I've been inspired by what gmonson and GtoRx7. are using for exhaust on their NA 20Bs, making surprisingly big power. So as the time nears to install my 20B, thoughts of exhaust and how to run come up.

I wonder has anyone ran a long primary exhaust on a 20B? Is the prospect of having to fit three separate pipes under the car a little unpleasant? Have only race teams had the opportunity thus far? Anyone ever done it on a street car, or as gmonson refers to his car, a GT? I like that.

Now for some car and engine specs.

Car is an 81 FB
trans is an S5 NA with FB tailshaft
engine is a very early 20B, #756 with the so called weak castings and shaft
streetported with about 2mm earlier opening secondaries and 1/4" later closing (pics in another thread), primaries slightly later opening and same closing as secondaries. Should idle kinda lumpy like gmonson's
compression is stock at 9.0
atkins 2mm apex seals
NF01 corner springs (purchased from Mazdatrix as they sell the real deal)
competition outer oil springs

now for the exhaust specs.

RB 3 rotor engine flange http://www.racingbeat.com/RX7-1975-1...ges/16184.html
RB disassembled road race header kit http://www.racingbeat.com/RX7-1975-1...ers/16212.html
three old school Rotary engineering glasspacks 26" long
alternative option of two repacked/rebuilt RB 2" presilencers originally from a dead "streetport" center section http://www.racingbeat.com/RX7-1975-1...ems/16398.html
RB 3" universal muffler http://www.racingbeat.com/RX7-1975-1...nts/16030.html
RB 3" universal "large" presilencer http://www.racingbeat.com/RX7-1975-1...nts/16434.html
two of these 4" round - 3" core Magnaflows melded together http://www.magnaflow.com/02product/s...e=main&id=9242
about four of these 3" flanges http://www.mazdatrix.com/getprice.asp?partnum=16192
already fabbed over the axle pipe in 3" 14 gauge 409 180 degree u-bend (weird they don't list it anymore, so this 45 degree bend will have to do) http://www.mandrel-bends.com/catalog...-bend-678.html

infact the RB 3" universal muffler is already installed in the car, as is the Magnaflow 4" round 3" core Magnapack in the mid location (right in front of the rearend), but this combo is way too loud. It makes so much bass it actually drowns out other important sounds. This has to be changed. It's the reason for this thread; to bounce ideas off the more exhaust minded here.

Here is what it currently looks like. Way too loud! I wouldn't recommend this.


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.


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.


Magnaflow Magnapack 4" round 3" core. Two welded together to make it 22" long. Bullet end.


Large RB universal presilencer. Nice 30° collector into 3". This collector is temporary while the 2 rotor is in there.


Magnaflow (Magnapack) 4" round with 3" core 14" long.



I chopped the ends off of two and made a single 22" long. It maxed out the space under the car.

Still not enough muffling. I'm junking this exhaust and starting over. I'll probably keep the muffler with its over axle pipe though. It turned out nicely.

 

I've got a positive story to relate. My friend's REPU had a crappy exhaust before I changed it to a long primary system. The old exhaust had a GSL-SE exhaust manifold with a 3" dump pipe welded to it (interesting that they did a decent job welding steel to iron, but it flowed really bad). Then it proceded under the cab in a thinwall 16 gauge 3" tube back to an RB large universal presilencer. Then up over the rearend to a supertrapp motorbike muffler. We took off the spark arrestor cap which gained power, but also lots more noise. It droned something fierce on the freeway or if you're standing next to it at idle. Kinda like my car! So I swapped in an RB road race header, two medium rotary engineering glasspacks that are kinda old, then a 2" to 3" collector and on to the RB presilencer as before. The results were stunning. Way quieter than before and can rev more freely now. It only has bass right at the tailpipe now. The rest of the truck is still kinda loudish, but far better than it was before. In all a smashing success considering it's got only minimal muffling in 3". The primary length is 50" to 51" to the crotch of the collector which is like a half octave of the recommended 100" to 102". Thanks j9fd3s for that little suggestion!

I'd like to experiment with a 20B long primary. If you've all seen what an RB streetport center section looks like:

I'd like to rebuild mine now that the presilencers are repacked, but add a 3rd pipe and presilencer.

Unfortunately I've only got two presilencers, but I do have three RE glasspacks but they're 26" long compared to the 20" long RB presilencers. They might be too long. I might be stuck using both RBs and one RE due to space constraints. I also require a quiet center section because the 3" universal muffler doesn't muffle as well as a 2.5" or especially the powerpulse main muffler. Have you heard a standard RB center section without the powerpulse muffler? It's scary loud! However with the rousing success of the REPU's exhaust, I'm feeling confident that I'll solve the noisy center section problem and the noisy bass problem associated with 3" exhausts, at the same time!

