View Full Version : GT28R with small exhaust - 300 WHP still possible?

04-13-2006, 08:29 AM
Hi guys,

I blew my T25 all to Hell and am considering stepping up to a GT28R. I'd like to see 300 WHP. I will have the following supporting mods:

Z-32 MAF
Nismo 550s
Forge Motorsports FMIC
Nismo FPR
Walbro 255
ROM Tune
Greddy Profec B

Here's where it gets interesting. Due to packaging constraints, I'm running a log style exhaust manifold. I have a full mandrel bent exhaust with a Mangiflow muffler but it's only 2 1/2 inch. At the connection between the downpipe and the exhaust, there's a ball and socket joint that chokes things down to nearly 2 inches.

With my current T25 set up, I see 223 WHP which seems to be about 10 to 15 WHP lower than other folks with similar set ups but better flowing exhausts.

So, what's the consensus? Assuming I keep my manifold and exhaust system, can I squeeze 300 WHP worth of exhaust through there? Would I be risking any damage or accelerated wear (turbo or otherwise) by trying to flow so much through such a small exhaust?

04-13-2006, 09:43 AM
Won't hurt anything except power and spool...

Can you not have a better flange welded on? A complete 2.5" exhaust at ~300rwhp wouldn't be so bad...but a 2" restriction...bleh.

- Brian

04-13-2006, 12:51 PM
Thanks for the reply, Brian.

The exhaust thing isn't really a flange issue. It's flex pipe issue. That ball and socket assembly at the end of my downpipe is where I get the flexibility in the exhaust system. There's no room to run a standard SS mesh flex pipe.

I just discovered that the compressor wheel contacted the inside of the housing. For the price of the rebuild, I might as well just upgrade to the GT28R. From everything that I've read/seen, it appears that the GT28R should be a straight drop in replacement for my T25. All the intake piping, downpipe flange, manifold flange, etc. should line right up. Anyone disagree with that?

04-13-2006, 07:03 PM
Have you thought about using OVAL exhaust tubes? You can get that extra few tenths of an inch and still keep that clearance restraint under control. I think Burns-stainless sells it.

04-13-2006, 08:29 PM
Have you thought about using OVAL exhaust tubes? You can get that extra few tenths of an inch and still keep that clearance restraint under control. I think Burns-stainless sells it.

Already have it! In the straight sections where the exhaust goes through the X-Member subframe. Awesome stuff! Most of the roadster guys are limited to 2" exhaust as the X-Member exhaust passageways are 2"x3". By running the Spintech oval tube through there I was able to get the equivalent of 2.5" but stay within the 2"x3" limitation.

Anyway, clearance through the X-member and out the back of the car is not the issue. It's coming off the downpipe and transitioning from the round downpipe to the oval tube, and having a flexible joint, all within about 5 inches that's the problem! http://forums.freshalloy.com/images/graemlins/ack2.gif

Well, I spoke with Jason at Hybridynamics today about the GT28R. I think it's the way to go. Even if my choked down exhaust and log manifold limit me to 275 or 280 WHP, it's still well above the 223 I have now. Sounds like the bolt up will be pretty painless. Just a few minor piping mods to make. Since my compressor wheel on the T25 hit the housing, I'd be looking at $500 or so to rebuild it anyway. Might as well drop another couple hundred and step up to a much better turbo.

04-14-2006, 04:46 PM
The GT28R isn't a hard install on a redtop with a T25, but it's not as easy as it is sometimes made out to be. There's a lot of annoying grinding/ghetto rigging etc to make it all fit.

If it'd take you 2-3 hrs to install another T25, plan on more like 10 hrs to do a GT28R install(in my experience) unless you've done it before and have all the tools right there and ready. It was just a lot of little crap that takes time to do right.

You're correct in that the GT28R is *MUCH* better than a T25, but you've got to step up to larger injectors, MAF, retune etc. which isn't horrible, but definitely adds another step of complexity. BTW - I have a good condition T25 if you want a cheaper solution and about your same power.

04-14-2006, 06:41 PM
Yeah, I gathered that from Jason. Definitely not a straight bolt in but it's something I should be able to handle. I've got all the saws, grinders, welder, etc to get the job done. It's just the cost that;s holding me back. That and do I really NEED more that 223 WHP in a car that weight 1800 pounds and has stock 1968 crap brakes...

PM'd ya on the T25.


04-14-2006, 07:57 PM
That is a sweet car. On my Datsun, I put the flex pipe in the straight portion of the exhaust. How about getting a mandrel bend instead of choking your pipe down and putting the flex pipe somewhere in the straight portion of the exhaust to separate your engine from the entire system. Here are the pics of my Datsun 280Z.


04-14-2006, 08:19 PM
Doh! Cant view the pics. Something about not having permission. Post 'em here on the FA picture post!

The only straight section of exhaust I have is oval, so unless someone makes an oval flex pipe, I'm SOL.