Do different turbos mean different amounts of boost?

I know this is a noob question, but I hear often, for example, that a b18b1 can handle 12 pounds of boost on stock internals. My brother has a 240sx with an sr20 (just got it in the other day!), and when I tell him that, he always argues that it depends on the turbo. On his SR, he runs about 10 psi for daily driving, and says it can handle 14.5 max. That seems like a lot, but he said that 12 psi with his turbo (it's a small stock turbo) is the same as 6 psi with a different turbo. Can someone please explain these differences to me?

You can read the post that I am "arguing" on, "What is your turbo boosting". But I do think that everyone can agree that your brothers statement is correct. Some others think it is for some other reason with defies me. But (I say) it is becuase the turbo he is talking about is a stock turbo and wasn't made to produce lots of power, so when he puts it way up to 12psi, it can produce 12psi at low rpm's, but at high rpms the engine is requiring too much air for the small turbo to put out. So the psi drops off accordingly (probably down to 6psi or so). So when you put a big turbo on there, you can set it to 12psi and it will give more power up top becuase when you set it to 12psi it does a much better job of keeping the psi at high rpms (becuase its bigger), thus the psi drops off alot less (if at all) so now he would actually be achieveing the 12psi throughout the power band.
So if he were to set the big turbo down to 6psi (like hes talking about), it would hold 6psi throughout the band and end up the same as the stock turbo set to 12psi (becuase the stock one only got 6 psi in the top ranges). Thus when he is pushing the motor, theyd both only be putting out 6psi, even though the small one is "set to" 12psi.

So when your brother says that "it depends ont he turbo" he means that if you set a small turbo too high (like 12 psi) its not actually going to be able to push 12psi all the time, so you wont actually be running your motor with 12psi all the time. So in that sense it does "depend".

Think of it this way, you can put on a blowoff and wastegate valve that only go off at 100psi, but if your turbo isnt big enough, its never going to get there. His small turbo isnt big enough to get to 12psi at high rpms, so hes not actually running 12 all the time. Even though its set to 12psi, its just not getting there. So yea 12psi is alot, but the stock turbo obviously can't do that, so hes not actually running 12psi and what hes effectivly getting is the 6psi (that he says it equivilant to), and thats a good conservative number.

HTH
Buzz1167
Jon N

Thanks for the info. I know it was hard to explain, but it helped a lot. One more question, though: if he's getting 12 psi at low rpm's and 6 in the top end, does that mean that his hp peaks at say 5000 rpms, and then just drops off?

Thanks for the info. I know it was hard to explain, but it helped a lot. One more question, though: if he's getting 12 psi at low rpm's and 6 in the top end, does that mean that his hp peaks at say 5000 rpms, and then just drops off?

Sort of. Turbos are all efficient at certain points. When you go outside of that efficiency, you start to lose horsepower and if you go out far enough, you can damage the turbo.

Wow, that really cleared things up a lot. Thanks.

How can you damage the turbo though? Surely if it's pushing too much boost the waste-gate kicks in and the exhaust fumes float harmlessly away out the tailpipe?

There's a question I've been meaning to ask. Does a turbo 'decrease' manifold flow at all? It must do, and in doing so, decrease performance at least a little bit. This is made up when the turbo kicks in, but what about before it does kick in?

Also, when the engine starts to rev above the optimal range, can the power curve drop 'below' the original non-turbo curve? Or will it always be faster, even when it gets waaay above the optimal range? I ask because type r hondas are designed to reach 9000rpm. That's where they reach maximum torque. It's no good putting on a turbo if it means I lose those precious revs.

When you get outside the efficiency of the turbo you can get surge which can do damage over time. I heard that on a couple years of the skylines, they were known to shatter the compressor wheel if you tried pushing too much boost. Also, if you spool up to quickly, this can also do damage.

Edit - you are correct that the wastegate will try to control boost, but that's what boost controllers are for, so you can boost past what is meant, which isn't always a bad thing.