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Help with header primary sizes


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I am ordering a BNM unequal length header and now must decide on the primary tube size. The header uses a 4 to 1 collector with a 2.25" output. I am using a Grimmspeed standard up-pipe and VF52. This is an autocross setup so spool is very important.

 

A) 1.5 sch10 which has a OD of 1.9, ID of 1.68

 

B) 1.5 sch40 which has a OD of 1.9, ID is 1.60

 

Area of 1.60 in = 2.01 in^2

Area of 1.68 in = 2.22 in^2

Area of 2.25 in = 3.98 in^2

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Spool is better with ELH. Also, comparatively speaking, the VF52 is slow to spool for AutoX. If you're only going to use it for AutoX, believe it or not, the Td04 is an excellent choice as it spools extremely quickly and you'll never use it all the way up. The Custom VF40 is also a good choice-slightly slower spool (about 100-200 rpm) but it pulls much harder in the middle and again, you'll never run it all the way out to redline.
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Spool is better with ELH. Also, comparatively speaking, the VF52 is slow to spool for AutoX. If you're only going to use it for AutoX, believe it or not, the Td04 is an excellent choice as it spools extremely quickly and you'll never use it all the way up. The Custom VF40 is also a good choice-slightly slower spool (about 100-200 rpm) but it pulls much harder in the middle and again, you'll never run it all the way out to redline.

 

I still have the stock VF46 that came on my Spec.B, it's currently installed on my daily LGT. The VF52 that I'm running on my auto-x Spec.B you actually built for me years ago.

 

So I could swap turbos between cars, put the stock VF46 back onto the auto-x Spec.B and put the VF52 onto the daily LGT. The auto-x Spec.B is a 6 speed so short gears and runs on E85.

 

Anyway, I'm leaning towards the smaller ID header primary tube. At 1.6" ID, they will still be 0.1" bigger than the exhaust ports on the heads. I know there are benefits to equal length header but I just can't do it...

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