FMIC = less boost?

[Slimg60 0]

Might sound like a really dumb question but I've noticed I have less boost (from the gauge) since I've fitted the front mount.

Is this normal? I've leak checked all the joins and they're all fine.

Any suggestions?

Cheers

[timg60 0]

I thought the same as mine is only making about 10 instead of about 14/15 psi. Have been advised by the garage to check for leaks as they think this is to low.

 

:censored:

[junkie 0]

A bigger intercooler will give some pressure drop but should feel more responsive when it usually gets hot and heat soaked on the old 1, you should still feel it been more consistent power and probably slightly quicker on the bum.

[flusted 0]

my boost levels got lower after the FMIC but mine is a tad on the large side

[Toad 0]

It's probably because your intercooler is loads more efficient, and pressure, volume and temp are linked.

 

A cooler charge, at a lower pressure is better than a warmer one at higher pressure.

 

Have a read of this ummm post by someone else....

 

Right. Engines need oxygen to burn fuel, for reference the oxygen content of air is about 20%. I'm going to try to work through the processes to get more oxygen into the cylinder so we can burn more fuel, and in turn make more power, which is clearly what we're trying to achieve ;)

 

I'm going to make a number of assumptions in this text, which will no doubt get picked up on, but I'd only get a headache and spend far longer in front of the computer than I actually want to if I went into every eventually. And I'm not an expert, just a plonker with no socila life, and a decent graps of GCSE physics :lol:

 

To make this all simpler, lets assume we have an engine with a cylinder with a capacity of 100cc, and using the assumption for the concentration of oxygen above, the volume of oxygen in the cylinder is 20cc at atmospheric pressure and room (20 degrees today, for arguements sake) I'll try to refer to the volume of oxygen at atmospheric pressure, i.e. if the actual pressure in the cylinder was 90% of atmospheric pressure, then there would be 18cc of oxygen (20cc x 90%)

 

As air is heated it expands, therefore a given volume of air at higher temperatures will contain less oxygen. We can prove this with the following equation.

 

(pressure x volume)/Temperature

 

Starting with a hypothetical naturally aspirated engine, the maximum volume of air you should be able to get into the cylinder is the capacity of the cylinder, at atmospheric pressure. There are several factors affecting this, how long the valve is open, how wide the valve is open, how many restrictions there are in the inlet path, slowing the movement of air and causing a vacuum. The air can be heated by conduction from the engine. However, clever fluid dynamics allows engine designers to design inlets that experience pulsing of air back and forth, so that at specific engine speeds, there is a mass of air travelling at speed when the valve opens, which helps with cylinder filling. We'll suppose though that due to restrictions, etc that we end up with the equivalent of 18cc of air in the cylinder at atmospheric pressure.

 

To maximise the volume of oxygen getting into a cylinder we can reduce the restrictions, or we can try to force feed the air into the cylinders. A given cylinder head will allow a given volume of air to pass through it. this is measured in CFM (Cubic Feet/Minute) clearly as outlined in the first paragraph we want to see the maximum volume of air passing through possible.

 

We can look at forced induction which, in simple terms is putting an air pump at the start of the inlet tract to provide as more available air to the cylinder. Firstly, forget preconceptions about boost = power, it doesn't automatically. More CFM = More power. If your air pump provides more CFM than your head can take, then there is a restriction in the system and pressure builds up, hence "boost pressure". Massive boost can be had by having a big capacity turbo or supercharger, and a badly flowing head. Technically you could see a N/A setup making more power than a badly designed head with massive boost. You would have to be a halfwit to get something so wrong though.

 

Using the equation (pressure x volume)/Temperature we can see that increasing the pressure in a constant volume will result in there effectively being more oxygen molecules. I.e if we fed our cylinder with double the pressure of air we would end up with twice the effective volume of oxygen, i.e 40cc. But... 1 bar of boost pressure in the inlet manifold does not equate to 1 bar of pressure in the cylinder, it's all down to the design of the inlet tract in the head. Well designed heads will benefit from increased pressure, badly designed heads will not.

 

Temperature of the air being fed into the cylinder also affects the volume of oxygen available, if the air is at double atmospheric pressure, but double the atmospheric pressure, then there is only the same effective volume of oxygen present. Therefore lowering temperatures is important. unfortunately, the process of pumping air tends to result in heat production, and therefore the air is heated as a result.

 

Cooling processes can be introduced between the air pump and the head, but, while we consider our favourite equation ((pressure x volume)/Temperature), if we decrease the temperature of the air, the pressure or volume will also seemingly drop, but, we will get more oxygen into the cylinder.

 

With a boost gauge connected to an inlet, it can appear that the boost pressure drops when the cooling method is improved, as the inlet system is generally a fixed volume.

 

I'm bored now and can't remember exactly why i started, but basically, what this illworded, halfbaked series of comments means is that more cfm at lower temperatures, with a higher pressure available in the cylinder is the way forward.