10% rule: Full analysis

UPDATE: As with many of these issues arising over a GP weekend, its a rapidly developing story.  The position given to me by the teams ast night, has since changed, as Charlie whiting considered the situation overnight.  For the balance of the British GP, Mercedes engined cars (McLaren, Mercedes GP, Force India) will be able to use their fired-overrun.  As this was pre-agreed with the FIA for reliability reasons.  However Renault Sports request for their larger overrun throttle opening was requested after the event had started.  Thus Chalrie Whiting decided that, as the technical regulations for the event need to be agreed before the event, Renaults request was inadmissible for this event.   Thus they have to meet the original technical directive on throttle opening and not the 50% they had lobbied for.  This leaves Renault having to run a mapping which is not optimal for reliability and Mercedes can run their mapping.

After much expectation on the effect of the 10% off-throttle limit, what transpired over the opening practice sessions brought more confusion than clarification. As practice got under way it transpires that the expected 10% limit was in fact not applied to all teams, nor was the dispensation to the different engine manufacturers communicated clearly to all the others. This brought much confusion to fans and media alike, as well as bringing a heated debate between Martin Whitmarsh and Christian Horner in the Friday press conference. Its been reported that Renault engines have been dispensation to run at up to 50% throttle when the driver is off the throttle pedal, and slightly less well reported that Mercedes engined teams are able to run a fired overrun.

However, the situation was explained to me by several key technical staff in the Silverstone pit lane. The communication and political issues notwithstanding, the status is at least technically clear.

Firstly I gained detail of what the proposed 10% rule actually consisted of. In order to prevent teams using off-throttle engine maps to continue to drive airflow over the diffuser for aerodynamic benefit, the FIA proposed a pair of changes to what’s allowed when the driver comes off the throttle pedal. Firstly the well known 10% limit on the throttle opening, but secondly a ban on injecting fuel into the engine when off the throttle. The intention of this pair of changes was to ban both hot and cold blown engine maps.

Of course this was the FIA proposal; the original date of the Spanish GP was delayed while the teams lobbied their cases to the FIA, giving their reasons why such changes were unworkable given the timescales and restrictions on development.

Now we need to understand what goes on within the engine when a driver lifts off the throttle and the subsequent effect that has on other aspects of the car. Unlike in road cars the driver in an F1 car doesn’t leisurely lift off the throttle and delay the braking phase. Instead the driver may be at near maximum revs, when he will simultaneously lift off the throttle pedal completely and hit the brake pedal hard for the initial downforce aided braking event. During the braking event the gears will be sequentially selected, further peaking revs as the car slows. This sudden closing of the throttles blocks off the inlet to the combustion chamber, but the cylinder will continue to pump up and down at a great rate. This creates huge stresses inside the combustion chamber and the vacuum created will suck air past the piston rings. This will rapidly slow the engine, creating too much engine braking effect, which in turns creates downstream stresses in the drive train and over-brakes the engine. The excessive engine braking effect will make the car nervous on throttle lift off, regardless of any subsequent aerodynamic effect.

So engine manufacturers find different solutions to ease the stresses and braking effect of the driver lifting off the throttle. In the seasons before EBDs there were several different strategies in place, the driver was able to alter overrun setting to tunes the cars handling, and driver switching between teams found the change in overrun settings needed some adjustment to both their driving style and sometimes with the engines settings. So overrun settings were already an issue before EBDs, and many strategies were already outside the intentions f the 10% rule.

Renault have been open and said their engine already runs open throttles on the overrun, this both eases the blow-by and stress issues, it also usefully cooled the exhaust valve, an alternative to using excess fuel to cool the back of the valve. This year the Renault sport are believed to be running as much as 90% open throttle on the overrun. This is what’s best known as cold-blown mapping. Earlier this season and through out free practice at Silverstone, the three Renault engined teams, had a distinctive loud overrun note, which continues briefly as the drivers picked up the throttle out of slow turns. As the throttles are open more than other teams, the induction noise is far greater.

Mercedes HPE, equally have their solution, this is the so called fired-overrun. When the driver lifts off, fuel continues to be injected into the engine and sparked within the combustion chamber. This offsets the engine braking effect created by the engine, giving a smoother transition from on throttle to the overrun when off it. As a result this means there is less engine braking effect. This gives Mercedes the freedom to define braking bias and KERS charging, without having to account for engine braking. Effectively decoupling the engine braking effect from the actual action of the braking system. As with Renault’s pre-EBD mapping Mercedes solution is analogous to the hot blowing mapping. At Silverstone the Mercedes engined teams had a particularly clean overrun sound. Where as Ferrari had far more cracks and pops as the engine slowed.

