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Question about overboost

Old 06-29-2011, 01:10 PM
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Default Question about overboost

Im learning alot about turbos now and trying to wrap my head around a few things. I bought Maximum Boost and have been reading it every day. I do have a question though and was wondering if anyone might be able to explain something. Can under hood temps that reach a really high number, or if you have a 100+ day with alot of humidity...can that cause a turbo to overboost? Like say you have it set at 10 psi, could hotter temps than normal (under hood or ambiant) cause it to spike to 12 or 14? If thats a possibility, how do you protect the turbo from doing that, in case your engine is not built to handle the extra boost?
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Old 06-29-2011, 08:50 PM
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Hoo boy. We all start from total ignorance and you are definitely on the right track to education reading Maximum Boost. Keep reading and asking good questions and you will know as much as you want sooner than you might think. I wouldn't worry too much about a hot day causing overboost spikes as much as I would about overheating.



You want to look up a blow-off (blowoff/blow off) valve.
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Old 06-30-2011, 07:33 AM
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The wastegate is designed to bypass exhaust gasses at a certain pressure no matter what the temperature or elevation. It will/should, if working properly, limit the amount of boost pressure at all times. There are other design and maintenance problems that could affect it's performance though.





Originally Posted by Stocker View Post
You want to look up a blow-off (blowoff/blow off) valve.
Why?
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Old 06-30-2011, 12:12 PM
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Duh? When does pressure
Originally Posted by thor420 View Post
spike
?



To regulate general boost levels, yes a wastegate is necessary. And a boost controller that looks like laser rocketships at HIN is sometimes added.



http://hondaswap.com/forced-inductio...stegate-36485/

http://www.c-speedracing.com/faq/07.php



********

The better question is why you feel the need to respond with so much attitude to what I say in so many threads, instead of just providing a helpful rebuttal if you think I was wrong.
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Old 06-30-2011, 12:36 PM
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Originally Posted by Stocker View Post
Duh? When does pressure ?



To regulate general boost levels, yes a wastegate is necessary. And a boost controller that looks like laser rocketships at HIN is sometimes added.



http://hondaswap.com/forced-inductio...stegate-36485/

http://www.c-speedracing.com/faq/07.php
A blow-off valve will NOT do that. When boost pressure spikes in the IC system, so will the pressure in the intake plenum while the TB is open. If there's pressure in the intake plenum, then what is going to tell the blow-off valve to open? It runs off vacuum, duh? So, no vacuum, no opening of the BOV.



And using a BOV to regulate boost pressures is NEVER to be used as a safe-gaurd. It's not built for that. Now, a pressure relief valve, operated by an adjustable spring pressure slightly higher than the boost level you intend to run, is a great way of preventing over-boost!!! This is used in several FACTORY boosted cars. I had to remove it from my Z when I needed to run more boost.




The better question is why you feel the need to respond with so much attitude
Asking a one-worded response (why?) to an off-topic post is not attitude. Since BOV's are never used to prevent boost pressure, you can understand my one worded question.

..... to what I say in so many threads, instead of just providing a helpful rebuttal if you think I was wrong.
I gave plenty of response to the OP's post. I couldn't give a response to your post because I had no clue what a BOV had anything to do with the topic at hand. Like I said, BOV's are not used to prevent overboost. Maybe on your turbo kit..... Oh, you don't have one.



Carry on.
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Old 06-30-2011, 04:11 PM
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First off, let's get rid of The Fast and the Furious term "boost". Let's use the accurate descriptive word of PRESSURE.

OVER-PRESSURE would be the more accurate term you're looking for.



My personal defition of "OVER-PRESSURE" is when air pressure inside the intake manifold exceed the value you have set/desired using your WASTEGATE.



How does that happen?



If the wastegate is installed correctly, and the exahust piping is done properly, you should never see Over-Pressure.

HOWEVER, if your wastegate is improperly installed, or under sized, or the Wastegate dump exhaust tubing is improperly sized, or poorly routed you can have problems with over-pressure.



