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GoFast!
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Discussion Starter #1
How did you get 159hp out of... 1 3/4" pipes?!! It goes against everything I've ever read about header size vs displacement. Do you have hi-comp with racing gas? Have you tried a larger pipe to see if there's even more? I'm totally amazed and baffled. :dunno:
 

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GoFast!
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263 Posts
Discussion Starter #2
Yeah I know, it's just that a 1 3/4" pipe is supposed to be choking a 124", and it's not.
 

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GoFast!
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Discussion Starter #3
SEDELUXE said:
well i dont know who said that . i know a lot of guys running 113,114 and 117 and 124 out there with fat cats and thunder headers.1/34 pipes maybe it has more to do with who tunes it rather then whats on it.
Well here's a quote, from RB:

Once the motors get bumped up in displacement and cnc ported cylinder heads are installed then the picture changes. Going from 80 or 88 cubic inches to 117/124/126/135 cubic inches represents more than a 50% increase in air being pumped out the exhaust port. A 1 3/4" or 1 7/8" design is just to small. Bigger motors require 2" primary tubes.
 

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skidmark316 said:
Well here's a quote, from RB:

Once the motors get bumped up in displacement and cnc ported cylinder heads are installed then the picture changes. Going from 80 or 88 cubic inches to 117/124/126/135 cubic inches represents more than a 50% increase in air being pumped out the exhaust port. A 1 3/4" or 1 7/8" design is just to small. Bigger motors require 2" primary tubes.

Im not saying RB doen't know there stuff hell I'm the last person in the world to question their info but in researching everything for every person that says one thing there is someone else saying just the oppisite..how big are the primaries on cage headers..normally 1.5 to 1.85..and most make more HP per CI than our Vtwins...just a thought
 

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GoFast!
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Discussion Starter #5
AZFXSTB said:
how big are the primaries on cage headers..normally 1.5 to 1.85..and most make more HP per CI than our Vtwins...just a thought
Hmm, a 124" vtwin would equate to a 492ci V8 engine. If you look at a header company, like say Hooker, they sell 2" or 2 1/4" primary headers for an engine of that size.
 

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skidmark316 said:
Hmm, a 124" vtwin would equate to a 492ci V8 engine. If you look at a header company, like say Hooker, they sell 2" or 2 1/4" primary headers for an engine of that size.
But how extreme is an engine of that size, I'm not disagreeing with your line of thought but if you take the HP to CI Steve is getting 1.27 per CI and put that to a street motor 1.27 x say 350 its 434HP thats not a crazy build at all and some OE's are putting out engines with those type of numbers and I'm sure with 1.75 or smaller primaries (or cast exhaust manifolds). Would you get more with bigger primaries i'm sure you would but at cost of low end TQ. Now using 1.27 x 492 is 624HP so I guess you could look at it from both sides with just pure numbers...If you look at HP per CI 1.75 primaries seem right in line with what I would expect but if you go by just CI and assume the same HP per CI it does seem small but what do i know im just bored;!pstwhore
 

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skidmark316 said:
How did you get 159hp out of... 1 3/4" pipes?!! It goes against everything I've ever read about header size vs displacement. Do you have hi-comp with racing gas? Have you tried a larger pipe to see if there's even more? I'm totally amazed and baffled. :dunno:
There is no "Optimum" pipe diameter for all RPM ranges. The 1 3/4 Pipes "Could" be the right size, depending on where in the RPM range you want your power. The optimum velocity for exhaust gasses (In a V-Twin) is approx 300 ft/sec. A larger pipe (Greater Volume) will slow down exhaust gases at lower RPM, and allow greater exhaust gas velocity in higher RPM's. THUS, A smaller diameter pipe will perform better in the lower RPM band, a larger pipe will perform better at a higher RPM band.

A 2" pipe will raise the RPM of your max torque/HP anywhere between 5-800 higher up the RPM Band, depending on the build.

It is true that at high RPM, the 1 3/4" pipes could restrict the motor a bit, but lets face it, how often do you rev your engine above say 5500 RPM?

I went with a 2" RB LSR on my ride (124"), but it is a Dyna, and relatively light in comparison to a bagger, and I rarely have a passenger (She has her own bike). As it is, I can usually break the tire loose with a quick snap of the throttle at 2500-2700 RPM, so I'm not sure more torque at that RPM would do me much good. In a heavy bagger ... especially riding 2-up often it would make more sense to have the power come on stronger in the lower RPM range.

That's why you see many opting for ... let's say ... a Fatcat over the Boarzilla in large inch builds. The Fatcat builds more torque where you use it.
 
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