Exhaust Size / Boost Creep / Turbulence... So many questions
 

OK I'll get right to it, my car doesn't make the power band i want it to. I hit 16psi at about 3000rpm, and that makes 210ft/lbs of torque which is awesome. Once it hits over 4500 RPM, my boost drops to about 12psi, and eventually by redline I'm making horrible HP and torque. I've since redone my timing map because it was screwed before, and the local dyno closed down so I can't fully tell what effect it had, but it definitely doesn't pull as hard in the higher rpm.

I think the majority of my problem is with my exhaust system. My downpipe is a custom divorced gas, that merges into 2.5". Here it is:
http://www.dbmiata.com/miata/turbo/IMG_4301.JPG

I had a magnaflow stock replacement catalytic converter (1.6L), and the downpipe was short in length to mate up with the cat, so we cut the flange off of the catalytic converter, and welded a length of 2.5" stainless to extend it so that it would connect to the downpipe. After the catalytic converter, I then have a really old OBX 2.25" exhaust.

It seems pretty obvious that my exhaust is far too small. I'm guessing it boosts really hard, hits the exhaust wall at about 4500rpm, and just kills the power. So I'm thinking come April when the car is back on the road, I'll get a whole new 2.5" system from downpipe back, no cat. This leads into my main concern, boost creep.

I presently don't have any boost creep, but I'm worried that since I'm internal wastegate, and it's a divorced gas downpipe, if I get rid of the cat and open up the exhaust, there won't be any turbulence or enough back pressure to stop the turbo from over boosting. I would like to believe the wastegate is capable of flowing enough, but I doubt it. The turbo is a T25 from a 1991 300zx. Going to an external wastegate is also not an option with my HKS manifold.

SOO.... sorry for the long message. Any experience with opening up a very tiny T25 turbo and overboosting? Is it going to be an issue? Would it be worthwhile to keep the magnaflow catalytic converter in the system as a means to create some pressure/turbulence, and just run a 2.5" exhaust after the cat?

As always, thanks for any advice.

Edit - Oh yah, I should also mention that my downpipe turbo flange actually does seperate the two chambers. It has an insert on it so that there is no turbulence created between the turbo outlet/wastegate outlet.

Your biggest issue is the cat, that's the biggest restriction, the muffler, then the piping size. You might have boost creep once you replace it all, just depends. Keep in min, that turbo doesn't have the best top-end anyway so even if you can maintain the boost output, you still will have a heavy drop in torque past 5K.

If you can, Begi will port the turbine housing/wastegate to help alleviate any creep for free once you open it up.

This is exactly what sbkcocker499 is dealing with right now on his greddy turbo/sg downpipe/3" exhaust. You will most likely creep. You can port out the wastegate and enlarge the flapper door to help alleviate the problem.

EDIT: Damn your sick ninja typing skills brain. I'll beat you one day.

That 300ZX T25 is a .49 A/R, isn't it? I don't think your boost drop and poor power at redline is at all related to your downpipe. If you want more power at redline, you have to get a bigger turbo and sacrifice a little low-end grunt.

I don't see a downpipe in the picture...

Thanks for the replies guys. It's just at red line I'm making less then 100ft/lbs of torque and about 130hp. I max out at 186HP at 4700RPM. This is my older chart at a bit lower boost (it peaked at 14psi before and held at 11. Now peaks at 16 and holds at 12. I have no idea where I'm at now that I have a proper timing chart.
http://carphotos.cardomain.com/ride_...0110_large.jpg

Stupid small ass turbo :) It's so much fun right off the line, but that's about it. I'm quite adverse to getting a new turbo since I would have to get a whole new downpipe made. I was an idiot and got this turbo for cheap, but it uses a 4 bolt flange for the downpipe and not the common 5 bolt. The only upgraded turbo's I can find to fit it are only sold in pairs (because this was only used on twin charged vehicles), or it makes them purchase a 5 bolt... arghthepain.

Yes I know there are boobies in the back.
:drool:

I had this problem after upgrading to a 3" on my begi divorced downpipe. I ended up removing my boost controller and its very solid now, but before boost was all over the place. Mostly if I just romped on it it'd creep so high and not stop that i'd hit boost cut (16psi) immediately.

If you think the cat & cat back are a restriction it is really easy to test. Unbolt the cat & cat back and take it for a rip up the street. If you notice a huge change you know the exhaust is the problem. That turbo will be working hard in the higher rpm..

I would look for a larger turbo.

Nice fab work btw. The welds on the downpipe look great!

Wow...your torque drops waaaay off. I blame the turbo. My turbo is smaller than most on this board, like yours, and I have battled this for months. COPS, exhaust, WI and about to do an adjustable cam gear.

The COPS made a noticeable difference...open exhaust made a LOT of difference (no cat...try this first), and I'm hoping that retarding the intake cam will help a little. Jury is still out on WI but I just finished finals and haven't given it enough tuning time.

I finally faced the inevitable last month though and ordered a compressor upgrade. I'm willing to sacrifice a little spool in order for it to keep pulling up top. I seriously shift at just after 5K rpms...even when trying to go fast. I want to know the glory of still pulling at redline.

Correct me if im wrong but can't you change the impeller and snail out for a bigger one with the same hot side with this turbo? I think someone on this forum did it and got really nice results.

Thanks! Though I sadly can't claim to have done it. I had my stainless welder friend do it up for me, and he's awesome. Too bad he moved to New York to work for midnight auto.

I would love to do something like that but I haven't found any information on it.

It's nice to know we're in the same boat. What compressor upgrade did you go with? Where did you get it?

Thanks again everyone.

or http://i38.tinypic.com/2ptebsl.jpg
:giggle:

Respect to gospeed81, and I'm curious to see how his turns out, but I'm not sure I'd spend my time or money on a just compressor upgrade for a little ol' 300ZX T25. Although I could be convinced by dyno plots. Remember that whatever air you stuff into the motor has to flow out the other end, and the tiny .49 A/R turbine is still going to be a big restriction for top end power. Even the .64 A/R GT2554R, which is small by this forum's standards, will shit all over a 300ZX T25. In Garrett's current lineup if you want to find anything with a .49 A/R turbine you have to look way down in the GT20 class. Go look at some flow maps on Garrett's website and you'll see how much that turbine is limiting you.

I don't think there are many good inexpensive upgrade options when your setup is built around a puny turbo with a weird 4-bolt outlet. But you could probably move up to something like a nice used S15 T28 without landing yourself in the poor house. They often trade for $400-500 on the nissan boards and have the same specs as a GT2560R- you get a ball bearing CHRA, bigger compressor, and a less restrictive .64 A/R turbine. It will be capable of ~40-50% more hp than what you're putting down now and I don't see the guys running GT2560s complaining much about lag either. Save what you were going to spend on the compressor upgrade and put it towards redoing your DP with a standard 5-bolt T25 flange that's compatible with a wide variety of turbos. :2cents:

It would be nice to drive out of boost sometimes. Crap, I can easily make boost in reverse...

Looks like it might be more worthwhile to get a new turbo at the moment then changing the exhaust system. I wonder if the 4 bolt and 5 bolt flanges are similar enough that I could just do a cut and re-weld onto a new flange.

I actually have a t25 sitting around with the .64 ar hotside. It is a 5 bolt but let me know if you're looking to upgrade

Thanks but I have to pass. I actually had a T25 SR20 turbo with .64 originally which I bought used, and it turned out to be totally screwed, so I rushed and bought this 300zx one. So yah, I'm done with used turbos :) Now a chinacharger, that's a different story.