Cats or no Cats?

[V8RangerBoy]

Please give me an explanation why. Just saying it doesn't mean anything. Maybe you don't really know. shady

Not to piss anyone off, but I see this EVERY TIME I come into the Exhaust forum, and I have yet to see ANYONE with a logical explanation of how backpressure helps power. I have to side with shadetree here. Or else someone needs to post a reputable reason, because it's just not logical. A two stroke uses quite a bit of flow technology to make power, but a four-stroke... not so much.

Engine builders run them because its.... FEDERAL LAW! Unless in closed-course racing.

They're on the Ranger you bought because 1.) the ECU needs them to operate properly (though its not hard to fool it either), 2.) it's federal law, and 3.) there's not alot of reason to lose them really, unless you're out to kill the environment ASAP. I took mine off because they absorb alot of heat, and heat = exhaust velocity, which = less lag time on a remote turbo setup like this. But they're really a moot point performance-wise when it comes to an N/A motor.

Really old cat converters were pretty inefficient - they absorbed a ton of heat and were quite restrictive for the system. Modern cats are far, far better. The "high performance, high flow" cats you see everywhere really aren't worth anything noticeable on newer cars over the stock converter either.

 

[93Range302]

alright well i'm leaving them on... and to clear things up .... an individual won't be fined in indiana for having no cats as long as they have a 10000 pound plate...

 

[Muddycowboy]

Not to piss anyone off, but I see this EVERY TIME I come into the Exhaust forum, and I have yet to see ANYONE with a logical explanation of how backpressure helps power. I have to side with shadetree here. Or else someone needs to post a reputable reason, because it's just not logical. A two stroke uses quite a bit of flow technology to make power, but a four-stroke... not so much.

heres the logical reasoning:

say you install a less restrictive exhuast on a stock engine. they engine will not preform as well that is the HP and torque will drop.

Because the engine is outside of it's stochiometric ratio. What is that? That's the chemically "neutral" balance of air/fuel ratio. It has been shown that engines produce maximum horsepower at 14.7:1 (air/fuel) by mass. The problem is that when you alter the backpressure of the exhaust system, the ratio will also be altered. If you reduce backpressure, the engine is allowed to flow more air out. More air out means more air in. If the fuel system is not compensated for the increased air mass, the engine runs lean. If the backpressure is increased, the mass of air leaving is decreased. Less air out means less air in. Again, if the fuel system is not calibrated for the change, the engine will run rich. and rich or lean engines do not preform better than and engine with a nuetral air/fuel ratio.

Also backpreassure effects the gas's velocity, gas with a higher velocity will create a vaccum to help pull fresh air/fuel in to the combustion chamber.

so the arguement can go either way, with out the proper engine tuning to air?fuel ratios and change in back pressure can hurt an engine.

the reason moded engines can run with straight headers(2 psi of parasitic pressure loss vs factory exhaust 15-20 psi parasitic pressure loss) is because they can supply the correct air/fuel ratio. a stock engine can only compensate so much.

 

[Wicked_Sludge]

do you happen to know what an O2 sensor is?

see, thats the good thing about fuel injection..the exhaust is constantly monitored and the fuel ratio adjusted thousands of times a minute to compensate for things like engine wear, health, and modifications. a high performance exhaust system is easily adapted to by the engines electronic engine control system.

an individual won't be fined in indiana for having no cats as long as they have a 10000 pound plate...

you are required to have cat converters on any vehicle originally equipped with them (your ranger) by FEDERAL law...this means the law applies to all 50 states regardless of your state or local laws. you CAN be fined:

from the Federal Clean Air Act:

"The Federal tampering prohibition is contained in section 203(a)(3) of the Act, 42 U.S.C. 7522(a)(3). Section 203(a)(3)(A) of the Act prohibits any person from removing or rendering inoperative any emission control device or element of design installed on or in a motor vehicle or motor vehicle engine prior to its sale and delivery to an ultimate purchaser and prohibits any person from knowingly removing or rendering inoperative any such device or element of design after such sale and delivery, and the causing thereof. The maximum civil penalty for a violation of this section by a manufacturer or dealer is $25,000; for any other person, $2,500.".

 

[Muddycowboy]

i mentioned the ability to recalibrate IE o2 sensor mass air flow sensor. a logical reason was asked for the effects of back pressure on performance and i gave one.

if you want to run no cats with out being illegal just punch them out. as long as there is no sensors after the convertor no codes will be tho

 

[Wicked_Sludge]

thats not a logical reason since the system will adapt itself with no mods whatsoever.

so again, you will not loose power by removing backpressure.

 

[Muddycowboy]

not every system is like that. i'll be more politically correct i provided a generic logical answer.

 

[Wicked_Sludge]

ALL electronic fuel injection systems are like that...including those used in the rangers.

 

[Muddycowboy]

what i stated is how backpressure effects preformance GENERALLY! im sure you know not all fuel systems are FI. there for a (not all) four stroke engine can be effected by the amount of backpreassure and exhuast gas velocity. i also stated than the arguement could go either way.

 

[Wicked_Sludge]

a carburated engines fuel is metered by how much air flows through the venturi. thus, more air in = more fuel in and stoich is maintained.

the arguement only goes one way.

anything else?

 

[shadetree]

Correct spelling is stoichiometric ratio. All this means is that there are 14.7 parts air to 1 part fuel for a 100% burn of the fuel. Max power is obtained at a lower ratio usually around 12-13 to 1, not at stoic. Most engines run at stoic only during light loads, it is constantly varying.

