Carbomer90

I was driving home late Thursday night cruising @ about 70mph in 5th gear. I accelerated lightly just to watch my boost gauge because I have had some boost problems lately, which I believed to be related to a clogged CAT. When I hit about 8 psi I got this sensation like someone shoved a banana up my tailpipe. The car shuddered, the exhaust note changed, (Like suddenly it became very restricted) and I started losing speed but maintained RPM’s, my boost gauge went to about 5 to 7 psi and wanted to stay there regardless of throttle position. (Blow offs sounded normal) No idiot lights came on oil pressure stayed normal as well as temp. When I finally got to my exit the light was red so I had to stop. Idle went to like 300 to 400 RPM’s I thought she was going to die on me. When I get home I am left to try to assess the damage, there is smoke pouring from the engine, like all the oil on the engine has been raised to a temp much higher than usual (I have had a slight oil leak for a while) nothing glowing red though.

Okay, so I sit on the couch thinking it’s either a blown engine or a piece of the brick inside the CAT moved to a very unnatural position, of course I’m praying for the latter.
(NOTE: The previous owner removed the air pump but left the CAT, I have planned to replace it with a MP as soon as that damn AEM ecu comes out.)
To test my clogged CAT theory I pulled out the jackstands and start removing the unwieldy beast.

Finally, to the question (thanks for hanging in there for me) okay I have the CAT off so nothing is connected past the DP, how much backpressure is required to rev normally? She started fine and idled normally around 900 RPM’s but when I tried to rev her up a bit she was slow to rev and just seemed weak.

The only MOD’s are the DP and Intake!!

Thanks
Damian

luneytune

Could the smoke just be a direct hit from your oil leak to the outside of the CAT? I have a similar problem in my transmission has a very small leak and every so often if builds up and flys onto the Cat producing a lot of smoke and worry (the first time).

As to your problem, I would bet you have some sort of clogging in the Cat due to the airpump removal. If I run real hard at above 4K for an extended time the exhaust pitch changes getting noisy and if I back off, it continues for a few minutes noisy then goes back to normal. It must be the Cat in my case it is 90kmi old.

But as far as back pressure, a rotary doesn't need any, only to mitigate boost creep which has nothing to do with the engine and all to do with the smallish wastegate. Your car should rev very freely with the cat off and very loudly also. IMHO

Eric Michel

Pre-cat installed? Did you check you spark plugs? Sounds like they may be fouled.

P'cola FD

I'd check your plugs. Interesting, though, that's incredibly similar to what my car has been doing. It almost sounds like it's running on one rotor. If your plugs don't do it, I'd do a compression check. If that turns out ok, then you can check everything people are suggesting on my $100 post. Mine has been leaking oil to, which I know is coming from the MOP, and I wonder if that could be the culprit.

Red Rotary Rocket

Is it possible the MAP sensor hose popped off while you were on boost in 5th gear?

Carbomer90

I checked the two plugs that are easily accesible and they looked ok I cleaned them while I had them out! I have gone ahead and ordered a resonated MP from Shane Racing just to troubleshoot with.

Where is the MAP sensor hose?

No pre-CAT just the DP!

Thanks guys!

luneytune

As you check with your resonated MP since you have no fuel richening (AEM or PFC) watch out for leaning out and detonation. Boost creep will be an issue!

Lost Time

MAP sensor is the little black box attached to the driver's side of the firewall with one vacuum hose attached, or should be attached.

Carbomer90

Thanks, Understood! I am going to make a restricter plate to manage my exaust flow until the f'ing AEM comes out!

Carbomer90

^^Bump^^

Carbomer90

Thanks I checked it, and it's still connected!! I'm starting to realize it is probably the worst....rebuild time.

Well, I bought the car with the expectition that I would have to rebuild it. Unfortunately my wife did not have the same expectation.....she is going to kill me. She wanted me to get a C5 but I wanted the FD. ****!!!

