based on early 1960's Mopar Logo

Building a Polyspherical (Poly or Polyhead)
Poly 318 decal  318 Motor

The Mopar polyspherical motor was Chrysler's attempt to retain the "heavy breathing" benefits of the Hemi head design while reducing the cost of producing the Hemi engine. Poly A 318 with 3 two barrel induction
Poly 318 valve cover decals Poly 318 air cleaner decals
Source: Art by Sky, Schuyler Wrobel.

The resulting motor, named the "A" motor by Chrysler, used polyspherical combustion chambers and was called the "poly motor" or "semi-Hemi" by Mopar fans.

For some reason these motors never really had a lot of speed equipment made for them or really caught on in the racing world.

But tens of thousands of these motors made their way into 1962 to 1965 Mopars. The 318 version is a great reliable engine, and many 1962 to 1965 Mopars are still propelled by a Poly 318.

Fast Forward

DaimlerChrysler has released a new motor to replace the famous "LA" motors that were introduced in 1964 as the 273.

This new (as of 1999) V-8 Mopar motor is a 4.7-liter V-8 called the Power Tech. (4.7 = 287 cubic inches)

"The heads feature 'fast-burn' combustion chambers that are 'almost' hemispheric. When viewed from below, the chamber has an Omega shape, but from the side it resembles an OHC Hemi. It even has laterally opposed valves, just like a true Hemi." (Source:

1999 Jeep Grand Cherokee 4.7L V-8 from DaimlerChrysler Press Release Poly A 318
The 318 "A" Motor we call the Polyhead.

Torqueing a poly head from a FSM

Old is new again?

Below are pictures of the head design used in the new 4.7 motor. Look familiar?

4.7 valves - Source: Mopar Action, April, 1999, page 67 4.7 valves - Source: Mopar Action, April, 1999, page 67

The premise, then, is using the 318 Poly as a base motor, is it possible to make the old Poly design into a torque monster? The notion is that, using headers, a turned down 360 crank, 11:1 pistons, and good carburetor, the poly will make 500+ HP.

Lucky for us one of the members of the 1962 to 1965 Mopar Mail List Clubhouse is doing just that. He has graciously agreed to share his project with everyone!

Gary Pavlovich writes:

There are several ways to go with the Poly and I am experimenting in building one now. You can turn down a 360 crank and nitrite it for surface strength, bore the block .090 (they take it easily) and sonic test for peace of mind, which will yeild a 360cid Poly. Or, order a 4 inch stroker crank (new Mopar Performance offering just out) and bore the block .090 which will yeild about 402cid of Poly. I am going the 402 inch route with 2.02/1.60 stainless valves (Ferria or SI), 10:1 JE or CP pistons, Scheider 288/.480 cam, custom headers by Spitfire Headers...

Initially, I was going to stroke it with the 360 crank along with the .090 overbore to yeild a 360 Poly which is documented by Bill Richardson of RAM Racing Engines, Lynchburg VA (an avid & knowledgeable 318 Poly builder) to produce from 350-500+ HP depending on your intent and budget.

However, after hearing of Mopar Performance's new 318/340 4 inch cast stroker crank (they make a 360 stroker crank also) I got greedy for cubes and thought this combo would be advantageous rather than the 360 because of the 3700lb.+ weight of my 1956 Plymouth 4dr.(my high school "first car" from 1968). I could use the additional torque from the 402 inch big stroker.

The new project will use a 1966 Charger rear end with 489 case Sure Grip, starting out w/mild 3:23 gears.

The Poly project would take a 360 size piston because of the 4.0 bore, however the valve reliefs are entirely different on a poly piston. I will have pistons made. After consulting with people more informed than I, a strong case was made for the 3.58 stroke 360 crank: "Years of research indicate the optimum 'bore to stroke' ratio is 90%, which means that with a 4 inch bore, the stroke should be 3.6 inches. The 3.58 inch stroke of the 360 is so close you could spit. In fact a .020 inch offset grind will bring it right on the money." (Don Dulmage).

Sound advice. This science is what Bill Richardson of RAM Racing Engines employs. I have chosen to pioneer the 'gruesome torque' aspect of using the Mopar Performance 4 inch stroker crank for my application."

My desire is that people can learn from my successes and mistakes to help them in building the Poly motor that suits their wants and needs. One thing is for certain, building a Poly motor is a cool way to go, with horsepower to boot.

