Piston Slap: Hocus Pocus…Focus???

Sajeev Mehta
by Sajeev Mehta
piston slap hocus pocusfocus

Zack writes:

Hi Sajeev,

I’ve been following a series of discussion on a MK3 Ford Focus forum; in particular I’ve been following the technical discussion about how to squeeze more power from the MK3’s new 2.0 GDI motor. Of course, this involves talk of CAIs, Cat-deletes (inadvisable), and free flow exhaust. One of the more curious things to emerge is…

…the notion that re-gapping the spark plugs can account for +5whp. I’m dubious to say the least that something seemingly inconsequential could generate that much power. It’s almost seems akin to slapping turbo badges on the rear lid expecting some similar black magic. This being an internet car forum there is much breathless back and forth, but few actually explanations. I was hoping you might have heard of this “trick” and whether or not there’s any validity.

I think for questions like this we need a carforum Snopes.com, but then again we have you. Thanks in advance!

Sajeev answers:

You sir, have made my day. Putting me on par with Snopes is a high honor indeed. That said, now I wonder if Snopes is as horribly inaccurate and clueless as yours truly on many, MANY occasions. (sad trombone sound)

Now about the spark plugs: I won’t say that a re-gap cannot possibly increase horsepower. I will say that it isn’t very probable. At all. Two things:

  1. Spark plug gap can make a huge difference, especially in forced induction (turbo or supercharger) applications where adding extra boost is on the table. But that low hanging fruit (i.e. extra power) is usually not there in factory setups: they normally hide the power in tame air/fuel/timing parameters in the engine computer’s tune.
  2. If it isn’t backed by a dyno sheet from a local tune shop, this is pure, un-stepped on, pharmaceutical grade bullshit.

Look, I’ve been messing with Ford products for a looooong time. And while not everything I do has been proven with dyno results, there’s always that low hanging fruit proven many times over with other’s dyno sheets: conservative factory computer tunes, intake boxes with inlet tubes significantly smaller than the engine’s throttle body and mediocre (i.e. quiet and restrictive) mufflers on inadequately shaped crush bend exhaust tubing (older models only). The first is solved with an SCT tune, the second is free (remove something) or requires a trip to Home Depot for a slice of PVC pipe/glue/black paint, and the latter is not a big deal with an exhaust shop and a muffler from a 2005-present Mustang GT.

But spark plug gap? The forums never show that as a credible performance modification. Perhaps GDI motors are a game changer, but I doubt it. That will be optomized to perfection by Ford’s engineers, the low hanging fruit will be the things mentioned in the previous paragraph.

Best and Brightest, you go right ahead and prove me wrong. Snopes ain’t got nothin’ on me. Or not.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.

Join the conversation
2 of 23 comments
  • Scoutdude Scoutdude on Jan 30, 2013

    Increasing spark plug gap on a boosted engine particularly if the boost level has turned up. The amount of voltage needed to create a spark is determined by a number of factors. The primary factors are the spark plug gap and the cylinder pressure. The higher the cylinder pressure the higher voltage requirement, the larger the gap the higher the voltage requirement. That is why it is common to reduce the spark plug gap when turning up the boost or adding boost to a previously non-boosted engine.

  • Rpn453 Rpn453 on Feb 07, 2013

    Always ask the question: why didn't the engineers - who have done extensive testing and likely know more about the details of this engine than anyone else ever will - do this? If you can't come up with a reasonable answer, then don't even think about it any further. When considering performance modifications, the typical answers would include: emissions, low-rpm driveability, NVH, and cost. Good luck tying a spark plug gap to any of those. Emissions aren't going to get worse by making the spark ignite the air/fuel mixture more efficiently. "If it isn’t backed by a dyno sheet from a local tune shop, this is pure, un-stepped on, pharmaceutical grade bullshit." I wouldn't even trust that. All they'd have to do is heat soak an engine for the first run then wait long enough to test with a cooler engine on the second run to show a big gain.

  • Tassos And all 3 were ordered by Fisker's mother. Seriously, given Fisker's terrible record of Failure in the past, only an utter loser, (for example, VGhost or Art Vandelay?), looking for a BEV terrible enough to be a proper replacement of his 11 mile range Fiat 500E, would order one of these. (apart from Fisker's mother)
  • Tassos And all 3 of them were ordered by Fisker's mother.Seriously, after Fisker's DISMAL record of UTTER FAILURE in the past, only a GOD DAMNED MORON would order this one.
  • RHD Any truth to the unconfirmed rumor that the new, larger model will be called the bZ6X? We could surmise that with a generous back seat it certainly should be!
  • Damon Thomas Adding to the POSITIVES... It's a pretty fun car to mod
  • GregLocock Two adjacent states in Australia have different attitudes to roadworthy inspections. In NSW they are annual. In Victoria they only occur at change of ownership. As you'd expect this leads to many people in Vic keeping their old car.So if the worrywarts are correct Victoria's roads would be full of beaten up cars and so have a high accident rate compared with NSW. Oh well, the stats don't agree.https://www.lhd.com.au/lhd-insights/australian-road-death-statistics/