Heads / int cc’s? Vs flow Vs cross section Vs air speed
 

A lot for one topic… POWER!

I just want to discuss the variables involved here as stated in the title. I’ve always liked the idea of maximizing flow with the least amount of space “cc’s”. Makes sense right… Take a 540 for example making peak power on the low to mid 6k’s. Personally id think about a 315 cc intake port would be close to ideal. What if you increased flow by say 20% BUT also increase int port cc’s by 20%. With the correct x section and ported intake manifold and increased air speed with smaller entry port entry 2.250 X 1.750 Good or bad? What I’m curious of is if the int port cc’s is just a ref number so to speak and can become irrelevant with other key factors in place?

(This is a discussion)

I dont understand the intake port volume thing, intake port CSA is what matters. Wallace racing calculator has been very accurate to determine the needed CSA.

This isn’t the OSO of 15 years ago. Good luck with that discussion lol. The answer is going to be buy Mercury. Most guys I know in the performance world nowadays don’t even know what prop pitch they’re running…

Lets say for discussion your intake has a CSA of 4 sq inches, flow 350 cfm, your port has a CSA of 5.5 sq inches and flows 450 cfm, its not going to play nice, turbulent flow etc
scenario two: lets say for discussion you have a 5 sq inch CSA intake runner and port, flows like 500 cfm , motors "small" like a 540, at part throttle and at wot until a certain RPM the fuel signals going to be terrible as it has no velocity, ie, ports big and lazy, fuels going to puddle, fall out of air, . Going to hit a certain rpm , smooth out and start working well, the question is, whats THAT rpm. If it takes off at 5500 and has to buzz too 7000 to run good, will be a miserable package for most normal pleasure boaters. The problem with boaters is the "bigger is better" idea, Ive had guys call me wanting to run profiler 370/380 whatever heads on a 509/540 in a boat sub 6000. Its irrelevant if a marine engine makes a big number at a unsustainable rpm unless your talking dry sumps, best of best parts, short rebuild intervals or a jet boat/vee drive where it will buzz to wot for 20 seconds 4 times a day. When you start getting torque peak much above 5000 rpms, unless motors are fairly "big" and making pretty good tq lower, it becomes "hard to get there".
Years ago a guy stopped by that seen me on here, he had a 27 fountain, built 540 done by a employee or ex employee of sterling. Had biggest cam Ive ever seen in a NA 540, had a dyno sheet showing like 740 hp at 6800 or somewhere around there (prob been 15 years so exact details are fuzzy). Long story short he tried multiple props smaller and smaller until it finally rpm'd (like 30, 28, 26 down to 24). Oil ran hot, had terrible cruising speed. His lack of tq made boat a turd, i think he turned the bearings to copper in less than 20 hrs and had that thing back out from all the foaming, oil heat, rpms using a wetsump and trying to spin 6500+, Smitty

I have been building large cubic inch BBC motors for about 15 years now starting with drag racing and moving into offshore boats. From a drag racing perspective you are looking for large flow numbers at high rpm. I was shifting at 8k in a 565 but my heads were flowing over 400 cfm. I currently have a 555 I pulled out of my offshore boat and put into my jet boat. This motor is is a 10.5:1 motor with Brodix BB2Xtra CNC heads with Jesel shaft rockers and a large Bullet custom hydraulic roller (.634 lift I think), also has a matching Brodix intake. I thought this motor would just make the jet boat a scary ride. I was wrong. I can't get the motor to turn over 5100 rpm because of the size of the boat and the impeller cut. The heads are too big and it can't get into the rpm it needs to make power. I am a firm believer in size does matter and larger is not always better. Are CFM numbers more important that the intake port volume CC's?

I am currently building a 496 to put into the boat but I am planning on using Profiler 290 cc oval port heads with their matching intake to make peak power in the 5500-6k range. There are a lot of great YouTube dyno videos of head comparisons and some of them are shocking as the smaller heads are making a lot more power. You need to match the motor parts to the intended use. I think people over carb too. Larger is not always better. I am talking N/A builds.

The discussions on this site is what keeps me coming back there are some smart people on this site.

So let’s say the CSA is 4.63 sq. Inches

At the flange its 3.85 sq in.

As a quick note:
Remember, raised intake ports are a little longer, so if the cross sectional area (csa) is the same as another head, the runner will pour higher. IE: Runner will have larger CC's. because of math. CC's is a function of length x width x length. For cylinder head port sizing we are considered with Width X height. This is referenced to as the CSA. Again, cross sectional area.

How much valve lift are you comfortable running? Ill tell you which head is best (on paper)

Which head would you indicate for 615 cid w/ .632 lift?

Id go to dyno loaded for bear, modded intakes, port matched but un modded intakes, etc. Dyno them all BUT knowing what I know about your heads, large ports, high flow numbers Id hate to see you KILL all the gains by restricting it with a intake thats too small. BUT the dyno will tell real story, sometimes results are opposite of what we expect. This is HYPOTHETITCAL for sake of discussion, lets say you got this 450, 480 cfm port flow, ports pretty damn big to get that number, no way around it. On dyno motors lazy and doesnt perk up till top of your power band (IM NOT SAYING THIS IS WHATS GOING TO HAPPEN but, your in prostock head flow territory), you bolt a intake thats Restrictive on, it makes your combination actually work better overall for some reason, ie, creates a restriction thats needed, something. Would be NICE to have those parts on hand to try, Id bring every possible intake you can to dyno and test them all. Maybe one that is ported close to your heads works best, you wont know till you dyno, Smitty