Porschefile

I'd really appreciate any insight anyone can give me. The whole issue with my car ('86 951) is with engine response. Basically, on the low-end from just off idle to ~2k rpm there is this "lag" to my throttle inputs. The engine just doesn't seem to want to rev almost as if your at low rpm in a high gear. It seems to slowly rev to ~2k rpm and takes 3-5 seconds, at which point the motor seems to rev fine above 2k rpm. The odd thing is I generate full boost of 1bar and everything runs very smoothly. There isn't any misfiring, detonating or otherwise rough running characteristics. I have an ebc, vitesse maf chips, #55 injectors, and am running 1bar. The a/f's are nice and smooth and just right at ~11.9-12.0 I believe. I had this issue before the chips, and with my stock injectors and have done the following to try and correct the problem:

-new plug wires, plugs (1 step colder, ~.6-.7mm gap), dist. cap, rotor, coil
-new knock sensor
-cleaned, wrapped, heatshielded and/or replaced much of the wiring under the intake (knock sensor wiring, etc)
-Iceshark battery ground, checked/cleaned all grounds
-new intake manifold gaskets
-new intake/intercooler couplers and hoses
-new vacum hoses
-venturi delete kit w/ ICV hoses and brake booster hose
-new fuel pump/filter
-new tps sensor

The odd thing is the car runs fine, is perfectly smooth, and does not smoke. I plan on doing a compression test by the end of the week. I also have a TB reseal kit I'll install soon. I recently dyno'd the car (I can post the sheets if anyone is interested) but I am baffled at the power loss. I think it was only ~219rwhp and 240rwtq at 1bar with VR maf chips and a stock 26/6. Everyone else with similar mods seems to be at ~250-260rwhp and ~280+rwtq. Does it sound very plausible that a clogged cat (I'm at 222k miles) could cause this power loss and lag? I'm still making 1bar though, and I would think a clogged cat would result in less boost due to the higher backpressure.

Has anyone else had any engine "lag" issues similar to what I'm describing? I have a stock exhaust with a straight through muffler and high flow cat (all 2.5") that I'm going to install in a few days and see if my theory holds any weight. I'm beginning to think that this is a timing issue and that the stock Motronic just sucks. Someone please help because I'm having a similar issue on my 911 (turbo 1000rpm laggier than it should be even with all new/good condition components, looks like too much timing retard on tuned chips) and I'm getting to the point where I want to say screw it and throw an aftermarket ems on everything. The car drives great at moderate or highway speeds. Even though the stock CR of 8:1 is fairly low, it's still a 2.5l and should have pretty decent low-end for a 4cyl. To put it bluntly, the car is a DOG to drive on the low-end.

jimbo1111

Clogged cat or crushed cat/down section would be my guess as well. You may want to look into exhaust leaks between the head and turbo. With that kind of mileage I would think that you have leaks for sure. I have seen leaks with less then 100k.

fast951

A restriction in the exhaust (clogged cat) could cause the extra lag. How's the air filter?
Did you check for any vacuum leaks?

Things to check (not in any order):
- Complete exhaust system from cyl. head to tailpipe. A leaky header, collapsed pipe all can contribute. (top priority)
- distributor cat and rotor
- inspect turbo for any damage or excessive play (the last item to worry about now).
- the TPS and adjust properly
- When do you make 1psi and 1bar boost in 4th gear (start at 2400-rpm).
- Is the FQS set correctly?

What's in common between the 930 and 951, both are laggy? Are you at high elevation? Fuel quality? ...

Ski

Do not expect these cars to come on boost below 2k rpm, it isn't going to happen. If you understand that the boost threshold on a 26/6 is 2400-2800...you didn't mention wastegate - are you still on the factory unit? Whose MAF?

streckfu's

I had the collapsed exhaust wall problem and it did not affect low end response. It did choke the hell out the car at higher rpms. I dyno'd at 225hp/250tq with the APE MAF and chips . Changed to a test pipe after verifying the inner wall of the up-pipe was collapsed and power rebound to 240hp/276tq.

