Poly 318 A-block Cylinder Head Porting

Poly A-block Cylinder Head Ported

Poly A-block Cylinder Head Ported

Introduction

This article is part of my larger engine-building series and focuses on porting heads for performance A-blocks. The A-block polyspherical combustion chamber design is excellent in many ways despite its short-lived use from the 1950s – 1966 until its reemergence in some 21st century engines. By the 1960s, Mopar likely restricted the design to A-blocks and abandoned it in 1966 due to the higher cost of production compared to wedge heads.

Of note, when compared to LA340 and LA360 wedge-head intake and exhaust runners, A-block runners appear small; however, it is important to remember that the polypherical combustion chamber design and combustion physics do not require runners as large as wedge heads. Still, A-block heads benefit from increased valve diameter, port matching, and full porting. Even modest hand porting done by a competent, careful mechanic at home feeds engines up to 450 HP. For the faint of heart not yet ready to dive into full porting, port matching will see cheap power gains since the intake and exhaust manifold ports are decently misaligned from the head ports.

In the case of the heads I’m using on my 390 stroker build, I chose to increase the valves to 2.02” intake, 1.60” exhaust, race valve job, and full hand porting. In this article, I detail how I went about porting the heads at home with a quality high-volume air compressor, die grinder, long-reach carbide bits (ball, rounded cylinder, and tapered point), stones, 80-grit sanding barrels, moveable light source, and container of water to cool the carbide bit. I do not have a flow bench to test the heads, like many hot rodders before me, but focus on pinch and turbulence points that people who have flowed A-block heads have discussed with me in depth. The key is to focus on matching the runners to the gaskets and porting back keeping the runner corners as uniform and open as possible, removing/smoothing as many humps in the casting as possible, and maintaining good taper into the bowls. Attention to the short and long turns of the bowls is critical as well. A light touch and working slowly and deliberately is key.

My intent in this article is to demonstrate what competent hot rodders who like to do their own work can do at home to get substantial improvement in performance without dropping thousands of dollars on a flow bench/software or with a professional porter. If I were looking to build a 600 HP A-block, I would likely spend the money building a flow bench or as a last resort farm out the task to a reputable A-block porter to get every bit out of the heads. I list three different porters in the flow numbers below.

I break this article into three parts: work I did before having my machinist cut the larger valves and install new guides, work after the preliminary valves job and guide, and work after the valve job was completed. The runners and a portion of the bowls can be ported prior to any valve work to lower the risk of damaging the new seats but leaving the machinist plenty of meat for the new seats and guides. After that work is completed, I go back in and blend the bowls and sand the bowls and runners. After the valve job is completed, I go in and do any final touchups.

Flow Comparisons

The following flow numbers at 28” water demonstrate the differences in CFM between factory and degrees of porting by different porters. 

Stock A-block (courtesy Total Performance for Gary Pavlovich)

  • Intake Valve: 1.85”
  • Exhaust Valve: 1.55”
  • Valve Stem: 3/8” no undercut
LIFT (INCHES)INTAKE (CFM)EXHAUST (CFM)
.1005146
.20011783
.300150106
.400172127
.500184142
.550187147

Hand Ported 1 (courtesy Total Performance for Gary Pavlovich)

  • Intake Valve: 2.02”
  • Exhaust Valve: 1.60”
  • Valve Stem: 3/8” no undercut
LIFT (INCHES)INTAKE (CFM)EXHAUST (CFM)
.1007049
.20014588
.300189113
.400203134
.500211151
.550216158

Hand Ported 2 (courtesy Douglas Porting Service)

  • Intake Valve: 2.05”
  • Exhaust Valve: 1.60”
  • Valve Stem: 11/32” undercut
LIFT (INCHES)INTAKE (CFM)EXHAUST (CFM)
.1007263
.20014498
.300204123
.400225146
.500240163
.600252171

Hand Ported 3 (courtesy Scott Glover)

  • Intake Valve: 2.02”
  • Exhaust Valve: 1.60”
  • Valve Stem: 11/32” undercut
LIFT (INCHES)INTAKE (CFM)EXHAUST (CFM)
.1007563
.200158105
.300218130
.400241171
.500250192
.550256198

