Gear ratios for datsun 720 street truck

[Ratrod720]

I want to put an amc 20 axle rear end in under my datsun 720 street racer. The stock axle is overgeared and I want the stronger ring gear of the amc 20. Also you can get a lot more aftermarket parts for the amc 20. I am going to put one piece axle shafts to make it stronger. My question is, what gear ratios should I run for 25 inch slicks with the  5 speed and a boosted Z22. Also, if any one knows the ratios for the transmission please post them so I can run them through my gear ratio calculator.   

[1001001SOS]

I was looking into this recently also and had this page bookmarked, may help for transmission ratios (note, info seems to be for AU market):

http://datsun1200.com/modules/mediawiki/index.php?title=71-series_Transmission

Edited October 2, 2023 by 1001001SOS

[datzenmike]

There are some mistakes on the years used so I wonder how accurate the ratios are. Is club1200 north American?

 

 

There were two gear sets used on the Z series engines in the 720s. Wide and mid ratios. Yours should be mid ratio. If you ever have the transmission out there's an easy way to find out what you have.

 

Mark the input and out put splines with marker paint. Place in first gear, turn input shaft 20 full turns while counting the output turns.

 

Wide ratio.... 5.5 turns

Mid ratio*...... 6.0 turns

Close ratio... 6.5 turns (used only on the L series '80-'83 280zx)

 

*Mid ratio

1st..... 3.321

2nd.... 2.077 (37.5% change)

3rd.... 1.308 (37.5% change)

4th.... 1.000 (23.5% change)

5th.... 0.833 (16.7% OD)

 

 

Differential...

The differential ratio installed when built is on the engine tag located below the right hood hinge on the inner fender. They look sort of like this...

 

 

Bottom line is:

TRANS/AXLE ... FS5W71B..........HF38*

 

HF38* is Nissan speak for 3.875 ratio

HF41 would be 4.11

HF43 is 4.375 and so on

 

[1001001SOS]

3 hours ago, datzenmike said:

There are some mistakes on the years used so I wonder how accurate the ratios are. Is club1200 north American?

 

It looks like an Aussie site actually, based on the classifieds. 

[Ratrod720]

Alright. I've determined that I have 3.87 gears. also the engine card says HE38 instead of HF38 like on Mike's card. What does that mean?  

[datzenmike]

HE or HF doesn't matter much. H is the Salisbury style differential housing

with the 'third member' or removable R&P as a contained unit. 

 

On 9/29/2023 at 7:28 PM, 1001001SOS said:

 

It looks like an Aussie site actually, based on the classifieds. 

 

So some of the years and facts might not match those in North America. ok.

[Stinky]

What sort of HP does your boosted 2.2 put out?

 

IMHO....your axle is plenty strong.

 

The ratio is determined on what sort of RPM that you are wanting to turn at the end.  The Q....end of what?

 

Over-geared....What do you mean?

 

If you mean not low enough, you can go, easily, to 4.11.  You also could go lower with a Hardbody axle....

 

Not sure why you'd want a dif wheel bolt pattern.   

[datzenmike]

The H-190 is good for almost 300 ft lbs of torque. At that amount one of the tires will probably break loose anyway and this will lessen the load on them.

 

If you were racing 1/4 mile you want the engine to reach maximum RPM power as you flash past the finish line. This is trial and error but if you are still in 4th then the differential could be changed from say a 3.889 to a 4.375 or 4.625 maybe even higher. That would be rough on gas mileage on the highway.

 

For street 'n strip where economy is somewhat important a more modest change to a 4.11 would give you a little extra more than stock. You could also swap in a wide ratio transmission that has a higher first gear than the mid ratio transmission but the same overall 4th for mileage. First would rev up faster helping the turbo to spool faster too.

[1001001SOS]

16 hours ago, datzenmike said:

So some of the years and facts might not match those in North America. ok.

 

Yeah pretty interesting stuff, nice catch. 

[Ratrod720]

The axle is strong enough. That's for sure. The AMC 20 has a pretty good aftermarket product base that the Datsun axle doesn't have. Also the ring gear is stronger than a dana 44.  If something has a good possibility of failing I like to replace it with something that I can get good aftermarket support for. Also, as stupid as it sounds, I like the ability to lock the rear diff for whatever reason (snow,ice,maybe launching on less than favorable surfaces). The datsun axle is great, but I would rather replace it with a part with better aftermarket support. Also, the AMC 20 is cheap. Like 50 dollars if you know where to look. I'm going to put disc brakes on it and its easier to find parts. 

 

The boosted Z22 isn't built yet so I don't know what kind of hp it will make. I going with a heavy cam, turbo making 16 pounds of boost, forged pistons, twin webers, fender exhaust, shaved head, and O- ringed block. That will give me a little bit of pepper. 🙂 

 

[NC85ST]

Just curious, why a Z22 and not a Z24? In making hp, I always thought, the more cubic inches the better.

[Ratrod720]

2 minutes ago, NC85ST said:

Just curious, why a Z22 and not a Z24? In making hp, I always thought, the more cubic inches the better.

The Z24s, in my experience, crack a lot of heads if you try to get a lot of power out of them. I also have more experience with Z22s and have a lot of spare parts available in my shop. If I do put a bigger motor in it I'll toss a twin turbo 3.0 I got out of a 300zx in it. I probably wont do that though.  

[datzenmike]

The Z heads are terrible breathers and getting air in and out is what makes power. To a point, a turbo pushing air in will help but you're already behind to begin with compared to a Z22 with an L head on it.

 

As to cams. Stock or mild cams work best as they usually have smaller overlap.

