So, this last week I ran into another RX-7 owner and of course started talking about 7's and volunteered to introduce me to some people that could help me out and knew what they were doing. So, of course I am with it!

I get there and they are Puerto Rican, Not that I am racist just the fact that we all know they KNOW Rx-7's and have a lot of us jealous. Anyway, they are looking at my car and telling me little things here and there that I can do. Now, I use to be a mechanic for a little while but I know that 7's are different and that you don't want a lot of back pressure. They told me that my 2.5 inch exhaust is too big. That I should go with a Racing Beat set-up because it gives it good back pressure that it needs. (BTW my motor is N/A) This blows my mind. and I am wondering if this is believed amongst other very well informed rotary heads.

I have RB Street Headers to 2.5 inch straight exhaust till after the axle then 3 to Tanabe muff. Way too loud. and looking at a comparison the street port RB package. I mean. I am not against doing it, if just to cut down on some of the noise but just wondering if this was a valid statement.

Off subject, I am stoked about my tat tomorrow of a rotary motor. I will post pics as soon as possible and see what people have to say.

 

A 2.5" is fine for you SP motor but is way too big for a stock motor. A stock motor only need a single 2". The RB SP exhaust uses 2-2" pipes and connects in the back before the mufler to one pipe. so most of the way it will be larger then a 2.5" but flow a little different.

 

Tell them to stop spreading crap the next time you see them. Back pressure is NOT good. Velocity, however, is. That is probably what they are referring to. You have a ported motor so the 2.5" is good, but like Chris said on an average rotary that would be too big.

 

Good point. I really only remember them telling me that the muffler has some sort of baffle I guess and that it just kept some "compression" compared to my straight through muffler and that it helped. It was kinda hard understanding everything exactly so I might have gotten a mis-interpretation.

 

if you are looking to quiet the car out with the RB SP setup then by all means do it. If you are hoping to get more power out of the RB SP setup then you will not. It will either be about the same or a little less. The RB system makes a compromise between power and sound.

The back pressure argument has been around for as long as people have been modifying cars. Basically the best system is one that balances the exhaust from both rotors. a good equal length header and an exhaust with the least amount and smoothest of bends. If you go too big with the exhaust, it starts to become a waste. Also if you get too big the air doesn't know where to go, the air gets too turbulent and you loose velocity like super82 says.

 

for example:
blow through a straw
blow through a paper towel tube
blow though a 4" pipe

the straw is ovious
the pt tube will have quite a bit more velosity
the 4" pipe will flow but at the end of the pipe there will not be the velocity. it will almost feel like there is no air moving at all.

 

So what would be a good size for a stock fuel injected engine?

I always figured that 2.5" was a good size...

 

you probably will not gain anything with an exhaust over 2.25" on a stock 13B.

 

Hmm, do you think it would hurt to make it 2.5"? If you're saying that the max is about 2.25" I would think that would mean that a .25 increase in diameter isn't going to benefit power but not take away that much either.

I ask because I am planning in the future to either beef up the N/A I have or drop a turbo motor in there and I rather not have to redo the entire exhaust system. You know what I mean?

 

you are probably fine doing a 2.5" if you do a built N/A and mild Turbo car it will be good. If you deside to do a monster turbo it will not be enough.

 

I have RB street header, 2.5" exhaust, and 2 Borla mufflers.

All the people who say 2.5" is too big is right. I really miss that 1HP that I lost.

 

We all know what, now?

 

GAWD DAMNIT EVERYONE SHUT UP YOU DON'T KNOW WHAT YOU'RE TALKING ABOUT. 2.5" IS NOT TOO BIG
SERIOUSLY DIDN'T EVEN ONE OF YOU READ MY THREAD? I run 3" into 4" into 3" and i gaurentee not one of you guys make as much tq or whp as I do...
https://www.rx7club.com/1st-generation-specific-1979-1985-18/racing-beat-exhaust-dyno-compairisons-823229/
Not back presure is or has ever been any good. It's an old wives tale and you're acting like old wives spreading milicious roumers. You want velocity in your exhaust and back pressure has nothing to do with that. think about how much air a rotary flows and the straw example means nothing when it comes to cars becasue you want to tune your exhaust for pressure waves and scavaging, and your mouth on a straw isn't doing that.


p.s. I'm Puerto Rican and I can gaurentee although I know my 7 not all of us know our 7's. I'm the only in my huge *** family that knows 7's.

 

^^^^ lollllllllllllllll

 

Is that when you compared the RB system to yours?

What are your engine mods?

It seems that the OP is running a stock ported motor and you are not. There is a big difference there

 

why would you want to run a full RB exhaust. These parts are designed to bolt up to a stock exhaust. In other words, the pipes are really small, say 1.75 ID.

 

3" is about 47% more cross-sectional area than 2.5" (assuming a .065" wall). The question is whether you think Adrian's barely ported 6-port 13B flows more than 47% more air than a stockport 12A. Somehow I doubt it.

 

Here's a question?

If you had a 3" inch system and then shrank it to 2.5" just before the muffler, would that give you the same effect as having a 2.5" exhaust pipe all the way along teh car, or simply cause turbulence?

I've never actually really realized how complicated exhausts are until I started deciding to make my own.

 

That over in Puerto Rico they have some of the quickest 7's.

 

The OP has a street ported 13b. my 13b is ported, but the port sizes remained almost unchanged. I have a 48 IDAw 43mm chokes that should flow about 600 cfm he's using a holley 650 cfm carb. I think it's fair to say that his engine should flow more than mine. Every piece of advise even mentioning 2.5" is baaaaaaaaaaad.... lol i'm so tired. I need 2 more beers.
Sorry, but the straw thing is just killing me. Say your lungs hold 1/2 cubic ft of air (they cant, they hold about .25 ft^3 total), and you can expell that air in 500 milliseconds. That's 60 CFM if you can keep that up for 60 seconds. A stock port 12a with 90% VE will flow 307 cfm at 6500 rpm. Huge flow differences, plus you use exhaust pulse tuning to help scavange the exhaust out of the port and you're not doing that huffing and puffing away. Now after you've woken up from passing out trying this little experiment please go throw your straw away.

 

So, is there an over the shelf, pre assembled or piece by piece, exhaust that I can get that will give me max hp yet not be so loud that when I pass by something I can't help but be glad I am inside the car.

 

Your best bet is the racing beat street port exhaust. It's going to be the quietest and highest HP "bolt on" exhaust out there, but if you actually want to make serious power you're going to have to design and fab up your own exhaust.