08-27-2004, 10:27 AM
KPWSerpiente Wrote:RJ says boost means nothing. He is correct up to a point. Telling someone how much pressure you are subjecting the charge air to means nothing unless they are familiar with the turbo you are running. Telling someone how much air you flow is always a better indicator of power being made. Jack was asking me a few days ago how is 20psi on this turbo better than 20psi on the Evo16G...obviously the pressure the air is put under is the same, but this wheel outflows the Evo3 turbo by a lot. I was logging 35 lbs/min of air on the evo16g when I made 320 at the wheels. I hope to flow over 45 lbs/min on this one...I'm sure you are capable of doing the required math.
Anyway, I really do need help. Scotty won't ever get to drive the car the way I dynoed it (I'm a liar, sorry) but he will be the second to get behind the wheel once I've installed, broken in, and tuned this thing.
-T
Congrats on the new snail. Boost is fun. Hope you like buying tires
About your answer to your friend about why 20 psi on two different turbo's is the same. Its wrong. It doesnt really have anything to do with how many lbs/min the turbo can flow. If it cant flow enough, it wont be able to support that boost pressure. The engine is what determines how much air flow there will be. Let me give an example with completely made up number. Lets say you have a small 1 liter engine with a turbo running 15 psi. The turbo is well matched to this engines, and it runs efficiently. You now put this exact same turbo on a 2 liter engine, again at 15 psi. It now flows twice as much and has to spin a whole lot faster to get to 15 psi. It is now way off of any of the center islands on its compressor map. Next, we put this turbo on a big 5 liter V8. Now it can't even flow enough air to get to that 15 psi. Notice the turbo is the same, the engine dictates different flow rates for a given psi. Looking at how much the turbo flows can give a rough indication to how good of a match it will be. Compressor maps are much better for this.
All of this becomes apparent when you look at a compressor map. The X axis is airflow. The formula you use to determine aiflow is Mf=DI x displacment in CI x (rpm/2)xvolumetric efficency. You can see how its the engine that dictates flow. There is no variable for how many lbs/min the turbo flows.
So, what makes two different turbo's on the same motor produce different amounts of power at the same boost level? The reason is efficiency (both compressor and turbine), having a good compressor turbine match, and a less restrictive turbine. Compressor efficiency is basically how much does the compressor heat up the air over what the ideal gas law says it has too. A modern well designed turbo will reach 80% compressor efficiency. A more efficient compressor will heat the air much less then say an old inefficient root type blower. The the turbien is efficient and is matched well to the compressor, less energy is needed to spool up the turbo. That results in better response and/or more power by being able to run a less restrictive housing. Which is the next reason, backpressure on the engine. A larger turbine housing will give more power by creating less backpressure on the engine, at the expense of spool up time. Some turbo motors with very efficient turbo's and big turbines can reach volumetric efficiency levels of over 100% while in crossover. Crossover is when the the pressure being created by the compressor is greater then the total backpressure on the engine. In effect you have negative backpressure.
Or maybe IM wrong, and dont know what im talking about. I like getting into a good technical argument. Always a good learning experience.
Happy boosting,
-Mike