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Discussion Starter · #1 · (Edited)
Little while back my 168k mile T25 finally gave up after overdosing on too much boost :cry:. Having sold my other car several weeks back, I needed to keep this car on the road and running until I could get the turbo rebuilt. So, the CHRA was removed, a block-off plate was installed to cover the hole:

oil line was plugged, coolant lines were fed into each other, and a 5" round air filter assembly meant for a carbureted V8 was attached to the intake elbow, and shazaam! The engine was converted to NA. (On a side note, the ECM seems to be handling it fine. MPG is actually on par or better than before. Car is not running rich either)

One of the very 1st things I noticed when the exhaust housing was out of the car, was how it takes the exhaust from the header, and literally chokes it down to a port about an inch in diameter! (think of a funnel) I also noticed that the "runner" had a oddly shaped, rough "hump" deep in the hole that looked rather restrictive, so it was removed and the inner walls of the runner were expanded, staying with the original "funnel" theme.

Before:

Notice the "hump" further in towards the upper area, accentuated by the shadows.

After:


As you can see, I smoothed out the "hump" to make for a more uniform transition, and enlarged the diameter of the runner as this was it's smallest diameter (choke point), and blended everything so the spent gases meet little resistance but follow the original concept. This was the best pic I could get to look right. It kept giving the illusion of the area I ported as being overly ported due to the lighting and the area being a bright silver, whereas deeper down everything was still dark. It's actually uniformly blended and consistent. If you look closely towards the bottom of the runner, you can see the opening for the hole leading to the wastegate flapper. I was also able to slightly unshroud the very beginning area (inside the runner) where the gases 1st hit the blades. There's actually a notch in the corner of the runner that's sort of "V" shaped. The bottom of the "V" is where the gases 1st hit the blades.

Other area:

If you remove your down pipe from a T25, you'll see the wastegate flapper to the left, and the main port to the right. It is possible to widen the overall exit diameter slightly (possibly giving better flow) as there is more than enough room for the down pipe's flange to still seat correctly. I opted to not do that though, and instead smoothed the inner edge flat (if DP is removed, looking down at the opening, it'd be from the 3 o'clock to roughly 6 o'clock positions). This pic shows from the inside what was removed. Look through the opening for the bright silver. When the turbo is spooling and the WG flapper is shut, the gases hit the exit area at an angle before leaving and entering the down pipe. This "blending" helps to try and keep the gases going straight into the down pipe rather than getting deflected at an angle before exiting. My drill broke after 16 years of faithful service as I was using it for most of the porting. Had it not broke, I would of worked this area more to give the gases a straighter shot out to the DP, possibly notching some of the flange area to straighten the flow.

All in all, the goal was to get the gases to enter and exit through the housing quickly and much more efficiently while not killing the original design which in itself suggests the "funnel" or "bottle neck" created by dropping the runner down in size is needed to spool the turbine quickly or properly. Before the porting, (Naturally Aspirated form with the center section removed- have to spell everything out as a couple guys will come in and knit pick and use my lack of clarity as a childish excuse to insight trolling :roll:) the engine struggled horribly above 2k rpms. 1st gear was kind of an exception as it would reach say 4k before completely falling on it's face. 2nd-4th had to shift around 3500. After porting, torque picked up quite a bit. Now when I give it gas at 2500-3500 rpms, the car actually responds and is noticeably making more power until around 4200 rpms. There's even a slight weight transfer to the rear which has been missed greatly LOL. Overall the car is more responsive and able to keep up better with the flow of traffic and not getting dusted by Chevy Aveo's :eek:. I'm wondering if this design "fault" is what separates the TD04's 300 HP capability and the T25's 250ish limitations. I fully expect once the center section is back in that the car will have greatly improved mid and top end breathing- wondering about when it'll start spooling though. Stay tuned as I'll definitely update ;ol;

I'm waiting for the GT2860RS compressor wheel and the T25 rebuild kit to get here. If I get enough interest from people curious about the effect of the exhaust porting with the stock T25 compressor wheel still in, I'll throw the kit in, and run the car a bit before swapping the GT2860RS wheel in (again, to keep stupid comments about GT2860RS wheel not being a direct swap or other flaming comments to slander me, I'll clarify: cold housing will get ported to accept GT2860RS wheel; CHRA will get balanced) to give an accurate idea of what the porting has done. So reply here if you're interested on the effects of the porting or not. One thing I'd like to point out, is with more air getting compressed into the engine, it surprisingly ignores that tiny runner, but in "NA" form, it's way too small to run and keep a healthy attitude on the street. If the performance that I picked up today after the porting translates to better boosted performance, this might give a little more life to this baby turbo, especially if it spools early and keeps going with with the bigger wheel in place :cheesy:

EDIT: The wastegate flapper was wired in the halfway position, so it's "open", but it's not open enough to restrict flow from the main port. Probably doesn't matter and might be better to be wired shut, but in the test runs in the next post it will have been the same before and after. Also, in a videos I'll post tomorrow after I make an actual "after" video, it should be clear just how helpful this porting is to exhaust flow. There's a steep winding hill near me, when the turbo was working, I could hit 50-55 before loosing grip from the turn; BEFORE porting I hit 30 MPH in 2nd and it'd fall flat until I reached the top of the hill; AFTER porting I almost hit 40 MPH on a test run earlier!

