It's a safety precaution built in by subaru. Design the car very conservative, and worry less about engine damage. Common business practice for any automanufacturer.

This can be fixed with the addition of some sort of aftermarket boost control.

 

it tapers for couple of reasoning.........safety is one....but the other is, if u look at the flow map of our tiny turbo, u'll see that it run out of steam, meaning it can't push any more air above the 5500k rpm range. so basically, it's out of it's efficiency range

if u go w/a bigger turbo....it'll be able to push more air, even at the higher rpm range, but then u're fuel system and cooling system can't keep up

 

Its a consideration of the stress put on the engine and turbo. 15psi@4000rpm is not as potentially destructive as 15psi@6000rpm. Engine heat and stress is not just a product of combustion, but the friction of moving metal and air.

As far as "can't pushing any more air", what's that supposed to mean? Turbo efficiency is a matter of air mass. The hotter your turbo gets (from internal friction, ambient heat, etc) the less effective CFMs it puts out because hot air is less dense. The TD04-13Gs puts out a "max" of about 360cfm, and I believe that's a rating well beyond the WRX's redline.

As you get higher RPMs you get more mechanical heat from a myriad of sources. So your calculations for keeping engine stability relative to boost cannot not be performed as though it was for a static environment. Its changing and that mechanical heat needs to be factored in. If you don't keep it balanced in some way, *kablooey*. Since Subaru can't control your driving habits (pushing revs, or not) and can't control the highest octane available to you... the lower the boost.

Well, it makes sense to me anyway

jason

 

Please read this

http://auto.howstuffworks.com/turbo.htm

 

Basically, the boost tapers off towards redline to rpevent detonation. The compressor falls out of it's efficiency range, as was mentioned, which raises intake charge temps. Combined with the injectors coming up close to full duty cycle, it's just a little more perilous than Subaru wanted to get. So they reduced boost, nothing breaks, and they still get their 227 peak hp. To be honest though, keeping full boost all the way to redline wouldn't result in much extra power due to the hot air being less dense. The actual number of oxygen molecules in each cylinder wouldn't increase much due to the lower density, and it would also be more likely to knock. This is why compressor efficiency is the single most important factor in making the most possible power in a given turbo engine.

 

Heh, your turbine engines have a big advantage over turbocharged internal combustion- turbines will run on damn near anything, from kerosene to race gas!

 

If you took the time to read page 3/design considerations it would have given you a brief glimpse into why things are the way they are.

 

What it really comes down to is this: You've all said parts of it (including myself) and missed the real mark. Not only, like I said before, are over all stresses FAR more significant during higher RPM, but if you are having heat issues and ultimately detonating you will have more detonations per second at higher RPMs. Basic math.

Read (among other things) Corky Bell's book on turbo system kit design. Its a bit old ("...ultramodern Acura Integra engine..." but the theory is pretty up to date.

jason

 

I've read it several times. I have yet to come across new information that isn't in that book, except at a very high level of math and physics. FYI for those who want it, the ISBN is 0-8376-0160-6. I got mine off of Amazon.

And the Acura he uses is an NSX... the buildup is pretty sweet. I just want to find out if they are still making Aerodyne turbos.