Open downpipe = Overboost?
05-13-2008, 10:55 PM
#1
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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
Thanks in advance,
--Wes
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05-13-2008, 11:32 PM
#2
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.
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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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?
-Ryan
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
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05-14-2008, 01:35 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?
-Ryan
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
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05-14-2008, 02:46 AM
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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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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.
I hope that makes sense, i need to take a technical writing course.
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05-14-2008, 10:47 AM
#9
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.
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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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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05-14-2008, 11:41 AM
#11
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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05-14-2008, 04:31 PM
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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.
I hope that makes sense, i need to take a technical writing course.
Get that **** fixed and walk his ***, Son!
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05-14-2008, 08:04 PM
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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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