Jeep Strokers

Am I the only one who has screwed up using this guide?
Having read the SAHPG (Standard Abrasives Head Porting Guide) it is my opinion that these guys are just trying to sell porting kits. There is much misinformation here and if you follow their guide I think it would be easy to do more harm than good. Luckily I started with a practice head off of an old 258 or I would have ruined my good head. I am sure there is more wrong with this article than what I found, but I have to point out one thing that I noticed. And then I will read it again to see what else they have lied about.
Simply gasket matching your head and manifolds will cause a large pockets in the transition from your manifold to your head.
This "pocket" will slow the velocity of air entering your C/C and hinder performance.
Here is how I understand it.
Imagine you are holding a long pipe. Now blow air though it...air will flow through this pipe uniformly and fastest (at greatest velocity) if the pipe is uniform through the entire length...Having the same cross sectional area through out the length of the pipe. Now, Imagine the pipe has a pocket (an area of larger volume) or a "bubble" in the middle of it. The air entering this "bubble" will have to expand causing it to slowdown. This will in turn cause the air behind the "bubble" to slowdown. And it will even pull at the air in front of the "bubble" causing it to slow down. In other words any pocket or "bubble" or any change in cross sectional area with in your intake or exhaust runners will cause the air going into the C/C to slow down. This means less air in the C/C...less air means less O2...and less O2 means a smaller bang...means less power.
And besides all of that I have also learned that the exhaust ports are small for a reason. It promotes low end torque. By hogging out the exhaust interface to the size of the gasket you are not only creating a pocket that slows flow but you are also doing away with all of the back pressure that your engine needs to produce low end torque.
And.....they don't even mention re-leaving the valves or removing edges made by the seat cutter. Two very important things to look at!!!
I encourage everyone to become as knowledgeable as possible before grinding away at your head.

I must have spent 20 hours gasket matching the 258 head before I read enough to learn that I was doing it wrong.

akadeutsch wrote: Simply gasket matching your head and manifolds will cause a large pockets in the transition from your manifold to your head.

Some people do actually gasket match.. really bad idea... one guy on here did it... should have seen the exhaust ports he had.

Also, its just a guide. I recommend just a clean up, port match and chamber polish, not much more.

Your consensus of velocity and cross section is sound. The trick is to not limit the potential amount of air that can be passed WITHOUT significantly losing velocity. Especially on the exhaust side where the moment of both vales being open at the same time can "draw" the intake charge into the cylinder. One of many reasons some engines respond to dual pattern cams.

As Silver has said, bigger is not always better.

As with everything there are trade offs. One of the major negative attributes that the I6 design has is a centrally located plenum and widely differently sized runners. The "new" style intake addressed this to a degree but is difficult to solve by the sheer nature of the design. Notice of the bottom of the plenum the "diverters ? This was a further attempt to help equalize the flow between the inside and out side runners by directing airflow.

Your balloon analogy has some merit (without getting into cam dynamics and such) by the evidence of reversion on the plenum floor.

I`ll throw another bone.

The stock manifold has a cross section of ~63mm at the opening for the TB. Thus the plenum is fed around the 63mm parameter. The airflow is distributed across the 63mm opening BUT the roof of the plenum creates a dead spot in the corners and the underside of the plenum itself.

Throw in the larger 70mm TB and you have now reduced the amount of "plenum roof" by virtue of the larger 70mm opening. The plenum volume has not changed but the dead spots and transition sections have.

Air speed increases because your feeding the small(er) cross section runner from the plenum. I contend that it ends up being more efficient because the plenum is now being filled more evenly because of the wider feed cross section of the 70mm inlet. Which would most advantageous to the outside runners.

Know wat I mean ??

There is a lot to improving flow and even the people that do it all the time need equipment to confirm it works the way they think from time to time. I just cleaned mine up a bit and smoothed them out some. I was afraid of doing damage. Extrude hone seems to be the hot setup and the honing paste likely flows very similar to the engine gases.

jsawduste wrote:Your consensus of velocity and cross section is sound. The trick is to not limit the potential amount of air that can be passed WITHOUT significantly losing velocity. Especially on the exhaust side where the moment of both vales being open at the same time can "draw" the intake charge into the cylinder. One of many reasons some engines respond to dual pattern cams.

As Silver has said, bigger is not always better.

As with everything there are trade offs. One of the major negative attributes that the I6 design has is a centrally located plenum and widely differently sized runners. The "new" style intake addressed this to a degree but is difficult to solve by the sheer nature of the design. Notice of the bottom of the plenum the "diverters ? This was a further attempt to help equalize the flow between the inside and out side runners by directing airflow.

Your balloon analogy has some merit (without getting into cam dynamics and such) by the evidence of reversion on the plenum floor.

I`ll throw another bone.

The stock manifold has a cross section of ~63mm at the opening for the TB. Thus the plenum is fed around the 63mm parameter. The airflow is distributed across the 63mm opening BUT the roof of the plenum creates a dead spot in the corners and the underside of the plenum itself.

Throw in the larger 70mm TB and you have now reduced the amount of "plenum roof" by virtue of the larger 70mm opening. The plenum volume has not changed but the dead spots and transition sections have.