I'll issue updates as my project proceeds.

UPDATE -- May, 2000

Here is a re-cap. I'll continue to add the rest of the details, parts sources and prices as they happen and editorialize where I erred and paid too much ...or could have used an optional part.

Thats it for now.

UPDATE -- July 6, 2000

I am still waiting on the headers, cam, and ARP head studs. Ordered the Rollmaster timing chain, Clevite bearing for the stroker crank today from Mancini. Presently working on the '66 Charger rear end to swap in place of the tapered axle unit. I have several cam choices; Schneider 284/288 .480 inch (224/230 @ .050), Racer Brown 288/288 (230 @ .050) .480 inch, Neilson 290/290 .492 inch (234 @ .050) all hydraulic. Can't seem to decide between em. All are regrinds except for the Chris Neilson cam. What would you go with?

Well, this project is certainly dragging on; having to wait for suppliers before I can proceed tests the patience.

Update June 2001

Gary P. writes:

I have written a two-part article on Poly motors for MOPAR MUSCLE Magazine. The first piece should appear in the Sept. 2001 issue: Poly ID, induction science, and interchangeable parts with the LA motors. The second piece is scheduled for the Oct. 2001 issue and outlines all the data on building a 402 inch Poly stroker motor; Poly head flow data (stock, mild port and polish, full port and polish), sonic test, dyno tests and all the necessary part numbers and sources for performance hardware.

I won't "spill the beans" on the details here, but I think the results were worthy of the undertaking and would encourage anyone to consider (or reconsider) building a Poly!

After the articles appear, I will share all the information I have gleaned to those who are interested. Meanwhile, I will help where I can.

EDITOR'S NOTE: The articles did appear in that magazine, to a very favorable response! Watch here for a review of that material in the future. The chart below is the first:

Stroker Poly Parts List
Description Part Number
4 inch Mopar Performance stroker crank for LA318/340 #P5007256
ARP rod bolts #ARP 1446001
ARP head studs #AR8.OL
ARP main studs #AR4.06L
Clevite hardened rod bearings #CB481H
Childs and Albert Durs Moly rings #934335
Clevite full groove main bearings #MS1344P
Melling hivolume oil pump #M72HV
Milodon small block windage tray #32230
Rollmaster billet timing set #MRE1262 Mancini Racing
BHJ street dampner #BHJMPIB816
Crower valve springs #683051
Crower keepers #861071
Crower bushings #90927
Manley retainers #2364516
Sealed Power valve stem seals #ST2012 rubber PC type
Nylen guides #6850
FelPro full gasket set for Poly 318 #FS 7734PT2
Flow Kooler hiperf. water pump #1701 Summit Racing
Weiand (new) alum. singlefour Stealth intake #7503 Summit Racing
Carter 850cfin Thermoquad carburetor N/A
Additional information:
Ferrea custom made stainless steel 2.02/1.60 valves, polished w/three angle valve job;
unleaded seats for the heads.
Rocker Arm Specialists blueprinted the rocker arm assembly; hard chromed shafts w/brass bushings,
trued all rockers to 1.5 ratio w/lockdown nuts.
New Chris Nielson Cam 290/.492 (234*.050) on 110 lobe center, high performance hyd. lifters,
moly pushrods from Chris Nielson Cams.
JE custom made forged aluminum pistons, data on file at JE, $595 thru a WD outlet.
Used Chevy lightweight .927 wrist pin; pistons weighed 473 grams each, rotary weight 1108 grams.
Stroker crank was polished and internally balanced.
ARP fasteners 18008263045

September 2009 update: I am currently working choosing a custom roller cam profile (or maybe saving the cash and using the Hydraulic flat tappet) and am engaged in bringing the roller rockers to fruition; the holdup on the rockers is our supplier/designer. I do have access to a trick/ulitimate custom roller rocker assembly but the cost is frightening.

Poly Engine Manifold Options


The Weiand intake is a single plane design which reduces torque and throttle response in stock or near stock motors while operating under 3500rpm.

The stock cast iron intake is a dual plane design which dramatically improves torque and throttle response in stock motors; be it a two barrel or four barrel manifold.

Stock motors do not operate much above 5000rpm and need the dual plane design for street driveability. However, the stock cast iron four barrel intake will be a good strip performer for the average street/strip Poly as well.

