VR Ute

[afterburner]

This is a 3" single, but at this stage the cat is only 2.5", which can wait for another day.
I originally wanted a twin cat twin system with a 3" single pipe over the diff, but for $300 for a near new 3" catback versus $2500 for a custom system I couldn't pass it up.

I run pacemaker headers-procoat ceramic coating, dual 3” exhaust to diff. The pipes originally dropped down in front of the diff, but it’s quite loud & attracts too much attention. I could only fit 2.5” over the diff, so I’ve added 2.5” tail pipes from the 3” before the diff, up over axles & out the back. It’s really quietened it down in the cab ( now you can have a conversation & hear the supercharger ) but there’s a noticeable drop in power with the 2.5 tail pipes on. Remembering this is a very high spec build 355 stroker with whipple supercharger so it really needs 3” all the way, but my point is I believe you will get far better performance out of yours with twin 2.5 rather than single, & you will run into the same clearance issue over the diff with a single 3”, so twin 2.5 will be just perfect. A mate & I spent many late nights under the ute making the exhaust with new pipe, donuts to cut & shape all the bends, flow master mufflers. Tacked it all, then removed to fully weld & paint. Included a cross pipe which is definitely a good idea with a dual exhaust. It sounds really good, & if you want to get louder & free up some more horses, it’s just 4 nuts to undo & two hanger rubber to remove-the extra power is not really needed-as it’s quite easy to be reminded to pay attention often with the aggressive power delivery, with the 245 wide rubber still not coping, driving through a coastal open road I travel often with lots of flowing corners it’s pretty common to find yourself counter steering out of turns as you accelerate out, the difference between rear wheel steering & grip is a very small amount of foot pressure. Hell of a fun ute to drive. But anyway, as I was saying, I think you’re best bet is twin 2.5

[brindo]

Absolutely agree.
From my experience there was a significant power increase going from a 3” single on the tonner to the dual 2.5”. Room under the VN – VS is a lot tighter but on my VS ute that had the Whipple, I had the 1 ¾” Pacemakers, twin 2.5” cats into two 2.5” mufflers before the diff. Both pipes went over the diff and merged into a single flanged 3” pipe with a resonator that I could drop out for the 1/8” mile. It had an X Pipe though which made it sound tinnie but was quiet enough and no drone.

If money was no object I would certainly have something similar on this VR. It’s already got the hanger on the drivers side for a twin muffler system.
The Manta systems are very well made, I just find them too loud. But for $300 for a second hand but unused system I couldn’t pass it up. It’s got more than enough power for a P Plater.

[seabreeze]

I love how exhaust debates for Commos are a never ending topic! Shame that HM Headers are no longer. I think they'd have cleaned up in this current 5 litre renaissance.

[vlad01]

Just FYI. I tried to run 2x 2.25 over the diff and it didn't fit, the width or height of them being side by side is 4.5". A max of 3.5" single does fit with carefully routing. On that car, I ended up going back into a single 2.5 which was enough for the power it was making, but the 2.25 side by side was a no go, even when being rerouted as best as possible to see if there was another way, fix one hitting point, make another and round I went. Even looked at one on top of the other, but there wasn't enough room between the axle tube and floor. 3.5" does work though.

If you have IRS, then all those issues go away as the pipes just go under the drive shafts. Live axle, is a PITA when it comes to exhausts.

End of the day, 3.5" should support around 450-550hp.


On my current project, the 2.5 single needs a bent pan hard bar to clear the exhaust, I can't move the exhaust up as it's already almost touching the floor right above the bar, so the only thing is to get one of those adjustable bars that all are bent for the extra room. Once you have that bar, 3.5" should fit. Dual side by side hits the diff bushing mount and the spring seat in the body iirc, or at least so close that neither have adequate room and end up hitting both from the movement the exhaust naturally has during driving.

[brindo]

I love how exhaust debates for Commos are a never ending topic! Shame that HM Headers are no longer. I think they'd have cleaned up in this current 5 litre renaissance.

I remember talking to HM about cat backs for my VS ute when I first got it. From memory they had a twin cat dual system but only in 2.25”. They said their dyno testing found that on a stock 5.0 litre it returned better results than a 2.5” dual system. They did the exhausts for the Group As so they'd have far more credibility than I ever will.
For the life of me can’t remember where I saved this photo from, but my VS ute had the same system from the factory. HSV headers, y-piece, single cat, but merging into a twin system, then back into a single after the diff. But I got rid of all that for a twin cat dual 2.5” when the blower went on. Must be more room in the utes with no fuel tank in the way but there was a factory offered dual system of sorts.

VS Maloo Exhaust.png

[vlad01]

Oh yeah lol, totally forgot utes don't have a fuel tank in the way, my bad.


That's interesting with the dual part way system there. My best guess maybe was to try and get more flow but also keep velocity up? logic tells me no though, or maybe a cheap hack way of getting an X or balance pipe via the cat itself? I am leaning towards that but not sure really.


My good understanding of exhausts breaks down after the collector section. Beyond that I only know what diameter is approx needed for the power and that's as far as I know. Lengths, mufflers, cats and various things like x pipe are all guesses for the most part.

There is very little maths and science publicly tested and available for the cat back, but lots of solid info for the the cat forward such in race cars with header and collector which forms the entire system to the side pipes. For street cars, it get way harder because there is no public established science for what the ideal cat back even is.


I think there is software like Pipemax I think it's called? that does do calcs for the entire system, not just the engine and headers and intake. I have considered buying it as the dynosim I have is good on the engine, but the exhaust side is very generic. Just a drop down of the description, eg, small headers, logs, large headers and stepped headers. But no volumes or diameters, angles, step sizes, collector specs etc... Pipemax does all of that in great detail, and I did read on some forum some guys discussing the tailpipe (aka cat back section) and it's results matched that for some Toyota performance factory car stock specs which suggests the modeling matches real life engineering.

[seabreeze]

Mate has a VR HSV enhanced wagon and it's the single cat/ twin muffler/ single resonator into oval outlet as above. I wonder if it was the cheapest way for HSV to offer a "twin" exhaust? Looks like all live axle from VN-VS had a single cat in pretty much the same spot.

I have this many pieced thing too. VN SV5000 headers.

[Deuce]

My good understanding of exhausts breaks down after the collector section. Beyond that I only know what diameter is approx needed for the power and that's as far as I know. Lengths, mufflers, cats and various things like x pipe are all guesses for the most part.

There is very little maths and science publicly tested and available for the cat back, but lots of solid info for the the cat forward such in race cars with header and collector which forms the entire system to the side pipes. For street cars, it get way harder because there is no public established science for what the ideal cat back even is.

This is what I use for exhaust system sizing:
https://forums.justcommodores.com.au/th ... er.221208/

I reckon HSV did the dual system shown above purely for the 2 mufflers if the mufflers were too restrictive to flow enough at the appropriate sound dampening level.
Solution, halve the job required for the muffler - I reckon the noise suppression would stay the same but the flow restriction would be avoided (by going dual muffler instead of single)

[vlad01]

Actually that's a great point.

[brindo]

Well I wanted this ute to be reliable and not be working on it all the time. Guess I got that wrong too.
Got home from work the other day to be told its now making a funny noise and isn’t driving. Lucky she got it in the driveway. Had to push it around 40 metres on the trolley jack to get it into the shed.
Initially thought it had failed the collapsible spacer.
But pulled it down today and its snapped the pinion in half.

Pinion Failure.PNG