Need some expert opinions - exhaust baffled!!

[Panelman]

Hi guys....

I am stumped on this one so here goes -

VZ SS Ute (2005 build)

HM Headers (short)
REMUS Racing System
CAI


1st Dyno 222 rwkw. 2nd dyno after edit- 230 rwkw.
Guys spent half a day on her and just couldn't get any higher. They were expecting (based on previous results on other VZ's) around 240 - 250 rwkw.

Anyway, I'd like your wisdom of the possibility of restrictions relating to the short HM headers. Should I exchange them for long 4-1's or perhaps go for DiPhilipo 4-1's or Tri y's. ??? Also hi-flo cats?

Bottom line- should have achieved more gain than 8 rwkw after the edit. Please help.

I am dissappointed, but not out. Cheers

[Sonnymad]

Mate,

dont worry bout the dyno figures,does the car feel good on the road ? does it drive smooth ? good fuel consumption figures? throttle response ? all dyno's are diff and give diff results,as long as the above requirements are met,dont worry bout the dyno figure.enjoy ur ride

regards sonny

[Mongy]

This goes a little deeper than what it appears on the surface. You have used the HM short header like this one

http://img.photobucket.com/albums/v516/mongy/HMHeadersVYII.jpg

If that is the case notice how the header bolts up to the original engine pipe. The original engine pipe between that point and the cat (where the O2 sensor is) is a very restrictive double skinned pipe. It might be 2.5" on the outside but is only 2" on the inside. To overcome this problem I fitted the Genie headers pictured. They still leave the O2 sensor in the original position but it is in the collector which is 2.5".

http://img.photobucket.com/albums/v516/mongy/GenieHeaders.jpg

You'll notice that they are longer and the pipe between the header and cat has to be cut away and a new 2.5" section is supplied with the headers. Also the pipe out the back of the cats that goes to the first join where the "cat back" bolts up to is also only a smaller pipe with crush points in it. Genie supply a replacement mandrel bent 2.5" pipe to replace this. When fitting these the cats have to be opened to suit the new pipe size.

So in actual fact you do have a bigger exhuast but it still has the original 2" restriction points. This could be one reason you are not seeing the gains you wanted. Just a thing to remember with the cats if you get them opened, they have an internal skin that is held in place by the original pipe, if you enlarge the hole (front and back) you must weld that skin in place otherwise it will come loose and rattle around driving you nuts. My advice is fit good quality legally compliant high flow cats, but that's just my opinion.

What Sonny said is right as well, don't get too hung up on peak dyno figures, it is how the car drives and what midrange it makes that is the most important factor. Look at one of the well known Queenslanders, he ran 12.2 with only 219rwkw. A dyno is really only a tuning tool to make sure you are going in the right direction with your modifications. Sorry for the long post but I hope it helps.

[cosmo vyss]

Sonny- good advice
Mongy - great post.

Have you compared your two dyno runs? The torque should be well on the improve compared to your pre edit. Its not all about the top figure.
Any decent exhaust shop worth there salt can open up your cats and replace that pipe for a small cost. You have gone the whole header route you may as well get the job fully completed and the gains tied in with a decent tune will make it all worth while.
enjoy your ride
jb