Porting 081 heads
11-12-2005, 07:00 PM
#1
Supreme Member
Thread Starter
iTrader: (2)
Join Date: Nov 2001
Location: Lakewood, CO
Posts: 1,602
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1994 Jeep Wrangler
Porting 081 heads
Ok.... I just bought some 081 heads that already were machined for 1.94" intake valves, have screw-in studs, guide plates, and LT-4 springs and have had a valve job.
That leaves me ahead of the game on the machining. It still needs alot of porting. Where do I start? I read the sitting bull thread. I figure I could just start buy straightening out the intake walls and laying a set of 305 head gaskets on there to find out how much I can take off around the new intake valves? Should I just take as much as I can around the intake valves while not going outside of the bore?
The next thing..... I was also thinking about increasing the compression ratio. These are suposed to still be 58cc how much smaller can I make the chambers and how much compression can my stock bottom end 85' LB9 take on premium pump gas?
Is this the general idea with de shrouding? removing metal on the wall of the combustion chamber where there is very little clearance along the side of the valve? Any help with any of this would be great... Im just trying to understand what I have to do before i start. I intend on taking my sweet time on this.
That leaves me ahead of the game on the machining. It still needs alot of porting. Where do I start? I read the sitting bull thread. I figure I could just start buy straightening out the intake walls and laying a set of 305 head gaskets on there to find out how much I can take off around the new intake valves? Should I just take as much as I can around the intake valves while not going outside of the bore?
The next thing..... I was also thinking about increasing the compression ratio. These are suposed to still be 58cc how much smaller can I make the chambers and how much compression can my stock bottom end 85' LB9 take on premium pump gas?
Is this the general idea with de shrouding? removing metal on the wall of the combustion chamber where there is very little clearance along the side of the valve? Any help with any of this would be great... Im just trying to understand what I have to do before i start. I intend on taking my sweet time on this.
11-12-2005, 08:12 PM
#2
Supreme Member
iTrader: (1)
Join Date: Jul 2004
Location: Calgary, AB, Canada
Posts: 10,763
Likes: 0
Received 4 Likes
on
4 Posts
Car: 1982 Trans-Am
Engine: 355 w/ ported 416s
Transmission: T10, hurst shifter
Axle/Gears: 10 bolt, true-trac, 3.73
make sure you scribe the circle of the head gasket, and don't grind outside that. also, any porting in the chamber will lower your compression ratio, so don't get greedy.
when porting, focus on the pocket, area closest to the valves, that'll give the most power. Are you sure they weren't ported before you got them? how about removing an intake valve and snapping a pic?
do your porting, and deshrouding, then cc the heads. then use a CR calc, and see how much you need to get milled off, probably not much. You don't see a lot of power from bumping up your compression much, just more pricey fuel...(or additives). I'd say 9.5:1 would be ok.
when porting, focus on the pocket, area closest to the valves, that'll give the most power. Are you sure they weren't ported before you got them? how about removing an intake valve and snapping a pic?
do your porting, and deshrouding, then cc the heads. then use a CR calc, and see how much you need to get milled off, probably not much. You don't see a lot of power from bumping up your compression much, just more pricey fuel...(or additives). I'd say 9.5:1 would be ok.
11-13-2005, 11:10 AM
#3
You can spend a little time on unshrouding valves, but perhaps should consider just how much you need to remove to be successful. Compare that photo to one of an aftermarket head that supposedly suffers no shrouding problems:
11-13-2005, 11:23 AM
#4
Another consideration is the valve bowls. You can usually safely remove a lot of the casting around the valve guide, so long as the length of the guide is not altered (Notice the difference in the exhaust guide in the photo).
Another significant restriction can be the ridge of material just above (behind) the valve seats. There can be a large funnel of material right behind the valve that effectively reduces the valve bowl size to 3/4 of the valve diameter. You can straighten the walls of the bowls from the seat right up into the bowl. The fastest way is with a mill, then some hand cleaning and blending with a die grinder. The intake valve bowl in the photo shows that difference.