With both engine manufacturers having long established overrun strategies that have critical impacts on the basic engine design or the braking system, it will be hard to rapidly switch to a very strict overrun mapping as demanded by the 10% rule. Both manufacturers lobbied the FIA to be allowed to retain elements of these old overrun strategies, while still emasculating their current strategies. The FIA have been able to see the mappings used in 2009 through to the current day, as the code is held by the FIA since the advent of the single ECU (SECU). They’ve been able to see the engines have had these long established mappings, but also how they have become more aggressive since the EBD has been developed.
So the FIA relented and although we will commonly call this the 10% rule, the actual throttle will allowed up to 50% and some fuel can still be injected and burnt in the engine. This sounds like a climb down by the FIA and unfair to different engine manufacturers. But the unreported events at Silverstone this afternoon are fairer than the picture being painted by the teams and the media. Its true that Renault were given their greater throttle opening, but also Mercedes were given their fired-overrun, but these dispensations have been given to every engine manufacturer, so Ferrari could have more throttle opening or Cosworth could develop a fired overrun. As I understand you can one but not both of these options, so no 50%-open with a fired-overrun.
Although the communication and timing of these clarifications appears to be wanting, the final rules clarification meets the basic needs of individual engine suppliers, but still maintains parity between the four parties involved. There is no doubt this allows some secondary benefit of greater flow through the diffuser on the overrun, but this is still greatly reduced over what’s been raced already this year. So there will be reduced aero effect and no further arms race in developing these aggressive strategies. After the furore dies we have been left with w reasonable compromise on reducing engine effect on aerodynamics, before the fuller bans comes into effect with periscope exhausts next year.

54 thoughts on “10% rule: Full analysis

  1. Pingback: Formula One Blown Diffuser Controversy Silverstone GP | Overtime Sports Report

  2. Whilst this gives the engines what they need in terms of cooling/reducing stress – which option gives the greater advantage in terms of ‘blowing’ the diffuser?
    It seems to me that the change in stability of dropping to 50% of cold blowing is a smaller change compared to a drop to 10% with hot blowing. Also is the definition of hot vs cold refer to injecting fuel or igniting it via the spark plugs – if it’s igniting it surely it will ignite in the exhaust anyway due to the high temperature if its in the correct mix?

  3. Finally , a balanced and clear explanation of the EBD situation. Thanks and keep up the great work with this site.

  4. So what if it turned out Renault needed it and Mercedes HPE didn’t. Would the FIA want to equalise the engine gas flow so the Renault powered teams don’t have a significantly better EBD system, or would they just say deal with it. I highly doubt they would let an engine give any significant advantage to one team, so how they can put rather arbitrary restrictions on them for this race with no proper investigation of which system is more effective is beyond me.

  5. So let me get this straight:
    Mercedes engines will be allowed to have 10% opened valves with hot-blowing, Renault 50% with cold blowing.

    That sounds like a massive advantage for Renault in my book. Because at the end of the day, Renault won’t have to change anything, right? At least under race condition they apparently never opened their valves more than 50% and they never used hot blowing.

    Mercedes on the other had did open them more and hence loses much more grip with the EBD. If that’s the deal then Mercedes powered teams might as well give up on this season…

  6. Thanks Joe, much confusion here over it all after FP today. Excellent insight into engine braking management too, as a bonus 🙂

    This affair typifies what I see as one of F1’s major problems coming into the Social Media age; so much money and effort is dedicated to aspects of F1 (marketing, of course, but also very limited team exposure via Twitter etc) with no direct routes for all the really interesting stuff like directives and ‘open secrets’.

    FOM needs to massively broaden the information flow away from team and sponsor activities, the start of which we’ve seen, but I still can’t read IQ for instance (I want to see what Sid Watkins has to say! And how aviation monocoque and survivabilty is of relevance to the FIA!) or know for sure the sporting and technical regs as there’s so much opacity where transparency and contact could explode this sport.

    I keep meaning to write a pitch to Mr E about this, far too much free time 😀

  7. if RBR were already using less than 50% throttle, surely the impact on them is more limited then on the Merc-powered cars?


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  9. Pingback: Row erupts over diffuser restrictions | F1 Fanatic round-up

  10. Scarbs mate! Yet again you are the F1 Fan/Blogger’s technical Seer.
    A great, concise, clear and factually impartial explanation of the latest EBD ruling developments overnight.

