Just wanted to note that the HKS downpipe MAY NOT be an appropriate modification for tracked street cars or extended HPD use.

At a recent HPD event at Thunderhill West, the downpipe split cleanly at the weld between the larger flange and the pipe. I suspect an initial crack developed around my 3rd 20 min session and during open pit, after around 4 laps, the pipe broke completely off and I very suddenly had an open pipe FD. I was very lucky to not have been going over any berms or the pipe could have easily caught and resulted in a pretty catastrophic situation.

I was able to take the DP off and staff at Thunderhill gave me a rough weld on it to get me home without blowing out my ear drums for the entire 2.5 hour drive back to East bay.



Car mods and history:
DP installed since 2016 but less than 3000 miles on it
However, has seen a number of HPD events in the past 12 months; around 6 track and 2 drift days

-PowerFC tuned to a little over stock (265)
-hi-flow cat
-racing beat catback

I see Pettit has a SS option which should have much greater resistance to heat than mild steel. Anybody ever have this experience with HKS or have any good alternatives in downpipes?

 

I think this may have been an isolated incident.... I wouldn't write off hks for this.

Did you have it bolted to the trans?

 

did you contact HKS?

 

Interesting, very rare I'd think for it to fail on that weld.

Also a little concerning as I'm about to fit one of these to my current build! Just looking at the welds now, penetration is excellent, I cant see how it could fail really.. As cr-rex says, was it braced back to the gearbox?

 

That's a super common downpipes and I've NEVER heard of one failing.

What do you have for the rest of your exhaust? A failure like that doesn't happen due to heat, that's from some sort of mechanical thing happening.

For anyone else reading this, there is nothing wrong with HKS downpipes. It's one of the better ones you can get for the FD. They've been selling them for well over 20 years and I've NEVER heard of a problem.

For OP, it's either just a fluke or something else in the system fatigued the downpipe at that joint. Very strange.

Dale

 

The rear bracket was bolted down.
The rest of the exhaust is RX7 Store hi-flo cat and a RB dual tip catback.

I was also very surprised at the failure but only have my own experience in this situation to go by. I wonder if the mild steel could be the culprit? My car runs fairly rich for safety due to use of the stock IC (distilled water AI installed but my IATs are still extremely hot) so I wonder if the excess fuel burning up in the exhaust may have over heated the metal. Not sure what the specs are for the HKS downpipe material.

Looking online, I see a couple alternatives in SS but am not confident that they'll match the fit and finish of the HKS downpipe.
Does anyone have experience with the Bonez or Pettit SS downpipes?
They look to have the O2 bung located in the same place as the Megan Racing DP, which I have heard is very in the way, but I'm only going by rough pictures online.

I did reach out to HKS just regarding what applications the DP should be appropriate for and they stated:

"The product of your inquiry is a track oriented part. 3103-RZ001 F-PIPE FD3S D75 MSRP: $240"

 

If you don't have a flex joint in your exhaust system, then you should get one. Exhaust systems needs to flex or the weakest point in the entire system will break and that's usually at a welded joint on headers or downpipes. Most stainless technically isn't even rated for constant 1600-1700F heat, but we seem to get by fairly well. Hopefully it was just a freak incident as HKS makes high quality stuff.

 

I'd let them know about the failure and ask what they recommend. Maybe they replace it.

 

I'd avoid rigidly mounting the rear pick-up off the gearbox if there's much track use. Found that stainless twin downpipes, can have growth enough to bend and break the mount, munt the bolt and blind hole...and crack the turbo flange after several laps too. A mild steel one should be less prone to that, but I still wouldn't screw directly into the box, if that's what is occurring.

 

Yeah I've never used that mount to the gearbox, I don't think it's necessary. That could have been a contributor.

In all fairness I'd get another HKS downpipe. I think you had a fluke and also it could be having it bolted to the gearbox and too much heat caused the failure. Just guessing though.

Dale

 

Yes... I agree. This will likely never happen again. Get another hks

 

For what it's worth, my SMB DP was bolted at the transmission and the bracket broke off at the welds after some use. I left it like that.

 

That is an anomaly
Both Bonez and Pettit are great options.

 

Thinking there would be as much evidence for increase risk of exhaust gasket failure by not using that downstream mount at the transmission, as there is evidence that using it was the cause of the OP’s downpipe failure.

