Cam shafts

Discussion in 'Modified Shizzle' started by Zed, Sep 18, 2020.

  1. Zed


    A different way of looking at it. :)
    There are other factors too.

    First the basics.

    It's all about peak torque. Because HP is torque x revs, having peak torque at low revs results on high HP at lower revs and "lower than possible" HP at high revs - a good spread of HP. That's a stock VW tactic.

    Peak torque is based on Volumetric Efficiency. Putting aside the myriad of other variables that effect VE, the cam does it like this...

    When the piston reaches the bottom sucking in air, the air is lagging behind and there is a vacuum in the cylinder. There is still a vacuum as the piston starts to rise. The ideal time to shut the valve is when pressure has equalised, just before the piston starts blowing it back out again.

    The higher the rpm, the later that equilibrium is attained so the later you want to shut the valve to take full advantage.

    A stock type-4 cam inlet valve closes at about 35° ABDC which produces torque and HP like this, though this is a 1600 twinport at 37°ABDC.

    The equivalent single port is worse on all counts which rather debunks that that theory of single ports being more "torquey".
    Last edited: Sep 18, 2020
  2. Zed


    Funny, those two have the same cam.
    Also funny, my stock cam 2.4 behaves like the single port 1600.
  3. Zed


    Here's a Scat chart. A common mild upgrade, the C25, closes the valve at 44°ABDC, 10 degree later than stock. I can't find any rolling road type graphs that might hint at the peak torque rpm.
  4. This is very interesting. What I find difficult in discussing cams is that the quoted timing depends on the initial lift used to measure it. Factory VW cams seem to be quoted at 0.040", 1mm, but aftermarket ones can be 0.020", which gives big numbers for the duration of course, or 0.050" which can't be compared with the factory figures. Then, if you want to compare, as I do, with other pre- and post- war 2 valve pushrod engines it becomes a complete nightmare as it's rarely quoted what the initial lift is. :mad:
    Anyway, it seems that the factory timing is quite tame, probably, and well suited to buses.
  5. Don't talk to me about camshafts Steve, it's a sore subject :mad:
    Zed and docjohn like this.
  6. The Type 2 1800 vs Type 4 1800 is interesting - altering the advance curve and the compression ratio increases peak power without changing the cam or the valve sizes.
    It doesnt change the peak torque or peak torque RPM very much.

    This is the high CR engine


    This is the low CR engine in a T2 . Both same stroke/bore. 20200929_180823.jpg

    BTW if you scale by displacement - 85HP with 1800cc , 110HP with 2332cc
    Zed likes this.
  7. Zed


    Interesting, the higher compression has saved it when the efficiency was dropping off. Big difference by 5000rpm - 90 v 120 Nm.

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