It's entirely possible that the engine can output more exhaust gas than the wastegate/piping can flow. This means more exhaust gas than is desired is routed through the turbocharger, spinning it faster and faster and producing more air pressure.



This can happen with both internal and external wastegates. Sometimes the dump pipe is so poorly constructed that the flaper valve that opens to allow the exhaust gas to by-pass the turbo is obstructed by exhaust piping. It can also be caused by the dump pipe being so poorly designed/constructed that it restricts exahust gas flow so severely, over-pressure occurs. It can even happen if the Dump pipe joins the turbo exhaust output in such a way that the exhaust flow from the turbocharger is impeding the exahust flow from the wastegate dump pipe.



Notice I haven't mentioned high under hood temps or blow-off valves. They don't have ANYTHING to do with the problem of Over-Pressure. Well, Let' me clarify that. It's possible that if your under hood temps close to the turbocharger/wastegate are SO HOT, that the diaphram/spring in the wastegate changes its performance, but in every instance I have seen/heard of when this happens, the wastegate opens EARLY, not late. Sot in reality, it caused UNDER Pressure, not OVER Puressure. But it is theroretically possible.



Now...about blow off vales. They exist for one purpose and one purpose only. To vent pressure from the intake piping when you close the throttle. Let's assume you have a setup that you want to produce 15psi. You would set your wastegate to open at about 15psi, and your BOV would be hooked up to your intake manifold. When the intake manifold see's vacuum, but the intake air piping see's pressure, the BOV opens and vents that excess pressure before it builds and damages your turbocharger or blows off one of your pipe connections (ive seen both happen).



If the BOV wasn't there, the turbocharger is still spinning at 60,000+RPM pushing air into the intake piping. But you just closed the throttle...so that pressure builds, and builds and builds...till BOOM!!!! You've either damaged the turbocharger itself (via reflecting pressure waves) or you blow off one of your intake hose connections. If your BOV is dumping intake air for any other reason, you're working your turbocharger harder than is necessary, wasteing energy, and also increasing your intake air temps unnecessarily.



The waste-gate(and BOV) stays closed until it senses 15psi of air pressure in the intake piping. When intake manifold pressure approaches 15psi, the waste-gate starts to slowly open, preventing the turbocharger from producing too much pressure. When the wastegate opens, exhaust gases are routed from the exhaust ports, thru the exhaust manifold, and out the wastegate into the dump pipe(thus around the turbocharger). If the wastegate is open 100%, and you and you're still at say 4000 rpm, you've got a problem child. Your engine still has 2500 more RPM (and thus that much more exahust gas to flow) but the waste-gate is 100% open. Now you've got an over-pressure situaton. Your wastegate is either too small, or your dump pipe is so poorly constructed(or under sized) that it can't flow that excess exhaust gas. The turbocharger will now produce more pressure than you intended/desired.



How do you fix this? If you've got an internal wastegate, you can move to a larger, external wastegate. You can also look at the design of the dump pipe off the wastegate and see if there are ways you can opitimze it (larger piping, smoother/more gradual bends, etc.).



Some people incorrectly assumine that an externally vented dump pipe (dump pipe doesn't re-join exhaust piping, but rather dumps the exhaust directy to the outside, usually under the engine or out a fender) fixes this problem. It doesn't. While an externally vented dump pipe will flow more exhaust gas than a poorly designed dump pipe that dumps back into the exhaust, a properly designed and instaleld dump pipe will be nearly as efficent. It may flow a tiny fraction less, but unless this is a race car, having externally vented dump pipe is illegal. Do it right.
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Old 06-30-2011, 04:28 PM
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well put.. sticky that sh*t.
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Old 06-30-2011, 04:36 PM
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+Rep for Random.
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Old 06-30-2011, 08:33 PM
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Originally Posted by JonGTR View Post
...


Gee I guess I was wrong. You are the soul of civility and the fount of all wisdom. My mistake.
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Old 07-01-2011, 07:00 AM
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Originally Posted by Random View Post
......


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