Another significant restriction can be the ridge of material just above (behind) the valve seats. There can be a large funnel of material right behind the valve that effectively reduces the valve bowl size to 3/4 of the valve diameter. You can straighten the walls of the bowls from the seat right up into the bowl. The fastest way is with a mill, then some hand cleaning and blending with a die grinder. The intake valve bowl in the photo shows that difference.
11-13-2005, 11:55 AM
#5
Sitting Bull has a lot of good information in his posts. Take the time to read that carefully, or cut/paste it into one document on your PC as a guide.
There are conflicting theories on whether gasket matching on the flanges is beneficial, detrimental, or insignificant. My personal take on this is that the factory runners on most of these heads are already so undersized, and offer so little material for alteration, that getting the ports to a regular average of cross sectional area from one end to the other is worth the effort. Make sure you understand that statement completely before going nuts with the mill and grinder.
In any case, it is important to keep the downstream side of the runners and ports no smaller than the upstream side. There should be no measurable steps in the system. If in doubt, keep the downstream side slightly larger. This may contradict the theories of wave induced cylinder charging and scavenging, but only a small percentage of the charge is actually wave induced. Conventional air flow dynamics still dictate what happens with most of the air entering the chambers. Ideally, the ports should be the same size all the way through with no points of significant enlargement or constriction along the way. That will allow both conventional dynamics and pressure wave induced flow to work in your favor.
If you study the design of most SBC heads before you start grinding, you should notice that some effort was made to maintain a uniform cross sectional are all the way through the heads. The ports start at the flange (and even upstream, in the intake base) as a narrow but tall passage. Somewhere within the intake port runner, usually just beyond the push rod area, this shape changes to a wider but shorter passage. The rate of transition should be such that any decrease in height should accompany an increase in width, so that the overall area is constant. The same volume of air can therefore flow through without constraint, and only have to slightly alter its shape and path (not velocity) to pass through the port runners. If you fabricate two-dimensional templates of the port runners in both the vertical and horizontal axes, then compare the two as a three-dimensional model, you will see that the factory at least tried to cast the heads so that the overall size (cross sectional area) was maintained.
Your porting work should concentrate on removing any obstructions and flaws to the idea of a constant area, and direct the flow to the valve more equally and easily. Bigger isn't necessarily better. Consistent flow should be your goal. Removing any obstructions to that is what you are trying to accomplish.
One method of improving air charge direction is to aim the charge at the back of the valve better, which can be done by raising the roof of the port. You can also expose more of the back of the valve to a direct air flow by eliminating as much of the short- or near-side valve bowl radius as possible. If intake air can flow more directly to all of the back of the valve without having to turn sharply anywhere, you should be improving overall flow.
The factory castings are typically bad in the are of short side radius. What's even worse is the "swirl" bowls that the factory experimented with, since they may indeed induce some swirl to the intake charge, but also present a narrowing of the intake ports before the valve.
On factory castings, it is nearly impossible to increase the port runner sizes so much that they are too large to maintain good flow. About the best I've been able to accomplish on factory castings while maintaining adequate material thicknesses is 197cc. That's not beyond the realm of "reasonable" for a 350-inch engine that needs good flow velocity to make torque (TPI) so long as the runner area is consistent along its length. On the smaller chamber iron heads (like 14014416) about the farthest I've gone is about 180cc before getting uncomfortable with their durability. In either case, that’s only about 20cc larger than the bare castings, and much of that is removed in the valve bowl area alone.
You can go as far as you like, but try to adhere to the flow conventions.
There are conflicting theories on whether gasket matching on the flanges is beneficial, detrimental, or insignificant. My personal take on this is that the factory runners on most of these heads are already so undersized, and offer so little material for alteration, that getting the ports to a regular average of cross sectional area from one end to the other is worth the effort. Make sure you understand that statement completely before going nuts with the mill and grinder.
