I have 223 that I'd like to put a 2 or 3 carb set up on, but I don't have the cash flow to purchase a 2 or 3 intake. I'd like to take a shot at building one from scratch. Are there any requirements for CFM, engine displacement, etc that need to be taken into consideration when fabricating an intake manifold? Thanks -MO
You could try and guess at the volumetric requirements of your engine at the RPM you want it to work best. Then try to relate that to the flow of the carbs you want to run. Or do it like everybody else and do what seems to be working for people with cars like yours. I made a manifold for my 311 cid GMC six and used three 270 cfm Holley two bbrls. Worked OK. Then I replaced them with 500s. Still worked OK. Proves i don't know nothing.
Here's a few I've done for inlines: The best advice I can offer is to use small sized carbs. Longer runner length gives better low end torque. A large common plenum is good for high RPM engine use. A smaller plenum gives better driveability and better throttle response. I prefer a common plenum over IR setups for the street. Good luck with your project.
Has anyone ever built a variable volume plenum? Maybe run it from a vacuum dashpot, so that if the vacuum falls, the plenum reduces in volume.
Quote ["The best advice I can offer is to use small sized carbs. Longer runner length gives better low end torque."] Quote Good advice. Here is a pic of a manifold I used to make quite a few of for model A's and B's. It used Ford 6 single throat carbs. It had excellent off idle response.
The typical twin carb manifold was made to use your stock carb, plus a second, identical carb. This meant you only had to buy one carb. Two stock carbs is plenty for a mild street engine, in fact you would have to build a pretty hot engine to need 3. You may be able to buy pre cut steel flanges, if not you can cut your own using a gasket as a pattern. It may be convenient to use one big flange and weld up headers and intake at the same time.
There are some basic formulas for intake runner design . . . they revolve around tuning the intake "pulse" when the intake valve closes and the charge runs back up the intake runner related to RPM. You can lengthen to tune for 1st wave or 2nd and usually 3rd is too long. Typically runner size is 80-85% of the valve area. http://www.wallaceracing.com/runnertorquecalc.php
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