Some weeks ago, I did a bit on the ‘Bike’ eco-Triumph that the magazine converted to run on alcohol and promptly tuned for top speed to the detriment of fuel consumption, which to my mind made the whole exercise daft.
That was in the back of my mind when the topic of fuel consumption came up the other day on one of the forums – a rider had just had one of the performance enhancing kits fitted to his litre-plus sportsbike, had it all dyno-tuned and set up professionally.
And when he came to ride it on the road, he found the fuel consumption had plummeted, something he clearly wasn’t expecting!
I’d come across this little hitch myself some years back, having bought my trusty GSXR750WN proudly sold with a loud system and a jet kit as a performance aid.
Having never ridden an un-tweaked model, I couldn’t tell diddly squat about whether the mods worked in terms of power output or delivery, but the bike swallowed petrol quicker than I swallow real ale. The fuel consumption hovering around the low 30s mark, way off the 40mpg figure I was expecting from magazine tests!
I’ll admit that I was somewhat spoiled by my old (and totally unmodified) 145 mph Yamaha FZ750, as it was blessed with an extremely efficient motor which would happily return fuel figures nudging 60mpg at motorway cruising speeds.
So I found the trade-off to a bike that had about 10hp and 10mph in hand over the FZ at a cost of nearly twice the fuel consumption rather less than appealing in terms of the hit on my wallet… not to mention the fact that the straight line performance was actually poorer, given that I was filling up twice as often and taking longer to get anywhere!!
So why does a bike with a performance enhancing kit actually need more fuel, when a reasonable guess would suggest that if the engine was working more efficiently you’d actually get MORE miles to the gallon after tuning?
Well, the answer is in that word ‘efficiently’.
An engine producing maximum power is NOT working at peak efficiency. Let’s explain a bit of basic chemistry.
Internal combustion engines burn fuel to create kinetic energy, mixing the fuel in the cylinder with oxygen in the air to create carbon dioxide and water.
Without doing the calculations or getting too complicated, a perfect air:fuel ratio for pure gasoline is 14.7:1 – ie, you’re mixing 14.7 parts of air to one part fuel. At this ratio, there is sufficient air in the inlet charge to completely burn all the fuel. Modern fuel has varous additives so the ratio is typically a bit down on this, around 14.1:1 to 14.3:1 according to some figures I’ve seen, but the result is the same – all the fuel injected into the cylinder will be burned leaving no excess of either fuel or oxygen.
Obviously, you don’t want too little air in the charge (or too much fuel) as some of the fuel will be unburned. Not only is it wasted, but it’s thrown out of the exhaust port and contributes to pollution, particularly particulates and toxic carbon monoxide. This is known as a rich mixture, and it burns cool and so limits power. This was definitely the case with my GSX-R, judging by the plume of black smoke on full throttle that looked like an F-15 on full boost, and the soot all over the back of the bike at the end of a long run.
On the other hand, you don’t want too much air in the charge either. Although this would effectively burn all the fuel, the combustion can run hot. Whilst in theory this could prduce more power, in practice it can melt pistons and also causes atmospheric nitrogen to oxidize to produce nitrogen oxides, another pollutant and a source of acid rain.
Now, it might sound like the perfect ratio is thus the stoichiometric ratio. In fact, until recently, due to relatively primitive inlet and exhaust design and lack of suitable materials to resist melting and expansion, combustion wasn’t complete and engines ran a bit rich.
Through the 80s, as engine and carburettor design improved, using innovations like four valve heads to improve combustion, so did fuel consumption improve too – or rather more accurately, more power was squeezed out of an engine without significant compromising fuel consumption!
However, motorcycle engines still produced more pollution than the equivalent car engines, so the emission levels were ramped down, leading to the demise of virtually all carburetted models in recent years, and the fitment of catalytic converters on most new bikes.
The latest injection technology allows very precise measuring of the air fuel ratio, with various sensors operating to keep things in tune. The one most riders will have heard of is the lambda sensor. This sits in the exhaust and measures the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas. A lambda of 1.0 means there’s a perfect stoichiometric burn, rich mixtures are less than 1.0, and lean mixtures are greater than 1.0. To keep the catalytic converter working properly, it’s important to keep the lambda figure close to 1.0.
So what’s all this got to do with fuel consumption issues after getting the bike tuned?
Well, we’re back to that word efficiency. In short, by throwing a bit more fuel into the motor than that perfect mix, we can get more power out of the engine. Typically, to get maximum power, we’ll want a ratio between 12.5:1 and 13.5:1. To quote one website offering motorcycle tuning:
“All motorcycles have an optimum air/fuel ratio for producing maximum power and torque. Factors such as emissions regulations, after market exhausts and air filters can alter your motorcycles fuel/air ratio resulting in loss of power and poor or
snatchy throttle response.
They go on to say:
“The correct air/fuel ratio number (say 12.9:1 or 13.2:1) depends upon many factors like combustion chamber design but what we want is that figure at every point in the curve (straight line).“
Dyno figures will usually demonstrate that there’s a power increase over stock, but dyno runs are done at full throttle. And
as most of our riding is done at part throttle, clearly we don’t actually need (or even want) all the power that could be delivered by opening the throttle all the way. You could get the same power increase by opening the throttle a bit more! And most of the gains at usually at peak revs.
Think about it, if you own a 600cc bike and up, how often do you have the throttle on the stop at peak revs?
Not very often I’ll suggest.
So what are the gains? Why would we want to do this? What’s the point of remapping if we can just open the throttle a bit further to accelerate at the same rate anyway?
The key point is that their “correct air/fuel ratio” is actually running rich – it’s only “correct” if you want maximum power at any particular throttle opening. On steady-ish part throttle, where we spend most of the time, the same “slug” of air-fuel mix is still doing about the same amount of work to push us the same distance at the same speed.
The problem is that if you remap your engine to run at the “correct” 12.9:1 ratio rather than stoichiometric 14.3:1 we’ll assume for modern fuel, the bottom line is you’re actually burning around 11% more fuel to do that same amount of work. If you were getting 40mpg before the tuning work, you may well find your fuel consumption is now around 35-36mpg! And if the motor’s set to run richer than that, it could be worse!
Are we just throwing fuel down the exhaust pipe? The reasons usually advanced for remapping is that it’s either to sort out the fuelling after fitting a new can, or to sort out power delivery problems that the standard mapping has failed to deal with. And dyno curves show remapping might well cure a hesitancy when opening the throttle, and if the ignition is remapped at the same time that may well smooth out noise regulation flat spots in the power curve.
You have to ask yourself would you really notice the improvement. It seems to depend on the bike, the rider and the usage.
On a race track – if you’re good enough to lap consistantly, I’d suggest you probably would notice the difference – better drive out of corners is often quoted as a benefit.
On the road – it would probably depend on your riding style and the bike. I’ve ridden a couple of the bikes supposedly cursed by flat spots and can’t say I noticed a huge issue with either of them, but another was like having a delayed on-off switch instead of a throttle and I certainly couldn’t have coped with that for long. So it’ll be a very personal decision.
But one thing we can be sure of – like most things in this world, we don’t get something for nothing.