EDIT 2: Forgot to mention there was a buildup of carbon deposits on the round surface that encompasses the exhaust wheel. It should be removed with a wire brush or similar as it reduces the tolerances of the wheel to the surface.
 

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Discussion Starter · #4 ·
Looks like a worthy free mod ;ol;
Yeah, it's funny but awhile back I tried to Google porting a T25 and virtually everything I saw said it's not worth it, gains won't be noticed, don't bother, just upgrade to a bigger turbo, etc. which seems to not be the case. *If* it can perform similar to a TD04-15 with an hour of port work and 30 minutes to pull the turbo out and unbolt 4 bolts to separate the hot side housing; seems like a nice alternative. But gotta wait and see how it acts once the CHRA is installed to be confident ;ol;

Nice thing was I could of ported more material from the walls, but stayed on the side of caution. It won't add 50HP or anything, but if it adds a little all the way up to redline and reduces spool time, it's a no brainer. Suppose I could buy a used HolsetHX35 for $60, but squeezing every last drop out what Saab gave us is more rewarding :cheesy:
 

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I'd also consider porting the manifold flange to the size of a 9k flange. Easy enough to do, just stick your t25 gasket onto your manifold and draw around the inside of it with a marker pen, then port out to that.
 

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Discussion Starter · #6 ·
I'd also consider porting the manifold flange to the size of a 9k flange. Easy enough to do, just stick your t25 gasket onto your manifold and draw around the inside of it with a marker pen, then port out to that.

Are you saying there should be a round gasket? I have an oval shaped gasket which appears to be factory. Bought an aftermarket one too and it's oval as well. My manifold has the divided runners with a round port that matched the turbo's inlet, and sticking my fingers up there it didn't feel like very much could be removed? Are you thinking of a different manifold?

I'm not in any hurry to pull the header as it looks like it'll be a royal PITA with the cheap bolts/studs they used as the lower obligatory right hand corner is already sheared :confused:
 

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The ng900/9-3 manifold exit is round. The 9k exit is oval and a lot bigger. You could port your turbo zorst gas inlet (which you've started) and manifold outlet to match.
 

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Like this.....

McVities said:


the turbine inlet could do with a little bit of a grind to bring it out to match the T25 gasket......

I used the gasket to port out the flange outlet on the gm manifold
before





after




I ground it back to get a nice smooth transition from the merge collector to the flange, without any rough edges, lumps, bumps or steps

[/quote]

Thanks to Rich T for digging the photos out for me.
 

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Discussion Starter · #9 ·
Ah, ok. You meant grabbing a 9k manifold. The plenum is divided to separate the exhaust, pulse right?

I dunno, I was looking at the outer flange material on the T25 (at least on mine) and for whatever reason it's stepped, where it looses roughly half of it's thickness in the flange material, meaning it didn't look like it'd support going too wide as cracking would be possible. Also, the walls of the runner appear to be about as thick as the stepped area (maybe a 1/4"). I'm wondering what year/model T25 that is from in those pics as I don't think I can port my opening that aggressively, and not really sure it'll help at all. I thought the exit of the header was oval like the gasket and was planning on widening the T25's hole until I realized the header was round, but spent some time looking over the design and concluded it wouldn't be able to match up to the gasket for reasons mention already. Wondering if the 9k's T25s were beefier or something?
 

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Ah, ok. You meant grabbing a 9k manifold. The plenum is divided to separate the exhaust, pulse right?

I dunno, I was looking at the outer flange material on the T25 (at least on mine) and for whatever reason it's stepped, where it looses roughly half of it's thickness in the flange material, meaning it didn't look like it'd support going too wide as cracking would be possible. Also, the walls of the runner appear to be about as thick as the stepped area (maybe a 1/4"). I'm wondering what year/model T25 that is from in those pics as I don't think I can port my opening that aggressively, and not really sure it'll help at all. I thought the exit of the header was oval like the gasket and was planning on widening the T25's hole until I realized the header was round, but spent some time looking over the design and concluded it wouldn't be able to match up to the gasket for reasons mention already. Wondering if the 9k's T25s were beefier or something?
Mate, I have no idea what your on about. I'm simply saying port your round hole where the exhaust gases come out of the manifold so its wider and smoother so it matches your gasket.
 

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Discussion Starter · #11 ·
The gasket I have is oval, not round. Both ports, coming from the manifold and obviously going into the turbo are round. Nevermind ;ol;
 

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I know back in the day i got a t25 off a regular 9000 cse (non-aero) and their t25's had that bigger exhaust housing (6cm2) like that found on the 9000 aero tdo4's...

