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Variable Valve Timing

Discussion in 'Technical Q&A' started by ATBNM3, Jan 26, 2004.

  1. ATBNM3

    ATBNM3 Formula 3

    Nov 17, 2003
    1,392
    Sunny Isles
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    Don Jackson II
    How does the Variable Valve Timing on a modern Ferrari work?
    On most car it is the intake cam the moves either mechanical or hydraulically. On a few cars both cams move.

    I know on Ferraris only the exhaust cam moves. What is the advantage to this, how is it controlled, and when does this come into play i.e. at what RPM or is it continuously variable?

    Also the Pop valve on the exhaust of the 360 Challenge Stradale how does it work and what is it for? It does make the car sound great when it opens. But what does it do besides esthetics?

    Yamaha uses something like this on there motorcycles they call it an Ex-Up valve. I’m wondering if it is used for the same thing.
     
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  3. Ferrari_tech

    Ferrari_tech Formula 3

    Jul 28, 2003
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    Malcolm W
    Each exhaust camshaft has a timing variator that is hydraulically contolled by a solenoid, which in turn is controlled by the Motronic electric control unit.


    The by-pass pneumatic throttle valves ("pop valves") are fitted to each exhaust outlet which are connected to the vacuum tank and is controlled by the by-pass system's solenoid valve. The ECU of each bank controls the solenoid valve which determines the opening of the by-pass throttle on the exhaust terminal - the by-pass valve is vacuum operated.

    The opening of the solenoid valve depends on the rpm and on the gear engaged, but only when the vehicle is in a full power condition. This system allows the engine power to be increased by an adjustment of the discharge counter pressure.


    MW
     
  4. ATBNM3

    ATBNM3 Formula 3

    Nov 17, 2003
    1,392
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    Don Jackson II
    Thank you very much for the response.
     
  5. ChrisfromRI

    ChrisfromRI Karting

    Jan 28, 2003
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    Foster, RI
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    Chris F
    What actually changes from an operating standpoint?

    Is this merely a timing advance change, or is valve lift and/or the duration of the valve opening/closing cycle affected?

    One of my sports cars has a technology called VTEC, and there are separate camshaft lobes (on both the intake and exhaust cams) with longer valve cycle duration and more valve lift that get engaged and effectively override the standard lobes by hydraulically engaging a center follower to its neighbor followers when the RPMs go over 6K, up to the rev limiter at 9K. The end result is good street driveability below 6K and race-car like performance (breathing) from 6K - 9K.

    My 308 GTB QV obviously has a timing advance curve applied by its electronic ignition according to RPM (a more reliable electronic successor to mechanical weights within the distributor), but what exactly does the modern system do to improve performance (breathing)?

    Thanks, and Kind Regards, Chris
     
  6. Ferrari_tech

    Ferrari_tech Formula 3

    Jul 28, 2003
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    Malcolm W
    Chris,

    The camshaft variators advances the exhaust cams 20° ± 1°, it has no influence on the duration of the opening of the valves.


    MW
     
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  8. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
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    Chris, dont confuse cam/valve timing with ignition timing....very different.

    To clarify MW's post, the exhaust cams actually retard by 10 deg, which is 20 crankshaft degrees, and when it does so is influenced by temperature, throttle, rpm etc. It's simply an oil chamber that switches it's pressure feed on and off, and is located inside the end of the camshaft, pulley end.

    Interstingly, the N-GT (and my 360 "speciale") engine have non variable exhaust cams. Simply using a solid lightened inlet drive bushing on the exhaust cam, but then timing it to it's retarded position.

    There have been two separate "free parts" recalls on the 360 Challenge cars for the variators to be replaced in fear of shearing.

    It happened on one of our cars before the recall, and Ferrari assisited with many new engine parts at reduced cost. All the other fleet were up-dated when parts were available.

    It's clever, but really is more for economy and drivability than outright power. Race cars almost never fall into the window of needing the timing varied.

