Building Mopar Power
in a 318 Polyhead Motor
Note: a page update for this page is delayed.
| 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. |
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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: http://www.car-truck.com/chryed/buzz/b121699.htm)
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The 318 "A" Motor we call the Polyhead. |
Old is new again?
Below are pictures of the head design used in the new 4.7 motor. Look familiar?
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 66 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.
- Harold at Spitfire will be sending the headers in about two weeks.
- I am currently researching ARP head bolts for the 318 Poly. Bob Mazzolini Racing (Riverside, CA) is doing the leg work on the ARP bolts and much of the other parts needed:
- JE 10:1 forged pistons and rings (prob. around $545 for the custom pistons)
- the superior design Milodon Windage Tray w/ARP studs ($79)... I will be studding the bottom end for added strength.
- a Rollmaster Billet Timing set assure accuracy and longevity.
- Cooling will be handled by a four core radiator and a trick "Flow Kooler" water pump (reputedly increases coolant flow up to 30%). This pump fits the pre-70 small blocks, including the Poly 318, with the correct water outlet on the driver side to match our early Mopars.
(It is not listed in any of the Summit or other popular catalogs but is available through them if you give them the correct
#1701 Flow Kooler part number. The cost is about $99 retail or $89 jobber cost, but worth it when compared to the cost of a new inefficient OEM unit).
- I will be running the Mopar Electronic Dist. kit with the chrome box. (One can save some bucks by purchasing a junkyard electronic dist. (about $10) in good cond. and recurving it. Add the Mopar Performance wire kit (P 3690152, $21), and a good used Orange or Chrome
box (swap meet price $10) and save a total of about $150 to apply to more costly stuff like custom valves and pistons.
- Also working on an 11" 2300 stall convertor for the pushbutton Torqueflite trans. I have a great source for Torrington bearing high quality convertors here in San Diego in any size or stall that is superior to most anything available for the street or strip at a very reasonable price.
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 re-consider) 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" 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 |
|
Poly Engine Manifold Options
Polyheads,
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
Bob Asay was my initial inspiration. After perusing his Polyspherical Engine Association Web Site I was incented to build my motor.
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).
Vendors who "speak" poly 318:
Gary Pavlovich -- glpavlovich@cox.net -- builds Poly strokers (See estimate!) and Poly 318 motors. Located in Lakeside/Santee, CA 92071
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
Poly and 50's Hemis Discussion Group
318 Polyspherical Article from the B.B.C.
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
