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Wayah Road Warrior
 
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Help a 930 Newbie out. Overboost/Rev Limiter Question.

OK, please no hijacks, if you can answer or contribute, great. If you have a tangental question or a great war story ...start your own thread.


I traded my stock 87 3.2 for a stock 87 930 (.8 Bar).

I drove my 3.2 as it was designed to be driven, I would hit the rev limiter at least once per outing.

Now I am in a 930.

Question 1. What is textbook Max RPMs for the stock motor?

Question 2. Does the stock 87 930 have rev limiter? If so, what RPM?

Question 3. At what RPM should the overboost switch cut the fuel?

Question 4. Should I just plan on shifting at 6,200 RPM to maximize the brute?


The reason I ask these questions is that I will also drive this car as it was designed to be driven but ... I do not want to grenade the engine if at all possible.

I took it out the other day to test for existence of a rev limiter. Nailed it in 2nd and at close to ~7k it fell on its nose. Felt more like no fuel rather than what I am used to with a rev limiter. I assumed overboost worked as designed.

Did it again but less abrubtly and took her up to 7k+ but did not feel any rev limiter or overboost so let off.

Both high rev situations were very brief. I was truly OK with the shutdown in scenario 1. That was what I wanted to see. The 2nd one concerned me. I wanted to see a repeat of the 1st run. That would give me the fail safe I want.

Additional datapoints:

Factory boost gauge pegged in both runs. Fun to look at. Impressive for the passenger. But for material information ..... worthless.

Also running a 935 Boost gauge with a tattletale. Not sure I am impressed with this either. Will leave tattletale at 1.2 and even 1.5 on a good hard run up and quick shift to 2nd. I doubt I really hit that much boost?

So there it is.

Your thoughts?
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Last edited by Shadetree930; 04-09-2008 at 05:21 PM.. Reason: Edited to add the smiley up front.
Old 04-07-2008, 06:00 PM
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Rev

There is a rev limiter under the Dr seat that shuts the FP's off at approx 7k, 7200 is stated on the wiring diagram. I believe you've reached peak torque at approx 5.5 - 6k so anything over that is just noise. From what I've read, I would not take a stock 930 to 7k on a regular basis.

Oh, The over boost switch monitors boost, not rpm.

Tim
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Old 04-07-2008, 07:13 PM
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Wayah Road Warrior
 
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Thanks, that was helpful.

I will modify my shifting habits to aim for a 6k max shift point. Makes the best use of the resources available whilst protecting my investment.
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Old 04-08-2008, 06:13 AM
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Wayah Road Warrior
 
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Is the rev limiter integrated into the DME?

This is different from what I thought was a distributor cap based limiter in my 3.2?

Or am I mixing apples and oranges?
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Old 04-08-2008, 06:21 AM
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First, overboost switch doesn't have nothing to do with revs. It only senses boost and not RPM. Once the boost is over 1.1 bar (roughly) it will cut fuel.

Second: Car makes max power at 5500 RPM with stock cams. It also has same weak rod bolts as OEM Carrera. You know, the ones that stretch at high RPM and make a nice hole in the case and your wallet.

Third: as far as I know, RPM-limiting function was somewhat flaky for earlier cars and would creep upwards sometimes.
Manal says: 6000 RPM continuosly, 6500 RPM in brief emergency. You said you revved it to 7000+ RPM?

Fourth:
Stock boost is 0.7 bar. Stock CIS and engine will tolerate 1.0 bar on a nice cold day...if you are pegging 1.5 bar your wastegate is either stuck and overboost switch is disconnected or there is something wrong with BOV and you have excessive boost peaks on overrun. First is much more dangerous than second....second will "merely" blow your gaskets from intake system.

It seems like you are on your way to produce nice long rebuild thread on this forum pretty soon.

Get my point?
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Last edited by beepbeep; 04-08-2008 at 07:35 AM..
Old 04-08-2008, 06:22 AM
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Wayah Road Warrior
 
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OK so something that protects the engine is working as designed.

I am inclined to believe it was the overboost switch since I did not hit 7,200 RPM and the shutdown happened on hard acceleration which would have a large boost component involved..

The high revs? Not as dramatic as it sounds. 7k was for maybe a second, probably less. More a spike than a constant RPM. Once, maybe twice since I have owned it. No funny noises from the engine post spike so it looks like I got lucky. One of the purposes of this post was to establish my shift point. 6k is now my high rev shift point. Unlikely I will be seeing a rev limiter in the future. No need since no incremental power at the high end. Unlikely I will be funding my mechanics retirement dacha.

How do I test that my wastegate is operating as designed? Short of a Youtube video of flames sprouting from my ass?
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Old 04-08-2008, 07:02 AM
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Rev

Quote:
Originally Posted by prials View Post
Is the rev limiter integrated into the DME?

This is different from what I thought was a distributor cap based limiter in my 3.2?

Or am I mixing apples and oranges?
No DME, it's CIS. The rev limiter is a "relay" (small module) under the dr seat that get's it's signal from the CDI, and when you hit apporx 7k it shuts down the FP's.

You have a spring in your wastegate that prevents your boost form going much over .7bar, so in theory, you should never hit your overboost switch, which as I stated earlier, has nothing to do with rpm's. If you fell on your face at 7k, my guess is you hit the limiter. I've never ever rev'd mine that high. If I am on it, I'll shift right around 5.5k. No need to go any higher. If you want to bounce your car off the rev limiter, then go buy the rotor with the rev limiter for a 914. I had one on one of my earlier 911's and it worked really well, as it comes in around 5.8k.
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Last edited by timc; 04-08-2008 at 04:48 PM..
Old 04-08-2008, 01:50 PM
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Originally Posted by prials View Post
OK, please no hijacks, if you can answer or contribute, great. If you have a tangental question or a great war story ...start your own thread.
You sound like a chick. Do your feelings get hurt easily?
Old 04-08-2008, 03:27 PM
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Wayah Road Warrior
 
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Yes, my feelings get hurt very easily and at least once a month I get really really b!tchy (like right now).

