boost pressure vs. volume questions

[AMink]

So I've read here that more volume will result in lower egt's, but that more boost doesn't necessarily mean more volume. I think that's what I read, at least. Wouldn't volume be equal at any given boost pressure on different turbos? I mean, if a stock turbo is making 18 psi, would a 40 or bigger making 18 psi be moving the same amount of air? If it moved more, wouldn't the pressure go up? Am I making sense? :-laf

 

[bmoeller]

The HX-35 has a smaller turbine outlet than the 40. The 40 is 4" vs. 3 or 3 1/2". Meaning less restriction= less boost required to move the same amount of air.

 

[AMink]

So do you need a higher boost to "push" the air into the combustion chamber? I always assumed (and we know about that!) that you needed the pressure, or boost, to shove the air into the engine. I understand the higher volume, lower pressure thing though. Thanks!

 

[Turbo Tim 1]

Since an HX-35 is working outside of it's effeciency range making say 42 psi the air that it compresses is hotter than the air of a larger turbo like an HX-40 making 42 psi, therefore the HX-35 air is less dense and has less air mass for cooling and combustion purposes. Take a look at these numbers for air temperature versus pressure:

Other factors equal, the volume of the air in the tire will change by 1/273 for each degree (Celsius or Kelvin) change in temperature. A span of 0 degrees F (-17. 8 C or 255 K) to 100 F (37. 8 C or 311 K) represents an approximately 55 degree C or K difference. Thus, the volume of a tire (considered 1 unit) would expand to about 1. 2 units. Likewise, the pressure inside would rise by a factor of about 1. 2 as well.



For example: The pressure inside a tire containing air at 80 lb/in^2 (5. 44 atm) at 0 F would rise to about 96 lb/in^2 (6. 5 atm) at 100 F.

So if the air out of your intercooler was 1 cubic foot at 0 degrees with one turbo and 1 cubic foot at 100 degrees with the other you can see the differences in volume after reaching combustion temperature right?



Quote from a HotRod Magazine: Every 100 degrees F reduction in air temperature increases air density 12-13 percent, greatly increasing engine power output.



If you do a search on Google for Air Density Turbo Efficiency you will find some pretty good reading.

 

[AMink]

Now I get it. More oxygen to burn in cooler air at same pressure. thanks for the info!