In any case, it is important to keep the downstream side of the runners and ports no smaller than the upstream side. There should be no measurable steps in the system. If in doubt, keep the downstream side slightly larger. This may contradict the theories of wave induced cylinder charging and scavenging, but only a small percentage of the charge is actually wave induced. Conventional air flow dynamics still dictate what happens with most of the air entering the chambers. Ideally, the ports should be the same size all the way through with no points of significant enlargement or constriction along the way. That will allow both conventional dynamics and pressure wave induced flow to work in your favor.
If you study the design of most SBC heads before you start grinding, you should notice that some effort was made to maintain a uniform cross sectional are all the way through the heads. The ports start at the flange (and even upstream, in the intake base) as a narrow but tall passage. Somewhere within the intake port runner, usually just beyond the push rod area, this shape changes to a wider but shorter passage. The rate of transition should be such that any decrease in height should accompany an increase in width, so that the overall area is constant. The same volume of air can therefore flow through without constraint, and only have to slightly alter its shape and path (not velocity) to pass through the port runners. If you fabricate two-dimensional templates of the port runners in both the vertical and horizontal axes, then compare the two as a three-dimensional model, you will see that the factory at least tried to cast the heads so that the overall size (cross sectional area) was maintained.
Your porting work should concentrate on removing any obstructions and flaws to the idea of a constant area, and direct the flow to the valve more equally and easily. Bigger isn't necessarily better. Consistent flow should be your goal. Removing any obstructions to that is what you are trying to accomplish.
One method of improving air charge direction is to aim the charge at the back of the valve better, which can be done by raising the roof of the port. You can also expose more of the back of the valve to a direct air flow by eliminating as much of the short- or near-side valve bowl radius as possible. If intake air can flow more directly to all of the back of the valve without having to turn sharply anywhere, you should be improving overall flow.
The factory castings are typically bad in the are of short side radius. What's even worse is the "swirl" bowls that the factory experimented with, since they may indeed induce some swirl to the intake charge, but also present a narrowing of the intake ports before the valve.
On factory castings, it is nearly impossible to increase the port runner sizes so much that they are too large to maintain good flow. About the best I've been able to accomplish on factory castings while maintaining adequate material thicknesses is 197cc. That's not beyond the realm of "reasonable" for a 350-inch engine that needs good flow velocity to make torque (TPI) so long as the runner area is consistent along its length. On the smaller chamber iron heads (like 14014416) about the farthest I've gone is about 180cc before getting uncomfortable with their durability. In either case, that’s only about 20cc larger than the bare castings, and much of that is removed in the valve bowl area alone.
You can go as far as you like, but try to adhere to the flow conventions.
Last edited by Vader; 11-13-2005 at 12:10 PM.
11-13-2005, 12:31 PM
#7
Incidentally, you probably already realized it, but you'll need to alter the intake bolt pattern (angle of the center two bolts on each side) and use self-aligning rocker arms or guide plates to run those heads on an '85 block. Everything else should present no problems. The only difference may be in the rocker covers, and thus the source for CCV air. Your original setup for CCV inlet air and PCV may require a little alteration.
Trending Topics
11-13-2005, 01:37 PM
#9
Supreme Member
You're getting way to carried away with the valve deshrouding. You've missed the whole concept.
Do not grind out where you drew on the pic in red.
Read my posts on porting. The area within 5/8" of the spark plug boss is where all the flow is. Only a little metal needs to be removed. Then only a small amount around the vlave to allow it to physically fit the chamber wall. Look at a vortec head or a dart pro 1 heads combustion chamber.
Do not grind out where you drew on the pic in red.
Read my posts on porting. The area within 5/8" of the spark plug boss is where all the flow is. Only a little metal needs to be removed. Then only a small amount around the vlave to allow it to physically fit the chamber wall. Look at a vortec head or a dart pro 1 heads combustion chamber.
11-13-2005, 01:48 PM
#10
Supreme Member
In the photo the green area is the area that needs deshrouding for a larger valve. The blue area for the exhaust is ok as is really even with a larger valve.
The red arrows only need a little grinding to clear the edge of a larger valve.
The green area is the high flow area of the intake valve.
Scribe a line arc 1.4x the radius of the valve on the combustion chamber in the green area using a compass with the point at the center of the valve face. This is the area the needs a **"little"** deshrouding radiusing work.
