Tech Talk

This section is to explain technical terms, which often get talked about but are not fully understood or even worse people have completely the wrong understanding and who then go on spread this misinformation to others. The items below are in no order of misconception and will be added to when time or comments from people suggest there is a need.

Pre-Ignition

Pre ignition as the term suggests happens before the plug has sparked and can be due to many factors, in the old days when the term “pinking” was introduced it was usually due to poor quality fuel or too high a compression ratio, hence the advent of high octane petrol. Other factors could be hot carbon deposits, or a spark plug of the wrong grade glowing red hot and self igniting. As can be judged pre ignition is not a common problem on modern cars, even tuned ones.

Detonation

Detonation is a more common problem on highly tuned cars. What happens here is a portion of the gas, usually at a corner of the combustion chamber furthest away from the spark plug, self ignites but AFTER the spark plug has fired. Detonation is a far more serious problem as it is harder to cure and potentially more damaging to the engine The pinking noise that is heard, is the gas ignited by self ignition, colliding with the ignited gas from the spark plug. With pre ignition, the collision of the burning gasses occurs before the piston has reached the top of the bore and so not at maximum cylinder pressure, this is why it does not create so much damage. With detonation, as it occurs after the spark plus has fired and the piston is usually at TDC e.g. maximum cylinder pressure, the effect is to make the coming together of the flaming gasses that much more powerful and so damaging.

The causes of detonation can be similar to pre ignition, e.g., too high a compression, only causing the problem of self combustion at TDC. The favourite cause of detonation however is too weak a mixture as a weak mixture burns more easily than a rich one, hence the reason for making a lot of engines over rich as a safety measure. On supercharged and turbo charged cars, the cause is often down to inlet air too high a temperature. The results of both, but more aggravated by detonation is the flame paths colliding and hammering the combustion chamber and more importantly the piston. You can readily see the damage when you strip an engine, if it is light detonation you will see pit marks in the combustion chamber and on the piston crown in the area it has occurred, this gives you some idea as to how to rectify the problem. In many cases it is totally terminal, the piston crown collapsing with a big hole in it and usually the pieces of piston destroying the valves and cylinder head.

Exhaust Manifolds

Regarding manifolds many things are said about the merits of 4-2-1 as compared to 4-1 as to the performance properties each has. The basic facts are that a 4-2-1 will give more low down power at the expense of a bit of top end and the 4-1 is the opposite, slightly less bottom end but more top. The problem for someone choosing which manifold to buy are not as simple as this, as the above comments are based on the manifold being built to set physics principals, that do not change, no matter who has built it.

The principals are too complex to go into fully, so below are the basics.

When you burn the fuel and it escapes from the exhaust valve is has a pressure wave, this wave has energy and it can be harnessed to suck more gas than normal from the cylinder, this is the reason for performance manifolds. These waves have a natural harmonics and pulses and you have to get these right to make them do their job. To their job the exhaust pipe diameter has to be correct and more importantly the length of the pipes between joints. To be correct, the length of the exhaust manifold before the gasses from the 4 cylinders come together should be about 32 to 34 inches from the head flange, assuming it is a 4-1 manifold. If it is a 4-2-1 manifold the first 2 pipes should join at between 16 to 17 inches, this is called the primary length, the next 2, the secondary pipes are the same length. The distance to the first silencer or Cat, should be the same distance away from the place the pipes all join. So why are these distances so critical? The pulses of gas move down the pipe until there is a junction and then reverberate back up to the back of the valve, if these lengths are correct the wave pattern exerts a suction behind the valve head. If these lengths are wrong the pressure wave gets distorted and so looses it’s suction power and in some cases it can actually be worse than the standard manifold, as the waves clash against each other ruling out any suction and even providing positive pressure to hold the gas in the cylinder.

I know many will say, how is this possible, the standard manifold is not anything like the lengths you have quoted. Yes, quite true, here comes a bit of the technical stuff again. The harmonics of the gas and pipe lengths are to do with multiples and divisions of these lengths stated. Lets take 32 inches as the correct length. If you made a manifold that the 4 pipes joined at 64 inches, no problem, if you made one that they joined at 16 or 8 no problem. If the first silencer was 8 or 16 or 32 or 64 inches from where the 4 pipes join together, no problem. Therefore the standard exhaust manifold will have been designed by the manufacturers to coincide with one of these lengths, even though it does look crap.

OK, so where is the problem. The problem is that a lot of exhaust manifold manufacturers do not follow these rules often due to space limitations under the bonnet or for ease of manufacture due to complicated bends and junctions, this being particularly applicable to 4-2-1 units.

So yes on paper a 4-2-1 should give you more bottom end power but if it is not built to the correct lengths it will not and may even be worse than the standard one. A particular car that was built that no one could really improve on the standard manifold was the 205 MI16. It was made of course cast iron but was worked perfectly. One reason I am mentioning all this is that some people were contacting us saying a manifold was available for the Saxo that was stainless steel and cheaper than our mild steel one. I rang the company up to ask details and, yes they could supply it cheaper but only if I bought 10 at one time, so the price problem disappeared. I then went on to ask technical specification as it was a 4-2-1, when given the dimensions of the lengths of pipes, I had to laugh, there was no way this manifold could work.

