Bike Engine Tuning

Everyone seems to want to know how to make their bike faster, or at least louder. First off I’ll give you some brief advice, then get into tuning a bit more longhand.

Louder

Cheap and noisy cans are ten-a-penny these days, but what happens if you buy one? If your bike has an annual inspection (MoT in the UK, for example), the can might make it fail. Legal cans may be pretty loud, but will have a mark on them – BS AU, E mark, something like that. Without this it comes down to the letter of the law, but most countries allow you to make your own can and it will be fine unless it’s too noisy. I’m not going to dwell on the legal stuff, you can figure it out yourself.
A noisy can will make your bike run lean, as more noise means less restriction, which means running leaner. Sometimes you can get away with no other mods as it won’t upset the carburetion too much, but that would mean you’d get no real benefit other than more noise; for it to run properly and make more power, you need dyno setup, and we’ll talk about that below. If you have fuel injection then this may be able to adjust itself to compensate, but very often not.

Keep Your Head Screwed On

As the mechanic in Mad Max says, “It’s not how fast you want to go, it’s how much money you’ve got.” Tuning costs money, lots of it. More so with more modern bikes, as the manufacturers have made so much progress, that the time has passed when you could just buy something and bolt it on, because in the 1970s bikes had a lot of scope for improvement. At some point it’s cheaper and easier to get a bike with a bigger engine – if you want to squeeze another 25% out of a 600, you might well be better off just getting the 750 in the first place. The times you end up going a long way with tuning, and spending lots of money, are when there are reasons you can’t do that – racing rules, for example, maybe you’re trying to tune a big bike and you can’t get a bigger one, or a larger-engined bike doesn’t have something you need in the smaller one. Next we’ll go through what you can do, but in order of increasing price (roughly). Once you get past the cheapest, biggest gains, then you get into the territory where you need to think if you want to go that far or get a bigger motor.
The other question you should ask yourself is, why do you want to tune? If you want to go faster, then anyone who’s turned a wheel on a racetrack will know that there is a lot more than power involved. Money spent on suspension upgrades, tyres, wheels and brakes are well spent, as these give you better handling, or at least give you more confidence in the ride so you can exploit the bike more. Learning to ride faster makes a big difference too. I don’t know anywhere you can buy bigger nuts though.

Downsides

More power means more stress on the engine and frame, which reduces reliability. Fuel economy is reduced. If you tell your insurance company about what you’ve done they might tell you where to go, or at least ask you for more money.

Dyno Set-Up

Any tuning needs to be set up on a dyno, or it’s money wasted. People often forget this when they see a cheap can for £100, as lobbing an air filter, Dynojet kit or Power Commander (or similar products), and set-up you could be talking more about £300 or more. A dyno is a rolling road, and the data from it and the sensors help the operator tweak the carburetion (or fuel injection) to optimise it. You can’t do it by riding the bike, at least not with any degree of accuracy, plus the dyno tells you how much power is being made. When it’s done right you’ll have spot-on carburetion, more power, and snappy throttle response.

Tuning Options

Ignition advancers are worth a look, but tend to give more bottom end than peak power. Things like the Nikko G Pack are usually to remove an in-built restriction in particular gears; this is built in at the factory for noise tests, so perhaps in 2nd and 3rd gear the bike uses a slightly retarded ignition map so it makes a little less power but is quieter. Removing this restriction won’t set the tarmac alight but it probably worth doing; often this simply means disconnecting a couple of wires so the bike doesn’t know it’s in 2nd/3rd gear.
A free-flowing exhaust (or end can at least), air filter and dyno set-up is step one. Choose between a full system for best performance, or an end can only for price. You can expect 5-10% peak power increase. Titanium cans are very lightweight, as are carbon fibre, but the latter is going to take more of a beating when you crash.
A big bore kit may be worth a look. This always gives more midrange but not always a lot of top end power as the valvegear etc. is still sized for the smaller engine, so starts restricting the higher capacity at high gas flows. Often big bore pistons are stronger and no heavier than the originals, even though they’re 1-2mm wider.
Probably next is cams. Hotter cams have altered lift and duration of the valvegear, giving more peak power, though usually at the expense of a chunk of midrange. They’re often referred to in “stages,” stage 1 being new exhaust camsshafts, stage 2 being both, etc. You can have yours ground to spec but it’s more usual to buy exchange items from Megacycle, Yoshimura etc. and just fit them. For the best performance you’d need slotted cam sprockets for fine alignment, but this is expensive a the associated dyno set-up is lengthy.
Gas flowing and raised compression can eke out a little more. The cylinder head gas flows are worked on to provide minimal resistance at high gas speed. Increased compression ratio gets more out of the mixture the engine sucks in. This is done by skimming the head or barrel; pistons may have to be machined more heavily to avoid hitting the valves. Higher compression means more chance of pinking or detonations, so you might be forced into using super unleaded. If you go too far, the usual is to add an extra cylinder base gasket to increase the compressed volume.
To balance out reliability problems you’ll also need other bits like stronger engine studs, forged conrods, etc. Beyond this you’re getting really serious, so junking the air filter for bellmouths, blueprinting the engine, or really crazy shit like turbos and nitrous could be on the agenda.
The best thing to do is think about what you’re aiming for before you start. Talk to tuning houses to see what they’ve got to say about it. Look on the internet for people with your bike to see how they’ve fared. Get yourself a very large piggy bank.