3.9 versus 4.0 - Defender Source
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  #1  
Old January 29th, 2008, 08:24 AM
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Douglas Wozny
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3.9 versus 4.0

Replacing the 3.9L engine in my 1994 D90 with a 4.0L engine. What kind of gas mileage can I expect out of the 4.0L? It is only a .1L (7 cu in) increase.
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  #2  
Old January 29th, 2008, 08:33 AM
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It's probably more a function of your foot that anything, so it will probably be the same.
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Old January 29th, 2008, 10:37 AM
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Are you going with a new or used 4.0?
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  #4  
Old January 29th, 2008, 11:40 AM
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Quote:
Originally Posted by The Wizard
Replacing the 3.9L engine in my 1994 D90 with a 4.0L engine. What kind of gas mileage can I expect out of the 4.0L? It is only a .1L (7 cu in) increase.
You should check the exact capacity of the engine.... The 3.9 is NOT 3.9L and the 4.0 is NOT 4.0L...... So there is not 0.1L difference. Check this out: http://en.wikipedia.org/wiki/Rover_V8_engine#3.9.2F4.0

Expect no difference unless you swapped the electronic that goes with the 4.0L (in which case, MPG should go up a bit).
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  #5  
Old January 29th, 2008, 12:01 PM
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My 94 and my 97 when new had basically the same gas mileage - don't know other peoples experience, but that was mine. (And now that I swapped a 4.6 for the 3.9, I still get the same gas mileage - about 13 to 14 - but I drive it like I stole it).
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Old January 29th, 2008, 12:08 PM
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I currently have a 3.5, 3.9. 4.0 and 4.6 every one of them get between 13-15mph.

Two things--a lot of the 4.0 blocks have issue with slipped/dropped liners--just something to be cautious of especially if buying used.

Second thing, all things being equal, get a 4.0 over a 3.9 since the block is crossed drilled for the main bearing caps and this would be good especially if you ever increase the power. That said, it is really just one of those "all things being equal" because I know of many very strong modified 3.9's and not a one has had any problem with the cross drilled caps.
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Old January 29th, 2008, 12:44 PM
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Quote:
Originally Posted by Davis
I currently have a 3.5, 3.9. 4.0 and 4.6 every one of them get between 13-15mph.

Two things--a lot of the 4.0 blocks have issue with slipped/dropped liners--just something to be cautious of especially if buying used.

Second thing, all things being equal, get a 4.0 over a 3.9 since the block is crossed drilled for the main bearing caps and this would be good especially if you ever increase the power. That said, it is really just one of those "all things being equal" because I know of many very strong modified 3.9's and not a one has had any problem with the cross drilled caps.

Added power issues aside, is the 3.9 setup a better bet for long-term ownership. They are often replaced in favor of larger engines, but over a lifetime of ownership, why do I feel the 3.9 will have less issues? It seems I hear of more problems from the 4.0 and 4.6. I'm ignoring the RRC 4.2.
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Old January 29th, 2008, 01:53 PM
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In actuality they are the same displacement engine. The 4.0 does upgrade a few areas such as better head retention, cross bolting of the main caps, and a much superior oil system. But it also requires switching all the front end stuff such as water pump, alternator and such because they go with the serpentine belts. Not to mention, only one year engine was made that takes a distributor, so good luck finding the hybrid timing cover you need.

Then you also get some negatives. The slipped liners can happen to 3.9's as well if you overheat them enough, but there were metallurgy problems with the 4.0's that can be serious and can only be solved with either block sealants or sometimes you just need to replace the engine block. It's also harder to get new bearings for it if you've resurfaced the crankshaft. They are out there, but almost nobody has them.

The 4.2 is actually the same block as a 3.9, just different rotating assembly on the inside. You can also go to a custom shop and have them do a stroker engine out to just under 5 liters on the 3.9 block. Something else to consider too is that some of the very late 3.9 blocks can be drilled out for the cross-bolting. They've already had the casting changes to the webbing on the block, they just never had the machine work done. But it will take somebody who knows what to look for to spot it.

-Hans
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  #9  
Old January 30th, 2008, 04:38 PM
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In a loose order of highest to lowest, from a cost/benefit perspective, how does one effectivley increase horse power in these engines?
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Old January 30th, 2008, 04:58 PM
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Exhaust/intake
4.6 with 4.2 chip
4.6 with RPI chip
Blower
Headwork

YMMV
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  #11  
Old January 31st, 2008, 09:24 AM
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Thanks Ron. I'll do my research in that order.

Clay
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  #12  
Old January 31st, 2008, 11:56 AM
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Quote:
Originally Posted by evilfij
Exhaust/intake
4.6 with 4.2 chip
4.6 with RPI chip
Blower
Headwork

YMMV
Anyone use a perfromance chip with a 3.9L?
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