The Novak Guide to

Powertrain Conversions


Fuel Economy

fuel_gaugeFuel economy has never been a trendy topic at Novak. It has always been an important part of our equations and suggestions for our customers Jeeps as well as our own.

There are some key misconceptions about fuel economy that run rampant that we will attempt to dispel, and then offer our best ideas and experiences about choosing a transmission and engine combination for your Jeep that will yield better economy and power than factory setups. In the end, it is very possible for customers to be able to choose a strategy that may rapidly pay for the conversion itself and you could end up with more power and higher miles-per-gallon for your Jeep than you might expect.

Fuel Economy Misconceptions

"A small (typically four-cylinder) engine will give better economy than a larger V6 or V8 engine."

This is not really the case. A four-cylinder engine is not often well suited for the weight, tasks and aerodynamics of many Jeeps. The requirements of these Jeeps often cause the four-cylinder engine to overwork itself, which is completely counter to the engine's efficiency. Because the four-cylinder may not have the torque and horsepower required, the driver will tend to run the engine in a higher RPM range, or to lug the motor at a lower RPM range with the throttle depressed beyond the point where the engine has the power to maintain speed. The stronger V6 and V8 engines have a greater economy advantage in that they have broader horsepower and torque bands, and are more likely to be tasked in the engine's more efficient operating range. Additionally, these engines carry reserve power that the smart driver can opt to use when they need it, and keep their foot out of the pedal when they do not.

There are two popular misunderstandings about engine size that should be brought to light. The first mistake many make is in thinking that a small displacement engine will invariably give better gas mileage. This is only true if the small engine is in a lightweight, properly geared, and semi-aerodynamic vehicle. A small engine in a heavy vehicle with "tall" gears will perform poorly and give bad gas mileage. Any engine, when worked to the point where vacuum drops low enough to operate the power jets in the carburetor, or to lug, will give poor gas mileage. If too small an engine is used for the work to be done, it will operate at low vacuum for longer periods and use more gas than a larger engine that would not be working as hard. The added benefit of the larger engine is its reserve power.

The second most common error swappers make is to convert to an engine that is too large, from both size and displacement, for the vehicle. While a Small Block V8 is a great engine, there are sometimes better choices for smaller Jeeps, such as Buick and Chevrolet V6's, Ford 2000cc and 2300cc's, GM "Iron Duke" Fours and the like. Big Block V8's and heavy old I6's should hardly ever be considered in short wheelbase Jeeps! When planning a conversion into a CJ2A up through the CJ5's, remember that you are dealing with a 2500lb. vehicle. This, by all standards, is light, and that is one reason why these vehicles prove to be the most agile in the world. Adding an overburdening block of iron to smaller Jeeps will give disappointing results in terms of handling, braking and of course, breaking – of several components directly and indirectly between the block and the vehicle. Besides, fit into the engine bay is usually so poor that the work soon looks as poorly as it was thought out in such situations.

The trick is to match engine size to the load, then only use the reserve power when needed. Engine torque output is essentially related to cubic inch displacement of any engine. The RPM that maximum torque is produced at is related to the length of the stroke of any engine. A 230 CID "under-square" engine will make about the same torque as a 230 CID "over-square" engine but will do so at lower RPM due to its longer stroke. (An under-square engine has a bore that's smaller than its stroke.) Many swappers and engine enthusiasts prefer the challenge of running an optimum V6 to the power levels of V8's, and then reaping the weight and fit benefits both on and off-road.

We get so many calls and complaints from customers running four-cylinders - that they don't have the power to overcome the wind resistance and mild highway climbs that Jeeps often see. The result of these motors is often poorer economy than other customers get from transplanted V6's and even some V8's.

"I need an overdrive or overdrive transmission to achieve better fuel economy / lower RPMs."

Not necessarily. Stay alert here and we will clarify this topic in a way that it is seldom considered. Gearing span is more important than whether or not a vehicle has overdrive. The definition of gearing span is the range of gears within a transmission from the first gear to the last. Many individuals knee-jerkingly think themselves into the paradigm that they must have a five-speed (often NV4500, etc.) if they are to get into an RPM range that is easier on their engines and wallets. The NV4500 and other five-speeds actually have a narrower gearing span than four of the popular four-speed transmissions we often work with here at Novak:

GM SM420 (span=7.0),
GM SM465 (span=6.55),
Ford NP435 (span=6.68) and
Ford T18 (span=6.32)

versus the

NV4500 (span=6.07) and the
NV3550 (span=4.23).

