Last month I had an exhaust shop install my Magna Flow high flow cat to go with my Flow Pro muffler. Well I had a 2.5" tail pipe with a 3" tip from 3A Racing. I had him bend up a piece of 3" pipe and replace the 2.5 set up. Would that be to much pipe causing lack of BP? I dont think I have quite the flow as I did before. Any ideas?

 

It's not a matter of backpressure, but the volume of exhaust gasses that your engine expells that determines what size piping to use based on engine displacement. Look at it this way:

Take a a garden hose and put a specific volume of water thru it. The water pressure in the hose will be a certain value as will the velocity of the water escaping the hose. Now, put that same volume of water thru a firehose. Is the water pressure value in the firehose going to be the same? Also, will the velocity of the water leaving the end of the hose be the same? The answer to both questions is "No".

So, getting back to your question about exhaust piping diameter. If you use too large of a pipe for your engine, exhaust gas velocity will suffer since the exhaust gasses expand and cool too rapidly in the larger pipe, which hurts performance. They slow down because they become more dense as they cool, and the exhaust scavenging becomes very poor. This causes the engine to have to "push" harder to expell the spent gasses, which destroys torque. It's not too little backpressure that causes this. That is a misnomer. I wish people would quit using that term.

Here's a chart that should help clarify what I just said:



Backpressure is good for 2-Cycle engines, but for 4 strokes like our engines, you want very little backpressure. On your engine, you shouldn't go any larger than 2.25" ID piping. And you should look for a shop that can do mandrel bends. Or, get a Jardine Cat-Back. They're tuned extremely well, use mandrel bends and are very inexpensive compared to other pre-made systems.

Good luck with whatever you decide to do.

Hope this helps.
G

EDIT: One more thing: tail pipe tip diameter shouldn't exceed the exhaust tubing diameter by any more than 1" or you'll get poor tone and it also affects the scavenging.

 

Thanks G for the help. Waited & waited for an answer and got a great one. Thanks. Ben

 

Yw

G

 

BBB, did you go down to 2.5" for the rear pipe? or are you at 2.25" now?

reason I ask is I have a 2.5" dynomax superturbo that I want to put on mine, but not sure if going that extra .25" would hurt the performance too much.

right now I have 2.25" piping and a 3A racing turbo muffler which has the straight through design.. I don't like the sound or the fact that it hangs off the back messing up my 4wheelin ground clearance.

so hopefully going 2.5" catback wouldn't hurt flow?

 

a 3" pipe is too big unless you are running a v8

 

thinking about 2.25 in and 2.5" out, but I guess that would give it a deeper louder tone though

 

After headers im debating if i should upgrade to 2.25 over my stock 2" piping. And probally a turn down at the muffler.

 

Hey fellas, I have a 85 celica 22re with no cat and just a muffler. If I remove the cat and go straight to the muffler on my 92 22re on 4x4 extended cab, will the computer knowledge throw a code? They both are stock and have o2 sensors in pipe parrellel with the bell housing,only one sensor. I don't have DEQ emissions checks here. With the airbox swisscheesed with a k@n, I think the cat is just more restriction off the ehaust gasses. Will this be a problem? I'm sure some of you folks have done this. Thanks Tim

 

It's been siad many times on this board...don't go any bigger than 2.25". You're not running a high RPM engine...shoot at 75 mph, mine only turns 3100 rpm. Opening up the exhaust to 2.5 or (OMG) 3.0 inches will diminish your low end torque and then you'll have to rev the engine to get any power. If you want to upgrade the exhaust, go with 2.25" and keep your revs below 4500 rpm. Your rods will appreciate it.

Of course...BBB, I know deep down, you want it to throw a rod so you can drop a small block into it.

 

Or a turbo.

Small engines that don't have a lot of torque down low need backpressure to get moving and with a too big exhaust on a small displacement NA 4 cylinder you'll actually lose horsepower on the low end and I've seen plenty of dynoed imports sporting little more than a 3 inch catback lose 10 horsepower or so and you should see the faces of those kids when they get the results.

I don't see anything bigger than a 2.25 inch useful on a natuarally aspirated 4Runner.