Interesting thread. As to why a larger turbo will produce more power at the same psi, I came across this post on an RX7 forum and I thought this makes sense. "As for the original question, consider an internal combustion engine as an air pump. To get it to consume more air, we pressurize the intake. 10 psi is 10 psi, no matter what device is being used to create that level of boost. If all other operating parameters were the same, power output would also be the same regardless of the badge on the turbo. However, there are obvious differences in output at the same boost pressure with different turbos, so how can these be accounted for? Assuming basic engine configuration is unchanged, there are two primary reasons which were mentioned or alluded to in earlier posts: 1.) Higher manifold air temperatures with stock turbo reduces air density, thereby reducing the mass flow of air. This is the ideal gas law in effect here. 2.) Higher exhaust restriction with stock turbo effectively cancels some of that boost pressure. A more efficient hot side will reduce that restriction, providing a greater pressure difference across the engine (intake pressure minus exhaust pressure) which will increase engine output. It's actually that pressure difference that is of greatest interest, but is not often measured at the auto enthusiast level. In turbocharged aircraft applications it's possible to see power output actually increase with altitude so long as a constant manifold pressure can be maintained and the intercooler(s) can keep the air temps in check. This power increase is due to the reduced exhaust back-pressure effectively pulling more air through the engine. This potential power increase is not often realized though, as lack of intercooler capacity often results in rising manifold air temps with altitude."
It does make sense M8, and a whole lot of it. Thing is, this has already been covered, exhaustively (no pun) on page 3. But this very valid point is totally lost on some who seem to be of very high authority on turbocharging and hydrodynamics theory and practise. Just remember, a bigger turbo magically causes the positive displacement device to flow more at the exact same pressure......sheesh! E
E, Your right on everything dude.. apart from this. Pressure is a result of density and temperature. Combustion benefits from more density and less temperature. The 'exact' same pressure from 2 different sized turbos will have a varying density and temperature to make the same pressure. One has more air, one has less. You could pour liquid oxygen into a cylinder, make the volumetric efficiency go through the roof, but it would have no pressure above ambient. Erik can give us a quick rundown on volumetric efficiency. Answers everything easily. Turbos, cams, revs, temperature, size.. all there.
Ah.....so that's the difference. A larger turbo will make the same boost pressure at a lower temperature. So that the volume of air entering the engine remains constant, it just has a higher concentration of oxygen. It's all becoming clear now!!!!!!! Edit: I know we have to compare with everything equal. But I suppose by adding a more efficient IC to the smaller turbo some of these diffferences could be offset?
Yeah the intercooler helps a lot. Pre-intercooler, the air has more temperature, more pressure (less density). Post-intercooler, the air has less temperature, less pressure (more density). An intercooler is a fine example of how air pressure and density varies.
Depends on the efficiency of the compressor... I was having a look last night at the comp maps of the 28RS and the 2558 which is similar to stock, and at 15lb/min @ 1.5PR the smaller turbo was ~3-4% more efficient... But as soon as you were over 20lb/min the 28RS was better, and it had a better range of efficiency too.
The point that has been missed is that a naturally aspirated engine draws air in under a vacuum. Hardly filling the cylinders to their full potential. No matter how large or efficient a turbocharger is, there will always come a point where the engine simply can't flow any more air. No matter what turbos are installed to the VG30DE(TT), as long as the boost level is restricted to the point where the injectors don't exceed safe limits no other upgrades or changes are required to drive the car. A number of you are making valid points based on your interpretation of the question, and it has been a good read.
Any idea what that would equate to in boost land?? At what psi would the 28rs become more efficient than the 2558?
Here's the maps, and sorry, I got the number wrong on the small turbo. It is actual the GT2252. You have to remember that this is NOT the same as the stock turbo, actual efficiencies are most likely very different. I can't say how much better an extra 3% would be, probably unmeasurable.
Steve, all that was covered on P 3. That last comment of mine was close to a throwaway line. Facetious at best. L8r E
Just finished re-reading through this thread again and it's been VERY very helpful. especially after taking on board all the concepts and theories etc discussed. Thank's heaps Tassuperkart, you've pretty much cleared up anything i needed to know in regards to my original question, and it makes complete sense to me as well, although i'm a bit fuzzy on some area's of understanding. I believe i have a grasp on it enough to understand how it all works together. I'm also kind of glad that there had been a bit of a debate as well, otherwise some of the information discussed may not have been written Glad it's also led to other's finding it useful. Thanks also to everyone else for the VERY helpful discussion.