MakoDHardie

It sounds like a really sketchy engine problem you've got there. The worse part about these things is that you don't always know exactly where the problem is coming from since there's a lot of stuff in there that could be going wrong. I don't think you'd be having trouble too much with intake or exhaust as far as air choking. Its likely that it has to do with electronics or fuel. If the sound you were hearing was really muffled (like somebody was putting a bannana in your tailpipe), it could be that you're not getting full air/fuel ignition. If you haven't fixed the problem yet, you could try running it without the tail pipe (but with something that brings the gases out of the engine enough to see them-and get your distance from that part of the car) on and rev it pretty well. Once it warms up, you'll start getting backfires if you're not igniting all your fuel. That's why you'd feel a loss of power. You could have a wiring problem, but If you have new materials under the hood or you can get a resistance meter and check to see if there's a break in one of the lead wires to the spark plugs. If the plugs aren't the problem, then get off the revving real quick- you're probably detonating. In that case, you'd be better off contacting the dealer or a rotary specialist. As long as you don't really kill your engine figuring out what is wrong with it, I doubt you'll have to pay a lot for a rebuild, but the money you'll put towards having your core pulled and inspected as well as having your ecu/electronic mapping checked would be well worth paying to prevent blowing your engine up. One thing there is to consider a suspect factor in this problem is the actual mapping itself. This could very well be an issue if you've done your intake and exhaust and haven't done any advanced computer programing to set the timing right. I'd put my money on the detonation, but then I'd also count on the simple problem of your timing being off. These two guesses could be part of the same problem and most likely are. If your timing is off, you're likely to be detonating. The problem could be solved with an APEX'i Power FC. Other AFC's could do the trick for you too, but I'd go straight to the Power FC since you can control every electronic aspect of performance in your car. Its anything and everything that your car is programed to do. In your favor, there are options which allow you to program intake/exhaust compensations. You'll have to tune the whole computer, since it replaces all the ecu tasks, but it would get you on track if timing is your simple problem. If its not that, then I'd say you could have a weak spark plug or a cut wire. Clean plugs doesn't mean they're not broken. If you get through all of that, you shouldn't be having any problems. If you are, there's something seriously wrong with your car and though a knock system from J&S would prevent detonation, you shouldn't be running a car that is dependent on one just to keep running. Knock systems are only there for safety precautions. Kind of like a pre-fire extinguisher. That's what I'd make of it. With a downpipe, intake, and exhaust, it's a lot of gas pressure resistance taken off. Good luck with it. Be careful checking for backfires! Let me know what you find.
-Mako

Carbomer90

Everyone:

Thanks for the great replies, we have a very good speciality shop here in Atlanta (Mazcare) I think I will drop it by there and let them check it out. With my work schedule I dont have time to work on it.

If she does need a rebuild I'm going all the way.......

Thanks again to everyone that replied!!

Here is a pic Before project "My wife is gonna kill me"

Carbomer90

One More.....

Carbomer90

^^^^

Tim Benton

iTrader: (1)

What's it idling at now? Did you ever throw the MP on to see what it did?

Tim

BrianK

my only comment on this issue is that removing *all* back pressure destroys your low end. I've never done it, so I can't comment on what to expect, but it's possible that the slowness (in the low end, anyway) was due to the fact that there was nothing to get the turbos moving.

hope everything turns out ok.

mmaragos

I'm not convinced this is true. A rotary is more like a turbine that a piston engine. What does back pressure provide to the engine and/or turbos (aside from a restriction to prevent boost creep)?

The more exhaust flow (the less the restriction), the better the response of the turbos should be, right?

I would think that loss on the low end could be due to improper tuning with a fully open exhaust.

Just trying to better understand...

rynberg

It's true. Ask anyone who has upgraded to a midpipe and free-flowing catback, especially those freaks who install the HKS 4-inch exhaust (don't worry, they won't HEAR me anyway ). It's all a trade off. Also, go to Mazdatrix's website and read what they say about installing a 3-inch exhaust on an FC.

http://www.mazdatrix.com/e1.htm (scroll to last couple of paragraphs)

skunks

how come you are trying to accelerate in 5th gear when you are only going about 70mph??? that is too low of a speed to be in that gear unless you are only cruising and trying to save gas or something! speed up to at least 100mph then shift to 5th.

btw, if your airpump is gone and you still have a cat, its dead by now. stop driving it and get a MP, its only 200 bucks!

mmaragos

OK. That is a NA FC.

What about a turbo? Why wouldn't your turbo spool earlier?

I have experience with a MP and high flow cat. I have swapped between the two. My turbos seem to spool much more quickly with the MP and the engine seems to rev slightly faster. I have notice no loss in low end, but that is on the street, not on a dyno.

mmaragos

I think that I have been looking at this from the wrong angle. The back pressure (or restriction) really is not the key here. The key is the exhaust velocity and finding the optimal exhaust velocity with the least amount of restriction ("back pressure"). Exhaust velocity will be affected by the restriction of the piping (pipe bends, cats, mufflers, pipe diameter). Larger pipe diameter equals less exhaust velocity, but removing bends, cats, and/or mufflers increases exhaust velocity.

There is a balance between all of these factors. A fully open exhaust with the correct pipe diameter would be the ideal solution. Too big of a pipe will result in too low of an exhaust gas velocity. Too small of a pipe will result in hitting the exhaust's "terminal limit" for expending gas, while the engine still has the capability to make more.

The MP having less torque than a high flow cat is either a tuning issue, or one of the piping diameter, bends and CB not being matched to achieve a higher exhaust gas velocity.

mmaragos

Might be related to exhaust port velocity. For any given flow rate, the velocity will increase when the pipe diameter decreases. More exhaust velocity = less turbo lag, which can produce more power down low (more boost at lower RPM).

But on the high end the smaller exhaust may not be able to keep up with the flow of the engine. It would be the flow limiting factor (hitting a "terminal" velocity), reducing power as compared to a more free flowing exhaust.

Seems to make sense. There could potentially be a point between a MP and high flow cat that would make more power through the entire band. Ensuring a high low end exhaust velocity to spool quickly and a good capability to maintain a exhaust velocity at the high end.

Back pressure may be one way to say it, but it makes more sense to me to look at the exhaust velocity. Although, I could be wrong.