What's wrong with revs on a J13 anyway?

[Ofeargall]

I posted something a few weeks back about wanting to possibly switch out my differential because A.) It's too loud and probably needs to go anyway and B.) I needed something to lower my revs (rpm) on the highway.

 

But now I'm thinking, do I really want to drop my revs or just embrace the rpm madness because, as any motorhead (or physics professor) knows - revs equals power!

 

Would it be smarter to freshen the engine and get a cam designed for high revs and run with it, or is the J13 just not designed to handle that?

 

Your thoughts are appreciated.

[banzai510(hainz)]

that small of motor migh not have enough torq to turn a higher gear diff

[Ofeargall]

Ya, that's what got me thinking about the higher rpm's and how they might not be that bad after all. I guess I'm just used to modern engines turning 2500 r's at 65 mph, not 4500 r's. The real question is, are the high rpm's that bad? Should I just go for it and maximize my plight with a better cam for high r's and tweak my carb and exhaust to increase flow?

 

Basically, do I seek the blessing of the high rpm saints or am I courting disaster because my J13 worships a low rpm god deep down in its journals?

[datzenmike]

The earliest 620 up to '74 with automatic uses a 4.625 ratio rear end. That would drop the revs slightly.

 

Check your rear end oil. If low it will make noise.

[sycloned916]

I have the same problem kinda need to get some different gearing in the rearend,but don't know what to get it out of. I'm running the 4.8?? and it ok to drive now with the sr but the J-motor had a hard time keeping up on the freeway. I'm looking for something in the 4.3?? to 4.1?? that will work for my rearend.

[datzenmike]

Early automatic 620s were 4.625, most 620s were 4.375 the '79 standard was 4.11. Two wheel drive 720s ran 3.70 and 3.90, long box automatics were 4.11. Look below the passenger side hood hinge on the inner fender, there is an aluminum engine tag with this info stamped on it... no guessing. Here's one from an '82 2wd with Z22 motor. The ink is gone but should say near the bottom:TRANS/AXLE: FS5W71B...... HF43. The HF means H-190 and the 43 means 4.375, a 41 would be 4.11 a 39 would be 3.9 and so on.

 

http://i138.photobucket.com/albums/q251/datzenmike/720%20stuff/720Large2.jpg[/img]"]

[sycloned916]

Early automatic 620s were 4.625, most 620s were 4.375 the '79 standard was 4.11. Two wheel drive 720s ran 3.70 and 3.90, long box automatics were 4.11. Look below the passenger side hood hinge on the inner fender, there is an aluminum engine tag with this info stamped on it... no guessing. Here's one from an '82 2wd with Z22 motor. The ink is gone but should say near the bottom:TRANS/AXLE: FS5W71B...... HF43. The HF means H-190 and the 43 means 4.375, a 41 would be 4.11 a 39 would be 3.9 and so on.

 

http://i138.photobucket.com/albums/q251/datzenmike/720%20stuff/720Large2.jpg[/img]"]

 

Thanks for that Info. Now the hunt begins!

[Ofeargall]

datzenmike, I saw this info on the other thread but, my big question is, do I really need to lower my RPM's?

 

Wouldn't it be better to keep my gearing and re-cam the engine for higher RPM's since the higher revs will give more power? I mean, why fear 5000 RPM if the engine is freshened and breathing properly? That's where the horses are anyway, right?

 

By the way, you are a huge help on this forum I've noticed. Thanks for all the info you provide.

[HRH]

I think the J13 is probably more suited for a forklift. Not bashing the motor, but it's just not a screamer, and even if you make it that way, it's probably going to be disappointing when you're done with it and it's not powerful enough. Go L series or KA, or VG, or something easier to deal with and find parts for.

[ggzilla]

The J13 is designed to run at high revs for long periods of time. I think you'll find it is in the torque band too on the freeway (good). Being a short stroke engine it revs nicely.

 

that being said, you don't need much HP to cruise at 60mph. A five speed is the cure, you can have your 4th gear torque when you need it and your extra fuel economy in 5th at other times.

[datsunaholic]

Yeah, the only problem is finding a 5-speed for a J-series. They were only available in Australia and New Zealand.

 

I have a J15 in my 520, so it handles a different drive gear (4.375s) very well. But the J13 will rev all day in the 5000RPM range.

