NAS D90 gearing and speed - Defender Source
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  #1  
Old June 23rd, 2015, 10:10 PM
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NAS D90 gearing and speed

Ok, this question has probably been asked 100 times (or more) on this site, but I can't seem to find a thread that fully addresses my question.

I just purchased a 1994 ST with the stock 3.9 V8 and a r380 trans.

Can this machine have the gearing reworked to go slightly faster? It cruises along nicely at 55 mph with about 2700 on the tach. It will go faster, but candidly I'm not sure what rpm level that I should stay at/under.

I know, I know, I know...I'm asking a tractor go 65 mph...it would just be really nice if I could make that happen.

If the gearing can be reworked, what does it take to do it, and what's a ballpark $$ figure I should prepare myself for?
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  #2  
Old June 23rd, 2015, 10:15 PM
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That is not a gearing issue. It is a power issue. You should be able to do 90. The engine needs work.
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  #3  
Old June 23rd, 2015, 10:30 PM
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Quote:
Originally Posted by Red90 View Post
That is not a gearing issue. It is a power issue. You should be able to do 90. The engine needs work.
Second that, also are you rolling on stock tires?
3000 rpm at 70 mph is normal... You don't want to go faster than that on a short tall vehicle.
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  #4  
Old June 23rd, 2015, 10:38 PM
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Wait is this about the truck with the 4" lift?
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  #5  
Old June 23rd, 2015, 10:44 PM
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Josh - You are correct. It's more like 3", not 4" as I originally thought.

Tire size is 285/75/16
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  #6  
Old June 23rd, 2015, 10:45 PM
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We need an FAQ for road testing defenders. A V8 should pull hard to 80 and start to taper off and drive stable and smooth.
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  #7  
Old June 24th, 2015, 01:37 AM
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But it might vibrate you to death depending on how the 3" lift was performed...
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  #8  
Old June 24th, 2015, 06:35 PM
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Good point about the vibration at higher speeds. I'm gong to attack the engine/power issue first. Then determine if I want to take the lift down, or back to stock.

I'm going to check the typical tune-up type items, but I don't that is going to explain my power issue.

The engine doesn't display any other issues besides top end speed (55+)...it sounds fine, drives fine. Any thoughts from the group on where to start diagnosing the engine/power issue? (Assume that I know very little about wrenching on engines...because it's true)
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  #9  
Old June 24th, 2015, 06:37 PM
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Spark plugs
Ignition wiring
Air filter
Any codes being displayed?
Any indication of a slipping clutch?
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  #10  
Old June 24th, 2015, 06:42 PM
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the 285's will throw off your gearing, have you check your speed with a GPS?
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  #11  
Old June 24th, 2015, 07:10 PM
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I second Phillips suggestion to verify the speed. I would also check what gearing you have in the diffs - if this thing is lifted, someone might have re-geared with a bias toward off-roading, not highway cruising.
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  #12  
Old June 24th, 2015, 07:45 PM
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If it is not tuning, it is most likely the timing chain or cam. The chains always stretch and kill power.
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Old June 24th, 2015, 08:06 PM
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Ask the last owner if he changed the gearing and for the love of god get rid of the stupid lift.
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Are there shocks that I can addjust up and down like my friends LX460? That would be very cool!
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  #14  
Old June 24th, 2015, 08:10 PM
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It has nothing to do with gearing if he is only doing 55. Maybe the speedo is a bit out, but I'm pretty sure he would know if he was doing 90....
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  #15  
Old June 24th, 2015, 08:11 PM
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I second Red90's input on the timing set. I rebuilt my engine due to poor performance and the only real issue I found was a very worn timing chain (not a fan of nylon timing gears). Its a pain to pull the timing cover, but it could solve your engine performance problem. I put a Kent cam and a Cloyes timing set in and have had no worries.

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  #16  
Old June 24th, 2015, 09:04 PM
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Check for a small slit in the large diameter hose between the MAF and the intake manifold.
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  #17  
Old June 25th, 2015, 05:36 AM
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Originally Posted by down_shift View Post
Check for a small slit in the large diameter hose between the MAF and the intake manifold.
please sir, go on...
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  #18  
Old June 28th, 2015, 11:13 PM
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Guys - thank you for all of the tips/ideas. Very much appreciated!

I spent some time going through the plugs, wires, hose to the MAF, etc. Everything checked out. I did not investigate the timing chain given the time involved in that fix.

I took it back out for a road test and got it up to 65 and 70 mph (still had power to go faster, but not necessary to do so). I was running about 3100 rpm at 65 mph and about 3200 rpm at 70 mph.

Does that seem reasonable? (keep in mind I'm running tires that are 285/75/16)

It seemed ok to me given that in this thread Carlos mentioned 3000 rpm at 70 mph was about normal.
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  #19  
Old June 28th, 2015, 11:29 PM
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Are you checking the actual speed by GPS or are you believing the speedometer?

There is a gearing calculator on Ashcroft's site that you can check on.
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  #20  
Old June 28th, 2015, 11:42 PM
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Had to work off the speedo today but my GPS was giving me the business. I think I'm pretty close on the speed based on how traffic was traveling and what the prior owner told me about the accuracy of the speedo.

Just got my GPS updated a few minutes ago, so I'm going to drive it again to verify the speed. Thanks for the tip on the Ashcroft site!
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