[steve25]

On stock D port big valve high heads that flow there norm peak of some 205 intake CFM @28", max FPS is about 350@28".

This with the bare port, bolting on whatever Intake Manifold can change things real fast!

[Navy Horn 16]

If the small valve heads have a blower attached to them....

[steve25]

Is that a question?

[rohrt]

I have never really understood the relationship between port velocity, air flow, and HP other then a certain velocity is needed to keep gas in suspension.

[steve25]

Let try to help it make full sence to you because it means near everything in terms of having any motor perform per you intended needs.

First off air has substantial mass, even though it only weighs in at .075 lb per cubic foot sticking your head out your car's side window at 60 mph will confirm this for you!

Air does not flow continuesly into a cylinder, once the Intake valve closes the inward flow stops.
This now stopped air mass in the cylinder's Intake tract needs to be re accelerated again once the valve starts to open again. But it takes energy to do that and at best you have only 14.7 psi of atmosphere pressure to do that in a normally asperated motor.

Now lets say we are running a Cam of only .390" lift, at this amount of lift we are not using the full flow capabilty of the Intake port.

Now,let's say at this. .390" lift that the port is flowing 180 cfm with a port air speed of 250 fps

Now where going to add more valve lift the easy way with a higher ratio rocker, so now we have let's say .500" lift which is making use of the full amount of air flow this Intake port has which for our discussion let's say is 210 cfm.

Now since we are moving more air thru the same size port the velocity has gone up and in turn it takes more air pressure applied thru the Carb and Intake tract to get it moving again, but nature imposes a limit on how fast the air can move thru any given size orfis.

This limit is about 55 percent of the speed of sound which is rated a standard temperature and pressure.
This is some 1100 fps at sea level ( once again 14.7 psi) at about 60 degrees.

The speed at which air moves thru the motor directly ties into throttle responce and how fast the motor gains rpm!

There is no 100% right fps for any given NA motor, the best you can do is to optomise it in the widest power band that your reworking / building a motor for.

For flat out NA race usage in a very narrow power band of 1500 rpm or so as in pro stock racing they build to attain no more then 375 fps, but try to run only a 1500 rpm peak power band in your street bruiser and there's a good chance that the 9 year old kid from the house next door will blow you into the weeds with his mini bike!

If you can get a Intake flow system to produce 450 fps 1000 rpm above the peak torque of your given motor then you would have built about the best everyday driver/ weekend strip runner that you can with the heads ( Intake tract) and Cam that you currently may have.

[steve25]

Please also keep in mind that once any given normally aspirated motor reaches its rpm of peak torque you have then also tapped out the heads flow capacity that has been set up by all the factors involved in doing such, like first on the limit list is the Cam lift, then Cam duration, Carb size and on and on thru a list of some 8 different things that are all interrelated .

The only reason HP numbers keep climbing with rpm rise after peak torque is reached is the fact that however many cylinders you have are firing more times per minute, but once the Intake tract reaches its sonic limit as I posted above in number 45 ,the HP numbers will then nose over very fast with increasing rpm.

Another way that a motor can reach peak HP is if it has a overly large intake system in which case peak HP is neutered by the amount of internal fiction the motor has over taking the amount of HP the motor is making, and this can also happen in combination with hitting the air flow sonic limit.

Even the best factory production Pontiac motors at best could only attain a volumetric efficiency number of about 85% ( closed Exh and no headers ) but building a full on race motor that makes full use of Exh collector scavenging in getting a early start to the Intake tract air flow moving again during the overlap phase can produce VE numbers of 130% which produces the same effect like super charging or Turbo charging the motor!

[slowbird]

I highly doubt any na engine has reached 130% ve. No Pontiac has come remotely close to that for sure

[Will]

Originally Posted by SRR

I once had a 70 Sport with the stock 350P with 17? (small valve) heads and one day a rocker stud broke. I had a fresh pair of 62's (big valve,screw in stud) so I bolted them on along with a stock Q-jet intake and added dual exhaust. 3.23 GTO diff. It felt like I added 100HP to the car!

You made a whole bunch of changes all at once. Chances are the bigger valves/better flowing ports alone were a very small part of the improvement you saw.

If I read your post right, you also changed gears to 3.23 at that time? Depending on what gear set you had before, that alone represents a BIG increase in torque at the rear wheels if you were coming from a 2.x gearset.

The 62s had smaller chambers so you raised compression. Freeing up the exhaust always helps. I'm going to go out on a limb and say raising the compression, freeing up the exhaust, and changing gears were the biggest contributors to the increase in power you felt. Next up would be the intake & carb, and lastly the ports & valves.