Something to think about in the pursuit of numbers... There was a magazine build I read a while back... I don't remember any of the specific parts they used, but the principle is what's illustrative. I think the engine was a 355 or so and the builders initially picked well matched components which resulted in an engine that made something like 485 HP with a full, robust torque curve. The customer wanted the magic 500 number, though, so he had the builders do what they had to do to get it. They incrementally swapped to bigger heads and intake, more cam, etc and did eventually hit the 500 humber, however that 15 HP top end gain came at the expense of 25-30 ftlbs of mid-range torque. It wasn't anything new, but was a good illustration of the classic trade offs of building an engine.

Personally, I thought the builders did a great job putting together a well matched, well sized combo to begin with and efforts thereafter were actually counterproductive for a street car.

 

Great post!! There is a trade off and you have to chose which side of it you want to be on. How heavy is your car, what gears and what application.
Matching the componants is the most under stated principal on this site.

 

that's what i'm going for, kinda......i can't really make up my mind on what i want to do. i don't want to just slap together some high-dollar shiny parts from "the usual" vendors like some people.

i'm thinking more and more i want a motor that does have a lot of down-low power. my 9" is going to eat up quite a bit of rear wheel numbers, so i'm not going to pursue the 600 mark, although that's the benchmark i'd like to hit. i'm looking at a 440 LSX with TFS 245's now. i like the power they make all across the board. i think with that setup my car would be where i need it to be. as far was weight goes, i've ditched the spare tire and that's about it. when the motor is done, i'll start setting the car up around it. i'd like to see a 9 second pass NA. the motor and the car's weight are all that's really holding me back right now, i've got the rear-end, tranny, and suspension to handle it.

 

You CANNOT slap a flow number on a head and EXPECT it to make "X" amount of horsepower.

I have seen 300cfm heads not get in the 10's(namely me) and heads that flow 257 run in the nines(both LT1 cars, both normally aspirated, both VERY different setups but it proves my point.)

I have also seen MANY 408's not make the power I make NOR do they run as quick as I do. But they have a 15* head and have 25 more cubic inches.

Head "flow" numbers do NOT necessarily reflect the power POTENTIAL of said cylinder head.

A flow bench cannot and will not reflect how a head moves air in a running engine. Why? Because it doesnt simulate load, camshaft,intake, intake tract, headers, exhaust, compression, rod ratio, piston speed,.....which ALL affect how the air moves through the head.

ANd BTW I have a 250cc intake port/23* head on a pumpgas 383 making near 600hp. A big port isnt necessarily a lazy port


David

 

Dave, I had Clyde's heads flowed, the ones that ran 10.40 @ 128, they flowed 245cfm at 600 lift according to the flow bench. Go Figure

 

flowbenches are like dynos to me, just a tool. they don't truly reflect real world performance. that's sort of why i made this thread, to see if there was a way for me to make an educated head selection based on something other than "flow @ .600"", which seems to be the benchmark standard by which a head is judged

some local idiot asked me, after i did my last head swap, "what do they flow at .600"?".....i kinda gave him a blank look and asked him what difference it made with my cam with less than .600" lift.....he didn't understand

so how do you keep a big port from being a lazy port? is it relative to cross sectional dimensions? or does it have to do with the actual shape of the intake/head port runner? probably both i'm sure...but still..???

 

Pressure differential can help. yes it's related to cross section, but also to the location of your minimum cross sectional area (I hope I'm saying that right, I'm still a learning newborn here).. you're also going to need to match your cam profile to your heads.. they depend on each other.

 

I would think that this is the single most important piece of information regarding cylinder heads, is Cross Sectional Area (CSA). Where is probably just as important, seeing how putting the minimum CSA over the short turn would be purely retarded.

 

One good indicator of how good, or efficient, a port is is a lift to diameter ratio.

300cfm @ .600" is great and all, but it's generally better on a 2.00" valve rather than a 2.165".

 

Right, I could also add that tapering the runner down right before the short turn would increase the pressure over the short radius, allowing flow to hold the turn at higher velocities where it might otherwise shear. Then let it expand into the bowl, past the valve, into the chamber. That has something to do with delta P right? (I'm such a noobie about cylinder heads, but I can't stop reading about them.. soon I'll order the tools needed to port them, and see if I can translate knowledge into results)

 

Call me a non-contributer to this thread but I want to throw a variable into the fray for my own knowledge as there seems to be "some" knowledgeable people in here. Forced induction heads. Is velocity as important concerning F/I heads or is the valve event more important? I am experimenting with my own setup so I guess I will find out myself but would like to hear some opinions on my above question.

 

.

Also got way over my head. Just a very basic knowledge of numbers, velocity & HP.
Tried reading everything but too confusing. So has it been determined CFM is equal when making power,
no matter how you get it. With everything being equal except head, will there be a difference in HP with a
2.00, 2.25 or 2.50 int valve if they have the same CFM. I was always taught, it's a combination of velocity & CFM.
In newbie language please.

.

 

Where did you get this informaton?

I suggest that you might consider finding different sources if they say that increasing velocity increases pressure. Mr. B would spin in his grave.

 

Maybe I said it wrong, but tapering the CSA down should slow velocity and increase pressure. I didn't mean to imply both would increase. For example if you are pulling (throwing a number out here) 500 fps through the port, and you're choking going higher velocities in the port because the air is starting to shear off the short radius, then increasing the air pressure over the short turn would force the air on the short radius to hold, because the pressure would be holding it there? Allowing more fps? I mean, it would probably be at the expense of flow BELOW it's shear point because you are making the minimum CSA smaller (or losing some velocity if you're just making the runner bigger) but if your engine and target RPM are demanding something the port can't do, the slowing the velocity over the short radius would help it.. and that can be done by increasing the pressure.. or am I just WAY off base here?

