Transmission information

[NicktheMillwright]

Hey everyone

 

Been searching for some information about the transmission and diff specs on the 1985 z24 5 speed manual. From what I gather there are 2 versions.

 

For the diff I'm looking for LSD options, even if that means swapping to a hardbody rear end.

 

My goal is to keep the truck as original as possible but as good as it can be.

 

I've also decided to keep the z24 stock, but again as good as it can be. On the fb group I saw a truck with several engine mods I want to adapt to my truck. Mainly a 272/272 cam and Weber 38 with 14mm spacer. Anything else I could do? ARP studs are also going on. Going to refrain from turboing it. Going for fun over fast.

 

Thanks

[datzenmike]

Same transmission just there are two lengths, 26" and 31.5"

 

Not a fan of LSDs. Good for straight ahead acceleration where the traction is poor and/or you have more than enough power. 

 

For a performance engine build the block doesn't contribute much it just has to be strong enough to not fly apart. Real power making is concentrated in the head and is related to how well it breaths air in and out.  It breaths just fine for what it was designed for (up to highway speeds) but both ports are low to the head gasket surface with a restrictive sharp bend at the valves and into the combustion chamber and this is their biggest fault.

 

The Z24 makes just over 100 hp. The better breathing 2 liter L20B makes about 95 hp so if you multiply by the displacement difference of 20% the L20B would make 114 hp if it was 2.4 liters. As you can see the Z24 is low by over 10 HP. 

 

A 272 cam will make some numbers but won't reach it's full potential. Old Z24 valve springs are pretty weak so get new ones with higher pressure. Try it but don't throw away your old cam. Never over carburate or over cam your engine.

 

I would concentrate on low speed torque increases or anything that makes breathing easier.  

 

The stock carburetor is rather under size. The 38/38 is a V6 or small V8 carburetor. I would go with the easier 32/36 Weber. Never over carburate or over cam your engine. It has to start well in all weather and temperature extremes and idle in traffic.

 

Take the Cat off and hollow it out or stick a straight piece of pipe in it's place.

For a muffler, a turbo style (NOT a see through fart can) something with some packing and sound absorption.

If replacing the exhaust pipe (2" I think on the 720) go to 2 1/4" that's a 26% increase in cross section, way more than you'll need.

 

Compression on the Z24 is a pathetic 8.25. You can mill the head to increase. If rebuilding use KA24E pistons for 9.52

 

 

Best bang for the buck performance increase without touching the engine is a differential gear change. Your gear ratio is on the engine plate under the passenger side hood hinge on the inner fender. Bottom line TRANS. AXLE ... FS5W71B..... HF38

38 is a 3.889 ratio a 37 would be 3.70 and so on. A 41 (4.11) or 43 (4.375) will rev the engine higher at the same vehicle speed. Revved up makes more power at that same speed. More power, more performance. Easy to change and reversible, no extra strain on the engine.

 

 

 

 

 

 

[NicktheMillwright]

Once again thanks for the info Mike.

 

So this is good news so far, I have a 32/36 already equipped and a set of KA24E pistons already in stock, so that's excellent news. When I do the full tear down I'll swap in those. Timing chain and tensioner ect ect all needs to be done. Still tracking down a water pump.

 

The exhaust needs to be redone anyway as it has more holes than Swiss cheese, so I'll keep that information in mind as well.

 

For the diff gear my plate reads HF37. Would the 280z gear set work in this application?

Another thing is I've heard is a "Wide range" 71b tranny. Any info on that?

[datzenmike]

The 280z or zx use independent rear suspension (IRS) and an R-180 differential. The 720 uses a solid axle with removable 3rd member H-190. Not in any way close to the same. The 720 did use 33, 35, 37, 39, 41, 43, 46 ratios. Forget 720 4x4s as a source. The earlier 620 primarily used 4.375s. 

 

Your 2wd should have the mid ratio 5 speed. only the Diesel or the Cab/Chassis option got the wide ratio.

 

First gear engine turns per single output turn comparisons

3.592...... Wide.... engine turns faster at same speed, better acceleration, better if working truck carrying loads like camper.

3.321....... Mid...... engine turns less faster at same speed, slower acceleration.

3.062...... Close.... only available in the '80-'83 280zx.

[NicktheMillwright]

Awesome. Thanks Mike. I may have a lead on a 46 diff but need to verify. Assuming a wide trans will bolt straight up I'll be on the hunt for one of those as well.

 

Another question. I've replaced pistons on good motors but assuming I have to have the Cylinders bored due to wear, how would I go about sizing pistons? From my Millwright experience it's simply taking going for a "larger" piston and having each Cylinder bored it its designated piston withing tolerance. Do correct me if I'm wrong.

[datzenmike]

 

Only the '73 620 automatic had the 4.625. The later D21 Hardbody and the WD Pathfinder had 4.625 but in H-233 and that's never going to work. The later Xterra also had a 4.625 but in C-200 and that's not compatible. I wouldn't go higher than 4.375 and they are plentiful.

 

Get the pistons first and give them and the block to the machinist. The cylinders can be honed out to the correct final clearance and finish. Let him worry about the measuring and boring. The piston to wall clearance is 0.001" to 0.0018" one to two thou.

[NicktheMillwright]

Yeah that was the plan with the pistons. More curiosity than anything.

 

So at my local wrecker they have a WD Pathfinder with the 4.625 diff. What is stopping me from pulling the entire Axle and using that under the truck? I know the housing is likely different.

 

Not that ive sought then out but I haven't seen a single 4.375 in lower mainland BC

[datzenmike]

WD21 Pathfinder is coil spring and link suspension and a few inches wider that the 720.

[Stoffregen Motorsports]

Head studs are absolutely not required for anything but massive boost. And even then, machining an o-ring groove in the top of the cylinders and installing o-ring wire, can handle most situations. Head studs make assembly and disassembly a pain, and I avoid them if I can.

 

A limited slip is a going to be hard to find. In stock form, LSD units have very low breakaway pressure so normal driving is not adversely affected.

[bottomwatcher]

LSDs were offered in the pathy. Also should have disc brakes and the thing is bombproof. That being said you will have to add spring perches and modify the driveshaft along with the e brake. As long as you aren't expecting drop in and you have the tools it can be done.

[NicktheMillwright]

Just looking for clarification

 

With the 3.70 diff currently in my truck, wouldn't I be technically putting more revs into a 4.625 diff if I swapped it? Therfore limiting my max speed and trading it for acceleration?

 

And the point with the Z24 was to keep the power in the lower end for breathability because this head flows like a wet paper bag?

[Stoffregen Motorsports]

Correct.

 

A 4.62 may be too low of a ratio, depending on tire size.