The Weiand single plane will need a larger cam, headers, and low rear end gears to restore and improve the torque lost over a dual plane. It serves the four barrel purpose when one does not have the OEM dual plane four barrel intake.

Why do they call it a high rise? A misnomer as I survey the intake runner height and design; probably called it a “high rise” because they raised the carb mounting pad.

I used the Weiand single four manifold on my first dyno run w/ the Poly stroker motor; easy 405HP (low ball number); but I had all the other “stuff” to make the manifold work.

Are there a better manifolds? Yes.

The second dyno tests will compare/contrast the Weiand single four with the Weiand dual quad.

The Weiand alum. dual quad WPD4D is also a single plane and the same design as the factory dual quad but it dramatically increases the HP over the single four because of the improved fuel distribution and multiple carbs (more cfm); the venturis are very close to feeding each intake port directly. However, the dual quad will require a larger cam (264/.450 and up), w/headers, and low rear end gears to reach optimum performance and will outperform the OEM dual plane single four barrel. The Weiand alum. dual quad is the ultimate performance piece at this time for the Poly motor. It will serve street duty fine and be a killer at the track.

Additionally, there is the Weiand three deuce intake WPD3D; same single plane design but with square carb mounting pad for later model four bolt flange carbs.

Lastly, the Edelbrock P600 three deuce intake is an excellent manifold because it is a very nice dual plane multiple carb manifold. It will outperform the single four if set up properly and retain the torque and throttle response most street cruisers are looking for with the WOW factor of multiple carbs. Most P600’s are built for the old style three-bolt Stromberg and Holley 97 carbs which limits the cfm and raises the cost of induction.

However, there are a few of the later production square flange four-hole Edelbrock P600 manifolds around; these will accept the later style two-barrel carbs (Rochester and even the Holley six-packs).

Bottom line, use the OEM dual plane four barrel when available for a single carb application. The Weiand single plane four barrel will suffice when a larger cam, headers, and lower rear end gears (3.55, 3.73 and preferably 3.91's) are used. Optimum performance with higher HP potential is achieved with the OEM cast iron dual quad or the Weiand alum. dual quad WPD4D which is king (in my opinion) of the performance intakes made for this motor.

The Edelbrock P600 three deuce (dual plane) is a great performer but like any multiple carb setup, you pay more for the carbs but when they are set up properly, it is a wild ride w/eye appeal.

Gary Pavlovich

December 2009: Gary Pavlovich writes: People ask me “What is a Poly Motor?” and I reply:

The 318A “Poly” engine that many refer to is nicknamed a “Poly” because the head design is a Polyspherical combustion chamber with canted valves (angled intake and offset from the exhaust -- not in the same plane and side by side as the Wedge chamber) as opposed to a HEMIspherical chamber.

The Plymouth Poly was produced from 1956-66 as a 277, 301, 303, 313, 318, and 326 (1959 Dodge only) and shares many of the same parts as the later 273 / 318 / 340 / 360LA Wedge-head engines. Sometimes the aforementioned Wedge-head engines are mistakenly called “A” engines, which can be confusing. NOTE: The 1956 (277, 303 Poly) to 1961 Poly 318 (+301,313,326 Poly engines) have a different bellhousing bolt pattern/oil pump/deeper oil pan/slightly different timing case cover/extra water jackets in the head and block/different starter and starter location.than the later 318 Poly engines.

Interestingly, the modern 4.7 Chrysler engines are a Polyspherical chamber design and not a wedge design; even the new HEMI’s are not true 90degree Hemispherical valve layout.

The BMEP, brake-mean-effective-pressure of a Poly engine is far superior to the Wedge chamber and slightly behind the HEMI: ‘...the poly engine had one absolutely unique feature: its volumetric efficiency. If the Mopar figures as found in the Plymouth factory manual are to be believed, torque of the poly 318 was 345 lb.ft. Now, take torque and divide it by cubes, then multiply this by 151, and you get brake mean effective pressure (bmep). Try it for yourself: bmep for the poly 318 is 164 psi. (Again, that’s the figure Plymouth gives). That is near to the theoretical limit for an unsupercharged engine (and that’s with the old log-type exhaust manifolds too) and is very close to the 426 Hemi’s bmep of 173psi, which had all those performance-designed components. The bmep figure purely reflects breathing efficiency (proportional use of the charge coming into the engine), and is a product of the head, as opposed to the block, which just needs to suck/blow as rapidly as possible without flying into little pieces....’ [Source of the quote is Bob O. from the Forward Look Discussion Section, a February 21, 2003 post titled Well, since you ask........]