Part of why I'm confident is the two RB presilencers were repacked with not only the standard pot scrubber material that RB uses, but also a layer of coarse SS wool all the way out to the shell, making them more like standard straight-through mufflers . You see RB uses pot scrubber material only and as such only muffles the high pitched metallic noises inherent in rotary exhaust, but they make lousy mufflers for any mid to low frequency noise. They last a long time, just dont muffle much. They rely on the powerpulse main muffler to do 90% of the muffling. I don't have that luxury unless I shell out $315 for a powerpulse and hope the slightly restrictive nature of it doesn't kill my high end.

On the flipside if I had left my ports stock, I could have gotten away with a 2.5" exhaust and probably been happy with it... for a while. The stock ports and stock TB/UIM run out of breath at around 5 or 6k, so I'd have a lot of low end and midrange, but not much above 6k. Now as a streetport I have to go 3" exhaust and will deal with the restrictive TB and UIM later, like Logan (GtoRx7) and gmonson did on their high flowing NA 20Bs making gobs of power.

Thoughts?

 

By the way just for completeness sake, here is a graphic I whipped up to show the Magnaflow mod. I later modified the bullet end to be a slip fit for heat expansion so it won't buckle in use on a rotary. Cool idea, just too loud for an NA. Would be perfect on a turbo though.



And here is a graphic for the current exhaust that's just way too loud for the street.


This whole exhaust would be perfect for a turbo. Just swap out the header for a turbo and it would be the right amount of volume for the street.

 

This graphic shows the proposed long primary exhaust.



Think it'll fit? Think it'll work well?

 

Seems long and heavy. How much does all these extra pipes and mufflers and **** weigh? How much more power will you get over using a "normal" header and one big pipe under the car like a normal car?

 

I'd imagine the power increase would be similar to comparing a Racing Beat single pipe system to their dual pipe "streetport" system. The difference is striking. Noise is also a serious concern as the current full 3" system is unusable.

Yes it does weigh more. As much as 20 pounds over the RB streetport center section, or whatever the third presilencer and length of pipe weighs. However it's low and in the middle of the chassis. Probably about the same as relocating a group size 24 battery to the passenger storage bin. Those are like 40 pounds.

I'll weigh the parts later when I get a chance.

 

yes! we did it and since the catalog makes such a deal about the presilencers, we expected it to sound nice and the volume be reasonable, but no....

your graphic is KEY BTW, fitting THREE presilencers under the car is going to be interesting, the length is ok, but its gonna be WIDE.

if you look at logans headers, he makes spaghetti in the front, so that he can get the header length he wants AND fit mufflers. it does seem a longer exhaust is better, i was thinking about going sideways, like the RB but more bendy. its a simple idea really, but i almost want to make two "L" shaped header tubes, and one goes on the front rotor, and the other gets inverted and goes on the rear rotor.

the only problem is that the rear rotor's pipe can't go far enough out without hitting the steering box go make it equal, so if they both make an S instead of an L, it might work. plus this gets me a longer header* but closer to the front of the car, so i get more room for megaphones, and mufflers and presilencers and post silencers and the drum section....

*it may well be that the engine wants a short header, in which case the above is all moot

 

Are you talking about Mr Peepers wanting a short header?

But yeah, on the graphic I just whipped it up reall quick like in MS paint to show what I'm imagining. But the header in reality may end up curving differently.

I'm not intertested in perfectly equal length headers the way Logan did them. I think the way RB did them for the road race header kit will be good enough. Heck I'll have long primaries so header length is less important than if it was short primaries. This leaves me with more room to fit the 3rd header pipe.

I have the two pipes from an RB disassembled road race header here to play with. We know RB bends these in such a way to fit just fine in the 1st gen chassis and just welds an engine flange and a small dual pipe flange on the other end (in a jig of course heh). So what I'm thinking is for simplicity's sake, I could take the front rotor pipe and move it forward to the front rotor of the 20B (that's 16cm) so I'd probably have to add some straight length somewhere in the middle. Then same thing for the rear pipe, move it to the middle rotor and have to add whatever in length (13cm I guess) leaving the rear pipe for me to fab from the u-bend that's included with the header kit.

The other option is to keep the two header pipes in the 13B position and fab the far front pipe from straight and u-bend pieces. It really depends on where I've got room when the two header tubes are in there.

Third option is to leave the rear pipe in the rear, move the front pipe to the front rotor and fab the middle pipe. Whatever fits.

I also have a fully welded RB road race header that I can throw in there first to get a good visual for where to add the 3rd pipe.

