Need help interpreting dyno data for 1990 stock 951

[MTemp]

Hello all,

Just acquired (well swapped for a Triumph TR4A IRS) a 1990 951 - this one is different for two reasons

1. I did not buy it as a wreck, strip it down, repair and refurbish it myself
2. It is newer than 1972

In fact it has only done 25k miles and has been lavished with tlc by Porsche at the behest of the previous owner - a local collector. The car in many ways is like new, but also doing only 25k miles in 24 years is not all good either. We have just returned from a 1200 mile road trip to central France and while the car ran well, it did feel a little gutless low in the rev range (to 4000rpm), never showing more than 1.2 barA boost. You can get it to show 1.8barA but only if you floor the accelerator and wait for the revs to get to about 5000. Now I've never driven a turbocharged car of any type and have no previous with 944's …… so I need a bit of expert input please. I have posted this on Rennlist as well.

Car is as stock as stock can be - down to the new Porsche battery! All hoses/belts replaced less than a year ago.

There is a friendly racing setup nearby with a rolling road so I went for a dyno run. I have attached the results below:

As you can see the peak bhp is close to stock (248 vs. 250), peak torque is low (215 ftb vs 258), and most obvious - the max boost of 0.75 barg is not reached until nearly 5000 rpm.

Now as I understand from my research to date (I have the Turbo Workshop manual, but not the first 4 vols) the boost on a 44/52 engine should reach max (about 0.8barg) at around 3000rpm and stay flat (ish) to at least 5800rpm. As you can see at 3000rpm on the chart the boost is still a vacuum of about .10 barA.

NOW …. I know dyno's are not that accurate in absolute terms, and comparisons are the order of the day, and the rpm measured is a dyno measurement from wheel rotation, so is also not accurate, but this still seems to suggest the boost is very late in arriving.

The rev limiter cut in at about 7000rpm on the chart, if that helps with calibration.

Any views/thoughts please.

Mick

Apologies the charts are showing portrait layout - why I don't know as they're landscape in my files.

[MTemp]

I have found a stock power/torque vs rpm curve for a 44/52. Seems I should have more like 140bhp at 3000rpm (vs 70bhp measured) and 185 ft-lb (vs 90ft-lb measured). The comparisons at 5000 rpm are much closer, so even allowing for dyno measurement inaccuracies it does seem there is something going on (or not going on!) at lower revs.

Any ideas what it might be and where to start - leaking waste gate after all these years?

Mick

[cockerpunk]

boost leak somewhere for sure.

[slowcorrado]

I agree, looks like some type of boost leak, or possibly an exhaust leak before the turbo. Do you hear any tapping, etc.. to signal a cracked header (especially when cold)?

[MTemp]

No, no tapping.

I'm thinking I should replace the waste gate, probably with a 2 stage and check all the vacuum lines as I do the swap. Thoughts?

[cockerpunk]

headers/exhaust leak isn't a bad idea either.

i had a cracked header, couldn't even tell because even though one cylinder was not even attached to the rest of the exhaust, my car still made 12lbs of boost ... crazy ... still blows me away. about a 2 inch hole in the exhaust, and still making substantial boost ...

[sketchers356]

Sounds like a weak wastegate to me. A boost leak would throw the fuel ratio off. Im thinking of getting the lindsey duel port for myself. It is a little more expensive than a Tial but it doenst need adapters.

[bh912e]

That doesn't sound right..... Too much hp for the amount of torque displayed. Take it to a drag way and tell us your mph in a 1/4 mile. I ran my car on both a mustang dyno and a track and can tell you that 248 hp on a mustang dyno (actual rear wheel hp) equated to 102 mph trap times..... This was with my 86 turbo with a fresh rebuild, vitesse chip, tial wastegate and cat back exhaust running 15lbs of boost.

[DannoXYZ]

Flywheel HP is a calculated guestimate based upon assumed drivetrain losses. We know from numerous rear-wheel HP tests that the 951 is one of the most efficient cars out there with only a 15% loss. Non-S cars typically get around 185rwhp and TurboS gets around 210rwhp.

