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Flat spot after new cat 

 

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 Post subject: Flat spot after new cat
Posted: Thu Sep 08, 2005 12:15 pm 
Tyre Shredder
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Bout 2 weeks ago i had some pacemaker 4480's and a hiflow cat fitted and now when accellerating i have a bit of a flat spot between 2k and 3.5k rpm
it never did it before. but the guy at the exhaust place said he's seen it before and resetting the computer fixes it, which it didnt.

has anyone had this before, or know whats causing it and knows how to fix it?

 

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Posted: Thu Sep 08, 2005 1:12 pm 
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Check 02 senser?? Did they silicone sealant ??

 

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Posted: Thu Sep 08, 2005 1:35 pm 
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and have u reset the ecu correctly?

 

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Posted: Thu Sep 08, 2005 1:47 pm 
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there no silicon on the o2 sensor.... how long do those sensors last???


yeah i reset the ecu correctly

 

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Posted: Thu Sep 08, 2005 8:25 pm 
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yeah i know exactly what your talking about, after i put on my 4480's i had that flat spot. ive reset the ecu and changed the oxy sensor and its still there.
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Posted: Thu Sep 08, 2005 8:30 pm 
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02 sensors can last a very long time. but usually after 80,000km or so they start to get a little "lazy" and take longer to react to a change. but usually ur ecu adapts to this, (adaptions) but replacing it can help.
as u canged ur exhaust this would have changed ur back pressure and this could have upset the motor, to cause the flat spot ur getting.
or it could be something unrelated to the exhaust
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Posted: Thu Sep 08, 2005 8:41 pm 
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fordgt290 wrote:
02 sensors can last a very long time. but usually after 80,000km or so they start to get a little "lazy" and take longer to react to a change. but usually ur ecu adapts to this, (adaptions) but replacing it can help.
as u canged ur exhaust this would have changed ur back pressure and this could have upset the motor, to cause the flat spot ur getting.
or it could be something unrelated to the exhaust


hows the back pressure effect the motor? i mean when the valves open the motor'll be running more efficently as the increased flow will be helping to draw the gases out of the cylinder head, and when the valves closed the back pressure won't make any difference as the valves closed.

 

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Posted: Thu Sep 08, 2005 10:46 pm 
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twr7cx wrote:
fordgt290 wrote:
02 sensors can last a very long time. but usually after 80,000km or so they start to get a little "lazy" and take longer to react to a change. but usually ur ecu adapts to this, (adaptions) but replacing it can help.
as u canged ur exhaust this would have changed ur back pressure and this could have upset the motor, to cause the flat spot ur getting.
or it could be something unrelated to the exhaust


hows the back pressure effect the motor? i mean when the valves open the motor'll be running more efficently as the increased flow will be helping to draw the gases out of the cylinder head, and when the valves closed the back pressure won't make any difference as the valves closed.


yes it will effect it because all cylinder exhaust port dont open and close at the same time the lack of pressure has freed the motor up alowing it to work easier but if your fuel is still the same as when it had more back pressure you will get flat spots were the fuel is to rich or lean and you notice it because you are gettin more fuel else were say higher in rev range were there is more fuel , and this is cause these motors run rich

or it could be other problem but yes i have flat spot too after new system, nothing a turbo wont fix lol

 

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Posted: Fri Sep 09, 2005 1:35 am 
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hrmmm well at least im not the only one with the prob, ill have a look round on the weekend and check over everything.

 

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Posted: Fri Sep 09, 2005 10:09 am 
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left a hole so big the exhaust isn't moving fast enough out the tips, those big bore cats all useful for blown engines or only good at higher rev range.

eg: think of it as a veccum cleaner, attach a large diameter hose and it won't suck.

 

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Posted: Fri Sep 09, 2005 11:11 am 
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Thats the problem I got now with too much flow now with Extractors, Cat and exhaust and I only notice any power gains above 3000Rpm like you said

 

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Posted: Fri Sep 09, 2005 4:44 pm 
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1 SLY 97EL wrote:
Thats the problem I got now with too much flow now with Extractors, Cat and exhaust and I only notice any power gains above 3000Rpm like you said
Have you got your old cat? I'll bet you when you weld it back on you'll claim back what you've lost between 2000-3000rpm.

you'll need to balance whats coming in throu CAI to creat vacum. and whats going out the rear, like a vortex effect.

I too came across some flat spots back then, I worked it out by keeping everything in proportion with the engine capacity.

 

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Posted: Fri Sep 09, 2005 4:53 pm 
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put a restrictor plate in between the cat and the exhuast at the oulet end, you what 2.5" so make a plate with a 2.25" hole in it, bolt it in-line and test the power.

 

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Posted: Fri Sep 09, 2005 5:11 pm 
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When I put in my zorst, headers and cat I never had that problem. Although the cam went in about 2 weeks later...

 

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 Post subject: flat spot.
Posted: Fri Sep 09, 2005 7:45 pm 
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I seem to remember Jim Mock saying you had to use smaller diam extractors to get the correct effect on these engines. And he has done a lot of diagnostic and developement work on the I6.
Not all extractors work well and some people would not know if the power was up or not they just think it is because they have extractors.
When you change one thing you have to change others to compensate for the original change you made.
Thats y not everybody can get power out of thier engines, its more complicated than most think. Extra power costs money, wether u like it or not u cannot get something for nothing

 

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