Fireworks At Night
Posted: 5th November 2009 | Author: @AdrianFRST | Photography: Unknown (via modifiedcars.com)
One of the most prominent aspects of racing and other highly tuned cars is their propensity to disperse copious flames from their exhausts, along with an assortment of pops, bangs, cracks and fizzes. Such behaviour is unsociable, intrusive, legally a grey area but most of all exceptionally appealing.
Your own car need not be borderline WRC spec to produce such effects, indeed some near standard engines are quite capable of such mischievous behaviour with a little encouragement. We’ll take a look at what can be done but first let’s delve into exactly what’s going on when we see and hear automotive fireworks.
Playing with fire - safety first and engine care
Clearly messing around with any of the devices or techniques detailed in this article isn’t going to be the safest thing you can do to your car, so exercise caution, ensure nothing is in the firing line of your exhaust and have a fire extinguisher on standby.
Also, if your car is equipped with any device which momentarily causes over-fuelling, be aware that prolonged use is almost certainly going to damage your engine. Bore wash and silencer destruction are likely to be the first casualties, so use in moderation. If you car is equipped with a catalytic converter this will almost certainly be destroyed quite quickly, so a bypass pipe is recommended - remember to keep it for MOT time though.
In short - unburnt petrol finds its way into the exhaust where it explodes - producing noise and/or flames. To elaborate this can happen due to fuel puddling in the intake ports on wide open throttle, when lifting off this fuel is absorbed back into the air flow and the engine briefly runs rich. As there is not enough oxygen in the combustion chamber the hot fuel travels down the exhaust and when air is available it burns. Cars where the cam overlap is high - i.e. the intake and exhaust valves are simultaneously open will pop and bang on overrun as the inlet manifold pressure drops below that of the exhaust, causing exhaust gasses to be drawn back into the cylinder and altering the mixture.
Ok great - how do I get my car to do this?
As explained above the source of these pyrotechnics is unburnt fuel, and no standard Fiesta will allow this to happen so we have to apply some trickery. Sometimes this trickery is employed for performance enhancing reasons and the popbangflames are a happy side effect. In other cases the gadgets are purely to generate visual effects.
Wild cams - lumpy when idle
A cam, or cams in the case of 16v engines, with a high overlap will make the car more prone to misfiring on overrun. If a suitable profile is chosen this should yield a performance increase, at the expense of a lumpy idle.
A helping hand but useless alone - the flamer kit
One trick which is as old as the hills, having been first employed on the hot rod scene back in the fifties is to place a spark plug in the exhaust, coupled with an additional ignition coil and activation switch. This is unlikely to do anything on a standard car, but will help ignite unburnt fuel that passes through due to other tweaks. One sure fire way to produce jet engine style blue flames is to fit a switch to kill the ignition, thus dumping all the injected fuel right though the engine and into the path of the sparking plug.
Machine gun fire on command - the soft cut rev limiter
This is a fairly simple box of electronics intended to do as the name suggests, but exactly how it does it is the interesting part. The standard Fiesta rev limiter works by cutting both the ignition and the fuelling to provide a soft barrier at the top of the rev range. Hitting this does nothing except slow you down a little until the revs drop. No drama, no noise, nothing. Effective but boring. The soft cut limiter on the other hand employs a much more interesting method - by cutting just the ignition, the injectors are free to maintain the supply of fuel which passes through the engine unburnt and into he exhaust. Depending on the type of ignition the sound made varies. Fiestas equipped with distributor-less ignition will make a rapid fire machine gun sound and those with a distributor based system such as the Escort RST’s MFI will produce a single backfire, shotgun style.
A common addition to the rev limiter is the full throttle gear change. This is an auxiliary switch, positioned under the clutch, which is activated when shifting and temporarily halves the car’s rev limit. This allows the driver to shift with his or her foot flat on the accelerator, improving gear change times. As the car will now bounce off the temporary lower limiter when flat shifting, each change will produce a sequence of rapid backfires.
Big noise for big pockets - WRC style anti-lag
The most costly and most extreme noisemaker device of all and certainly the most expensive - a turbo with a stronger shaft and cut back blades and maybe even an uprated exhaust manifold will be required.
There are several different methods of ALS, however all of which are add ons for aftermarket management - the functionality is not able to be added to the Fiesta’s standard ECU.
How ALS works
Under normal circumstances when the throttle is closed the turbo slows down, meaning that when the throttle is opened again, the turbo must spin up to speed - this is turbo lag.
Essentially ALS is designed to keep the turbo spinning when the throttle is trailing (less than 25% open), be this on gearchange or when lifting off for a corner. It does this by increasing the amount of exhaust gasses fed to the turbo by means of retarding the ignition to around 40 degrees from TDC, and sometimes cutting the spark for each cylinder by 75%, so that combustion occurs in the exhaust manifold. At the same time extra air is added via a throttle bypass valve or air injector. In its “mild” form ALS can simply cancel out the vacuum which normally occurs at idle, or more severe setups can supply positive boost - sometimes as much as 25psi. When the throttle is opened again past the quarter mark, the ECU instructs the ignition timing to revert back to normal and the bypass valve or air injector to shut off.
Typically the fuelling also runs rich when ALS is operational to try and reduce the sky high temperatures inside the exhaust manifold and turbo. Such temperatures along with the physical shock of the explosions are what contribute to such short turbo and manifold lifespans on ALS equipped cars. For this reason its use is largely restricted to competition vehicles.