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Waste gate straight pipe vs. factory muff pipe

So I was up at the Hershey event and a guy was selling T-304 stainless waste gate straight pipes. I bought one since it was a great price----$50 instead of the $100-125 I have seem floating around on the board.

I had recently dropped my whole exhaust system, took off the air pump assembly and installed a GHL Euro muffler. The waste gate straight pipe was a 5 minute change out... thanks to new bolts,nuts and anti-seize when I replaced everything last month....anyway here are my thoughts on the straight pipe replacement and the effects on performance...


The straight pipe opens slightly as it should as the car begins to drive .6-.8 bar boost... nice sound... not sure it was worth it from a performance perspective...again it makes a harrowing sound.

The performance of the factory pipe I believe makes the car feel a little stronger because of the internal cone structure within the muffler can....it creates back pressure in the exhaust and thereby keeps the boost or the feeling of boost up.... seems logical to me...any thoughts??

If I replace the spring- 1 bar...I still will have the bleed off... just maybe a little later in "under boost yahoo.."
Again thoughts????
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Old 05-08-2007, 04:57 PM
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The point of a waste gate is to dump boost - immediately. The spring and valve should be doing all the work, not the muffler. A muffler might delay boost creep but that is a band-aid for a worn spring.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
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Old 05-08-2007, 06:14 PM
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Brian... thanks for reply... So my question should have read like this....

as you are on boost and you peg your boost gauge at .8 you hear the waste gate thunder as you proceed up the power curve.. if you pull back on the throttle and pressure drops below .8 the gate will close...understood.... now when you are on full boost and the gate is open is there enough pressure from the exhaust engine load to keep the boost up at .8 and have the gate open or are you going to see bleed off by the waste gate to where you arent getting the same power factor.... it just seems that there was more momentum power with the old silencer WG muffler than with the straight pipe... I attribute this to the restriction in free flow of the factory WG muffler....am i off base here...

Anyway thanks for all your contributions to the board...

JM
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Old 05-09-2007, 04:28 PM
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think of it this way... inside the wastegate you have two halves. One side has the spring, one side has a vacuum chamber. The spring keeps the outlet valve closed. When you get under boost, which is 'pressure', it forces against the spring and overcomes it, allowing the outlet to open, dumping excess boost. If it didnt, it would overboost and you blow the heads off your engine. As long as the turbo is spinning, 'under boost', the pressure vs spring rate, either keep the outlet valve open or closed. In theory it should either be open or closed, but im sure you could measure it moving to some degree. So practically, when the pressure overcomes the spring, valve open, when you decel and boost goes away, spring overcomes pressure and closes outlet. It shouldnt 'waiver' or flutter. It should either be open or closed.
Old 05-09-2007, 06:21 PM
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so even with WG open and under boost the system is still able to charge the intake...at the approx .8 bar level. I guess I am thinking that with the WG open I would be loosing some level of performance because of the diverted exhaust and that the exhaust system couldnt keep up with delivering .8 bar boost to the intake.....the original WG muffler just seemed to make the car a little faster on boost with the WG open....I view this more as obstructed flow and hence the fractional increase in exhaust pressure fed to the turbo ( which may not register on the boost gauge)


thanks for your reply.
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Old 05-10-2007, 04:32 AM
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As you accelerate the exhaust gas spins the turbine.
When the intake charge reaches a pre-set point, such as 0.8 bar, the waste gate opens and diverts enough of the exhaust gas charge that the turbine cannot exceed the 0.8 bar intake charge setting.
The waste gate valve can slightly adjust how much exhaust escapes in order to keep the signal presure at 0.8 bar.
The waste gate closes when the exhaust gasses slow to a point below the 0.8 bar signal threshold.
I hope this is not too simplistic. Once the turbo is spooled it doesn't take a lot of exhaust flow to maintain 0.8 bar.
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Old 05-10-2007, 05:15 AM
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That was the explanation I was looking for and totally puts into perspective... Thanks again Brian.....do you run a straight pipe off of your WG??
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Old 05-10-2007, 06:54 AM
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I am guessing that your wastegate is working better than ever and that what you notice is a reduction in your boost levels BECAUSE you are bleeding off boost very rapidly whereas before maybe you were building up more boost before releasing it, due to more restrictive exhaust.

In that situation you could go to a 1 bar spring and retune you air/fuel ratio, and the problem should be solved.

And what on earth sounds better than a straight wastegate pipe, anyway!

I just got the K+N EVOM intake, I can't wait to hear that sucking in air and the wastegate pipe bleeding the air back out again....
Old 05-10-2007, 07:13 AM
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JM,

I was at Hershey as well, selling parts and respending the money! Wish we could have linked-up. Sae the guy selling the WG pipes.. almost bought one also.

Did you make it to last weeks Dorki run? I did the meet & greet at the Starbucks..but not the run. Didn't have the Turbo with me as it's still apart.

Oh.. and I run a bar spring w/straight (and Short) WG pipe.

P-
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Old 05-10-2007, 08:27 AM
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Yes, I run a dump pipe off the waste gate.
I have a few I can sell. $65 if anyone is interested.


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Old 05-10-2007, 10:29 AM
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I'm not convinced that one can "feel" the difference between a stock WG muff and a straight pipe.

I think you also asked about 1 bar spring.
I'm definitely not a fan of upping the boost .3 to .4 w/o checking for correct CIS system operation and health of the engine.
Since these things go meltdown rather quickly, I'd check and verify - and learn - as much as I could about my car and it's health before 'cranking the boost'.

My 2 cents, maybe it helped or not.
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Old 05-10-2007, 10:53 AM
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My pipe looks just like Brian's!
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Old 05-10-2007, 11:02 AM
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Thanks to all... Craig your point is well taken on the 1 bar spring....my "feeling" is perception tempered by the significant noise the straight pipe makes..

I plan to go back and forth between the two pipes and see if I still perceive a difference ....or ...it was just the fact that I had accomplished allot of work on the car with the new Euro muffler...had a blast driving it up to Hershey and when I got home swapped it out and while it sounded fat...it didnt feel the same as it had before the swap.

Again thanks all...
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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 05-10-2007, 06:13 PM
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Thanks to all... Craig your point is well taken on the 1 bar spring....my "feeling" is perception tempered by the significant noise the straight pipe makes..

I plan to go back and forth between the two pipes and see if I still perceive a difference ....or ...it was just the fact that I had accomplished allot of work on the car with the new Euro muffler...had a blast driving it up to Hershey and when I got home swapped it out and while it sounded fat...it didnt feel the same as it had before the swap.

Again thanks all...
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1987 930 -Guards Red
2001 740I sport Blk
2011 F-150 supercrew Blk
2014 X3 2.8 white
"What goes around comes around..."
Old 05-10-2007, 06:18 PM
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