Bob Asay was my initial inspiration. After perusing his Polyspherical Engine Association Web Site [Editor’s note: Offline as of November 2009] I was incented to build my motor. [Note: 2011: back online Polyspherical Engine Association

Don Dulmage gets credit for encouraging me to procede and with patience in bearing with copious and circuitous e-mails soliciting his advice and expertice.

A lot of parts interchange between the polyspherical A 318 and the LA series of motors (318, 273, 360, 340).

Two Articles by Gary Pavlovich:
How To Give A 318 Poly Engine More Power - Mopar's Rodney Dangerfield
The Mighty 318 Poly Gets The Last Laugh!

Gary Pavlovich’s Tunnel Ram Poly 318 Prototype:

318 poly tunnel ram 318 poly tunnel ram 318 poly tunnel ram
318 poly tunnel ram 318 poly tunnel ram 318 poly tunnel ram

Poly Dyno Test April 2011

I had a nice engine break-in session on the dyno the other day with one of my 390 cubic inch Poly Stroker builds. We made 412HP@5600rpm and 438ft.lbs.torque at 3700rpm. As is common with the Poly 318 engine the HP and Torque curve was very high and flat. We averaged 428ft.lbs. of torque from 3400rpm to that will set you back in the seat and keep you there; The average horsepower from 3700-5700 rpm was 375. Not bad for a “throw-away” motor.

Specs: Cast iron dual quad w/500cfm Edelbrocks, 9.5compression, Hyd. Poly Cam with 290/.482 inch Advertised and 23-@.050 inch , 1.94/1.60 inch 11/32 inch Stainless Valves, a modest head flow of 208/151cfm @.500 inch lift, 16deg. initial adv. 34 total...should make a few more HP and Torque once the rings get used to their new home.

Polys in the wild

Building a Poly 318 Stroker Motor
by Gary Pavlovich

If you are looking for this in your Mopar poly build: pump gas engine with enough horsepower and torque to smoke the tires at will but not unmanageable street manners, that is the Poly engine must start easily, not require extensive tuning to maintain driving pleasure, not require more than a 3500rpm stall convertor and a 3.91 rear, read on. Your 1962 to 1965 Mopar will be a nice looking, hard running, sleeper car, with that nostalgic 1962 - 1965 Mopar look and Hot Rod stance, (perhaps with aftermarket rims and 235/.60-14 or 15 Drag Radials; Hoosier/BFG/MT); that is, enough mods to stay within your budget without compromising your “performance.” You will need to assess your true street to strip driving ratio as well.

A lot of people with a Mopar with a Poly 318 in it think about putting a later year 360 Mopar engine in their 1962 to 1965 Mopar. But I would choose the 318 Poly in Hot Street modified form or a 360 Stroker Poly.


In my view people do not need more than approximately 350-375 REAL horsepower to reach a nice street B body Mopar

HOT Stock Stroke Mopar Poly

360 Stroker Mopar Poly

I suggest writing down your planned buid with the cost of parts and machine work next to the “plan.” Be honest with your goals/usage and do not let any claim of rear wheel HP and Torque cause you to stray from building a “fun” package. Like I said, 350+ REAL HP will give you traction issues that you can easily/inexpensively harness; any more than that will require more money for suspension mods.

Field Report from FuryUs63, the owner of a 1963 Plymouth with the 360 Poly stroker option installed:
“The 318 Poly is the first small block engine I’ve owned. I’ve never had a LA wedge. All my B/RB wedge engines were unique in some way. This stroker motor revs up like a SBC [small block Chevy], you have to watch the tach close. I now see what my engine builder was planning. A 4 inch bore / 3.60 inch stroke being the best rotating combination. He did it with a 3.58 inch stroke and the blueprinting did the rest.

The Poly stroker engine comes alive when you start it, step on the gas and you’re at 2400, push the pedal down and that’s were the stroker Poly is different. My other engines you hear carbs open up and that big pull of power going through the gears. On the Poly you do not hear the carb open up, it is just open, you don’t feel the big pull of power, it feels like it’s at full power and the car has to catch up to the engine. You’re so busy in the cockpit watching the road looking at the tach and pushing the next button and then the afterburner kicks in. The sound has a little to do with clouding the thinking. Parked next to my Max Wedge all you could hear was the Poly.