Now that we have that out of the way, can you tell me how to test that my wastegate is working?
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Last edited by Shadetree930; 04-08-2008 at 05:24 PM..
Old 04-08-2008, 05:13 PM
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Plagerized information!

I'm stealing this information from JFairman, as it was imparted to me last week:

"Only 4 little 12mm nuts hold the wastegate on. It would be a good idea to remove it and carefully and slowly give it some air pressure and see if the valve opens.
It should retract or move inwards off it's seat.

You can use a blow gun, but don't hold it tight against the hole - put it next to the hole and slowly close your fingers around it while letting air escape all around so you don't over pressure it and rip the diaphragm.

An air pressure regulator turned way down is a good idea if you have one. You only need around 15-16psi to open the valve with a 1 bar spring in there.

If it doesn't open you know the diaphragm is toast.
The valve guide lasts a very long time but eventually wears out too."

Hope this helps! It helped me! Thanks JFairman!

PRIALS: Sorry to hear about your monthly problems! LOL
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Old 04-08-2008, 07:18 PM
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As per factory manual.

1. Loosen and remove control line to WG.
2. Run engine at idle speed. There must be noticable vacuum in control line. With increasing engine speed, vacuum changes to pressure (charging pressure).
3. Insert a plug in disconnected control line, so that engine will not stall while checking WG operation.
4. Carefully blow compressed air into control line opening of WG. Listen for sound of WG opening.

Caution
Do not use more than 1.5 bar (21 psi) air pressure.

Check bypass line warms up (cold engine).
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Old 04-08-2008, 07:27 PM
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Quote:
Originally Posted by prials View Post
Yes, my feelings get hurt very easily and at least once a month I get really really b!tchy (like right now).

Now that we have that out of the way, can you tell me how to test that my wastegate is working?
Old 04-08-2008, 11:14 PM
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Bingo! That is the detail I was looking for.

Thanks all!
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Old 04-09-2008, 05:45 AM
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Most of the time, it's enough to just supply compressed air (or just blow trough it) trough vent hole (on the top of wastegate bell) and see if it vents further down.

If it does, diaphragm is ruptured.

That being said, a faulty/stuck BOV will give you boost peaks on overrun despite having fully functional wastegate. If your boost-sensing line (going to boos gauge) is connected before throttle, a stuck BOV would let turbo build pressure way over what wastegate would allow. It will do that due to impeller inertia. Reason that BOV exists is to recirculate air on shifts so this doesn't occur.

You can also test WG by just hitting third gear and watching the boost gauge. If it hovers around 0.7 bar (for stock spring), it's OK.
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Old 04-09-2008, 07:26 AM
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Wayah Road Warrior
 
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Thanks beepbeep. I will try the 3rd gear test 1st.

I just got my nails done and don't want to break one whilst wrenching. .
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Last edited by Shadetree930; 04-09-2008 at 08:49 AM..
Old 04-09-2008, 08:10 AM
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Hey way to start out this thread in a positive manner, you'll get lots of advice that way....

With that being said, make sure you check the line going to the wastegate as well, if the compressed air opens the wastegate. I had problems with the wastegate not opening, and it was due to a very small split in the line going to my wastegate.

Bill
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Old 04-09-2008, 03:34 PM
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I went back and put a at the end of the opening sentence. The intended humor was initially lost on some but it came out later on.

But anyway .... Went out and did the 3rd gear test. You would all be proud of me .... I never went over 6k RPM. I know my engine internals thank me.

It appeared to me that boost dropped of sharply at 7 bar under an aggressive 3rd gear climb up the speedometer (according to the boost gauge with the tattletale). I am going to interpret that as a sign of a properly functioning wastegate.

The boost gauge in the tach was less helpful. It would go up to 1 bar on hard acceleration. The 935 gauge appears to be the 'go-to' gauge?

If I am running two guages, are they both going to work accurately? Or am I supposed to ignore one (the one in the tach)?
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Last edited by Shadetree930; 04-09-2008 at 05:46 PM..
Old 04-09-2008, 05:13 PM
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You can do a search, but I am pretty sure the factory boost gauge under the tach can be somewhat unreliable. It is also going to depend on where the boost gauge is plumbed into, as to how accurate it will be. Do you know where the 935 gauge is tapped into?

Bill
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Old 04-09-2008, 05:38 PM
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Quote:
Originally Posted by BSiple View Post
You can do a search, but I am pretty sure the factory boost gauge under the tach can be somewhat unreliable. It is also going to depend on where the boost gauge is plumbed into, as to how accurate it will be. Do you know where the 935 gauge is tapped into?

Bill
Bill, I concur. Prior research had led me to believe the factory gauge was merely an interesting ornament in terms of dead nuts accuracy.

Not sure where the 935 gauge is plumbed but it seems to be more reflective of the sound and fury going on in the back of the car. If I am going for the accuracy gold standard, where do I want to see the plumbing tap into?
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Old 04-09-2008, 05:44 PM
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Quote:
Originally Posted by prials View Post
Yes, my feelings get hurt very easily and at least once a month I get really really b!tchy (like right now).

Now that we have that out of the way, can you tell me how to test that my wastegate is working?
Hilarious. You sound like my sister. I love it.

At fear of hijacking... I am sincerely interested in this thread as I have all the same questions but haven't posted since I haven't driven my 930 since January (I don't have it in my possession right now).

Carry on.
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Old 04-09-2008, 05:48 PM
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