RB20DET 5sp swap in D21 but kept stock auto axle = crap??

[s13_240_rb20]

I have a RB20DET 5sp swapped into a 1993 D21 2wd auto pickup. I have not changed the rear axle so according to the doorjam, I have a "HF37" rear axle. If the net tells me right, my auto axle has a gear ratio of 3.70. My RB engine would crank up to 150mph fairly easily in my 240sx using a r32 vlsd, which had a gear ratio of 4.36, if the net tells me right. Sooo...my rear axle should be turning my wheels even faster than my r32 differential turned them, right???? I don't know...

So my biggest complaint about the performance of this swap is the top end of the rpms in each gear.

In 1st, I can reach around 25mph at 5800 rpms
In 2nd, I can reach around 40ish at 4400
In 3rd, I can reach around 65mph at 4000
In 4th, I can reach around 70ish at 4000 (YES>>> ONLY 70ish)
In 5th, I can reach around 80ish at 3000, using WOT. If I stay WOT for a mile, I sometimes reach 90ish at 3500.

As you can see, this is no longer a fun vehicle. Keep this in mind and read the chart above again: In my 240sx, I could redline EVERY gear @ 7500, but I never pushed all the way to 5th gear redline...too fast...over 150ish at 6000rpms.

I know nothing about gear ratio's except what I just typed here, and what I just read on the net:

determined by driveshaft turns / 1 turn of the rear wheels (understood)
"HF37 axle is only used in weenie 2wd Frontiers" (according to the net, but my hardbody's doorjam disagrees)

What complications come up when using an auto axle with hi-power 5sp transmissions? What can you tell me from what I have explained? I want my power and speed back I would love a solution that includes a LSD upgrade/swap...lol

--TJ

[seang]

I knew the aerodynamics of the D21 were far worse off than the 240sx, and unless there is another variable here, this just proves the extent.

[s13_240_rb20]

Aerodynamics can explain some loss in acceleration and even top end, but it doesn't explain why I can't redline the first couple of gears.

[seang]

It is like WOT is not really WOT at all. Is your TPS alright?

[s13_240_rb20]

It is like WOT is not really WOT at all. Is your TPS alright?

My tps is fine. I have even tried adjusting it forward and back to see if it would improve any part of the performance..had little improvement. I posted this question here in the truck forum to discuss the truck's stock axle (all other suggestions welcome, but I am still confused).

According to the ratio's, my hf37 axle only needs 3.7 driveshaft turns to turn the wheel one time. My engine/trans were designed to spin the driveshaft 4.36 times to achieve one turn of the wheel......so when paired with the truck's 3.7 geared axle, my wheels should be turning 17.83% faster than my transmission's speed sensor is calculating. Final result would be, say, ...speedometer saying 100mph but my axle would be turning the wheels at 117mph.

My speedometer doesn't work right now, so I did all the calculations on paper.

So in my current setup, 100mph from my RB20 transmission is probably turning the 3.7 axle at the fastest rate that Nissan would even fathom that 3.7 axle to turn = 117.83mph.

So I ask again: Is it possible for the rear end to be physically limiting the potential of the engine's upper rpms?? Am I on the path for exploding my differential? lol

and, last but not least,: does it cause any problems or make a difference if you use a rear axle from an automatic and pair it up with a 5speed transmission??

--TJ

[s13_240_rb20]

I saw a 4x4 hardbody the other day, so I waited and asked the guy if I could see his door jamb. he said both tires spin in the mud, so I am hoping his is lsd....

can anyone tell me from experience if a CA41 axle is lsd? and does it bolt right up to a 2wd 1993 automatic hardbody? and does anyone around texarkana/little rock or inbetween have one for sale pretty cheap?? I wanna try it!!!!!!!

btw...I mentioned my HG37 axle supposedly is "found in wennie 2wd pathfinders"....the same page that is plastered all over the net says that the CA41 is found in 720's....I thought 720's didn't look just like hardbodies...lol

--TJ