The smart installer will use these great four-speeds and their low, compound gears to their great advantage by choosing a tire size and axle ratio combination that will allow the Jeep's engine to spin in its optimum range of power and efficiency for cruising speeds, and then have an off-road ace up their sleeve with the super-low compound first gear when it is time to hit the hard trails. What's more, adaptations from these four-speed transmissions (mentioned above) are more affordable, and they are adaptable to nearly all engines of importance in the Jeep world. you will probably spend significantly less money choosing the smart transmission, tire size and ring & pinion ratio than you will going with an expensive and poorly calculated transmission. An additional and ancillary advantage of this method is that the taller (numerically lower) axle ratios offer a stronger ring & pinion combination for your axles. We cannot underscore this whole strategy enough.

One more issue on this wise: overdrive transmissions are not directly driven, but use a total of four helical gears on the input, cluster and main shafts to transmit power; helical gears trade quietness and smoothness for heat gain and power loss. However, a direct-drive transmission connects the input and mainshafts together, for a more efficient transfer of power. It is estimated that 2% of the engine's power can be lost through an overdriven transmission.

"I need a diesel to achieve good fuel economy."

Diesel engines are a great idea for Jeep swaps. However, they have seldom materialized successfully past the idea stage. The lack of applicable, light-duty domestic diesel engines makes these swaps a discouragingly difficult process. Foreign diesels are usually quite advanced, but are difficult to find and maintain. If you do manage to import one of these engines, US diesel fuels have a higher sulphur content than foreign purposed fuel, and generally incompatible with foreign designed and produced injectors. We have researched this topic enough to be hopeful that these swaps will one day be more feasible than they currently are.

If you are considering a diesel engine for the sake of novelty, you may find that it requires more commitment and cash than novelty is worth. If you are seeking them for fuel economy or efficiency, you may consider a modern, clean burning fuel-injected gas engine set up against a properly geared drivetrain.

This is a sticky topic, and one apt (as experience would testify) to be misunderstood by the overzealous.

We get calls and mail (and have for many years) from individuals who say "Hey! Let's put a diesel in a Jeep" with the same enthusiasm and sense of pioneering discovery that one would attribute to a scientist having discovered cold fusion.

Diesels are neat. They power some amazing equipment. There is no doubt that diesels can offer great economy, but the question is, "What diesel engines are available for Jeep conversions that are:

  • common across the nation

  • the right size

  • the right weight

  • have a broad enough operating range for a multi-use vehicle such as a Jeep

  • affordable to buy

  • compatible with transmissions that must also meet the above requirements

If you are lucky enough to meet these requirements, than you'll have managed to find an engine that we and hundreds of our smart and ambitious customers have not. This should not be such a revelation to any reader of this article that knows of the tumultuous history of diesel engines in American cars and small trucks. Contrast this to the very great success of European and other foreign countries progress in engineering great diesels that are made for cars and small trucks that are efficient, compact, broad revving and clean burning. They have understandably had the impetus to produce such engines as their fuel costs have traditionally been significantly higher than ours here in the US.

Injected vs. Carbureted
This question has been at the forefront of the conversion world for a while now. There is something great about a simple, clean, unencumbered carburetor and simple ignition system. However, there is something excellent about a modern, self-adjusting, efficient, operate-at-any-camber fuel and spark delivery system.

No doubt that many individuals are in their comfort zone with the earlier hardware, but distill it down to the basics and it is the same essential thing that was going on in 1903; getting fuel and spark into the cylinders with the right mix and timing. There is no way around the conclusion that fuel injection systems do this better and in a broader range of conditions. Old iron is really cool, but this author has lived squarely during both carbureted and injected eras, an I see fewer breakdowns than ever, and have been in the bays and at the wheels of enough injected vehicles to know that they use less fuel to generate more power and in a cleaner, more reliable manner than their predecessors.

We get an occasional call from customers that have found a beautiful Vortec V8 and ask if they can put a "simpler" carburetor on it. This has every distinct disadvantage that we can think of: increased parts cost, decreased efficiency, driveability and reliability. Don't even think about it. Fuel injection (especially GM fuel injection) is much easier to work with than too many people think.

In time, we are likely to see the requirements of the above list become fulfilled. In the meantime, people should not be too quick to discount the progress and efficiencies attained by a century of progress in the design of the gasoline engine. With the advent of modern and intelligently engineered engines that have computerized, well breathing, closed loop fuel injection and exhaust systems, we actually have some really excellent engine conversion choices.

