3 inch cat back on N/A ?
Ive got the collected header, not the true dual. I have a 2.5" dynomax catback on right now, its fairly quiet, i like it for the most part. But i keep hearing about how 3" is ok on NA's and wont hurt performance, and then I hear the opposite a lot too. Ive been wanting to go single exit for a while now, but I dont want to go through the hassle of buying crap to get this welded up if its just going to hinder performance.
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Originally Posted by anewconvert
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
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
There is a point where backpressure, however slight, out weighs any benifit you are getting from scavenging.
Traditional scavenging doesnt exactly happen in a rotary anyhow, because both exhaust ports are essentially open and flowing exhaust at all times, therefore they try to occupy the same space at the collector more so then on a piston motor , insted of neatly stacking one behind the other. Because of this a rotary requires a larger collector, and larger piping after the collector then a piston motor.
The main thing the collected exhaust is giving you is pressure wave tunning.
The violent initial opening of the exhaust port (still in the expansion stroke) does generate pressure waves that can be reflected and used for scavenging by a proper collector.
High Hp NA rotaries run 4 inch megaphones, not 2.5 inch,..
Originally Posted by drago86
As far as I'm concerned the overlap is even more reason to run a larger exhaust, Especially on a bridge port.
There is a point where backpressure, however slight, out weighs any benifit you are getting from scavenging.
Traditional scavenging doesnt exactly happen in a rotary anyhow, because both exhaust ports are essentially open and flowing exhaust at all times, therefore they try to occupy the same space at the collector more so then on a piston motor , insted of neatly stacking one behind the other. Because of this a rotary requires a larger collector, and larger piping after the collector then a piston motor.
There is a point where backpressure, however slight, out weighs any benifit you are getting from scavenging.
Traditional scavenging doesnt exactly happen in a rotary anyhow, because both exhaust ports are essentially open and flowing exhaust at all times, therefore they try to occupy the same space at the collector more so then on a piston motor , insted of neatly stacking one behind the other. Because of this a rotary requires a larger collector, and larger piping after the collector then a piston motor.
Second: You arent going to SEE much backpressure in a 2.5" exhaust, so opening it up further is only going to harm you. Not only is it going to lessen the impact of what scavenging is going one, but it will slow the velocity of the exhaust, causing it to cool down earlier in the exhaust system, increasing its density and causing back pressure in an of itself. That is THE reason that you dont oversize exhaust on an NA engine. Velocity is key, even if you arent getting any scavenging.
The main thing the collected exhaust is giving you is pressure wave tunning.
The violent initial opening of the exhaust port (still in the expansion stroke) does generate pressure waves that can be reflected and used for scavenging by a proper collector.
High Hp NA rotaries run 4 inch megaphones, not 2.5 inch,..
The violent initial opening of the exhaust port (still in the expansion stroke) does generate pressure waves that can be reflected and used for scavenging by a proper collector.
High Hp NA rotaries run 4 inch megaphones, not 2.5 inch,..
Pressure wave tunning is......? Scavenging! When you have a high pressure wave the front will be the high pressure, the rear lower in pressure. This will cause the next wave, at high pressure, to go chasing after the first wave. (high goes to lower) this is scavenging. thats what you are describing. Even in a seperated system like the true dual system you will find this happening. Its effect is increased using a collector or megaphone.
Fianlly, you are comparing apples to oranges. The starter of this thread doesnt HAVE a high hp rotary. And further a high hp rotary isnt using a 4" exhaust. Its using a megaphone that increases in diameter over a distance in order to increase the low pressure effect.
Years of people building NA motors, from piston to rotary, has demonstrated that an exhaust CAN BE too large. 3" exhaust on a stock port and probably streetport engine is too large and wont perform as well as an smaller exhaust. Will it be an improvement over stock? YES. Will it be as good as an appropriately sized exhaust? no.
BC
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