All kidding aside, the torque converter, rear gear, matching the cam’s profile allows this engine’s torque curve to move 3600 lbs at any RPM range. Horsepower is for bragging rights, a good flat torque line through the RPM range.”—Nick T., FuryUs63

Poly 318 stroker Poly 318 stroker

Poly 318 stroker Poly 318 stroker

Above: Pro-Street and Race Ready 360 Poly Stroker: a hydraulic roller cam and professional tunnel-rammed engine where the exterior design and carefully-chosen components mirrors the engineering “inside” the motor of Barrett Lash. (August 2011)

Poly 318 head bolts measurementsMopar Poly 318 head bolt measurements (Thanks Rich and Roger!)

Inspired by the Poly 318? Check out Michaels’s Poly Hop Up 1965 Belvedere!

Also see Michaels’s What Is A Mopar 318 Polysphere And How Come Nobody Likes It? and FAST EZ-EFI Dual Quad Installation on Michael’s 1965 Plymouth Belvedere

Poly Head Engine Resources Page by Mark Polk who is rebiliding a poly 318 in his 1971 Dodge W100 Sweptline Power Wagon project.

Vendors who speak Poly 318 Mopar Engines:

Gary Pavlovich, located in Lakeside/Santee, CA 92071 helps members of the 1962 to 1965 Mopar Mail List with technical data and parts source information to help in their Poly 318 or Poly Stroker build. Contact Gary Pavlovich at his email: Contact 318 Poly Gary  Put '62-'65 Mopar in the subject line.

Craig Fountain in Australia sells aftermarket parts for polyspherical engines, for example finned aluminum valve covers for the 318 and 354 Chrysler Poly, four different styles of air cleaners that fit the 97-94 carburetors used on a polyspherical tri-power, single or two four-barrel carb air cleaners, multi carb finned fuel logs, as well as various other hot rod components.-- is the contact email.

Bill Richardson at Ram Racing Engines in VA. has Poly stroker kits.
Chrisman’s C.A.R.S. in Santa Anna, CA.
Eckman Machine in Rexburg, ID.
Junior Thompson in Norwalk, CA.
Performance Carz in Australia
The Supercharger Store helped create a Procharged 318 Poly Small Block Mopar Supercharged in New Zealand.
Procharged 318 Poly 1 |  Procharged 318 Poly 2 |  Procharged 318 Poly 3 |  Procharged 318 Poly 4 |  Procharged 318 Poly 5
Chrysler Power webstore New Poly parts (September 2014)

Toth Performance High performance engine building for street or race, Cylinder head porting;[use link below] all work done on a "Super Flow" bench. Toth Performance now [2018] offers a Mopar Poly engine service building stroker poly engines! Just needed are your block and heads. Full service poly head porting is also available. Prices vary due to your power levels desired. Call Bruce at Toth Performance to discuss and get a quote, seven two four 290 | 2497
Mopar Polyspherical (Poly or Polyhead) 318 Motor Head Work by Toth Performance (May 2016)

Poly and 50's Hemis Discussion Group

318 Polyspherical Article from the B.B.C.

Mopar Polyspheres: Often Overshadowed By Their Hemi Brethren, They Might Be The New Cool By Jays Torer, an article about Mopar Poly engines on Street Rodder Magazine Web Site

318A Poly Pictures

Poly Gas Pump

318 poly gas pump Poly brand gasoline pump,
with the gas price set at 29 cents per gallon!
(At the Hemi Hideout)
Poly gas pump

April 4, 2000 Page Version 1. Revised April 14, 2000; May 13, 2000; July 6, 2000; May 3, 2001; June 5, 2001; January 16, 2002; January 26, 2002; March 15, 2003; July 28, 2004; July 28, 2005; November 23, 2005; May 10, 2006; January 16, 2007; November 4, 2007; January 23, 2008; September 30, 2009; October 20, 2009; December 18, 2009; January 30, 2010; February 14, 2010, April 24, 2011; July 20, 2011; August 10, 2011; January 19, 2012; September 30, 2012; October 15, 2012; December 9, 2013; January 27, 2014; February 14, 2014; March 27, 2014; September 2014; January 15, 2016; May 24, 2016; September 7, 2019

1962 to 1965 Mopar Home Page Return to Tech Tips Page