I was just curious about this. If I put my evo in 3rd and mash it around 2500rpms and I hit 25psi why if I'm racing starting from 1st and when I shift from 2nd to 3rd I only hit 23 or so in 3rd? Is it because I'm already higher in the rpms when I go to 3rd and the boost has already started to taper or what?

 

Do you have a manual or electronic boost controller?

 

The reason why this happen's is becuase you have a stock turbo at high rpm's it starts to taper expecially when you in the power band also boost change's from the weather, hummidty,heat,and cold climate's how much boost is your controller set to and were does it taper to.

 

Have you tried in top gear to see if it peaks even higher than it does in 3rd?

 

+1 newbie to turbo cars would also like a better understanding of this topic.

 

some reasons are 1 if you dont have a boost controler sometimes your boost will spike the way to fix this is get a boost controler duh 2 now this is just something i have heard of if your car is dynoed they always do 3rd gear pulls and i have heard this is why 3rd is the best just heard it dont quote me on it

 

hm, im still not getting this haha.

Is it more efficient if the boost is tapered during high rpms? also

another noob question that is on topic
doesnt more air mean more hp? so keeping the boost at that level keeps it as strong as possible right?

 

Depends strongly on conditions and turbo the stock turbo really loses its efficiency quickly as you turn the boost up. You can see a turbo's efficiency map by checking out the compressor maps available online. Once you outflow the turbo its going to taper and there is nothing you can do about it except get a larger turbo that flows more air. Airflow requirements go up as you increase any mechanical efficiency beyond the intake, IE exhaust, valves, cams, etc...

More airflow generally means more horsepower but turning the boost up doesn't mean your pushing more air through the engine necessarily. Back pressure from the exhaust pushing that pressure up can keep airflow gains lower if your trying to push the turbo too hot. Also if the output of the turbo is nothing but extremely hot air you wont see much in the terms of gains for power.

 

Boost refers to the increase in manifold pressure that is generated by the turbocharger in the intake path or specifically intake manifold that exceeds normal atmospheric pressure. Atmospheric pressure is 14.7psi or 1.0 Bar. Anything that hits positive PSI will be considered boost. This is also the level of boost as shown on a pressure gauge, usually in bar, psi or possibly kPa. This is representative of the extra air pressure that is achieved over what would be achieved without the forced induction. Manifold pressure should not be confused with the volume of air that a turbo can flow.

Boost pressure is limited to keep the entire engine system, including the turbo, inside its thermal and mechanical design operating range by controlling the wastegate which shunts the exhaust gases away from the exhaust side turbine.

The maximum possible boost depends on the fuel's octane rating and the inherent tendency of any particular engine towards preignition. With appropriate calibration and efficient charge cooling, relatively high boost pressures can safely be attained[citation needed]. Ethanol, methanol, liquefied petroleum gas (LPG) and diesel fuels allow higher boost than gasoline, because of these fuels' combustion characteristics. Race fuel can be used such as c-16 that has a octane rating of 117. To obtain high boost levels, all elements have to be upgraded such as larger fuel pump, bigger injectors, lower compression, right air/fuel ratio, and head-gasket.


From wiki by the way, it all depends on engine load and enviroment also related to wastegate and ECU mapping if tied into it.
For example in the winter time you will need less boost because the air is denser then in the summer, so you can do more with less. In the summer you will need more.

 

it's design to keep your engine safe and reliable, adding a manual boost controller (MBC) or an electronic boost controller (EBC) will help this issue, however theoretically you would want your car to be also tuned for the added boost so you air fuel mixture is able to keep your motor from knocking (detonating) due to a lean condition!