Teach me about Turbo

[shft22]

I am finally test driving my 87 Turbo, this is my first gas turbo car and have a few question on what I’m seeing from a boost point of view. I installed a mechanical boost gauge.

On full throttle there is no boost below 3k rpm, I believe this is expected.

Staying on full throttle boost gauge register about 12lbs at 3500 rpm, but as rpm raise the boost starts getting lower to about 7lbs at max rpm, is this normal?

While cruising the boost gauge register a vacuum not even zero, why am I pulling vacuum at cruising speed? A little throttle does generate boost right away.

Any education you guys can throw my way would be greatly appreciated.

[johnb]

I am finally test driving my 87 Turbo, this is my first gas turbo car and have a few question on what I’m seeing from a boost point of view. I installed a mechanical boost gauge.

On full throttle there is no boost below 3k rpm, I believe this is expected.

Staying on full throttle boost gauge register about 12lbs at 3500 rpm, but as rpm raise the boost starts getting lower to about 7lbs at max rpm, is this normal?

While cruising the boost gauge register a vacuum not even zero, why am I pulling vacuum at cruising speed? A little throttle does generate boost right away.

Any education you guys can throw my way would be greatly appreciated.

It sounds close to normal. You should definitely see positive boost before 3K rpm. If you're sure that you're not, then you've got some troubleshooting to do, like looking for leaks, ignition tune up etc. Really though it can be hard to tell with your eyes on the road (as they should be). You might be getting some positive pressure below 3k rpm and just not noticing it on the gauge. The best way to test is cruising in 4th gear at say 2k rpm. Then stand on the throttle and you can glance down at the gauge periodically - you should see a good amount of boost before 3k rpm, and I would say you should be hitting max by 3k or very close to that.

The 87 turbo doesn't hold max boost to the redline, it does drop off. Here's a graph with some numbers from the official Porsche technical docs (pdf attached):

Screenshot_2026-02-01_14-44-38.png (160.97 KiB) Viewed 343 times

The 88 Turbo S and all Turbos from 89 on hold full boost all the way to the max rpm - that's how they make their extra power (peak boost is the same pressure as the early Turbos).

The vacuum gauge shows 0 when the intake is at atmospheric pressure. When you're cruising, the throttle is mostly closed and the engine is under low load, so it's normal to see max vacuum or close to it. As soon as you tip the throttle in the pressure in the intake jumps up to atmospheric pressure very quickly. If you keep opening the throttle, then positive boost should start to build immediately (though slowly at first). So your description sounds pretty normal to me.

[Tom]

+1 on johnb's comments. I'd just add that it's pretty common for wastegates to get old and sluggish as these cars age. The valve stops sealing well and the spring gets old -- causing it to build boost slower and hold less boost -- i.e., pretty much what you are seeing. So after doing all the basics -- vacuum checks, tune-up, etc., if you are still seeing sluggish boost like you just described, I'd consider the wastegate. Tials are great options, but you can also try to coax more out of yours with shims, banjo bolt restrictors, check valves like the Lindsey boost enhancer, etc. to mask some of old wastegates... Welcome to turbo-world.

When I first bought my car back in the '90's, it felt very much like how you describe. I bet I added 100hp with chips, test pipe, boost controller, and banjo bolt restrictor. Went from a slow roll-on of power to a pretty explosive rush of boost. Totally transformed the car... Just beware if you start chasing power -- it never ends with these cars.

[shft22]

Thank you for the information, I will check as John suggests.
It will take me a little to do, right now I removed the cluster and all other dash part to get them cleaned and fixed up. It was depressing me looking at a dirty, cracked dash, every time I went for a test drive.

[rheidtman]

+1 on johnb's comments. I'd just add that it's pretty common for wastegates to get old and sluggish as these cars age. The valve stops sealing well and the spring gets old -- causing it to build boost slower and hold less boost -- i.e., pretty much what you are seeing. So after doing all the basics -- vacuum checks, tune-up, etc., if you are still seeing sluggish boost like you just described, I'd consider the wastegate. Tials are great options, but you can also try to coax more out of yours with shims, banjo bolt restrictors, check valves like the Lindsey boost enhancer, etc. to mask some of old wastegates... Welcome to turbo-world.

When I first bought my car back in the '90's, it felt very much like how you describe. I bet I added 100hp with chips, test pipe, boost controller, and banjo bolt restrictor. Went from a slow roll-on of power to a pretty explosive rush of boost. Totally transformed the car... Just beware if you start chasing power -- it never ends with these cars.

Agreed - suspect a leaky wastegate or other possible vac leaks. I've rebuilt a few of the OE wastegates and it's never fun - most times, the studs are brittle and snap, extending the repairs. I do believe I have one on hand and the aftermarket versions are much better. Do be prepared to be frustrated as this seemingly 'simple' job usually turns into a much longer exercise.

[shft22]

I did check the car for vacuum leak with smoke and couldn’t find any. Here is a picture of the waste gate, it is aftermarket and it looks relatively new

IMG_8500.jpeg (2.48 MiB) Viewed 303 times

[Tom]

Looks like a Lindsey Clubgate. What are you using for boost control?

[shft22]

Boost controller looks stock, that black vacuum hose from the top of the waste gate goes into the cycle valve control (is this the right name for it)

Regardless, by you guys description I believe my car is working as it should, there is a vacuum while cruising but at the slightest throttle input it goes into positive boost. The one thing I have to test for sure is at what rpm the boost comes in.

[rheidtman]

Boost controller looks stock, that black vacuum hose from the top of the waste gate goes into the cycle valve control (is this the right name for it)

Regardless, by you guys description I believe my car is working as it should, there is a vacuum while cruising but at the slightest throttle input it goes into positive boost. The one thing I have to test for sure is at what rpm the boost comes in.

Yes, the wastegate looks fine... the challenge remains not maintaining full boost. Do also check the diverter valve to ensure it's not sticking open and bleeding boost. If there are no other pipe leaks, then the turbo appears to be just getting weak and not able to maintain boost. I do rebuild these K26/6 and have a set of new billet wheels on the shelf.

[shft22]

Acording to documentation I got with the car, the PO, back in 2002 replace the turbo with what looks like a rebuild unit from 944online, I was able to trace the invoice part number to a 944online part number. I also have a picture from the mechanic that did the work that shows the "new" turbo, and looking at the one in the car it does look very clean.
For now I think I am ok with the way it is, I am concentrating on getting the car up to standards, mostly around the interior, front suspension, air conditioning and a few other items.
I know at some point I will start going down the more HP is needed rabbit hole, I have been down that path before with other cars, but for now I am staying away from that.