rb20 turbo dropping boost

[the_garynator]

I searched around and asked a few people, but nobody I talked to had a running rb20...

So there was an rb25 turbo on my car when I bought it, about 6 months ago I blew the turbine up because of a boost leak...I had a stock one laying around so i put it in. Boosts fine, has been working great since, but the first thing I noticed was that when I turn the boost up, it drops back down to 10 once I hit around 7k rpm.

At first I thought it was my boost controller, but it worked fine with the rb25 turbo when before it died, and I even swapped the boost solanoid to my backup one...still doing it.

Didn't really do much with till recently because there was always something else more important going on with the car. I finally got things straight recently and took another look at this issue. I read around and thought that maybe it was a weak wastegate spring. So I swapped the stock 10psi one for the 7psi wga off of the rb25 turbo, which I knew worked because the rb25 turbo would hold 14psi all the way to 8300rpm.

Still no cheese. If I run the vacuum line to the intake manifold instead of the compressor housing it seems to hold a bit better and holds 14psi till about 7k when it starts to drop off. Right now it's run to the compressor housing and it has dropped down to 10psi by 7k rpm. This seems strange to me since if it was the boost controller, you'd think it would be dropping to 7psi since the wga from the rb25 turbo is 7psi instead of the stock one at 10psi.

I know the rb20 turbo doesn't make top end power as well as the rb25 turbo, but I was always under the impression that it was because of flow, and that the rb20 turbo would hold boost up top, but was just less efficient, therefore not making as much power. One person I talked to said it was probably a limitation of the turbo, but to my knowledge, a turbo should keep making boost until something breaks (i.e. turbine), not that it will be efficient/make more power over a certain point.

So my question is (for those of you that are or have run a stock rb20 turbo at higher than stock boost) do your stock rb20 turbos hold full boost all the way to redline? If they do, what do you think my issue could be? I know it's not a boost leak or something, I've tested my plumbing and there aren't any boost leaks.

Thanks in advance.

[onepuff]

It is just the turbo's inadequacies.

[the_garynator]

Not trying to argue, just trying to understand...Wouldn't the turbo just spool higher to make the extra boost? Obviously it can spin faster because if it was spinning as hard as it could it would have blown the turbine apart or something...is this flawed logic or is there something I don't know?

[240z4u]

Sounds like your turbo won't flow enough air at that pressure.

Turn the boost down and see what happens.

Your sure you have no leaks?

[the_garynator]

I've tried running without the boost controller. with the stock (10psi) wga, it ran 10, all the way to the limiter. With the 7psi wga that's on it now holds 7psi all the way up.

I'm fairly sure there's no leak since I've replaced all of the vacuum lines since I switched to this turbo and I've had the I/C pipes appart a few times and replaced about half of the couplers, and it has done this the whole time. I haven't tested it with a air compressor yet, but I doubt I would have had a leak this entire time.

I'm guessing you guys are correct that the turbo just can't keep up because I cannot think of anything else that would cause it...but it still doesn't make any sense to me because of the stuff listed in my last post.

I really need to find another rb25 turbo...just wish I could find one that's not super overpriced at the moment lol

Anyone else's stock rb20 turbo drop boost at high rpm? It just seems that I'm the only one who has had this problem with an rb20.

[onepuff]

Here ya go

http://www.skylinesdownunder.c...t=427

You can usually find a lot more info on the Aussie skyline sites by the way. NICO is great but the guys over there have them stock in their cars and have been playing with them since they came out. Lots of good info about the RB engines.

[the_garynator]

Thanks! I can't believe I didn't find that thread before, now I feel dumb lol.

[MATTs14]

Mine pulls ten till about 6 and then slowly drops. Im assuming its just that my turbo is pretty old. Cant expect too much out of an almost 20 year old turbo lol

[the_garynator]

Yeah, I hear ya...I'm just trying to get the most out of the lil ol' stocker till I can find another rb25 turbo. Have you tried running your wga vacuum line over to the intake manifold instead of the turbo? I tried that and it seemed to help a bit, it held full boost like an extra 250-500rpm.

[MATTs14]

Where exactly did you run it to on your intake manifold?

[Carl H]

wg line to the intake manifold will lead to a stabler boost signal but off the turbo will give quickest response.also revving to 8k and pushing 14psi out of a stock turbo is pointless...no power to be had due to turbo and cams.not to mention the ecu is NOT mapped for that kind of revs, its only mapped to 6900.

[the_garynator]

Where exactly did you run it to on your intake manifold?

I removed the "air regulator" a while back (not the iacv/aacv), so I had an open vacuum port on my manifold (mind you I have an aftermarket intake mani). I also tried t'ing the bov line this worked also, although you need to have a decent sized line, otherwise the bov won't open as quickly (i.e. you need to loosen it up a bit more to avoid compressor surge).

CarlH:I know it was pointless to run it to 8g's, but I was just wondering why it was dropping off...I've been shifting at a bit under 7k since I put this turbo on because I know the stock turbo and cam combo doesn't do much over 6500-7k.

Also, I'm not doubting you but I've never heard of the ecu only being mapped to 6900. What does it do once you're past that point?

Why would it be any less responsive running the wga to the intake manifold, if anything, wouldn't you get a bit of boost spike since it would take slightly longer for the wga to see boost?