If I were going to modify a t25 I would definitely source out one with that bigger exhaust housing and port match the exhaust manifold to match... Would definitely help in lowering EGT's when you're spinning the hell out of it...

just my two cents... ;ol;
 

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The gasket I have is oval, not round. Both ports, coming from the manifold and obviously going into the turbo are round. Nevermind ;ol;
Right, now this is getting silly, one last time.........

Your manifold has a round hole yes? Port it to the shape of the t25 gasket.

Your t25 turbo had a round hole that you've opened up a bit yes? Well port that to the shape of the t25 gasket too. You will then ensure a smooth transfer of zorst gas. It's that simple. No round Gaskets or 9k manifolds or any other stuff complicating things. Make the round holes on manifold and turbo more like the shape of the inside hole on the t25 gasket. Use the t25 gasket as a template. Please say you understand?
 

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Discussion Starter · #15 · (Edited)
Right, now this is getting silly, one last time.........

Your manifold has a round hole yes? Port it to the shape of the t25 gasket.

Your t25 turbo had a round hole that you've opened up a bit yes? Well port that to the shape of the t25 gasket too. You will then ensure a smooth transfer of zorst gas. It's that simple. No round Gaskets or 9k manifolds or any other stuff complicating things. Make the round holes on manifold and turbo more like the shape of the inside hole on the t25 gasket. Use the t25 gasket as a template. Please say you understand?
Ken I understand. What I'm trying to say to you is there isn't enough material to grind away for that, which I was trying to say in earlier responses. The pics you posted look different than my exhaust housing ;ol; Besides, I'm fairly satisfied with the out come. Been busy so I'll add an "after" this afternoon. Think you'll agree the difference is pretty noticeable.

Also, look at the picture with my block-off lid. The flange is thinner in the middle area and the actual runner is a smaller outside diameter- all of this means that I can not port the opening to the size of the gasket. Maybe the 9000's T25 had a larger housing like someone else suggested, but mine doesn't. Don't appreciate the condescending nature.
 

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Ken I understand. What I'm trying to say to you is there isn't enough material to grind away for that, which I was trying to say in earlier responses. The pics you posted look different than my exhaust housing ;ol; Besides, I'm fairly satisfied with the out come. Been busy so I'll add an "after" this afternoon. Think you'll agree the difference is pretty noticeable.

Also, look at the picture with my block-off lid. The flange is thinner in the middle area and the actual runner is a smaller outside diameter- all of this means that I can not port the opening to the size of the gasket. Maybe the 9000's T25 had a larger housing like someone else suggested, but mine doesn't. Don't appreciate the condescending nature.
Reading your earlier posts again it seems obvious to me that you didnt understand what i was saying. If i seem condesending i apologise but after i took the time to ask a member from UKS to dig his old photos out, then me stick em on here and repeatedly explain myself perhaps my patience was wearing a little thin. For the record mate plenty of folk have ported 93-900 manifolds etc. I would ask Martin9-5t to post some photos up of his but i fear it would all be in vain......

Anyhoo, i appreciate the work your trying out/testing as ive been known to bodge the odd thing together and give it a whirl myself ;)

Keep up the good work ;ol;
 

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Discussion Starter · #17 ·
Reading your earlier posts again it seems obvious to me that you didnt understand what i was saying. If i seem condesending i apologise but after i took the time to ask a member from UKS to dig his old photos out, then me stick em on here and repeatedly explain myself perhaps my patience was wearing a little thin. For the record mate plenty of folk have ported 93-900 manifolds etc. I would ask Martin9-5t to post some photos up of his but i fear it would all be in vain......

Anyhoo, i appreciate the work your trying out/testing as ive been known to bodge the odd thing together and give it a whirl myself ;)

Keep up the good work ;ol;
Problem I was having understanding was the fact I just don't have the material to remove, meaning if I tried to go oval with the header AND the turbo as you suggested, it'd ruin both the turbo's housing and the manifold. If there was more meat to grind away on both, then sure it'd possible. Which was why I was asking if you were suggesting a 9k manifold earlier.

So, where ever those pics of yours were dug up from, it appears that's only possible on the larger housing, and NOT the housing on my 99' 9-3. If I tried to go oval, it'd literally ruin the wall and there'd be holes in the runner leading to exhaust leaks and failure. Hopefully that makes more sense to you now ;ol;
 

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Discussion Starter · #18 ·
After Porting Video

Been busy but here it is:

http://www.youtube.com/watch?v=cClvrLYWNko&feature=youtu.be

0-60 was almost 4 seconds faster and the hill climb was 14-15 MPH faster as well. Feels much more like an average 4 cyl. Torque is much more noticeable around 2k-3.5. Temps were 5 degrees hotter @89 today, as opposed to 85 the day of the "Before" testing. Hopefully this equates to a slightly healthier T25 once everything is put back together ;ol;

I also paid attention to keeping everything as uniform as possible. Bigger isn't necessarily better. Also, the car's mileage is slightly more on today's "After" video... just to keep the stupid "haters" comments at bay.
 
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