Like I said, I'm pretty new to learning about cylinder heads, so I may not be saying things correctly literarily. I've been reading whatever I can get my hands on for several weeks solid, and have a notebook full of notes, and a folder full of port and chamber pics, which is absolutely ZERO in this field! but my head is still trying to make sense of all of it.. So go easy on me

 

Let me take a shot at an explanation:

Air (or a fuel/air mixture that is mostly air) is a compressible fluid (CF) unlike a liquid (water, oil, gasoline only) which is an incompressible fluid (IF). Flow is volume per some unit of time (Cubic feet per minute (CFM) for example).

Bernoulli said that with a CF velocity and pressure are inversely proportional, or as velocity increases, pressure decreases. That's an important point.

I may have misunderstood what you meant by "tapering the CSA down". I took it to mean making the CSA smaller, not larger. If you decrease the CSA of a conduit (tube, port, pipe, etc) and maintain the same flow (particles per minute), thru that area, the velocity of the particles increases and the pressure decreases. The increased velocity usually makes sense, but the decrease in pressure may be counterintuitive to some folks. If you want to get into the "why" it's about Conservation of Energy.


Think about why a carburetor has a venturi, or area with a smaller cross section than the bore of the carb. The air passing thru the venturi speeds up, the pressure drops and fuel is "sucked" into the airstream at that point. Of course it isn't really "sucked in" but rather it is pushed in from the float bowl which is at atmospheric pressure which is now higher than the reduced pressure in the venturi. Mother Nature is a proper old girl: she doesn't "suck" nor does she "blow". She is all about pressure differentials (delta p).

So if you want more pressure somewhere in a port, you need to slow down the air, not speed it up. FWIW, I don't think multiple CSA changes along a port (bigger to smaller to bigger, etc) does much good. It probably does more harm, but that's another topic.

OK, if you buy into Bernoulli's pressure/velocity explanation, your thinking is probably off-base, to use your words. If you do not buy into it, but if that's how Ma Nature actually works, then what you think doesn't really change what actually happens. Ma Nature is a tough ol' broad. It's her way or the highway. That's why she is Mother and not Father Nature.

 

I read this summation of what Bill Jenkins said in a book of his, by a very renowned cylinder head porter.. "you should "funnel" the mixture, that is use taper from the plenum's entry ...then "neck it down" inches before the short turn apex and bowl area..and let it expand to the short turn apex + bowl ...in that way, you slow the velocity and turn the velocity into pressure before the short turn apex and bowl ,so it can turn, so it increases pressure differential between bowl and cylinder .

I took that as saying tapering down the port (decreasing CSA) would increase the pressure. Can the engine system at the point the cylinder is on it's intake stroke increase the velocity in the intake port, or will it back up in the more narrow entrance before the STR for more pressure? I guess it probably should speed the air up.. so I'm not sure how to take that interpretation.

I didn't think of Bernoulli's simple formula here, and the carb venturi REALLY put an image to it I could understand, I thank you for that. Delta P where velocity increases creates a depression in pressure.. high ---> low, a law of nature.

I've got soo much more to read, I'm sure I can hold an intelligent discussion with you... oh... 2-3 years from now

 

Tapering the port smaller increases the velocity and decreases the pressure until you EXPAND the CSA. It's the EXPANSION (or making the CSA larger[EDIT] if that is something a head guy wants to do. Some guys may not.) that slows it down and increases the pressure in the bowl. A higher pressure here (than in the cylinder) pushes the air in past the valve. Make sense now?

OK, another example: Engine cooling. The air inlet in the front of the car is relatively small. Velocity is basically vehicle velocity. Just behind the "grille" or air inlet opening the CSA increases rapidly to at least the area of the radiator. Result, air slows down a lot, pressure rises and pushes the air thru the radiator into the engine compartment which is at or above atmospheric pressure. All those rubber seals around the radiator and it's ducting are there to keep the pressurized air from going anywhere except thru the radiator. If you remove the seals your engine won't cool as well. Bernoulli, although dead for 226 years, sees to that.

See my siggy quote from an old engineer friend, Tom Ankeny.


Jon

 

I understand it now.. it's tapered down until a point before the short turn, then the CSA gets bigger before the STR, so it can increase the pressure.. got it..

But how far in front of the STR does this need to take place for it to be effective? I'm sure that's dependant on something, and I'm sure it also has a limitation on how far upstream that can be placed.. I'm sure I'll come across that information eventually..

good stuff Old SStroker!

You've helped more than you know...

 

From what I understand so far, having an expansion in the port is not a good thing. I think Darrin Morgan has even recognized this. When air is forced to expand into a larger space, it loses energy. Energy is what fills the cylinder and increases "volumetric efficiency", hence Darrin's "pressure recovery" theories. I think Vizard says something about an "expansion chamber" in the port...

Most of the guys I've learned from have said to have a gradual taper from the port entry that "converges on the valve". But, I'm still a pup as far as cylinder head porting is concerned. I've got another 10 years or so till I'm any good.

 

Yeah but you are damn sexy so that counts for a lot too! You and me and Gray all know they already made a movie based on you!

No one else in this thread can say that about themselves!!!

Also if Jessica ever get's tired of this site maybe she can go out in a blaze of glory and free those big beautiful twins of hers for the whole world to see.

It might just change everything but when you have cans like she does people do what you say.