Is this a good thing? Or will I lose power if I go with a 3inch catback. My car is a 87 N/A. Thanks

 

very little, i dont think youll notice the difference from a 2 1/2 setup to a 3 inch catback setup. Everyone has their own opinion. Go with what you think, i slapped a corksport on my n/a and i love it

 

O ok, because I was talking to a guy at a tuner shop he said it will do good cause the car is a Rotary and it needs to breath, But today I went to another tuner shop and the guy said that it would make me lose power so... I dont know just want to know from people who knows.

 

if you run JUST the 3 inch catback it probably won't hurt too much. it would be better with a 2 1/2 catback. BUT it you have a header or run without cats then the 3 inch catback would not be very ideal...2 1/2 ftw

 

I had a pacesetter which collected to 3". I had that running to a 3" motoria pre silencer (no difference in sound silencing) to 3" motoria catback.

on a s4 you'll have no back pressure to open the 5/6 ports. So they'll always be closed. If you wire them open then the car will be absolutely GUTLESS under 2000rpm with 3" exhaust.

I did see an increase in gas mileage though. To combat the gutlessness the muffler silencer was installed. This restricts the 3" opening to 1" and adds enough back pressure to make the car roll easier.

if you are going to run this setup, make sure to keep your air pump and tap a line into the air pump's exhaust tube and attach that to the 5/6 ports so they still function.

This setup with a streetport was the fastest n/a I've ever drove/ridden in. It felt like my T2 up until 4K rpm. Put the smack down on many cars, even a few turboed.

 

I just have some headers on my Rex with stock Cats. Stock engine with an intake. So will 3in be to big or no?

 

2.5 is plenty.. dont overkill the n/a u will loose low end and possibly overall power

 

O ight thanks

 

yea agreed.. you 2.5 most of the way down you can open it up to 3 on the muffler end. Thats how the corksport is built. 2.5 then 3 at the end

 

Technically you want to decrease the size of the exhaust as you go further back to keep velocity up us volume decreases aqnd density increases. All increasing the exhaust at that end does is slow down the exhaust gases causing things to back up.


BC

 

he is right there... but who does that anywas lol

2.5" piping 1.5-2"muffler lol

 

Or, the other way around:

3 inch pre silencer to 2.25 dual catback (2.5 at the collector)

 

Yeah but they don't make 1" fart cans!

 

You know, there are alot of studies concerning backpressure. I suppose the correct answer would be "whats more important? HP or Torque?". Smaller diameter exhausts tend to produce more torque due to scavenging (low pressure areas behind a pulse that helps "Pull" the subsequent pulse out). Larger diameter exhausts tend to make more HP and less torque due to the ability to exhaust larger amounts of gas at high rpm/throttle applications.

So whats important to you?

 

Don't Listen to the man! (turbo owners) They are just trying to keep you down!

2.5 Is fine for mildly moded, keep in mind however that the rx-8 uses 3 inch, so if you are approaching/shooting for rx-8 power levels or above, 3 inch is the way to go.

 

Be careful comparing the renesis to the 13b. The renesis has 0, or near 0, overlap due to the side wall exhaust port. Because of this there is very minimal exhaust scavenging happening.

With the exhaust port on the housing there is inherent overlap in the 13b. This means that as the exhaust leaves a low pressure effect helps to pull in the intake charge once the intake ports are uncovered. If the exhaust is too large the low pressure event is less powerful and you end up recirculating exhaust gases and that leaves less room for intake air/fuel mixture. Further the intake velocity is lower, which reduces the power output further.

A 2.5" exhaust is capable of moving a LOT of exhaust. A 3" is overkill on a stock port, or even streetport, NA engine. A bridgeport is probably the only reason to need a 3" exhaust on an NA. Even then I would question if its NEEDED as opposed to just being something to talk about.

BC

 

Doesn't matter after the cat, the cat provides so much restriction that no matter what you do after it scavanging isn't going to be effected.

Even if a 3in exhaust provided an addition 2hp above what you would get from a 2.5 (which is doubtfull), it won't be worth the extra noise you would get.

 

If you keep the cat then yes, you are right. It wont do anything for you. i highly doubt you would even see 2 hp. I was operating under the assumption that the cat had been removed since its such a restriction.


BC

 

I have an NA with a 3" exhaust. Was running with stock intake and exhaust ports and stock manifold.

Now I have a rebuilt engine going in and it has turbo exhaust sleeves that are 5 mm larger than stock with a ported intake, i still have the stock manifold right now though.

With the stock exhaust ports it was fine, i don't know what it will be like now though.

BTW its a turbo catback from a greddy 20G kit. Bought it used for $200.

 

Ive been considering this for a while now. Ive read up some on scavenging as well. I have an RB header on a stocker port engine, cat is being replaced with a test pipe, I have a 3" straight pipe with an N1(with a silencer) that came off an FD that ive been wanting to try and put on to see how it would run. Id have to get another test pipe with different sized flanges welded up first. Still though, id like to know more about 3" exhausts on NA's as well.

 

i have a 86 n/a, all stock for my daily driver, and i just want more horsepower, but not huge numbers, and a good sound. would 2.5 piping from cats back give me this?

 

Just get a pipe coming from a header to a 3" greddy muffler if you dont mind a deep yet loud sound. I have no cats eaither btw

 

Define good sound. that is going to be almost 100% a function of the muffler you use.

Dont worry about geting huge hp, you wont. it will be an improvement, but it wont be huge. 2.5" would be more than enough. If i were you and was looking for a nice sounding, daily driver I would go to racing beat and get there dowpipe and catback. Its a bit pricey, but there is the improvement and the sound is agreeably nice and mellow. Not loud and raspy.


BC

 

Which header? The collected header or the true dual header?

In the end its not going to be a huge difference. However if you were out to make the most hp you could I would look into a smaller diameter catback. In either case that silencer is just going to plug things up. It silences by backing up the exhaust and forcing it through a 1" to 1.5" hole. If you pull the silencer you see its JUST a piece of metal with a tiny hole.

BC

 

Thats why you need good piping to a good muffler, thats the only way. A good muffler will take out the high end raspy tinny sound that make it sound like a honda civic.

God do I miss my RX-7.