Header for the SP20

[twilightprotege]

maybe you got confused with the general 4-1 headers are high end power compaired to 4-2-1. that is true, but high end power headers have much much longer primaries - like 80-90cm long. the obx header is only 40cm long max so it'll act more like a 4-2-1 design than a traditional 4-1 design.

 

[coldie123]

OK did some research, and the all the info i could find say that longer primaries give more power in lower revs while shorter primaries give more power in high revs. Here's a few links
1
2
3
Makes sense, since speed = distance / time, and given that the speed of sound is constant, distance must decrease with time. So as you increase revs (higher frequency) the time between sound waves decrease, so the distance (tube length) to maintain resonance must decrease also.

 

[twilightprotege]

those links are true for a racing sense (ie big big power numbers and pretty much open exhausts), but you'll see from the 2nd link and the 2nd picture and when they talk about it, that the lengths are too short to use wave length tuning. this will be the same for the obx. you will find that also because of the more restrictive path of the obx (ie they all merge together very quickly) that it'll give you enough back pressure to loose it's 4-1 characteristics (ie the high rpm tendancies). dont forget the tube diameter has lots to do with it as well

i still beleive that the obx will yield more low end power than the awr/wildcat etc. the obx gets the power back from the backpressure it generates...

 

[twilightprotege]

very interesting links though. thanks for clearing that up for me.

 

[syd_speed]

Hey Coldie, that's a fine looking header, if you don't mind, I am going to modify my exhaust system as well and I wanted to know if we could start a group buy on the OBX header.

I plan to do my whole exhaust system, is there anyone you can recommend me to in Sydney area? I have already tried Bankstown Exhaust, Hitech mufflers and Liverpool exhaust.

 

[coldie123]

Twilight, yeah I think you're right about the primaries being too short for significant energy wave tuning, although I don't fully understand the concept yet. I originally wanted to get the ractive header because it had longer primaries - its collector is after the 90deg bend, under the engine, instead of on the downpipe like the obx's. And its actually been dynoed and shown to give good gains. However the ractive ones seem to be in very short supply, so I'm settling for an obx instead.

As for the collector, I also agree that it looks too short and doesn't allow the gases from each primary to blend smoothly with each other. Hence it would probably cause a bit of turbulence and decrease gas velocity. But I think thats a bad thing because high gas velocity and its high inertia is what gives you low end torque, not backpressure. From what I understand backpressure is always robbing you of power because its pushing against the flow of exhaust gases. Its a myth that you need backpressure to maintain low end torque. I read a brilliant article about this, but can't find the link atm. The link is actually in one of the threads here on this forum.

Having said all that, I truly hope that your right, and the obx does give me some much oomph down low, but the more I read, the more it seems that it will only give noticeable gains above 4000 rpms I just don't want to get my hopes up high.

 

[coldie123]

Hey syd speed, if you want to get an obx header too, theres a group buy going on at the moment here. I'm already signed up and its probably gonna close in a week or so. The shipping quotes I've gotten from Ken are $52 for airmail (4-10days) or $28 for surface mail (6-8wks) thru USPS. I'll ask him if it'll be cheaper to ship 2 together if you wanna save on shipping.

I'm planning on doing a full exhaust too, but i haven't actually gotten many quotes from exhaust shops. This header is the first step, then a catback, then a midpipe with hiflow cat. The only rough prices I've gotten are $320 for a stainless muffler installed, $350 for a mild steel catback or around $650 for a stainless catback. Both from shops around the mascot/botany area. I can't recommend any since I'm new to this too
What kinda prices did you get from hitech and liverpool? I've heard good things about hitech.

I'm actually considering getting a racing beat catback from the US, and yes I know its gonna be expensive and only gonna give 5whp gains, but I really want something quiet and deep, and the rb is proven to be just that. I'm wary of getting a catback from an exhaust shop because I don't have time to keep taking my car back to the shop to try different mufflers and hotdogs until they get the note just right.

 

[sabrelli]

Damn, i want this, and i dont!!!!!!!ARRGGHHH

how much custom duties are we lookin at boys???

 

[sabrelli]

I got my catback exhaust done at Livo Exhaust...370 all up.
im never goin back there again. Bastards

 

[sabrelli]

There are some very good exhaust places down in the gong.
Few of my friends have recommended me goin there to get it zorst work done.
And another bonus is price!much more cheaper than wot the guys charge in Sydney

 

[coldie123]

LOL! c'mon theres a shiny obx header in the states right now saying *buy me* *buy me* to you! why are you hesitating?

I have no idea about customs duties, but I can't help but think that a bigger, heavier package is probably gonna attract more attention and more likely to get charged duty. So I'm not sure if its worthwhile combining shipping. Also USPS has restrictions on the max length and girth of a package that it can send to Australia so probably the most you can combine is two headers in one package.
So why is Liverpool exhaust so bad?

 

[twilightprotege]

coldie, backpressure is good and bad. for unagressive cams, backpressure is bad. but when you start to get cams like mine where there is a fair bit of cam overlap, i loose a lot of power down low because of the lack of backpressure. because the intake and exhaust valves are open at the same time, a bit of the intake charge goes straight out the exhaust valves. backpressure would keep that in and give me much more lower end power. however, when the engine is reving high, the cam overlap and lack of backpressure helps because it sucks in the intake charge, then closes the exhaust valve before it has a chance to go out the exhaust valve.


but otherwise, come on guys - jump in with this. you know you want to! it's a great price!

 

[sabrelli]

Well if i get it, im gonna look elsewhere for insurance.

Im with aami, they wont insure my car!told me to get quote from justcar!.

 

[chipa]

Thats nuts! They didn't have a problem with me when I indicated I had al these things already on the car! Mind you, age and driving history can be a bugger.....?

 

[sabrelli]

im 27, my driving record is immaculate.
9 years, no claims, no fines in past 5 years.
i already have exhaust, lowered, and mags listed.
was when i told them extractors, theyre like no no, sorry bugger off.

got a quote thru suncorp andrew, it, comes at 1650!, thats 400 more than wot im payin now!

 

[chipa]

I'm 26 with the same record. I've got tinting & exhaust listed and they didn't give me a hassle when I originally told them I had extactors (I didn't have the headers but I told them in advance so I didn't have any problems, I changed the exhaust specs later.) I'm paying somewhere around the 500 mark if I remember correctly! Only thing different I can see is that I'm in Albury. The car is under a novated lease, but still listed as private not business.

 

[sabrelli]

maybe i got a trainee when i rang them up!
damn 500 for insurance...so cheap even thou ur all the way down there on the border!

 

[chipa]

It is a guess but I'm fairly sure its quite close. I'm not complaining
But I am a bit worried what will happen if I ring them up to re-add the headers....

 

[sabrelli]

hahaaa, well im gonna ring again later, if i get same answer...then its confirmed.
AAMI doesnt like headers.

 

[coldie123]

I wish i could get insurance for $500 too
Same story for me, 26yrs old, perfect driving record for 9 years, and aami refused to insure me for any mods, even 17" mags. So I went with Just Cars, only $150 more than AAMI and best of all, unlimited mods =)))))
GIO is also pretty good, they have better coverage (e.g. free windscreen replacement and contents insurance), but they're about $250 more than AAMI.