    I’ve been a big fan of F1 for many years (early Eighties onward), and personally I am very motivated and intrigued in the analyisis of the Technical / Regulatory side of Sport. As a profesional engineer, that sorta follows my aptitudes. The FIA create many opportunities per Season for rule confusions and sudden rulle deviations, and your timely and ‘on the money’ technical insights and insider information, is really fisrt rate.

    Once again, in the midst of this latest FIA ‘rule change’ storm (nay furore), you manage to cut through the veil of heresay and lay-posturing (fanboy stuff), and properly explain the details of the situation – with factual imaprtiality as always. Kudos man.

    Your BLOG takes pride of place as first on my Web Applications ‘F1’ shortcuts.

    Jack Flash – Brisbane Australia.

  11. Hello Scarbs,

    Great article! It really has opened the proverbially can of worms. As stated in the presser the FIA have bought this circumstance on themselves by changing the rules (or providing a ‘clarification’) mid-season without reasonable time for input by the manufacturers and subsequently in good time making a clear ruling.

    BTW enjoy your shows with Peter Windsor on TFL

    PS seems your name is a bit difficult 😉

  12. Do you think 50% cold blow = 10% hot blow? Did Renault teams (esp. RBR) use hot blowing off throttle? if all these times they are using cold blow, then they loose less in terms of percentage compared to Merc powered teams.. correct? or all these times the Merc powered teams only use like 20% opening? How much air is moved using either solution?

  13. Another excellent article Craig. It does seem that whilst there may be some justification for allowing more off-throttle levels, it does seem as if the FIA are modifying their rules to suit the engine manufacturers and not the other way round as should be the case.

    The complaint, most vocally expressed by Christian Horner, that rules are being changed mid-year is a red herring. He has cited the F-Duct and double diffusers as examples. However, both were very clever interpretations of the rules. The FIA, via Charlie Whiting, have deemed that the cold blown diffuser constitutes a movable aerodynamic device as it relies on the drivers foot position on the throttle.

    It could be argued that by the FIA taking this on they have themselves created the current confusion and it would seem they are now trying to appease the manufacturers. I understand the Mercedes fired over-run is only on four cylinders and produces a less significant blown effect. Maybe this is why Martin Whitmarsh was so vexed yesterday during free practice and the subsequent press conference.

    Keep digging!

    • The FIA, via Charlie Whiting, have deemed that the cold blown diffuser constitutes a movable aerodynamic device as it relies on the drivers foot position on the throttle.

      No they haven’t. EBD are still allowed. This ludicrous, pointless, haphazard mid-season restriction only alters how the EBD works when the driver lifts his foot off the throttle. If the FIA had determined that EBD as such were illegal, every team would have to have periscope exhausts, starting today.

  14. Pingback: Fresh diffuser rules row | F1 Fanatic round-up | PooZ

  15. Do any of the engine off throttle maps result in the generation of any down force while the car is STATIONARY? If so, they work in an analogous manner to the fan on the Brabham BT46 and are illegal.

    It makes no difference whether the device that moves the air is a fan or a piston

    • This isn’t being addressed at all right now – this is a tangent from the current discussion. The discussion is whether or not the diffusers can be blown off throttle or not, not whether or not they can be blown. They aren’t going to be banned because they are analogous to the fans, however true your argument may be.

  16. Would it not be better to mandate a move of the exhaust to a neutral position for the next race to stop EBD fully? That way the engine manufacturers can keep their reliability maps but there is absolutely no advantage?

    • That’s a pretty big disadvantage to all of the smaller teams… only the largest teams would be able to move their exhausts for next race, and in Renault’s case, it would be a fundamental change to their car design… in 2 weeks?

  17. Pingback: 10% rule: Full analysis « Scarbsf1's Blog

  18. If the teams (or should I say Renault Sport and Mercedes HPE) want to continue running with open throttles even when the driver lifts off – in the name of reliability then let them do so ; but only if they also re-route the exhausts out through the top of the bodywork and nowhere near the diffuser.

    That way the engines will be as relaible as they were, which keeps the teams happy, but also reduces the EBD effect, which keeps the FIA happy as this was their aim all along.

    As teams will now continue to use (and carry) 10% more fuel but to no aerodynamic or performance benefit – indeed a performance disadvantage because of the weight of that extra fuel – you may find that they suddenly find a way to run their throttles the way the FIA want…

    Great explanation as always, thanks for all your hard work.

    • If all the teams (not just Renault and Mercedes powered) were mandated to re-route the exhaust, then I believe Renault and Mercedes wouldn’t mind.

      If it’s only Renault and Mercedes powered teams that have to do that, then it’s unfair to them, since everyone can still use EBD but they can’t. You do know that half the grid are using Renault+Mercedes engine and half the grid are using Ferrari+Cosworth engine.

