Turbo spin-up Queston

[ratherBfishin]

I was told to go with a 3.5" downpipe if I did not have a chip or programmer.
4" if I did. The reduction in backpressure would cause the turbo to spin up slower at low speeds and make it sluggish. Why is it not sluggish with a 4" DP when in the economy or performance mode with a chip or programmer?? I really like this one...

[Ditty]

I don't think that is correct.

Who told you that? A performance shop? Sounds to me like they are trying to sell you a 3.5" DP now...and have you come in an buy a 4" DP when you get a programmer. Truthfully, Turbo diesel's will not suffer if the post turbo exhaust is larger than stock...no matter what the size. I say, go with the 4" DP. It's not going to effect anything negatively. You'll actually get faster spool up.

[ratherBfishin]

I've read it on posts on this site & I was told the same by Magnaflow. Thanks.

[CSIPSD]

There full of crap... your turbo builds all the back pressure it needs... 4"...

[pipelayerjdh]

I would go with the 4" also. You may want to add more mods later. Even thoygh 4" is overkill for most mods, I say go with the best you can get.

Turbos do react to backpressure. You won't notice it much with a chip or tuner, though.

[CSIPSD]

Originally Posted by pipelayerjdh

I would go with the 4" also. You may want to add more mods later. Even thoygh 4" is overkill for most mods, I say go with the best you can get.

Turbos do react to backpressure. You won't notice it much with a chip or tuner, though.

Show me a single dyno run that shows a drop in TQ with a 4" down pipe... Keep looking, I'll wait right here...

[pipelayerjdh]

Like I stated before, you won't notice it with a chip. The extra fuel hides it. I'd probably have a difficult time finding a dyno run of a stock tuned truck that only supports the backpressure issue. I'm just going by my experiance. When I was running stock injectors and no tunes, the backpressure lost when i went to 4" straight pipe was enough to delay spool-up by about 500RPM. I added a muffler and got it back. I have also tested this on an old 12v Cummins and a 7.8L Ford. All results were similar.

[sttstroke]

I dont know how the two compare but I went with 4'' and the engine seems like it dont work as hard for the power if you know what I'm tryin to say.Good investment.

[ratherBfishin]

I talked to Edge today - makers of EVO2 programmer. I told them my interests were two conditions:
1)Economy while towing my 19' boat (3200#) with my slide-in camper (3400#) installed
2)Economy while unloaded in the city & highway.

I told them that I was upgrading to a Tymar/Donaldson intake system, a 4" turbo-back exhaust and their EEF1120 programmer.

I asked- if these are the primary driving situations, should I use a 3.5" or 4" downpipe?

Edge responded that for the towing & economy mode (my situations) the
3.5" downpipe might give better overall performance as opposed to a 4" downpipe with respect to turbo spin up time and acceleration at low speeds.

Note that my primary focus is on fuel economy and towing for my load conditions. If I wanted top performance in acceleration and speed, then I would choose the 4" downpipe.

I also talked to the people at Magnaflow. They had a general comment of using a 3.5" with the stock settings and a 4" with a chip (performance). They had no opinion they were willing to share about economy settings with a programmer.

I'm not trying to cut myself short, just trying to match what's available to my driving needs.

Thanks for all the responses.