    The transition between varied and non varied timing creates a flat spot, which is smoothed out by the centre butterfly in the plenum chamber, in conjunction with the operation of the variable inlet port system.

    Many road cars would benefit from retarded exhaust cam timing, to produce power higher for longer in the rev range, at the expense of smoothness and low down torque.

    In any one make "challenge" series, where gains of 0.5% in any area may move you 5 grid places, it's an area worth exploring.....
     
  9. jmillard308

    jmillard308 F1 Veteran
    Silver Subscribed Owner

    May 29, 2003
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    John Millard
    Ok Philip, I mostly understand all that - what would be the effect of retarding the exhaust cams on my 308 (carbs, standard apart from muffler)?
    If worthwhile, how best to effect it?
    Don't mind losing down low to win up high.

    John
     
  10. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,183
    By retarding the exhaust cam three things happen. A) the pressure in the cylinder is converted into crankshaft power for an additional 10 degrees, B) the valve overlap period is reduced by 10 degrees, C) because the valve overlap period is reduced, the engine response to header resonances is greatly reduced. A) is worth less than 1% of added power, B) reduces emissions, and C) prevents a big flat spot in this highly tuned engine.

    Since this engine does not spend a lot of time between idle and 3,000 RPMs, it is not surprising that there is not variable timing. Just like you don't find variable timing in F1 engines.

    I would argue that redarding the exhaust cam would increase TQ in the low RPM (retarded) region by supressing bad header harmonics.

    But I don't see why you loose smoothness, can you elaborate?
     
  11. ChrisfromRI

    ChrisfromRI Karting

    Jan 28, 2003
    230
    Foster, RI
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    Chris F
    All right, so it is essentially the same as what we used to do with a degree wheel when installing a race cam, except that it is not a fixed cam advance/retard after installation, but varies continuously during operation based on input from the ECU. This would clearly affect overlap, and could be used for relatively minor modification of the cycle, though probably not do all that much for improved performance without also being able to affect lift and duration -- which to me were the really important specs when choosing between cams.

    I guess the VTEC is also a compromise to give driveability with performance by switching between two cam profiles, but it is clearly not variable between the two -- thus the compromise in only having two states.

    Thanks for the clarifications!

    Kind Regards, Chris
     
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  13. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
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    John

    You can simply move the pulley dowels to give your desired cam timing......you know that! It's installing the degree wheel to verify everything that takes the real time. I think the figure for minimum change on the dowels is about 2 crank degrees, but there are different cams and pulleys with anywhere from 3/3 holes, to 5/18?. Changing the exhaust timing of a carb car is worth a little, more when done in conjunction with a tweak to the inlet, but rather than outright gains, your main gains are in the position of the peak power and torque in the rev range. So, YOU decide where you feel your gear ratios don't suit, and then decide to rev higher, or use a different gear, then which way to tweak the timing. It's a huge effort for little gain.........but if you're racing many identical cars it's worth having.

    Mitch

    Retarding the exhaust timing, makes the engine begin to recompress its exhaust gases on its exhaust cycle momentarily at low engine speeds. Recompressing a spent gas won't give any more power at all, it'll try to stall the rise of the piston. This causes the "bub bub bub" noise and roughness. As soon as the speed rises enough, the period of recompression is so short that it's unnoticable. As the speeds rises further, the self scavenge effect becomes more, and produces more power due to better evacuation of spent charge.

    Header resonance is something that is very audible but ultimately not very influential on specific output.

    How much it's worth is an endless debate, but even if it's only 1% or less......I'd take that, and put it with all the other 1%'s I can find!

    Since we received our very first 360 challenge and tested it with a professional, we've developed it to be generally 2 seconds quicker per 60 by compiling all the 1%'s from every area of the cars!
     