    • “But the unreported events at Silverstone this afternoon are fairer than the picture being painted by the teams and the media. Its true that Renault were given their greater throttle opening, but also Mercedes were given their fired-overrun, but these dispensations have been given to every engine manufacturer, so Ferrari could have more throttle opening or Cosworth could develop a fired overrun. As I understand you can one but not both of these options, so no 50%-open with a fired-overrun.”


      • I saw that, but it does not say what solution are they using. And if you read carefully they have different sound than Mercedes or Renault engines.
        But I still wonder what are they using.

    • I too would like to hear what these teams are doing. I assume that they are at a disadvantage compared to the Mercedes/Renault teams?

      • Who can be behind 10% rule?
        It can’t be Red Bull (Renault) because they were good as where they was.
        It can’t be Mercedes (McLaren).
        Then there was rumors Mercedes gain most while Brawn told they planned to install new floor at Silverstone. So rumors completely misleading.
        Some small team? No way – they all voted to lift ban.
        And who is left?

  19. Thank you for this greattly detailled analysis.
    Do you know if it is possible to have the official documents on this subjects ?
    There is nothing on there website.
    Thank you in advance

  20. Thanks for this clarification Craig.

    What I find interesting is the fact that teams are raising these issues about reliability etc now. They knew this was on the cards since just before the Spanish GP, why all of a sudden the problems now?

    Whatever issues the teams have with this ruling should have been clarified long before this event. Period.

    If RBR claims that they need this for engine reliability, why did they not provide this information to the FIA before this race weekend?

    The teams are clearly playing games with the FIA here, they are simply looking for excuses to keep their advantage. What a mess this has become…

    • Remember that they have not been able to run the cars in testing under this new set of rules. It’s entirely possible that they detected excessive exhaust valve temperatures in the practice sessions for the first time and brought the issue to the FIA’s attention at that time.

      But this is truly a mess, either way.

  21. One thing has been bothering me. Engine development is frozen for years and nobody used off-throttle engine maps, but engines were still reliable. Suddenly after a year of using it, manufacturers are saying they can’t revert back without loosing “critical” reliability.
    How can that be, if engines are supposed to stay more or less the same? Or are teams just bluffing to keep the performance gains?

    • Current engines were frozen in 2009 when 18000 RPM limit and 8 engines per season rule was introduced. Ferrari and Mercedes were allowed to change engines for reliability, but they “accidentally” increased power and that led to another set of changes on engines to match Mercedes engine, which was the strongest.
      So yes, they could have reliability problems.

    • Renault have been using throttle opening to cool their exhaust valves since the inception of their current engine, according to the reports. Using that airflow to blow the diffuser has only been around since last season however.

  22. Latest twist seems to be, that the FIA offered to put off the clampdown altogether from next race until the end of the season, but only in cas ALL team agree on that! Wonder what HRT, or Ferrari for that will say about it. Not very likely then, right.

    A test for FOTA cohesion?

  23. Pingback: An Appalling Exhausting Affair | LiteralF1

  24. “Renault have been open and said their engine already runs open throttles on the overrun, …This is what’s best known as cold-blown mapping.”

    “Mercedes HPE, equally have their solution, this is the so called fired-overrun. When the driver lifts off, fuel continues to be injected into the engine and sparked within the combustion chamber.”

    Silly question, but wasn’t it Renault who said earlier this season that they used 10% more fuel at one race due to the EBD? Does that mean they were still injecting fuel but with no spark? To me, that would be against the rule of having an internal combustion engine, because combustion is being induced external to the engine.

    As an aside, to me, if you harvest energy by KERS, the engine must use no fuel at that time, otherwise it’s not an energy recovery system, just a further waste of fuel.

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  26. Did anyone else notice Nigel Mansell’s (The driver-steward at Silverstone) comment post-race to the BBC about some teams having a special high-temperature carbon fiber developed by NASA? Do we know who these teams are and where it’s being used? (diffuser and side-pods I’d guess)

    • I think he means Glass Ceramic Composites, these can withstand +1000c temperatures. They are extremely expesive and banned in areas except exhaust heat shields. Pyromerals material ‘Pyrosic’ is the product believed to be widely used by F1 teams.

  27. I remember Damon Hill saying he need to be on throttle when cornering. EBD tech was new but Newey had it down even then.I’m not sure if the Williams was using a clutch then, and I’m surprised the drivers of that era aren’t speaking about. James Bond stuff

  28. Most of the advice that I’ve gotten on here has been pretty good, but just like the tomatoes in produce you don’t always find good ones. But thanks for the information everybody.

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