KnightSports and AutoExe twin tube downpipe comparison
 

A week ago there was a post about a hood deal on an autoexe downpipe. After first not being able to find it, I realized it was listed as an exhaust manifold not a downpipe, so I found it and bought it.

couple years ago I had bought 2 knightsports twin tube downpipes of yahoo Japan auctions. No real reason other than it was a good deal and I’ve always thought they looked cool.

I had wanted a knightsports for my FD, but at the time, looked like they no longer made them, so AutoExe was the alternative, but they have been out of stock for a couple years now too. So maybe they are no longer available new.

so now that I have both in hand, there are a few differences.
1. knightsports has larger diameter twin tubes
2. knightsports merges to 3”, autoexe is more like 2.5” (I didn’t measure precisely)
3. Merges in slightly different locations
4. Head unit where it attaches to the turbos is made differently
5. Flanges are a little different. On both, I found the flanges to be thinner than I would have expected. Not like a Pettit downpipe with a 1/2” flange.
6. one of the bungs is in a different location. Not sure what the small bung is for. O2 bungs are about same location.


https://cimg7.ibsrv.net/gimg/www.rx7...fe9b75bb7.jpeg
Knightsports on the right, autoexe on the left.
https://cimg8.ibsrv.net/gimg/www.rx7...62a8c81a8.jpeg
https://cimg9.ibsrv.net/gimg/www.rx7...1b1130dde.jpeg
https://cimg0.ibsrv.net/gimg/www.rx7...1d4898404.jpeg
https://cimg1.ibsrv.net/gimg/www.rx7...e11a7a670.jpeg
https://cimg3.ibsrv.net/gimg/www.rx7...b8a965a97.jpeg
https://cimg4.ibsrv.net/gimg/www.rx7...327bc1b9b.jpeg
https://cimg6.ibsrv.net/gimg/www.rx7...7d3d96608.jpeg

One of the things I like about the KS is the collector is a smoother transition and the cast upper section is a bit larger. Still have the dip for the steering rack on the rhd cars. I believe the small bung is for an egt sensor. They were a popular add on over there on the island. For some reason, a lot of the ks twin tubes have had the lower support bracket cut away. Not sure if this was the people at the yard just being lazy, but it’s a common theme I’ve seen with them on the used market. They weight a bit too. Is the autoexe the same weight?

Link to my thread on the KS exhaust. Had some pictures of mine that’s been restored and coated. See post #20

https://www.rx7club.com/3rd-gen-gene...ation-1155590/


~ GW

Back to back dyno comparison coming next?

haha. Probably not. I doubt there would be a noticeable difference.

Nice, this is in my planned upgrades if I can ever find one for sale without being an insane price. Are you going to get them blasted and coated? I'm curious to see how well they clean up.

I'm going to have 1 of the knightsports coated and use it with my BNR twins. Either JetHot or Embee (waiting on quotes now).

Will likely sell the rest. I think they'll clean up well, no signs of deep corrosion that I can see. They also make nice garage wall art haha, but should probably just sell and declutter

I have a Knight Sports dual tube on my car, have had it for some time. It replaced a Bonez 3" downpipe.

I SWEAR I picked up power going to the dual tube seat-of-the-pants wise. I don't have any data or anything though.

BTW I had my downpipe coated in Jet-Hot 2000 ages ago, it still looks brand new. That's also an awesome thing that can be done with old downpipes. That Bonez I had I got cheap because it was "dirty looking" - I think I paid $50. Coated it and it was better than new.

BTW it's worth putting an extra O2 bung for a Wideband on before you get it coated, trying to get that coating off to weld a bung on is NOT FUN and borderline IMPOSSIBLE.

Dale

I went with jet hot 2500° on mine. It looked similar to yours in condition before coating. I’m pleased with the results. Fantastic company to work with and communication was excellent.