[72240z]

the J13 will rev all day in the 5000RPM range.

 

3500rpm to do 80mph in my d21 makes me nervous, 5k would be panic attack time. I'm just not mentally prepared to listen to a motor in a truck I'm driving turn 5k continuously lol.

[DanielC]

The J13 engine is a pushrod engine. That is why Nissan developed the OHC L-series engine. It is a much better engine, that likes to rev higher. Five speed transmissions are available for it, also

How many main bearings does the J13 have, does it have five, or three?

If I were you I would consider putting the L-16 or 18 engine in your truck.

Most 521 trucks with the L-16 ran a 4.375 rear axle ratio.

Going to a rear axle ratio less than what you have may cause your top speed to go down, unless you are going downhill, because the engine cannot "pull" it

[lynchfourtwenty]

dont u FS5W71B me mike! :lol::fu:

[datzenmike]

Not counting transmission losses, to go 50MPH you need about 10-15 hp most of which is used to push the air out of the way. There will also be some drag from aerodynamics, tire rolling losses etc. To double your speed you will need 4X the power to do so, not twice. What ever speed you want to go you have to be revving high enough to be making enough power to do this.

 

Since you don't seem inclined to motor swap, (and I don't blame you) you could make improvements on what you do have. Your motor is small and rev happy so give it more fuel/air and more exhaust pipe to get rid of it. Maybe a bigger carb and a better intake, perhaps a pair of small SUs. Must not over do this is a small motor with modest needs. This is one instance where I would suggest a header and a very slightly larger diameter exhaust system. Don't forget that when you double the diameter of a pipe, it has four times the cross sectional area, so a small increase is all you need. Going from a 1.5" pipe to a 1.75" pipe is a 36.5% gain!!!!! Over a third larger.

 

To help coordinate the extra fuel and exhaust an upgraded cam is also in order. More will be realized with a cam change if you already have the intake and exhaust already modified. This should open up the power in the mid range and up so keep that 4.875 rear end.

 

Optional:

 

Keep a look out for another J13 head and rebuild it. Port and polish it, maybe mill it down slightly, new seals and gasket match the intake and header. Porting for good flow adds power that is built in never wears out.

 

See if you can locate a J15 from outside the US, there was a J20 used in Japan. No replacement for displacement.

Edited November 20, 2009 by datzenmike

[69srl311]

Same issue here. The '66 520 I daily drive would scream at hiway speed. I thought that first gear was very low, I figure it is designed that way for a heavy load I'll never pull. I pulled the 4.88 rear and put in a roadster 3.90 in it. now I can comfortably cruise at 75mph. No 1/4 mile screamer but than what would one expect with a 1.3 liter 3 main bearing engine w/ 60hp!

 

[datzenmike]

J13 with 3.90 and it goes 75?

[sycloned916]

I got my J-putter up to 74mph with the stock 4.8 gears,thought the motor was going to go through the hood:confused:. The 3.90 you should be maxing the speedo out.

[Ofeargall]

WOW! I figured this would be somewhat 'controversial' but I didn't expect quite so much conversation.

 

One option I've considered (talked to a guy in Oregon who gave me the idea) is to put a Volvo overdrive on the end of the tranny. (Mentioned this over at Datsun Pickup Lovers...) I've got a 82 Volvo that could be a likely donor on that.

 

I'm a bit committed to the J13 for the sake of balking the conventional reasoning that you always have to swap out for an L engine. Chances are this commitment could be my ruin, but I would like to try I guess.

 

Thanks for all the collective brainpower. We'll see what happens next. I may go through all this and end up caving into the L.

[69srl311]

75? Hell, it will go 80 if the wind is calm and the ground is level! Just don't sneeze!

[fisch]

Look into what folks do to hop up MGBs! The J and MGB (b-series) should swap. MG guys have been known to put J and E heads on their brit blocks.

 

If you want to get crazy you can get into cross flow head swaps. Moss superchargers (though the don't recomend it for a 3 main) and if you got 5-7k to kill, you can put a BMW motorcycle DOHC head on it!

 

Ofcourse for a fraction of the money you can get into other engine swaps that won't just bolt in. But will still probably come out cheaper!

 

Can you imagine?

 

Start digging!

Edited December 9, 2009 by fisch