Mercedesben

I took my 951 to emissions testing yesterday and had some fun driving around until it started raining, but I noticed something funny with my boost.
I have my newly installed VDO boost gauge the car is currently stock except for autothority chips.
At full throttle 3rg gear the gauge will peak at 16psi and then drop to about 13psi at full throttle. Same in 4th gear. Is this normal? Any ideas?

I am eagerly waiting for my tial wg from Sfr maybr this will solve the problem.
Ben

hosrom_951

Stock cars do drop boost at upper rpms (5,500+ in my car). Reason for this is that the wastegate spring is weak and the turbocharger looses steam in that range.

Normal........

Dal Heger

Yes, it's normal. The DME is programmed to allow "overboost" for a short time to give you a bit more power to pass cars on the autobahn. It then dials back the boost for extended cruising at WOT. Porsche did this for durability reasons.

It's normal and controlled by the DME. You can get around this by putting in a manual boost controller, but then you lose a major safety feature of the engine, the ability for the DME to dial back boost when it encounters knock! Besides, that's one of the only ways you'll know that something is wrong with the engine, the boost will be limited to only 1.2 bar. Remember, there is no "check engine" light on these cars.

I still don't know why there seems to be this misconception about the wastegates being bad. Unplug your wastegate, it will hold 21-24 psi of boost, enough to blow your engine up in no time. If the wastegate seat is cracked it can be repaired, the wastegate is rebuildable.

Dal.

Porsche-O-Phile

What they said - a number of fixes are available for this including stronger wastegate springs, shimming the wastegate, new wastegates, new chips / controllers, etc.

Porsche-O-Phile

I think there's a secondary type of overboost protection where the DME can kill fuel flow also, or am I mistaken about this? The KLR is the knock detection computer / system and that will regulate the action through the primary wastegate "signal" line via the cycle valve, but I thought a secondary overboost protection was also available through the DME (fuel cut-off).

RPG951S

No one has mentioned this yet, so: Do NOT use a manual boost control with those Autothority chips.
They are not designed for it, the chips are expecting a falling boost curve (just as you have).

Alpine951

K26 runs out of air in the high rpms which is why boost drops. I would be interested to see if you can hold boost in the rpms with a k26 and a Tial.

Mercedesben

Dont worry, once I have the wg installed I will hook up the mbc and dump the autothority chips. TT is going to help me with my setup and we will dyno the results before and after MAF.

Jason_86_951

This has been beaten to death.
A new wastegate, even a Tial, will NOT completly fix this. THe stock K26 won't hold higher boost untill redline unless, it's brand new or rebuilt to perfect tolerances, and even then it will still drop a psi or 2.

A new wastegate will HELP, but not CURE.
Cheers,
Jason

Porschefile

I'd highly suggest not wasting time with MBC's, and just get an EBC that has a peak hold function. Might as well do it right the first time. I've tried a couple MBC's, but none have increased performance as well as the Blitz dual sbc spec r I just put on. It has a boost gauge built in, also it has a peak hold function. My 951 is completely bone stock, and it holds max boost to redline! I've run up to ~12psi and it holds flat all the way to redline (I'm using a 3bar fpr and FQS set to +3% fuel just in case). The top-end is significantly better as a result.

daniel951

I plan on doing the same thing with the EBC

NZ951

Have you seen Laust's k26/6 boost results?

Danno

The only time I've seen a K26/6 hold flat boost is with an electronic boost-controller. That's because you need a variable boost-control signal that takes RPM into account. So the signal is not a straight in -> out function of boost, 15psi @ 3000rpm -> 10psi to the wastegate, whereas 15psi @ 6000rpm -> 0psi to wastegate. Manual controllers can't do that.

On Laust's car, we've got a new wastegate design that actually increases the valve clamping force with RPM... At one point, boost was going crazy at 15psi in mid-range to 30psi+ by redline, had to modify the design a little...

hosrom_951

Dal Heger: I did not say that the stock wastegate is bad, i said that the spring is weak. Porsche MADE IT THIS WAY, even BRAND NEW 944 Turbo Wastegates will still bleed off. They are designed that way

Danno

Yup, there was a webpage that had a quote from the manual that contrasted the difference between the Turbo and TurboS. It explicitly states something to the effect that the Turbo starts dropping boost at 4500rpm or so while the TurboS doesn't until 5300rpm??? Was it on John Pohl's S2 site?