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Old 07-11-2004, 02:55 PM   #1
legacy2003
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Default so i have a 2.5" exhaust, does that mean i need a 2.5" high flow cat and header?

like the title suggests... should they be all the same diameter for maximum flow and no restriction? is there any companies that make headers and high flow cats this diameter? most that i have seen are 2.25". i figured i should have the same diameter from the header back?

-mike
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Old 07-11-2004, 04:26 PM   #2
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I have heard of some people going up to 3" exhaust. But I put a 2.5 on there to keep the backpressure up. the exhaust is just more free flowing.
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Old 07-11-2004, 07:38 PM   #3
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2.25 is what you want on an RS

anything bigger and low end torque will suffer.

Jay
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Old 07-11-2004, 09:14 PM   #4
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okay, well i have the 5zigen fireball catback which as far as i know is 2.5"? can anyone vouch for that? im wondering if it would be better to get header/cat that are 2.5" instead of 2.25" or does it really make a difference?

-mike
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Old 07-12-2004, 05:19 PM   #5
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Unless you get a cam that produces power higher in the rev range, skip the 2.5 exhaust.

1. The header that you will want to get will not terminate in a 2.5" collector.

2. Putting larger pipe on the end of a smaller pipe does not gain anything.


cheeRS,

Greg
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Old 07-12-2004, 11:32 PM   #6
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TWE offers 2.5" full exhaust systems
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Old 07-13-2004, 12:23 AM   #7
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so basically youre saying that if i were to throw a stromung high flow cat (2.25"?) and a borla header (hypothetically) in with my 5zigen catback i would not get any more or less gains than if i went with a header that was 2.5" and a high flow cat that was 2.5"? i was basically just wondering if i would get maximum gains if i were to keep the whole exhaust from the motor back (header, cat, catback) at 2.5" rather than having a 2.25" header and cat, then a 2.5" catback.

-mike
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Old 07-13-2004, 01:03 AM   #8
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What he's saying is that you'll lose low-end torque by running anything larger than 2.25"

BTW, you wouldn't be interested in a 3" Rallispec intake/Akimoto filter/Cust. vac line setup for an '00 RS, by any chance? I tried to adapt it to my car - no go.
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Old 07-13-2004, 09:46 AM   #9
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Quote:
Originally posted by legacy2003
so basically youre saying that if i were to throw a stromung high flow cat (2.25"?) and a borla header (hypothetically) in with my 5zigen catback i would not get any more or less gains than if i went with a header that was 2.5" and a high flow cat that was 2.5"? i was basically just wondering if i would get maximum gains if i were to keep the whole exhaust from the motor back (header, cat, catback) at 2.5" rather than having a 2.25" header and cat, then a 2.5" catback.

-mike
Unless you plan on hotter cams (something other than the variety of cams that do not trip CEL's, ie mild, stage 1 ect) stick with the 2.25 exhaust. The reason is that your engine is built around midrange torque, but putting a 2.5 exhaust on the car you are adding a part for high end. You are not matching parts and the overall performance will suffer. If you put the 5 Ziggen exhaust on the end of a 2.25 cat pipe there will be little difference either way. If you already have it, use it, if you are going to buy it, unless it is a phenominal deal buy the matching diameter.

If you plan on cams that push the torque curve towards higher in the rev range, the 2.5" exhaust would be a way to go. With the cobb spicy camsleaving the stock redline alone you could use a 2.25 exhaust. If you plan on raising the redline then th 2.5 is the proper diameter.

I have a TWE header on my car with spicy cams, a light weight flywheel, a Cobb style intake and essentially a straight pipe exhaust with a 45" glass pack. I previously had a SYMS header with an exhaust with a 40 series Deltaflw muffler. Both of these exhausts were/are legitimate 2.5" systems (the header dump is 2.5", I am not just putting a 2.5" pipe on a smaller header).

I had the cams in the car with a stock exhaust while waiting for the SYMS headers. When they were installed, there was no noticeable difference in low end, or midrange torque, yet the high end was fantastic.

