What causes overboost?

[VonHanstein]

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.

[Ronnie's.930]

Is the wastegate signal hose sill attached on both ends? If so, you might want to check it for holes, cracks, etc.

[mark houghton]

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.

[87 930]

Sounds like you triggered your overboost sensor mounted in the charge air cooler which shut your fuel pumps off like it's supposed to.

[JFairman]

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.

[DSPTurtle]

I bet it was nice and cool weather.

[16Volt]

Originally Posted by DSPTurtle

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.

[VonHanstein]

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.

[VonHanstein]

Sorry, thats 3-4 degrees Celcius.

[T77911S]

Originally Posted by 87 930

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.

[VonHanstein]

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.

[DSPTurtle]

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!

[VonHanstein]

"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.

[T77911S]

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).