krackle82

If a heads intake port will flow, say 220 cfm, is it ideal for the intake manifold runner feeding that cylinder to also flow 220, or is what the manifold "flows" insignificant? The reason I ask is I don't subscribe to "the bigger the better" and I don't want to waste money on something I dont need or is potentially counter-intuitive to what I'm trying to do.

DeltaElite121

Depends on the purpose. For an all out drag motor yeah it is because port velocity isn't as critical since we are dealing with strictly a higher RPM motor. Your average street car however needs that extra port velocity to help build power/torque at a lower RPM.

This gets into the classical dual vs single plane intake debate which has be brought up many times across the internet on various boards.

Drac0nic

The cross sectional area, length and RPM you want to turn are the biggest factors in determining which intake to run. Plenum volume is also in there, but somewhat less controllable except say spacers. Bigger is far from always better, here's an interesting table to look at.

users.erols.com/srweiss/tablehdp.htm

Keep in mind as a rule of thumb you can make roughly 2HP per CFM on a decently set up 8 cylinder engine that's turning adequate RPM. This table lists the minimum RPM to take advantage of the flow of your head properly given a certain CID, so from that info we can practically draw a line over to figure out what RPM we need to make a set amount of power.

As an example that means that if your intake port flows 200 CFM and you have a 400CID engine (50CID/cyl) you would need to spin 4444 RPM at a minimum to make peak power and if things are set up decently you should make about 400hp. You can make more than this indicates, but it's dependent on the quality of the engine build. Race car engines (you know, people with cubic dollars) spend tons of money so that they can break out of this generalization.

Runner CSA determines where you're going to make peak torque, as an example here's an easy to use calc for that:

https://www.rbracing-rsr.com/runnertorquecalc.html

This is actually a pretty close calculator in many cases. Exhaust tuning, intake tuning etc. can spread tq out over a wide area, but in general this will get you in the area of peak torque if everything else is relatively close (not trying to feed a 400 through stock exhaust/manifolds or something.)

Lastly, runner length has to do with peak HP. You have a resonance that determines the RPM you're going to make peak HP at based on your CI and partially on intake valve events. Even radical cam changes only fudge it a bit though. The reason has to do with the amount of time it takes the intake pulse to travel to the end of the runner, and back to the intake valve. When that timing is perfect it effectively supercharges your engine in a narrow area. You can get this effect from the exhaust as well, it can effectively "suck" a new charge into the cylinder due to resonance. Complex topics for sure.

Do you have an application in mind?k

krackle82

Hey thanks for the info, guys. I am building a street/strip (mostly street) small block in the future and I'm doing all my homework and lining up all my little duckies now. I know the questions I'm asking wont really pertain to a street motor, but I want to make sure I'm not selling myself short. Budgets tight and I'm working with what I have, which is a 2 bolt main 1pc. Gonne be a 355 and decked to 9. Im looking at a stock replacement crank from Scat and will most likely use rods and pistons out of a truck with either reworked 305 HO heads or early vortecs if i can find them and a cam around 220* @ .050 and .480-ish lift. I've got to the point where Im looking into intakes now and Im sure an aftermarket one will do me just fine with some port matching.

Damon

If this is one of your first performance builds, I would ALWAYS suggest you "follow a known-good-recipe" and not try anything outlandish.

The Vortec heads will blow those 305 castings into the weeds. It's not even close. Use the Vortecs.

With typical flattops (make sure they have a 1.56" pin height, not the 1.54" pin height used in a lot cheap rebuilder pistons), a .039" gasket and zero-decking the block, you should be right around 10:1 compression with Vortec heads.

Agreed, a cam in that range would be about right for something leaned more toward street use. If you wanted to lean it more towards strip use you could bump it up into the 230* range.

Nice dual plane, like a Performer RPM or similar; some headers and and a good flowing exhaust behind them and you will be pushing up into the 400HP range with a sharp tune.