Fast forward a year and I sold the SYMS header/exhaust and put the stock back on. Didn't notice anything different except that the high end suffered. Then the TWE system arrived. WHen I put it on I had a noticeable increase in lower RPM driveability (around 1000 RPM), that can only be explained by an increase in power. WIth the light weight flywheel, parking lot manuvering and less than 7 MPH is a royal PITA with either the SYMS or the stock exhaust. WIth the TWE it is only a problem under 1000 RPM. It will not jerk you around nearly as much, and can power out of the jerk-jerk-JERK-JERK cycle that starts when you get to slow. With the other exhausts it was impossible, the only way to avoid the jerks was to push in the clutch. I use to have to keep the car above 2000 RPMs to reliably accelerate in traffic, and never shift before 3000. Now there isn't any need to watch teh RPM's, you just choose the gear acording to how fast you plan on accelerating.

With the TWE there is no longer a rush of power between 3,300 and 4,000 RPM, the torqe sneaks up on you instead. It smothend the power delivery across the intire rev range. Due to the charecter of this header, and the fact that when you accelerate hard your RPM's never drop below 4,000 RPM, losing low end is a good trade off. You can go to far with it obviously, but putting a 2.5 inch midpipe back on a car that is mostly stock doesn't make much difference either way.

I don't know if the TWE headers on the TWE site react the same way as the one that I have, but with the experiance that I have with the few headers/manifold it makes a bigger difference with header design than with piping diameter. My car gained off idle torque with a larger exhaust. exhaust gas velocity is clearly less than the stock exhaust, but the power is there none the less. It could be the fact that the pirmaries are ~50 inches long with cross over pipes connecting primaries from cylinders that fire 360 degrees apart, as it is a newer design than waht people are familure with that started the whole 2.25 vs 2.5 debate many years ago.

If you are really worried about output, seriously concider ditching the Borla plan.

cheeRS,

Greg
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Old 07-13-2004, 02:19 PM   #10
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In theory, you only need as big of pipeing as what comes out of your block. I think its 1.5" on a legacy, then grows to 1.75". Going over an Inch in stock pipeing is Honda Way tooo loud and creates a loss in back pressure.
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Old 07-13-2004, 03:30 PM   #11
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I imagine that you mean the exhaust ports of the heads. If you mean the cylinder bore (the "holes" in the block) that is the funniest thing that I ever heard.

That is your theory I suppose? Nobody has ever produced and marketed a header that way because it doesn't work. The dump is always larger then the primaries (and secondaries if equipped).

As for the primary being exactly the same size as the exhaust port being the only way to make power, that has been proven to be false. Many many people and manufacturese have found that putting larger primaries than the exhaust port on a manifold provides small gains with no loss in performance. The "step" produced by the head inside of the larger primary acts as an anti-reversion dam keeping exhaust from re-entering the combustion chamber contaminationg the incoming intake charge at lower RPM.

Your car is produced this way stock. As are many other cars straight from the factory.

As for putting a larger exhaust on a Honda, ever think that could be one of the reasons why they are faster than you?

(They have more HP. There for burn more fuel and air thus producing more exhaust. Exhuast diameter is dependent on output, and not displacement)

Even though this is the NA forum, I would like to see what the turbo guys think of your "theory". Why don't you go post it there?

cheeRS,

Greg
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Old 07-14-2004, 01:09 AM   #12
legacy2003
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Quote:
Originally posted by Section 8
Unless you plan on hotter cams (something other than the variety of cams that do not trip CEL's, ie mild, stage 1 ect) stick with the 2.25 exhaust. The reason is that your engine is built around midrange torque, but putting a 2.5 exhaust on the car you are adding a part for high end. You are not matching parts and the overall performance will suffer. If you put the 5 Ziggen exhaust on the end of a 2.25 cat pipe there will be little difference either way. If you already have it, use it, if you are going to buy it, unless it is a phenominal deal buy the matching diameter.

If you plan on cams that push the torque curve towards higher in the rev range, the 2.5" exhaust would be a way to go. With the cobb spicy camsleaving the stock redline alone you could use a 2.25 exhaust. If you plan on raising the redline then th 2.5 is the proper diameter.

I have a TWE header on my car with spicy cams, a light weight flywheel, a Cobb style intake and essentially a straight pipe exhaust with a 45" glass pack. I previously had a SYMS header with an exhaust with a 40 series Deltaflw muffler. Both of these exhausts were/are legitimate 2.5" systems (the header dump is 2.5", I am not just putting a 2.5" pipe on a smaller header).

