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Out of curiousity why does everyone use 2.5" on the crossover pipe

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Out of curiousity why does everyone use 2.5" on the crossover pipe

Old 02-01-2013, 03:10 PM
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great info. subscribing
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Old 02-01-2013, 05:36 PM
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Great thread phil, now if we can get guys to stop putting these itty bitty turbines on LS motors we'd be getting somewhere, and using the right cams too!
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Old 02-01-2013, 09:32 PM
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Well you need itty bitty turbines when you have large drive killing pipes before the turbine. I've got a 1.50 AR on my 460" and I swear I can use a 1.70-1.75 if someone made one.
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Old 02-01-2013, 10:24 PM
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I think you could even go bigger than that. Have you seen what some of the gt55 guys are doing in x275 with the gt5518s and the huge housings Jose makes for them?
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Old 02-02-2013, 01:39 AM
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So if I take my 2.5 inch exit manifolds neck them down to 2.25 all the way to the merge I will get essentially more focused exhaust gases?

It's like putting a tip on a blow gun, you take it off and you have no air pressure it seems. Once it's on a focusing the air you can sweep your whole garage clean in a few passes...same concept correct?

Now answer me this if you guys could. If I merge the 2 2.25 pipes into a 3 inch pipe then into my t4 flange will that still maintain this velocity the 2.25 pipes created? The 3 inch section before the t4 flange will only be about 3 inchs long.

The 3 inch pipe is pretty much an exact match to a t4 flange.
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Old 02-02-2013, 09:03 AM
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So do you not think that dropping from a 2.5" manifold collector to a 2.25" is going to mess with the exhaust flow at all? I guess a nice transition piece might work. kind of like the aforementioned nozzle, lol.
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Old 02-02-2013, 12:10 PM
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Originally Posted by Sarg View Post
So do you not think that dropping from a 2.5" manifold collector to a 2.25" is going to mess with the exhaust flow at all? I guess a nice transition piece might work. kind of like the aforementioned nozzle, lol.
How come you keep asking the same questions over and over? Mess with flow? Of course you dont want to use an abrupt coupler.
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Old 02-02-2013, 12:33 PM
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Originally Posted by 95bowtie View Post
So if I take my 2.5 inch exit manifolds neck them down to 2.25 all the way to the merge I will get essentially more focused exhaust gases?

It's like putting a tip on a blow gun, you take it off and you have no air pressure it seems. Once it's on a focusing the air you can sweep your whole garage clean in a few passes...same concept correct?

Now answer me this if you guys could. If I merge the 2 2.25 pipes into a 3 inch pipe then into my t4 flange will that still maintain this velocity the 2.25 pipes created? The 3 inch section before the t4 flange will only be about 3 inchs long.

The 3 inch pipe is pretty much an exact match to a t4 flange.
the volume of the 3" is 11% smaller than the 2 - 2.25 pipes together,,,so it might not hurt, as long as the 3" pipe fits exactly into the flange
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Old 02-02-2013, 01:29 PM
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Here's some pics of my 2.5 to 2.25 crossover/hotside and my t4 merge. I used some cones reducers after the v bands



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2.5 v band with a cone reducer to 2.25"


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Old 02-02-2013, 02:47 PM
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Originally Posted by LS1NOVA View Post
How come you keep asking the same questions over and over? Mess with flow? Of course you dont want to use an abrupt coupler.
Because I am not getting a straight answer. I am getting analogies about running, information in regards to merging the pipes together, and information about non-divided housings. I have always been told that stepping up in piping size like a step header will help with scavenging and negating exhaust pulses back towards the cylinder. Stepping DOWN from 2.5" collector into a 2.25" pipe (lets forget the word crossover as it does not apply) would be the reverse of this.

The reason I am very curious is I have seen quite the opposite in some of the turbo systems I have built and helped design on 4 cylinder motors. When going from a 1.68" primary leading to a 2.25" pipe merging into the T4 sized turbo (much smaller turbo) it was slower to spool and did not hold boost as well as the system that had a 2.0" primary that lead to a 2.5" collector merging into the turbo. Same turbo same motor. This was on a 2.5 liter Subaru motor. The smaller pipe, smaller collector system did not work as well on the T4 turbo. I just can't imagine that on a motor that had more cubic inches that it would not need to breath a little more than a 2.5L 4 banger.

What specific data is out there in regards to smaller vs larger piping in a divided turbo system? Again you are talking about merging into a 3" merge. I am not merging. Each exhaust gas pulse is going from the cylinder to the turbine wheel. They are only hitting one at a time. So there is not merge, each pulse is hitting the turbine individually.

