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Hey guys,
Building a 68 resto mod. EFI on a 289.
I have a set of full Length headers with the O2 ports in them with 3" collectors.

I want to put a H-pipe on it or X-pipe, haven't decided yet. What I need to know is how far back the exhaust the connection between the two pipes needs to be.

1. Is there a math equation to figure this out based on flow numbers and pipe diameter? Or is it just a standard location a certain distance back from the rear of the block? What is this distance and if that's not where I measure from where do I?

2. How long should the connecting tube be? (Distance between the driver and passenger side pipes where they are joined together.)

3. What's the advantages/disadvantages to the H-Pipe Vs the X-Pipe?

I haven't found information on this in my google hunt so asking here.
I plan on going down to 2 1/4- 2 1/2 for the rest of the pipe and still going over the axle and out the tail end with the pipes.

I'm lucky and actually have access to used single, double, and triple chamber flowmasters to see which nets the proper amount of flow to back pressure ratio for HP and torque so I'll tackle that next but I want to get the cross pipe location correct for the H-pipe and re-cutting and welding pipes till I find the right spot is just a waist of time and money.

Any assistance appreciated.
 

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Wow, an EFI to a 289. That's exactly what I had decided to do for my summer project, but I don't have a particular car in mind as of yet. For now I'm just tracking down parts and doing research on it to figure out it is I'm going to build the 289 up.
 

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I vote Lakewoods. I hope to run side exhaust but I doubt they'll be Lakewood style since those are more costly than just a piece of 2 1/4 exhaust pipe with a 45 degree cut sticking out of the side. Think AAR 'Cuda style.
 

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I have never had an issue with any set distance from the collector. The purpose of the "H" or "X" pipe is to allow equal presser for both sides of the exhaust. Typically the closer to the collector the better as it will allow equal pressurization to happen as soon as possible. As for the "H" vs. "X" design. That is just about spacing from pipe to pipe. The "X" may yield a tiny bit more performance. I personally run a "H" setup on mine with 2 1/2" pipes. It allows me to tuck the pipe just behind the belly of the trans. Thus keeping everything up and out of the way of those nasty speed bumps. But also makes it easy for routing the rest of the exhaust out the back without having to make any other "adjustments," for clearances of anything else.
 
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