  14. Sir Gus

    Sir Gus Rookie

    Feb 9, 2004
    1
    Someone correct me if I am wrong,
    doesnt retarding the exhaust cam actually increase the valve overlap, not decrease as previously said
     
  15. rorschach23

    rorschach23 Rookie

    Feb 6, 2010
    16
    1.How reliable are these systems?

    From the Ricambi site, they are called "phase variators' and of course are pretty expensive.
    Example on the 599GTB. (has variable valve timing on the intake and on exhaust cams)
    http://www.ricambiamerica.com/parts_catalogs.php?M=FE&P=&V=diag&I=111004

    http://www.ebay.com/itm/Ferrari-Enzo-Inlet-Phase-Variator-187820-/120239606563?hash=item1bfed6cb23&item=120239606563&pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr

    http://www.ebay.co.uk/itm/FERRARI-INDUCTION-PHASE-VARIATOR-PART-193779-/120755321312

    2.Just out of curiosity, is there any way to 'delete' them?
    For example i know from BMW with their variable valve timing - VANOS - you can delete by replacing the sprokets with fixed ones and of course modifications to the ECU. (used often for greater reliability when bulding a track engine etc)

    But is it doable on such engines like ones from the 599, F430 etc ?

    Thank you.
     
  16. bill365

    bill365 F1 Rookie

    Nov 3, 2003
    3,319
    Chicago area
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    Bill
    I am of the opinion, that the reason VVT is absent in F1, is only because it is specifically banned.

    Regards,
    Bill
     
  17. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,183
    Not at all what I was talking about. What I was talking about was when the exhaust valve opens while the piston is still decending on the power stroke. If the exhaust gas pressure drops to essentially atmospheric before the piston starts upwards, then there are minimal pumping losses on the exhaust stroke. If there remains pressure as the piston starts to rise, then power is sucked out by the pumping losses.

    At low RPMs there is plenty of time for the cylinder pressure to achieve atmospheric pressure before the piston gets to BDC. At high RPMs, there is some remaning pressure, and this robs HP. Now, if the header harmonics deliver a low pressure wave at the back side of the exahust while the piston is rising, the lower than atmospheric pressure in the cylinder returns/cancels much of the pumping losses. If this low pressure wave arrives when the intake valve is opening, it can suck fresh mixture into the cylinder before the piston starts downwards, more power is produced on the subsequent power stroke, but the pumping losses have already been taken.

    I said nothing about roughness, merely the timing of the valve train.

    The header system can deliver on the order of 4-5% over an ill-designed but otherwise not overly restrictive system.

    The intake system (velocity stacks and helmholtz resonators) can deliver 20%, about 4X what the header system can deliver. Ferrari only tapped 10% of this, the rest is waiting for someone (like me) to tap. I have a design, just have not had the time to build it.

    Given that the 360 non-challenge goes 0-60 in 4.1-4.2 seconds, you would have to be at 0-60 in 2.2 seconds for that statement to be accurate. I doubt that. Perhaps you want to restate what you did achieve in a more regular english vernacular.
     
  18. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    7,183
    By opening the exhasut valve later and closing the intake valve earlier, more TQ is developed at low RPMs. The cyclinder operates with higher "dynamic" compression.

    Opening the exhaust valve laater means the exhaust closing is also later
    Closing the intake valve earlier means that the intake valve is opened earlier.
    Later exhaust valve closing and earlier exhaust valve opening means more overlap.
    More overlap at low RPMs does almost nothing (in a negative sense) to power production as the header has not delveoped significant resonances nor has the helmholtz resonator or velocity stacks developed significant resonances. I have no idea what more overlap at low RPMs does to emissions. Certainly if this persists into the mid-range (when the header comes on song, and the resonator and velocity stacks start to function, emissions suffer as fresh mixture is sucked across the top of the cylinder and out the exhaust.

    The ideal situation is where the opening and closing events can be timed individually, varying both durration and overlap simultaneously with RPM and throttle position. Given that one can control the opening and closing times of valves, the utility of a throttle is obviated.
     

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