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Turbocharged slant six: cheap power and gas mileage (Part I)

by Mike Holler

I did a turbo installation on a slant-six Duster about 5 years ago and took pictures. My goals were increased performance and mileage. Both goals carried equal importance. Mileage went from 18 to 23 mpg with the turbo. Here is the story.

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Allpar notes that turbochargers should be used with caution... they can easily lead to over-lean conditions that can destroy engines.

I'm sure that this will come as a surprise to many of you, but turbo charging your car can actually increase fuel economy. Many gear-heads have installed turbochargers on their hobby car/truck and noticed that fuel economy suffered. But it wasn't necessarily the turbocharger's fault. There's this Jeckle-and-Hyde phenomenon that occurs when humans discover new-found power. They use (abuse) it diligently. Chrysler, Saab, Toyota, Volvo, GM, Ford, Mazda, Nissan, Volkswagen, Porsche, Mercedes, Mitsubishi, and others offered turbo and non-turbo versions of the same car with the same engine. Driving conditions being equal, most turbo versions could get better mileage than the non-turbo versions. [Smokey Yunick built a Fiero and an Omni based on a draw-thru style turbo system and got around 250 HP and over 80 mpg!]

I have read repeatedly in other magazines that turbo charging is very difficult, troublesome, and most importantly, very expensive. One such article said that to turbo charge a late-model Chevy Lumina would cost around $20,000! There is a BMW M3 turbo kit out that starts at $22,000! (And yes there are expensive options!) The cheapest number I've seen in print ads is still over $2,000. This is not budget power and economy in my book. I will show you how I did it for under $300! Really!

"So how does a turbo work? How will it give me so much power while still delivering reasonable fuel economy? How big of a turbo do I need? How do I install one?" I'm glad you asked.

A turbocharger is a centrifugal compressor. All fans draw air from the low pressure side and blow it into the high pressure side. A supercharger is such a device. A household fan uses an electric motor to drive the blower, creating the pressure differential. A supercharger uses the engine's crankshaft and a belt to drive the blower. In both of these examples, there is a consumption-of-energy cost for the pressurized air. The fan uses electricity from the wall outlet, and the supercharger uses engine crankshaft horsepower.

Turbochargers are unique in that they utilize normally wasted heat energy to drive them. As the exhaust exits the cylinder head into the exhaust manifold, it is still burning and expanding, especially under load. With the turbo, we are "putting a turbine at the bottom of the waterfall," so to speak. The hot expanding exhaust gasses are used to drive a fan blade, which is connected to another fan blade at the other end of a common shaft. This other fan blade creates our positive pressure, called "boost."

What is our limiting factor when determining maximum boost? Well, you could just keep cranking up the boost until the engine blows up. Then go get another identical engine and keep the boost at the last best setting. Or we could rely on hundreds of thousands of dyno, racing, and street tuning hours that have shown safe levels on engines similar to yours. Generally speaking, if you have a compression ratio over 10.0:1 with an aluminum head or 9:1 with an iron head, you might not want to consider turbocharging. Your safe boost levels will leave you with very small power increases. But the lower the compression ratio, the more boost the engine can tolerate. Again, this is a generalization.

Other factors also contribute to how much boost your engine can handle like overall engine condition, do you have any stress cracks or manufacturing flaws in any of the high stress components (pistons, rings, crankshaft, connecting rods, etc.), the shape of the combustion chamber, the ignition timing curve, quality of gasoline used, overall drive train strength, final drive ratio, cylinder head and manifold restrictions, carbon build-up on pistons and valves, and so on and so forth.

The turbo will have the ability to produce enough boost (pressure) to completely grenade most engines. Because of this, precautions must be taken to keep the boost at safe levels. This is accomplished by the use of a waste gate. Most waste gates are connected to the exhaust side of the turbo. They are activated by a spring loaded diaphragm. When the boost reaches a preset level the pressure overcomes the spring tension and pushes on the diaphragm, opening the wastegate. This forces a 'door' open and allows exhaust gases to exit to the tail pipe, bypassing the turbo impeller. A Hobbs switch can be adjusted to different pressure settings. Use the Hobbs switch to activate a factory waste gate solenoid from a factory turbocharged car, and route your waste gate signal line through this solenoid. Want more boost, turn the screw on the Hobbs switch.

If fuel economy is your first priority when turbo charging, I recommend using a very low boost. Most non-intercooled production turbocharged vehicles use boost levels starting at 6 psi. For the best fuel economy, keep boost around 6 psi. I'd recommend starting at 4 psi and experimenting from there.