But like you said it's gonna be wide under there. Plus I only have two presilencers and three glasspacks; the car currently has one 26" glasspack in the cat location and it actually fits really well. It's far too long to fit in the mid location though. Max length back there is 22". I know because I maxed it out with the Magnaflow 4" round. So a 20" presilencer will go there. plus the two inches will be useful in coming up with a 3 into 1 collector before the flange.

A new thought just occured. I use the assembled RB road race header and then use one pipe from the dissassembled kit for the front rotor. Simple. Elegant. This leaves me with the other pipe which should contain enough mandrel bent curves to hopefully complete the whole center section without having to buy anymore u-bends.

 

yes exactly! i think the front rotor pipe gets a straight shot, and the other two bend, and it'll be close....

so with peepers, the BOOK wants a 89" header, which is fine with me, but i think i'm going to go to the dyno and cut pipes until the ENGINE is happy.

and if the ENGINE is happy with a short header, then I get more room for mufflers, which my neighbor will be happy with, so when she comes over and says does this thing have mufflers instead of saying, yes its got 3, i can say something else

 

I already have a prepared 3 rotor engine flange. It has a set of 2" tubes welded with .035" mig wire (I had a local rotorhead named Lloyd do it for me). Because the wire used was larger than what RB uses, it caused some shrinkage/warpage to the flange. I had to relief cut it between the front rotor and middle rotor sections so the stud holes would fit again. I also had to correct the warpage so it sits flatter on the gaskets.

Turns out the relief cut was a good idea because now I can easily use it with the fully welded road race header and you're right, the road race header tends to curve kinda close to the engine, but the front pipe has a 2" straight shot and then there is some additonal straight pipe on the disassembled header tube. That's like a 3" or 4" straight section exiting the port. It'd be nice if they were all straight for several inches. Oh well.

Youe you should get some cheap aluminum 16 gauge tube so cutting is really fast and easy. Then when you're happy with a length, redo it in thickwall. But remember the thickwall will have a smaller ID. Hmm if you're **** I guess you could do the tube cutting experiment with thickwall. Just more expensive and takes longer, but it wouldn't need to be redone.

 

well i need about 6' of pipe to do long primaries. so if i buy 3 4' RB pipes, and cut one in half, i get the following lengths without having to cut anything

header is about 26", collector will say has about 2" or pipe.

so i get, header + collector = 28"
add a 2' section and you get = 52"
add the 4' section and its = 98'

with no cutting!

 

That's using your noggin! I thought you'd have to test it in 1 foot increments or something.

 

yeah i had a dream about it last night (ive had dumber), and if i can clamp together a couple combo's like that, it should save some time, and then hopefully it'll give me some direction!

 

Will be intresting to see results

 

Thanks McCarthy. How loud is your exhaust?

 

In the interest of equal time, I have an older idea to express. This one involves going with a short equal length collected header, collecting into 2.5" which megaphones out to 3". Then on to my RB large universal 3" presilencer, then the Magnaflow 4" round and so on.

Now because the RB 3" universal presilencer has a shell of only 18 gauge thickness, it's way too thin for NA use, unless you like a lot of noise leakage. Same for the 16 gauge shell of the Magnaflow. I 'd go ahead and cut the 18 gauge shell off and replace with a 14 guage length of sheet steel lightly bent to follow the curve of the end caps. Likewise the Magnaflow would simply receive a 4" 16 gauge tube, slotted and spread lightly to fit over the existing 4" shell , then tacked in place.

The above could work rather well but only if the noise level at the tail pipe is acceptible, as it would remain unchanged. The mods listed above would only address the noise leakage concerns/complaints into the passenger area and outside the car.

Having had others drive the car around the block and listening for how loud it sounds over a bit of distance (so the noise leakage from the presilencer and magnaflow are no longer a factor), I'd say the noise levels present at the tailpipe are still too loud for comfortable around town type driving.

So it would appear my only options are a turbo, or my long primary idea hoping the two repacked presilencers and/or three glasspacks do a better job than actual RB presilencers in the center section, and that the main muffler muffles the rest of the noise adequately. If not I'll be forced to purchase an RB powerpulse main muffler. Just wondering if I should purchase it now and get the pain overwith. Like pulling off a bandaid, it only hurts once.

 

I only had it running sortly because of the faulty mircotech but it wasn't as bad as i thought. Not that that helps you really !!!! If i get mine ready before you do this i'll throw some DB levels your way

 

I would appreciate that!

 

i still find it a bit weird that yours is really loud INSIDE the car, and mine isn't. its loud OUTSIDE the car, but its fine inside.

well its kinda loud inside now that the mufflers are broken....

 

Maybe it's because mine is an S model and doesn't have the sound deadening a GSL has.

 

mine was too and it was even the same blue as yours!

whats the build date? mine i think is an 8/81? last 6 is 610627

 

1/81 520986

It's pretty early, like the engine!