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Engine Facts

Guest BoBOKik

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Guest BoBOKik

1. Do I need to warm my engine up before working it hard?

Definitely. Engines are designed to operate at a given temperature and damage can result if they're made to work hard from cold.


2. So should I start my bike and go for a cuppa while it warms up, or just ride off gently?

Open to debate. Some prefer to leave the bike idling for a miunte or so before riding away. The idea is that letting the engine idle places minimum stress on the cold, poorly lubricated components and gives oil a chance to circulate and protect them.


The other school of thought is that it's best to ride away gently as soon as you start the bike, to burn off any excess fuel, as leaving unburnt petrol in the bores could cause damage. All engines need more fuel in the mixure to start (bikes with carburettors had a choke to achieve this, fuel injected bikes take care of it themselves). However, petrol is a very effective de-greaser and if any is left unburnt in the cylinder it can wash the oil from the bores, leaving the pistons scraping along them. This is only an issue at start up, when the engine is running rich. When it's warm (or the choke's off) you should have the perfect amount of juice.


3. Sometimes I give my engine a real shoeing, but does this do it any harm?

Provided the bike's in good condition and correctly maintained, no. Making an engine work hard can actually do it good. The pistons are sealed onto the bores by the piston rings. These sit in grooves on the piston and are not complete circles, they're open at one end, meaning they can expand.


To seal the rings to the bore, some of the pressure from combustion gets into the groove behind the rings, and forces them out, onto the cylinder walls. If the engine is not used hard, deposits can clog this groove, preventingthe ring from expanding. When that happens, compression in the cylinder drops, making it inefficient and less powerful. But if there are any weak spots on your engine, a caning will expose them very quickly.


4. What happends if I don't warm the engine up?

You can accelerate wear by running a cold engine. When you start your bike, it takes a few seconds for the oil to circulate. Until then the components are relying on the residual oil that has 'stuck' to them for protection. Engines and oils are designed for this, but only under low or no-load conditions. The oil reaches the top of the engine last, so the camshaft bearings, cams and tappets are protected by only this residual oil at first.


The other thing to consider is the expansion factor. Metal expands as it gets hotter and the clearances and tolerances within the engine are all designed to be right when it's warm. For instance, the gears on the primary drive (the cogs that take the power from the crank and transmit it to the clutch) will be tight up against each other when the engine is cold. As it warms and the crank cases expand, the gears move very slightly apart and the correct clearance is established.


5. What's the differenc between torque and power?

The best way to put it is that torque is the size of the bangs and power is the number of bangs per minute. More technically, power is a measure of work done, torque is a turning moment (that of the con-rod on the crank).


6. Why does my engine have a 'sweet spot'? It's at much lower rpm than peak power.

What you're feeling is peak torque. It feels good because that's where the engine is at its most efficient. It all comes down to volumetric efficiency.


Highest volumetric efficiency occurs when the cylinder is taking in the maximum amount of the fuel mixutre possible. The reason peak volumetric efficiency occurs at a single point in the rev range is down to the size of the inlet tract, the inertia of air in it and the phasing of the inlet valve opening.


Pressure waves bounc up and down the exhaust pipe and the idea is to hava a low pressure area right behind the exaust valve as it opens, 'sucking' the burnt gases out quickly and creating a lower pressure in the cylinder, helping to draw more fresh charge in.


All of these factors come together at a certain rpm and that's where peak torque is. So when people talk about 'tuning' an engine, it's a good word, in the same way that you might 'tune' a set of organ pipes so the frequencies produce the right note. It's all the same idea.


7. How exactly does oil protect my engine?

Basically it prevents metal-to-metal contact in the moving parts of the engine. It also helps with the cooling, and, with the help of the filter, removes debris. It's under most stress in the palin bearings. There are few rollers or ball bearings - most bearings within the engine are plain. The 'big end' bearings that connect the con-rod to the crank are good example of plain bearings.


In a plain bearing, pressurised oil is used to keep the rotating surfaces apart. The high pressure needed are created by the motion of the components. The crankshaft drags a 'wedge' of oil under itself as it spins, creating extremely high pressure. Think of a car aquaplaning. The water it hits is under no pressure, but the wedge effect of the wheel hitting it creates enough pressure to lift the car off the road.


The pressure is much higher than the oil pump puts out - enough in the big-end to stop the con-rod hitting the crank under combustion. the clearance between the bearing surfaces is critical. If they wear and the gap is too big, the wedge won't work and metal-to-metal contact can happen, hence (in the case of the big-end) the knocking noise.


