Bad cat= bad boost?

Wyldchyld · 09-05-2005, 03:01 PM

I have been chasing a low boost prob (only goes to the 2 on stock gauge in WOT) and I have replaced the vacuum lines, BOV, and I have verified the wastegate is not stuck open. I placed my hand over the exhaust for about 15 seconds and it never got hot. Could it be a bad cat and would that affect my boost?

AaronM · 09-05-2005, 03:55 PM

Uhh, the "2" on the stock guage is 2 bar or 14.7 PSI above atmospheric...

Unless you mean just the second tic-mark after atmospheric.

Wyldchyld · 09-05-2005, 04:03 PM

OK..14.7 Going WOT seems a little steep just to get that 14.7, but I wouldnt know since I have been battling this car since I bought it. I dont know what it takes "normally" to achieve this.

AaronM · 09-06-2005, 08:15 AM

Stock boost was roughly 1.75 bar. If you're getting 2 bar, the car's modified. And it's a turbo. Of course it's going to take WOT to get full boost.

Aaron

ae1969 · 09-06-2005, 08:19 AM

What RPM are you hitting full boost? (<3K is what you are shooting for)

AaronM · 09-06-2005, 08:49 AM

Quote:

Originally posted by ae1969
What RPM are you hitting full boost? (<3K is what you are shooting for)

Note that the RPM only applies at WOT in a higher gear. Should hit full boost at 3K in 3rd and above at WOT.

Wyldchyld · 09-06-2005, 10:46 AM

Here is the deal: My (stock) gauge will only go to the one bar when I am in 3rd gear at 4.5k rpm. When I shift to fourth and push it to the floor, the boost shows about 1.8 at 4.5k rpm. When I am in second I cannot even get it to go to one bar, the rpms just shoot up fast. Now I have been in another 951 nor have I ever seen this one running completely as advertised so I don't know if this is right. Doesn't seem right to me though.

ae1969 · 11:05 AM

Ok you are bleeding off boost.

If this is a stock 89' things to check.

1. You replaced the vacuum lines. Check the nipples at the cyclic valve under the intake. The will develop cracks over time. It is not an 'if' but 'when'. You develop a crack and you will have a leak.

2. If this is the stock wastegate. On your car it will begin bleeding immediately. Crazy thing for porsche to do...........but thats the way it was designed.
The spring in the wastegate will also weaken over time.........again not 'if' but 'when'. There are ways you can shim the wastegate to keep it from bleeding at low boost as well.......

3. Worst case scenario. Pull your intercooler pipe and take a look at the blades in the compressor side. In some cases the fins can get shredded if something comes flying through....and you are probably wondering what that could be...............well if its a stock vehicle there is a bolt in the airbox that is notorious for coming loose if you have had the box out of the car.

4. OH also exhaust leak at the crossover will give you this problem.........

5. A loose intercooler pipe as well..........but will give you driveability issues.

On your car it should build boost to ~1.8 on your guage at ~<3K and bleed down ~1.5 at redline ........... in 4th gear.

This is normal behavior.............building full boost at 4.5K is not.

P.S Also a bad cat can give you low boost as well........ but if you want to rule it out........... pull the cat and the exhaust from the down pipe and give it a run.............but then again............ it better have a 'CAT' stuffed in it to give you this behaviour.

Wyldchyld · 12:30 PM

Quote:

Originally posted by ae1969
Ok you are bleeding off boost.

I KNEW IT! I cant wait to see this thing in action. I checked the turbo and it looked great and spun freely. The J boot did not have any cracks either. The intercooler hardpipes are just fine too. As far as I can tell the cycling valve nipples aren't cracked. I put my hand on the pipe coming from the wastegate and it never gets warm, so I am ruling that out. Hmmm..
**EDIT** If I can rule out the intake side, would adding a MBC help? I was going to add this as my first mod anyways.
Even if the wastegate was going out wouldn't the MBC give me the boost I want?

AaronM · 09-06-2005, 02:50 PM

Simple question: Does the diverter valve (aka "blowoff valve") close?

It's a fairly common failure and if the diverter is stuck open everything else can be right but it won't build boost. For the cost of the part, I'd say to start with the diverter valve. Especially if you get good vacuum at idle.

Aaron

ae1969 · 09-06-2005, 03:42 PM

Quote:

Originally posted by AaronM
Simple question: Does the diverter valve (aka "blowoff valve") close?

It's a fairly common failure and if the diverter is stuck open everything else can be right but it won't build boost. For the cost of the part, I'd say to start with the diverter valve. Especially if you get good vacuum at idle.

Aaron

He replaced it on his first post...............

..........

An MBC would help isolate the problem. If the valve is kaputz then an MBC will help. IF the wastegate spring is weak........then you may need to shim it. Although you are right that the pipe would get hot if it was excessively bleeding so you may just have a shot cyclic valve (common problem)........

........When you do get an MBC...........

1. Hook up another boost guage. The stock one is ok but it does not give you the resolution you need when you start fiddling with boost changes....

2. You can make one by buying A $20 piece at home depot or borrow one from someone.......

3. Don't go higher than 11 psi (~0.8 bar). If you are running stock chips...........avoid the urge to bump up the boost.

Wyldchyld · 09-06-2005, 03:51 PM

Roger. Ran up to LR and bought a MBC tonight. Thanks for the help, I will keep ya'll posted on how it turns out.

