calculating intake runner length

[ap72]

When calculating runner length for ITB's where does the runner length end? I know the average port length is figured into it, but do you stop at the carb flange, the butterfly, the carb opening, the velocity stack?

I'm just trying to figure out how long to make my intake runners.

Entire engine specs are:
Ford 2.3L Lima engine
3.78X3.13 4cyl
9.5:1 compression (I know its low)
ported head 1.89X1.59 valves, not flow tested
288/288 (ADV) 240/240 (.050") .500/.500 cam on a 112LSA 108 ICL (Elgin 1104 cam)
Mustang exhaust manifold to 2.5" exhaust
Manual trans

Keihin 40mm CVR carbs from a zx9R

[MODNROD]

End of stacks.

Gotta love those ZX/GSXR/Busa throttle bodies.........

[ap72]

End of stacks.

Gotta love those ZX/GSXR/Busa throttle bodies.........

Keep quiet about that, this set cost me $30 and I'd like the next set to be around that price too.

Seems like every 4 banger in the UK is running "bike carbs"

[MODNROD]

OOPS!!!! Sorry......

I got a couple of sets in the shed. You never know when the urge to hotrod something will hit......

[BlitzA64]

Not to Hijack but along the same line, does cross section of the runner have an effect? I use the Pipemax length on my 2.5" IR Fuel Injection but most all others in my class have substatially longer ram tubes. Maybe they are tuning the intake for a lower rpm? I use the tip of the bell to the valve for the length........

[#84Dave]

Technically, the length of the induction tract runs from the intake valve seat to about .5" beyond the top of the bell/velocity stack atop the carb/throttle-body. That's where the compression & expansion waves 'turn around' to go back down the IR induction tract. If there's taper in the runners of the intake manifold, the induction will 'tune' to a shorter length than the actual physical length of the runner.
-Dave-

[Troy Patterson]

Not to Hijack but along the same line, does cross section of the runner have an effect? I use the Pipemax length on my 2.5" IR Fuel Injection but most all others in my class have substatially longer ram tubes. Maybe they are tuning the intake for a lower rpm? I use the tip of the bell to the valve for the length........

I'd recommend experimenting to see what your engine likes best. Computer simulation / analysis is a great tool and Pipemax may be excellent, but if you are in competition by all means experiment to "qualify" the results and to see if there isn't something more.

Troy Patterson blog tmpcarbs.blogspot website TMPCarbs.net Engine Professionals Magazine's TMP Tuning article, pg. 44

[ap72]

This is not competitive, I just figured since I have to make a manifold I'd try to get as close to theoretically ideal as I could. Its just another one of my homebrewed experiments.