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Open downpipe = Overboost?

 
Old 05-13-2008, 10:55 PM
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Default Open downpipe = Overboost?

So, a friend of mine with a mach 1 called me out (I know street racing is dumb but the road was closed) so I pulled my test pipe out. Well, my wastegate was set around 10 psi and i had an MBC set up for around 12 psi. So, we lined up and launched and i hit overboost (200kpa) hard, shifted, hit overboost again and stopped. I don't exactly understand what happened. Can someone enlighten me? Maybe i am missing how the wastegate actuator actually works.

Thanks in advance,
--Wes
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Old 05-13-2008, 11:32 PM
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That's the problem with the increased flow your regulator can't control boost well enough so you overboost badly. Go read Paul's old thread on this with his MSM kit he had on his car a while back.
You need some exhaust restriction or intake restriction to cure that. Port your wastegate and add a helper spring to the actuator if needed.
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Old 05-13-2008, 11:37 PM
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Hmm, sounds like i just need to get a real exhaust instead of running stock for daily driving and open DP for racing.
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Old 05-13-2008, 11:38 PM
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The spool, however, was ridiculous with open dp.
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Old 05-14-2008, 01:12 AM
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Hmm.. but an open downpipe should be optimal. Maybe the problem is that your MS settings in one way or another are set up for the stock exhaust and the change to far less restriction needs to be acounted for in your settings...
I don't know enough about this stuff yet to give serious input, but it doesn't make any sense why you 'need' an exhaust vs. just unbolting the exhaust, the problem is that settings are allowing the overboost to hit, no?
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Old 05-14-2008, 01:35 AM
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Originally Posted by ThePass View Post
Hmm.. but an open downpipe should be optimal. Maybe the problem is that your MS settings in one way or another are set up for the stock exhaust and the change to far less restriction needs to be acounted for in your settings...
I don't know enough about this stuff yet to give serious input, but it doesn't make any sense why you 'need' an exhaust vs. just unbolting the exhaust, the problem is that settings are allowing the overboost to hit, no?
-Ryan
No. MS cannot account for a shitty wastegate. If you have a 10psi can, and boost control set for 12psi, no MS setting will account for a 15psi spike. You need to port your wastegate. I have the same problem on cold nights with my 3" DP and 3" catback; during the day I can scream along at 15-15.5psi, but if I try and do a highway pull at night I smack the 16.5psi overboost pretty consistently.
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Originally Posted by codrus View Post
Basically I've come over to the camp of "If something is a reliability problem on the track, just ask Andrew and do what he says".
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Old 05-14-2008, 02:46 AM
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Originally Posted by Savington View Post
No. MS cannot account for a shitty wastegate. If you have a 10psi can, and boost control set for 12psi, no MS setting will account for a 15psi spike. You need to port your wastegate. I have the same problem on cold nights with my 3" DP and 3" catback; during the day I can scream along at 15-15.5psi, but if I try and do a highway pull at night I smack the 16.5psi overboost pretty consistently.
Sav, Educate me: I'm unclear about correlations betweem different air temps and wastegate response.. Is the overboost problem typically from internal wastegates Only?
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Old 05-14-2008, 09:30 AM
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When the air is colder the charge is more dense, therefore, you create more boost. The size of the internal wastegate port is fairly small compared to the turbine outlet, therefore, when the exhaust is more dense, the exhaust has a tendency to flow through the larger turbine outlet instead of the wastegate outlet. This is alleviated by using an external wastegate because they respond faster, have a larger port, and are farther away from the turbine outlet.

I hope that makes sense, i need to take a technical writing course.
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Old 05-14-2008, 10:47 AM
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Basically, because you've decreased the restriction on the turbine, the exhaust gas is not forced out of the wastegate.. either because the wastegate is too small, or the actuator is too slow, or both.

This is pretty common on a number of cars, DSM's and Subaru's come to mind. This is when they either go to a ported wastegate with bigger flapper (EVO3 16g with 34mm wg) or they go external.

My favorite solution to the problem is to add more fuel.
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Old 05-14-2008, 11:14 AM
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My wastegate was tightened a lot to get that 10psi, so i loosened it and put my MBC back on. Hopefully i can control my boost a little bit easier now.
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Old 05-14-2008, 11:41 AM
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I had the same problem when I went from a 2.25" exhaust to my 3" exhaust. With the smaller exhaust I saw a constant 8psi. Now with the 3" my boost spikes to nearly 10psi. I've kept an eye on my wideband and it hasnt become an issue.
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Old 05-14-2008, 01:21 PM
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After reading this I'm glad I'll be going from my 2.25" exhaust to the 3" at the same I will be going to an external wastegate system.

So I take it the mach1 won?

Vash-
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Old 05-14-2008, 04:05 PM
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Originally Posted by Vashthestampede View Post
So I take it the mach1 won?

Vash-
Haha, yeah, technically he did, he is up for a rematch though! His car is basically stock with intake and exhaust.
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Old 05-14-2008, 04:31 PM
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Originally Posted by wes65 View Post
When the air is colder the charge is more dense, therefore, you create more boost. The size of the internal wastegate port is fairly small compared to the turbine outlet, therefore, when the exhaust is more dense, the exhaust has a tendency to flow through the larger turbine outlet instead of the wastegate outlet. This is alleviated by using an external wastegate because they respond faster, have a larger port, and are farther away from the turbine outlet.

I hope that makes sense, i need to take a technical writing course.
That makes perfect sense. I guess it didn't really occur to me that more air in = more air out.


Originally Posted by wes65 View Post
Haha, yeah, technically he did, he is up for a rematch though! His car is basically stock with intake and exhaust.


Get that **** fixed and walk his ***, Son!
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Old 05-14-2008, 04:41 PM
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Originally Posted by elesjuan View Post
Get that **** fixed and walk his ***, Son!
One of the main reasons i am building this car, to smoke all the guys with stock mustangs/TA's/camaros/corvettes that think they are tough ****.
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Old 05-14-2008, 08:04 PM
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Originally Posted by wes65 View Post
One of the main reasons i am building this car, to smoke all the guys with stock mustangs/TA's/camaros/corvettes that think they are tough ****.
Haha. I can't wait myself!

Its a lose / lose situation for anyone that ever runs against a miata. Nobody will brag they beat a miata, and nobody will ever tell anyone they got beat by a miata.
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Old 05-14-2008, 08:16 PM
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Originally Posted by wes65 View Post
Haha, yeah, technically he did, he is up for a rematch though! His car is basically stock with intake and exhaust.

Look out, Cobras can get over 50HP from a K&N intake. Now thats poor design.
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Old 05-14-2008, 08:24 PM
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Originally Posted by compy View Post
Look out, Cobras can get over 50HP from a K&N intake. Now thats poor design.
I hope you're talking about the 03/04 Cobra..
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Old 05-14-2008, 10:44 PM
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I hear you on the stock exhaust. I do the same. DD with it on and race with it off.
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Old 05-14-2008, 11:17 PM
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Originally Posted by elesjuan View Post
Haha. I can't wait myself!

Its a lose / lose situation for anyone that ever runs against a miata. Nobody will brag they beat a miata, and nobody will ever tell anyone they got beat by a miata.
omg so true
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