To port or not??

[afterglow]

My 69 Mach 1 is being painted and the engine is out of the car...perfect time to decide exactly what I want to do with it. It's a mostly stock 351W from a truck. Current upgrades include Weiand Stealth intake, Holley Street Avenger 670cfm carb, CompCams 262H, Durapsark, MSD 6A, Hooker Comp longtubes, C6 tranny and 3.8 Truetrac rearend.

Future plans may include building a 393 stroker and installing AFR/Canfield heads. Some people have suggested I port my manifold and stock heads. Are there any implications doing this now if I plan to go with aluminum heads in the future?? Should I go for it??

 

[CraigMBA]

The AFR's are so good, I don't know why you wouldn't just buy a set now and be done with it.

 

[afterglow]

Cost is the major issue. I still haven't completely paid for the bodywork and the paint. And the heads aren't the only things I'll need...I'm planning on a 393 stroker so that'll mean a crank, rods, pistons, rings, new cam, new carb...aside from the heads.

I'll do it in time....but not today.

 

[CraigMBA]

Cylinder head porting is, IMO, something to be left up to pros.

A pro is somebody with a flowbench who knows how to use it.

Considering you can buy a set of GT40xs for under a grand and the AFRs for not much more I'd be inclined to go that way.

If you wind up having your heads ported and the valvetrain needs any parts replaced, get ready to spend at least a couple of hundred on parts.

You've got enough irons in the fire now. Finish the bodywork and paint and save your pennies to do the motor right.

 

[RUSTYNUT]

Port matching your intake and heads is one of the few mods easily done by the average guy in his garage. Use the intake gasket as a template . Just make sure you use the same brand of gasket as there is some differance between brands. How much differance it makes in a mild street engine who can say, but it's got to help some. Just a tip, keep your cabide cutters lubed with atf when working on aluminum or they will plug very quickly. Check the car rags out, it is a fairly common how to with pics.

 

[brianj5600]

Gaskets are made for sealing, not port desgn. I would just take out all the rough edges in the bowls and imperfections on the walls and that's it. Don't even mess with the intake unless there are some obvious lumps on the walls. Wait until you get you aluminum heads and port match, not gasket mattch them togather.

 

[RUSTYNUT]

I agree about just cleaning up the rough edges, but to port match the intake to heads you need to eliminate the miss match of three items. The heads, intake, and the gasket. Hold the gasket up to to your heads then to your intake. the opening of the gasket is almost always larger. If it is not the same or it is smaller try a differant brand. To smooth out the transition between these three parts the gasket is the key. You aren't designing ports, just using the gasket factory spected opening as an easily tranfered template. You should only go about an inch in from the opening. Porting of the rest should left to pros with flow testing equipment as on many occasions adding material will make the parts flow better. Another tip, when I am buying heads or intakes I bring a intake and exhaust gasket along. You might be surprised how much core shift in castings are allowed out of the door. The gaskets placed on the part will make even a small shift stick out like a sore thumb.

 

[Max Power]

The intake side of a 351 is not the problem, it's the exhaust side. Port the thermactor bump out and leave the rest for competent aftermarket heads.

 

[danny clemens]

The intake side of a 351 is not the problem, it's the exhaust side. Port the thermactor bump out and leave the rest for competent aftermarket heads.

I agree 100%

 

[bnickel]

The intake side of a 351 is not the problem, it's the exhaust side. Port the thermactor bump out and leave the rest for competent aftermarket heads.

whilw i do prefer ro port match the heads to the intake, Max is correct in that until you fix the exhaust side you shouldn't touch the intake side. and depending on how much work you do on the exhaust ports will determine how much work should be done to the intake side. if all you do is remove the speed bump in the exhaust port and smooth things out a bit you shouldn't mess with the intake side. you have a to do a lot of work on the exhaust port of a SBF head to warrant any work on the intake side, though, like i said i still like to port match the heads and intake since you are really only increasing flow becuase you are smoothing that flow out by removing the mismatch.

