Cat back question
Rethinking Catback size....
I have now changed my way of thinking this whole thing through.
What you really want to do is use a 3" DP (Free flow Cat or no Cat) FIRST..then we can talk about the CB size. I have 3" Catted DP installed.
I don't think *most* people consider GAS THERMODYNAMICS with this set up. Basically exhaust temps rapidly DROP as they exit the DP. When this happens the gas volume CONTRACTS and LESS pipe diam is thus required from the CAT BACK.
Our *cousin* the MSP 3
2.5 vs 3" exhaust - Mazdaspeed Forums
using the principle that colder air takes up less space, we can figure out if the exhaust coming from your turbo actually NEEDS a pipe bigger than 2.5" at the CBE. as the air leaves the turbo, you're looking at air heated to about 1500F. by the time it gets to the CBE, the temp has dropped hundreds of degrees. let's just use 800F (and this is a really conservative estimate, i bet) for simplicity sake.
according to the ideal gas law: pV=nRT
this implies that if we keep the amount of gas constant (n), the volume it takes up at a given pressure should vary proportionally to the absolute temp of the gas in question (in kelvin).
1500F = ~1090K
800F = ~700K
this suggests that the gas@800F will only take up about 64% of the space it did at 1500F.
area of the 3" cross section = ~7.065
area of the 2.5" cross section = ~4.9
this means that a 2.5" diameter hole is about 70% the size of a 3" diameter hole...compare that with the 64% required, and you see that a 2.5" diameter pipe is more than enough for spitting that exhaust out the back. so,this basically means that as long as your transition from 3" pipe to 2.5" pipe was smoothly done, you'd have a hell of a time figuring out the difference between a 2.5" CBE and a 3" CBE.
feel free to correct my math, since i wrote this up really fast and didn't really double check it, but you should catch the drift of my logic. this is the reason why i maintain that 3" aftermarket CBE on the MS3 are purely there for sound reasons.
Another post on this topic:
VW GTI Forum / VW Rabbit Forum / VW R32 Forum / VW Golf Forum - Golfmkv.com - View Single Post - turbo exhaust theory - 3" dp to 2.5" cb
after reading up on [turbo] exhaust theory / pipe sizing just wanted to hear what people had to say about this exhaust setup: 3" dp to 2.5"cb (for stock turbo).
the 3" dp get's rid of the stock dp's restriction and helps move the exhaust gases away fastest (over stock dp, 2.5"/2.75" dp)
then as the exhaust gases cool a bit (though I really question the amount the temps can drop from the dp to the cv) you 'squeeze' the exhaust gases through a 2.5" cb for a quicker exit out of the system.
What you really want to do is use a 3" DP (Free flow Cat or no Cat) FIRST..then we can talk about the CB size. I have 3" Catted DP installed.
I don't think *most* people consider GAS THERMODYNAMICS with this set up. Basically exhaust temps rapidly DROP as they exit the DP. When this happens the gas volume CONTRACTS and LESS pipe diam is thus required from the CAT BACK.
Our *cousin* the MSP 3
2.5 vs 3" exhaust - Mazdaspeed Forums
using the principle that colder air takes up less space, we can figure out if the exhaust coming from your turbo actually NEEDS a pipe bigger than 2.5" at the CBE. as the air leaves the turbo, you're looking at air heated to about 1500F. by the time it gets to the CBE, the temp has dropped hundreds of degrees. let's just use 800F (and this is a really conservative estimate, i bet) for simplicity sake.
according to the ideal gas law: pV=nRT
this implies that if we keep the amount of gas constant (n), the volume it takes up at a given pressure should vary proportionally to the absolute temp of the gas in question (in kelvin).
1500F = ~1090K
800F = ~700K
this suggests that the gas@800F will only take up about 64% of the space it did at 1500F.
area of the 3" cross section = ~7.065
area of the 2.5" cross section = ~4.9
this means that a 2.5" diameter hole is about 70% the size of a 3" diameter hole...compare that with the 64% required, and you see that a 2.5" diameter pipe is more than enough for spitting that exhaust out the back. so,this basically means that as long as your transition from 3" pipe to 2.5" pipe was smoothly done, you'd have a hell of a time figuring out the difference between a 2.5" CBE and a 3" CBE.
feel free to correct my math, since i wrote this up really fast and didn't really double check it, but you should catch the drift of my logic. this is the reason why i maintain that 3" aftermarket CBE on the MS3 are purely there for sound reasons.
Another post on this topic:
VW GTI Forum / VW Rabbit Forum / VW R32 Forum / VW Golf Forum - Golfmkv.com - View Single Post - turbo exhaust theory - 3" dp to 2.5" cb
after reading up on [turbo] exhaust theory / pipe sizing just wanted to hear what people had to say about this exhaust setup: 3" dp to 2.5"cb (for stock turbo).
the 3" dp get's rid of the stock dp's restriction and helps move the exhaust gases away fastest (over stock dp, 2.5"/2.75" dp)
then as the exhaust gases cool a bit (though I really question the amount the temps can drop from the dp to the cv) you 'squeeze' the exhaust gases through a 2.5" cb for a quicker exit out of the system.
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