My HKS decat downpipe on my STi V3 has corroded (mild steel..) so I'll probably replace it - might be able to repair. The HKS d/p is a closed neck d/p and I'd have thought an open necked pipe would deliver better performance. However my STi V3 managed a very credible 326bhp@7,500rpm recently, only mods being full decat and K&N filter. Don't want to screw this up by fitting an open necked d/p - anyone shed any light on the performance differences between the two types?

Ta.

 

IMHO:
Very little.
The open neck id fundamentally better when the waste gate is open/bleeding excess but against the huge flow of gases from the main turbo outlet i doubt it interrupts much.
My dp on my mild modded Sti v3 is close to closed and runs 380 x 360, even managed a 407 x 380 once!

The modern St Steel open necks are cheap enough from many sources, so I would buy one, wrap it for at least 75% of it's length and bolt it on tight.

Graham

 

Closed neck here too. If you have further power aspirations it might be a good time to change to open-neck.

 

Thanks chaps, I think I'll opt for one of the cheap stainless one's this time. Probably one of the 3" open necked one's. Had hoped I could weld up the HKS pipe but having removed it I don't think it's worth patching - there's a lot of corrosion that'll just give me more bother in the future.

One Q though - the exhaust temp sensor at the bottom of the downpipe, most of the stainless one's don't seem to have the threaded boss - was this summit that was particular to the V3?

 

My dp has a threaded bung for the gas temp sensor, but if the dp is decat i think it is redundant.
You need to ask a mapper IMHO to be sure.

email AndyF?
Graham

 

V3 and early V4 Sti's had a sensor fitted at the bottom of the downpipe but you needent worry about having it in.

Russell

ps - how's the Legacy? Running well now?

 

Cheers chaps, realise now this sensor is only for warning that the cat is on fire, now added to my ever growing list of bits that Subaru fitted that the car really doesn't need....I've a cupboard full of heatshields and standard air boxes etc.

Russell, Legacy's running fine now thanks, shame you couldn't manage the SIDC rr day in Ellon the other month, would've been nice to see your car on the rollers. My v3 STi managed a credible 326 bhp - not bad for a standard car that's been a daily driver for the last 7 years. Watching the threads on SIDC as well - my advice is to ignore what folks are saying..