Talk to me about exhaust

I've got a high speed miss when I use my scratch-built exhaust system that I don't get when I use the factory "system" (no mufflers, just head pipes, O2 bung, and cat). Car accelerates fine, but if I cruise at any given speed for a few minutes and then increase the throttle (like to maintain speed up a hill), the engine hesitates a little until I really put my foot down and then I can feel it kick back in with the other cylinder(s) that weren't making power before. Not throwing any codes.

I'm pretty sure I have some small leaks between the engine and the O2 sensor with the scratch built rig, I'm planning on pressure testing it on Wednesday.

Something else that occurred to me, though, is that the scratch-built system has the muffler openings pointing about 30 degrees forward from parallel to the back axle, whereas the factory setup shoots straight out the back. How critical is it that the exhaust tips point back? The bodywork limits the direction I can point the mufflers, but I could still make some turn downs to point the flow toward the sides or the rear...

In the old days, a less restrictive exhaust would slightly lean the fuel mixture. Resulting in a bigger main jet needed. Today, the O2 sensor should take care of it.

An exhaust tip pointing forward is pretty unusual, you could be getting more backpressure from air entering the exhaust tips.

In reply to GameboyRMH:

Would forward facing exhaust tips result in air finding its way backwards toward the O2 sensor? That's what I was getting at.

I've got bent tubing ordered to kick the tips out to the sides and down, and I'll check for leaks ahead of the sensor later this week.

if you have exhaust leaks between the cylinders and the 02 sensors.. it is going to run funky and once you go to WOT.. it goes into closed loop (ignores the 02 sensors). I think once you chase down the exhaust leaks, you should be fine

In reply to mad_machine:

Okay, how sensitive is the sensor to leaks? A couple of pinholes going to make a difference? How tight am I going to have to get this puppy?

In reply to JohnInKansas:

Pretty tight. What is it on btw?

yamaha wrote: In reply to JohnInKansas: Pretty tight. What is it on btw?

I hope it's on a mid-90's Ford 5.0 because mine does EXACTLY what you describe!

Leaks would have to be before the O2 sensor to effect the running. After, it will just sound crappy.

Its on an EJ22 from a '91 Legacy.

Here's the beast in question.

LOTS of welds, some of them in hard to reach places. I may have to cut it back apart to get to a few places.

In reply to ebonyandivory:

My '95 Mustang GT did stuff that sounds similar until I installed an EGR block-off plate and installed a plug on the header side to remove the EGR tube. Which causes a new set of issues but not the lean-miss during cruise.

Or you could have some other problem.

JohnInKansas wrote:

So...no access to any kind of tube-bending machine huh?

I believe I would have packed them babies with sand and bent them.....

It's not too late to start over and do it right.

In reply to JohnInKansas:

Good Lord you have a lot more patience than I do. Even if I am doing something " custom" I just buy a used EBay special header and cut and bend that into shape.

But I digress, In my opinion yes you have one or several leaks. That or reversion is playing games with your part throttle areas of your fuel maps. I can't remember exactly what the load points were on NA subies, but your open/closed rpm transition should be somewhere around 3600 rpm. And until your load/rpms put you in open loop you have to keep that rear o2 happy.

I don't think its your exhaust. That is all.

I don't get accused of doing things the easy way.

I set about making this manifold knowing that it might be a total wash. Wanted to play with pie cuts, and figured I'd give it a try. I've got all of about $60 invested in it, so it'll be a neat conversation piece if nothing else. I'm sure its leaky, but I won't know how bad until I can pressure test it (Wednesday at the earliest). Based on my findings, I'll either say 'to hell with it' and use the other exhaust or I'll patch it up until it works better.

65mph in 4th (top) gear is about 3400 or 3500 rpm, so I'm betting the open/closed loop transition is what I'm running into.

In reply to N Sperlo:

Coming from you, that sounds like sarcasm.

The open/close loop should throw a code from the upstream sensor.

No actually this time I'm serious. I don't think its the exhaust. Have you been playing Forza with me?...

In reply to N Sperlo:

There's only one O2 sensor. No upstream and downstream.

Tisn't me playing Forza. I've got a PS3.

Pre-obd2? The lone ranger O2 would throw a code for the open/close loop, I would expect.

Only code I've got in the last month is #35: Purge control solenoid - abnormal voltage.

Per the source I used to do the vast bulk of the conversion: "You will not be using this valve. It is designed to recapture fuel vapors and send them into the intake manifold during idle. The valve will still be present on the intake manifold and must be in place to prevent error code." The valve is still in place, so I'm not sure why I'm getting the code, or if it has any bearing on my current issues. I got it occasionally with the other exhaust pipe, and had no such issues then.

Honestly we are all pulling suggestions out of a hat. A possible tool may be some data logging. Some scanners have the necessary cables to read your car. Another possible suggestion is that you car has ( I think) the Denso 76 pin ECM like more recent Mitsubishis. Evoscan might be a data logging option that you could use. I don't personally have a def file for your car though. Also you would need an adapter cable to fit the 9 pin SSM 1 diagnostic plug those cars had.

Is this not in a Subaru?

Its not in a Subaru anymore.

I don't think I have a 9 pin SSM diag plug, you just get codes by counting LED light flashes.

I understand that this has shifted from "do you reckon my exhaust is the problem" to "this could be a billion things". I'm still pretty sure its exhaust related, so I'm going to pursue the exhaust angle and see what happens. Any other ideas y'all can give me about what it could be would be great, because it gives me other things to keep in mind (I've got a dull job, I need things to think about for 8 hours a day).

Your car is trying to give you a seizure.