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February 2006 4x4 Tech Questions - Techline

Posted February 1, 2006

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Four Wheeler
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Question: I have a '77 Jeep CJ-7 and want to know if it is possible to put a diesel engine in it.
Clint Kartchner
Mule Creek, NM

Answer: Anything is possible, but not always practical. Jeep has actually used a diesel in Jeeps before. From 1961 through 1969, they used a Perkins four-cylinder with 192 cubic inches (3.15 liters), which was rated at 75 hp and 114 lb-ft of torque. I don't think that many were produced for the U.S. market, as I have only ever seen two-and one of those was on the Jeep assembly line in 1964. In the early '80s (and maybe longer), Jeep offered in their overseas market another four-cylinder diesel in the CJ line but it was never imported to the U.S. Then in 1986-87, the Cherokee line had a Renault aluminum-block and aluminum-head four-cylinder turbocharged diesel. This was a 126ci (2.1L) motor which made 85 hp and 132 lb-ft of torque. Again, not a very popular option, so finding one would be quite difficult. I have heard of and even seen a few conversions using a Nissan six-cylinder diesel, but can't tell you much about them.

In Montana, there is a flatfender Jeep built by Advance Frame Works (406/538-4996, that has a Cummins four-cylinder diesel in it. While it's a great engine, it does weigh in at over 900 pounds! Some special considerations have to be made in regards to this extra weight as far as frame, suspension, and axle use are concerned.

If any of our readers have any information on diesel swaps for Jeeps, I sure would like to hear from them on the subject.

Question: One of my favorite columns is Willie's Workbench. In the November '05 column explaining carburetor sizing, there is a small error in the cfm calculation: Half the engine size times the maximum rpm (corrected for units) should be multiplied by the VE (%), not divided by the VE. Dividing by a number less than one (1) will increase the product. Here, a VE less than unity should decrease the estimated airflow of the engine. This is properly calculated by multiplying by the VE. Willie's example gives the correct value if multiplied by the VE (i.e., 665 x 85% = 565, not 665 / 85% = 565 as written). On the other hand, 665 / 85% = 782.

Another factor that I've found important, other than shoe size, is the size and type of air cleaner used on the carburetor. Since many designs can reduce the effective airflow through the venturis, a larger carburetor may be necessary to achieve the cfm predicted by the calculation. As Willie said in his limited space, there are many variables, some easier to quantify than others.
Doug Taylor
Rio Rancho, NM

Answer: Thanks for the good catch. I went back and looked at the original copy, and while I wish I could blame the error on the copy editor who may have changed the "multiplication" for a "division" sign, there is no one to blame but me. I screwed up. (OK, maybe my wife who typed up my scribbles did it wrong. Yeah, that's who I can blame!) Actually, I should have caught it when I proofed it before I sent in the story.

Hey, I like it when readers catch my mistakes and keep me honest, but I sure hope my mistake didn't cause someone to buy a carburetor that is way too big for their engine.

Oh, and you are so very right in that the air cleaner can have a major effect on airflow, especially those that drop the filter down slightly below the carburetor's air horn where the air has to make a turn to get into the carburetor.

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