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Stroking a Jeep 4.0L Motor

By Tony Lopez

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Stock cam (top) and Crane Cam (bottom)
The camshaft becomes the next concern. The few people I spoke to about building this motor told me that if you have a non-HO 4.0L, you should spend the money on upgrading the cam. I have heard that the 4.0L HO cam can be used, but again, this is just hearsay as far as I am concerned. I spoke to Crane Cams about this project and they recommended their part #753901. The specifications compare as follows.

The main concern with installing this aftermarket camshaft is the increased lift. Will this increase in lift cause the valves to contact the tops of the pistons? As we can see from the table below, the maximum increase in lift occurs on the exhaust valves and it goes up by .079”. However, this concern is not all that different than when building any other motor with an aftermarket camshaft. The stroker actually creates a bit of an advantage in this particular case and you can see how in the previous discussion about connecting rods. Because the piston actually ends up lower in the cylinder than with the stock crank and rods, I have actually created more room for the increase in valve lift.

Stock HO Camshaft
Crane part#753901 Camshaft
Lift – Intake
Lift - Exhaust
Gross Lift - Int/Ex

I decided that the Crane cam would be ideal for this project and ordered one. Another thing that needs to be considered with the Crane Cams is shimming the rocker arm bridges. The bridges need to be shimmed to help compensate for the increase in lift so the preload on the lifters is not excessive. If this is not done, the lifters could be damaged and you’ll end up doing it over again.

Fuel and Compression Ratio
One of the concerns that arise when stroking a motor is compression ratio. Compression ratio is the ratio of the volume of air in the cylinder when the piston is at the bottom of its travel to the volume at the top of its travel. A compression ratio that is too high can cause detonation. Detonation occurs when the spark ignites the air/fuel mixture normally, but rather than burning evenly, it explodes. This is due to higher compression in the combustion chamber than the fuel was designed to operate with. The gas that we can find at the local gas station can run reliably in motors with compression ratios as high as 10:1 and some believe as high as 11:1. If the compression ratio does exceed these figures, the engine can only operate on pricey race fuels. This is not very desirable for a daily driver. To avoid this, I needed to confirm that the compression ratio was in an acceptable range. To calculate the compression ratio, take:

cylinder volume + combustion chamber volume
combustion chamber volume

This formula yields a compression ratio of 9.4:1 for the 4.5L HO stroker motor. This keeps it well within the acceptable range for using normal fuel. The stock 4.0L HO has a compression ratio of 8.8:1 and Jeep recommends a minimum of 87 octane fuel though here at altitude in the Mile High City we regularly run them on 85 octane without any trouble.

Notice the 258 connecting rod (top) is shorter than the 4.0L rod (bottom).
The assembly of this motor is no different than any other. The only concern I had was the valve to piston clearance because of the aftermarket camshaft. This proved to not be an issue. The 258 crank does not have any clearance issues fitting is the block or with the piston skirts at the bottom of their travel. The only problem that I had was the previously discussed harmonic balancer spacer that I had to make. I also had to bore out the block .030” oversize due to excessive clearance with stock pistons. This increased the total displacement to 4.7L and the compression ratio to approximately 9.5:1. I wasn’t planning on doing this but it shouldn’t create any adverse affects. Everything else went together smoothly.

External Compatibility Issues
Because the stock 4.0L block is retained, all of the factory accessory brackets bolt right on. The stock transmission bellhousing is used, as well. The 4.0L flywheel must be retained because it has the notches on the outside edge for the crankshaft position sensor. The fuel injectors that I used were for an HO engine. It is my understanding that the non-HO injectors will cause the engine to knock but I do not know that for sure. The stock intake manifold and exhaust header were retained. No other modifications need to be done because the Jeep thinks it still has the old 4.0L under the hood.

Does it work?
Now that I have shown that the parts will actually fit together, the question now becomes does it work? The answer is an enthusiastic yes! I could not be happier with this motor. It does everything that I set out to do. The highway drivability is great, and with 4.10 gears and 35” tires, I cruise on the highway at 70 MPH turning about 2200 RPM in 5th with ample power to spare. This engine will idle over rocks that stall a 4.0L HO in an otherwise identically-equipped Jeep.