 

Z24 aren't known for cracking the heads but they blow head gaskets at about 100k mile intervals. This is avoided by re-torquing the head bolts every year. A Z24 turbo will make more than a Z22 turbo because of the displacement and at lower RPMs.. Stroke sets the engine redine. The longer the stroke the lower the redline.

 

L20B/Z20....... 7,000

Z22................. 6,700

Z24................. 6,350 

[Ratrod720]

44 minutes ago, datzenmike said:

The Z heads are terrible breathers and getting air in and out is what makes power. To a point, a turbo pushing air in will help but you're already behind to begin with compared to a Z22 with an L head on it.

 

As to cams. Stock or mild cams work best as they usually have smaller overlap.

 

Z24 aren't known for cracking the heads but they blow head gaskets at about 100k mile intervals. This is avoided by re-torquing the head bolts every year. A Z24 turbo will make more than a Z22 turbo because of the displacement and at lower RPMs.. Stroke sets the engine redine. The longer the stroke the lower the redline.

 

L20B/Z20....... 7,000

Z22................. 6,700

Z24................. 6,350 

I want to put a U67 head on it. I think those work best with boost. I've been having trouble finding a U67 that wasn't a piece of shit around here for under 1500. Thats one of the reasons the motor aint put together yet.

[datzenmike]

$1500 for an L head!!!!!!!!! Christ I'm rich!!!!!

 

The U67 head will make more power NA than the Z head for sure. It a far better breathing head. There is a nice compression boost also to 9.8.

 

There's a lot more to it than the head. You'd need the manifolds, the engine brackets that lean the L series over to the right and an L20B timing cover. You'll then need an L series transmission to match this new engine position. 

[Stinky]

Been thinking about this....won't a stock rear end hold up to a SBC?  

 

IOW, when people swap in a V-8, it holds up doesn't it?

[datzenmike]

Again up to 280 ft. lbs. of torque.

[Ratrod720]

18 hours ago, datzenmike said:

$1500 for an L head!!!!!!!!! Christ I'm rich!!!!!

 

The U67 head will make more power NA than the Z head for sure. It a far better breathing head. There is a nice compression boost also to 9.8.

 

There's a lot more to it than the head. You'd need the manifolds, the engine brackets that lean the L series over to the right and an L20B timing cover. You'll then need an L series transmission to match this new engine position. 

Yeah I know it is kind of a hassle to get an L head on. I am going to fabb up my own motor mounts, and completely redesign the engine bay. The booster and master cylinder are in the way, and maybe the steering column. I'm going to make a new exhaust manifold for it as well. I hope I don't have to put a new tranny on it. I wonder if I could swap the bellhousing to keep the shifter angles acceptable. 

[Crashtd420]

14 hours ago, Ratrod720 said:

Yeah I know it is kind of a hassle to get an L head on. I am going to fabb up my own motor mounts, and completely redesign the engine bay. The booster and master cylinder are in the way, and maybe the steering column. I'm going to make a new exhaust manifold for it as well. I hope I don't have to put a new tranny on it. I wonder if I could swap the bellhousing to keep the shifter angles acceptable. 

Yes you can swap the bellhousing.. you just use the L series that matches the angle of the head.... 

 

[datzenmike]

Again, you don't have to reinvent the wheel. Get an early 521/510 exhaust manifold. Nissan spent a lot of time an money on a cast iron header development for the L series. The only difficulty is the two into one down pipe that does not exist anymore. Make one of those.

 

 

Watch out. Later L18s they changed the manifold and did not keep the 1 and 4 separate from the 3 and 2 pipes fully separate. Below you want the one on the left where 1 and 4 is separate from the 2 and 3. The one on the right has two outlets but all ports are jumbled together.

 

 

Cast iron 'header' fits perfectly first try, never rusts or wears out, quieter, runs cooler, cheaper. Only advantage a made or after market header has is it's lighter. Again if you're good at welding pipe all you need make is that 2 into 1 down pipe.

 

The 71B 4 or 5 speed front case swap will convert a Z to an L series transmission.

[Ratrod720]

23 hours ago, datzenmike said:

Again, you don't have to reinvent the wheel. Get an early 521/510 exhaust manifold. Nissan spent a lot of time an money on a cast iron header development for the L series. The only difficulty is the two into one down pipe that does not exist anymore. Make one of those.

 

 

Watch out. Later L18s they changed the manifold and did not keep the 1 and 4 separate from the 3 and 2 pipes fully separate. Below you want the one on the left where 1 and 4 is separate from the 2 and 3. The one on the right has two outlets but all ports are jumbled together.

 

 

Cast iron 'header' fits perfectly first try, never rusts or wears out, quieter, runs cooler, cheaper. Only advantage a made or after market header has is it's lighter. Again if you're good at welding pipe all you need make is that 2 into 1 down pipe.

 

The 71B 4 or 5 speed front case swap will convert a Z to an L series transmission.

I hear you. I just want to have something a little simpler. I don't really want to track down a manifold, modify it, and then try and fit the turbo to it when I can just make my own. I don't mind building an exhaust manifold. Thanks for the info though. You have been very helpful, but I would like to make my own.  🙂

[datzenmike]

Ah that's right turbo. Look at after market KA24E turbo exhaust manifolds. The bolt patterns and ports almost match and can be ported to match.

[dew]

Just my first thoughts, 2cents... if

you are swapping heads with boost seems KA24e head would be 1st choice, if can find one. Without boost, import a KA20de for your Z22, but that  may be more than most prepared to do.

[datzenmike]

Note that if a KA head is used there is an oil drain back hole at the rear that empties into the water jacket if used on an L or Z series block. There are two so just TIG the left rear one closed.

[dew]

Thanks Mike, hoping someone would fill in... W/O robbing topic completely, to raise compression in that combo, can one just bore for KA 10.5 compression pistons (plus valve relief) and rods or does that put piston too hi?