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post #1 of (permalink) Old 08-08-2003, 09:22 PM
LilRed
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No Cat, Higher RPMs

I just removed the catalytic converter from my 92 Sidekick and replaced with straight pipe. Now it runs at higher RPMs and has noticeably less power. With cat, 55mph = 3000RPM. Without cat, 55mph = 3400RPM. No codes are showing so I don't think its the O2 sensor. The stock muffler and exhaust pipe were previously replaced with a "torpedo" muffler (which is pretty close to a straight pipe but has great ground clearance!) and 2" tailpipe. The cat was damaged and full of pieces before I took it off. A friend suggested I don't have enough back pressure. Could that be it and if so, any fixes besides a new cat? If that's not it, what else might it be?
Thanks in advance for any help. I'm new to the board and have learned a lot just from reading the posts!

red
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post #2 of (permalink) Old 08-08-2003, 09:29 PM
 
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Re: No Cat, Higher RPMs

Does this vehicle have an auto transmission?
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post #3 of (permalink) Old 08-08-2003, 11:30 PM
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Re: No Cat, Higher RPMs

Yes, its an automatic, 1.6L 8 valve.
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post #4 of (permalink) Old 08-08-2003, 11:56 PM
 
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Re: No Cat, Higher RPMs

Despite what most people say, no vehicle REQUIRES backpressure to operate properly.

The problem comes from exhaust velocity. If the exhaust diameter is too large, the velocity of the exhaust slows down, thereby negating any effect that the exhaust manifold ("header") has on exhaust scavenging (Timing individual exhaust pulses to create a suction effect).

If your engine is stockish, then it won't create enough exhaust gasses to utilize the 2" diameter. Add a header, cam, intake, and ignition, and you'll be laughin'!
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post #5 of (permalink) Old 08-09-2003, 01:58 AM
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Re: No Cat, Higher RPMs

Casper -- BINGO!!!

Absolutely correct - backpresure is always detrimental.
In fact, too open an exhaust not only stops the scavenging effect, but the pressure waves can actually push the spent gasses back in! Just the opposite effect of scavenging.

But I wonder about something else too - the only way the RPM is different at a certain speed is if the gear ratio's been chanmged.
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post #6 of (permalink) Old 08-09-2003, 02:27 AM
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Re: No Cat, Higher RPMs

RPM change with auto trans in high gear = torque converter not locking up now. [img]images/graemlins/confused.gif[/img]
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post #7 of (permalink) Old 08-09-2003, 09:33 AM
 
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Re: No Cat, Higher RPMs

You guys need to read your posts more carefully.
THe first statement you say is that "no vehicle needs backpressure to operate properly" then you say, "if the exhuast is to wide in diameter then the engine wont scavange properly" Those two statements contradict each other, am I wrong?

Buy stating that haveing a wide daimeter exhaust could have the engine not scavange properly, causing a negative effect, well isnt that back pressure? Just trying to figure out what you mean.

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post #8 of (permalink) Old 08-09-2003, 10:28 AM
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Re: No Cat, Higher RPMs

Sorry if it wasn't clear.

ANY spent gasses left over from the previous firing prevents new air fuel mixture from entering the chamber. That obviously lowers the effectiveness of the cylinder.

If you had no manifold - just an open head, you'd still have a little left over "spent" gasses in the chamber, not alot, but some. But you can't run it that way. When you shut it off the cold air will make the hot valves look like pretzels. And the neibors seem to complain too.

So we add a system of manifolds and pipes to carry it off.
Pulsations inside that pipe system cause pressure nodes and negative pressure nodes along it's length. Those nodes, or peaks, will either help or hinder getting the gasses out.
(Visualize the pressure wave in front of a car at speed, and the negative pressure wave behind it. Now picture it with another car behind that one.)

The way a header works is it is actually is "tuned" so the negative pressure point "sucks" (scavenges) the gasses out for a nice clean chamber = more power. It does that by utilizing the pulses or pressure waves to create a suction at the cylinder in question (like behind the car.)

Unfortuneatly those node points, or peak suction and pressure points change as RPM is changed. They change in intessity as well as position up and down the pipe's length. And the size (dia) of the pipes as well as the lengths change those points too.
If the size or length is wrong, it can actually put the peak pressure point near the exhaust port, thereby pushing the exhaust gasses back in the chamber -- not good. Pushing exhaust back in is called backpressure. That's the referance made about being too big.

A stock system is designed to operate throughout the expected RPM range, as well as reduce noise as much as possible. It's a compromise situation at best - no RPM's are at their peak potential performance, yet all work fine.

A performance header is designed to operate at one particular RPM. Often they increase power radically at their designed speed, but actually inhibit power at other speeds.

Now the next point - the entire breathing system operates as a "system." That includes not only the exhaust, but the chamber shape itself, valving, cam, the intake manifold (size/shape/length/flow etc.) as well as the throttle body or carb and the air cleaner system. They all work together - or end up fighting each other.
After all - the engine really is just a big air pump. The more air you can get in and out (mixed with fuel and fire of course) the more power you have.
It is possible to use all "performance" parts and end up with LESS power if the wrong combinations are used.

Fun experiment - get a temperature chalk marker used for aluminum welding. Run a line along your exhaust pipe starting at the manifold.
Drive it at a constant speed for a few minutes. Quick shut it down and crawl under to look - where the temperature is the greatest is the peak pressure points. Where it's the lowest is the negative points.
Now try it another speed - they change positions - more than you'd expect. That's the principle of headers.

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