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question about cams

Discussion in 'Ferrari Discussion (not model specific)' started by scuderia47, Feb 4, 2004.

  1. scuderia47

    scuderia47 Karting

    Nov 5, 2003
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    Bill
    my question is about cams in an engine. i know how they function and what they do, but my question is what are the differences in engine characteristics between say, having 2 or 4 cams in a V12 from a 275GTB? why would it be advantageous to have one over the other? thanks
     
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  3. Steve Magnusson

    Steve Magnusson Two Time F1 World Champ
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    Jan 11, 2001
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    The disadvantage of having only 1 camshaft to perform both the intake and exhaust functions is that you need extra levers/mechanisms in-between the cam and (at least some of) the valves it actuates --- so:

    1. the lever adds to the inertia of what you're trying to move so that can put a lower limit on max RPM (things start floating or flexing),

    but

    2. even if you've got a great lever design that doesn't mechanically limit the max RPM (and you ignore friction), oscillating a lever (at a constant rate) takes power due to its inertia, whereas, rotating a camshaft at a constant rate does not take any power (even though a cam has a large inertia).
     
  4. PSk

    PSk F1 World Champ

    Nov 20, 2002
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    Er, how about not as much power ;)

    The other point that has not been raised yet is that with a 2 cam design such as the 275 GTB/4 you can position the spark plug in the centre of the combustion chamber ... with the 275 GTB/2 it has to come in angled from the side.

    In the end when you design a cylinder head the order of priorities are IMO:
    1. Combustion chamber shape.
    2. Valve size and number.
    3. Port size and flow.
    4. Spark plug location.
    5. Valve actuation.

    Thus IMO the method of driving the valves is pretty low priority compared to other things that directly result in power and efficiency, etc.

    (Note: There are plenty of single overhead cam engines that do not have any rockers, but to do this the valves are in line and thus the combustion shape has been compromised ...)

    In the end the less things you have between the camshaft and the valve:
    1. The more efficient (less mass, less friction), and thus
    2. Stronger, thus reliable.
    3. Less build cost.

    Against that you have to turn 2 camshafts instead of one, but when you think about the forces against a single versus 2 cams with the same number of valves it is pretty close to even, thus the friction value for the 2 cam would I guess be only slightly higher.

    Driving the cams would be very similar ... just another pulley.

    Pete
    ps: Also most importantly 2 cams just look cool ;)
     
  5. snj5

    snj5 F1 World Champ

    Feb 22, 2003
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    And for us amateur engine tuners, you can vary intake and exhaust valve timing independantly with separate intake and exhaust cams - adds a lot of flexibility.

    And did someone mention it just looks cool?

    :)
     
  6. PSk

    PSk F1 World Champ

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    Man why didn't I think of that!!!!????, a bloody good point snj5! :)

    Pete
     
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  8. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
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    Consider a V8 with a single cam between the banks. The cam has 16 loabs on it, and requires a rather smooth force to turn it.

    Consider a V8 with DOHC. Each cam has only 4 valves to actuate, resulting in periods where no cam is being manipulated, and periods where very large forces ae involved. This engine has 4 such cams. It requires more energy to turn these cams than it takes to turn the SIBC cams. And that is ignoring all the journal and drive mechanism friction.
     
  9. PSk

    PSk F1 World Champ

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    Agreed.

    Pete
     
  10. mk e

    mk e F1 World Champ

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  11. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
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  13. ferrarifixer

    ferrarifixer F1 Veteran
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    !!!!

    err Mitch, how about comparing the overall gear ratio between the Vette and the 355......

    This whole discussion seems to prove that all engines would have much more power and far less friction if they had no camshafts or valves whatsoever......now there's an idea......
     
  14. mk e

    mk e F1 World Champ

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    It might be true, but I don't think so. The timing chain sees a much smoother load in a pushrod engine, but the camshaft and therefore it's bearings see all the pulses. The only way it will be better is if a valve from each bank is operating at the same time between the same 2 bearing jounals so there is some load canceling. I really think that a biggest part of the fuel ecnomy difference between a corvette and a ferrari is the final gearing. They roll at something like 1800 rpm instead of 3000. Also, they are much closer to their peak torque number at cruising speed than a ferrari is. That says the engine is operating efficiently, where the ferrari with high-flow 4v heads and big cams is not.
     
  15. gabriel

    gabriel Formula 3

    Well, thats entirely correct. The two stroke engine type will produce far more power than it's equivalent 4 stroke. -But trying to get a 2 stroke to pass any type of emission standards is one of the big reasons you don't see many, if any, of them anymore in production.

    Anyone remember all of the 2 stroke motorcycles that could accelerate so hard that you needed clips to keep your feet on the pedals?
     
