which intake port is better?

bc · Post by bc » Mon Mar 07, 2005 4:02 pm

Which intake port would you run based off of these flow numbers and valve sizes? The engine has a bore of 4.185.

The first on has a 2.15 valve.
.100 81.4
.200 151.7
.300 221.2
.400 269.6
.500 308.2
.550 317.1
.600 322.2
.650 330.2
.700 333.9

The second intake port has a 2.19 valve.
.100 77
.200 151
.300 225
.400 278
.500 321.3
.550 332.5
.600 339.5
.650 343.2
.700 345.1

May first guess is the second one with the 2.19 valve is the better one. If you add up the Discharge Coeffient and average them their averages are equal. Both average .643 but the one with the 2.19 valve averages 9cfm more. What do you guys think?

panic · Post by **panic** » Mon Mar 07, 2005 4:17 pm

A better comparison would be plotting the flow against the curve area of the cam in use.

Rick360 · Post by **Rick360** » Mon Mar 07, 2005 4:41 pm

There are obviously many more variables to consider than the info given.

Port volume and/or cross sections would be important to know. If the ports are identical except the valve and seat, the port CSA's could be too small for the flow, increasing velocities too much. Or if the port was too big it might bring the velocities up where they need to be.

What engine size? What rpm range? What manifold?

Rick

bc · Post by bc » Mon Mar 07, 2005 6:17 pm

From what I know of the 2 ports they within 10cc of one another and the min. cross section is almost the same between the 2.

dbusch · Post by **dbusch** » Mon Mar 07, 2005 8:08 pm

what is the minimum cross section? 345cfm seems very low for a head with a 2.19 valve. What kind of head is this?

bc · Post by bc » Mon Mar 07, 2005 8:28 pm

They are Pontiac heads. The min. cross section I believe is around 2.70".

dbusch · Post by **dbusch** » Mon Mar 07, 2005 9:13 pm

With a 50* seat at 91% you should have a min cross section at the pushrod of 3" with a decent 15* head. Maybe you cannot get that much. Is this a 23* head? If it is then i can understand the low valve efficiency. With those angles the chamber always seems to be in the way. Opening up the cross section at the pushrod is only good if you are utilizing most of the valve curtain area. IF you are only running .700 or .750 lift you wont be needing cross sections as large at the pushrod area. The 2.19 valve would help low and mid flow which is a good thing running lower valve lifts. Run .900+ and you must open the cross section up equal to the throat size and even larger over the short turn. I would actually prefer the 2.150 valve with .900+ lift, especially if you cannot open up the cross section restriction. The 2.190 would have very low velocities at the throat since it cannot move any air at the higher lifts. So, with a low lift deal and cross section limitation, the 2.19 is ok. With high lift, run the 2.15 port since it will have more uniform velocities. What size engine are these heads for and what rpm are you planning on? 2.7" could limit you.

I'd like to hear some inputs on this too from the real wizards on this forum!!

bc · Post by bc » Tue Mar 08, 2005 10:59 am

This is a Pontiac head for a Pontiac. I believe the heads will be run on a 412cid engine (this is for a friend). Rpm will be around 6500 to 7000.

dbusch · Post by **dbusch** » Tue Mar 08, 2005 11:22 am

looks like the 2.15" head would be good for 7000 rpm and the 2.19 will be good for slightly more, like 7200, since its average cross section will be a little more. When you said Pontiac, i figured you meant a Pontiac aluminum small block chevy head, not a cast iron big block head. I doubt you will be running a ton of lift, either, right? The 2.19 head seems better for you i think. Make sure you put a super victor Pontiac manifold on there to make use of those heads! Check the manifold for cross section near the gasket, that it is at least 2.7"...