Wyld

ae1969 · 09-06-2005, 04:20 PM

Quote:

Originally posted by Wyldchyld
Roger. Ran up to LR and bought a MBC tonight. Thanks for the help, I will keep ya'll posted on how it turns out.

Wyld

You live close to LR.

SICK just SICK

My car is half LR........fuel rail, intake, intercooler pipes etc...

AaronM · 09-06-2005, 06:22 PM

Quote:

Originally posted by ae1969
He replaced it on his first post...............

Oops. I fail reading comprehension.

Still, it's very odd to be bleeding that much boost in lower gears and yet still manage 2 bar in 5th as he seems to be doing. I wonder if he's tested the diverter valve though. Replaced new or not it should be tested to make sure that it will hold vacuum and that it closes. It just seems to me that if the WG were opening then it should never get to 2 bar. I mean, a fully failed (open) CV will have the WG opening at roughly 1.2 bar, if he's seeing 2 bar under any circumstances I don't understand how the system could be bleeding to the atmosphere anywhere.

At this point I'm curious from a strictly technical standpoint.

Wyldchyld · 07:48 PM

Anyone have a Lindsey MBC in their car? I am wondering how many turns it takes from being fully open to get 10psi. I drove it a little after I installed it and the stock gauge is quick to show atomospheric, but I have turned it eight times and it wont go to .8 (1.8 on my gauge). I am a little weary of turning it too many times w/o advice. Thx
BTW..I went ahead and double-checked the diverter valve and it is 4.0
Wyld

ae1969 · 09-06-2005, 07:46 PM

Quote:

Originally posted by AaronM
Oops. I fail reading comprehension.

Still, it's very odd to be bleeding that much boost in lower gears and yet still manage 2 bar in 5th as he seems to be doing. I wonder if he's tested the diverter valve though. Replaced new or not it should be tested to make sure that it will hold vacuum and that it closes. It just seems to me that if the WG were opening then it should never get to 2 bar. I mean, a fully failed (open) CV will have the WG opening at roughly 1.2 bar, if he's seeing 2 bar under any circumstances I don't understand how the system could be bleeding to the atmosphere anywhere.

At this point I'm curious from a strictly technical standpoint.

If the wastegate opens early we are losing that critical energy to have early spool up. In 5th gear under heavy load he is building enough energy to build up the boost he cant develop in the lower gears. It just takes longer for that boost to build and probably does not hold for very long. MOre of a spike.......

As you mention a bad diverter valve would make him loose valuable boost as well. A simple test at sucking on the vacuum nipple will test to see if its working.......and a visual to see if the rubber diaphram is ripped. I am assuming that it is working at this point......but a second check never hurts.

... using the MBC (bypassing the CV) will show us if there is an issue with the CV. Normally if the CV totally failed or starting to fail we would see little to no boost like you mention.........

I feel his issue is with the wastegate and or crossover leak at this point....... but you never know.

The only way is..... if I could see it for myself.

AaronM · 09-06-2005, 08:06 PM

Quote:

Originally posted by ae1969
If the wastegate opens early we are losing that critical energy to have early spool up. In 5th gear under heavy load he is building enough energy to build up the boost he cant develop in the lower gears. It just takes longer for that boost to build and probably does not hold for very long. MOre of a spike.......

As you mention a bad diverter valve would make him loose valuable boost as well. A simple test at sucking on the vacuum nipple will test to see if its working.......and a visual to see if the rubber diaphram is ripped. I am assuming that it is working at this point......but a second check never hurts.

... using the MBC (bypassing the CV) will show us if there is an issue with the CV. Normally if the CV totally failed or starting to fail we would see little to no boost like you mention.........

I feel his issue is with the wastegate and or crossover leak at this point....... but you never know.

The only way is..... if I could see it for myself.

I think I see now. I was thinking the WG would either be full-open or functioning, but if it's sticking somewhere in the middle it suddenly makes sense.

I kept thinking diverter valve because it bleeds boost back into the intak tract, so it would lose pressure, but not all the pressure. Convoluted reasoning I guess.

Thanks for the patience!

ae1969 · 09-06-2005, 09:10 PM

Quote:

Originally posted by Wyldchyld
Anyone have a Lindsey MBC in their car? I am wondering how many turns it takes from being fully open to get 10psi. I drove it a little after I installed it and the stock gauge is quick to show atomospheric, but I have turned it eight times and it wont go to .8 (1.8 on my gauge). I am a little weary of turning it too many times w/o advice. Thx
BTW..I went ahead and double-checked the diverter valve and it is 4.0
Wyld

I have used one............and as I recall... on a single port wastegate (stock one) you turn the knob all the way clockwise for lowest boost and then set it incrementally higher turning it counterclockwise until you hit the desired level.

Now it comes all of of as sudden. So easy on your pedal.

You did bypass the CV when you installed the MBC. From banjo bolt to MBC to wastegate............

ae1969 · 09-06-2005, 09:13 PM

............ or you could just clamp the wastegate line.

Then just modulate boost with your foot.

Wyldchyld · 09-07-2005, 04:00 AM

LOL.. I feel like clamping that sucker just to see what the heck real boost in this car feels like.