I have chatted with several folks that spoke highly of the Vast Piggies, but I do havelingering questions....

I was told by these folks that the Vast Piggie will eliminate excessive heat directly behind the turbo...Is this because they hollow out the precat that sits behind the turbo?

The precat is sort of egg-shaped...If it is hollowed out, wouldn't that cause a bit of restrictive air flow in itself because it is not a straight thru pipe? From what I see, it does not look like a smooth transition from the precat to the pipe leading to the main cats?

Wouldn't it be better to cut the precat out all together and weld in a piece of straight pipe?

Am I missing anything else? Thanks for your input!

OOOO

 

The cat is removed, so there is no longer any restriction. The oval shape is the shape of the turbo & wastegate housing.

 

You're points are logical-- but.

The turbos themselves are the main restriction for the engine.
The exhaust (and hence, the precats) are the main restriction for the turbos.

So, yes, removing the precat restriction will improve flow, reduce backpressure and result in quicker purging of the gas and less heat-soak of the turbo. A lot of difference?.,.. I'm guessing not.

You're also right about the flow, the open precat housing will result in turbulence. On the other hand, it gives a little bit of a surge tank for the turbo to exhaust into during initial spool up. A well-sized pipe (big!) would be the ideal solution, but again, I'm not sure the difference in performance between the two will be easy to measure/discern.

Its probably a poor analogy, but I had a 3000GT VR4 with similar characteristics- 3L twin turbo configuration with precats and main cats. Gutting/replacing the precats was very very common, and generally produced 15-20HP with very little difference shown between those that hollowed out their own precat housing, and those that bought a downpipe with a tubular pipe.

 

Yes, no one will debate that. The pre cats, are inches from the turbo. They are a big restriction.

It releases a LOT of pressure right off the turbo, it also greatly reduces heat buildup at the turbo. Yes, they do make a big difference on the 2.7, most people see around 20whp, & a lower & more broad torque curve.

The DP opening is the same size as the turbine housing, where the cat was is not that much bigger, & has a nice taper down to the pipes. most aftermarket DPs are not that much bigger, I have ASP DP's on my that are the biggest out there, it keeps the pipe size the same from the turbo housing then tapers after the cat to mate up @ the exhaust.

Yea, it's about the same for the S4, some aftermarket DP's show more gain, but that might be do to the HFC VS stock cats. It usually comes down to $200 for piggies, VS $1500 for aftermarket.

 

It also appears that AWE Tuning and Millteks DP's have also eliminated the precats as well which would further explain what Vast does.

All of your input has been great! One of my big worries is the labor cost to install them. I have looked underneath and it is tight. I can only imagine a full days work to get them installed.

I would say Vast has a distinctive edge mainly for the price alone.

Thumbs up fellas, and again, Thanks!

OOOO

 

I think there is a write up somewhere on the web for a cat replacement and for piggie install, IIRC. ALthough it may have been 4.2 rather than 2.7T since that's what I searched for. I do remember that the person yanked a good bit of the front suspension. That, plus the likely less-signficant improvement for the 4.2 changed my mind and I decided to leave alone for now. If I had the 2.7T, I think I'd want to get in there, instant spool is great..