85 brat catalytic converter offorad use

[Bolinkxr]

I looking to get the most power i can get out of my stock e81 so I ordered a weber kit and I wont to drop a the catalytic converters. My ? is can you take out both cats or do you need the ypipe cat to make it run well, or is there a difference in power by keeping it .

[Mr. Brat]

I pulled my y-pipe off and gutted it. EA81's like haing back pressure, if they dont have it will backfire (mine did it). I had a gutted cat to 2" pipe into a single in dual out magniflow. Sounded really good and barely had a backfire on a hard/high downshift.

Just my .02

[GeneralDisorder]

There's only a single cat on Brat's - the other is a resonator. Unless one has been added of course.

 

You can gut them - doesn't affect anything. All mine have been gutted for years. It will not cause backfireing if done correctly. That's generally a symptom of exhaust leaks.

 

GD

[NoahDL88]

Removing the cat will only increase power if the cat itself is bad.

 

If you do end up removing it (which I would recommend against, as it is good for the environment) Be sure to wear eye protection, gloves and a good mask/respirator. If you don't you'll get cancer.

 

Engines don't "need" back pressure, we've been over this before, and I think it bears correcting. Engines need proper exhaust flow velocity, which in an optimal system is around 300 cfm. When the flow is less the air stagnates and creates back pressure (BAD) when it gets too high it creates too much internal friction and blocks flow IE back pressure (BAD). When the flow is between 250-300 as it should be it helps evacuate the cylinder's used gases and increases power.

[GeneralDisorder]

Removing the cat will only increase power if the cat itself is bad.

 

Yes - that needed to be said and I overlooked it. The cat does not hurt performance. But I have found that most of them are either clogged, broken up and rattling, or straight burned away leaving on the metal straps and wire mesh (clogging the outlet because it's a jumbled mess). That, in conjunction with not having to test my rigs, has led to them being gutted to regain lost MPG and eliminate rattleing, etc. On my hatch where the contents were gone and the mesh/strapping crammed into the outlet I gained at least 3 mpg after removing the "blockage" from the cat housing.

 

GD

[Bolinkxr]

im new and i found the search and did some reading I guess its all about the back pressure. The big ? is do you need it or not some people think you need back pressure creates for low end power . I went to the muffler shop down the street and he told me take the the y cat out and put in a resonator backwards so the louvers catch the exhaust and makes a little back pressure . im not sure if it will make it not back as much i think im going to go that way no catalytic converter thanks guys

[NoahDL88]

I'll say it again, its not back pressure, its proper exhaust flow velocity. I have an automotive engineering degree and have been wrenching since highschool.

 

The exhaust shop wants to sell you exhaust, if they were coffin salesmen they'd tell you to start smoking.

 

Backpressure is a misnomer for flow velocity, kind of like a vacuum doesn't suck, the high pressure flows to the area of low pressure.

[987687]

The exhaust shop wants to sell you exhaust, if they were coffin salesmen they'd tell you to start smoking.

 

Haha, wonderful analogy...

 

I think a lot of the back pressure confusion comes from people getting confused up with 2-strokes. I know I've had my share of back pressure conversations that ended up with people mixing things they'd heard about 2-strokes into 4-strokes.

[TeamCF]

Being in CA I'm betting you have to do the DEQ thing.

So I'd recommend that if your CAT is good leave it alone, get a second Y-pipe and gut that one for wheeling. Then you can easily swap for testing.

 

This is what I did. And makes for less headaches when new tags are needed. Just a quick swap.

 

I'd leave the CAT on all the time if I could. But the past has taught me that I'll almost always bash it in then have to gut it out anyway the first time I take it up FB5 out at TSF. (lots of rocks) And then buy a new one every time I need to test.

 

Just my .02

[playtimeover]

I looking to get the most power i can get out of my stock e81 so I ordered a weber kit and I wont to drop a the catalytic converters. My ? is can you take out both cats or do you need the ypipe cat to make it run well, or is there a difference in power by keeping it .

 

I think there is a difference if you keep it.

[WoodsWagon]

There was a news story about an EA81 wagon that started a forest fire a few years back. I think it came up here, there was impressive pictures of the river of aluminum flowing downhill from the burned out shell of the car.

 

Supposedly they got it high centered in the grass and the cat lit the grass on fire. They had no fire equipment, so they just ran away from the burning car. It took out a bunch of forest before the fire was controlled.

[NorthWet]

+1 on the "no engine needs backpressure". The whole backpressure myth probably sprung up due to misunderstanding of using pressure pulses to tune the powerband. These are reflected pressures, going both positive and negative, and transient in nature. Backpressure is merely resistance to flow, and only causes the engine to work harder to overcome the pressure.

 

Further OT: Exhaust flow speed is also not important in and of itself. It is an indirect measure of other factors. Case in point: If you port the exhaust to open air (ala WWII aircraft V-12s) it becomes irrelevant. (But you lose the ability to use pressure pulses to powerband-tune.)

 

Back on topic: If the cat is badly deteriorated, you probably aren't hurting the environment much if any by gutting it. You could also assuage your conscience by adding an aftermarket cat.

Edited November 7, 2010 by NorthWet
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