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Reconfiguration of 308/328 cam belt tensioner pulley?

Discussion in 'Technical Q&A' started by dave80gtsi, Jan 12, 2006.

  1. dave80gtsi

    dave80gtsi Formula 3
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    #1 dave80gtsi, Jan 12, 2006
    Last edited by a moderator: Sep 7, 2017
    There's certainly been a lot of animated discussion over the years concerning the limitations of the 308 / 328 series cam belts. But I have yet to see anyone discuss the possibility of increasing, at least theoretically, the life and reliability of these belts via a simple re-routing of them onto the opposite side of the tensioner pulley.

    One of the several design goals for long belt life is to try to flex the belt as little as possible. If you look at the OEM routing of these belts, for each belt revolution, the cam and crank pulleys form the belt into a convex "teeth in" orientation. But note that it is the tensioner pulley alone which pushes the belt into a concave "teeth out" position. So, the placement of the belt on the left OEM side of the tensioner pulley causes a unique configuration for the belt, forcing it to flex over center at this position only in the belt's travel.

    However, if the belt were to be re-routed onto the right side of the tensioner, then it seems to me that the belt would be subjected to much less bending stress during use, which would logically contribute to belt longevity. See the (admitted, crude) picture below highlighting the proposed belt re-routing.

    This idea would require the replacement of the existing smooth faced tensioner pulley with one which had matching teeth. Perhaps the overall length of the belt might need to be adjusted a bit, or maybe the pulley O.D. would need to slightly increase so to be able to properly tension the belt. None of these issues strike me as significant design problems to sort out. It also seems as if the end result ought to come pretty close to still being able to fit beneath the OEM cam covers.

    In fact, it's hard for me to visualize any genuine justification for the OEM routing of the belt on the left side of the tensioner pulley in the first place, compared to the proposed alternate method.

    ... certainly, I can't have been the only one to have ever brain stormed this idea, right?
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  3. CraigFL

    CraigFL Formula Junior

    Jan 17, 2001
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    Of course the engineering answer would be that there needs to be a minimum amount of wrap on the pulley -- defined in degrees. If the belt were wrapped as you're suggesting, there would be less wrap on both the exhaust valve bank and the driving pulley from the crank and less as the belt gets tighter.

    The other problem would be that you really can't tension a toothed belt with a toothed pulley like that. If you think about it, it would require an extremely precision adjustment track for the pulley because of the precision distance that is required between the adjustable pulley and the exhaust bank and the precision distance that is required between the adjustable pulley and the driver pulley.
     
  4. Lusso5

    Lusso5 Karting

    May 2, 2005
    69
    Houston, Texas
    If you use tension to push the belt outwards, as you're proposing, it would decrease the contact area on the outer cam pulley, and the crank.

    Edit: Sorry Craig - just read you mentioned the same issue.
     
  5. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
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    Having been reading and learning, It seems the 308 has a bit more trouble with belts than the 348 and 355. Although they have had a little trouble as well. My feeling, having really looked at this and compared the system to other cars, is that the 308, and also the flat 12 engines, use a system that you only see on Formula type racing engines from the mid 60's to late 70's. I do not know what type of cam drive was used after that period. And regardless of what techology appeared later, this is the design these cars had, and if we want our cars to remain realiticaly stock, this is where we are at.

    My feeling as to why the belts have short life, in comparison to other makes of cars such as Japanese cars, is because the Ferrari has smaller diameter pulleys, which wrap the belt in a tighter radius. This, coupled with generally higher engine speed is hard on these belts. Yes, a S2000 may rev as high, but I will just about bet it has larger pulleys. It is my belief that Formula engines, and Ferrari in general, used the design to effect a performance advantage from the engine, and not as a way to save money. Small diameter pulleys made of light weight material can rev faster with less inertia, the same as some would lighten a flywheel, or pistons, or a crankshaft.

    As for what I would like to see, I want my car to continue to "appear" stock. Whatever is inside it, or under wraps is of no real consequence, but I do want it to at least look original. I have read several posts in the past, on this and other websites, of a fellow who made newer style pulleys of aluminum, designed to take a more modern belt tooth design, but it never got off the ground and he gave up. I think something like this would help the cars keep from ripping up belts, but unless there is enough desire, the idea goes nowhere. I also think the tensioner could be altered, or replaced altogether, with somthing that takes a larger diameter idler pulley, so it has more radius, and turns slower. New pulleys on the cams could be made with nice degree wheel faces, and a nice pointer could be attached to make checking and adjusting timing easier. Maybe make the pulleys easier to adjust so timing could fall to exact degrees instead of only 3 as it is now. All of this could fit under stock belt covers and we could all rest happy.
     
