Need Advice on Exhaust System for 13B NA
 

I'm going racing with a bone stock '87 13B NA, and I need some advice on the best way to upgrade the exhaust system. I want to get the biggest power increase I can, and the only limitation I have is I need to be at 92 db or less from 50 feet away from the car.

I've been looking at some of the Racing Beat stuff, but I really have no idea where the noise would be at, and I'm not sure if there's something better out there. Any advice would be greatly appreciated.

Get the header for sure ...the whole exhaust isn't that loud (idk the db's) but i like the less wieght of a single exhaust instead of rb duals.

I run the Racing Beat header and dual exhaust on my street ported 13B, with straight pipes installed instead of the center muffler (so I only have the two rear mufflers), and my car blows 99db at 50 feet. With the center muffler installed, you might make 92db. If not, put a turndown/turnaway on the exhaust tips and you can probably fool the meter enough to be OK

on my ITS car i ran the RB header with two straight pipes merging into one Ultraflo muffler with an adjustable turn down tip. i don't know about hitting 92db but the power is definitely there.

A car at my track has an OBX header, magnaflow stainless 4" body resonator/glass pack and single flow-pro max muffler.

I can't remember the exact decibel count but i know it was less than 95. If you use a dual setup out back it may be even quieter.

That car dyno'd 173whp with fresh-ish mild street port, 850cc secondaries and factory computer. I don't know how good that power is (I'm used to modded turbo numbers) but at the time we thought it was alright.

I still haven't found any system that sounds better than RB. I'm running an RB header to a Bonez Superflo cat to a stock Y-pipe (RB's doesn't have larger diameter or cleaner transitions) to a pair of RB mufflers. It flows pretty well and noise level is 92db at 18 inches. My only real gripes are that a better header would make more power and my catback is only 5lbs lighter than stock.
For absolute max power, I recommend a custom header. All of the OTS header designs appear to turn too soon after the port exit, most have poor/undersized collectors and/or unequal length primaries.
My next system (I AutoX in CSP) will be a custom header to 2.5" cat for street duty (and either a resonator or an expansion chamber for race duty) to a 2.5" single catback. I expect that a RB Power Pulse muffler behind one of their pre-silencers should produce a reasonable tone without too much weight.

When I ran a street ported 13B ran racing beat intake with Holley 650cfm and on the exhaust side custom measured headers with resonators four of them keeping seperate pipes 57 mm I think until a collector muffler before rear axle and a serviceable custom box at the back. Was running very competitive lap times. Booze was within rules at the time the rear silencer was able to open so you can pack with fibre and stainless steel wool. I'm busy designing a box at the moment based on the classic megaphone design

ISC header, 3" outlet into a Borla SS muffler, 3" to a Borla oval muffler in the rear. Suspend rear with chain because system grows about 3" on track. If necessary I can get you muffler numbers.

According to the reading I've done here and elsewhere, pipe sizing in an NA should be planned as close to 250fps as possible at peak flow, or slightly larger. Anything larger might contribute slightly to top end with progressively smaller gains and larger losses down low due to lack of exhaust velocity.

That said, a 2" (50mm) .125 wall pipe should flow enough for about 130RWHP, so perfect for a true dual system on a max effort street port or bridge port motor that's going to spend a lot of time around the rev limiter.

A 3" system with .125 wall pipe should flow enough for around 320RWHP, so perfect for bridge port peripheral port setups, but it seems a bit much for a stock or street port.

A 2.5" single should flow enough for about 220RWHP, which is already pretty ambitious for a stock port or street port motor.

Of course these numbers change a bit if you run thinner-walled stainless pipe. Speaking of which, does anyone know how thin you can go on a stainless system before they start having issues?

after playing with a bunch of FC's over the years, Mazda used 50mm ID pipe, and this actually seems to be a good size for a stock, or street ported car. the restrictions seem to be the cats, and mufflers

so anyways, 50mm ID i think was a 2.25" OD pipe.

it does seem like by the time you get to a 3" pipe, it gets hard to muffle, and i think its past the point of big returns

From experience, I cannot recommend any system more than the RB cat back with a straight pipe from stock headers back. After my cars rusty stock pipes met a grisly fate with a particularly energetic backfire, I got the RB system and was very happy. The increase in power with no emissions equipment and a free flowing car back couldn't be beat, especially with the bearable in cabin noise and excellent tone.

I can really only regurgitate what I have read on here over the years plus the evolution my current system which is the only one I've ever had. Popular opinion is the RB true dual is the best off the shelf system for a stock port car. Ported engines need the primaries collected at some point. Where depends on the porting and intake setup. I don't know what db numbers it is at. If you start with this system you can put a collector in it later and even convert to a custom single if you chose and/or if you port your motor. It gives more options as you will have the road race header and pre silencer allowing you to play with primary lengths in the future.

As to my system it started with their(Racing Beat) collected header and pre silencer to their Y-pipe to stock mufflers. Power gains were good and noticeable. Sounded like crap above 4000rpm. Don't know the db's but it was louder than my current setup inside the car. I currently have the old HKS sport cat back but it hasn't been on the market since the early 2000's but the RB mufflers with this setup should give about the same gains maybe better but are a touch louder. These mufflers made a noticeable difference over the stock cans. None of this probably helps without db numbers.

I would get the true duals for the above reasons as there is probably nothing better out there( off the shelf) without building something custom and tuning it on the dyno.

You might look at Defined Autowork's header if you have lots of money( it's $850)and you'll have to fab something custom from there back. It is all stainless and looks to collect mid chassis, but with no pre muffler provisions meeting 92 db may be a challenge.

Also if your class rules allow get a way to tune fuel to go along with the exhaust to get the most gain.

What straight pipe are you running instead of the center muffler, custom? I'm looking to do the same thing