Low Comp

[SteSE]

Why when going forced induction is it better to have a lower compression?

[UweM3]

Why when going forced induction is it better to have a lower compression?

FORCED induction. You are forcing MORE mixture in.
i.e. a NA cylinder is sucking in 500cc of mixture and the upstroke compresses the mixture with the CR, the whole lot gets hot and wants to ignite on it's own (also known as knocking or pinking).
To avoid this you have to use high octane petrol and the correct igition timing.

If you now FORCE 600cc into the same cylinder with the same CR, the upstroke of the piston would compress it and it get's too hot and will ignite itself. You either need a higher octane petrol than NA (which is impractical and ££££) or you reduce CR to avoid this.

This is a VERY VERY basic explanation, there is more too it. Hope this answers your question

[SteSE]

Thanks for the reply, yes it does answer my question however raises some more.

High Octane Petrol has a better threshold to ignite over normal petrol. i.e you can compress it/heat it more before it ignites?

Compression can be altered by head gasket thickness and or piston type. How does this work, the thicker the head gasket the lower the compression or vice versa. Also is it the top of the piston/head that affects the compression or length or something else?

Why is a higher compression more suited to N/A applications or is lower compression better for both N/A and forced?

[appletree]

Thanks for the reply, yes it does answer my question however raises some more.

High Octane Petrol has a better threshold to ignite over normal petrol. i.e you can compress it/heat it more before it ignites?

Compression can be altered by head gasket thickness and or piston type. How does this work, the thicker the head gasket the lower the compression or vice versa. Also is it the top of the piston/head that affects the compression or length or something else?

Why is a higher compression more suited to N/A applications or is lower compression better for both N/A and forced?

Higher octane is better and gives more lee way before knock/pincking occours.

compression is the "gap" in simple turms between the top of the piston and the top of the cylider. thicker gasket/ shallower piston = lower comp as the gap is bigger.

Higher Cr is generly better for N/A and lower for boosted, in boosted applications the CR has more affect on the low end torque off boost.

its abit of a comprimise, low enough compression to run boost but not so low that you loose low end torque.

hope that helps

[GeoffBob]

Why when going forced induction is it better to have a lower compression?

Because of detonation. Detonation in fuel is physically very similar to what happens (as the name suggests) in an explosive. It is very different to the process called combustion. Detonation occurs much faster than combustion and releases all the energy of the fuel against the piston crown, combustion chamber, valves and edge of the gasket with a ferocious smack, comparable to a severe hammer blow. Detonation is, therefore, in a word, bad.

The temperature at which a mixture of air and fuel will detonate is inversely propotional to its density. That is to say, the more you compress it (increasing its density), the lower the temperature at which it will detonate. And unfortunately, as you compress a gas, you also raise its temperature.

There are two ways to compress a mixture of air and fuel to the point where it detonates. One is to increase the compression ratio of the engine, the other is to compress the air fuel mixture in advance of pumping it into the combustion chamber (as in a forced induction engine), and then compress it some more during the compression stroke.

The advantage of forced induction is that it is possible to pump a greater mass of air into the engine during each induction stroke, which (with proportionately more fuel) enables the engine to make more horsepower. However, to compensate for the already compressed air being pumped into the combustion chamber, the compression ratio is generally lowered to avoid detonation. Typically, a low compression engine will be able to tolerate high boost (without detonating), and vice versa.

Why is a higher compression more suited to N/A applications or is lower compression better for both N/A and forced?

Because, so long as you haven't reached the point of detonation, a higher compression ratio engine yields more horsepower for a given amount of fuel.

The resiliance of a fuel to detonation or knock is indicated in the UK and Europe by its Rated Octane Number or RON (93, 95, 98 etc..). The RON of a fuel is in no way a measure of the energy content of a fuel. However, the greater the compression ratio of an engine the greater its ability to convert the chemical energy in the fuel into mechanical energy (so it makes more horsepower). However, since a higher compression ratio engine requires a higher RON fuel to avoid detonation, there exists the general misconception that a higher RON fuel will make more horsepower in any engine.

[clarko74]

Interesting stuff this.

Slightly ot but... if in a n/a engine it is generally better to have a higher compression ratio why did bmw build the facelift cars with a lower comp engine than the pre-facelifts?

[Morat]

For some reason they were worried about the catalyst cars requiring lower compression ratio.

[Jhonno]

Fuel availability?

[GeoffBob]

For some reason they were worried about the catalyst cars requiring lower compression ratio.

So far as I am aware that's exactly what happened. The switch coincided with the introduction of unleaded fuel, required for use with a catalytic converter. Tetraethyl lead was the primary additive used to boost octane in petrol from 1923 to 1975, and became illegal in most countries in 1995. There were concerns that without it, and the resultant 2 RON drop between leaded and unleaded (due to the cost of octane improving additives such as methyl-benzene), that the higher compression engine would suffer detonation within some markets.