CNC Ported (courtesy Scott Glover)

  • Intake Valve: 2.02”
  • Exhaust Valve: 1.60”
  • Valve Stem: 5/16” undercut
LIFT (INCHES)INTAKE (CFM)EXHAUST (CFM)
.1007267
.200141104
.300211133
.400248175
.500263198
.600266217

Work Prior to Valve and Guide Machine Work

Disassemble the cylinder head (see my head disassembly technical article). Having a machinist clean and magnaflux the heads before porting can determine if the heads are useable; in the case of these heads, I chose to do most of the porting work prior to having the machinist clean and magnaflux the heads since they visually looked solid and I did not mind practicing porting if the heads turned out to be unusable. As it would turn out, they magnafluxed fine.

  1. Focusing on the intake runners first, clean the gasket surface, apply Dykem Blue or similar, and secure with 3/8” bolts the same gasket used to port the intake manifold (see my intake porting technical article). Mark the gasket with a sharp scribe (Figure 1).
  2. In Figures 2 and 3, note that most of the material on these heads needs to come off the sides, roof, and corners. The corners, particularly on the floor, have a wide radius that can be tightened to widen the runners and ease the short-turn. Also, note the hump on the floor, referring to the sectional Figure 3. I will address this hump in a following step.
  3. Using the rounded cylinder bit with the head placed where I can work comfortably on the roof and walls, I begin taking off material from the opening moving toward the bowl evenly. The casting has a crease down the runner walls like the sectional view of an opened book, so I work on taking down the hump on each side of the crease before moving to making sweeps with the bit from floor to roof (Figure 4). I focus on maintaining the gasket corner radius back into the bowl, but be careful not to carve a trough into the floor/roof. I will work all intake runners in this fashion before needing to change bits.
  4. Once I have the roof and walls (except for the very top corners) close to where I want them, I use a combination of the round and tapered bits to smooth the areas around the guide, being careful not to get tunnel vision and dig into the wall when struggling to reach a spot between the guide and wall from the runner side and instead flip the head to work from the bowl side for access. I leave plenty of material around the guide for the machinist and will go back in and contour after the new guides are installed.
  5. With the head positioned where I can work comfortably on the floor, I place a permanent-marker dot on the roof above the appendix of the floor hump. I take a measurement from the dot to the floor with a telescoping bore gauge (Figure 5). Using the rounded cylinder bit, I move side to side flattening the hump until I have removed .0625” measured with a feeler gauge (Figure 6).
  6. Working from the gasket mark back, I remove material from the lower walls and floor keeping the corner radius consistent.
  7. There will be two sharp angles in the floor, one on each side of the portion I flattened. I smooth these angles and work the short turn down into the bowl using my finger to feel the contour until I am satisfied with the runner porting (Figure 7). I finish the rest of the intake runners in this fashion.
  8. With the head positioned where I can work comfortably on the chambers and bowls, I note the rough casting/machining and any pinch points (Figure 8). I will leave most of the meat near the seats for the machinist and focus on contouring the areas deeper in the bowl.
  9. Using a combination of the ball and rounded cylinder bits, I shape the bowl to smooth out any waves and to contour around the guide. I use the tapered bit to clean up the wall and ceiling on the tight side of the guide. At this point, the bowl is finished until I complete contouring after the new seats and guides are installed (Figure 9).
  10. Clean the exhaust gasket surface, apply Dykem Blue or similar, and secure the exhaust gasket with 3/8” bolts. Mark the gasket with a sharp scribe (Figure 10).
  11. In Figure 11, note that the exhaust runners on these heads require removing a lot of material with 1/8” typically needed from the floor and one side. However, the sectional Figure 12 shows that there is not much material between the floor and water jacket near the short turn.
  12. To address the floor, I use the rounded cylinder bit to remove 1/16” off the entire floor back to the turn and the remaining material off the thickened gasket flange, creating a very slight hump in the floor since I want to keep the floor as thick as possible to stay away from hotspots and cracking from the exhaust temperature. I ease the transition from the floor through the short turn, using my finger to feel the contour.
  13. Working up the walls, I open up to the gasket line. In the case of these heads, the runners shrunk inward at the gasket flange, so opening up the wall to the gasket line actually straightened out what had been a bulge in the runner. Keep the gasket corner radius consistent all the way trough the short turn.
  14. In Figure 13, note the bulge in the wall above the short turn. While I do not take out all of the bulge, I flatten it considerably and smooth it. Also note that the walls have a crease similar to the intake runners, but I cannot remove all of this crease without creating a bulge in the runner that will pinch down at the gasket; I smooth out the crease and leave it.
  15. In Figure 14, note the shelf in the bowl. From all the flow tests I know of, this shelf does not impact flow for street porting, so I simply ease the edges and leave it.
  16. With the head positioned where I can comfortably work on the roof, I flatten the roof and radius the corners to the gasket line, carrying the corner radius into the bowl.
  17. Similar to the intake side, I use a combination of the ball and rounded cylinder bits to smooth out and around the guides leaving most material for after the new guides are installed. At this point, the runners are completed until after the new seats and guides are installed where I will finish working the guides and sand the runners (Figures 15 and 16).
  18. With the head positioned where I can work on the chamber and bowls, I note the severe obstructions in the bowls (Figures 17 and 18). Using the ball bit, I remove much of the shelf tapering from the seat to the guide but leaving meat for the machinist. I use the tapered bit to remove the sharp edge at the guide keeping most of the guide meat for the machinist. At this point, the bowls are completed until after the new seats and guides are installed (Figures 19 and 20).
  19. Looking at the combustion chambers (Figure 21), note that the end chambers have casting “arrows” between the seats that I knock down with the sanding stone (Figure 22). After the larger valve seats are installed, I will go back in and finish easing any edges in the chamber.
  20. As part of my oil system modifications (see my technical article), I chamfer the two drain holes to assist with oil drain-back (Figure 22).