The area you marked off for destruction is a very low flow area and will only serve to make a huge weakened chamber wall.
The red arrows only need a little grinding to clear the edge of a larger valve.
The green area is the high flow area of the intake valve.
Scribe a line arc 1.4x the radius of the valve on the combustion chamber in the green area using a compass with the point at the center of the valve face. This is the area the needs a **"little"** deshrouding radiusing work.
The area you marked off for destruction is a very low flow area and will only serve to make a huge weakened chamber wall.
11-13-2005, 02:01 PM
#11
Supreme Member
Don;t assume that are 58cc's now. If they have larger valves now the combustion chamber will be a little larger.
If you're willing to run the car on 92+ octane gas you can go to 10:1. I would not go higher.
You can mill off approx .060" or until you hit the intake valve seat. Usually the intake valve seat sets the milling limit on these heads. More than enough room to adjust the compression ratio. be sure to measure and accurately calculate your cr. do not depend on approximations and guessing. Many replacement pistons are "destroked" .020" and require the block to be decked to maintain even the stock cr.
If you're willing to run the car on 92+ octane gas you can go to 10:1. I would not go higher.
You can mill off approx .060" or until you hit the intake valve seat. Usually the intake valve seat sets the milling limit on these heads. More than enough room to adjust the compression ratio. be sure to measure and accurately calculate your cr. do not depend on approximations and guessing. Many replacement pistons are "destroked" .020" and require the block to be decked to maintain even the stock cr.
11-13-2005, 02:38 PM
#12
Supreme Member
Join Date: Aug 2001
Location: Caldwell,ID
Posts: 5,389
Likes: 0
Received 2 Likes
on
2 Posts
Car: 2005 BMW 545i
Engine: 4.4L N62B44
Transmission: 6spd auto
Axle/Gears: Rotating
vader you kinda outdid your self here.
I remember seeing someplace this guy was porting a set of heads. he removed about 2-3cc of material. the gains in flow where through the whole lift range. at .400" he gained a good 20-25cfm though. I wish I could remember where I saw this or what heads he was using.
most of his work wasn't porting to make larger or anything like that. instead what he focused on was reshaping like vader was talking about. he took the ports and removed obstructions, straightend the flow path, and redirected the air in a little more efficient manner with a goal of removing as little of material as possible to keep good street manners.
vader I always though in an ideal head the ports should slightly taper going towords the valve. granted a little more work and I doubt worth it for the first time porter. not talking steps or anything but just a very slight decrease in port area.
and vader do you think you could help with ideas on portin a CVCC head?
I remember seeing someplace this guy was porting a set of heads. he removed about 2-3cc of material. the gains in flow where through the whole lift range. at .400" he gained a good 20-25cfm though. I wish I could remember where I saw this or what heads he was using.
most of his work wasn't porting to make larger or anything like that. instead what he focused on was reshaping like vader was talking about. he took the ports and removed obstructions, straightend the flow path, and redirected the air in a little more efficient manner with a goal of removing as little of material as possible to keep good street manners.
vader I always though in an ideal head the ports should slightly taper going towords the valve. granted a little more work and I doubt worth it for the first time porter. not talking steps or anything but just a very slight decrease in port area.
and vader do you think you could help with ideas on portin a CVCC head?
11-13-2005, 04:21 PM
#13
Supreme Member
Thread Starter
iTrader: (2)
Join Date: Nov 2001
Location: Lakewood, CO
Posts: 1,602
Likes: 0
Received 0 Likes
on
0 Posts
Car: 1994 Jeep Wrangler
hmmm... well i guess I got some more reading to do. And concerning your other comments vader, I have a lower intake from an 89' TPI lying around I will use to avoid bolt hole issues. I also have a set of center bolt valve covers from my 87' parts car. My rockers are not self aligning, but the heads already have guide plates installed. I will take some more pics once I have them apart.
Thread
Thread Starter
Forum
Replies
Last Post
355tpipickup
Alternative Port EFI Intakes
3
08-29-2015 10:47 PM
