HSV Extractor - cat modification

[VF_CalaisV]

I'm after a cheap power up and I want to keep the car emissions legal (and not worsen any scraping issues). So I am thinking of getting some VE HSV extractors and cats for my VF V8 Calais.
The previous owner put a dual 3" DiFillipo exhaust on leaving the standard exhaust manifolds and cats.
The size of the cat outlet compared to the 3" pipe looks really small. So I was thinking of just simply increasing the cat outlet diameter, the section only about 5-10cm long, up to 3" diameter, this will then match the cat back.
I was thinking that this could be one of the main restrictions of the HSV cat since the exhaust comes out hotter after the cat (= greater volume/pressure). Which means increasing the outlet diameter could free up more power than increasing the inlet side.
Has anyone tried just this relatively simple mod? Any thoughts?
Also what is the material of the following:
HSV extractor pipes
HSV Extractor flanges
HSV Cat can
HSV Cat inlet and outlet pipe
I think the cat can (on HSV and the regular Holden V8 exhaust) could be stainless since from most pics they seem less rusted compared to the extractors. But everything else looks like mild steel.

 

[monstar]

I'm after a cheap power up and I want to keep the car emissions legal (and not worsen any scraping issues). So I am thinking of getting some VE HSV extractors and cats for my VF V8 Calais.
The previous owner put a dual 3" DiFillipo exhaust on leaving the standard exhaust manifolds and cats.
The size of the cat outlet compared to the 3" pipe looks really small. So I was thinking of just simply increasing the cat outlet diameter, the section only about 5-10cm long, up to 3" diameter, this will then match the cat back.
I was thinking that this could be one of the main restrictions of the HSV cat since the exhaust comes out hotter after the cat (= greater volume/pressure). Which means increasing the outlet diameter could free up more power than increasing the inlet side.
Has anyone tried just this relatively simple mod? Any thoughts?

I did this as an early (2012) experiment in exhaust modification.

Modified down pipe for more HP at top end at the expense of stock 'torque ring' restrictor.

 

[vr304]

U could always get some hi flow cats and pipes made up to suit the rest of the exhaust system and do away with the hsv cats altogether (using the hsv headers)

 

[monstar]

U could always get some hi flow cats and pipes made up to suit the rest of the exhaust system and do away with the hsv cats altogether (using the hsv headers)

I think the point is maximising the 400 cell cats also upon inspection still looking stock.

 

[VF_CalaisV]

Thanks Monstar, it was your posts from back then that made me think about the mod. So I am just looking at doing the cat outlet not the inlet.
What size did you increase the inlet and outlet to? Did you measure the power increase? I'm guessing the benefits of just modding the outlet might be up to two thirds the gain of doing both the inlet and outlet...
Did you know what type of metal the different parts were?
Below is a pic showing the need to increase the cat outlet pipe size!

 

[426Cuda]

What rear mufflers are you using? Coz anything but stock mufflers means it's gonna be loud and it's gonna drone.

 

[VF_CalaisV]

Hi Cuda, at the moment it's it stock VF 6.0 manifold and cats behind that it's full 3" difillipo.
Drone is only slight. Do you think adding HSV extractors and cats with a modded 3" outlet would make much difference to noise?
I read I think from one of monstar's old posts that the hsv extractors could actually reduce drone since they are closer to proper tuned length pipes and the pulses come out separately.

 

[blackve76]

I did this in 2007 on my Ve SS so can be done think the power difference was marginal but it is a restriction point. If you want to keep stock cats and tune go for it

 

[VF_CalaisV]

Thanks the main power increase will be from the difference between the HSV and GMH parts. I'm just thinking that while I do that mod I can get a couple more kW by changing the outlet pipe size. I definitely don't want to make a cal change to turn off the cat DTC.

 

[monstar]

Thanks Monstar, it was your posts from back then that made me think about the mod. So I am just looking at doing the cat outlet not the inlet.
What size did you increase the inlet and outlet to? Did you measure the power increase? I'm guessing the benefits of just modding the outlet might be up to two thirds the gain of doing both the inlet and outlet...
Did you know what type of metal the different parts were?
Below is a pic showing the need to increase the cat outlet pipe size!

Looks like a haggis!
That’s actually not a bad thing, means flow gets accelerated past that point as it expands into larger pipe. Seems counter intuitive I know but it is the same reason there is a merge point at the end of proper header primaries. For example my current setup has 4” megaphone with a reverse cone into 2.5” outlet.
Take a step back from the HSV and cat combo and see the “down pipe” after the primaries leading into the cat is technically the correct size for a stock engine to produce maximum torque - 1.75” - likely skewed to useful everyday torque at the expense of tits ablaze horsepower. This secondary pipe is ideally to run unobstructed before sound and pressure expands (gas slows sound reflects) at a tuned wavelength.
I thought this was all theory until with the modified cats in place I threw more valve lift and area, ported intake and throttle body and several hours trying to tune more hp on the dyno, to hardly any effect.
After fabricating and logging I found the problem is actually not the 400 cell substrate or the down pipe, rather the angle of entry not flow restriction in/out of the cats. I did some diagrams and calcs here somewhere but on entry the pressure waves were hitting a brick (ceramic substrate) wall at an obtuse angle causing an early reflected wave - incorrect timing for exhaust scavenging.
To test this I used a simple 2.5” mandrel bend and 400 cell barrel cat flanged to catback. Voila, was able to realise more power out of the setup. At the expense of everyday torque.
Torque requires velocity for proper scavenging so smaller 2.25” mandrel increased the acceleration, and 200 cell stubby barrel cat as far back as possible (before catback flange).
By enlarging the inlet it sounded more gutsy and boomy in cabin, but lost torque for everyday. Retrograde drone in effect but at least you could hear the pulses better, and there was a +5 kw difference at 5700.
Difference to sound out the back that I expected from opening the outlet to catback was not that much different from what I recorded. Maybe raspier, louder.
So inlet and outlet to 2 1/2” is first, was going to do 3” glad I didn’t. Back those days was not a calculated decision it just made sense looking at end of HSV headers and flange to catback.
I think the cat shell is aluminised steel, down pipe and flanges are hiigh carbon.