 

[brianj5600]

Enlarging the intake port at the gasket will decrease the velocity at that point. The air must then be accelerated again as the port narrows. Since this area is not the point of restriction, making it bigger will hurt flow. The slowing and the accelerating of the air makes the port less efficient. I would not cut on an itake if I were going to put it on another head. AFR heads have a shape that should not be change to match a 40 year old gasket. They developed the port to flow w/o dead spots and if you cut the intake it may not be usable later. Also when I talked to Edelbrock, they said that a slightly smaller intake exit may help reversion at lower RPMs and only cost a couple HP up top.

 

[bnickel]

the problem with not part matching the inatek to the heads is that quite often you have a port in the intake manifold that is smaller than the port in the head, effectively creating a dam, if you will. also notice i said port matching not gasket matching, there is a difference but most people think of them as one in the same. if you do happen to end up with a port in the manifold that is smaller than the port in the head then that is fine but it is also not often the case. in any case, i still always prefer to port match the heads and intake, NOT gasket match, port matching is a little more work in that you have to actually bolt the intake to the heads and record the difference in the ports and the correct the mismatch

 

[CraigMBA]

The intake side of a 351 is not the problem, it's the exhaust side. Port the thermactor bump out and leave the rest for competent aftermarket heads.

This is good advice. If I HAD to use the stock heads this is all I would do. It's just about the only modification a DIY home user can make and not screw up.

In addition, I'd backcut the valves 30 degrees if I had it apart. Frankly, this is probablly more useful than the porting, and it probablly doesn't cost anything.

 

[CraigMBA]

the problem with not part matching the inatek to the heads is that quite often you have a port in the intake manifold that is smaller than the port in the head

I think you have that backwords.

If the intake port is bigger than the head, THEN you neck the port down and cause problems.

Going from a smaller intake port to a larger cylinder head port is much more desireable. Think of the expansion chambers on the pipes of two stroke engines.

Having them matched is optimal. For a typical street engine it probablly doesn't matter.

 

[bnickel]

I think you have that backwords.

If the intake port is bigger than the head, THEN you neck the port down and cause problems.

Going from a smaller intake port to a larger cylinder head port is much more desireable. Think of the expansion chambers on the pipes of two stroke engines.

Having them matched is optimal. For a typical street engine it probablly doesn't matter.

you're right i typed it backwards. if the intake port is bigger than the port in the head you basically have a dam that impedes the flow. just one of those days.

i would also back cut the intake and exhaust valves as well. it also helps a lot to clean the casting flash out of the valve openings in the heads as well.

 

[afterglow]

Is there a consensus?? Work on the exhaust side first, back cut valves and clean rough spots and that's it??

Can anyone guide me through this?? Pics would be great!!!

 

[brianj5600]

The bump on the downstream side of the exhaust valve should be reshaped. I would make it look similar to some of the aero bicycle helmets. That allows the exhaust on the backside of the stem to flow smoothly back together. I would do the I and E while I'm in there. The short side on the ones I did were horrible. There were distinct ridges that I smoothed and I was a little more agressive on the exhaust side, especially the roof. I'm not sure we will all agree on this though.

 

[afterglow]

Is this something that can be done with a Dremel and a few carbide tips or is it better left to the pros??

 

[brianj5600]

You could smooth things up and help it out, worth while IMO, but the reshaping of the thermactor bump would take a long, long time. I used an electric 1/4 die grinder from Sears. Have you removed the valves to see how ugly the ports are yet?

 

[CraigMBA]

Is there a consensus?? Work on the exhaust side first, back cut valves and clean rough spots and that's it??

Can anyone guide me through this?? Pics would be great!!!

http://www.mustangandfords.com/techarticles/28898_400_hp_302_small_block/photo_02.html

I've been Joe Dirt dead broke before more times than I'd like to admit. So dead broke, once when in college I burned an exhaust valve. I took it apart, bought ONE head gasket ONE valve and did a one valve valve job, reassembled it, and drove it for five more years. Just so you know where I'm coming from.

I back cut the valves, work on the bump on the exhaust side out if I get real frisky, and quit. If I want to make more power, I go work overtime, sell blood, rent out a room in my house, yard sale, collect cans, or whatever else I think I can do and buy AFR 185s, bare, back cut the valves and buy good springs from whoever I get the cam from. For a street/strip car you can cheat and buy the assembled pieces. I'm a road race guy and don't get that luxury.