I want to go to 3" NA. I'm building a streetported 12A in the next couple of weeks. This is for my '81 FB. Is 3" too big for any 12A or just a 12A with stock ports?

The port size I plan to do is gargantuan. I can show you pics of the template and a couple of completed ports a little later.

I have all the exhaust components I need to complete the exhaust system in 3" from 12A road race header to 3" flanges and mufflers. It currently has a 13B road race header collected to a single 2" RB thickwall pipe and a mild ported 4 port 13B with a 7" Camden. It runs out of breath with a quickness! The SC and 13B are coming out for other projects to make room for the extreme ported 12A. I'll eventually go to a huge ported 4 port 13B and an RB holley 600, but for now the 12A is nearly ready to be built and it's been a while since I've done a nice 12A.

My question for you guys is about the wisdom of going 3" now for the 12A or waiting until I do the 13B. Will my low end, what little there is with a giant streetport, suffer at 3"? I plan to supplement the low end with either a stock flywheel (26 pounds) or an RB light steel (13 pounds) so I don't hate it for daily driver duties.

 

Some specs on the engine.

'83-'85 rotating assembly (the lightest stock assembly you can get for a 12A)
Y plates front and rear with a '74 intermediate plate.
Port size bordering on about as big as I'd want to go for a street car.
exhaust ports matched to sleeves
Holley 390 with open plenum spacer (so all 390 CFM are available for the engine to ingest)
rare 70s high flow intake manifold with or without channels (gotta research a little more)
Carter 110GPH fuel pump (the one with 1/2" fittings)
Holley 1 to 4 psi regulator
stock flywheel or RB light steel (whichever you guys think would make a better daily driver while still allowing quick reving and spirited driving)
stock or upgraded clutch disc and pressure plate combo that matches the flywheel I choose
electric fan for less rotating weight

One more question. I have access to an RB aluminum underdrive front pulley/hub thing. Should I use it? Will the waterpump and alt spin fast enough at low RPM?

 

Stick with 2.5". Easier and cheaper to fab, the 3.0 will just make more noise and less velocity.

 

I can't stick with 2.5" because I'm not currently running that size. Nor do I have the components to go to 2.5".

I've got one shot at going from 2" to 3". No in between on this project.

Future plan is an NA 20B, so 3" is inevitable. Just curious whether I should go to 3" now with the streetported 12A, or wait until I do the streetported 13B to build the 3" system.

 

I would go with the 3" then, seeing how your future is everyone's dream car

 

go with the 3"!!!! what are you gonna use ar a collector from the road race header? also what are you using for a pre-silencer and muffler? im planning my exhaust system now on my rx3 and i am torn between a 3" or dual 2"... i want it to free flow but i dont want the noise too awful loud!!!! im getting old i guess

 

3" eh? since its "only" a side port this is good.... noise is the problem with a 3"

my current 3" system is a longish (35" they aren't equal) primary, into 3" pipe. this leaves me with 2 mufflers (glasspack, oops and a borla), and its a bit loud.

i suggest either running a longer system (like 90" like the mfr setup), this lets you run mufflers in the primary pipes, its 2 pipes, so you get like twice the muffler under there.

also since its "only" a sideport, you can probably pick a quieter or even 2.5" rear muffler.

my system is built totally wrong (i don't know what i was thinking...) and its only a little too loud, it sounds like the gardener hit 88mph...

the new system, is 2" pipe into 2 looooong straight thru glasspacks, into a better collector at about 90" (if we have time we're gonna tune it on the dyno) and rear muffler will be removable, but stay the same for now.

 

Ah, well the end goal is for an NA 20B with a basic streetport where exhaust port sleeves are swapped to a set of T2 sleeves and ported out to match. This precludes the use of a long primary system. HOWEVER I am doing a long primary system in the REPU. Not sure how long it will be yet, but it will have two RB 2" presilencers side by side (not staggered like the RB streetport center section) to allow them to collect a little sooner so I have more room for mufflers. The REPU will collect to 2.5" under the cab and go into an RB universal 2.5" muffler, then I will test listen to it, and if it's too loud, I'll add another 2.5" muffler, possibly a Magnaflow under the bed. Yeah I came up with this idea about a week ago so I haven't had a chance to update my REPU thread yet.

As for my 3" system in the '81 FB, 73rx313b asked which components I was going to use. I'll start by saying the current system has a 13B road racer header, then through an RB dual pipe flange with dual 2" collected into a single 2" about 6" long and at the RB prescribed 30° angle (just like their collected headers), then into an RB 2" presilencer that's basically rotted out inside. After that it connects to a stock '83-'85 single stock 1 7/8" (or 2"?) connecting pipe that goes up over the rear end and then into a 2" Borla. It's not too loud.

PercentSevenC used this system for about a year with no complaints except it was a bit too small. It made the time while he was borrowing the 7" Camden somewhat unproductive above 6k. Even his Weber 45DCOE seemed to run out of breath at or above 6k. This system does flow, it just runs out of breath to early. But for a 12A, might be ok if it was stock ported or nearly stock. Since I plan to make the ports really huge, and have a stock flywheel to retain some of the low end driveability, I feel I can jump on the 3" sooner than later.

Oh, you'll like this part. The RB presilencer is on its last leg. Almost all the packing has rusted away. The outer shell was smacked and dented by a previous owner's failed driveshaft, too. It came from a dual pipe streetport center section (it was the rear). Its mate presilencer (the front one) was completely hollow inside so I took a chance and cut up the center section. I rebuilt the front presilencer with stainless dish scrubbers and steel wool. Test ran it on the GLC as a rear muffler and it actually quited that beast down better than the previous half dead rotary Engineering glasspack. The glc is getting the Borla as soon as I can swap the '81 to 3", so there's something of a sense of urgency.

