what is the significance of port velocity or how is it used when putting together a new combo? whats a good number?

If there is not enough velocity then the fuel falls out of the air and does not burn properly. Too much and it can slam the fuel into the outside of the curves and seperate it that way. I don't know what numbers to shoot for I just tend to stick with what works.

300 fps seems to be the magic number for most to
shoot for

ok, so is there a way to pick the ideal bore,stroke,rod length and port cross section using the math for port velocity?

I covered the subject in my big block book.

Some of the best head porters in the world don't agree on an absolute best number to shoot for. Just like everything else, there is more to it than just shooting for a magic number. 300 fps is a good general starting point but it can change per application.
Keith

yes andy, I read some of the first chapter about that. what I was looking for was what stroke length and piston speed best matches port velocity or volume. want to pick the most efficient stroke for my head.

Use the chart in the book to match CSA with displacement. That is about as good as you can do without spending the money to buy a simulation package. If you want to get into the subject a little deeper then pick up a copy of PipeMax.

You can't really do much better than the simple chart that I put in my book unless you know some specific data on your heads. You would need to have a velocity probe to know if your heads have any dead area. If you don't know that type of information you just have to assume that CSA is CSA and so then you use the chart to see where the torque peak will be.

In the simple view the only thing you care about is CSA and displacement. A more advanced simulation will take a look at stroke vs. bore since a larger bore will provide some extra beathing capability. You need PipeMax to look at that. Rod length doesn't matter much but PipeMax will take it into account.

Like most other particulars, it is design specific.

For example.....

Try to up the port velocity over a bad short turn and see what happens.

My guess is that unless a person is building a high end motor like pro stock or a " class" type motor this stuff is best left in the hands of an accomplished head porter who is armed with the basics of a combo and it's intended use.
The longer the stroke with a given head the lower rpm a motor needs to be spun to make peak horsepower, and peak torque will come in sooner as well.
I would plan the combo around how much you plan to invest in valve train components, and how long you want the motor to live. Those will dictate how big an arm can be used with the intended head.
All the above assumes the OP isn't building a pro stocker or a trick " class" car

I map out ports with a velocity probe to see how fast the air moves at several lift points at various areas of the port. Getting the air to slow down or speed up to get it where you want it is the trick part.
The average velocity is just one aspect of it. Port shape and size are very important. Velocity and CSA can give you an idea of where peak power will occur but it wont tell you everything that is important.
Kept

The chart in the book answers the question for most guys. If they are shooting for more than 1.5 hp/cubic inch then they'll need to get more sophisticated tools. Airflow measurements, velocity measurements, swirl, tumble, etc. would all be helpful for higher power combos.

The book is $20 and answers most bracket racer questions. PipeMax is $50 and handles most race engine builder questions. After that the money gets more serious.

I've built 900 hp big blocks with not much more than PipeMax and a credit card. You have to assume that the head porter knew his stuff when you buy an off the shelf ported head but that is just the way it is. Not very many guys have the money to buy heads which are flowed, mapped, fixed, etc.

My port measures 1.330x2.155 csa would be 2.866. Wallace calculator figures bore and stoke and csa to give limiting port velocity. Mine is 559cfm. How do I use this number to figure proper stoke or piston speed to match my heads. Andy your graph tells me max torque word be 5080, which is close as my peak is actually 5100. I'll check my pipe max program later. I have everything for next build but crank rods pistons.

So based on those numbers I take it that you have a 400 inch engine and standard port big block heads? That would give you a torque peak at about 5100 rpm.

If you increase the stroke then the torque peak will drop according to the chart. A 3.75 stroke makes a 451 so the torque peak is now at 4500 rpm. A 500 inch engine with stock port heads will have a torque peak around 4000 rpm.

An average bracket racer just needs to figure out what RPM they want to cross the stripe at, how fast they want to go, what RPM they are willing to live with or afford and then build the combo that fits.

With a standard port head you probably can't get enough piston speed to worry too much. The bigger the stroke you put in there then the lower the power band so the problem kind of solves itself.

If you go too big with the stroke then you'll end up with really high gears and you'll have some trouble getting the car to leave hard. But if it is a stock block you'll have trouble putting too big of a stroke in there so once again, the problem kind of solves itself.

If you just have std port heads then there isn't much point in overthinking this. A 3.75 or 3.90 stroke crank works pretty good and will make decent power. The power band will be low enough that you won't need expensive valve train stuff but you'll make enough power to make it a fun ride.

360 small block with indy 360-1 heads. so you think a 3.79 stroke would be a good combo/most efficient with these heads? better than a 4" stroke?

Dave if you have PipeMax dont you put your engine
combos through it to see what you get... biggest thing
is getting the volumetric efficiency correct

haven't used it in awhile. I lost it when my computer crashed. pipemax just sent program yesterday. i'll check it out.