Work after Valve and Guide Machine Work (write-up in progress)

While requiring more trips to my machinist, I have my machinist rough cut in the larger valves and then give back the heads for my blending before he finishes the valve job. If I accidentally hit the 45-degree seat cut while blending and sanding, there won’t be an issue since the valve job will remove any damage.

  1. The enlarged intake seat will cut down into the bowl leaving considerable ridges and a nasty cliff on the short turn (Figure 23). Using the ball and tapered carbide bits, I blend the new seat just below the 45 degree seat cut into the bowl being careful not to hit the 45-degree seat cut and then blend the bowl into the runner (Figure 24). Keep in mind that there are bulges in the runner walls at the mouth of the bowls for the pushrod/bolt holes and water jackets (Figure 25). While I have seen these bulges removed for ultimate performance using a CNC program, it requires bushing the pushrod hole and epoxying the runner since removing the bulge will break through the wall. Rather than focusing on removing the bulge completely, I smooth the radius into the bowl and back into the runner for easier air flow; this method is good for 450 HP.
  2. For the intake short turn (Figure 26), I smooth the cliff back into the runner floor keeping in mind that this area is thick enough to allow for a more gradual ramp (Figure 3). I focus on not cutting the corners of the runner too much into the seat area since it is easy to undercut the seat here. When finished, the turn should be gradual and smooth (Figure 27).
  3. After all the carbide work in the intake and exhaust bowls and runners, I hit the surfaces with 80-grit sanding barrels being careful not to get into the 45-degree seat cut. Some people go even finer on the exhaust, but I don’t.
  4. Turning my attention to the combustion chamber, I drop in used/junk 2.02″ and 1.60″ valves to protect the seats (Figure 28). On multiple chambers, there is a hump at the exhaust seat (Figure 29) and around the seats that require smoothing. Working with fine cone stones in the die grinder, I remove the hump at the exhaust, between the seats, and the ridge around the seats. As the final touch, I hit the entire chamber with 120-grit abrasive pad balls (think Scotchbrite on a stick) in the die grinder to smooth my stone work and knock off any sharp casting ridges (Figure 30). My goal is not to polish the chamber but to eliminate any hot spots keeping in mind that any material removed increases the chamber volume and, thus, decreases compression ratio, which I don’t want.

Check back for details on the exhaust side once the hardened seats are installed

Acknowledgements

I want to thank Ray and Ben Bell for sharing photos of the A-block head they cut up into sections. Thanks to Gary Pavlovich for chatting with me on the phone about porting and sending me flow charts and some reference photos of his porting work. Thanks to professional head porter Greg Douglas for chatting with me about valve selection.