  6. Artvonne

    Artvonne F1 Veteran

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    I should also point out, that for any new design, such as what I have just proposed to gain any realistic support, it has to be affordable. 308 owners are cheap! I think the last sytem I refered to was priced at something like $2K, and I think thats what really killed it. If a new set of all 6 pulleys, with a new tensioner that runs a larger idler could be sold for under $1K, I think it would gain interest. I think also, that the belt should be widened as far as it can go, and a belt of highest strength be used, that would offer the greastest life expectancy. I dont know if it would still make sense to allow the belts to go longer periods between changes, but at least we could rest easier knowing the belts wernt going to go south at 18K miles on the 51st week of the fourth year.
     
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  8. lusso64

    lusso64 Formula 3

    Apr 12, 2004
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    Paul,

    One of the other differences between 3 series F cars and other makes is the profile of the teeth. F cars use a square tooth and many others use a round tooth. Replacing all the pulleys with round toothed versions and a suitable belt would help longevity and reliability.

    One could also increase the diameter of the current tensioning pulley so that the bend radius is decreased.

    Of course, a really trick setup would be to have a small tensioning pulley BETWEEN the cams, and a third tensioning pulley on the "straight" leg and have all 3 tensioners hydraulically controlled to give variable cam timing. Now that would be a project :)
     
  9. Artvonne

    Artvonne F1 Veteran

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    I think you missed part of my post, as I was speaking of a more modern type of belt as well as a larger idler pulley to both reduce the radius of the bend as well as make the idler run slower.

    As far as variable valve timing, boy, I dunno. Seems like a lot of extra stuff to make work. But you are correct, three idlers linked together could drastically alter valve timing and also alter each cam indepently. But no doubt figuring that out would be very expensive.
     
  10. lusso64

    lusso64 Formula 3

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    Sorry - I was agreeing with you :) I did not express it clearly at all.

    I suspect that with some clever engineering, the 3 idlers could be linked mechanically and the whole mechanism driven from a single actuator. It would be expensive though....
     
  11. Artvonne

    Artvonne F1 Veteran

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    Okay, I been thinking. I know, dangerous, lol. Okay, the belt is being pulled from the left side, so everthing after the right cam is the slack side. Lets say you had two more idlers, but they were on thier own, with no connection to the idler. With one on the outside of the belt between the drive pulley and left cam, and another between the two cam pulleys, if they hinged, you would advance and retard the two cams simultaneously, but in opposition to each other. The belt tension would need to be held by a dynamic tensioner as the slack side would be constantly changing in length.

    Lets look at the rear bank first. If the first idler was outside the belt, and moved out to allow more belt between the drive pulley and exhaust cam, it would advance both the cams. But, if at the same time, the center idler would back out, at a 2:1 ratio, it would also retard the right cam an opposite and equal amount. Movement of the assembly one direction or the other would accomplish what you are suggesting. You could retard the exhaust, while simultaneously advancing the intake. And if you wanted it real simple, you could make it operate off manifold vacuum. Now just make the front bank set work opposite (the left cam is intake, right exhaust) through some type of linkage. It might all even fit under the stock belt covers.

    Nobody is probably ever going to do this, so its probably just acedemic, but it could be possible. You could have a hotter cam timing under throttle, and a milder setup at low throttle. But with all the belt trouble we have now, complicating it probably wont help. Four more idler pulleys and dynamic belt tensioners couldnt really be better. Simple is better. I hope this isnt a thread highjack.
     
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  13. Verell

    Verell F1 Veteran
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    krobar,
    I support where you're going. The previous serious attempt was Ed Gault's belt system. He made 10 sets & sold them for $1k each, so he was talking the same kind of $$ you are.

    If I remember correctly, it took him almost a year to sell the last couple of sets, even thru eBAY.

    They were nicely done, with a modern belt design. The tensioner pulley was large enough so that the cam belt cover had to be cut out to allow a bit of it to protrude.

    I couldnit come up with the $1k at the time, or even a couple of years later when he made the last set available.

    About a year ago, I contacted Ed to see if I could get the design drawings for his system so I could try making it or a varient. He made a major effor to discourage me. Later a friend of me told me that he was really concerned about possible legal issues in case something went wrong.

    (This is all in the Fchat archives, so not really anything new.)

    Something to worry about tho, is assuming you have a new system. How do you establish the belt change interval? Even the big mfgs have had trouble getting this right. People are going to be a tad upset if you say the system is good for say 5 years, 50K miles & they start going after 4.5 years or 45 K miles...

    I never could come up with a good answer for this one.

    BTW, if anyone has one of Ed's systems they'd like to unload, please contact me!
     

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