Exhaust Systems

The same problems regarding pipe sizes and lengths apply to exhaust systems, this is why all manufacturers (until Citroen started with VTR) have the Cat or before Cat’s came into being, had an expansion box or silencer just about 3 feet away from the manifold (or a multiple distance of same). So taking off your first silencer and putting in a straight pipe is unlikely to do anything for performance, unless you happen to be lucky that the rear silencer position just so happens to be the correct length away. This is most unlikely as rear silencers are fitted anywhere that room allows. So what about taking off the cat from under the car, the car goes a lot better without it, yes, it does but this is due to getting rid of the restrictions of the Cat, nothing to do with pipe lengths and gas harmonics. So whilst taking off the cat is good it is not as good as taking it off and replacing it with an expansion box in the same place. This is why our BTB exhaust system is so good, not only is it massive bore all the way down to prevent back pressure but it also has an expansion box in place of the Cat. I know this system is expensive in comparison to many others but none has such a big bore, and none have an expansion box, plus you have to buy a De-cat pipe to make fair comparison to price.

Understeer And Oversteer

These are two of the most misunderstood terms in motor sport or more correctly I should say that people get them mixed up. These two problems should only occur when the car is driven hard.

Understeer is when you turn the steering wheel and the car wants to drive straight on, even though the amount of steering lock you have on, should steer you around the corner. This is the problem with nearly all front wheel drive cars.

Oversteer is the opposite, you apply little steering lock but the car goes around the corner as if you had use a lot more, this being the normal for rear wheel drive cars.

Of the 2 problems, oversteer on a rear wheel drive car is the most pleasant, as by good driving, the car can be balanced on the throttle to hold the line and get the power down. This oversteer problem is attributable to the fact that on a rear wheel drive car you are pushing the car along and so when you go around a corner, the change of direction the front, makes the rear wheels want to drive straight on, thus pushing the tail of the car out. Understeer is the opposite problem, on a front wheel drive, the front wheels pull the car and when you get to a corner they want to go straight on. The other thing that aggravates it is that FWD have a weight split of 80% on the front wheels and only 20% on the rear, so that the rear does not have much grip to prevent this. On most rear wheel drive cars you usually have about 60% on the front wheels and 40% on the rear, so the problem over oversteer is not as severe, this is why the BMW's handled so much better in the British Touring Car Championship.

Can these problems be cured, yes but only to a limited extent, ideally you want the car to be equally balanced, e.g. 50% weight on the front and rear but this is impossible except in a mid engine car. The way to reduce these problems is by altering the suspension set up, there are no set rules here, it is trial and error with suspension springs/ torsion bars and Anti roll bars changes. The only rule of thumb that can be given is that on an under steering car you either soften the front or stiffen the rear, or a combination of both and do the opposite for an over steering car.

It is much more difficult to get the balance right on a FWD car as the higher % of weight on the front wheels makes the problem more difficult and if not careful you can go to far and get a very nasty situation of snap over steer. This is when the car is driving around the corner perfectly and you are really pressing on and then with no warning the back steps right out and you spin off the road. This usually happens so fast that it is impossible for the driver to catch it.

Turbo Dump/Blow Off Valves

We get many people ask for dump/blow off valves as they want to make their car make a noise like a high boost turbo car. Firstly, these valves can only be fitted to turbo cars and cannot be fitted to any normally aspirated car and there is something wrong if they are needed for a supercharged engine. On all standard turbo cars they have what they call a waste gate to stop excess boost pressure, as if they did not, the engine would blow up due to too much pressure. The type of waste gate fitted to standard turbo cars do this by venting the excess exhaust pressure, not boost pressure, to reduce the speed of the turbo and so reduce boost pressure and with this type of system no blow off valve or noise is heard. The only systems where you have a blow off valve fitted are cars such as evo's,mr2's skylines etc where dumping or blowing off boost between gearchanges reduces boost lag by allowing the turbo blade to spin free with no pressure against it other than this the usfullness of the said dump or blow off valve is limited to severe cases where removing boost in a supercharged engine is not available by means of a waste gate.

With a supercharger the size of the charger should be matched to the engine performance and because superchargers are mechanically driven you know the boost pressure rises fairly uniformly up to a maximum and as are utilising all this boost pressure you do not want to waste any. For those who insist on making "the noise", yes you can fit a supercharger that is far too big and waste all the excess air but it is an expensive way to produce a noise. The only time it maybe worth doing the above is if you wanted maximum boost low down the rpm range and then you would have to have a dump valve as the pressure high up would be too much. Having said this most people want the car to accelerate as quickly as possible from a standing start, what is the point of having massive bhp low down when all it is going to do is cause wheel spin and transmission problems.

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