Ok, so I'm getting close to installing my 13b w/48mm Weber DCOE and need some input on the size of my exhaust pipe.
I am using the Racing beat dual exhaust header and have the weld on flanges for a custom collector to go from 2x 2in to whatever size single pipe.
Should I use a 3in or should it be a larger size. ***No emissions required***
Going with MagnaFlow components, so I will adjust my choices based on the pipe size.

So give me some feed back on what would be a good size to go with.

Thanks!

 

2.5 inch should be plenty for a non turbo car.

 

I don't see any need for anything larger than 2" ID. 3" is for Turbo applications.

 

Just to make it clear, I am intending to collect "two" 2inch pipes together from the header into one larger one.

Are you saying it's not necessary to increase the size of the exhaust?
Interesting....

So Doc.....who's your evil twin in the first post? LOL!

 

There is no need. You are just going from 2 pipes (just like a road race header) to 1 pipe as the outlet.

http://racingbeat.com/FRmazda1.htm

 

Well that might be what racing beat says, but remember that SDJ's header collected into 2.5" and always made at least 10 whp more than RB long or short primary exhaust.

 

well im using a rb header to 2 1/2" pipe with two borla xs mufflers that are 2 1/2 in and out.

altho i am doing this because theres a turbo involved but n/a should be greatbe if
3" may be ok if you had a peri port.

 

Yes, But he is using a RB header, not a SDJ.

When RB designed their systems I'm sure that they took into consideration the maximum amount of HP gain with the matched pipe size. I'm sure that SDJ also did the same and that is why with their header they are using 2.5".

It would be interesting to show if there was a difference in HP gain on the RB header with 2.5" tubing.

 

Well he's using the RB long primary header and a custom collector. the problem with the RB header is the shitty collector. so if he uses a decent collector that collects into 2.5" I think he'd make more power.

 

Ok slammed it's up to you to do some dyno testing here. 2" VS 2.5". :-)

 

id say 2.5 cause it cant really hurt to go only a little bigger, but it can hurt to go too small. even with out hard data, you can play the odds.

 

Makes sense to me to use 2" at the collector if you use 2" primaries. When one rotor is breathing, the other isn't. They are phased 180° apart you know.

 

Ahh but are the runners equal length though? This what makes a better performing exhaust system. Though i don't know what he want in a collector though, a better flowing one or a higher volume collector.

This one


Vs this one


It's hard to say what the exact performance difference is going to be.

 

im running the rb race header to custom collector to 3in back and dont have any complaints.

isaac

 

This all helps, I currently run a RB header collected to 2in and then 2-1/4in the rest of the length on my existing 12a nikki carb setup, so I'm not going to go any smaller with a 13b. I already have a RB collector for the dual header now, I just checked the I.D of the RB header pipe and it's 1-7/8 per runner not 2in.
If I stick with a 2.5 all the way to the rear of the car I should not have any issues.
The problem is, do I want to run my port actuators or just wire them open all the time. If I go 2 inch I can make a pickup tube and have the pressure I need for proper operation(maybe). The magana flow components flow very freely and create little back pressure so do I give up the exhaust tone I like for port acutator performance? Hard choice, Magnaflow sounds sooooo good!

 

i say wire em open, its all about enjoying your car.

 

I searched but I couldn't find it... it's in the archives, but someone dynoed his car with the 5&6th ports functioning perfectly and with them wired open, and he made more hp and tq across the entire RPM band with them wired open. The only thing functioning 5&6th ports improve is gas mileage...
Back on topic Lloyd you need to ditch that collector. It looks terrible. You're better off running the RB streetable header then running with that collector. It looks like more of a restriction than a collector. Something like this would be perfect

SDJ FTW...
since you can't buy 'em anymore why not try and make your system ad close to their design as possible. that's my plan...

 

I plan on fabbing one up that will have no constriction.
Thanks for the tip Adrian!

 

What is the connecting tube in the middle of the 2nd pic? Wouldn't that mess with exhaust flow?

 

No worries, not using that design, close but it will not close like that.
Picture 2 funnels that are melded together, the smaller tapers will be the flange that connects to the dual header outlets, the wider part of the funnels at the top will be melded to a single pipe, that will be the desired diameter with no reduction.

Simple geometry

 

Does anyone in the states run twin pipes joining into one larger diameter pipe at the diff? I currently run a 13b large extend port with 2.5 inch extractors into 2 inch pipes joining into a 3 inch single at the diff with a stainless dumpy exhaust tip.

 

I believe he was asking about the H pipe in this pic

An H pipe has been used in the muscle car scean for years. It used to equalize pressure in the exhaust flow which in theory gives a much flater torque curve. SDJ Dyno sheets have proven that yet again. The few SDJ dyno sheets i have seen give IMO a 25% flatter torque curve than a RB exhaust.

 

I say 2" or 2.25" at most. I used 2" in my GLC with a large streetported 13B and the powerband went well beyond redline if I chose to rev it that high. However, I don't have dyno sheets to prove this.

Another thing to keep in mind: the larger the pipe diameter, the louder it'll be. You may be losing a lot of power just getting the noise down to acceptable levels. No way to know what's best without some real dyno time, though.

 

nm

 

It's all about torque for a good streetable car. If you're more concerned with higher RPM HP and could care less about low end torque, then by all means run the biggest (or just big) pipe you can fit. I know that rotary engine's aren't known for their impressive low end torque, but I think I'm still going to stick with 2.25" and 2.5" max when I get around to adding my turbo. I want a car that'll perform in traffic when I need it to, at RPM levels that don't draw unnecessary attention...

Just plan out your mods and consider what the goal and main purpose of the car is. If it's going to be street-driven and is mostly stock under the hood you may want to keep the 2.25" or even go down to 2". The exhaust alone won't make the car-it's all the mods working together with each other...