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cams and ramp rates

Discussion in '308/328' started by Ferraripilot, Oct 21, 2009.

  1. Ferraripilot

    Ferraripilot F1 World Champ
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    I see some pretty radical cam grinds out there. Some slam the valve open and slam it shut with a seemingly fast ramp rate, while some are more gradual yet have a higher peak lift figure and similar duration.

    Per my novice reviewing of this per what a 2v stock head flows, a cam with a faster ramp speed and long duration at peak lift or near peak lift would be better than a cam with similar/same duration yet higher peak lift with peak lift having a short duration.

    Can anyone comment on what does what to rpm bands in these engines? I would think a cam with high peak lift would be highly dependant on the head flowing at that peak lift, otherwise what's the point. And if it could flow well at that peak lift figure, the power would be very peaky and with a narrow power band. The downside of a fast ramp rate as I see it would be that it might be harder on the tappets. comments?
     
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  3. Ferraripilot

    Ferraripilot F1 World Champ
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    An example of this is the P6 cam vs a webcam grind I know of.

    P6 has a somewhat slower ramp rate of 290 intake dur @ .020 and 270 @.050 with peak lift of .374.

    webcam grind much more radical ramp rate = 290 ADVERTISED dur (@ .000) yet 266 dur @ .050 with .360 peak lift
     
  4. duck.co.za

    duck.co.za Formula Junior

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    I stand corrected but I think the P6 is 290 at .000 . I put a degree wheel on the end of the cam with a dial gauge on the lobe and got 290 . I can check for you in the morning
     
  5. Ferraripilot

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    #4 Ferraripilot, Oct 21, 2009
    Last edited by a moderator: Sep 7, 2017
  6. mk e

    mk e F1 World Champ

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    You’re entering a dangerous world of black magic…….or that is often the way it seems at first glance.

    You are correct the opening the valve past where the flow stops increasing doesn’t help….that statement comes with a huge BUT though. You can’t stop the valve instantly once you’ve gotten it moving so in order to open the valve to peak flow as quickly as possible you need a significant amount of additional travel to slow it down and stop it. If you try to stop at peak flow lift you need to begin the deceleration before you reach peak flow so you haven’t spent all the time at peak flow that you could have regardless of what duration you are talking about…does that make sense?

    Ramp rates. Faster is better for performance and slower is better for life so every cam made is a compromise of so sort. Now to confuse the issue a little bit, you can’t really tell what the ramp rates are from the basic cam specs, the only way to know is get a cam doctor of similar file for the cam you are interested in….which is what I’m holding on for the lobe options I’m considering for my project.
     
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  8. mk e

    mk e F1 World Champ

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    I don’t think you can just put a dial indication on a cam lobe and get the correct duration. The cam acts on the tappet which is basically flat on top and has a diameter, so the cam is not acting at the centerline most of the time. To get a good measurement you need to put the cam in a head and put the dial indicator on the bucket or valve….I think.
     
  9. Ferraripilot

    Ferraripilot F1 World Champ
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    "To stop at peak flow lift you need to begin the deceleration before you reach peak flow so you haven’t spent all the time at peak flow that you could have regardless of what duration."

    makes sense.
     
  10. AZDoug

    AZDoug Formula 3

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    I can tell you that GM used fairly gentle ramp acceleration, even on their most high performance grinds for reliability and trying to reduce warranty repair expenses.

    Aftermarket cam grinders have put faster lifts on identically timed and gross lift lobes, in effect increasing the average valve open area, and thus flow, per rotation. The limiting factor on a GM cam, is the fact that the lifter is of relatively small diameter, too much ramp too fast will cause the cam lobe to act on the edge of the lifter, which will destroy both quickly.

    The large shim surface on your Fcar doesn't have that limitation, so that leave parts reliability. For one, I am sure hollow, sodium filled valves don't like to be thrown open and slammed shut.

    If you could make your valve opening look like a half square wave, you could increase average CFM flow by close to 100% as compared to the half sinusoidal normal grind.

    What fast opening does, is allow you to get either the same net flow with less duration/overlap, in effect broadening the torque range of the motor, no loss at high RPM, yet better breathing at low RPM.

    Doug
     
  11. Ferraripilot

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    I see some of the boxer race engines running well over .420 lift and the engines are horribly peaky yet make great power. Sounds like they are limiting themselves like this?
     
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  13. duck.co.za

    duck.co.za Formula Junior

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    Good point . Does the cam doctor machine not run a roller straight on the lobe ??
     
  14. AZDoug

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    Peakiness is generally a function of lobe spread angle and duration, get a real tight lobe spread with a lot of overlap, and reversion, or plain old air/fuel-in-and-straight-out-the-exhaust happens at low speed, killing power down. OTOH, you get nice evacuation pulse effect and ram type cylinder filling with tehse same cams at higher RPM.

    Doug
     
  15. Brian Harper

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    On an interference engine there might be other reasons you don't want valves to be opening too fast. Sometimes those clearances are already pretty tight.
     
  16. CliffBeer

    CliffBeer Formula 3

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    The ramp (aggressive or gentle) also affects max rpm. An aggressive ramp will cause the valves to float at a lower rpm than a gentle ramp. Why? Because the valve accelerates to a faster speed in it's stroke and thereby has more momentum. Of course, stronger valve springs (or other closing mechanism) are used to try to counteract this effect but that causes more wear and strain on the valve train components.....

    If valves didn't have to change directions things would be a lot simpler. The inlet opening could be opened and closed much more quickly and duration and overlap could be reduced, or at least more easily optimized for the particular load and conditions in the combustion chamber.
     
  17. AZDoug

    AZDoug Formula 3

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    Or use light weight high strength materials like titanium, that lets you keep normal spring rates and have very fast valve reaction. No free lunch; inexpensive, engine life or performance, pick any two.

    Doug
     
  18. 2dinos

    2dinos F1 Rookie

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    IIRC (and this is from a long time ago) the basic acceleration algorithym options for cam designs were based on cycloidal, sinusoidal, and trapezoidal. Where the science meets art is to get the valve off the seat and open as far as possible when the flow will be best without having the machine tear itself to pieces. I recall getting into this for my 308 because Ferrari had these really cool recipes for cams ie. Daytona / 246 / P6. I wanted to stay with a profile that was developed by Ferrari. My logic was the effort to put them in and set them up should be backed by engineering I have much respect for. I think the 308 cam was based on a trapezoidal profile. A great book for this subject is Kinematics. If you can find something by the author Shigley, you've got a great text.
     
  19. Racing-Baker

    Racing-Baker Karting

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    any adresses on cams ??
    i'm still searching...
     
  20. Ferraripilot

    Ferraripilot F1 World Champ
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    Grinds? Webcamshafts has quite a few nice grinds available and they do nice work.
     

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