  16. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    8,010
    Consider the SCIB V8 engine. As one valve is released from maximum lift the vavle spring presses on the cam lobe and transmits energy back into the cam in the same direction the cam is turning. This spring released pressure is immediately available to a different lobe just raising the valve off its seat.

    In the DOHC V8 engine, that same energy has to travel out the camshaft, accross the belts/chains into the gear drive, accross to the other gear drive, up the other belt/chain and finally into the other camshaft.

    While the SCIB engine can efficiently transfer this energy, the DOHC cannot.

    While the final gearing plays a part, if a Ferrari were geared to rev-out in 6th gear at 250 MPH like the Vette, it would have so little TQ available in normal driving situations (i.e. 60 MPH would be 2000 RPMs) that 6th gear would not be considered very useable.

    In addition, the low peak RPM band of the Vette engine allow it to be above 90% peak TQ just above idle RPMs. So, A) the Vette makes more TQ due to its displacement, B) the Vette engine is optimized for lower RPM operation, C) the gearing puts the Vette at maximum efficiency at interstate legal speed limit, D) at these RPMs the vette engine uses 1/4 to 1/6th the power in the valve train as does the Ferrari at the same speeds and in the same gears. This lack of waste shows up in fuel economy.

    I can get 21 MPG in my F355 at cruise speed (65 MPH) but I get 31 MPG in my Vette at 65 MPH. Since both cars have similar tire sizes, and both cars have similar coefficients of air resistance, and both cars have similar frontal areas; both cars have to develop the same thrust to move the car down the road. The extra fuel the Ferrari uses is used to turn the valve train and turn the extra gears (drop gear, crown gear) in the transmission.

    At the other end of the speed spectrum, the LS1 engine runs out of revs in 5th gear at 172 and cannot pull more speed in 6th due to its fuel economy gearing. The F355, on the other hand, runs out of revs in 6th at 183. The Vette uses 5th gear for all acceleration from 120 through 172, while the Ferrari has 2 gears; 5th goes from 120 through 150, and 6th goes from 150 through 183.
     
  17. snj5

    snj5 F1 World Champ

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    I love these high level FChat tech discussions.
    They're so good I'm going to have to shower.
     
  18. mfennell70

    mfennell70 Formula Junior
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    Nov 3, 2003
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    I'm sure that the valve train and gears are contributors but there has to be far more to it than that. Frictional losses are higher for everything at the higher rpm. Whether valvetrain is a disproportional part of that seems open to debate unless you have some references on the subject (I can't find any). The biggest difference may be that the Chevy designers simply sweat fuel economy where the Ferrari guys do not.

    Does your Corvette have an instant economy readout? It would be interesting to see the different values at 65mph on a flat road in 6th, 5th, 4th.
     
  19. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
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    I have often thought that the Vette has a super economy mode that is only activated when the cruise control is managing the throttle. I can only get 28 MPG at 65 when I am controlling the throttle, but touch the cruise button, and bang, in 6-8 seconds, the FE goes up to 31 MPG. I am sure that the Vette engine controller shuts down the fuel injectors on overrun, as the fuel economy pegs out at over 99 MPG.

    Reciprocating losses go up with the square of RPM, while frictional losses go up linearly with RPM. At the low end of the RPM band, the Ferrari has more frictional losses, at the high end of the RPM band, the SCIB engine has higher reciprocating losses.

    In addition, the Ferrari engine runs 105 PSI oil pressures (and might move 2 or even 3 times as much oil through the engine per unit time as the Vette engine since the F355/360 engine heads are cooled more by oil than by water. The 3 oil pumps HAVE to take more power to run than the 60 PSI single oil pump in the Vette.

    6th 30-31 MPG
    5th 25-26 MPG
    4th 21-22 MPG (twice the revs as compared to 6th)

    In effect, I can get about the same fuel economy in 4th gear in the Vette as I get in 6th gear in the Ferrari (interstate constant 65 MPH).
     
  20. mk e

    mk e F1 World Champ

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  21. Dale

    Dale F1 Veteran

    Oct 7, 2003
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    Phil good point im sure the electromagnetic or electro-hydraulic valve control will be in road cars of the future(if theres any oil left)
    renault has patented such tech,toyota has played with it to,
    think the popet valve may be here to stay though,
    Dale.
     
  22. ferrarifixer

    ferrarifixer F1 Veteran
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    I think Mitch should leave his laboratory for a few hours and hug some trees or something......

    Somebody save him from himself, please!

    Yes, I think pneumatic valve closing control will be used before long. As for pneumatic or hydraulic opening, I think it'll be a while before that's in road cars.

    Rotary valves though.........I'm surprised they haven't appeared in cars, great, simple, low friction devices.

    I'm sure Mitch will do an experiment to investigate the reasons!!

    Good on ya Mitch, you're a gem!
     

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