~ GW

I did a TON of research on these duel tube DPs a few years ago, and it's a fun rabbit hole to go down. I'll add some of my findings:

First, the AutoEXE is made by Maxim Works, and last I checked, you can still get both new. The other outlier is the Mazdaspeed one. Bryan did a comparison of the sizing of the piping many years ago:

Bryan definitely knows his stuff (I've bought several things off of him in the past) and I mean this as no disrespect, but I think he is wrong on the MS. When I went through my library of HyperRev and Carboy mags, the MS pipe was always listed as 54mm X2. Closer inspection of the images used was always peculiar to me as well. The promotional images for the MS DP were always the same as the AutoExE/Maximum Works images.
It is my belief that the Mazdaspeed was also made by Maximum Works. I believe this was confirmed in a conversation I had with someone who had the MS version of the DP in their hand. We discussed several aspects of the DP and this is where my suspicions of the MS version being the same as the AutoEXE/MW version began.

Now, I could be wrong. Again, Bryan was/is super knowledgeable on this stuff. I have only actually held the KS version in my hand, so take that for what its worth. It is also possible that the person I discussed the MS version with was sold an AE/MW one and it was not a true MS (whether this was intention or not, it really wouldn't be relevant). So again, it may be possible that there is a true third MS version out there, but the evidence I've seen (both physical and circumstantial) lead me to believe there were only 2 truly different versions of the duel tube downpipe: Maximum Works and Knightsports.
[sidenote: I vaguely remember another company with a duel tube DP in some HyperRevs, but again the images were the same as the AE/MW version, so I think that was also just a rebadged MW. I can't for the life of me remember what shop it was advertised as though.]

Sorry if this is a little beyond the scope of this thread.

Interesting. I was planning just to install the haltech wideband into the bung that's already there. Now I need to go measure and see if it fits.

It fits in the stock narrow band location just fine. Chances are you will exceed the max temp rating of the Bosch and Ntk sensors, even with a heat sink. From experience it does last a while in the stock location but eventually fails.

~ GW

Yep, a wideband will lead a very hard life in the stock location. I have mine at the very end of the downpipe before the flange to the midpipe/main cat. It's much happier there.

In the stock location best case you will go through O2 sensors faster, worst case you will get the sensor too hot and the wideband will error out and stop reading.

BTW, when I said Jet-Hot is bullet proof, I'm not kidding. I took a bench grinder with a grinding stone to it and hit it HARD, ZERO change, did not come off. Had a buddy with a coated downpipe that wrecked the car, the downpipe was heavily dented from going over a concrete curb, the Jet-Hot was still there and fine. We actually got the downpipe back out into shape and the coating was STILL THERE and good.

Dale

Also the total volume of the downpipe being so much bigger (like equivalent to 120mm) makes a lot of sense with regards to making power.

Turbos LOVE huge exhausts right off the turbo. Turbos work on a pressure differential, you have high pressure exhaust gasses before it and low pressure empty volume after the turbine wheel. The more you can drop that post-turbine pressure the better it spools. I think having the "expansion chamber" type head on the downpipe is also a great thing too.

It would be nice if there were more of these out there but that is obviously not an easy pipe to make.

Dale

I've got KS one stashed away. Saving it for when I get old and want twins again. Maybe I'll never get that old, but either way, it's good garage art.

Turns out my mechanic was the first KnightSport dealer in Thailand back in the day :)

I did some math to see how these compard to each other, plus to a single pipe at 3" diameter. check my math/thinking but here's what i came up with (some comments copied form the posters above):

Knightsports:

• merges to 3”
• KS = 60mm x 2 = 120mm
• Cross section area of both pipes = 5654.86 mm sq

Autoexe:

• is more like 2.5” (I didn’t measure precisely)
• autoexe - 54 mm x 2 = 108mm
• Cross section area of both pipes = 4580.4

MS:

• 50mm x 2 = 100mm
• Cross section area of both pipes = 3930.6 mm sq

Pettit or other 3" single tube downpipes:

• Cross section area of both pipes = 4560.4 mm sq

So going purely by the area inside the pipes, the Knightsport one appears to be a clear winner. But wait, there's more!