 

I don't track my car, but now I'm worried. I have a stainless steel Pettit downpipe. It came without any kind of rear bracket. I asked Cam whether the back end of it needed to be attached to the transmission like the OEM pre-cat was. He said no, it was so light that it didn't need it. Later, I worried that the flexing of the exhaust system vs. the engine could put all the strain on the downpipe's (spotty-looking) weld at the turbo flange and lead to failure there, so I fabricated a clamp/bracket out of stainless steel and attached the back end of the downpipe to the transmission. It never occurred to me that the downpipe would grow and shrink enough from heat to matter.

 

@Retserof
I don't think you should be too worried.
I think my case is a pretty rare occurrence and most likely due to a combination of the following factors:

• Aftermarket engine mounts (Banzai Racing) that shift the engine position slightly that may have led to additional stress at the weld joint
• Extremely rich tune resulting in excess fuel combusting in the exhaust that led to higher than normal temps
• Long periods of sustained boost at redline further exacerbating the temperature issue
• Thunderhill West has a few berms that you have to go over at very high speed, which coupled with hard set Ohlins, probably jolted hanging components like the exhaust quite a bit

Am curious how the Pettit downpipe is though. Do you have any issues getting a 14 on any of the nuts? Is the O2 sensor position in a good place?

 

Thanks for the reassurance. I got the downpipe as part of a group buy and put it on 16 years ago and don't remember much, but took notes. I seem to recall that Pettit supplied mounting hardware with the downpipe and included Allen-head bolts (cap bolts) to replace the 2 lower OEM studs. The bung for the O2 sensor was easy to reach and had no clearance issues. I don't know if Pettit sells the same pipe today. Anyway here are my notes:
Using a 10” breaker bar on a 14mm deep socket, I was able to get the nuts off the top 2 turbo studs and one off the bottom. The 4th stud, lower front, came out of the turbo housing once the nut was backed off a ways. I removed the other (rear) bottom stud to create adequate clearance for the new downpipe.

I had a clearance problem getting an allen wrench on the cap bolt in the bottom rear position of the downtube where it bolts to the turbo. I obtained a ball-head allen wrench socket in 8 MM and was able to get the 2 cap bolts to proper torque (36 ft-lbs). I used a regular or deep 14 MM socket with the torque wrench on the new self-locking nuts on the 2 original studs on the top. I installed the original O2 sensor in the downpipe without any problem.

 

I bought the Pettit DP in 2012 and have put 30k hard miles on it, some track miles. The turbo flange fits 3 oem inconel studs and Dale Clark's copper lock nuts and washers just fine, the fourth hole requires a stainless or titanium allen key bolt. That bolt is on the bottom left corner behind the pipe so the socket head cap bolt also requires Bondhus Stubby L Wrenches to get a proper torque on it.
I have 1 other complaint about the Pettit pipe. Fitment is good, but if you wrap the pipe it will rub on the chassis.

 

I wrapped mine and it does not rub. Where does yours make contact?

Bottom view.

 

It's been a while since the car here had twins, but for the current and past exhausts, we used a rubber strap hanger (fibre reinforced) to support and cope with any movement. They should be available for a few bucks from any exhaust shop if concerned.

Like to have more rather than less support in most circumstances here. Ripple strips and high speed compressions and a bit of G can overcome the mid and rear exhaust rubber mounts we've found......stresses the whole system with a muffler dragging around on track.

 

American-made downpipes like Pettit's use a mandrel-bent pipe welded to a flange that bolts to the turbos. Problem with that is access to 2 of the fasteners is a chore, many times you have to remove the stock studs and replace with nuts and you have pretty tight clearance to work with it.

Japanese-made downpipes all typically use a cast top piece that bolts to the turbos then a mandrel bent pipe welded to it. The reason is RHD cars have a steering shaft right there and it's near impossible to bend a pipe to clear it.

Also, the nature of the design of the Japanese-made downpipes mean you have a straight shot for all 4 fasteners holding it to the turbo. That means easy access with a socket and ratchet, no turning it a few degrees at a time with an open end wrench.

From what I can tell over the years there's little to no difference performance-wise between the 2 downpipe styles.

Dale