Is putting a 3” catback on a 2.5” exhaust (hooker y pipe and intermediate pipe w cat delete) any performance improvement?

 

Only if the current system is restricting the output of your engine.

 

The y-pipe is only 2-1/2" after the pipes come together? Also, do you have headers?

 

Yeah it will help a little bit.

It's an old wives tale that the exhaust is only as good as the single biggest restriction. If that were so then you'd see 0 horsepower increase until you did the entire exhaust system front to back. Just sounds silly when you put it that way, huh?

The exhaust system is really only as good as all the restrictions combined. Take away a little bit of restriction from somewhere and it helps a little bit. Take away more and it helps more. But not all things are created equal. You might get small gains by improving one area, larger gains in another, and even more if you do both at the same time.

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No. For stock engines, exhaust flow upgrades are a waste of money. Yes, the cat makes a little difference but now the car can be quite stinky.

 

that’s actually no longer true, I measured it, 2.5 all the way. Yes I have the hooker 2460 headers

 

Ok cool, is there any way a 3” could hurt performance? My motor will be a low end torque one so some say you want back pressure? Is this true sounds kinda like another myth to me. I’ve got the 3” headers to 2.5 y pipe to 2.5 intermediate pipe and then to 3” catback. I already bought the hooker so no going back it’s too bad it’s only 2.5 though

 

I’m not sure I agree with that but my motor isn’t stock either. 2.25 crush bent is restricting no matter how you look at it

 

crush not only restricts but causes turbulence, which is also restrictive. expanding the system like you describes won't hurt it, but the effective id will be the smallest space inside the system.

 

There is no such thing as "back pressure". There is just pressure and it's not directional. Period. If there was back-pressure, then how come nobody ever talks about front-pressure and left pressure? Their long lost cousin, under-pressure, might feel left out of the discussion too.

The exhaust fluid (gases) moves because of pressure differences across the whole exhaust system. The high pressure side is the combustion cylinder, and the low pressure side is atmosphere at the exhaust tip. And what happens in between, inside the pipes, has a lot of influence on how quickly (time wise) the engine's volume of exhaust gasses can get out to atmosphere. Anything that impedes the engine's ability to expel exhaust from the cylinders will rob some performance (because exhaust gasses left in the cylinder don't burn during the next combustion cycle).

So really it's about fostering an environment where the engine can expel exhaust in time before the exhaust valve closes. If you have pipes in the exhaust system with a cross-sectional area that is too small to support the required flow rate, then it's going to impede the engine's ability to expel exhaust from the cylinders and rob some power. But air is a tricky little bastard and does some weird things. Going too big with the exhaust can be detrimental too. It's about finding the right balancing act for your engine.

Now in addition to that, there are high-speed waves that travel through the exhaust (caused by the violent things that happen inside the engine) and there is a large pressure change across the boundary of the wave front. Those waves leave the engine but can get reflected back at the engine by objects inside the exhaust pipes, similar to how a wave of water reflects off a wall. There are 8 cylinders generating waves, so waves are bouncing all over the place inside the exhaust, combining, canceling, and doing all kinds of neat things. If one of those waves runs back into the exhaust cylinder right when the exhaust valve is open, then it's going to impede emptying of the cylinder and have an effect on engine performance. But if the waves are timed to help empty the cylinder, then wonderful things happen. So why talk about this? Well, because sometimes people get unusually good results even with undersized exhaust and this is the reason why. Actually they just got lucky, but maybe we now know one of the reasons behind their luck.

 

thank you for taking the time to type all that it’s very helpful and makes a lot of sense!! Do you think 2.5 is big enough for me (rv cam, different air cleaner, advanced cam timing, hooker 2460s and y pipe) I would guess about 200hp/300tq and I don’t plan on modifying it more after

 

I've owned several vehicles over the years where i only did an exhaust upgrade. Imho, a better exhaust than stock is always an improvement. Would you see an increase on a dyno? Maybe, maybe not. But it can't hurt, so why not?