"A fuel-efficient engine cannot be a powerful engine"

Blatantly incorrect. A fuel-efficient engine is nearly by definition, a powerful engine. An engine that best uses an optimum fuel/air mixture to the greatest power advantage is powerful. The rest of the equation is whether the engine is scaled right for the job at hand.

We'll use the 400 horsepower C6 Corvette as a shining example here. Its LS2, 6.0L engine achieves 23 MPG (city/freeway combined - cruising MPG's is at 29!) while other sports cars of its class fare more poorly in power vs. economy. It beats out the Viper, the Porsches, and the Ferraris, achieving as much as 50% greater efficiency when comparing horsepower vs. mileage.

There is a bigger difference between the engineering of engines than most would assume.

"A low-emissions system is a fuel-efficient system."

Just because a motor vehicle power source has low emissions, does not necessarily mean that it will be highly efficient. Emissions controls refer more to the evaporative and exhaust filtration systems than the native ability of an engine to best vaporize and spark its mixture to the highest level of efficient burn and creation of transmissible power.

Some engines (and some transmissions for that matter) create more heat when what they should be creating is more power. Additionally, many motors send unburnt fuel into the catalytic converter for the purpose of finishing the job the engine was supposed to do in the first place.

In our long experience of cooling engines, we have discovered interesting differences in the classic V8's, including (from hottest to coolest burning) AMC, Dodge, Ford (302 & 351) & Chevrolet. Some engines are notorious for needing larger radiators and/or oil coolers than a similarly displaced and power producing engine of another design and brand. Heat is (as per Carnot) a byproduct of power, but also a product of inefficiency.

"Lower octane fuels cost less."

Many individuals unwisely install cooler thermostats in modern engines, thinking they are doing the motor a favor, or trying to cover up a cooling inadequacy in an insufficient radiator.

Newer engines are designed to run hotter than their predecessors, and to their benefits.

Not necessarily. Higher octane fuels have higher potential energies than lower octane fuels. The higher the octane, the longer the burn. The longer the burn, the more power is generated during the combustion stroke. Therefore, fuel economy is increased. The math can often be such that 91 octane fuel costs less to use despite its higher initial cost at the pump.

A caveat here: Higher octane fuels can only release their increased potential energies in engines that have higher compression ratios, and that run at hotter temperatures. Compression ratios of 9.5:1 and higher in concert with engines that run higher temperature thermostats are required to make the best use of these higher grade fuels. This is why there has been such a trend over the past several years to run engines hotter, typically into the 210 F. range. Another benefit of this hotter operating temperature is both in increased burn and cleaner exhaust emissions.

And now, a departure from physics to economics...

"The costs of petroleum are controlled by Big Oil and corrupt sheiks."

Well, there is no love lost here for either of the above, but more sinister and problematic than corruption and profiteering is the stone-cold fact that the cost of petroleum is actually quite stable compared with precious metals, commodities and a weighted basket of world currencies. Given the predicted fall of the US Dollar due to central bank money printing to pay for US debts, occupations, corporate bailouts and governmental inefficiencies, the price of oil is at ~$140. instead of ~$35. per barrel, as was the case when the dollar was stronger a few years ago. The 800 lb. gorilla in the room is the perpetuated misconception that the fuel costs foisted upon Americans is a result of anything other than policy ineptitude vis a vis the laws of economics.

Practical Suggestions for Fuel-Economy in your Jeep

Choose the Right Transmission

The right transmissions will have a broad gearing span for both efficient cruising RPM's and a good gearing for off-road use. Don't forget that the torque converter of an automatic is much like a gear reduction unit - the automatic of your choice will not list a compound low gear, but lower gearing is effectively achieved by the differential of the power transmitting fluid. In terms of automatics, you may consider a TH350-C, which features a lockup torque converter. Even better for economy is the TH700R-4 and its updated electronic progeny, the 4L60-E. These feature an overdrive gear and a lower first gear than just about any automatic shift transmission available. Additionally, their design and construction is world class tough and refined.

The 700R4 / 4L60-E and Mileage Benefits

A good part of GM's reasoning in developing the 700R was to allow more of their vehicles to meet ever stricter mileage and emissions requirements. It was an effective strategy. By converting to the 700R, your Jeep may realize significant mileage benefits as well. It is not uncommon for swappers of this transmission to see a 30% drop in fuel consumption if the Jeep sees highway use.