Air speed increases because your feeding the small(er) cross section runner from the plenum. I contend that it ends up being more efficient because the plenum is now being filled more evenly because of the wider feed cross section of the 70mm inlet. Which would most advantageous to the outside runners.

Know wat I mean ??

Yeah. Yeah. I am considering having my TB bored to 63mm but I have never considered going any larger. As for the intake runners. I could see attempting to enlarge the outer runners to compensate for the longer travel. but not without some measure of progress or detriment.
I have just seen this SAHPG referenced more times than I am comfortable seeing. I am doing some porting now and I'm glad I started with a junk head. The transitions all look like tulips on my first try.

akadeutsch wrote:Am I the only one who has screwed up using this guide?
Having read the SAHPG (Standard Abrasives Head Porting Guide) it is my opinion that these guys are just trying to sell porting kits. There is much misinformation here and if you follow their guide I think it would be easy to do more harm than good.

I read at least three DIY head porting articles before attempting to port heads myself.
As far as the Jeep 4.0 head goes, here's what I'd recommend:

1. "Pocket port" the valve bowls/throats to remove the casting ridges/flash, making the surfaces as smooth as possible. This makes the biggest difference.
2. Remove just enough material from the short side radii to make them smooth. Don't cut them back.
3. Remove some material around the valve guide bosses and shape them into a teardrop.
4. ONLY gasket match the intake ports. Do NOT gasket match the exhaust ports 'cause that'll make them way too big.
5. Just remove about 1mm of material from the chamber walls in a small area where they are shrouding the exhaust valves.

Cheromaniac wrote: 5. Just remove about 1mm of material from the chamber walls in a small area where they are shrouding the exhaust valves.

Better yet, put the head on the bare block and scribe the cylinder outline onto the head. Then, when you polish your chambers, don't go past that line.

I was going to use the old head gasket, but scribing the block is a better idea. Thanks

Retlaw01XJ wrote:Better yet, put the head on the bare block and scribe the cylinder outline onto the head. Then, when you polish your chambers, don't go past that line.

This is spot on. Many people think they are unshrouding the valves by hogging out the chamber walls to match the HG.... I did as well in the past... In this application it's not the best idea. As with many motors, the head gasket bore is considerably larger than the actual cylinder bore. A 4.0 chamber opened up to a 4" bore head gasket will create a sharp edge between the valve/chamber wall and the upper edge of the cylinder bore creating turbulence. The only way to reap gains in this instance is to have the upper edges of the cylinder notched near the valve to match the chamber and promote smooth flow. Or, like Retlaw said.. Place head on bare block and scribe finished cylinder diameter onto the head and only open chambers to the line. This is where head alignment dowels or similar would be very beneficial.

I posted this on my build page as well but I think it belongs here as well.
Here is how I did an accurate port match taking as little material off of the head and manifold as possible. I started by going to ACE hardware and picking up a sheet of 28 gauge aluminum that is big enough to trace the entire plenum gasket onto. Once I had my sheet I cut it to size with tin snips and then used a hammer on a cement floor to pound it straight again as the tin snips tend to warp the finish. First I marked and cut the holes to attach my piece of aluminum to my intake manifold. Then found bolts with the appropriate shoulder width to support my new aluminum "plenum gasket" to my intake manifold with no play to speak of. I then drilled a hole in the middle of the manifold ports beginning to open them up for sizing. Then I used a carbide cutting bit to open the holes that I drilled in the ports to the EXACT size of the actual intake manifold ports. Use care here as your carbide bit will not hesitate to cut through your aluminum intake manifold. When all of your new "plenum gasket" ports are opened to the size of the manifold port you have what looks like and exact copy of your plenum gasket made out of 28 gauge aluminum. Now place this new aluminum "gasket" onto your head. It should fit very exactly over the guide pins on your head. You will see areas that dont exactly line up. Scribe those areas onto your head and then remove the thing you made. Now remove material from your head out to the scribe lines that you just made. Then place the aluminum gasket back on the head. This time, remove more of the aluminum gasket to EXACTLY match the port interface on your head. Now, take the aluminum gasket off of the head and put in back on the intake manifold. This time it will not line up. Scribe the new outline of the head transition onto the intake manifold and remove your aluminum gasket. Remove material from your intake manifold out to the lines that you just scribed and you are done. Now, mount the intake to the head and stick your finger up the C/C and into the intake ports and feel the transition. The 1/8 inch step that used to be there is now a smooth transition and you only took away as much material as was necessary. After I was done with this I also camfered the port transitions ever so slightly and I think this helped with the alignment

Pretty sure it says in that article to almost never port match the exhaust...

Are you recommending that we leave the very obvious step only on the exhaust side? I understand that a change in the runner volume requires a change in either velocity, density, or both of the air traveling through it. But this is minuscule in comparison to running into a wall of iron. Because I understand this concept I outlined the way that I was able to remove the step while changing the runner volume as little as possible. "Pretty sure" I understand the concept and also, "pretty sure" if you are able follow my lead you will be happy that you did.