KAAMA

iTrader: (1)

This question is for all the air flow guys or those familiar with air flow. I have heard it said that there is a small window when it comes to proper head intake runner cc size vs cubic inches on a naturally aspirated MARINE engine. Can any of you guys tell me where that window is, or what the perameters are when running up to let's say 5500-5800rpm? The formula being: (Intake runner cc size DEVIDED BY cubic inches). I will give you a couple of examples...

269cc head on a 454cid comes out to .59
269cc head on a 509cid comes out to .52
310cc head on a 454cid comes out to .68
310cc head on a 509cid comes out to .60
310cc head on a 540cid comes out to .57
357cc head on a 565cid comes out to .63
335cc head on a 565cid comes out to .59

Mcollinstn, JimV, cstraub, rmbuilder, Tomcat and anyone else who knows, and has experience with air flow please chime in and enlighten me if you desire. I don't mind if responses get a little spirited here as long as you are passionate about your arguments. I am just looking for some insight, understanding, pro's & con's on this theory/formula. Thanks

PatriYacht

Interesting comparison KAMMA. Maybe that's why we're all looking at cams for our big inch motors that would also work well in 454's. The ports flow the air. There's no longer the need to hold valves open longer to fill large cylinders.

dyno

Mark step away from the bug spray!!!!you got the best of everything now!!

rmbuilder

Kaama,
It’s my opinion that addressing only the cc to displacement ratio does not provide adequate information to arrive at a value that is consistent enough to determine the best cylinder head. The variables of port cross sectional area, flow characteristics, and port velocity differ from head to head and manufacturer to manufacturer. We have a program that graphs CC to flow ratio. By examining flow @ relevant lift points (example .200”-700”) we arrive at a “flow value” (CFM per CC) for each individual port to be used for evaluative purposes. This way instead of comparing peak flow values we can establish a reference number that includes the low/mid/high lift value to ports of equal/unequal volume. This also applies to the exhaust side and the resulting I/E ratios. When you establish what flow is required per your displacement then you can address which head will deliver the required flow through the (lowest volume) highest velocity orifice practical. This is just the tip of the iceberg, I’m sure Jim V and others could contribute volumes to the topic.
Bob

PatriYacht

Bob, did you get my p.m?

mopower

[QUOTE=KAAMA]This question is for all the air flow guys or those familiar with air flow.

269cc head on a 454cid comes out to .59
269cc head on a 509cid comes out to .52
310cc head on a 454cid comes out to .68
310cc head on a 509cid comes out to .60
310cc head on a 540cid comes out to .57
357cc head on a 565cid comes out to .63
335cc head on a 565cid comes out to .59

QUOTE]
I have found there is no simple solution since different heads with the same runner size can flow differently.
Take the volume of one cylinder times 1/2 the max rpm( since it only sucks in every other stroke). This is a good flow number for the port. This , to me , is more important than size.
Keeping the port as small as possible on a N/A engine will give to the highest port volocity and therfore the best charge. Long story short

KAAMA

iTrader: (1)

[QUOTE=mopower]Morpower,

Okay, so then more volumn (from a larger cc intake runner) doesn't mean much? For the sake of arguement let's assume that we have TWO different cc sized heads with both heads flowing exactly the same flow numbers and going on the same CID engine with the same cam. Keep in mind that this is for a marine application. Let's say the target RPM is 5500-5800rpm. Is the smaller head going to give the better performance even though it doesn't flow the same volumn as the larger cc sized head?

Let's take your formula as you have described---my bore size and target RPM for instance: 4.600" X 2900rpm= 13340 Is this correct? What does the 13340 equate to or mean?

cooltoys61

925cc x 2900 rpm for 454. Equals 2682500. Still don't know what that means,,,but that's what he said. 268?

KAAMA

iTrader: (1)

Thanks for the input so far fellas---keep it coming. By the way, anyone is welcome to respond to my latest question(s) in post #7.

SeaRay Jim

A comment/question from the peanut gallery pertaining to question in #7.

Wouldn't velocity come into play regardless of volume and be critical because you're trying to fill the chamber as much as possible, as quick as possible based on how long you have the valve open? Intake side anyways.....

You know, it kinda makes something simple, fill the chamber as close to 100% as possible at every RPM with just the right A/F mixture, sound complicated. LOL