Random question; Does your boost gauge needle ever hit .8 (the max)? What does it top out at?

I've noticed my boost gauge never goes above about .5... (.4 is top dead center, .8 is max). What does yours go to under a full 0-100 run? Should it be hitting .8? (so something is wrong with mine?). Would altitude matter? (I'm in Salt Lake City, so ~4200 in the valley, up to about 7000 up in Park City (or higher when venturing around mountain trails).

 

0.8 bar from ~2200rpm onwards in my CTT with power kit (same tune and intercoolers as CTTS). No altitude where I live though.

Getting 0.4 bar at ~1800rpm which is pretty nice. Love the bottom end on this motor!

 

Altitude shouldn't matter.

Should get to .6bar at WOT then taper off to .4bar or so at redline. (assuming 957 turbo)

Possible you have a boost leak or leaky diverter valves.

 

Turbo S is pegged far to the right of max, tapers off towards higher RPMs to prevent runaway overboost.

When my N75 valve/sensor was bad, it would limit to 0.5 only.

 

This is excellent news! There may be easy horsepowers to be had.

Fingers-crossed there is an inexpensive remedy that will restore to stock (new-to-me more horsepower).

is this an easy thing to diagnose? Will obvious things be throwing codes? I’m not throwing any related to this at the moment, but about 100 miles ago it threw a CEL with:
- P1248 - Basic boost pressure adaption
- P0299 - Turbocharger underboost
- P2181 Cooling System Performance

cleared, haven’t returned.

this is a new to me 2009 CTTS with the only mod I know of being exhaust that goes stock primary cats straight back to magnaflow cans (no 2nd cat, no suitcase muffler).

What could be tried? Is this professional shop territory for diagnosis(likely so, I’m thinking…)? Anything a reasonably competent DiYer could try?

 

Ah, I see! Sounds like a boost leak hunt and replacing N75 and diverter valves might be a good start.

I have no idea how to do this. 😢, but the internets are my friend…

https://rennlist.com/forums/cayenne-...rbo-955-a.html

anybody in SLC like turning wrenches and accepts beer for payment? ;-)

 

Mine does not easily get to .8 bar unless I'm in sport mode, then it's almost like a light switch....

 

My turbo S pegs the gauge at 0.8 - and that was before the mods. I now have a separate bost gauge and with the tune now goes to 1.2 bar so 50% higher than stock, making the built in dash gauge just about useless, as it pegs almost immediately under load and heavy pedal.

If you're not seeing 0.8 or close to it, I'd replace your diverter Valves that might be bleeding off pressure prematurely. I have the Evolution Motorsports billet ones and they seem great.

 

n75 is silly easy- right next to throttle body.

id check your charge pipes from the inter coolers to the throttle body, including y pipe, for leaks while you’re in there. Check the screw clamps make sure they are all tight and properly affixed

 

My 04 CTT has always pegged the gauge at .08+ and holds most of it till redline. Over 250K miles!

 

Mines hitting +0.8bar on reconditioned turbos with no cracks at the wastegate!!


I suspect many may have such bad cracking from the volute through to the wastegate outlet they likely arent making proper boost. (photos on this one arent as good though)







Now the really bad one this is a left turbo an its cracked clean through to the outside on the centre section side!

 

Interesting. I was going off my memory, haven't driven Helga in a while. May need to get her out for a spirited run this weekend and check out the boost. She definitely doesn't feel fast to me, but I attributed that to her heft and my numbness to speed given my other fast vehicles.

 

OK, so apparently there are a lot of threads related to this topic already. (Apologies for not finding them sooner).
(future self: come back, edit this, and add some easy links to help errbody...)

I also have an update on some easy diagnostics that can be done, that I didn't see covered elsewhere:
- using a mighty vac, we can test the circuits related to the N75 to rule that out
- using a couple of pipe pinchers (I have not idea what these are properly called, see pics), we can determine if the diverter valves are "at fault"

TLDR; a quick test can fairly definitively point to diverter valves if they are the issue, and the butt dyno will easily feel the difference (at least, in my case). Just clamp off the diverter valve return pipe (effectively forcing boost into the intake instead of diverting back to turbos), and go for a WOT pull.

