Thaddeus

Hi.

I seem to detect an occasional occurence of a 'flat spot' in the torque curve on my car. Seems to hit between 3800 RPM and 4400. (My boost usually comes on around 3200 because of my LBE)It doesn't always happen, but it's a puzzling thing. I thought maybe it was temperature related, but I always wait until the engine has achieved normal operating temps before I open it up; So I am at a loss. Intermittent problems are such a pain to diagnose!

The car is still plenty quick, even during the flat spot, but I'm used to having that power just come on and stay on, I'm wondering if this is normal for these cars-- a known bug-- or if there is an error condition I can correct. It may be that the car has always done this, and I'm just more attuned to its behavior after driving it 2 months... but I don't think so.

Anybody else's car do this? Comments, questions, suggestions all welcome.

Car specs:
87 951, Autothority St2 chips, LBE (set at 9 psi), stock turbo, tuned and running great (except for the occasional flat spot), 94 octane gas, K&N filter in stock air filter housing (not the cone).

~cheers~

Thaddeus

keith

Mine has a similar condition. I was planning on going through my ignition system (cap/rotor/wires/plugs) to address this...

I am hoping that will clear it up?

TomH

When you say "flat spot" do you mean a point at which car stumbles during acceleration. I tried everything to cure my stumble/misfire, and it turned out to be the main ignition wire off the coil. I had it routing too close to other metal things (primarily the headlight motor arms/linkage.) By routing it straight down from the dist. cap, and then over to the coil (furthest point possible from other grounded metal), my misfire disappeared. Sometimes its so damn easy.

Thaddeus

No, it's not a stumble or a misfire. It's not a bog. It's more like... the boost comes on hard, then not so hard, then hard again. It's not a malfunction so much as a zone where the car just isn't accelerating as hard as it was. Like I said, the car runs great, and maybe this is just the way they are... I would just like that vicious boost to be consistent throughout the boost range. And, it doesn't always happen, just sometimes...

Thaddeus

txhedg

I had a similar problem of a "stumble" in my car in the same 3000-4000 rpm range. For me it ended up being insufficient fuel pressure on the fuel rail. I have custom chip, MAF, etc, so I probably barely had enough FP when the stock FPR was new, but it seems like over time it may have dropped the effective pressure it put on the rail. I was only getting 28 psi (<2 bar), when stock FPR should have given me about 37 psi (2.5 bar).

I added an adjustable FPR ($169 from Paragon Products), set the FP at 3 bar, and the car ran perfectly, with no stumble. You can also go with a cheaper, 3 bar fixed FPR, but the adjustable one allows you o better tune the car for optimal performance.

We "diagnosed" (using the term VERY loosely) the problem by clamping the fuel return line slightly, and then reving the throttle (while sitting still).......even with that mickey-mouse way of increasing FP, you could hear how much smoother the engine was as it revved. Just be carefull not to (1)crack the fuel return hose, or (2)let fuel spray out of one of the the connections due to the increase in back pressure.

If you go to the link below, and click on FAQs there is a good description of how to easily make a FP gauge for the end of the fuel rail.
www.funcarsonline.com/frames/944board/944board.htm

Can't be sure thats your problem, but it worked for me....and BTW, having a flat spot, or stumble as the turbo is really kicking in is a major PITA.

txhedg
'86 951 track car

2 Robs 944 Board

txhedg

Oops, your follow up posted while i was typing....I guess you can disregard my advice since its not a stumble

Thaddeus

No, your reply is much appreciated. This my be the crux of the issue, since it's more like a performance tweak I need than a fix... Do I need to upgrade any other parts to account for the adjustable pressure if I ramp it up (replace hoses, rail, anything like that?

TIA

Thaddeus

BTW, how does a person get rated on this board? I see some people sporting the 5 stars and I'm JEALOUS.

keith

Talking about me?
I guess I have perfect attendance!

txhedg

No, the Adj FPR just bolts right in place of the old FPR. Rail, hoses, etc can remain as is. Its a 10 minute install.

You will need to somehow monitor the FP on the rail when you install the AFPR. Once you get the new AFPR adjusted, you can remove the FP gauge, but its tough to know even where to start with the new AFPR if you don't have a gauge on the rail. Any shop should have one they can bolt on, but if you install the AFPR yourself (VERY easy), then you will need to know what level the new unit it providing as you turn the nut to increase or decrease the FP.

Also, if you are going to install the AFPR, and are working on tweeking the engine performance, an Air/Fuel Ratio meter is not a bad thing to have either. They are about $100 (see Huntley Racing or Lindsey Racing web sites). The A/F, coupled with the AFPR will allow you to turn up the FP to assure you are not running lean when under full boost.

Hope that helps

txhedg