The motor spools up much faster than stock, though I believe this is due to the cam more so than the stroke increase. I have left many sports car owners scratching their heads wondering how a Jeep could leave them behind, let alone one with 35” tires on it. I have to use 87-octane gas or the engine will knock. These are just driving impressions that are very subjective, though. Is it actually that much better? I’ll let the numbers do some talking.

The Numbers
All who drove it knew that it was better than stock. We wanted to know how much better, so to find out, I looked through my list of wheeling buddies and found one with an identical Jeep, or as close to identical as I thought I could find. My Jeep is a 1991 Wrangler with 18,000 miles on the new motor at the time of testing, 5-speed, 4.10’s, and 35’s. The nearly identical Jeep belongs to Dave Jepsen and is a 1994 Wrangler with 89,000 miles, 5-speed, 4.10’s, and 33’s. Both Jeeps have stock intake and exhaust systems. The plan was to do some side by side racing down major streets and through school zones in Denver. The local law enforcement wanted to race with us, though, so we needed another plan. I borrowed a set of 33’s off of Cole Ford’s Project Cross Trainer. Then, using a G-Tech meter, we ran both Jeeps through a series of horsepower test runs as well as 0-60 tests.

StrokerThe G-Tech meter is basically an accelerometer that you calibrate for the vehicle’s weight. It compares how fast the vehicle accelerates to its weight, and spits out a rear wheel horsepower figure. This number is the horsepower that the vehicle is putting to the ground, not what the engine is producing. A good portion of the horsepower at the crankshaft is lost in the drivetrain due to friction. However, with the rear wheel horsepower (RWHP) for Dave’s Jeep with a known crankshaft horsepower, I can calculate what percentage of the crankshaft horsepower is lost. Then with that figure and the RWHP of my Jeep, figure out an approximation of my Jeeps horsepower at the crankshaft.

The tests went something like this. Both Jeeps were weighed to get an accurate figure to input into the G-Tech meter. A flat test track was used to eliminate any errors due to slope. I drove both Jeeps to eliminate any errors due to differences in driving technique. Each Jeep did 12 horsepower runs and 3 0-60MPH runs. For the HP tests, I threw out the high and the low and averaged the remaining 10. The three 0-60 figures were all averaged. The tests were conducted at 6,000 feet elevation because that is where I live.

The stock Jeep put an average of 94.6 horsepower to the ground. Its 0-60MPH time averaged out to be 12.41 seconds.

The Superstock Jeep (still trying to think of a cool name) put an average of 119.9 horsepower to the ground. It averaged 10.1 seconds for 0-60MPH.

Using the stock Jeep’s RWHP (94.6HP) and factory horsepower numbers (181HP) at the crankshaft, I can approximate the crankshaft horsepower for my Jeep. This figure comes out to be 229.4 horsepower. This figure is probably slightly inflated due to the mileage difference on the motors but it does give a good approximation of the improvement.

The 4.0L engine is a great foundation to build a Jeep around. However, like everything else on a Jeep, some of us see a need to improve upon it. The right combination of factory parts allows us to do just that with minimal costs over a standard rebuild. It produces enough horsepower to keep up with many V8’s and when I break an engine mount in Moab, I can just go to a local parts store to get a new one rather than having to call Advance Adapters to have one shipped to me. For those of you with older YJ’s or CJ’s with the 258, read Cole Ford’s article on swapping in a 4.0L. You already have the parts you need for this upgrade. I want to stress that I did not just throw this idea together. I spent a lot of time researching this project so it would work. Jeep has given us the parts to make the perfect rockcrawler motor. We just needed to figure out how to put it all together.


Tony Lopez

Tony Lopez is a staff-writer for Rockcrawler.com and lives near Denver, Colorado. Tony is a licensed pilot and avid 4-wheeler. Tony is the one in green.

Contact Tony at yjlopes@rockcrawler.com

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