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Boost tapers off (16psi down to 14)

Old 12-16-07, 07:23 PM
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Boost tapers off (16psi down to 14)

Under WOT in 3rd gear, my car will indicate just about 15.5 psi before the tial wg cracks open, and hold approximatly 15 psi until about 6krpm, at which point the boost begins to slowly taper off down to a 13.8-14.2 at redline as indicated by the haltech.

What can I do to solve this issue? I was thinking of replumbing the Tial wastegate in accordance to the following diagram;

http://www.tialmedia.com/documents/w..._wginstall.pdf

Currently i have a spring and ball style MBC (w/ pinhole vent) connected to the WG's side port. I want to move it to a T'ed off fitting on the top port as shown in the diagram and then vent the wastegate to atmosphere. I would set the MBC to about 7 psi relief to start, in attempt to match the 8 pound spring which should get it to about 15psi.

I feel this will accomplish my goal of stabalizing 15-15.5 psi until 7500rpm. Any ideas?

My second idea is to get a larger compressor but thats just too costly at the moment.
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Old 12-17-07, 07:32 AM
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Umm, I'm no expert on turbo systems, but is your turbo big enough to sustain that sort of boost to redline? It sounds like a normal high-rpm roll-off to me.
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Old 12-17-07, 08:11 AM
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Im starting to think it not big enough, however my wastegate doesnt even sound like its closing when the boost tapers off so I dont really know.
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Old 12-17-07, 08:30 AM
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Originally Posted by SaabGuy View Post
Under WOT in 3rd gear, my car will indicate just about 15.5 psi before the tial wg cracks open, and hold approximatly 15 psi until about 6krpm, at which point the boost begins to slowly taper off down to a 13.8-14.2 at redline as indicated by the haltech.

What can I do to solve this issue? I was thinking of replumbing the Tial wastegate in accordance to the following diagram;

http://www.tialmedia.com/documents/w..._wginstall.pdf

Currently i have a spring and ball style MBC (w/ pinhole vent) connected to the WG's side port. I want to move it to a T'ed off fitting on the top port as shown in the diagram and then vent the wastegate to atmosphere. I would set the MBC to about 7 psi relief to start, in attempt to match the 8 pound spring which should get it to about 15psi.

I feel this will accomplish my goal of stabalizing 15-15.5 psi until 7500rpm. Any ideas?

My second idea is to get a larger compressor but thats just too costly at the moment.
With a MBC, there is no feedback loop to maintain your targeted psi, getting a 1 bar spring (external wastegate) might help. I would tweak MBC with the 1 bar spring to try to maintain the 15.5 psi to redline. The real answer is EBC, Greddy Profec type S would solve your problem.
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Old 12-17-07, 08:35 AM
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https://www.rx7club.com/single-turbo-rx-7s-23/question-engineers-499149/

Some good dialog with respect to boost control
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Old 12-20-07, 08:50 AM
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I'm fairly sure this Tial 38mm WG is using an 8 pound spring with nothing holding it down; meaning the top WG nipple is open.

What would be a suggested configuration for setting this up more correctly for 15psi?

Thankyou
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Old 12-21-07, 04:58 AM
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Originally Posted by SaabGuy View Post
I'm fairly sure this Tial 38mm WG is using an 8 pound spring with nothing holding it down; meaning the top WG nipple is open.

What would be a suggested configuration for setting this up more correctly for 15psi?

Thankyou
You need a real boost controller. MBC is worthless. I'm sorry but they are. They either over or underboost. Your problem is so common I couldn't evn think of the answer. at first I thought about asking you if you put a gasket under the entry for the wastegate where it bolts on but then saw you were getting boost so I figured you didn't. Which you shouldn't there should only be that ring with the step in it no gasket. Don't worrry it stays sealed. That's one super common problem.

BTW if your TO4B can make 15 psi at only 7500 you got a broken turbo. That's plenty of turbo for that low of an rpm.
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Old 12-21-07, 07:18 AM
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I was also thinking about hooking a boost solenoid to my haltech e6k which is much less expensive than a true ebc. The haltech can control duty cycle vs engine rpm which I think could be alright...
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Old 12-31-07, 10:01 PM
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I've had some engine builders tell me that it is usually beneficial to taper boost off as you near redline. Neither of them gave a reason though.
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Old 01-02-08, 08:31 AM
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Originally Posted by cjernigan View Post
I've had some engine builders tell me that it is usually beneficial to taper boost off as you near redline. Neither of them gave a reason though.
I think they mean it's "safer" then it creeping...
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