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Discussion Starter · #1 ·
Hey there. Can anyone help me out with what is the largest diameter dual exhaust tubes I can run over my convertible chassis plate on my 65 without them hitting on something?
 

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I'm 2.5 too in my early 65 Vert. If I had to do it again, I'd go with 2 1/4" . If for nothing else, but a little more clearance
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2.25" definitely fits better with no contact issues. Anything larger isn't necessary until you start getting up around the mid-300hp range.
 

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Discussion Starter · #5 ·
2.25" definitely fits better with no contact issues. Anything larger isn't necessary until you start getting up around the mid-300hp range.
Hi LotsOf68s. I will be running a 347 crate with about 410HP, so do you think the 2.25inch would be too restrictive?
 

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67 Fastback T5 331 TCI Frt End, Canted 4 link rear susp
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I don't know if this helps, but this episode of Engine Master's compared 2.5" to 3" on a 600+ hp engine, and the difference was only 13 hp. Don't know how that extrapolates down to 2.25 versus 2.5 on a 400hp engine, but I guess it also depends on whether or not you would miss / notice the missing hp.
 

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Hi LotsOf68s. I will be running a 347 crate with about 410HP, so do you think the 2.25inch would be too restrictive?
At that power level yes, that would be too restrictive. With 2.5" pipe I would recommend polyurethane engine and trans mounts to limit engine/trans movement as much as possible, as well as a competent exhaust shop that can build the system carefully so it's well centered and use good quality hangers, as it will be pretty tight.
 

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Discussion Starter · #10 ·
At that power level yes, that would be too restrictive. With 2.5" pipe I would recommend polyurethane engine and trans mounts to limit engine/trans movement as much as possible, as well as a competent exhaust shop that can build the system carefully so it's well centered and use good quality hangers, as it will be pretty tight.
Thank you. I have the poly mounts for all positions so should be okay.
 

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2.5" pipes fit, but takes a bit of finesse to clear the tight confines of the tunnel and driveshaft. Guess it depends on who would be building your system and what you start with. I built a stainless Pypes kit (slightly modified for the convertible) for a friend of mine. If you're shopping for an off the shelf system, 2.25" might be your best bet, otherwise it can be accomplished without rattles and fitment issues either way.
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Hey there. Can anyone help me out with what is the largest diameter dual exhaust tubes I can run over my convertible chassis plate on my 65 without them hitting on something?
Well, if U wana run huge 4" pipes, or something like that, go to a muffler shop and have them take measurements and tell U what'll fit.
U realize the larger the diameter the pipe the more U'll pay. Furthermore, large dia. pipes will hurt the scavenging effect of the exhaust gasses.
So actually, you have to select a diameter that is going to best benefit what size engine and power levels U have.
Ya just don't slap on 3 or 4 inch exhaust pipes on a Ford 289/302, even if it's supercharged! 🤔
 

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2.25" definitely fits better with no contact issues. Anything larger isn't necessary until you start getting up around the mid-300hp range.
[/QUOT
Yup, 2 1/4 inch exhaust tubing is all a Ford small block needs.
I used 2 1/4" for my 350 horse power 289, back in 1983. If doing it again, I might go 2 1/2", but that would be the maximum I'd ever use for a sb 289 or a 302.
U get into a 351W or more, that's modified, U may want a bit more. But once U exceed a certain diameter with any engine, U'll actually hurt performance. Use your head, not your wallet! 🤔
 

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Well, if U wana run huge 4" pipes, or something like that, go to a muffler shop and have them take measurements and tell U what'll fit.
U realize the larger the diameter the pipe the more U'll pay. Furthermore, large dia. pipes will hurt the scavenging effect of the exhaust gasses.
So actually, you have to select a diameter that is going to best benefit what size engine and power levels U have.
Ya just don't slap on 3 or 4 inch exhaust pipes on a Ford 289/302, even if it's supercharged! 🤔
I know scavenging is a widely held belief, but Engine Masters tests show maximum HP with no exhaust pipes after the headers. Maybe scavenging takes place where the header pipes meet in the collector, after which any pipes are simply restriction.
 

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I know scavenging is a widely held belief, but Engine Masters tests show maximum HP with no exhaust pipes after the headers. Maybe scavenging takes place where the header pipes meet in the collector, after which any pipes are simply restriction.
Top end horsepower is absolutely about eliminating restriction. But lower speed torque is affected to some extent by effective scavenging, and most of that is indeed dependent on header/collector design. When it comes to pipe size after the collector, if it's not a restriction that limits your top end horsepower, it generally won't reduce low end torque either, especially when it comes to most hot street small blocks that aren't particularly heavy on torque.
 

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It should also be said, running larger pipe than necessary is not productive, and will generally result in louder exhaust with more undesirable drone than a properly sized system. Most people don't actually have a 400hp small block, so most people don't need to run 2.5" duals with their average 302.
 
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