T/A Medic

iTrader: (1)

I was thinking what does it matter if you have two different heads with different size ports.

for arguments sake both heads flow exactly same everywhere...

but

ie, Head 1 has 35cc more intake port than Head 2...

Same amount of air enters the cylinder correct? shouldnt make same power?

What does it matter if for example 320 cfm enters the heads faster through a smaller port... or slower through a bigger port... ?

Or is there something im missing somewhere?

Thanks.

jrp

iTrader: (16)

velocity

T/A Medic

iTrader: (1)

velocity??? care to explain what you mean???

320 cfm wether it enters slow or fast into a combustion chamber/cyl is still 320 cfm isnt it?

or does the cfm we get on our flow sheets not directly relate to how much air is entering the cyl? meaning smaller port higher velocity will allow more air into the cyl. ?

LSONE

iTrader: (1)

LSONE

iTrader: (1)

think of it this way, make a cone with a small sheet of paper. if you blow into the small end, you can hardly feel any air comming out of the large end, BUT if you blow into the large end, you can feel a ton of air going out the little end.

just because two sets of heads flow the same numbers doesnt mean they will have an equal ability to fill a cylinder with air in the short time the valve is open.

Cal

iTrader: (1)

Another thing is the one with higher velocity is going to work better at low rpm. Better streetability. More velocity also means more turbulence inside the combustion chamber, which means you don't have to run as much spark timing to get a given amount of power.

Cstraub

iTrader: (1)

JRP wins the prize.
For example sake, lets say the intake valve on a cylinder on an engine is open 1 second at 5000 rpm. You have a 240cc port head and a 210cc port. Both flow the same CFM Port velocity is 500fps on the 240 head and it is 650 on the 210 head. Now which one will give you better cylinder fill in the amount of time you have?

Chris

BBQLS1

No. You have to compare flow at various lifts. Really the best way to compare is two graph the flow vs lift and the one with the most area under the curve may be what you want. And we still haven't considered velocity in the port (very very very important.) And the cam being used in an engine. If a head flows 310 at .700 lift and your bumpstick is only lifting the valve .510 lift then you are not getting the full use of the head. The easiest way to sumarize what is important: "The goal is to move as much air through the smallest port possible" - Warren Johnson

P.S. combustion chamber design is important to. Heads are very complex. More so than they look.

xxxhp

Good explanation . I think the comparison should be between a 240cc head with 500fps & more CFM (~330) versus a 210cc port with 650fps & less CFM (~310). In this case which head do you think would fill the cylinder better?

CHIEFMB2

iTrader: (3)

picture this: Piston approaching TDC on the exhaust stroke. The intake valve starts to open at so many degrees of the crank position BTDC. As it opens and the exhaust valve closes at so many degrees of the crank position. Considering the motor is turning some higher RPM's and we have little exhaust gases contaminating the intake charge. Again, as the intake valve is opening the charge is starting to enter the cylinder. The amount of charge at .200 and .300 is entering, we need velocity, we need CFM at these lifts to start packing the cylinder. As the piston moves down the cylinder and we have the intake valve opening to .400 and .500 the amount of charge increases and hopefully the velocity is there also(piston speed helps suck the charge into the cylinder). The piston still in a downward motion and valve at max lift, we see the charge packing the cylinder by way of velocity and volume. As the valve starts to close and the piston slowing down and changing directions at BDC. The piston starts it's upward motion into the compression stroke with the intake valve almost closed, trying to get in as much charge as possible before it closes using velocity. Now the intake valve closes at so many degrees ABDC. Now lets look at the charge inside the cylinder. Where is the charge in the cylinder? To maximize the combustion, the charge needs to be dispersed throughout the whole cylinder. This is where combustion chamber design and principles of Tumble and Swirl play a big role. Swirl and Tumble is another chapter in itself. So, back to the piston moving up the cylinder, the piston is now compressing the charge. Now we have made it back to TDC on the combustion stroke the spark ignites the charge(depending on the timing of the spark, depends on when it will ignite it). If we maximized the amount charge we could put into the cylinder then we will have a strong explosion of the charge(compression ratio will also have alot to do with the tightness of the charge at the time of ignition). Now we are making power. This is why velocity and volume needs to be maximized at all lifts of the intake valve. So anyone can make a intake runner shine, but can they port a set of heads to complement a particular cam. That is where time consumption and a flow bench with velocity tube come into play.

Adrenaline_Z

Another consideration is the bench testing, and testing method used to
get the numbers.

If Shop A uses 28" of Water with a pitot tube, and Shop B uses 28" of water
without a pitot tube, the results are skewed. It's a nice marketing tool so
be careful.

Further still, a flow bench uses a constant flow of air to measure CFM.
Engines create reciprocating air waves and pulses. The tuned length of
the head runner and intake runner air going to act EXTREMELY different
on the engine as RPM changes.

There are gains to be made by tuning head and intake runner length which
does not get acknowledged on a flow bench.

And...when you add fuel mass to the equation, port flow characteristics
change. Thankfully with EFI setups, the fuel doesn't have far to travel,
but it's still a factor.

The charge dispersion is normally controlled by the quench height and chamber
design.

As the piston nears TDC on the compression stroke, any charge between the head
and piston should be designed to move into the chamber. Squeezing the
charge out of the quench area is when turbulence and swirl are most pronounced..