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Datsun 620 Jatco 3n71b questions


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#1 DirtnastyDatty

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Posted 30 December 2017 - 10:09 PM

So I daily my 620 and i spend a lot of time on the freeway, it being a 3 speed, the rpms are pretty high and I don't have a tach but just going by the sound of the engine (sounds like 4500 or so), I don't want to go faster than 60. The point of this discussion is to ask if anyone knows the ratios of a Jatco 3n71b, but any other info is greatly appreciated. I've been driving/tinkering with my 620 for the past 2 years but I'm only 18 so I'm still working on my knowledge of vehicles in general. Once again, any advise/info is greatly appreciated, thanks

#2 datzenmike

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Posted 31 December 2017 - 07:29 AM

We get complaints of trucks 'high revving' all the time. It's just sound and while it seems high it really isn't and certainly nothing to worry about. The Nissan designers surely took driving your truck at 70 to be well withing their limits. The L20B is red lined at 7,000 RPM the L18 higher than that, and the L16 even higher. You can safely run this high but I doubt you will get there at top speed. If the engine is in good shape, full of oil and water it can rev this high indefinitely. But lets take a look at it....

 

On the highway you would certainly be in top gear* (3rd ) and this means the engine would turn one time for every turn of the driveshaft. In reality there is a certain amount of slippage from the fluid coupling of about 100 to 150RPM but negligible. The differential will be 4.375 ratio or 4.375 engine turns for one turn of the rear wheels. The 620 normally has a 6.00 X 14 tire size or 175/70R14 which is 23.65" in diameter. A little online calculation and you would have to be going 73MPH to be revving 4,500RPM. To go 60 MPH it would only be revving 3,729 RPMs. So you see that although it sounds high, it really isn't much at all. If your tires were replaced with something with a larger diameter these numbers would be even lower.

 

JATCO 3N71B ratios

1st....... 2.459

2nd...... 1.458

3rd....... 1.000.... highway speeds*

Rev...... 2.182


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#3 Stoffregen Motorsports

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Posted 31 December 2017 - 10:00 AM

My wife's commuter is a 2006 Scion xB and it has very short legs. She has 100 mile commute and she likes to drive fast, which means when she's cruising at 80mph, the poor little bastard is spinning at about 4300 rpms, for 100 miles one way. It is annoying, but the car has over 260,000 trouble free miles on it.

 

L motors are a bit more clunky than a modern engine, so I bet it isn't as smooth up there in the 4500 rpm range, but it isn't likely to kill it.

 

If you want to drop the rpms, install a 5spd trans or re-gear the diff, or get larger tires, but that may look goofy.



#4 DirtnastyDatty

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Posted 31 December 2017 - 11:54 AM

I feel honored having Datzenmike reply haha, but thanks for the info guys, been afraid of blowing up the l16 going past 60, engines in good shape, just leaks a little bit of everything but I keep a jug of everything in the bed. Once again, thanks for the info

#5 datzenmike

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Posted 31 December 2017 - 12:14 PM

Keep tuned up, lots of fresh oil and cooling system working and full. You'll NEVER blow one up. The theoretical 'red line' for a 73.7mm stroke engine is 8,100 RPM. You're barely half way there. The L16 has to rev in order to make enough power to push through the air at 70 or even 80 MPH.

 

 

 

F4e52N7.jpg

 

Above are the main bearings from a 150 hp KA24E and an L16/18/L20B engine. Which one's which? Well the left side will fit your 76 hp L16

 

 

I would just live with it, knowing there is no harm being done.


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#6 DirtnastyDatty

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Posted 31 December 2017 - 01:20 PM

Wow, good to know, once again can't thank you enough for the info