More:
N75
1) N75 controls waste gate actuators (bleeds exhaust past the turbo, effectively bypassing turbos). (Helpful info for beginners). So let's check it is working. First, let's check the vacuum connections, then the function of the wastegates (sort of).
2) grab a mighty vac or similar and pull the bottom vacuum lines off the N75 (plainly visible under beauty covers, driver's side of throttle body inlet). You may like spring clamp pliers for the spring clamps, but needle nose or others just fine. We want to put vacuum on the line plugged into the bottom of the N75, after unplugging it from the N75 (so, pointing away from the N75). Pump about 10 PSI vacuum and see if it holds. It might slowly decline over 30 seconds or so (I think that is OK, but would expect experts to weigh in, knowing this forum...) Upstream is a controller what would apply vacuum to this if it wanted wastegates opened. So we are checking if that circuit can even give the N75 a vacuum "signal." In my case, the vacuum held, so we moved on. (In another case, it might not hold, so then fix that vacuum circuit so your N75 has a chance of working properly).
3) passenger side of throttle body is a tee that sends vacuum signal to wastegates. Disconnect the one that dead-ends into the tee (coming into the tee from the rear of car). If we disconnect this, then the wastegates stay closed and we should see max boost. Tthe tee is just above the 15 in this diagram), and is connected to the "output" vacuum line from the N75.
4) Take that for a WOT pull. In my case, it did not fix the problem, so on to the diverter stuff, below. But if that had then generated full boost, we could conclude something is not getting the "vacuum signal" to this tee, and it is upstream so either N75, or the control valve upstream of that, which is under the intake cover, I believe (I'm sure someone will clarify...)

Quick Test Diverter valves
1) Diverter valves recirculate boost pressure from charge side back to input side, effectively keeping the turbo spinning without feeding the engine more boost (yet) (here is some beginner info)
2) So, if our diverter valves are not working properly, and are constantly "open" and recirculating boost back to inlet, we are effectively wasting boost that we should be using.
3) using a pipe pincher (what is the name of this darn tool?), crimp off the return pipes. They are the obvious big ones come from the side of the diverter valves, and very easy to access with fender liners removed and tires turned lock to lock (depending on side). (You may be ablet to get at them with partial or no fender removal, but mine are already off, so...)
4) closing this off effectively says - all your boost are belong to us. (I make no warranty about whether this will damage your car, it did not appear to damage mine, but I would definitely not daily drive it like this.
5) warm-up first, then WOT pull. For me, the boost needle pinned to .8 nearly instantly, and only floated down towards higher RPMs. The butt dyno was very pleased and the difference is quite noticeable. I expect the driveability to be even better when working properly than with the pinch in place, b/c the diverter would keep the turbo spinning off throttle, which would reduce turbo lag when I call for power again... So this is a partial "fix" as a diagnostic only. Attached videos show difference between .4 boost and "full" .8 boost. Went from about 7.6 0-60 to 6.6 0-60. (I'm at 4K altitude in SLC, and about 84K miles. I wonder if we can still get it in low 5s... or even 4.7 like it was supposed to be when new?)
7) don't forget to take the pipe pinchers off!
6) So I am off to replace diverter valves. I'm going OEM b/c $140 / pair at local dealer (and cheaper if I'm willing to wait, which I'm not). Others debate upgrading to performance ones that chase $300 / pair... up to you I guess.



some reference diagrams:
Id5z Supercharging 1 Pressure Pipe (autoatlanta.com)
M48.51 Supercharging 2 Control Line Pressure Line (autoatlanta.com)



pipe pincher on driver side

 

I like your posting style with diagnostics. I wish it was the N75 valve for you though lol.

 

A pair of OEM DVs from FCP Euro for < $100.