 

As for the 3" components, I have the "large" RB universal 3" presilencer, a Magnaflow 4" round with a 3: core (this thing almost doesn't muffle anything, so I may redo it later, but I needed something small to fit where the rear presilencer would be in the RB streetport center section), then for the main muffler I have a 3" RB universal muffler.

Not really sure how loud this will be so I'm lucky I get to test run it on a 12A first, as their ports tend to be a little quieter than 13B sized ports. Also since the header is collecting basically where an RB collected header would normally collect, it shouldn't be much louder than a collected header. Compare this to the RB dual pipe center section and you'll quickly discover that collecting the pulses early keeps them quieter. The same will go for the 20B header, which will look like an RB collected header, but be three 2" pipes to a three way (120°) 30° collector into a single 3" pipe.

For now I'll take my existing flanged collector and copy what RB does on their FC collected headers. I'll smash down a short length of 2.5" thickwall pipe and chop off the collected 2" single pipe at the right spot to make for a smooth transition from dual 2" to single 2.5" Then the 2.5" single pipe will start to expand to a 3" medium wall pipe. From there it will go into the RB "large" presilencer and so on.

 

I'm looking into expansion chambers as a means to cut down on the sharp pulsing of rotary exhausts. What I need to do next is compare the size and shape of the expansion chambers in 2" and 2.5" RB presielncers and see if there's any difference. If the 2.5" one is bigger (longer), I'll scale up the 3" expansion chamber to suit. Otherwise I'll sort of copy what's there.

I discovered the size and shape of these expansion chambers last year when I had to do surgery on a 2" presilencer that snapped a stud. I cut open an access panel next to the broken stud, cut out the stud, replaced with a bolt of similar size/length and welded it in. Then I added a piece of similar 4" steel to cover over the access hole and it was good to.

I put it in the '76 Cosmo, a car where you want quietness, and it passed the test. Only problem now is the Cosmo's exhaust is far too small. It runs out of breath even sooner than the '81 due to its ultra quiet and ultra restrictive stock muffler and its 1 7/8" over the axle pipe. So the 2" presilencer is coming down and a 2.5" presilencer is going in its place. The rest of the system will get upgraded to 2.5" as funds permit. Dave Atkins' Cosmo has a 5" Camden, a 2.5" presielncer and an RB powerpulse muffler from an '81-'82 (this is the one they sell with the streetport center section because it has dual inlets that collect to a 2.375" pipe which has the same ID as a 2.5 thickwall pipe). I'm going to get away with a 4" round Magnaflow in front of the rear end where the stock resonator is located, and a Magnaflow 5x8 oval 18" long muffler in the rear. I'm hoping to keep some quietness.

Wow, how did this get off on the Cosmo?

 

the cool thing is you're going 3 rotor, sooooOOOo you have 2 BIG advantages.

1. the 3 rotor is quieter, more pulses.

2. it sounds AWESOME.

for just the 3 rotor, you might not really want more than a presilencer and a muffler that sounds good. on a 2 rotor that's tricky, you can get good sound, but volume is a problem.

so the RB presilencer/magnaflow might be a little loud with the 12A, but it'll be SWEET with the 3 rotor...

 

If you stick with 2", you'll have to redo it anyway for the 20B, so might as well get that out of the way now.

 

Sounds like you've already kind of decided on the 3" system. That's what I'd go with. I sort of doubt 3" will end up being too big to the point where you lose power. The only downside is more noise. But hey, if it's too loud, you can always use the Cosmo or the REPU for your daily driver until the 20B is ready.

 

Sounds like you made up your mind. Kinda like the drunk trying to get his sober buddies support for picking the fat chick in the bar, we all know you're not gonna listen.....

 

i ran into this problem on my repu i have 2.5"mufflers ready to go but i will be turboing it and will need 3" for good hp gains.i bet the trucks gonna be pretty loud as a n/a but this is the cheapest route cause then i dont have to do the exhaust system twice.. soo go 3"

i had a 2.5"exhaust system with a borla on my fb and loved the sound over rb systems
heres the mufflers i chose for my 3".
http://www.summitracing.com/parts/BOR-40085/

 

Thanks for all the input guys. I have an old resonated dual tip I could put on the 3" main muffler and use it while 2 rotor. It would cause a minor restriction while aslo absorbing some sound. Then when I go 20B I'll chop it off and replace with a 3" dumpy.

I'll give it a test listen before I do anything of course.

 

Yes, the fat chick got laid!!

 

I knew you just wanted confirmation! Any resonator will help......the future turbo will be much quieter though.

 

I'm glad you uh agree. lol

 

3 inch ftw

 

Ok here's what I've decided. I'll test drive the car around the block on the 2" system to try to perceive when it runs out of breath. Remember the current setup has a mild ported R5 13B with a 7" Camden. Once I've established a baseline I'll fab up the 3" system.

It may be a little while as just today I discovered that my 4" round Magnaflow (that was going to be used in front of the rearend) doesn't muffle for anything. I'm going to elongate it and make it wider like something out of dj55b's thread. I'd prefer a 6" x 4" oval, but the floorboard won't allow it so I've gotten creative. I'll copy the external shape of an RB center section but with a 3" core and an expansion chamber on the front like a presilencer. It's gonna be sweet.

 

if you have one, put a pressure gauge in the exhaust, its a little weird that the butt dyno and gauge agree, you'll see pressure go up and feel power go down linearly.

 

I have a boost gauge and an O2 bung. I'll see about getting something hooked up. Awesome idea. Thanks.