FIGURES

Poly A Block Cylinder Head Porting Layout
Figure 1: Poly A-block Cylinder Head Porting Layout

Poly A-block Cylinder Head Intake Runner
Figure 2: Poly A-block Cylinder Head Intake Runner

Poly A-block Cylinder Head Sectional
Figure 3: Poly A-block Cylinder Head Sectional (photo courtesy Ray and Ben Bell)

Poly A-block Cylinder Head Intake Runner Porting
Figure 4: Poly A-block Cylinder Head Intake Runner Porting

Poly A-block Cylinder Head Intake Runner Porting
Figure 5: Poly A-block Cylinder Head Intake Runner Porting

Poly A-block Cylinder Head Intake Runner Porting
Figure 6: Poly A-block Cylinder Head Intake Runner Porting

Poly A-block Cylinder Head Intake Runner Ported
Figure 7: Poly A-block Cylinder Head Intake Runner Ported

Poly A-block Cylinder Head Intake Bowl
Figure 8: Poly A-block Cylinder Head Intake Bowl 

Poly A-block Cylinder Head Intake Bowl Ported
Figure 9: Poly A-block Cylinder Head Intake Bowl Ported

Poly A-block Cylinder Head Exhaust Layout
Figure 10: Poly A-block Cylinder Head Exhaust Layout

Poly A-block Cylinder Head Exhaust Runner
Figure 11: Poly A-block Cylinder Head Exhaust Runner

Poly A-block Cylinder Head Exhaust Sectional
Figure 12: Poly A-block Cylinder Head Exhaust Sectional (photo courtesy Ray and Ben Bell)

Poly A-block Cylinder Head Exhaust Runner
Figure 13: Poly A-block Cylinder Head Exhaust Runner

Poly A-block Cylinder Head Exhaust Runner Shelf
Figure 14: Poly A-block Cylinder Head Exhaust Runner Shelf

Poly A-block Cylinder Head Exhaust Ported
Figure 15: Poly A-block Cylinder Head Exhaust Ported

Poly A-block Cylinder Head Exhaust Ported
Figure 16: Poly A-block Cylinder Head Exhaust Ported

Poly A-block Cylinder Head Exhaust Bowl
Figure 17: Poly A-block Cylinder Head Exhaust Bowl

Poly A-block Cylinder Head Exhaust Bowl
Figure 18: Poly A-block Cylinder Head Exhaust Bowl

Poly A-block Cylinder Head Exhaust Bowl Ported
Figure 19: Poly A-block Cylinder Head Exhaust Bowl Ported

Poly A-block Cylinder Head Combustion Chamber
Figure 20: Poly A-block Cylinder Head Combustion Chamber

Poly A-block Cylinder Head Combustion Chamber Ported
Figure 21: Poly A-block Cylinder Head Combustion Chamber 

Poly A-block Cylinder Head Oil Holes
Figure 22: Poly A-block Cylinder Head Oil Drain-back Holes

Mopar Poly 318 A Block Cylinder Head Porting
Figure 23: Poly A-block Cylinder Head Large Valve Machining

Mopar Poly 318 A Block Cylinder Head Porting
Figure 24: Poly A-block Cylinder Head Intake Ported

Mopar Poly 318 A Block Cylinder Head Porting
Figure 25: Poly A-block Cylinder Head Intake Ported

Mopar Poly 318 A Block Cylinder Head Porting
Figure 26: Poly A-block Cylinder Head Intake Short Turn before Porting

Mopar Poly 318 A Block Cylinder Head Porting
Figure 27: Poly A-block Cylinder Head Intake Short Turn after Porting

Mopar Poly 318 A Block Cylinder Head Porting
Figure 28: Poly A-block Cylinder Head Combustion Chamber before Blending

Mopar Poly 318 A Block Cylinder Head Porting
Figure 29: Poly A-block Cylinder Head Combustion Chamber before Blending

Mopar Poly 318 A Block Cylinder Head Porting
Figure 30: Poly A-block Cylinder Head Combustion Chamber after Blending