Boost Creep

[float_6969]

Ok, so I recently bought a brand new S15 SR T28 SpecR turbo. It's the nice one with the Inconel exhaust blades and the built in wastegate divider. Ball Bearing, the works. This turbo is GREAT. I love it. The transient response is nearly instant, and it spools quite nicely. But I was having a pretty major problem with this new turbo, that I wasn't having with my old T25. Boost Creep. And not just a little bit. I was hitting 14psi + by redline. That's a 7+ psi creep from 4K - 7K. I didn't like it and was trying to resolve it. Now don't get me wrong, the car is REDICULOUS at 14 psi, but the power band is very non-linear, and trying not to light up the tires in first is impossible. You've got to short shift it at 3K, or only give it part throttle. So, I started tinkering around. Here was my process...

-First, I was referencing the boost for the wastegate actuator from the turbo outlet elbow. I drilled and tapped a hole and screwed a fitting it there. It works great and the vac line is all of about 3" long. But I thought that somehow this might be causing a proble, and since I wasn't running it like that on the T25 and I wasn't having problems with the T25, I moved it back over to the manifold. It didn't help and I think it actually made it worse. So I put it back like it was.

-Second, I thought that perhapse the actuator was fouled and not opening fully. So I removed it, made sure it was moving freely, and then used a tire pump and a low pressure gauge to see at what psi it was opening. Dead on 7psi.

-Third, I checked the wastegate flapper to make sure that it could freely open and close fully. No dice there either, it worked perfectly. So then I reattached the wastegate actuator, and used my tire pump again. Same deal, 7psi, and it was opening up.

-Fourth, at this point I was out of ideas. I hit up google and typed in "boost creep", or something to that effect. What I got was a bunch of DSM sites. Apparently these guys fight this alot? I guess the problem is that the opening for the internal wastegate is often too small to flow enough air for the turbo to stay at the set boost level and the turbo, ofcourse, creeps. So here was my 4th test. I unhooked the wastegate actuator arm from the wastegate flapper so that the flapper was free to move. Then I drove the car. First of all, it was totally gutless below about 4K. Coincidentally, this is the same RPM where my creep started. Only it wasn't coincidence. From 4K-7K RPM, the boost crept from about 7psi to 14psi.

Now if everything was right in the world, I should only see about 5-8psi @7K RPM with the wastegate just flapping in the wind. There is a few reasons why this happened, and I'm going to explain them so that you guys can save some of the headache that I went through.

I changed from a slightly modified SR T25 (larger compressor wheel, same housing) to an S15 Spec R T28. The SR T25 is actually a pretty decently sized turbo. The exhaust and compressor housings and blades are about the same size. This is good for effeciency. Also, for whatever reason, it looked like the opening for the wastegate in the exhaust housing was larger on the T25 than it was on my T28. The T28 is a slightly mis-matched turbo. The compressor housing and blade are noticeably larger than the exhaust housing and blades. I am also running a High Compression motor. This tends to give more exhaust and the exhaust tends to have more energy. I think those things in combination with the smaller opening on the T28 is overpowering the opening on the wastegate, not allowing enough gas to bypass the turbine blades, causing my tremendous boost creep. This also explains why it gets worse as the RPMS increase.

At this point, I now have 2 options. 1. Restrict the exhaust down so that it can't flow as much air. This will minimize the problem and is probabally the cheapest and easiest way to fix it. But IMHO, this is a bandaid, and I'll be sacrificing power and spool time. 2. Remove the turbo, AGAIN (I'm a pro at this now), and enlarge the opening for the wastegate. IIRC, there was A LOT of overlap between the opening in the housing and the flapper.

I'm hoping to perform the latter operation later this week/weekend and I'll let you guys know how well it worked. I'm anticipating this completely correcting my problem.

CLIFF NOTES: If the opening for the internal wastegate is too small, you'll get crazy boost creep, driving you nuts for at least 2/3 weeks before you finally figure it out.

I hope that my little "lecture" will save someone some time and trouble in the future.

Ryan

[LasVegas240sx]

Putting my CA in in about 2 to 3 weeks. I'm thinking about aftermarket turbos, if I have this problem I'll know for sure.

I bookmarked this thread, thanks for the info.

[themadscientist]

block up the flapper and have an external wastegate welded to your manifold.

[JDMSIL80]

My GT28R didn't do that, the pre-set boost was about 6 psi.

[Dattebayo]

I thought he had a GT28RS?

Anyway, I'm inclined to agree with Mike here. Unless of course you are going to use your Dremel and mickey mouse it into the size you want... (no offence, I know you know your way around a wrench)

Personally I would trust the work to a machine shop and that money would be better spent on some sort of upgrade like a external wastegate. But good information is always helpful.

[float_6969]

block up the flapper and have an external wastegate welded to your manifold.

It's an SSAC manifold. I somehow think that punching a big hole in that crappy manifold isn't gonna help it's structural integrity. Plus I'd have to buy an external w/flanges, gaskets, ect. Not that I have a problem dropping a couple hundred bucks on my car, but if I can fix it by simply enlarging the opening, I don't see the reason to spend money I don't need to. I figure that in the worst case scenario, I screw it up, so I've got to weld the flapper shut and get an external. I figure it can't hurt anything to try.

