MyShibbyZ28

So, I am really starting to get into the dynamics of how cars work, the geometery, math, physics, all that fun stuff. I have a question I don't quite understand yet though.

I gave Lloyd a quick couple PM's and asked about how big the ports can get on heads. He said if the ports get to big the lower torque gets killed, but raises upper horsepower. The racing 215cc heads are probably what a lot of people concider big.
A stock engine runs at roughly 80% efficiency, right? I found some math stuff to do (I love math ).

CFM = CF of engine x heighest RPM / 2
CFM = .202456 x 6000 / 2
CFM = 607.368
.8 CFM efficiency = roughly 486 CFM

Now divide that by 8 and that is how much each cylinder should take correct?

486 / 8 = 60.7368

I have an SLP CAI on and found some other formulas and junk to figure out how much that flows, but long story short, it flows a little over 1000 CFM. More than enough. So, going by some other flow charts I've seen, they flow at max a 275 CFM.

Here's the question I do not understand, how can a port too big hurt the engine's low torque? I mean, the cylinder can already get 275 CFM but only really needs 60, isn't that overkill? I understand the cylinder needs the air at a rapid rate, but 275? This is where I thought of something, I dont know if it is right or not.

If the engine can get all the air it needs really easily, like say the heads were flowing 400 CFM just for this purpose of discussion, does it put the engine into a free wheeling mode? Like it doesn't need to work to get the air or power, so that's why it hurts the lower torque and raises higher HP?

Another thing I read is, long tube headers can be bad depending on the application? I thought it was good to have no back pressure at all? Or is that the same concept, the engine is getting everything it needs to basically just spin, not work.

I'm having trouble understanding the concept of vacuum and back pressure.

flatlander757

I think it has to do with velocity as well because since the same amount of air is going out of the head through a larger port, it is not moving as fast.

That's why you can port the hell out of a set of heads but if it isn't done correctly it won't do much good.

MyShibbyZ28

How does velocity factor into it? If it is getting the same amount of air needed, just a lot easier, shouldn't that add more power? And it's still going to go through the same sized valve so it'll always have velocity to a certain extent, it's just how well can the air get to the valve i would think. Still very confused. Maybe Lloyd should get in here and tell me everything

flatlander757

I'm not entirely knowledgeable on the subject, but it adversely affects the scavenging that is needed to maximize power.

It's good that you like math... I sleep in that class a lot

flatlander757

by freewheeling... do you mean like when you are pedalling a bike as fast as you can and you get to the point where it's like pedalling with no load and your feet fly off and you get your shins torn to hell and you get your legs pulled under the pedals/rear pegs and you fly forward only to be thrown over the bars and have the seat knock you in the *****?

or something else?

MyShibbyZ28

something like that...

To be more precise, I mean "free wheeling" as in yes pedalling without any effort. Like going down a slight hill I guess, enough so you don't have to pedal to keep the same speed.

Wait...I think I just had a break through...

Say you being on the bike is the engine, and how bad the hill slopes is the size of the port. Is it something like that? Something like the more down sloped the hill gets (bigger port) then the less energy (torque) it requires to stay moving? So if you had a slight hill, enough so you need to keep pedalling to stay moving, that would be ideal? So it'd be like porting the head, and making it just big enough to get a boost of power yet still keep the torque to stay moving?

If that's at all close, then on to my next question...How does backpressure fit into this picture?

flatlander757

I suppose that's a good analogy... I'm sure someone will butt in to correct stuff but it makes sense the way I see it with "muh redneck braynz"

Backpressure isn't what is important... it's how well the scavenging effect occurs. If the headers have equal length primaries with say an X pipe it should keep sufficient scavenging. Just to clarify, scavenging is basically when an engine lets out exhaust, behind it is a vacuum, which helps to suck the NEXT exhaust pulse that will be coming.

MyShibbyZ28

So no backpressure is good? You just want scavenging. Or can't you have one without the other? It seems to me that you'd HAVE to have an x-pipe to create scavenging so that way one bank is basically drawing the gas out for the other bank and vise versa, or no? I got the vacuum/intake thing understood, now I just need to understand the exhaust part.

Injuneer

Your analysis, that leads you to the belief that the runner for each cylinder only needs to flow 60cfm is in error. The numbers are OK, but the answer is misleading.

In your example, 60cfm is the average over 2 strokes (one crank revolution) of the engine's combustion cycle. But air is only flowing in the runner when the intake valve is open. And, the flow rate is not even constant over that period, since air flow starts very low as the valve opens, and peaks when the valve reaches max lift. Peak flow is way higher than 60cfm.

And, just because the typical flow bench shows that runner can flow say 250cfm at 28" H2O pressure loss, doesn't mean that 28" pressure loss represents the actual condition in the intake runner. That's just a pressure loss standard that makes it convenient to compare RELATIVE flow capacities of different heads. And, you can't really compare steady state flow to pulse flow.

You also need to understand the difference between volume rate of flow (CFM) and mass rate of flow (#/HR, grams/sec, etc). You need to maximize the mass (~weight) of air in the cylinder. If the air flows through a passage that is too small, it "loses" pressure.... and hence the density of the air drops, and even though you are still flowing a certain number of "CFM" of air, you are not flowing as much "mass", and its mass of air (actually, the 21% that is oxygen) that determines how much fuel it can burn and how much enrgy can be released by the combustion. On the other hand, the smaller volume runner may increase velocity to the point where it aids air/fuel mixing, combution chamber turbulence, and other things that improve combustion efficiency.

Finally, you need to understand that saying "I have an SLP CAI on and found some other formulas and junk to figure out how much that flows, but long story short, it flows a little over 1000 CFM. More than enough." is meaningless. A flow rate is of no value unless it is accompanied by a corresponding pressure loss. When air flows, it loses pressure due to the friction in the pipe. That loss reduces the density, and you lose power if you lose pressure. An SLP intake does not magically flow 1,000cfm with no pressure loss, then experience some significant pressure loss ar 1,001cfm. Its losing pressure at 1cfm or 10cfm or 100cfm.... but that loss may be so insignificant that you can't measure its impact on the pressure at the engine end of the system. The important thing becomes the flow level at which the losses in the system become significant.

rskrause

To elaborate on one of Fred's points wrt "flow rate". Intake components are typically measured at 1.5" Hg pressure drop, a standard that came into use for 4bbl. carbs. The reason why 1.5" was chosen in no way contradicts the truism that intake flow restriction (which is the cause of pressure drop) is always bad. It is based on the fact that a typcial 4bb;. carb needs ~1.5" pressure drop to meter fuel properly. To interpret a claim that a part such as a CAI flow "1000CFM" is meaningless unless the test conditions (specifically pressure drop) is specified. But in any case, what ever the heads flow it's desirable to minimize upstream resistance to flow.

Rich

1racerdude

http://web.camaross.com/forums/showthread.php?t=411824

Go here and maybe ya will get some insight.