Bridgeport 12A EFI Exhaust Question
 

Hello All,
I bought all the exhaust parts i needed from RB to make up my exhaust. I am using the uncollected road race RB headers and i bought the universal 2.5 presilencers, one per rotor.
Given the size of the presilencers, putting them side by side would make the exhaust system very wide so i know they get staggered, one a bit further behind the other as shown on the RB website. Does anyone have experience at what length i should have the presilencers since they are the first expansion chamber causing the exhaust wave to return for better exhaust scavenging?

I have my car up in the air currently,has RB streetport exhaust,i could measure how far back from header flange each presilencer is if that'll help. Can take a pic or two if needed.

Thanks. That would be greatly appreciated.

The most forward presilencer is 5.5" from flange. Some pics..

They mostly get placed for where there is room under the car. No real magic lengths involved. They're both about equidistant from the bend in the exhaust, and they're pretty close to the flanges too.

I would have imagines that since its the first expansion, the pressure wave distance would have mattered. In RB site they say bridgeport with long primaries should be 115 to 118 inches in length, that would be the distance my y pipe collects into the 1, right?

Thanks :)

If you look at the pic,the other presilencer is also 5.5" from rear flange. This system collects in/at the muffler,never measured the length...

If it is, it's just a coincidence. RB put the Y where Mazda put the Y on the '81-82 RX-7 exhaust system, which had two smaller pipes between the catalysts and the muffler.

There are no magic lengths involved in the RB long-primary system, it's just shoving as much free flowing muffler as can be shoved under the car, period.

If you ever looked inside a RB presilencer you'd cringe. They're actually partially a chambered muffler. There's a large, square edged open area in the front, kind of like a firearms silencer. But they do work soooo well. (The expansion area is probably what works to kill the high frequency noise)

I heard the old RB muffler was chambered?

It isn't. Well, maybe the old ones but I think it was the pace setter that was chambered, and should be avoided. The RB has an actual straight through core, packed with SS pot scrubber material that lasts a good long time.

peejay is correct about the RB presilencer. It has an expansion chamber at the front to allow the supersonic pulse to suddenly expand and loose some energy. It is a very smart design, which can be copied.

My presilencers are both paper-thin and rusted through where it was welded to the adjacent pipe. Buckets of muffler mud and thermal wrap can only do so much :) I bought a pair of Magnaflow 18"x2" core mufflers with a plan to replace the presilencers with those. I have heard that the new RB exhaust systems don't fit well and are stainless steel instead of thickwall mild steel. Not good.

I have one of the new center sections. The presilencers are SS but the thickwall tubing is still mild like before. The sound is virtually the same but it is much* lighter. You have nothing to worry about by getting a new center section.

*10 to maybe 20 pounds lighter

I have an 84-85 chassis. I needed to hammer part of my heat shield up in one or two places. I suspect I'd have had to do this even on an older system like yours, considering where the presilencer was contacting.

I've only installed one long primary system on a "virgin" car, and it was a '79, it required all heat shields removed, except of course for the one next to the fuel tank.

They were all gone from my '84 so I have no idea if they would fit or not with the shielding. Same for my '85, the heat shields were long gone when I got the car.

the street port system never fit with the heat shield

my FC HKS system has mufflers like this too, its got a short perforated section, and then an open chamber, and then another perforated section. they are pretty quiet and have a nice tone too.

its an old 50mm sport system, so its more like a stock replacement system than big power

I've only ever installed RB long primary systems on cars with heat shields. :)

I already bought all the parts. Yes the presilencers are SS and the tubing is .120 mild steel. If there is no rhyme or reason then ill just do long primaries as RB says at 113". Thanks for the reply

You'll discover 75 to 78 inches is easier to do.

Thanks Jeff. I always thought the 113 was really long since it ended right at the rear axle. Didn't leave much room for a y pipe and muffler. Ill do the 75 to 78.

RB puts the Y pipe in the kickup to the rear muffler. Makes it a pain to re-engineer after it all cracks because it's poorly supported.

Just copy how RB did it. Look at pictures of the powerpulse muffler. Notice where the Y pipe collects into a single secondary pipe at 2.375" (you can collect into 2.5" if you want to) which is above the rear axle, and then travels a little further until it reaches the muffler. I believe this is at or around 75 inches from the header but it has been years since I measured this.

They say to measure from the middle of the header where the intermediate plate would be, all the way to the crotch of the collector. They recommended from 100 to 102 inches to be ideal for one of their setups. Now measure a typical short collected header for a 1st gen. These are about 24 or 25 inches. This gives 1/4 octave. I've built a custom 50 to 55 inch long primary (well, medium primary) which would be half octave. Thus we can conclude that the RB long primary "streetport" system at or around 75 inches could be considered a 3/4 octave and could still work. I've also done 5/8 octave on REPUs at around 61 inches or so which also works fine.

We're dealing with pressure waves and the speed of sound here, so I'd say proper exhaust tuning, like that of a musical instrument is rather important.

You also don't want to collect into too large of a secondary because the sudden expansion can produce an "air slug" which can actually hurt flow at low RPM. You also must consider the exhaust gasses will contract a bit as they cool which is why RB goes from thickwall 2" OD in the center section which has a 1.75" ID, into 14 gauge tubing in the aft section with an ID of 1.6". These collect into a single 2 3/8" OD (2 1/4" ID) and will work fine without a noticeable air slug on engines ranging from a stockport 12A up to a bridgeport 13B. Or at least a half BP 13B like peejay's. Plus it's quiet enough for the street and still sounds really good.

I'd personally just get the RB long primary if I were you. Hopefully your custom job will work out. But yes I have built a custom mid primary for a 1st gen and it was ok. Collector at 50 inches. Would I do it again? Nope. :)

I haven't had a half bridge since 2012. ;)

Full bridge these days?

Since August 2012, yep :)

That lil motor just keeps on going.

Far better than the nightmare year I had in 2010, I think it was, where I did five rebuilds in one season amongst three different engines. Whichever year it was that had a fresh build last me all of 400mi. I think that engine died because it was a early opening streetport 6 port experiment engine and it was bogusly slow. (That or a bunch of dust killed it because I wasn't properly maintaining my air filter.)

Jeff,
Thanks for the post. This is not a street car, its track only. I enjoy the custom work, ive done everything on it so more custom is no bother. I asked the question because i know exhaust tuning is important, i just thought if someone had the numbers already, i would not have to do them. Thanks again.