Yes, you lose back-pressure with a 3" exhaust. THIS IS A GOOD THING. Back-pressure is the enemy of performance (if someone tells you otherwise, it's a clue that they don't actually know what they're talking about). What I think you actually want to know is whether you lose too much velocity with 3", and the answer is no, at least not for a 13B. The only drawback is significantly increased noise.

Don't do dual 2.5" pipes. If you want an uncollected system, use dual 2".

 

it will fit no problem. just run them to the axle, collect them into one, then go over the axle.

 

Well i was bout 98% set on the Weber 48, i just got off the phone with a buddy and he told me i will have a big problem tunning this thing. He said i would need to get two 02 sensors and have them put into my header and run to widebands to get a proper tune. Is this true? I have searched youtube and this forum with NO luck. I see hundreds of people running Webers but cant find any solid info on there install and setup and how they tune or what they use to tune. I dont want one rotor running to lean than the other or being to rich. I could go with a Holley and be done with it, but i hear Webers are better. If you guys can point me to a good link for some reading that would help. I am a big boy and can read, but also need help understanding.

 

Well, you can do it that way if you want. But as long as you use the same jets for both rotors, it's not likely that they'd be too far off from each other. Most people just use a single O2 sensor after the collector or in one of the primaries.

Having run both, I like Webers better than Holleys. They're simpler carbs, and it's much easier to change jets. Plus, Webers don't have as much trouble with lateral g-forces. However, Holleys have the advantage of the possibility of having a choke.

 

I love my weber. The jets are extremely easy to change in minutes and they are very versatile carbs. Only need one O2 sensor unless you are running dual webers.

I have not dealt with a holley.

 

Why are you so set on running super long primaries? use a race header and a well designed collector. 1 o2 sensor just after the collector. If you want to build a PPort then by all means go with 100" primary header length, but in a SP you aren't going to see appreciable gains.

 

i want to loose back pressure! mazda has a chart, on a stock 12A you loose like 8hp for every psi of backpressure in the exhaust, for the PP it looses more hp per psi.

mine is the roadrace header into 2x2"x31" L straight thru glasspacks into a collector into a borla. its reasonably quiet the borla is now the obvious weak link, the glass packs made it sound buzzy and tinny.

like i say sound is a little subjective, i think on a real DB meter it would be right around 100, so its like a loud honda or something

the problem tuning the weber is that there is no instructions, and just about everything you read on the internet is wrong, and or conflicting.

the carb itself is simple and very easy to tune. 1 wideband is all that's needed.

PM me with your email, i have a weber tuning guide that makes it really easy

 

Ahh, the claassic debate on backpressure..........

In general backpressure is bad. PP or forced induction = Backpressure is very bad

N/A application - Backpressure helps torque (to a small extent) but a little is healthy.

Have a 6-port? Better not go 3". You will need the backpressure to open the ports.

High Hp applications, etc, things are different animal, but for a street port-street driven motor, I wouldn't kill myself trying to avoid it. RB factors in some BP for their exhaust. People have criticized them, but hey, they spent a lot on R&D for our cars. Your 2 1/2" pipe is more than enough if you ask me. I'd take the money you would spend on a 3" and buy a nice pre-silencer!

 

True, 6-port engines use exhaust backpressure to open the auxiliary ports (though it's easy to work around that). However, it is incorrect to say that backpressure increases torque, in any situation. Torque is reduced by backpressure because it prevents the combustion chamber from being properly scavenged. Doesn't matter if it's a stockport or a peripheral port (although the more overlap you have, the more important scavenging becomes). Exhaust gas velocity is the reason we don't build headers with huge primaries, not backpressure. Velocity helps scavenging, backpressure hurts it.

Now, I will grant that things get more complicated when you add a turbo into the mix, but that's an entirely different discussion.

 

Really the decideing factor on my exhaust choice is if the guy still has his Road Race header when i have the money,I can get the whole deal for pretty cheap, including mid pipes and muffler. If not it has to be all new.I still have my muffler from my 12a set up so i would only need header back. When i first bought my car it had the stock exhaust with the Racing Beat 465 and intake. To be honest it seemed to have more low end pep with that untill i changed to the 2 1/2. I guess it depend also on the bends in the pipe. Sound is not a issue, just worried about achieving the most hp i can get. I am still wondering about tuning the Weber. If i get it new , should i get it from RB? What 02 sensor to i need?

 

I like, and recommend, the Innovate LC-1.

 

regardless of the brand, just make sure it's a wide band o2 sensor.

 

I feel like I should say something. LOL

 

Didn't I start it for you at 7stock? Thought I did.

 

hmm, yeah i know i got to fondle it, but i don't recall if you started it or not. if you did start it and i don't remember it can't be that loud....

 

Something to keep in mind when looking for HP targets is that dyno results vary based on the type of dyno, test conditions, and operator skill/experience. Google "dyno accuracy" and you'll see some articles on the topic. Dyno Jets, for example tend to provide higher HP numbers than Mustangs --> http://s14.zetaboards.com/RaceJax/topic/6611584/1/ (see post #7).

The real test is how well the car accelerates under the conditions that matter to you. The dyno is a good tool for comparative tests on the same car. For example, you change the exhaust and make a dyno pull -- did the numbers increase? The absolute number the dyno provides, however, should be taken as just an "estimate".

 

That is true. I just would like a rough estimate of what my car will do.Time will tell. I made up my mind and i am going with the Weber 48 with Racing Beat Race header with dual 2 1/4 back and Stage 1 clutch

 

pm me with an email, and i'll send you the easy weber tuning guide, it'll actually work for any carb, but it is weber specific

 

Why would you want to use such big primaries?

 

What do you mean by big primaries ? Are you meaning the Weber 48?

 

No, you said you want to use 2.25" uncollected exhaust. Why? It's usually not recommended to go larger than about 1.8" ID for a stockport or streetport. Even peripheral ports only want 1.9-2" ID according to Racing Beat's chart. Go too big and you risk killing your torque at the low-end.

 

If you are thinking about running true duals, be forewarned they sound horrible.

 

the header should just be the 1.75" ID (inside diameter) stuff RB sells. which is 2" O.D. (outside diameter)

the MUFFLERS can be 2.25 though, that is a legal move. the concern with mufflers is flow, and noise absorption, not pipe size.

old school setup is

1. run the RB header, into 2x2 (or 2.25")x 31" long thrush glass packs, http://www.jegs.com/i/Dynomax/289/24...rentProductId= these are straight thru, no louvers. into 2x2.5" rear mufflers of your choice. dynomax makes the super turbo which flows enough.

its actually surprisingly quiet, even on bridge or PP engines. the bridge/PP motor will want the bigger glasspacks, and maybe 2.25" pipe after the header.

 

When after reading the replys, i remeber when i first bought my car it had the stock exhaust with the Racing Beat carb setup.It would take off like a bat ou of hell, and go sideways in 2nd gear. I put on my header and ran 2 1/2all the way back and it lost that low end power. But up higher rpms it screamed. So from what you guys are saying 2 or 2 1/4 with some kind of muffler in between is what i need? Also duals all the way back wont sound right?

 

Just collect to a single 2.5", use at least one presilencer and a large-bodied straight-through main muffler. Alternatively, a long-primary system with a presilencer in each primary, again collecting to 2.5". like a clone of the RB "streetport" system. In either case, use only RB .120" wall pipe, especially before the collector. These are setups proven to work well.