NA 20B header collector with expansion?
 

I've decided to ditch my old plans to run a 2.5" system in favor of a full 3" system on my NA 20B.

This is the header as it sits.

https://www.rx7club.com/forum/attach...d=230548&stc=1

The question is whether to collect at 2.5" and then expand to 3", or simply cut the collector up a ways where it's about 3" diameter and add on the 3" pipe from there?

Thanks.

I think I'll go with collected 3" like gmonson's Burns collector.

That's actually a very good question. A 2 rotor works fine with an area equivalent to a 2-1/8" ID pipe but would a 3 rotor work good with a 2-1/2" pipe area? I'd like to think it would but now we get into the fact that the pulses are spaced every 120 degrees rather than every 180 so we'd need a larger pipe to accomodate it. I don't think you'd have any issues with a 3" pipe. A 2-1/2" to a 3" expansion would probably work fine too. Each will probably have some advantage over the other at some point but without testing it's hard to say where with any degree of certainty. Often times simplest is best. We know 2-1/2" is great for a 13B so I say go with the 3"

2 1/4" ID collector is the well known size that works well for high hp NA 13B's, I think some where between 2 3/4" and 3" is your sweet spot for a 20B.

You should also think about having a 4" expansion area after that for the best results. :)

And after that you can split it to dual 2 1/2 mufflers for quietness..

Awesome. Thanks guys.

Engine is a never opened early (we're talking very early) #756 so we know it has the weak shaft and side plates blah blah blah So it's staying NA at least for now until I can rebuild it. Then I want to port it just enough to make 300 hp NA like gtorx7. I feel if I spend time researching the exhaust now, it will still be adequate after the port job, even if maybe a little big on the current stock ports. I'm certainly willing to try it and compare to the old system in the GLC.

The header you see above was a creation I came up with to fit in the GLC. The only exhaust components I had easy access to back then were some 2" RB thickwall pipe and RE glasspacks, so I gave them a try. :) It sounded decent. That pipe size actually worked reasonably well on the stock 20B ports, believe it or not. The MegaSquirt wasn't tuned yet so it didn't really have much above 5k, but according to j9fd3s, that's not too far off the mark for stock ports. The low end torque was phenominal. There was no way to cool it though, and working on the engine in that small bay was a chore. I pulled everything out and went with a ported 4 port 13B, and even that tended to idle hot in traffic on warm days. Since then I've swapped in a bigger fan and a Camden 5" SC; so now, at least when it's not boosting, it should run cooler.

Oh yeah, I should mention it's going in a 1st gen this time. :)

'81 S model with GSL (LSD with disc brake) rear end
Modded crossmember to clear the oil pan
Still need to fab a front engine mount, still debating solid mounts or competition motor mounts
NA tranny with RB 225mm light steel flywheel, 225mm SS pressure plate, 225mm HD disc (expecting pedal effort to be about a 7 or 8 on a scale of 1 to 10 where 10 hurts the ball of your foot and 1 is light enough to push by hand easily)
Walbro 255lph (got from eBay in '05 so could be a cheap chinese knockoff)
GSL-SE gas tank
MegaSquirt ECU
Gonna try a stock short radiator and front mount oil cooler because they are known to be of decent capacity
2" primary header pipes collected at the correct 30° angle into a 3" 14 guage pipe (probably).


Still debating which exhaust components to use. We know as pipe size goes up, so does the noise. With 3" I'll need to add an additional muffler right before the rear end. I'm thinking it'll go like this: header --> collector --> 3" RB universal presilencer --> short length of pipe --> 4" or 5" round Magnaflow muffler with 3" straight through core --> 3" mandrel bent curve up over the rear end --> RB universal 3" muffler --> some sort of tip under the bumper, or maybe a curved dumpy if still loud.

The cliff notes from above is this parts list:
RB 3" universal presilencer in either medium or large (whichever fits).
http://www.racingbeat.com/photos/16434.jpg

Magnaflow round muffler with a 3" core. The outer shell size is 4" or 5". I need to research this better.

Lastly, the final muffler in the system will be the RB universal 3" muffler (the one they recommend to be used with a turbo, and the 2.5" version with NA, lol).
http://www.racingbeat.com/photos/16030.jpg

Knowing what I want, I know what I don't want out of this system. I don't want drone on the freeway. My old system was loud inside the car and I won't have that again. That's why I mentioned 14 guage pipe above instead of the usual 16 guage that everybody going 3" uses. I'd rather use some thicker .120" wall stuff if I can find it locally. It's twice as thick as the stainless 16 guage .063" stuff that RB sells with the caviat that their stuff be used with a turbo, knowing that a turbo kills much of the noise that would leak through it. As I will be NA, I feel thicker pipe is necessary to prevent droning inside the passenger compartment, and I'm willing to accept the weight penalty. Then again we're only talking maybe three feet of the stuff spread out between muffling components, running nearly the whole length of the car. Better yet the thick stuff is probably only necessary before the first presilencer anyway.

I think I'll look for a 2 or 3 foot length in the .120" wall stuff locally and buy a 14 guage mandrel bent u-bend from a company that PercentSevenC found, which also sells 16 guage stuff but no 12 guage (or 11 guage - whatever .120 is lol it's late). I bet I can find .120" in 3" locally...

3" it is!

https://www.rx7club.com/attachment.p...1&d=1273091188

Sorry about the welds. Best I could do with a flux core welder.

The lump you see at the bottom is a nut to allow different inserts held with a bolt.
https://www.rx7club.com/attachment.p...1&d=1273438644

Over the axle part worked out pretty well. Those are air pump brackets. Lots of good metal in them. Special thanks to Lloyd for doing the welding on the 3" flanges. :)
https://www.rx7club.com/attachment.p...1&d=1273438644

Magnaflow Magnapack 4" round 3" core. Two welded together to make it 22" long. Bullet end.
https://www.rx7club.com/attachment.p...1&d=1273438644

Large RB universal presilencer. Nice 30° collector into 3". This collector is temporary while the 2 rotor is in there. I'll swap to thickwall later on when I redo it for the 20B.
https://www.rx7club.com/attachment.p...1&d=1273438644

Magnaflow (Magnapack) 4" round with 3" core 14" long.
https://www.rx7club.com/attachment.p...1&d=1273440600

https://www.rx7club.com/attachment.p...1&d=1273440600
Not enough muffling so I chopped the ends off of two and made a single 22" long. It maxed out the space under the car. :D