I had the cams in the car with a stock exhaust while waiting for the SYMS headers. When they were installed, there was no noticeable difference in low end, or midrange torque, yet the high end was fantastic.

Fast forward a year and I sold the SYMS header/exhaust and put the stock back on. Didn't notice anything different except that the high end suffered. Then the TWE system arrived. WHen I put it on I had a noticeable increase in lower RPM driveability (around 1000 RPM), that can only be explained by an increase in power. WIth the light weight flywheel, parking lot manuvering and less than 7 MPH is a royal PITA with either the SYMS or the stock exhaust. WIth the TWE it is only a problem under 1000 RPM. It will not jerk you around nearly as much, and can power out of the jerk-jerk-JERK-JERK cycle that starts when you get to slow. With the other exhausts it was impossible, the only way to avoid the jerks was to push in the clutch. I use to have to keep the car above 2000 RPMs to reliably accelerate in traffic, and never shift before 3000. Now there isn't any need to watch teh RPM's, you just choose the gear acording to how fast you plan on accelerating.

With the TWE there is no longer a rush of power between 3,300 and 4,000 RPM, the torqe sneaks up on you instead. It smothend the power delivery across the intire rev range. Due to the charecter of this header, and the fact that when you accelerate hard your RPM's never drop below 4,000 RPM, losing low end is a good trade off. You can go to far with it obviously, but putting a 2.5 inch midpipe back on a car that is mostly stock doesn't make much difference either way.

I don't know if the TWE headers on the TWE site react the same way as the one that I have, but with the experiance that I have with the few headers/manifold it makes a bigger difference with header design than with piping diameter. My car gained off idle torque with a larger exhaust. exhaust gas velocity is clearly less than the stock exhaust, but the power is there none the less. It could be the fact that the pirmaries are ~50 inches long with cross over pipes connecting primaries from cylinders that fire 360 degrees apart, as it is a newer design than waht people are familure with that started the whole 2.25 vs 2.5 debate many years ago.

If you are really worried about output, seriously concider ditching the Borla plan.

cheeRS,

Greg
the borla was hypothetically. im still trying to decide what header to go with. right now my system is stock except the 2.5" catback. so basically there would be no difference if i were to go with a 2.5" header and cat vs. a 2.25"? i just wasn't sure if i should go with all the same diameter. i dont want to ditch the exhaust because i like it a lot. i was most likely going to do 2.25" header/cat, then my 2.5" exhaust.

-mike
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Old 07-14-2004, 09:11 AM   #13
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No. Its not that simple.

With MY header (I think that there are 4 of them now, I also think I have the only high output version, HO, 2.5") there is significent differences from the other header I have tried. It proves that piping size (velocity) is not the only determining factor in off idle torque that has been preached since the beginning of this board. The SYMS header that I had previously (that was 2.5", with a 2.5" exhaust) had a dead spot off idle.



With a stock (or any of the cams that keep the ECU free from CELs) you will want to use a 2.25 header and cat pipe. Putting the larger 5 Ziggen cat back on will be fine. Matching the diameter is less important than having a proper sized header and cat section.

An equal length header is the only header you should be looking at.

cheeRS,

Greg
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Old 07-14-2004, 01:38 PM   #14
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yeah i know i should be looking at an equal length header... who, besides twe, makes an equal length? i don't want to break the bank and i know twe stuff is pretty pricey. i know that equal length in general is steep. btw, what cams are you running and how much did you pay?

-mike

Last edited by legacy2003; 07-14-2004 at 01:44 PM.
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Old 07-15-2004, 09:31 AM   #15
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I believe Brullen's are equal length.
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Old 07-16-2004, 11:59 AM   #16
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Quote:
Originally posted by legacy2003
yeah i know i should be looking at an equal length header... who, besides twe, makes an equal length? i don't want to break the bank and i know twe stuff is pretty pricey. i know that equal length in general is steep. btw, what cams are you running and how much did you pay?

-mike
TWE is the least expensive equal length header. Cobb has one also.

cheeRS,

Greg
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