Ref:

http://www.mummbrothers.com/SRF_Stuff/Secrets/Driveline/Air_Fuel.htm

Backpressure doesn't add power to anything. It is a restriction in the exhaust system. The system may calibrate for the pressure, but the engine doesn't need backpressure to perform correctly, and will do much better without it.

With modern day engines, removing backpressure is not a problem. shady

a carburated engines fuel is metered by how much air flows through the venturi. thus, more air in = more fuel in and stoich is maintained.

the arguement only goes one way.

anything else?

Actually, on carbed engines, if the backpressure was high to start with, and the system is opened up, the carb may have to be re-jetted to prevent a lean condition, which could lead to burned valves/pistons if the condition is not corrected. shady

 

[Muddycowboy]

well i have the dyno charts and literature for the effects on both carb and FI syatems if you would like a zerox copy or perhaps just a reference sighting id gladly provide

 

[Wicked_Sludge]

Actually, on carbed engines, if the backpressure was high to start with, and the system is opened up, the carb may have to be re-jetted to prevent a lean condition, which could lead to burned valves/pistons if the condition is not corrected. shady

your right shady, just like if a FI engine is modified beyond the capability of the stock injectors to keep up with the demand...they would need to be upgraded. for the most part, light modifications like cat-backs dont require this.

muddy, id like to take you up on your offer. i trust details such as vehicle, engine, mileage, a/f, ambient air temp, fuel pressure, vacuum/boost, barometric pressure, ect. will be available.

 

[Muddycowboy]

here is a direct quote from the author of my college txt book.

Exhaust modifications are done purely to alter the mass of air that can exit the engine (duh). Here's a story for you. A friend of mine put a new, uncorked, exhaust system on his truck. The engine is an otherwise stock, injected, small block. He put the exhaust on, and his fuel efficiency went way down, torque went down the drain, and his horsepower was basically non-existent. "But I don't get it. I was supposed to get 10hp out of that."

Why did that happen? Because the engine was outside of it's stochiometric ratio. What is that? That's the chemically "neutral" balance of air/fuel ratio. It has been shown that engines produce maximum horsepower at 14.7:1 (air/fuel) by mass. The problem is that when you alter the backpressure of the exhaust system, the ratio will also be altered. If you reduce backpressure, the engine is allowed to flow more air out. More air out means more air in. If the fuel system is not compensated for the increased air mass, the engine runs lean. If the backpressure is increased, the mass of air leaving is decreased. Less air out means less air in. Again, if the fuel system is not calibrated for the change, the engine will run rich.

As far as normal engines are concerned, alterations on the order of 1.3 parts of air to fuel would result in significant loss of performance. And yes, that's loss; no matter which way it goes. Contrary to popular beliefs, lean engines do not produce more horsepower and/or torque than neutral. So taking an engine, tuned for a specific amount of backpressure, and altering it, would result in the engine running outside of its stochiometric range. The engine would thereby run substantially lean, and be running quite poorly.

Now note that I said it was an injected small block. You might be thinking "well doesn't the ECM calibrate itself for the differing air mass?" Yes, it would. To an extent. Once you start getting somewhere in the range of about 1.5:1 outside of neutral, ECMs are typically unresponsive. As a general rule a code would be thrown and the ECM would just swing to it's maximum ability

this is the information i am going off of.

 

[Wicked_Sludge]

huh. those are funny looking dyno charts

a friend of my buddies sisters best friend's hearsay is hardly a scientific method (thats right, what you read in books isnt ALWAYS the way it is in the real world...just look at bumble bees if you dont believe me). there are a number of things that could have caused said power loss...including a poorly designed aftermarket exhaust.

now heres a few real world examples for you from first hand experience:

'93 ranger. true duals, equalizer, cherry bombs, rear exit. no loss in power or noticeable change in fuel economy.

'89 escort. stock HO header, cat delete (until i can afford a replacement), stock v-8 mustang muffler right off the header dumped right under my butt (again, until i can afford to fix it). power and economy are both greatly improved from the plugged/kinked/dented stock system that were on the car...but match my identical '89 escort.

'68 jeep wagoneer. true duals, no EQ, straight pipe with stacks behind the cab. no change in power. never figured economy on it but didnt "seem" to drop.

you seem hung on stoich. allow me to clarify for you. stoich is simply when there is just enough air in the mixture to completely burn the fuel in the mixture with no excess oxygen or fuel left over. its much less important to power than it is to emissions and economy.

heres some good crapper reading material for you:

CLICKY

as you can see, peak power is usually achieved at a much-richer-than-stoich ratio. in fact, EEC only attempts to achieve stoich under very light (cruising) engine loads. under moderate loads, the mixture is porously richened somewhat as a safety zone from pining. at full throttle, O2's are ignored and the computer goes full rich.

EEC has this thing called "adaptive fuel strategies". the PCM likes to play with the a/f ratio (rich to lean) from time to time to see how the engine responds. in effect its measuring the engines volumetric efficiency (which accounts for not only worn engine parts such as valves and rings, and differences in fuel qualities...but minor performance enhancers such as intake and exhaust work).

now for "limp mode". EEC continually compares what its sensors are currently telling it to a baseline reading stored in its KAM. when it perceives a sensor to be way "out of wack", it will assume something has gone wrong and substitute the sensors input from its memory. when you make a performance change such as exhaust work, you need a new baseline for your sensors since your engine is now flowing more efficiently and readings will be different from what they have been in the past. know how to erase old baseline readings from the KAM? unhook the battery