On this particular dyno, I'm not sure what parameters the operator entered to arrive at flywheel HP, but most likely it assumed higher than actual drivetrain-losses.

As for slow boost, could be any number of things. Typical test-procedure is:

1. [b]test turbo.[b] remove intake tubing so you can see the turbo-inlet. Spin the turbo by hand and confirm it's smooth. Axial-play on the turbo-shaft should be 1,5mm or less. Radial-play should be barely perceptible, maybe 0,1mm if you're able to feel that. Some turbos have bent-shafts and it binds at certain points of rotation. I've seen turbos with destroyed turbine-wheels from exhaust-nuts dropped into the headers. This can't be determined without removing turbo from car. Go on to other tests.

2. clamp hose between cycling-valve and wastegate shut. Do gentle runs with increasing throttle to see how much boost you get. If you get immediate and fast boost over +2,0 bar, then the cycling-valve is bad. The normal off-state for the valve is to let ALL manifold-pressure directly to the wastegate to open it and dump all boost. This is a failsafe state in case something goes wrong. Then under normal operation, the KLR flips the CV on/off with a duty-cycle like injectors. This sends that manifold-pressure gets away from the wastegate, allowing it to stay closed more and generate boost. There's a 3D map for the CV with TPS-position Vs RPM for boost. If this pinched-hose test doesn't give you mad amounts of boost, then the turbo can be bad, the wastegate is bad, and/or the crossover/exhaust pipe has a collapsed inner wall.

3. replace wastegate with block-off plate. Remove the wastegate and block off the opening in the crossover pipe and exhaust with a flat plate. This removes the wastegate from the system and determines whether the wastegate or exhaust is bad. Again, do some gentle runs with increasing throttle to see how much boost you get. If you get mad amounts of boost, then the wastegate has weaked spring it opens from exhaust-pressure (there are ways to replace the spring with much stronger unit). IF you still get no boost, then either the exhaust is collapsed or the turbo is bad.

It's really difficult to determine the collapsed inner wall issue without cutting open the exhaust pipe. On one car, after replacing exhaust with aftermarket 3,0" system, boost went from 2psi max to +20psi!!!

[MTemp]

First thanks to all for their time and input.

By way of an update:

Lots of checking, installation of a new 2 stage wastegate and manual boost controller, set to 1 bar on a calibrated external gauge, and a modest chip upgrade (Superchip in the UK) to just get rid of what I understand was the deliberate Porsche de-tune to make sure the 911 Turbo out-peformed the cheaper 944. Adds about 35 bhp apparently.

.... And we have a rocket ship. Boost comes in about 3500 rpm, and off we jolly well go! I plan to take it back and do an 'after' dyno run, but having too much fun in the sun at the moment.

Best guess is the stock wastegate spring was weak allowing premature bypass, but doing its job later in the rev range so stock bhp achieved, but late. All hoses, cycling valve etc checked and fine as I expected with a car that has had so much Porsche OPC attention.

Thanks again

Mick

[smudo]

Sounds good. It seems that you are happy with the outcome I have a similar problem. The issue was narrowed down to either worn out original wastegate spring or burned out cross-over pipe. My new Tial wastegate should ship in early next week. After that I will bring her to the shop for measuring as I want to replace the original exhaust with 3'' one (with resonator and muffler). When it will be done, I will chip tune it. Can't wait till its done. I hope that it will solve also some other minor issues.

[fejjj]

Sounds good. It seems that you are happy with the outcome <img src="http://forums.pelicanparts.com/ultimate/smile.gif" border="0" alt="" title="Smilie" class="inlineimg"> I have a similar problem. The issue was narrowed down to either worn out original wastegate spring or burned out cross-over pipe. My new Tial wastegate should ship in early next week. After that I will bring her to the shop for measuring as I want to replace the original exhaust with 3'' one (with resonator and muffler). When it will be done, I will chip tune it. Can't wait till its done. I hope that it will solve also some other minor issues.

You will need adapter plates for the Tial and I have a set if interested. Please PM if interested.