I just read this post that i am pasting at the bottom, He seems to lead the belief that you may lose low end if you go all the way with a 4 inch, so if this is true then I can go with a free flowing exhaust and 4 inch in and 4 inch out of it and run a 68hp chip and be ok as long as i have the ptro of course, it just interested me when i saw that you may lose anything by adding a 4 incher to your turbo, what i guess i am asking is is this true? here it is?


"Gentlemen, It has been some time in coming but I now have the means to upgrade my 2k PSD with exhaust, A-pillar gauge array and performance chip. In reading posts on this and other forums I need some guidance on the down pipe (ID/OD) selection. Some users praise the 3" to 4” down pipe and others the 3" to 3.5” setup. What are the benefits of the 4" over the 3.5”? I have read that the 4” will drop your low-end torque, is this so? I have an automatic transmission and don’t want to loose the low-end benefits when pulling. I want to purchase the right products the first time. Any help and guidance will be greatly appreciated."
Thx, Jeff
Some call it a 6-pack, I call it a support group.

thank you

usmc1775

 

A diesel is less sensitive to back pressure than a gas engine but it does effect it. The reason is a smaller pipe means the exhaust velocity is faster. Too low a velocity (at low rpms) and you lose power. Too small a pipe and it creates restriction, resulting in power loss. A 3.5 - 4" pipe is a good middle ground. 4" should not cause problems... generally anything larger such as a 5" will give no benefit and can cause low end power loss. The 4" should not be troublesome because the muffler and cats give enough exhaust resistance .

As to 68HP, do you mean 78HP? Superchips has about 40/60 (1994-1997 PSD's) and 60/80 on the 1999+ PSDs. Diablo is roughly 40/60 with 1994-1997 and 54/78 for the 1999+ PSDs... its HP numbers are generally a tad lower than the Superchips but the torque is bit more.