You would need to take into account the losses due to drag slowing the gasses in the ones with 2 tubes. Need the mechanical engineers to weigh in on that, but I suspect the twin tubes will lose efficiency, possibly very significantly, compared to the the single tube ones. Also, all of these are limited by the 3" collector at the bottom (or ~2.5" for the Autoexe). Also, not sure what the splitters/joiners at top and bottom of the double tubes ones do to the flow. So having 2 pipes in between may not help much?

Not a comparison of twin tube downpipes, but I have an HKS 75mm downpipe and Knight Sports twin tube downpipe. I measured the ID of the distal opening of the of both and the Knight Sports pipe is a few millimeters bigger. I can't remember the exact numbers, but when I get a chance I will measure them again and post them.

cut the inboard half off both of them, some thick card stock sheet to form the pattern between the two outer halves, some 18 Ga. T321 sheet to copy the pattern onto and cut out with a jig saw, and weld them into a single oval. :suspect:
.

AFAIK the Auto-Exe Maxim Works DP and the MS DP are actually the exact same. Maxim works made it for MS. The mm difference reported is probably OD/ID.

pretty sure you are correct. Saw this earlier today.

https://cimg5.ibsrv.net/gimg/www.rx7...19a03937f.jpeg

Wow $1,000.

How much are KnightSport DP's selling for?

I don't think they make the knight sports anymore. And I wouldn't order the one above without first calling to see if they are in stock. I've found some of these sites have been out of stock for years.

I will have to do some measurements on my single large bellow SMB. As I remember, it is around 6-6 1/2" at the bellow (flatten however), 4-4 1/2" round at the start of the pipe, and slowly tappers to 3-3 1/2" at the flange. The dual channel pipes would have more flow (greater total cross section), but less flow efficiency (laminar vs non-laminar flow) as you split the flow and then merge the flow. As the turbulence increases, the velocity decreases. These dual pipes would most likely flow greater volume (cfm), but by how much would be interesting.
Mike


https://cimg2.ibsrv.net/gimg/www.rx7...09746ca327.jpg

I have a KnightSport one I pulled off my car. Considering having it cleaned up and sold, just wondering what they go for

Yep. I found one of the images I was alluding to in my post above
https://cimg4.ibsrv.net/gimg/www.rx7...6799acc7eb.jpg
shows the diameter as the same as the autoexe and also uses a picture that I've seen used for the autoexe as well.

Good info and don’t want to sidetrack. I also know some DP’s don’t have them and many haven’t had issues. But in the Autoexe’s favor, it does come with the mounting ear for the transmission support. Something i prefer in a standard DP and maybe worth mentioning…especially with the thinner flanges.

not a twin-pipe but I was surprised to see another big 4" SMB and it's noticeably different from mine so here it is compared to HKS.
My SMB is LHD-only as it doesn't have the steering column squished area, expands to 5 inches then 4 inch (100 mm) pipe until the last couple inches where it tapers to a 3.5 inch (90 mm) flange. Exhaust is 3.5 inch from then on.

So flow area of the 4" (100mm) section would be 7854 sq mm and tapers to 3.5" (90mm) 6362 sq mm

https://cimg0.ibsrv.net/gimg/www.rx7...ff58ee5c7e.jpg
https://cimg3.ibsrv.net/gimg/www.rx7...561b111bf1.jpg
https://cimg8.ibsrv.net/gimg/www.rx7...438e9fc4c6.jpg
https://cimg1.ibsrv.net/gimg/www.rx7...8aa58711d5.jpg

That's a beast

SMB has some top quality stuff. I have a high flow cat I bought from them like 15 years ago, still going strong.

