hoohaa

So I'm reading through this very informative page on VG30ET compressor maps which was linked from the FAQ, and I have a few questions.

First, what exactly is the significance of the "knee"? Am I reading these maps correctly if I assume that the horizontal line representing each boost level which branches off of the knee is to be read from left to right, indicating air flow along the continuum? I am assuming that the position of the knee along the X axis represents the lower rpm which is specified in the key, and the end of this verticle line on the X axis represents redline. Is this correct?

I am still unsure how to determine the rpm at which each turbo will begin producing effective boost from these maps. I have read through everything linked in the FAQ about compressor maps and I am still unsure. This is very important to me because my goal is not to have the very best 1/4 mile, but to enjoy significant boost along a wider rpm range. I noticed on the above linked page about midway down that there was one turbo the author listed the following comments about:
* Goldilocks says Too small use the 76.2mm!
* No lag rally type application good to 20psi.
* Notice that at low boost it is never in the sweet spot (not until 20PSI).
A close up of the compressor map can be found here: http://www.mygen.com/users/dbruce/my...R71.vg30et.jpg

I am not quite sure what type of boost is preferred in rally racing versus 1/4 mile, but the "no lag" comment caught my attention. Let's just say I am aiming for 10 psi max boost. I notice that the 10psi line is never in the "sweet spot" of the compressor map, but that the entire knee to redline (?) continuum falls well within the curved lines of the map.

What is it about this map that told the author of this page that the turbo would have a "no lag rally type application good to 20psi"? I am not seeing a very significant difference between this map and the others of comparable sized turbos that would indicate less lag. What am I missing?

For the application I described, what should I be looking for in a compressor map? I mentioned that I want boost over a wide rpm range. By this I mean that I would be happy to give up air flow above 5500 rpm to gain boost at a lower rpm. Should I look for a compressor map in which the knee of the 10 psi range falls within the "sweet spot"?

To recap my questions:
1. What is the significance of the "knee" and the line that follows it in a compressor map?
2. How do I determine the rpm at which each turbo will begin producing effective boost from these maps? (I made some assumptions above on this subject, if I'm way off I might need a good explanation in layman's terms)
3. Where did the author of the above linked page get the stuff about the GT30R 71mm, 56 trim, 0.50A/R being a good turbo for "no lag" rally type applications from the compressor map?
4. Given my goal (low rpm boost), am I correct in assuming I want the knee to fall in the "sweet spot" of these maps?

hoohaa

Yep, back up there.

Nealoc187

iTrader: (23)

1) The knee is the point at which your turbo reaches it's set boost level, before the knee is where it is spooling up to that boost level.

2) You can't, that is the problem with compressor maps. they can't show you that, and only experience of others with that same or similar turbo and/or educated guessing is useful for estimating it.

3) no idea, didn't read that specific portion. Probably he experience of others with regards to how fast it spools.

4) That is probably a good assumption. You want the RPM range you'll spend most of your time in to be in the sweet spot with the knee on the left side of the sweet spot.

Bags

iTrader: (3)

good job in the help neal and thanks

hoohaa

Thank you for the reply. That is very helpful. There's a pretty steep learning curve with some of this stuff, but I'm doing the best I can.