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Old 02-03-2013, 02:59 PM
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Where do you get a good cone style reducer? I have only found the cheap Walker type.
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Old 02-03-2013, 03:00 PM
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Cone engineering FTW
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Old 02-03-2013, 03:02 PM
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Originally Posted by MUSTANGBRKR02 View Post
Where do you get a good cone style reducer? I have only found the cheap Walker type.

Rps has them
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Old 02-03-2013, 03:07 PM
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Woolf aircraft, cone eng and rps sell nice transitions
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Old 02-03-2013, 03:11 PM
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Originally Posted by Sarg View Post
Because I am not getting a straight answer. I am getting analogies about running, information in regards to merging the pipes together, and information about non-divided housings. I have always been told that stepping up in piping size like a step header will help with scavenging and negating exhaust pulses back towards the cylinder. Stepping DOWN from 2.5" collector into a 2.25" pipe (lets forget the word crossover as it does not apply) would be the reverse of this.

The reason I am very curious is I have seen quite the opposite in some of the turbo systems I have built and helped design on 4 cylinder motors. When going from a 1.68" primary leading to a 2.25" pipe merging into the T4 sized turbo (much smaller turbo) it was slower to spool and did not hold boost as well as the system that had a 2.0" primary that lead to a 2.5" collector merging into the turbo. Same turbo same motor. This was on a 2.5 liter Subaru motor. The smaller pipe, smaller collector system did not work as well on the T4 turbo. I just can't imagine that on a motor that had more cubic inches that it would not need to breath a little more than a 2.5L 4 banger.

What specific data is out there in regards to smaller vs larger piping in a divided turbo system? Again you are talking about merging into a 3" merge. I am not merging. Each exhaust gas pulse is going from the cylinder to the turbine wheel. They are only hitting one at a time. So there is not merge, each pulse is hitting the turbine individually.
So 2.5l, how much boost, stock heads? How much rpm did it pull?

What data can u share on performance difference between the two?
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Old 02-03-2013, 03:12 PM
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Thanks
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Old 02-03-2013, 03:20 PM
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I got it from cone engineering, but i think i would try the RPS stuff, they didnt offer them when i was building it.

you could also cut a pipe and make your own with a welded seam on the bottom, wouldn't be terribly difficult.
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Old 02-03-2013, 03:23 PM
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When I did my testing. I did steady state pulls from 3000-4000rpms under a load measuring boost every .10 seconds and boost was recorded down to the tenth of a #. Motor pulled about 200 rpms per second. On a 438", the best results were with a 1.75" header and a 2.5" crossover.
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Old 02-03-2013, 03:50 PM
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Originally Posted by Phil99vette View Post
So 2.5l, how much boost, stock heads? How much rpm did it pull?

What data can u share on performance difference between the two?
I got out of the Subaru business about a year ago and sold the laptop and tuning software, so I do not have any logs. But we were tuning it on the street with the same stretch of road. The first set up it would not hit full boost (22 psi) in 4th gear til almost 4700 rpms. Boost would fall off in the upper rpms above about 6000 no matter how many duty cylces we gave the wastegate. Heads, cams, everything were stock. We were pulling to 7000 rpms. Switched to 2" primaries and larger up pipe and it was hitting full boost about 4100 rpms and holding boost til redline. I know it is not the same motor, but that was just my experience. We got the idea to switch to the larger primaries after seeing that some of the major players in the Subaru market were going to 2" primaries on their headers and showing good gains.
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Old 02-03-2013, 04:31 PM
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Originally Posted by Sarg View Post
I got out of the Subaru business about a year ago and sold the laptop and tuning software, so I do not have any logs. But we were tuning it on the street with the same stretch of road. The first set up it would not hit full boost (22 psi) in 4th gear til almost 4700 rpms. Boost would fall off in the upper rpms above about 6000 no matter how many duty cylces we gave the wastegate. Heads, cams, everything were stock. We were pulling to 7000 rpms. Switched to 2" primaries and larger up pipe and it was hitting full boost about 4100 rpms and holding boost til redline. I know it is not the same motor, but that was just my experience. We got the idea to switch to the larger primaries after seeing that some of the major players in the Subaru market were going to 2" primaries on their headers and showing good gains.
Something there just doesn't add up. Running a 2" primary for 38ci x 22psi of airflow just dont compute in my head. What kind of backpressure were you guys seeing before and after?

Using that model as what works, We probably need to run a 2.25" header primary or even 2 3/8" which I just dont buy. Thats what you run on a 600+CI motor which has about double the CI per cylinder and more than 2.5 x the airflow.
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