Car runs rough and lean with the cut outs open

[ttocs]

As the title says I have been getting use to my cut outs and been noticing that it runs VERY different depending on if they are open or not. Up till this point I have been running them open and at first the motor ran lean and a bit rough but it seemed to get better the more I drove it. At first the AFR was reading high with them open but it seemed to settle down.

Today I closed them for the first time and notice it was running a tad bit rich for a little while again until it warmed up and then it was fine. I was sitting at a light and decided to open them up. When I did the idle dropped about 200 rpms and the AFR started reading lean. As the light changed and I took off the car started to buck and even backfired a little bit but was running rough and had lost power. I closed the valves and then it seemed to again smooth out back to normal but didn't run perfect again until I restarted it next time.

90mm lmaf
75 mm tb
ported cobra upper/lower
trick flow 170cc TW heads
1.6 rr
TF stage 1 cam
bbk shorty headers
magnaflow high-flow catted-X
Cut outs are after the cats and then finally the dynamax side exhaust mufflers.

Make any sence? Something a tune would help?

 

[Wichers123]

The ecu takes a bit to learn major changes in the afr's due to being straight open. The bucking comes from lean afr's"

 

[rz5.0]

Like i told you yesterday.. Doubt your running lean.. You prob did your cutouts to close. Now your sucking fresh air causing a lean reading.. Wide band and O2 sensors.. Causing your issue..

 

[ttocs]

wouldn't it be hard to suck fresh air all the way back through the cats? The cut outs are on the other side of the cats and with that being said I know a lot of people that put them on the other side of the cats and run it with out problems. Do I need a tune or what can I do? If you know the problem help me find the solution.

 

[CC'S95GT]

I wonder if it due to loss of back pressure.
Can the cutouts be stopped at partial open? Or are they either full open or full closed?

 

[rz5.0]

I can't say 100% that's the problem.. Just what i think. They might be able to retune it for you.

 

[ttocs]

If it was right next too the AFR sensor I could see that happening but going though hot cats idunno if I can see it happening. The cut outs are right in front of the muffler which is probably the biggest point of restriction from the design. I too thought about the loss of back pressure and if that could be effecting the EGR. Would it not make sense that if the cats are open and the back pressure goes down that when the egr opens to allow the exhaust gas back into the intake that there will not be as much exhaust gas being fed back through?

It is possible to open them a little or all the way. As soon as they are open you can see the gauge go. I will try to take a video tomorrow to show it happening as I can run my gauges in graphical mode to show the change.

 

[MustangChris]

theres a lot more to exhaust than people think.

back pressure, head loss, heat transfer, turbulence, etc. etc. etc. -- all effect flow.

Once you've modified some of that your engine *may* have to "push" the exhaust out, opposed to allowing it to scavenge out.


A proper exhaust takes someone who knows the science behind it to design it... they can calculate these gains and losses and how they effect the engine...

 

[ttocs]

can anyone help? I think this would be one of the first times that no technical advice was given, just a bunch of theory's of what might be happening.

 

[RichV]

Cut outs aren't something that a lot of people do, so this is kinda uncharted territory.

Try pulling codes, if the ECU thinks something is not right, it should spit some code to maybe start diagnosing this. I've run open exhaust after a non-catted H (side exhaust) and never had issues.

 

[ttocs]

Never had the cel light come on but I can check it.

 

[Slykin]

I've been working on my car lately and test driving with no exhaust past the x-pipe without any noticeable difference. I don't change between no mufflers and mufflers while driving though...

 

[ttocs]

Yea see that is my question I know a lot of people that take the mufflers off and don't notice anything short of more noise. my cut outs basically just cut out the muffler while using the same tip.

 

[MustangChris]

No one has tech help because you're the only member with cut outs...

we have to theorize what's up.... Lol

 

[joekd]

It is being caused by lack of back pressure, when I had my fox 5.0 I dumped it after the OEM catted mid pipe and noticed the same thing, just not as bad as you describe

Now the big question is how is anyone going to tune for it? They can't have the tune change anytime you open or close valves so they would just have to find something "close enough" for both scenarios

Definitely would have to be a custom tune, no mail order tune would be able to predict that

 

[rz5.0]

Dint want to say you guys are wrong.. But how is back pressure going to cause a car to run lean.. I can see a lost b in performance..

 

[ttocs]

The shop I take it to for tuning is an exhaust/dyno shop so I am going to call in the morning and see what they say. If its as easy as a tune that's great as I had been considering going and getting it done anyway as I was having ignition issues that made it hard for them to do it last time. Thanks for all the help/theory's as I am not a motor expert so too see it confuse you guys means I am way out of my league. Didn't rio just get cut-outs installed and I though in about the same place?

 

[rz5.0]

I'm curious to what you find out.. Keep us posted..

 

[Wichers123]

Dint want to say you guys are wrong.. But how is back pressure going to cause a car to run lean.. I can see a lost b in performance..

Old school efi man.. It's extremely slow in the learning department

 

[rz5.0]

Exactly..
You can run just about any exhaust type and size and the car will hardly know the difference.. Unlike the newer cars were you have to tune for the slightest changes you make..