I think to get the best out of a downpipe having an expansion chamber type setup makes a huge difference. Going large on the downpipe will pay off in any power level. Also the JDM downpipes seem to flow just as well as the US ones. even with the funky top to clear the JDM steering rack. Personally I prefer that design since it's a straight shot to get to the 4 nuts holding the downpipe on with a ratchet, you don't have to do 1/8 of a turn on a rear bottom nut slowly to get it off.

Dale

So turns out there is a laser cleaner locally that will take small jobs. I just dropped off the downpipes for cleaning. He did a test spot while I was there and it's just amazing. The surface corrosion just vaporizes. Will post pics when I get them back. I wish I had taken a video.

Dale, do you have any pics of your O2 sensor? I'm going to send one off for JetHot coating and will have a bung welded on before sending. But my engine is out of the car without turbos on it so can tell where the clearance needs to be. Any chance you have a picture and distance in relation to the lower flange that connects to the midpipe?

Also. When cleaning it, found a stamp that says Maxim on it. Wonder if there will be any Mazdaspeed or AutoExe branding...

Check out post #20 in the thread I tagged for pictures of placement. Between 3 & 9 o’clock on the inside of the collector. Thread is M18 x 1.5 for the bung.


~ GW

Thanks. Might go with that. For some reason I though Dale mentioned he usually goes down further by the rear flange.

also…. Results from the Laser cleaner.

before and after:

https://cimg2.ibsrv.net/gimg/www.rx7...42cb14fcc.jpeg
https://cimg9.ibsrv.net/gimg/www.rx7...fb8d8c414.jpeg

Unfortunately, the rear mount blocks you from doing it any further reward. I believe yours is like most I’ve seen which are missing the mount. So you could go further back towards the flange. Of course you could always put a bung on the other side of the connection in the midpipe or cat piping. Doing that will start to get into the trans crossmember area making the degree angle more critical. I run the banzai mount and it takes up more room than the oem one. Here are some pics of the downpipe installed that I found years ago when searching. You can have a better visual and figure out what would suit your particular set up better.

~ GW


https://cimg6.ibsrv.net/gimg/www.rx7...1be33b356.jpeg
https://cimg7.ibsrv.net/gimg/www.rx7...7d6efeb72.jpeg
https://cimg8.ibsrv.net/gimg/www.rx7...9ac444a1d4.png
https://cimg0.ibsrv.net/gimg/www.rx7...926c72f2d0.png
https://cimg3.ibsrv.net/gimg/www.rx7...21b6c5392.jpeg
https://cimg4.ibsrv.net/gimg/www.rx7...d1fa7964c.jpeg
https://cimg5.ibsrv.net/gimg/www.rx7...02a9df4bb.jpeg

Awesome thank you for the pics. Yes, 1 of the KS has the ear and 1 does not. The autoexe has the ear as well.

The further down the downpipe, within reason, the better. O2 sensors last longer and stay more accurate when exposed to less heat. This is according to an old friend of mine I trust.
Mike

Now that they are cleaned off. Can definitely tell a difference in manufacturing (not quality related, mainly superficial).

maxim/autoexe/Mazdaspeed is tig welded and it is beautiful. Knightsports is clearly mig welded and not as pretty. Maxim also has the JASMA tag, and a Maxim tag. No markings on the knightsports as far as I can tell.


https://cimg5.ibsrv.net/gimg/www.rx7...b80316ace.jpeg
Maxim
https://cimg1.ibsrv.net/gimg/www.rx7...18eae38f2.jpeg
Maxim
https://cimg9.ibsrv.net/gimg/www.rx7...95c14e92d.jpeg
Maxim tig welds. They look this nice on every weld on the pipe.
https://cimg2.ibsrv.net/gimg/www.rx7...42a0b3599.jpeg
Knightsports tig welded

Bump, does anyone know what a Yumex is?

https://cimg2.ibsrv.net/gimg/www.rx7...e9e86ad34e.jpg

the pics in the exhaust section are all Mazdas...