sunsetsandturbos

OK heres the run down.
R32 GTS-t RB20DET

ran fine but I wanted it to run better.
I installed the factory turbo off of a rb25det
does anyone have maps of these two turbines (rb20 and rb25)? Or at least sizing info?

the rb20 turbine had a nipple right on the compressor housing where the wastegate plugged into. when i bought the rb25 turbine, the nipple was not there and the wastegate vacuum line had been cut, so i didnt know where it plugged in. Guy in a garage told me that it only needed a vacuum, didnt really matter where from. So I teed off from the bov. Now the turbo wont go past .5 boost. Was in a performance part store and the guy in there told me the vacuum needed to come from before the throttle body, but after the intercooler. drilled and tapped a nipple into the intake, plugged off the tee to the bov, and still the same. No more than .5. max at 7000 rpm
Just for giggles, and for troubleshooting, i unplgged the wastedate and plugged up the vacant vacuum on the intake and ran the car without the wastegate hooked up. Sure enough at about 4 grand it was passing 1.0 boost. So the problem is defintely the wastgate opening too soon. This can easily be fixed with a boost controller, but my question is why is it doing that?

and yes, i did hook the wastegate back up and am not driving around at over 1.0 boost. It would be fun, but only for about 10 to 20 minutes,

The wastegate is the oem one that came on the oem rb25 turbine. why would it open sooner simply because i put the turbine on a rb20?

oh yeah, i installed an aftermarket (hks) turbine outlet pipe at the same time. but that shouldnt have any effect on boost pressure, right? only spool up speed.

Looking for more of a lesson here. Any and all help would be greatly appreciated.

Thanks.

sunsetsandturbos

Will reply to my own message here, just in case anyone else has ever had this problem.

The boost controller did work. Now it will boost up to .9 to 1 bar, but spool up speed is still rather slow. I guess its because its a bigger turbine, but it doesnt spool full boost until about 4000 to 4500 rpms. I know it will spool faster because when I unplugged the wastegate it spooled up much faster.

I am going to buy an aftermarket (hks) wastegate acuator and see if that helps. But for now, I am ok with it spooling as it does, for now.

After talking to lots of other people though, I think i can answer the original problem (not spooling past .5 bar). With the larger compressor, it was pushing more air. This created more air in the intake, as it is supposed to do. Although the pressure was .5 bar, the amount of air passing by the wastegate line was increased by the larger compressor. Thus, the wastegate opened sooner. The original compressor spooled up faster because it was smaller (90% of this is turbo basics), and this is why it felt like it hit harder. Wasn't hitting harder, only sooner. Now, with the larger compressor, the amount of air is greater than the smaller compressor at a given pressure. This being said, more air needs more fuel. New injectors in the works. But the computer will not deliver more fuel. Apex safc also in the works.

Hope this helps someone else. This can be a very basic explanation of why a larger turbo, larger injectors, bigger maf, bigger this or bigger that can be a good mod and you may get a few horsepwoer out of it, but everything needs to be matched and tuned (hence the term "tuner") together. Its an art I guess. And Im a long way from being an artist. But Im learning. The biggest turbo you can fit under the hood is not always the best one (seldom is). The best one can be found though, by figuring out the air requirements of your particular engine (or what its going to be in the near future), and placing those requirements at points on a compressor map.

Well, thanks for reading. Any comments, suggestions, or more info would be greatly appreciated.

Thanks,