Since I asked the question in my other thread and didn't have anyone answer. I kind of need an answer here soon.

Truck is a 5.2L with shorty headers, and a highflow cat that is getting replaced. Cat is 3" out, and my old muffler from when I had duals was 3" inlet, dual 2.5" outlet. Right now since I did the old highflow and my cat back at different times, I used 2.5" adapters to use the stock midpipes.

Collector, 3" in / out cat, 3 to 2.5" adapter, 2.5" stock mid pipe, 2.5 to 3" adapter, 3" in muffler.

My question is what is going to happen if I have a 3" midpipe vs. a stock sized 2.5" mid pipe? Quite frankly I don't know what to do. It would be easier on the pocket to just do the 3" mid pipe, but will that hurt me low end wise...? Would it be more benificial for me to keep the 2.5" mid pipe size. Need some discussion here guys if you have experience.

Thanks in advance,
pcfixerpro

 

I believe your low end would be fine or better and you'd lose on top end. I think it is short / wide = low end, long / skinny = high end.
But with all the 3" you have now and dual out, it probably won't have too much of an impact.

 

Yeah the dual 2.5" pipes are cut off so it is dual turn-downs before the axle. I actually thought it was backwards from what you said. Loss of low end with larger pipe and thats what I was afraid of. I think I decided 3" is the answer so I can save $30 worth of exhaust adapters and have it clean underneath. Keep it simple. Thanks Aubrey for the help

 

In my experience with 2 stroke engines and tuned exhaust - short and wide for high rpm power and small and long for lower rpm power.
Does this apply to our trucks? You decide.

If your cat out is 3" and your muff in is 3" then your should have a 3" pipe in between.

 

If it's a single pipe from the cat to the muffler, 3" is not going to hurt you. If you were running dual in, I would say no, but not a single. Back-pressure is largely how you get your tq, so a big loss of back-pressure (running pipes too big) will lower your tq specs. I read somewhere (maybe on Magnaflow's site) that a single 3" exhaust system is good enough for up to 500hp.

 

I have a 3" midpipe that the muffler splits in to a dual exhaust. I noticed a little low end loss and a little high end gain. Overall I wouldn't say it made too much of a difference though, so if its cheaper for the 3" I would do it

 

Thanks for your help. Yes it is cheaper to do the 3" mid pipe and as you said ben2080 if it doesn't make that much difference... ill give it a shot. Im also going to try to shorten the length of that mid pipe a bit so I may break even in pipe volume anyway. Stuff is getting put in next friday. follow the "cel with highflow...wtf" thread for updates.

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All the research I've read would suggest that having differnt size pipes going between sizes several times is bad for performance. These aren't nascars or anything so I would doubt that the one or two horse differnce would be a big deal but, if you want to get technical. The first problem is 3" is way to big for a 318. A 360 just barely falls in the realm of being a big enough motor to use 3" exhaust. The reason is because of gas velocity. You want the gas velocity to be unchanged as it goes through the exhaust. This means that it's the same temp from begining to end, there no bends to overcome and no restrictions. All of wich are nearly impossible to acheive on our trucks. Look at a nascar exhaust it starts out round and by the end of the exhaust it flattens out to about a 4" pipe that is one inch high. This is to control gas temps to keep the gas moving at a constant velocity. By having the gas move at the same rate through out the exhaust it will alow the motor to breath better.

But like I said these aren't nascars we are building I doubt that what you have done would cause any problems.

 

iceman is right. forget about backpressure, the concern is exhaust gas velocity. wether 3" is too big for a 318, however, is debatable. it would really depend on the RPM operating range. while these trucks are normally driven between 1500 and 3000, they sometimes will see as high as 5200, although i wouldnt consider this high by any means, i think it is enough to fill a single 3" pipe.

Since your other parts are 3" i would just finish it off the same size, the torque differences you will notice on a low performance engine like this will be un-noticable. all you would gain is sound.

 

I have 3" from the cat out, and it does fine. If I were to do it again, i'd probably go a little smaller.