Additionally, the transmission features a lock-up torque converter to allow a direct, non-slip connection through the transmission. This has the benefit of further fuel savings and cooler transmission operating temperatures. However, it is crucial that this lockup mechanism be properly connected to the ECM and brake lamp circuit. Stand-alone computers are available on the aftermarket for its control. Some control units even allow for the lockup to occur at a given speed.

Lock-up converters in automatic transmissions give the advantage of direct engine lockup through the otherwise fluid-coupled transmission. This function works either by the ECM or a vacuum switch. Earlier versions without ECM use a vacuum switch connected to ported vacuum. The transmission receives the signal and applies fluid pressure to the clutches in the lock-up converter. The system switches off due to a signal from the brake pedal. If you run the transmission with the converter unlocked all the time it will run a little hotter. Keeping it cool is important. The advantage of this lock-up converter is increased fuel economy and decreased heat at cruising speeds.

If you would like to simplify the stock setups, torque converter controls are available through racing supply retailers in a variety of styles. These facilitate the proper installation of the TH350 and 700R4.

The on-road advantages are obvious. For the off-roader, the ability to manually switch the torque converter on is excellent in that is allows for compression braking.

Choose the Right Tire Size and Axle Ratio

As discussed above, this will put your engine in its optimally efficient range while cruising. There is no better place to do this than on the Novak Gearing page, here.

Choose the Right Engine

A V8 that has the crawling torque of a straight-six-cylinder and the efficiency of a V6 cruiser is better than a torque oriented I6 or V8. While these engines do have an initially good torque boost, they do not have the legs required to get out and run without creating too much heat and wasted fuel.

Remember here that power is economy. A quality V6 and V8 can be a very efficient and broadly usable platform. Nearly all straight-sixes and oversize V8's cannot inherently achieve the economies of power.

Next, we highly encourage you to consider a throttle body or multi port injection engine. Chevrolet / GM Generation III and Generation IV engines have proven to have impressive amounts of power and economy and have set the standard as the most intelligent swaps yet. We remain hopeful that the other up and coming modern engines can do as well.

We get an occasional call from customers that have found a beautiful Vortec V8 and ask if they can put a "simpler" carburetor on it. This has every distinct disadvantage that we can think of: increased parts cost, decreased efficiency, driveability and reliability. Don't even think about it. Fuel injection (especially GM fuel injection) is much easier to work with than too many people think.

If an individual is looking for suggestions, we will offer the following:

Consider the Cost of Turning 4wd Systems

Some four-wheel-drives simply have wasteful designs in their systems. There is a cost to running all-wheel-drive transfer cases and axles. Consider using a transfer case that can completely disconnect the front axle and only transfer power to the rear when you do not need four-wheel-drive. All-wheel-drive transfer cases and other similar variants are expensive to run - no to mention the cost of worn tires, viscous couplings and other trappings of these "consumer friendly" systems.

your Jeep is already likely to have an efficient transfer case. However, some models turn far more parts than necessary to drive on paved roads.

From earlier Jeep Wagoneers to Jeep Grand Cherokees with fancy and complicated transfer cases and axle systems, convert to free-wheeling hubs and simpler transfer cases and watch your economy bump up by a couple points.

Keep it simple and then give yourself the credit you deserve for knowing when you really need four-wheel-drive.

Engine Choice and Emissions

your Jeep is considered to be a “Light Truck” by most jurisdictions. As such, you can usually source your engine from a GM truck or SUV without failing your emissions certification. However, this again is according to local laws and your research is encouraged. Car engines do burn cleaner and may be more affordable as well.

LS1 & Vortec V8 EngineSummary

The final myth to be dispelled is that conversions are expensive. Quite the opposite. For decades, we and our customers have been performing powertrain upgrades, and many that can pay for themselves within short periods of time. Before the topic of fuel economy came into vogue again, we and many others have been steadily concerned about efficiency and operating costs. Run a cleaner, stronger and more broadly purposed engine and transmission in your Jeep and you will find that your performance, maintainability and driving enjoyment will increase along with the savings.


A TJ Wrangler with a 5.7L LS1 Chevrolet V8 as sourced from a Corvette. At 400HP, and as much torque across a broad RPM range, it had the impressive ability to plant the driver deeply into the seat. This was our first LS1 to TJ swap and what was so challenging then has become second nature and well documented. Fuel economy was up five points from the 4.0L I6 that it replaced.