MTemp

Hello all,

New here but longtime user of 911S registry, Pelican and DDK in the UK. Just added (well swapped for a Triumph TR4A IRS) a 1990 951 to my stable - 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. It was suggested on other fora I would get that here!

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.

konakat

Sounds consistent with what you would see from a larger turbo being installed. You should be seeing less power since published HP doesn't include drivetrain losses. A larger turbo would also take longer to spool like you are seeing.

fejjj

Go to Rogue Tuning and purchase the M Tune Kit and your car will be completely transformed!!

MTemp

Not yet fejjj, I want to understand how it should work in stock format first.
Am I right that the turbo is boosting much later than it should? The factory manual says max boost at 3000rpm?

If so what could the cause be?

Aftermarket goodies come later …….

V2Rocket

I wonder if your bypass valve is leaking the boost out?
Maybe wastegate is stuck open?

turboS_swede

Also, the ECU will limit boost depending on throttle position. So unless you're at WOT you won't get full boost by default. Prior to dyno testing, one should check that the WOT is in fact WOT before diving down the winding road of ECU tunes, vacuum lines, wastegates and other potential rats nests.

MTemp

I assume WOT = Wide Open Throttle? I will check this but given this car has been crawled over by Porsche technicians every year of its life I think it's a good bet that all the basic adjustments are in order.

As I said even when you press the accelerator straight to the floor in 2nd or 3rd say then similar to the dyno run you don't show any positive boost until above 3000rpm, and max only at around 5000rpm. All I'm trying to establish is is this normal (sounds like it isn't given the Porsche manual says max boost at 3000 rpm) and if not what are the potential causes so I can approach the problem methodically.

Does anyone have the factory power/torque/boost curves for a 44/52 engine for comparison?

Thanks
Mick

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 where best to start?…… once I've checked WOT is correct!

Mick

Paulyy

Cmon guys, find the problem before modding.

Are you 100% sure the turbo is factory?
Check the AFM if it's in working order
wastegate might have failed
vacuum lines could be disconnected

With a good setup engine, you can get 1 bar (2bar on the gauge) at 3000 rpm with the following:
chips
dualport wastegate

MTemp

Thanks Pauly,

Yes I am sure the turbo is factory, as is everything on this car - and I cannot see it has ever been replaced in the (extensive) maintenance records that came with the car. Certainly looks original from the outside.

I will check the vacuum lines as suggested, but it looks like I need to check the waste gate and the AFM. Any tips for the latter?

Any other potential causes?

Mick

black944 turbo

Sounds a little slow spooling to me as well. First thing to check and the easiest is the bypass valve. It is located on rt. side of the engine compartment going between the upper boost hose and the j-boot. Take the vacume line off the top and put another one on it and either use a hand vacume pump or you can suck on it. It should hold without much pressure.

This is how I test it and may sound grose.
1. suck on the end of the vacume port either on the valve or with a vacume line attached.
2.Either hold your finger or your tongue over the end of the vacume line-port once it is open and see if it holds.

These things fail often and quickly and can cause the exact issues you are having.


We need pics of this low mileage car!!!!!!!!!!

Chunkerz

Does it have a bigger turbo installed? I'm not super familiar with the 951, but there should be some markings on the turbo to identify which one it is. It's buried under the intake so getting to it and finding these markings might be a pain.

There could also be a big boost leak preventing the turbo from working at maximum efficiency. Do you hear any odd whistling noises while it's under boost? There's a guide somewhere on this site posted by Van that shows you how to make a plug to pressurize the intake so you can look for boost and vacuum leaks. Find a post by Van, click on his username, click on "view profile" in the drop down menu, click on the link that shows threads started by him, and look for a thread titled "intake pressurizing" or something like that.

I'll see if I can find it for you when I get home.

I couldn't find the thread, but here's a video he posted on how to do it:

raleighBahn

I have a stock 89 951s. Yes you have a problem somewhere, it should be coming on hard at wot much lower and stay there. The bypass valve is easiest to replace and not expensive. I had mine recently replaced and all the low boost came back. One symptom of it is when you are in high rpm and shift to the next gear, the car slows dramatically in between shifts (losing momentum). It is because the bypass valve not working correctly and the air (blocked by closed throttle) comes back and slows the turbine down.

Check for vacuum leaks too, and yes your wastegate could be weak and opening too early.

Your instinct to not start modding the car is correct until you are certain you don't have a problem. A healthy 951s is a screamer with plenty of power right out of the box. Not many unmodified turbo S spec cars still out there.... they are getting parted out, catching fire, and tracked at a high rate. Speaking of fire, get those fuel lines replaced.

MTemp

I'm away on business at the moment, but will post pics when I get home. All the fuel hoses, vac hoses, fuel regulators, belts etc have all been replaced by the PO and with OEM parts. This car is in 'new' condition in that sense, but it is also clear that something is worn and tired. I will work systematically through the suggestions on here over the next week or so and report back.

Thanks to all.

Mick

Darryl H

A bit off topic but where in Surrey are you based? I'm in Camberley if you know where that is