The slow response below 2krpms seems pretty normal to me. At what rpm are you seeing full boost? Do you have a dyno to show us?

toddk911

"You may want to look into exhaust leaks between the head and turbo. "

How do you locate those?

"Does it sound very plausible that a clogged cat (I'm at 222k miles) could cause this power loss and lag? I'm still making 1bar though, and I would think a clogged cat would result in less boost due to the higher backpressure."

No. Your turbo, if it is capable, will just work harder to make that 1bar of boost. Which is why people see such big gains getting cat out. It is not the cat, but all the new boost that is found This allows yo utu turn the boost setting down and the turbo makes 1bar much eaiser and more effeciently now; also allowing cooler charge air temps, resulting in better response and more hp since the turbo is not pumping super hot air into the motor because the cat has it so over worked.

toddk911

"Basically, on the low-end from just off idle to ~2k rpm there is this "lag" to my throttle inputs."

So drive at no less then 3K

toddk911

Daniel: "Changed to a test pipe after verifying the inner wall of the up-pipe was collapsed and power rebound to 240hp/276tq."

Is this the upper pipe coming off the downpipe before the cat?

streckfu's

I think we are talking about the same thing. It is the exhaust pipe (not crossover) that connects to the downpipe and incorporates the cat.

Look here:Porsche PET online and select 9442...Main Group 2 (Fuel system/Exhaust system)...Service Group 02 (Exhaust system)...Illustration 202-20...Item 6 (Part Number 951.111.94.11).

The failure is at the upward pipe section. It is a dual wall pipe in which the inner wall is prone to collapse restricting exhaust flow and limiting power. If this failes, I would expect it to allow the turbo to spool a little more quickly from low rpms but then quckly begin to choke it due to increased backpressure.

Porschefile

I just put in a new dist. cap and rotor. All ignition components are new. I've checked the turbo by hand and there is no play. The compressor wheel looks in great shape and has no visible wear, so I'm sure it's been replaced at some point considering I have 222k miles. I've checked and I'm not leaking from the header. Also, my TPS is adjusted properly.

I generate boost perfectly. It starts to build boost at about ~2k or 2.2k rpm and makes full boost right around 2.7k rpm. See, this is the real confusing thing. I am still on a stock wastegate however, I am having NO boost related issues. The car generates boost just fine. The problem I'm referring to is the engine response and has nothing to do with turbo response. Basically, off boost and below 2krpm the engine just doesn't seem to want to rev. I know the stock CR is pretty low at 8:1 compared to many other turbo 4 cylinders, however are these cars that much of a dog off boost? The odd thing is occasionally the car will seem to gain back a little low-end off boost. Could a clogged cat cause this lack of engine response off boost? After driving plenty of smaller displacement Japanese turbo cars, even much smaller 1.8l and 2.0l turbo cars seem to have much better off boost engine response/power. That doesn't make sense considering these motors are much larger in displacement.

streckfu's

I think you are describing every engine with a large turbo.

See above post.

Even though the Japanese cars are smaller in displacement, they have much smaller turbos which respond more quickly (they also run out steam very quckly). The 951 has a decent size motor compared to many other cars so the response is slow at low rpms.

Based on your earlier post: I would encourage you to look at teh cat pipe. Either the cat could be clogged or the downpipe inner wall collapsed.

RKD in OKC

Recheck your knock sensor and wiring. It is very sensitive and without a proper signal the ingition will not be advanced at all. The workshop manual says to check it with an osciiliscope.

Porschefile

Thanks for the ideas guys. I'm going to go re-check my knock sensor and switch out with another stock exhaust. for the time being.

black944 turbo

What dyno where you on. Those sound like fine numbers for a mustang dyno. I had almost the same mods and almost the same numbers. I put it on a different dyno and put down what everyone else puts down. Most people go by dynojet numbers.

Porschefile

It was a dynojet. The operator did mention they had tested various cars between their dyno and another shop's mustang dyno with similar numbers, so it sounds like this dynojet does read pretty low. From what I've heard, mustang dynos tend to read ~10-15%+ lower than the average dynojet.