porting a set of heads questions
12-11-2011, 09:16 PM
#1
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porting a set of heads questions
I picked up a set of 706 heads for 40 bucks the other day and was thinkn of porting them myself but not gonna do the valves this is poor mans job here would like to know how you guys do it yourself and how much you and how much you can take out, the mileage is unkown but will do new valve seals regardless
12-11-2011, 09:40 PM
#2
I'll try to keep this as brief as possible because there's not enough time to type everything you need to know.
Don't grab a carbide burr and hog out the entrance.
The biggest flow restriction of 98% of heads out there is the short turn radius. Focus on just smoothing this with a cartridge roll to take out any rough edges or sharp drops. Most of the time, the factory bowl cut leaves a bad ridge that hurts flow over the short turn.
Don't "raise the roof" of the port or make trenches around the valve guide- theres no need to do that and it will hurt airflow if you raise the roofline above the port entrance height.
For your low lift numbers, you will depend 90% on the valve job and 10% on the port itself. For high lift numbers, its 90% dependent on the port, 10% on the valve job.
Don't "polish" anything and don't use anything smoother than a 120 grit cartridge.
Have a machine shop put a 30 degree backcut on the intake valves. Its easy to do, and picks up low lift airflow. If you had a flowbench and centerless valve grinder, you could find out exactly what angle flows best for your combo, but 30 works for 85% of the stuff out there.
Put a radius on the face of your exhaust valves. Do NOT put a backcut on the exhaust valves. try to leave the short turn of the exhaust port as rough as possible so that it "grabs" the air to make the short turn instead of overshooting it and going turbulent on the roofline.
Don't grind out the valve guide. Try to make a "airfoil" if possible, but don't make too much of a fuss about it. Typically a good job vs a bad job on the valve guide is 4-8 cfm.
Don't grab a carbide burr and hog out the entrance.
The biggest flow restriction of 98% of heads out there is the short turn radius. Focus on just smoothing this with a cartridge roll to take out any rough edges or sharp drops. Most of the time, the factory bowl cut leaves a bad ridge that hurts flow over the short turn.
Don't "raise the roof" of the port or make trenches around the valve guide- theres no need to do that and it will hurt airflow if you raise the roofline above the port entrance height.
For your low lift numbers, you will depend 90% on the valve job and 10% on the port itself. For high lift numbers, its 90% dependent on the port, 10% on the valve job.
Don't "polish" anything and don't use anything smoother than a 120 grit cartridge.
Have a machine shop put a 30 degree backcut on the intake valves. Its easy to do, and picks up low lift airflow. If you had a flowbench and centerless valve grinder, you could find out exactly what angle flows best for your combo, but 30 works for 85% of the stuff out there.
Put a radius on the face of your exhaust valves. Do NOT put a backcut on the exhaust valves. try to leave the short turn of the exhaust port as rough as possible so that it "grabs" the air to make the short turn instead of overshooting it and going turbulent on the roofline.
Don't grind out the valve guide. Try to make a "airfoil" if possible, but don't make too much of a fuss about it. Typically a good job vs a bad job on the valve guide is 4-8 cfm.
12-11-2011, 09:50 PM
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That's good info thanks I'm mainly doing the intake and exhaust runners I have a mill at work and going to be taking my time taking little at a time with the runners tring to smooth it out as much as I can
12-13-2011, 05:51 PM
#7
I really wouldn't worry about doing a "gasket match" so much as I would worry about the short turn and valve job.
On heads that I've done, (and others I know that port heads) they flow 310cfm and didn't even touch the first 1.5" of the port. Its all in the short turn. If you do open up the first part of the port, you won't gain anything, but you will raise your peak rpm up because with the extra volume, velocity will decrease and you will need to turn more rpm to pick up what you've lost.
On heads that I've done, (and others I know that port heads) they flow 310cfm and didn't even touch the first 1.5" of the port. Its all in the short turn. If you do open up the first part of the port, you won't gain anything, but you will raise your peak rpm up because with the extra volume, velocity will decrease and you will need to turn more rpm to pick up what you've lost.
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01-04-2012, 10:34 PM
#10
Came back to show you a couple of easy things that pick up alot of flow numbers while keeping the port small to keep velocity high (since we only have 360 cubes or less)
This works with all LS cathedral port heads. This does not work with the L92 casting or the LS7 casting. For some reason, removing the small bump (bolt boss) on those heads tends to hurt flow or not improve flow at all.
When you reassemble the heads, just use a bolt thread sealer on the rocker stand bolts that do go into this hole. The bolts will not protrude into the port- in the 2% of cases they do, they don't hurt the flow more than a couple of cfm at the top of the flow.
stock port. Note the giant zit on the roofline of the port. That things gotta go.
better view from outside.
using a roughing carbide, remove the majority of the hill. Do not try to blend the roof or port walls yet. Do not worry about the corner radii yet.
use a double cut carbide burr and start blending the roof and walls. Keep them straight- use a valve with the heat cut off as a "straightness gauge" to stick in the port periodically to check your slope.
blend blend blend. When you have something that looks good enough to run with just the carbide touching it, THEN get your cartridge roll and do the "final" blending of the port.
I have no pictures of the cartridge roll- as its the last thing that I do in any port.
Once you have successfully tackled this- look at the "air dam" sitting on the right side of the guide in the first picture. Streamline that and try to copy the factory design thats on the other side. If you just do those SIMPLE 2 things, you will be in the 285+ neighborhood with the factory valve job and a 2" valve. Most of the time its over 290, but I have seen casting flaws on heads that had a deep factory bowl cut or other blims.
This works with all LS cathedral port heads. This does not work with the L92 casting or the LS7 casting. For some reason, removing the small bump (bolt boss) on those heads tends to hurt flow or not improve flow at all.
When you reassemble the heads, just use a bolt thread sealer on the rocker stand bolts that do go into this hole. The bolts will not protrude into the port- in the 2% of cases they do, they don't hurt the flow more than a couple of cfm at the top of the flow.
stock port. Note the giant zit on the roofline of the port. That things gotta go.
better view from outside.
using a roughing carbide, remove the majority of the hill. Do not try to blend the roof or port walls yet. Do not worry about the corner radii yet.
use a double cut carbide burr and start blending the roof and walls. Keep them straight- use a valve with the heat cut off as a "straightness gauge" to stick in the port periodically to check your slope.
blend blend blend. When you have something that looks good enough to run with just the carbide touching it, THEN get your cartridge roll and do the "final" blending of the port.
I have no pictures of the cartridge roll- as its the last thing that I do in any port.
Once you have successfully tackled this- look at the "air dam" sitting on the right side of the guide in the first picture. Streamline that and try to copy the factory design thats on the other side. If you just do those SIMPLE 2 things, you will be in the 285+ neighborhood with the factory valve job and a 2" valve. Most of the time its over 290, but I have seen casting flaws on heads that had a deep factory bowl cut or other blims.