Lab Rat

Gill Welding
Nov 14, 2017
6 min read

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

Target AFR 16 12.7 14.93

Actual AFR 19.9 10.3 15.02

Average Fuel Economy in 4th Gear was 20 MPG at 65 MPH. Average Fuel Economy in 3rd Gear was 26 MPG at 55 MPH. Average Trip Fuel Economy was 23 MPG.

Coolant is in Fahrenheit. MAP is in kPa. TPS is in percent. IAT is in Fahrenheit.

Cold starts on the truck were flawless, but it was decided to pull the truck's engine in favor of installing it into Lab Rat.

After the engine and side winder style exhaust was installed in Lab Rat, we performed a few test runs and netted the same fuel economy outcomes as the truck. Lab Rat was producing average trips of 23 MPG and oil consumption of 1 Qt per 40 miles. At this time we received our mild compression head that was milled to produce 44 CC chambers. We decided to test what a simple head swap on such a worn out would do, but what we found were burnt valves that caused two dead cylinders.

With the new head on, we set out immediately to begin testing the engine. The sound was instantly different. The strength of the motor notably increased in all RPM ranges, and the low RPM miss was nowhere to be found.

The average fuel economy of lab rat has notably increased since the head swap. The better burn and revived cylinders has produced averages of 27.5 MPG. A 43-mile Interstate trip with east Tennessee hills produced 27.3 and 65 MPH Average. Lab Rat's differential gear is 2.76, and the tire size (after a wheel change) is 235/45/17. Valley floor (slow rolling hills) economy at 40-55 MPH averaged 36 MPG with peaks of 39.67 MPG held for over a minute.

On a 16.1-mile run, with a one-mile hill climb & descent, the following data was pulled.

Max Min Avg

RPM 4,339 432 1751

MAP 92.8 14.8 43.07

TPS 71 1.2 12.12

AFR 19.8 10.9 14.82

Target AFR 14.7 13.4 14.6

Coolant 198.7 127 182.8

IAT 129.6 79.4 104.7

Spark Adv 36 18 27.55

Fuel GPH 49.444 -5.729 8.176

Average MPH was 47.45.

Total Log Time was 1583.4s.

Coolant is in Fahrenheit. MAP is in kPa. TPS is in percent. IAT is in Fahrenheit.

Currently the Mk-II Intake manifold is for sale. The Mk-II refines many points in the design. The major changes are fuel injector angle and fully CNC produced parts.