porting and polishing?

[rr41mag]

I'm getting mixed signals some say yes and some say no? Clifford performance says no with their buildup kit they sell. On the other hand there are plenty of videos on u tube on how to do it. I'm an old small block chevy guy and I know it helps them out. I can't see why it wouldn't. I've started on this AMC 258 i6 / 4.2l head and already have two cylinders done. Anyone out there done this?

 

[Kane]

Even with small block Chevys they have certain heads that just aren't worth the time and investments. I feel the stock AMC 258 i6 / 4.2l head is along that thinking. I have rebuilt a few 258s and stayed away from porting on all of them. You may gain a small amount of power but you cannot mill out that terrible turn in the runners. If i were to port a head i would start with the 4.0 head and build on that. I bet you would find some good gains with it.

 

[duffer]

I would be skeptical that you would actually see any gains where you most need them-the bottom end of the rpm range. You could in fact reduce torque where it is most desirable if you slow the intake charge velocity with the bigger ports.

I like playing with small block Chevys but even with those, if you are looking for bottom end torque, big heads and porting are counterproductive unless you have a really lot of cubic inches.

 

[IOPort51]

I kind of like to open the ports to match the gasket on the head and the manifolds. Can not say it is all that important.

I have always thought of port and polish as a practice of people that will win or loose over .0001 seconds. So if you have done everything imaginable to the motor port and polish will make a difference.

For my Jeep? not so much.

In my book the primary focus is dependability and I do not see this adding anything to this column.

 

[gutthans]

Other than taking blue dye and matching the port opening to the gasket (on intake AND head) I wouldn't waste time doing anything else.

Eliminating gasket overhang and getting smooth flow between the manifold and port at the gasket is the only place to spend any tim IMHO. That might produce one hp per cylinder (for a total of 6) possibly, but NO, I wouldn't get into the port or bowl at all.

 

[ThisGuyUKnow]

I'm not an engine builder so this is only a slightly educated opinion.

I think you would be wasting your time porting and polishing a AMC 258 i6 / 4.2l head. The runners dogleg from the intake manifold to the valves. This is going to naturally slow the air no matter what you do. A port and polish would help open up some airflow but with that dog leg I cannot imagine that the gains would be at all worth the money or time.

Since you are doing the Port and polish yourself, you would see a lot better results if you just went to the local junkyard and grabbed a 4.0 head and cleaned it up/rebuilt it with the Port and polish. It seems like you are capable of doing that work yourself and generally the yards sell heads for about 50 a pop so you may be able to do it cheap.

 

[tarry99]

I'm getting mixed signals some say yes and some say no? Clifford performance says no with their buildup kit they sell. On the other hand there are plenty of videos on u tube on how to do it. I'm an old small block chevy guy and I know it helps them out. I can't see why it wouldn't. I've started on this 258 head and already have two cylinders done. Anyone out there done this?

rr41mag,

I think there is a resounding "No" to the porting of that head.......but in reality the porting of any head done by anyone without the knowledge and experience is really a waste of time..........In the hands of an experienced person the AMC 258 i6 / 4.2l head could be worked over to perhaps make some gains.........but the porting has to be done correctly........The trick is to increase air and fuel flow volume while increasing port speed while still matching all ports & bowls to the same cc volumes & flow.......working on the short turn radius is most important......sometimes a port will not flow enough unless a larger valve is also used......and then a larger cam with more lift and a higher ratio rocker to open the valve quicker and so on is needed........and it never ends!

Without the proper tools including a flow-bench you’re really wasting your time. And as others have said you could kill that head in just as much time as one could perhaps help it............

Now almost all porting on racing cylinder heads is done via CNC robotics......They have even eliminated the need for hands on prototype cylinder work as with the new stealth type multi axis 3-D programs , they can tell you all you need to know in a virtual environment on your screen! ...there is hardly a spot that is touched anymore by a die grinder held by a human. This in itself insures that all ports are the same size and volume which has dramatically cut the time and cost for a Super Hi flowing set of cylinder heads as compared to the days when hundreds of man hours were required for a set of good heads that folks literally defended with their lives.........

 

[rr41mag]

rr41mag,

I think there is a resounding "No" to the porting of that head.......but in reality the porting of any head done by anyone without the knowledge and experience is really a waste of time..........In the hands of an experienced person the 258 head could be worked over to perhaps make some gains.........but the porting has to be done correctly........The trick is to increase air and fuel flow volume while increasing port speed while still matching all ports & bowls to the same cc volumes & flow.......working on the short turn radius is most important......sometimes a port will not flow enough unless a larger valve is also used......and then a larger cam with more lift and a higher ratio rocker to open the valve quicker and so on is needed........and it never ends!

Without the proper tools including a flow-bench you’re really wasting your time. And as others have said you could kill that head in just as much time as one could perhaps help it............

Now almost all porting on racing cylinder heads is done via CNC robotics......They have even eliminated the need for hands on prototype cylinder work as with the new stealth type multi axis 3-D programs , they can tell you all you need to know in a virtual environment on your screen! ...there is hardly a spot that is touched anymore by a die grinder held by a human. This in itself insures that all ports are the same size and volume which has dramatically cut the time and cost for a Super Hi flowing set of cylinder heads as compared to the days when hundreds of man hours were required for a set of good heads that folks literally defended with their lives.........

Yep

 

[Goatboy4570]

This is where the parttime tinkerer and the Hotrodder take divergent paths, personally if it flows air i'll look for a way to do that more, that incluedes my vacum cleaner, dirt bikes, lawn mower, mini bike, intake manifold an the blow gun that i use to clean parts with not to mention what ever head is on what ever motor im building.

When you port a head it extends the useable power band of a motor without having to put in a bigger cam.
If your biulding a rv motor a ported head is gona make it pull harder in the mid without losing bottem end.
If for no other reason then to say "i didnt comprimize" the time it would take is equal to what some slaker would waste watching some dumb reality tv show.

So to the op i say this, just do it man!

 

[Lumpy Grits]

The AMC 258 i6 / 4.2l in our jeeps was based on a low-RPM tractor engine.
They are 'red-lined' at 4500 rpm.
Waste of time to P&P.
LG

 

[ThisGuyUKnow]

So to the op i say this, just do it man!

I'm gonna go ahead and take the opposite stance on that statement.

 

[rr41mag]

Other than taking blue dye and matching the port opening to the gasket (on intake AND head) I wouldn't waste time doing anything else.

Eliminating gasket overhang and getting smooth flow between the manifold and port at the gasket is the only place to spend any tim IMHO. That might produce one hp per cylinder (for a total of 6) possibly, but NO, I wouldn't get into the port or bowl at all.

some of the runners have almost 1/2 that can be removed

 

[hdgapeach]

No to porting and polishing.....for a different reason for me.

Porting and polishing on any head should be left to folks that have the experience and the associated tools to perform the job. I say this because P&P jobs can absolutely trash an otherwise good head. Remove too much material from the wrong location in the head and you risk cracking the head in places that it will not be possible to repair. Engine running torque will cause this to happen to thin places in the runners.

Looking at cross section pictures of various stock and aftermarket heads will show a good example of why the aftermarket head business is so good. Most of the aftermarket brands have more material to work with in the weak sections that the stock heads are manufactured with.