enough back pressure

[Grof]

I was wanting to a high performance catalytic converter with 2.25" straight pipe on my 3.0l 94 Ranger and was wondering if there would be enough back pressure for the engine to run right. And would this modification effect the computer.

 

[nate09]

ideally you want zero back pressure. what you want is exhaust scavenging. being a 94 you only have a pre cat O2 sensor so it wont affect the computer.

 

[chazzone]

Clearly, you don't want "zero" back pressure.

Don't worry about the cat providing back pressure, as long as the pipes are the appropriate size, you'll be fine.

The computer should be fine with adjusting to the new conditions. It'd be best to disconnect the battery fora few minutes to clear the program, and let it re-learn based on the new exhaust flow.

 

[BlackBII]

Clearly, you don't want "zero" back pressure.

Ideally, you DO want zero 'back pressure'

 

[Hahnsb2]

Be careful, without backpressure your engine will explode, you'll also get 0MPG and have about 3 ft lbs of torque...

 

[Frank The Tank]

The engine will spontaneously combust? nah it wont

You have to have cats because it's the law, that's it.

I ran open-headers on my drag car for the last 7 years and it ran fine i actually put flowmasters on it because I kept getting pulled over.

A good exhaust system actually pulls air from the engine when it's hot.

Frank

 

[Grof]

Thanks for clearing this up for me

 

[chazzone]

Ideally, you DO want zero 'back pressure'

Sorry, it is more complicated than one simple answer.

A properly constructed exhaust system provides a range of pressure differentials, depending on the specific area and where it is in the combustion cycle.

Too low a pressure immediately post exhaust valve will draw a portion of the intake charge out of the cylinder.

This is why they run pipes on high end drag cars.

This is also why good headers are designed or tuned to take advantage of the various exhaust pulses.

On the other hand, Frank is correct in saying that a well designed exhaust will create negative pressure in the downline to scavenge the exhaust that has left the runners, although too much will effect the upper end, lowering torque and efficiency.

However, he is wrong when he says that cats don't do anything. They are necessary to cut down on engine emissions, which effect the air that we all breathe.

Its about time that people woke up to the fact that if you drive, you have a responsibility to keep your junk running right, and not polluting any more than necessary. If you've got an oil leak or your cooling system leaks, you need to get it fixed. The same for the cats.

 

[BlackBII]

Sorry, it is more complicated than one simple answer.

A properly constructed exhaust system provides a range of pressure differentials, depending on the specific area and where it is in the combustion cycle.

Too low a pressure immediately post exhaust valve will draw a portion of the intake charge out of the cylinder.

This is why they run pipes on high end drag cars.

This is also why good headers are designed or tuned to take advantage of the various exhaust pulses.

On the other hand, Frank is correct in saying that a well designed exhaust will create negative pressure in the downline to scavenge the exhaust that has left the runners, although too much will effect the upper end, lowering torque and efficiency.

However, he is wrong when he says that cats don't do anything. They are necessary to cut down on engine emissions, which effect the air that we all breathe.

Its about time that people woke up to the fact that if you drive, you have a responsibility to keep your junk running right, and not polluting any more than necessary. If you've got an oil leak or your cooling system leaks, you need to get it fixed. The same for the cats.

OP asked about having enough backpressure. Simple question. I gave him a simple answer. Backpressure is something that you do not want

Not to get into a pissing match...but my response still applies. Ideally, zero 'back pressure' is what you want. Backpressure is always bad...

You do make some good points in your post, and none of which are contradictory to my post, whether you see it or not. We're on the same page here. I never did see any mention of Exhaust Gas Velocity, which is something that you want to be as high as possible. Too large or long of a pipe, and you lose velocity. Too small or short of a pipe and you also lose velocity. (This ties in directly with the high/low pressure you mentioned, however, not many people on this site will understand that explanation)

Properly tuned runners (meaning the right size and length) will help to scavenge the exhaust gas with pulses from the previous combustion cycles creating the negative pressure you were referring to. However, any kind of backpressure will slow these pulses and will limit the scavenging effect.

I am a little confused on your statement of the "Exhaust pulling some of the intake charge out"...We are talking about 4 stroke passenger car vehicles here right? That only rev up to ~7K? Where the exhasut valve fully closes before the intake valve opens?