If you go to a 4 inch turbo down system but keep the factory cat, the pipe must be reduced at the cat and then increased back to 4" again downstream. Doesn't this create a temporary restriction that negates the 4" pipes coming to the cat? Wouldn't it be just the same flow if you went from the cat back???? I hope someone can answer this as my friend and I were debating this over coffee this am. and would like a definitive answer from a higher authority! Thanks

 

most DP's are only 3 1/2 diameter and goto 4" after the cat or cat-delete pipe

 

Does this mean that the downpipe from the factory is smaller like 3"? I'm not aware of the size of the factory pipe. But still the question is,
is the reduction then from 3 1/2" to 3" at the cat any different than just having the stock pipe to the cat then a 4" tailpipe? It just seems that the stock cat would restrict the exhaust no matter whether or not the downpipe was 3 1/2" or 3", but that's what I'm wondering about. Is it worth changing the downpipe or not????

 

I am pretty sure the stock DP is 3" and goes to 3 1/2 at the cat.

 

Delawhere?,

Yes the reduction would be a bad thing flow wise for sure. Consider the MBRP systems, for one thing they have a cat provision so that the factory cat remains totally stock and bolts in place like original, no cutiing or mangling it to make it work etc. The second item is there is an available cat test pipe that you can get to use it for off road and or racing, it bolts in with two bolts and one clamp so it's easy to swap out when you need to. The MBRP has been flow tested, it was done on my truck in fact, and the 4.0 down pipe did not help with EGT reduction or HP gains at all. Their system has a 3.5 inch down pipe that connects to the 3.5 inch cat pipe, that then expands to 4.0. NO restrictive bumps in the flow this way. The systems are available in aluminized, 409 and 304 stainless and also in duals if you want. Just call us if you have any questions etc.

Mark @ DPPI

 

As Mark states a good mandrel bent full system is the best for proper flow...

BUT, if you look at the cat, it is actually is a good flowing flow thru design that has inlet and outlet "funnels" that can be trimmed to match the exact size of your pipe on either side....

Some have even done dyno and drag strip testing to prove that there is little or no change with or without a cat

AND
since it DOES help the environment that I use everyday,
since it is upwards of a $10K fine,
since it has little effect,
since my 03 6oh's turbo whine was too loud without it,
I had the muffler shop trim my cat and put it back into the system ...

 

Welcome back JD