2001CamaroGuy

iTrader: (6)

Could someone explain the reasoning behind the “more CFM is better” philosophy of heads.

ex: engine

388 CI big bore LS1

1 Cubic Foot = 12" * 12" * 12" = 1,728 Cubic Inches

388 CI = 388/(12*12*12) = 0.225 Cubic Feet

0.225 Cubic Feet * 7000RPM/2 (4 cycle engine only pulls air every other rotation) = 785 CFM

785 CFM/8 cylinders = 98.1 CFM/cylinder (@ 100% V/E)


Looking at the required CFM per cylinder mathematically, a 388 engine that runs up to 7000 RPM would only pull 98CFM per cylinder. Why then does it matter if your head's intake port flow 200 or 500 CFM (its all excess right)?

verbs

It's not better if you have to sacrifice velocity to get there.....don't forget it's what the heads flow through the intake that counts, not just through the heads. So if a 350cfm head flows 300cfm through the intake and so does a 320cfm head, then yeah it's pointless.

2001CamaroGuy

iTrader: (6)

but even that 320 CFM head is WAYYYY more than the engine can actually pull in, how is it helping make more HP?

FASTONE

Because an engine is an airpump, the more air you get in and out the more hp that they"ll produce.theres a formula of about 2.2 hp per cu. ft. intake airflow, of course this will vary just say your heads flow 300cfm you could make about 660hfwhp,this is if your heads are the limiting factor I mean you have to have good intake, exhaust,cam timing etc.This is pretty much the limit,I mean your heads will always determine your hp. In your formula you figured cfm with the intake valve open 100% of the time, the valve is only open a fraction of the time, also at 7000rpm its opening and closing 3500 times a minute,so you can see why you need a head to flow more air than 100%VE.Hope this helps you understand why you want all the flow you can get without killing your velocity,thats why smaller port heads that flow real well make more hp, its kinda like a ram effect with smaller port the air has to travel faster to maintain the volume so the air behind the intake valve pushes the air down to the valve even when its closed so when the valve opens again the air is trying to get past the valve into the cyclinder.Theres a lot to heads,porting,etc, but it all comes down to getting the air and fuel in and out as guickly as possibly.

2001CamaroGuy

iTrader: (6)

but.......what I'm missing is:

how does a head that can flow twice as much air as the engine can injest make more power? The engine CAN NOT pull in 300 CFM yet a 300CFM head makes more power than a 100CFM head. WHY?

FASTONE

What your are missing is that 300cfm is at .600 lift the valve is only open that much 1 time on the intake cycle whereas its open 2 times all other lifts going up then going down:example opening OK .050,.100,.200,.300,.400,.500,.600 now intake is closing.500,.400,.300,.200,.100,.050 now intake valve is closed, this is happpening 3000 times a minute at 6000 RPM's or 50 times per second .Air is not flowing at max lift but a small percent of this lift cycle, also heads are tested at.028 ins. of vacuum when you have your gas mashed down you may have .002 ins. of vacuum a LOT LESS!!!

racer7088

That would be if each head was flowing for a true 720 degrees of rotation or another words non-stop but it doesn't. It breathes mostly for only around 240-290 degrees of intake duration of those 720 degrees a 4 cycle goes through or about 240/720-290/720 which is 33-40% of the cycle or about (98CFM/.33) - (98CFM/.40) or about 245-295 CFM when they are actually breathing. They AVERAGE 98 or so in your formula at that rpm which isn't that terribly far off but again, you were looking at AVERAGE flow.

If a head flows 330 CFM at 28 inches H2O (one psi) pressure drop than if it is only really flowing 295 CFM it will do do at a lesser pressure drop or anotherwords flow denser air than the lesser port did so you will pick up power from the less restrictive port.

66ImpalaLT1

Your math assumes the engine intakes air for an entire revolution, then skips one. In reality it brings in air for less than 1/2 of a revolution then skips one.

2001CamaroGuy

iTrader: (6)

yeah, but I also mentioned that my math was for 100% VE (so trying to take that into account). Thus I came up with the problem of "if the engine only needs X amount of air, how do heads that flow 3X amount of air (on a flow bench) make more power?"



basically what I'm getting from everyone is:

"Because the valve spends such a short time off the seat, you need more flow capacity (lack of resistance to flow) to be able to actually get the air moving and into the chamber before the valve shuts. Its not that the 350CFM head will flow more air than the 300CFM head, but rather that the 350CFM head has less resistance than the 300CFM and therefore you are able to better take advantage of the time the valve is off the seat."

does that seem right?

gun5l1ng3r

I understood the last one

Iv_z28

Sort of. You need to injest your 100CFM, and your 300CFM head would easily do it IF the valves where open all the time. However if you take into account that the valve is actually open ~240/360 = 2/3 of your calculated time, and then out of that time spend at the max lift (300CFM) a miniscule amount of time, the rest being spent at much lower lift & CFM, then you end up with needing ~300-350CFM. Take a typical cam and do the integral of CFM over the angle.

That's why it's important to look at the whole area under the CFM curve, not the peak #, and that's why it's important to match cam and heads.
However I think you make a valid point in that it wouldn't make any sense to have a 10000cfm head&intake system for a typical cam out there.