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What causes overboost?

Hi Guys.

I was driving to see a friend the other evening and was stuck behind a truck.
When I found a gap I went to overtake in second gear and floored the throttle.
I'm guessing the revs hit 6300-6400 but may have been a touch higher, when it suddenly felt like I'd stopped dead or been hit up the rear, it was only fractions of a second but quite a heart in mouth moment. I then changed up to third and eased off back to a cruise.
I didn't see the boost gauge till the following morning and the tell tale was sitting at 1.4bar. I run a 1bar spring in a Tial46 wastegate. I've never felt this happen before and wonder what could cause this. I have the 1.4bar boost cut off switch and standard 6pin spark box.
Any ideas?


Thank you.


Robert.
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Old 04-10-2014, 12:11 PM
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Is the wastegate signal hose sill attached on both ends? If so, you might want to check it for holes, cracks, etc.
Old 04-10-2014, 12:18 PM
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Scares the holy crap outa you the first time you experience overboost shutdown, doesn't it? +1 on Ronnie's suggestions for the first place to look.
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Old 04-10-2014, 12:51 PM
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Sounds like you triggered your overboost sensor mounted in the charge air cooler which shut your fuel pumps off like it's supposed to.
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Old 04-10-2014, 12:51 PM
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If the above mentioned hose isn't leaking and the banjo fitting it connects to on the wastegate isn't loose and leaking then maybe your wastegate needs some TLC.. or replacement.
Old 04-10-2014, 12:57 PM
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I bet it was nice and cool weather.
Old 04-10-2014, 01:04 PM
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Quote:
Originally Posted by DSPTurtle View Post
I bet it was nice and cool weather.
Initially I had problems hitting the overboost sensor, I fixed some vacuum leaks and had the cars mixture/timing reset and that issue (among others) went away.

Now if the car isn't fully warmed up (or its really cool and dense out) and I get on it I will hit the cutoff every once in a great while. My thinking that is if you are running a one bar spring you are bound to hit the cutoff eventually due to boost surge.

Like others have said, check the WG lines first. I have also heard of worn WGs binding and not opening as well.
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Old 04-10-2014, 01:47 PM
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Thank you very much Chaps.

Brand new Tial 46 about 2000 miles ago so I hope it's not that.

Yes, it was probably 3-4 degrees ambient driving cross country. :-)

I shall check my hoses and fittings next.

Thank you again.
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Old 04-11-2014, 06:15 AM
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Sorry, thats 3-4 degrees Celcius.
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Old 04-11-2014, 06:16 AM
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Quote:
Originally Posted by 87 930 View Post
Sounds like you triggered your overboost sensor mounted in the charge air cooler which shut your fuel pumps off like it's supposed to.
thats what i was thinking. but he may have also gone higher in the RPM's and hit the rev limiter. i have mine set to 6500 and have hit it doing the same thing and it surprised me too. did not think the revs were that hi. in fact, i was passing someone too.
another thing, if you hit 1.4bar, you KNOW it. sounds like a bag of marbles in the motor form all the detonation, i am ashamed to say i know from experience and there is NO excuse. if i had the overbooost sensor then it would not have happened.
also, i assune the "tell tale" you are talking about is the boost gauge. is it the factory one? if so, it is not very accurate.
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Old 04-11-2014, 08:08 AM
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As you all well know, when the boost comes on, the revs rise so fast that I didn't really have a chance to hear any marbles/detonation.
Foot down, hang on, wham. It probably took about the same time as reading that last sentence.
It's all my fault I know, but those mental blood pumping seconds are part of what these cars are about and I'm addicted. :-)
It must have given 935 pilots mental meltdowns going to over 7500 through the gears.

I know the factory boost gauge isn't the most accurate so I have a North Hollywood Speedo 935 type gauge in place of the clock.

Just out of interest, I don't suppose any of you know what the little triangular key used for reseting the tell tale is called. I don't have one but need one and don't know what to search for.
As to hitting the rev limiter, I don't think I got to the red line. I always thought that would be the point to trigger the limiter.
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Old 04-11-2014, 02:12 PM
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They're called "little triangular key thingy".
Todd/OSI930 might be able to find you one...
You should have a rev limiting rotor in your dizzy. 7,000 or 7,200 rpm, I forget which. At least your overboost sensor did its job!
Old 04-11-2014, 02:18 PM
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"They're called "little triangular key thingy".

Thanks DSPTurtle, I'm always amazed at how technical things can get on this forum. :-)

I'll get searching.
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Old 04-12-2014, 04:27 AM
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i am 80% deaf in my left ear, I could hear it detonate, and it was as quick and as short as possible. if you hit 1.4, i dont know how it would not detonate. (i am still hainted by that sound).
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Old 04-14-2014, 02:38 AM
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