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Lab Rat

Updated: Nov 10, 2020

     This 1973 Dodge Dart Swinger soon earned its nickname of Lab Rat after we began our R&D endeavors into the world of everything slant six. The Chrysler 225CI Slant Six is our R&D weapon of choice for Lab Rat.

     Efforts started with the original engine, a Holley 500, a modified two barrel intake, an A904 transmission, 120,000 miles, and about 15 years of sedentary gunk. The entire trunk pan of Lab Rat was rotted out, so we immediately set to gutting the trunk pan and rebuilding the frame.

     After a new frame was built and a 15 gallon fuel cell was mounted, we took a break in our project to enjoy the work.

     The Holley 500 was always too big for the stock slant, but it worked wonders compared to the original single barrel. The 500 ran rich, netting a dismal 15-17 MPG. A rejet of the 500 produced much better cruise economy, but the power dropped back at peak. The 500 showed us the need for proper fuel control at idle, cruise, and WOT, but with the fuel runner design of the slant it always forced you to choose one side of the triangle. We knew there was a better way, and so began the imagination of our Aluminum EFI MPFI. 

     From the start of the project we wanted an injector per port. Fuel distribution could never be solved with a throttle body injected setup, and infinite tunability only comes from multi-point injection. We wanted the parts to be readily available so that DIYers could easily order parts that they needed. We bought a used 1995 Jeep 4.0L intake, fuel rail, throttle body, and fuel injectors for assessment. We began to study the runner spacing and injector spacing; this soon showed us that the intake could not be adapted and the fuel rail could not be used. Further efforts and study resulted in the realization that we needed a test engine for mock-up. After acquiring an engine for mock-up, we began the lengthy process of drawing plans for a custom MPFI manifold.

     We started with flange designs matching original thickness and port matching to the gasket. Everything was placed as precisely as a dry unwelded run could be. Next we worked on runners, positioning them onto their intake flanges & coping them so the log would be cradled, creating a ramming venturi effect.

     The final result was the Mark-I intake manifold. Its length and width was designed to fit in stock applications for all early slant six cars and trucks. The Mark I supports a stock Jeep 4.0L throttle body, EV1 style injectors, a tapped vacuum port, and a 10 Lb weight reduction over the factory cast iron.  

     Mounting the Mk-I intake manifold was a breeze, with no modifications required. Our exhaust manifold intake heater was already capped off with a welded plate. We used a Lokar cable with a custom bracket for throttle hook up, Jeep 4.0L fuel injectors 19 lb-hr (rated at 23-24 Lb-hr with fuel PSI increase), a custom fuel block off (sold in our store), a MagnaFuel regulator (set at 55 PSI), a Ford EDIS-6, a Megasquirt II, a 14Point7 Spartan Wideband O2, an Electric Fan, and a Walbro fuel pump. 

     The difference the Mk-I made was substantial; power was smooth across the whole RPM range and was available on demand. Free revving of the engine was vastly different due to full control of spark timing and fuel AFR, but the removal of the stock mechanical fan perhaps helped the most in this area. Throttle response on the road was absolutely fantastic, but the age of engine and how long it had sat soon caught up to it. 

     Lab Rat's original engine lasted a few months before sludge halted the oil to the rocker arm shaft and also blocked oil to a rod bearing. The project was at an impasse until another engine was acquired or the current one was rebuilt.

     Enter the 1985 Dodge D-150 Slant Six 4-Speed.

     The truck had as 180,000+ miles & consumed 1 Quart every 40 miles due to worn rings.

The truck had a constant misfire due to oil mist and a burnt valve. Nonetheless we still obtained valuable data off the trucks test runs. The typical test run with rolling hills produced 21.5-23 MPG average and as high as 26 MPG on flat valley floors. Below is a 50+-  mile test run to show its abilities.

Ambient Temperature was 81 F.          

                      Max         Min         Avg

IAT                 183         99.4       116.4

Coolant         N/A         182.9      194.4

Fuel Flow      7              -0.7        1.767

RPM               3,259      950       2,086

MAP               97.9         14.3       58.61

Ignition Adv   34            5           28.84