace belts but I have never done that on any engine I've ever worked on, belt or chain-driven. I just mark the cams' position before removing the belt/chain. If they shift a couple of degrees when removing the belt/chain, I just rotate them back to alignment to install the new belt/chain.

    If I'm working on an engine that I have never worked on previously, I rotate the crankshaft to TDC and check that the cam timing marks align properly before removing the old belt. But in subsequent belt/chain changes on the same engine, I just mark the cams/change the belts in whatever position the engine came to rest when I last shut it down.

    But as stated earlier, do what you are comfortable doing! ;)
     
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  21. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Lawrence A. Coppari
    I have had both cam pulleys move on my 328 at the same time. Since I only have two arms, I cannot hold one in place, turn the other with a wrench to realign it, and slip the belt on at the same time. Do what works for you until it doesn't.
     
  22. ferrariowner

    ferrariowner Formula 3

    Feb 21, 2014
    1,155
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    Ron
    The paper trick is much simpler.
     
  23. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
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    Lawrence A. Coppari
    If you have the cam covers off.
     
  24. Lawrence Coppari

    Lawrence Coppari Formula 3

    Apr 29, 2002
    2,191
    Kingsport, TN
    Full Name:
    Lawrence A. Coppari
    That's exactly what I have except your semicircles are trapezoids on mine. Works well.
     

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