Now that you know how we'll get the extra power, let's look at the added economy factor. As I said earlier, we are harnessing normally wasted heat energy to drive our turbo. Put another way, we have found a way to use more of the energy in the gasoline we are already burning. Our engines have what is called parasitic losses. This is the power it takes to draw in the air/fuel mixture, compress the air/fuel mixture, force out the exhaust gasses, drive the camshaft and oil pump, etc. We are overcoming the losses from the intake stroke by pressurizing the intake charge. As you can see while driving at steady highway speeds, the boost/vacuum gauge reads about atmospheric pressure, or at least at a lower vacuum level than when non-turbo. This means the engine isn't working as hard to draw in the intake charge.

Of course there is a trade-off. If you drive the car like you did before the addition of the turbo, you will most likely see better mileage. If you thoroughly enjoy the newfound power of the turbo, you will definitely lose mileage.

I had a 1970 Duster that has gone better than 44 miles on a single gallon of gasoline through a 2-barrel carburetor, and with substantially increased power over stock. But this isn't the car we are going to turbo charge. For this special project I had to have a special car. So I went out and bought a . . . 1970 Duster. This one has not undergone any major modifications and remains relatively stock. The only upgrade is the addition of Chrysler Electronic Ignition from a '74 Duster.

As for sizing your turbo, most turbo experts will lead you to a turbo that is sized entirely too large for what I was trying to accomplish. If you drive a factory turbo equipped car, you will feel the turbo start to kick in around 2500 RPM. Full boost is felt around 3000 RPM. The problem is that I have never revved the Duster's engine over 3000 RPM. So if I sized the turbo according to OEM formulas, the turbo would get hardly any use. I need the boost where I drive; between 1200 and 2500 RPM. In order to get this I need a turbo that is "too small" by the expert's professional opinion.

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My rule of thumb for sizing turbochargers for power and economy is to get one (used?) from an engine sized for between 60% and 80% of your engine size. This means that for my 225 cubic inch slant six (3.7 liter) I will need a turbo sized for a 135 to 180 cubic inch engine (2.2 to 3.0 liter). Because the 225 has an extremely long stroke of 4.125", I went with a 2.2/2.5 1980s Chrysler application Garrett T03 turbo. With the extra long stroke, I can't rev my engine as high as the short stroke engines. On the other hand, an extremely short stroke engine may need to go with a turbo that is closer to rated size, as the short stroke will lend well to higher revs.

The first step is to find a place to mount a turbo on your car. This may be under the back side of the engine, between the engine and firewall, on top of the engine, or next to the engine. Several installations I've seen involved relocating the battery to the trunk and mounting the turbo in that general area. It is quite possible that there is no room for one on your car.

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Next find yourself an appropriately sized turbo. A possible alternative to one big turbo is to use two turbo chargers of half the size needed. A good example would be to use two 2.2 turbo chargers on a 350 (5.7 liter) or 360 (5.9 liter). A pair of 2.2s gives you 4.4 liters of breathing. This is about 75% of the 5.9 liter engine.

To find a turbo, hit some local salvage yards to see what they have and what their prices are like. Take some wrenches with you so you can check for wear and tear. Some things to look for are oil on the inside of the exhaust (the cast iron) side. This could indicate seal problems in the center cartridge. Remove what is needed so that you can spin the impeller shaft. It should spin freely. Check for side and end play. The shaft assembly should stay pretty much stationary. Any play would indicate worn bearings.

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If you find one that is showing some minimal wear, it can be rebuilt. Just make sure that the impellers have not hit the housing. If this occurs, the turbo is junk. Turbo City sells rebuild kits for just over $100 for many popular turbo chargers. For what it's worth, I paid under $40 for my turbo at the local Pick-N-Pull in good shape. I've seen them on eBay for similar prices.

When installing your turbo, make brackets to mount the turbo to the engine using a head bolt, accessory mounting bolt, motor mount bolt, or something else durable. You must mount this bracket to the engine, not the body of the vehicle. The engine is going to flex because of the motor mounts. The bracket needs to keep the engine and turbo connected together, not the vehicle and turbo. Don't rely on the exhaust head pipe to support the turbo. This is asking for trouble. Once suspended by the bracket(s), have a machine shop or metal fabrication place make up a 1/2″ thick flange that matches your turbo charger's inlet flange. You may be able to modify your exhaust manifold the way I did and bolt the turbo up direct.

Go to Part II! Confused? Something missing? See Slant Six Turbochargers Revisited!

Also see our 2.2 turbo page.

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