8. I'm changing my oil this weekend. Should I go for the most expensive stuff I can find?

You could, and it wouldn't do any harm, but you could be wasting money. Correctly identifying which oil to use in your bike is the most important thing.


Oils are rated by the American Petroleum Institute (API), an independent body that tests oils to given standards and grades them. The grades is S for spark ignition engine (petrol to you and me) and another letter. The further towards Z you go, the better the oil is. For instance, a Suzuki GSX-R600 needs SG oil. Remember that , regardless of how much it costs, Tesco SG oil has passed the same tests as Castrol SG oil.


However, the API doesn't differentiate between car and bike oils. Bikes oils are tested by the Japanese Automotive Standards Organisation (JASO) but, annoyingly, most bike manuals specify the oil by its API rating for your bike, but make sure it has a JASO rating on the tin. Car oils will have no JASO rating at all.


Finally, check with dealer/owners' club to see if your bike has any strange tendencies. BMW boxers, for example, are happy only on mineral oil


9. Can I top my bike up with car oil?

God no. Car oils are no good in bikes, where the oil has to lubricate the gearbox as well as the engine and the shearing action of the teeth literally tears car oils apart. Also car oil's friction inhibitors will play havoc with the wet clutch in your bike.


10. My dad recons slipping the clutch on the move is a bad thing. Is he talking rubbish?

He's right. Slipping the clutch will wear it out more quickly. Perhaps more importantly, because bikes use the same oil to lubricate the engine and gearbox, you will contaminate this oil with all teh debris that's wearing off the clutch plates. This is one of the main reasons that race teams use dry clutches, which are outside the gearbox.


11. When I miss a gear, find a false neutral and then hoof it back in with a horrendous crunch, am I doing any harm?

Maybe a little, but just the occasional miss shouldn't do too much damage. Some gears in teh box are permanently locked to the shaft but others are free to rotate, then locked to the shaft with cogs when they're needed. Cogs have castlellated sections on the side to match holes on the gears, moved along shafts by forks and the action of your toe. False neutrals are the cogs missing the holes, the sickening crunch is the cogs crashing back into the holes. There's no real way round it, but keep the clutch pulled in, take your time and always go up the box, rather than down.


12. Do clutchless changes damage the box?

Not if they're done properly. Balance the throttle so the engine is neither accelerating nor decelerating. With no load on the gearbox the cogs can disengage and engage easily. But if you don't back off, you're just wrenching them apart and crashing them back togather.


13. When I'm riding I hear loads of rattles, knocks and noises that worry me. Am I paranoid?

Probably. A knackered engine will be very obvious, even to the untrained ear. If you're thinking 'is it there or isn't it?' it porbably isn't.


For instance, fuel injectors click as they open and close, which can sound like tappet noise. Some emissions systems such as Suzuki's PAIR valve will make odd noises. But if it's something major, like a big end knocking, you'll know about it.


14. How many miles will a bike engine do?

Up to 200,000 miles is not uncommon from a number of models, though the camchain in normally changed between 40,000 and 100,000 miles, which is a fairly big job.


15. Bike engins are complicated and jobs like checking the valve clearances are time-consuming. When my bike's gone into the dealer, how can I can tell if work's been done?

There's no real way of telling. The only sure ways are to either use a garage you trust implicitly or stand and watch the mechanic. You can mark components that would have to come apart to get to the valves. Or you could measure the clearances yourself, but adjusting valve clearances on modern bikes is tricky as it involves removing camshafts, measurement, calculation and re-assembly.


16. What will happen if the valve clearances aren't adjusted correctly?

Ducatis aside, valves are opened by rotating cams and closed by springs. If the gap between the cam and the valve is too big, the valve will open later and close sooner than it should. This will affect power and efficiency. The top of the engine will also rattle. However, this won't do any real damage. It's far worse when the gap is too small. The valve can't close fully, so the compression within the cylinder will be low. In extreme cases, the piston will bang into the inlet valve, or an exhaust valve will 'burn out' as it needs to shed its heat to the valve seat.


17. What does different colour exhaust smoke tell me about the state of my engine?

White smoke is steam/water vapour. When the engine is cold you'll get this, loads of it on a cold day. Don't worry about it. Black smoke shows and over-rich fuel mixture. Suspect an incorrect carburettor set-up, blocked air filter or a choke stuck on. It's unlikely on fuel-injected bike.


Blue smokes is oil smoke. If it happens on start-up then clears, it's probably worn valve stem seals. If it happens all the time, it's worn rings or bores. Or you're on a two-stroke.


Bikes - Feb 04

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