My GT28R didn't do that, the pre-set boost was about 6 psi.

I have a feeling from the look of the housing, it was somehow under bored when they drilled the opening for the wastegate in the exhaust housing. As I said before, the difference in the size of the openings was noticable. I feel like I'm simply going to rectify a manufacturing mistake.

I thought he had a GT28RS?

Anyway, I'm inclined to agree with Mike here. Unless of course you are going to use your Dremel and mickey mouse it into the size you want... (no offence, I know you know your way around a wrench)

Personally I would trust the work to a machine shop and that money would be better spent on some sort of upgrade like a external wastegate. But good information is always helpful.

The GT28RS has a .86 backside, mine is the .64. It's closest to the GT28R. Remember though that the GT28R doesn't have the wastegate divider and I don't think it uses the inconel turbine blades.

As I said before, I'm leary of the external wastegate for the afore mentioned reasons. What I will probabaly do is remove the turbo, then remove the exhaust housing. Then I'll take it over to my friends machine shop and use the drill press to enlarge the opening so it's even and centered. That way I've got machine quality work, for "Micky Mouse" money. It's a win-win situation!

[Dattebayo]

So are you going to try the same hole size as the T25? Im guessing that would be a safe place to start seeing as how the trim and housings are comparitively the same/similar...

I also wonder if the flappers are the same size... Maybe if it doesnt work you could swap in the T25 flap...

Im just thinking out loud here, so if I missed anything sorry.

[r34 gtr]

hey, my GT28R has that wastegate divider thank you very much. i also dont experience any boost creep to my knowledge, though it may do it by a pound or so. regardless, its too small to matter much. im using the stock o2 housing and manifold. oh, and i was under the impression my gt28r was exactly the same as a s15 t28....

- Tim

[ca18det_boy]

you still wanna switch wastegates and see if that helps man?

[f s t caz]

here's how one guy solved his boost creep on his Z. and the follow-up post

and boost creep is also a very common thing in the subaru community. for them, its several psi in the upper gears and its fixed by either opening up the wastegate port by removing material or by smoothing out the edges of the hole for a better/less restrictive flow out the port and around the exhaust turbine. you figure you kind of need to convince the air to go out thru the port, so taking away the 90degree cuts would seem to do alot.

[float_6969]

Well it worked! I used a carbide bit and my dremel. I think it took me about 2 hrs to do it. I'm not seeing any creep whatsoever. Here are some things I learned from my experience....1. Somehow mark the outside edge of the wastegate flapper. I thought I could just eyeball it and be fine, but I've got a small area that is leaking because it's just barely past the wastegate edge. 2.Don't buy 3 carbide bits @ $9.00 ea. It only takes one and it will still look brand new afterwards. 3.Do this sitting on a chair, at a bench or table with good light. I was on the floor, with my lap, with bad lighting. My neck and eyes hurt after 2 hrs. of doing this. 4.There is A LOT of material, and you can really do a lot of back cutting and smoothing out edges. I think that work by itself might have resolved the issue. The wastegate is placed in such a way that the exhaust gasses aren't going to be real apt to want to flow out of it.

All said and done, I'm very happy about it. I think I can repair my "over boring" without having to remove the turbo again. It's just barely bigger, so I think I quick touch with the welder and smoothing up with the dremel will fix it. I had also bent the actuator arm to make it looser. I'm going to try tightening it back up first and see what effect that has.

As it sits, with the over boring, instead of fully spooling by about 3000rpm, it's more like 3500-3750rpm. Not bad but noticable.

Anyway, I'll let you know how my various repairs go.

[undertaker]

Excellent post! I have a high comp. CA with S15 turbo and also have boost creep but only about 4psi (10 - 14) and had guessed the port was too small, now I know for sure porting will help

[float_6969]

For S&G's, I pulled the actuator loose from the flapper to see how much boost it would build by redline (7500 rpm). It made it right up to .5 Bar, wich isn't bad at all. I'm actually going to mess with the actuator tonight and see if that fixes it. If not I'll have to pull the dumppipe back off and fix it.

PS-Undertaker, what are you using for engine management?

[jamik_69]

dont know if this helps at all but heres another option the dsm guys use id like to try it no need to get the extrnal w/g manihttp://www.atsracing.net/ats_g...t.htm

[undertaker]

I'm just using a SAFC II and standard ecu for management

[sjbsuperman1425]

sorry to bring this back from the dead, but this only pertains to turbo's with the divider correct?

[float_6969]

Man, I forgot about this post!

There are some things I need to clarify for those reading this. I've learned a lot since this post. First and foremost, I have a high compression CA18DET (10.2:1). This was an accident and I didn't know about it at the time I wrote this post. I thought the pistons I had sent to CP to have copied were 9.5:1 and I later (after the motor was built) found out they were 10:1. Second, is that I was running pump gas at this stage, and due to the high compression I was having to run A LOT of timing retard and REALLY rich to prevent detonation. This combination of compression, timing, and fuel led to a situation where there was A LOT of extra energy in the exhaust that was the main cause of my boost creep. Although I can't test it, I'm VERY sure that if I were to run higher octane gasoline, or E85, add timing back in and pull the fuel back, my boost creep issues would have corrected themselves.

And in answer to your question, if you've got boost creep, I doubt the divider would make a difference.