Poly 318 Exhaust Manifolds and Headers

Both factory exhaust manifolds and aftermarket headers are available for the A-block, although bolt-on header options are limited to one company. However, I detail below multiple header options. From the factory, A-blocks came with two types of cast iron exhaust manifolds: a center dump and a rear-exit log (Figure 1). The casting designs vary depending on the vehicle and year; for example, early generator vehicles included a generator bracket cast into the manifold, and some export engines had a choke hot-air port on top of the left log exhaust manifold. The cylinder head exhaust ports are spaced evenly, so manifolds can be used on either side. In general, cars took a log manifold on the left and center dump on the right, while trucks took center dumps on both sides. 

Factory Exhaust Manifolds

Here is a list of exhaust manifold casting numbers and their descriptions that I have compiled, although there may be other castings I haven’t come across yet. 

1945095 – Center dump w/out generator bracket (1956 – 1961 or so)

1945094 – Center dump w/generator bracket (1956 – 1961 or so)

2129197 – Center dump w/out generator bracket (about 1962 – 1966)

2268189 – Left log rear dump with narrower flat front (likely 1956 – 1961)

2532415 – Left log rear dump with wider flat front (1962 – 1966)

2858331 – Left log rear dump with carburetor choke hot-air port (1962+ Canadian, info courtesy Brian Woodard)

Mopar Poly 318 A-block Exhaust Manifolds
Figure 1: Poly 318 A-block Factory Exhaust Manifold Examples (photo courtesy T. Woods)

Performance Tube Headers

Bolt-on Options

As of 2022, the only aftermarket headers available as direct bolt-ons are from TTI Performance Exhaust and Headers (Figure 2 and 3). They are full-length headers available for 1962 – 1967 B and 1965 – 1967 C bodies. They are quality pieces but are limited in fitment outside these year and body ranges.

Poly 318 A block Headers
Figure 2: TTI Poly 318 A-block Header (photo TTI)
Poly 318 A block Headers
Figure 3: TTI Poly 318 A-block Headers (photo TTI)
Options Requiring Modification

Luckily for those with fabrication skills or willing to pay a welder, some non-A-block exhaust ports align closely with the A-block and can be made to work by cutting off the flange and welding on an A-block flange. Flanges can be cut out of 3/8″ mild steel plate using the A-block gasket as a template or purchased through Hells Gate Hot Rods. Another option that works on paper but often presents fitment issues is an adapter plate that bolt to the head that are then drilled and tapped for the non-A-block header bolts; while the idea sounds good and will work in some configurations with less restrictions such as trucks, the additional spacing out of the adapter system presents clearance issues in many cars since the adapters push out the headers down and toward the fenderwells, steering components, frame rails, etc. In the end, it will take researching header photos, scrutinizing dimensions, and eventually physically trying to fit the headers. If all else fails, build-your-own header kits are available.For those wanting to do a little fabrication, prime donor headers include

Ford 289, 302, and 351 Windsor and 351 Cleveland (Figures 4 – 6)

The best bet for A-block headers is the Ford 289, 302, 351W, and 351C since tubes are dead on the A-block 4-1/2″ center to center port configuration, are rectangular ports, have a wide range of configurations available for many Ford vehicles, come in 1-1/2″ – 1-7/8″ primary tubes, and some can be inexpensive. However, from my research 289 – 351 Windsor headers are limited to 1-1/2″ and 1-5/8″ primary tubes below the $800 range. 351 Cleveland headers have very limited options at 1-3/4″ and 1-7/8″ for under $300. For example, when I began researching and purchasing headers to trial fit in my 1956 Dodge coupe build, I quickly found a lack of 1-3/4″ headers for the Windsor family and settled on a pair of Flowtech 12118 full-length headers made for a 1970’s Gran Torino 351C (Figure 4). The right header fit nearly perfectly, but the left header ended up having serious clearnace issues with the steering and stater motor that modification wouldn’t overcome. I moved on to a pair of Flowtech 12164 mid-length headers (Figure 5).

Poly 318 A block Ford Headers
Figure 4: Ford Small Block Headers (photo Flowtech 12118, 1-3/4″)
Figure 5: Ford Small Block Headers (photo Flowtech 12164, 1-3/4″)
Figure 6: Ford Small Block Headers (photo Hedman 886600, 1-5/8″)

Chevrolet 1997 onward LS (Figures 7 – 8)

Chevy LS (Gen III and IV small block) headers would be my second choice since they have some variety, although not as much as the Ford headers and often more expensive. A benefit of the LS headers is that they make them for LS swaps into older GM vehicles, so they are available for setups with tight clearances. A downside is that they are round ports (the A-block has rectangular), and they are limited in their 1-3/4″ and 1-7/8″ primary tubing for far more options at 1-1/2″ and 1-5/8″. For example, the Hedman 69050 (Figure 7) would likely be a perfect fit for my 1956 Dodge build, but they only make them in 1-5/8″ which will kill power for my particular A390 stroker.

Figure 7: Chevrolet LS Headers (photo Hedman 69050, 1-5/8″)
Figure 8: Chevrolet LS Headers (photo Hedman 69040, 1-5/8″)

Gen I Hemi 331, 354, 392 (Figure 9)

Lastly, Gen I hemi headers can work, but most are block-huggers and lake-style and tend to me more expensive than comparable Ford and Chevy versions.

Poly 318 A-block Hemi Headers
Figure 9: Gen I Hemi 331, 354, 392 Headers (photo Patriot Exhaust H8210)

Vintage Headers

As a historical note, Spitfire Headers made A-block headers (Figure 9), and sets come up for sale once in a very long while. However, the semi-“Y” design stuffs cylinder 1 into cylinder 3 and 2 into 4 with a sharp 90 degree turn to deal with clearances in cars, killing potential performance. The headers are a lot better than factory logs and a little better than factory center dumps. Considering their limitation and how rarely they come up for sale, I would rather focus on adapting a set of non-A-block headers or going with TTI.

Poly 318 A block Spitfire Headers
Figure 9: Spitfire Poly 318 A-block Headers (photo courtesy Richard G.)