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Flat Spot around 5000RPM
http://www.teamswift.net/viewtopic.php?f=32&t=61755
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Author:  stelios [ Sat Jun 18, 2016 1:15 am ]
Post subject:  Flat Spot around 5000RPM

Hey all, so I have my Sprint running quite nicely now that I've had a bit of time to iron out the kinks. 2 inch exhaust really livened things up! I however seem to have a bit of a flat spot around 5000RPM. I'm wondering if this is due to the piss-poor factory porting? It pulls very strong up until 5000 then it kind of dies out and seems to run out of breath. Is there anything else I should check before pulling the head off to port? I plan on doing it anyway, but if there is something else that could be causing this then I'd rather fix that first of course.

Author:  blueturbofly [ Sat Jun 18, 2016 7:41 am ]
Post subject:  Re: Flat Spot around 5000RPM

what year are we talking about?
check the simple things first- even without porting it should pull to redline.
fuel pressure could be okay at idle, but lean off at higher revs.
plugged fuel filter/ pinched fuel line, pressure regulator not holding. weak fuel pump? weak coil? spark plug wires have too much resistance?
use the factory recommended spark plugs. maybe one range colder if running lots of boost.
restricted air filter? clean or replace. double check your base timing
many different items I would check before tearing it apart.

Author:  stelios [ Sun Jun 19, 2016 12:35 am ]
Post subject:  Re: Flat Spot around 5000RPM

Sorry, should have given more details. It's a 1991 Chevy Sprint Turbo.

According to my AFR (narrowband, but better than nothing) it stays rich when on heavy throttle as I would expect it to.
Fuel filter just got replaced with new lines, relocated to the engine bay because the stock spot is fucking awful.
I'll check the plugs tomorrow, I can't remember what heat range I have in there TBH.
Air filter is new, K&N unit.
Base timing is set to 10.

Any other suggestions?

Author:  Woodie [ Sun Jun 19, 2016 4:41 am ]
Post subject:  Re: Flat Spot around 5000RPM

Could it be too much timing? The computer is backing everything off in the 5,000 range because of too much advance?

Author:  stelios [ Mon Jun 20, 2016 9:22 pm ]
Post subject:  Re: Flat Spot around 5000RPM

Dialing back timing seemed to have done the trick, I'm surprised, though maybe I'll play with it more. When it was more advanced it pulled really hard up until the flat spot. Now that I have turned it back a little, it doesn't pull quite as hard but at least it's linear now all the way up to 6500.

Author:  Woodie [ Tue Jun 21, 2016 5:40 am ]
Post subject:  Re: Flat Spot around 5000RPM

YAY !!!

I fixed one boss, I fixed one!

Author:  stelios [ Wed Jun 22, 2016 1:44 am ]
Post subject:  Re: Flat Spot around 5000RPM

Thanks for the help! I'm beginning to wish that I had ported the head before I put it all back together, ah well. Planning on installing a recirc valve and ordering a +6 cam gear pretty soon, so hopefully that will liven things up. I'd also like to make a new downpipe and get a larger/less restrictive cat, but all in time I suppose.

Author:  Rustyoid [ Wed Jun 22, 2016 1:01 pm ]
Post subject:  Re: Flat Spot around 5000RPM

Are you referring to a recirculating blow off valve?

viewtopic.php?f=32&t=60403

I would avoid recirculating as you will probably end up blowing your engine oil seals. Unless you are venting the valve cover to the atmosphere, preferably through a catch can.

If you use the stock 3 way pcv system you will blow your seals!

Author:  stelios [ Fri Jul 01, 2016 3:57 pm ]
Post subject:  Re: Flat Spot around 5000RPM

That was an issue on MK1 Turbos from what I understand. Mine is MK2.

Author:  stelios [ Mon Jul 18, 2016 11:57 pm ]
Post subject:  Re: Flat Spot around 5000RPM

stelios wrote:
Dialing back timing seemed to have done the trick, I'm surprised, though maybe I'll play with it more. When it was more advanced it pulled really hard up until the flat spot. Now that I have turned it back a little, it doesn't pull quite as hard but at least it's linear now all the way up to 6500.


I may have spoken a little too soon. It still seems to pull timing a bit at high RPM. I'm wondering if my fuel pump is weak, and leaning out at high RPM...
Where the timing is, it shouldn't be detonating, it's set just above 8 degrees. I'm thinking of grabbing a Walbro fuel pump, and a new fuel pressure regulator just to make sure things with fuel pressure are working properly. Is there any difference between the turbo and NA fuel pressure regulators?

Author:  blueturbofly [ Tue Jul 19, 2016 7:47 am ]
Post subject:  Re: Flat Spot around 5000RPM

you installed a k&n airfilter? check the maf sensor door for binding, or oil residue build-up. if you change the angle of the maf, it screws with the afr. it uses a swing-gate with contacts to change the afr.
mine have always worked, so I never really screwed around with them.

Author:  stelios [ Thu Aug 04, 2016 12:25 am ]
Post subject:  Re: Flat Spot around 5000RPM

Yea I have an K&N filter. What's the procedure for taking the MAF apart? I've always heard that they are a bitch to have to work on.

Author:  Rustyoid [ Thu Aug 04, 2016 12:30 pm ]
Post subject:  Re: Flat Spot around 5000RPM

http://www.teamswift.net/viewtopic.php?f=32&t=37317

Author:  stelios [ Tue Jan 24, 2017 8:05 pm ]
Post subject:  Re: Flat Spot around 5000RPM

Super old thread, but I thought I should weigh in on what ended up fixing it in case someone else has similar issues. It ended up being timing, it was slightly too advanced, and the knock sensor didn't like that, which caused the flat spot due to retarded ignition. Dialed the timing back a little and it smoothed right out.

I also in the mean time picked up an advanced timing gear and camshaft from 3-Tech, and what a difference that made! Highway cruising is an absolute breeze. Highly suggest it for anyone looking to get more out of their T3. It does roughen up idle a bit though, which is a bit of a tradeoff, but very worth it in my opinion.

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