Port Velocity??.... Dennis Moore
 

Dennis,
Is there a way to qualify port velocity when looking at volume numbers on a certain intake port?? How do you compare heads for velocity? Or must you take the manufacture/headman's word for it...

Thanks

Dick

Boy that is a good question.
I think that if the cylinder head experts were honest with us they would admit that the biggest factor in port velocity is the volume of the port. The smaller the faster.
They can speed up port velocity small amounts by recontouring the port slightly on the flow bench but I really don't believe that they can make that much of a difference (not as much as the advertising would want us to believe).
Looking at the latest automotive Pro Stock drag racing engines you will see that they no longer use huge ports but more modest size ports with better camshafts and intake manifolds. This means to me that they can't get the port velocity they want with the large intake ports, they are forced to make the ports smaller to increase velocity.
To pay someone extra money to make a large port flow slightly faster in order to allow the air/fuel mixture to be coaxed into the combustion chamber by engine vacuum is really a waste when you have a blower/supercharger forcing the air/fuel mixture into the engine.
All that extra work improving air flow with small tedious grinding at strategic points in the port to slightly increase air flow velocity for a naturally aspirated racing engine is fine if you are racing against someone, money is no object and you need every little edge you can get.
The average marine engine really doesn't see an improvement from these little changes, you can lose twice that much horsepower from having the wrong size jets in the carb on a hot day!
I hope this doesn't make anyone mad!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

Mr Gadgets, You have me thinking, and I have another question for Dennis. If I am reading this right, you are saying that it doesn't make that much difference on a blower/supercharger application correct? I am curious because I am a newbe to the supercharged motors.
Also, when you match the intake ports to the intake manifold to prevent turbulence in the path, I am wondering, about the difference in the port velocity and size of the intake runner as compared to the port velocity and volume of the head port. Is there an optimuim size for the intake runners? Maybe just splitting hairs again and wouldn't make as much difference on a blower app. Just looking for a base line to compare intake specs for a particular head.
I've been reading your BBC Marine book, and you have done a great job! Thanks.

When the piston starts on the downward path it must first start the column of air/fuel mixture moving inside the intake port. Only when this column starts moving can it then start the column of air/fuel mixture moving inside the intake runner. Finally the air/fuel mixture can be pulled through the carburetor. If the volume of air inside the intake port is large it is slower to start moving, delaying the pull on the intake runner. If the intake runner/intake manifold volume is large it delays the start of moving the air through the carburetor. This results in a lag or delay in moving the air/fuel mixture through the carburetor delaying throttle response.
Smaller ports will accelerate the movement of the air/fuel mixture through the intake ports, the intake runners/manifold and the carburetor.
Of course the most important factor is to have an intake port/manifold combo that just supplys all of the air/fuel mixture needed for the engines size and rpm and no larger. This will give the engine maximum power with minimum throttle delay.

When you have a supercharger blowing the air/fuel mixture into the cylinder you are creating your own air/fuel mixture velocity and you have no delay. When supercharging an engine you don't worry about port velocity created by the piston on its downward path. The port shape and size that is so important for port velocity, or stated another way, COMPROMISED for port velocity in a naturally aspirated engine is no longer a problem.

An example would be that the intake port floor must be raised to form a smooth even radius to create/help a high velocity air/fuel mixture to make the turn down into the valve pocket. When you are making your own port velocity (with a supercharger) you can increase port volume by making the port as large as possible by grinding the bottom of the floor flat (no compromises).

I hope this helps someone!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

Thanks Dennis, It does help me understand some of the differences in the blower motor setup. Some of the things that have been so critical in my past NA motors are no longer an issue. So when I need to rip into my motor (502) I can focus on the different areas that will make the most increase. Also helps me understand more about the NA motors when I am helping my buddies hop up their motors, both cars and boats.

The only way to test port velocity would be by putting a pito tube into the runner while you've got the head on a flow bench, and checking it at various locations. You're main concern is having a port that works properly in your application with your combination of parts. Remember that you're only at max lift for a very short duration, while you're going to see your mid lift numbers longer. Shaping of the ports, and the finish can have a dramatic impact to your overall outcome. Running a Pro Stock style cylinder head on a 9:1 boat motor wouldn't work any better than your cast GM head, it would probably even be worse. Run the smallest head that will flow your volume requirements. This will keep your air velocity, and fuel atomization at there highest.

Thank you Dennis!
I am in the NA world so velocity is critical. The heads I have now do have the floor filled to increase the short turn radius, to increase port velocity. I am wondering as cubic inches increases is this a hindering factor. At what point can you step away from a filled floor intake port and still have enough torque to get the boat on plane and sustain a sufficient cruise speed?
If the motor is Injected i would think the lag in throttle response may benifit from the injection process. I guess the real question I am asking is how do you determine how big a port to use? How do you determine your air requirments with out a hit and miss approach. I would assume that bore stroke possibly rod ratio would be some of the factors involved as well as cam timing. I am sorry but I am out of town and left your book at home.. so if you could indulge me I would appreciate..
The info you have explained so far is great. Seeing that I already have my basic engine in use, any changes in heads etc will have to follow what I started with.. And you are correct, I dont have an endless supply of money. So prudence is the key here, but of course the need for set me back in the seat acceleration and max warp speed is the end goal.. Well boating is the goal.. just how fast we go is dependant on money..

Thankyou for your input..

Dick

Dick, you are asking the question everyone has been trying to figure out since the first 4 cycle engine was designed! I can only say that you must find a good working engine combo that someone has found from testing different combinations and copy it.
Like supercharging, multi port fuel injection engines do not rely on port velocity as much as carbureted engines (although more than supercharged engines).
Multiport fuel injected engines, particularly with long manifold runners, work very well with large intake ports. The maximum horsepower is the same with a carburetor or an MPI EFI but I think that the MPI EFI engine is much less sensitive to port velocity than a carbureted engine maintaining much better low speed torque while using larger intake ports and manifold runners.
Again, I would think that compromising high volume for high velocity when designing an intake port would be less of a consideration with a MPI EFI engine.
Sincerely
Dennis Moore

Thanks Dennis,
That is good news on my part. I will continue the task of searching.. Your input has been valuable..

Dick

A motor at 14 psi boost is no different then running N/A at one atmostsphere below sea level discarding the heat /mechanics from the blower. When the piston moves down the bore, it creates a negetive pressure, it doesn't know what is on top of the motor. Moving at thousands of feet per second the incomming charge does not "see" the top of the piston until the piston comes to a stop at BDC. This is where the charge is mixed from turbulance caused by the heavy air/ fuel column in the intake tract crashing into the piston. The density of the column depends on the pressure on the other side of the restriction. The size of the restriction is a factor as well. Keep in mind that the intake valve is still opened while the piston moves up towards compression stroke. This extra filling is known as ram tuning which also effects volumetric efficiency.
By changing the port size it speeds up the air/charge in the intake and gives the air time to stabilize before reaching the venturi/restriction and also relieves the venturi from the pressure spike. I respectfully dissagree with the big port theory. Why would the top fuel guys be bolting their blowers on tunnel rams? as always my opinion