Mopar taxis, 1935-2003

The reliable 3.5 V6

1997 Plymouth Pronto
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Plymouth Volare Duster / Slant Six Modifications and Performance Upgrades

Rear End Swapping

The 1977 rear axles' spring perches are offset towards the driver's side, so if you want to use them on later models, you will have to grind off some of the rear of the brake plate and possibly some of the spring/shock pad to avoid hitting the spring.

Slant Six Power

I started modifying the slant six because 90 hp was not enough. First I added a four barrel carburetor and headers, but then I went on to camp, comp, and head work. This still did not provide the power I wanted, so I returned to bore the engine more and use a more aggressive cam.

 The first build up consisted of new bearings and rings, and shaved .100". The head was given a full port street/strip polish, 1.88" intake and 1.5" exhaust. Also shaved off .060". I used a Competition Cam 425" L 252 Dur. 110 C-line along with my Clifford 4-barrel intake and split headers with dual exhaust. I added a Mopar Performance Distributor and Orange ECU and Accel BIG YELLOW coil. I have had this motor up to 6200 rpm many times with NO problems.

This was a really good street set up; power, gas mileage, and good sound. I also used Holley red fuel pump and 1/2 line all the way and a K+N velocity stack. I ran a mix of 6 gallons of 100 octane aviation fuel with 12 gallons of 92 octane pump gas. I used 100+ octane booster when aviation fuel wasn't available.

Second Build Up

  • Bored the Block .060" and a Clifford .498"/.530" lift 280/290 duration
  • 108 C-line. Ignition this time will be Jacobs Street/Strip.
  • Added side pipes, hood scoop.
  • Other than this the motor has NEVER been balanced, and retains the stock vibration damper. It has not yet been driven and is still being dynod.

I added the cam and bore because my head flows best at .500"L, so my .425"L cam was way to small for the head's capability. I figured that since the head was to be pulled I should bore it as well, and paint it red instead of blue. Getting the right cam to start with would have saved lots of cash.

Dyno Testing

You can get a slant 6 on any dyno! All you need is a manual bell housing, clutch plate, and a place that's nice enough to actually want to dyno the motor. Dynoing ranges in price, I paid about $50 an hour. (which usually includes hooking the motor up). I spent a total of about 17 hours on the dyno (3 of which the dyno operater gave me free) and we got the motor to make 240hp and 270lbs. ft. torque, but that wasn't even near max, it was just a smooth run. I got tired of the dyno because I was out of money and the motor didn't seem to get very far in the 17 hours it was on. I would suggest on something like this is to try and tune it the best you can in the car first, then bring it to a dyno and optimize it.

1. Using only the primaries of an 850 Double Pumper and a Clifford 4-barrel intake with 28 degrees BTC and part throttle: 236 hp at 5250 rpm, 258 lb-ft at 4250 rpm. The low timing was used because the first time the motor was dynod it bumped the bearings on two cylinders.

2. We tried a 750 Dbl. pumper (primaries only) and it made 20 hp less. Also we tried a Offenhauser intake with the 850 and it made 30 hp less due to the smaller runners and plenum area.

The grooves on the intake may hurt fuel distribution, so 3 and 4 cylinders get lean while 1 an 6 get rich. I was told by some slant six racers to grind out the grooves and play around with the jetting, but I was also told to use a 600 double pumper; all in all, something isn't right, the motor falls on its face when the secondaries open on all three carbs. I suspect the fuel distribution rather than overcarburetion.

 3. This time we ground out the grooves in the intake and tried a 750 vac. sec. Holley. It revved up smoother, so then we tried my 600 vac. sec. and misc. jets and it spittered. Next we tried a 750 double pumper and that didn't work either. so after 4 times on the dyno this is what we got with 92 octane gas, 23 degrees BTDC, and a 2 " spacer. The result: 240 hp @ 5000 rpm and 270 lb-ft @ 4000rpm.

It didn't want any more timing for some reason and a 2" spacer made 10 hp more than a 1" spacer.


I think I may have made a mistake about the rear end swapping!! I finally got enough money saved to start working on my Duster again and I took out the 8.25 rear end (1977) and layed it side by side with my old 7.25" (1979) and measured every possible measurement and they are (within 1/8" or so) right on the money! I can't explain why the wheels were offset on the car, I measured the backspacing and everything on them also and they were right on. but I should no FOR SURE by middle of August what went wrong since the car should be on the street by then!

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Mopar taxis, 1935-2003 The reliable 3.5 V6 1997 Plymouth Pronto