Cruising rpm

[Repiv]

Is there even a 4.38 rear diff that works with the z?

[Rustyfriend]

Regardless of what rear you have, I suggest getting a 5-speed in there. The 4-speed has terribly short gears. I had one in my 280z after I converted it to manual and 65mph with a 3.54 rear was about 3500rpm. Once the 5-speed was in the gears lasted a lot longer and dropped rpm significantly. Can't tell you for sure before I also swapped in a new harness and the tach is not working.

[Repiv]

The 4speed doesn't sound like it'd be that bad with a normal 3.364 rear. I think I want a 5spd though with all the interstate driving I do. 81-83 5spd with a 3.7 or 3.9 rear seems like the sweet spot.

[blue72]

Is there even a 4.38 rear diff that works with the z?

 

Yes, it was an optional gear set, or could have come from another Nissan vehicle (720 for instance). You can always pop the diff cover off and read the numbers that are stamped into the ring gear, or count the teeth on the ring and pinion then do the math yourself.

 

I've currently got a '78 5-speed mated to a 3.9:1 rear and I quite like it. Offers good acceleration, and I'm taching 3700rpm @ 75 mph which returns surprisingly good mileage.

[Zmanco]

Are you sure the tach is accurate? The early 240Z tachs can easily be off a few hundred rpm. They are adjustable.

 

Still, it's not too hard to jack up the rear and count how many times the drive shaft turns for each turn of a wheel. That would get you pretty close on the diff ratio in short order.

[Repiv]

How do I tell if my tach is off and how do I adjust it? It idles at about 900 rpm which seems normal.

[Zmanco]

If you can find another tach that's known to be accurate, you could do a temporary install and compare the two.

 

I would suggest comparing the two at cruising revs, maybe 3k and 4k rpm. At idle I doubt the error will be large enough to be accurately measured and adjusted. FYI, mine read about 300 rpm high at 3k rpm meaning it was ~10% off. At 900 rpm that's only 90 rpm error which I don't think you'll be able to measure.

 

There is a potentiometer that is adjusted with a small screwdriver through a hole in the back of the tach. Hint: only do this with the ignition off. Otherwise you will damage the tach. Ask me how I know.

[Daeron]

If you can find another tach that's known to be accurate, you could do a temporary install and compare the two.

 

I would suggest comparing the two at cruising revs, maybe 3k and 4k rpm. At idle I doubt the error will be large enough to be accurately measured and adjusted. FYI, mine read about 300 rpm high at 3k rpm meaning it was ~10% off. At 900 rpm that's only 90 rpm error which I don't think you'll be able to measure.

 

There is a potentiometer that is adjusted with a small screwdriver through a hole in the back of the tach. Hint: only do this with the ignition off. Otherwise you will damage the tach. Ask me how I know.

 

That is freakin awesome to discover about a car thats been a part of my life since I was born almost 30 years ago.... Thanks!

[Tony D]

S30's were available in the JDM with ANY ratio from 3.7 to 4.38 STOCK (and in some cases with LSD)

 

My buddy has a 1975 Fairlady Z with a STOCK 4.11 and a three speed automatic. With an L24, triple mikunis and keeping a steady 65mph that car will get 28mpg driving from Phoenix to LA.

 

There is no concern for a car turning 4Krpms at 80 mph, or even at any desired cruising speed. The design range for cruise in a Nissan L is anywhere from 3000 to 4000 rpms.

 

Gear / Tire the car appropriately. If you continually cruise at 80 mph, gear for that rpm range. Lower rpms means sluggish top-gear acceleration compared to a gearset with a lower numeric ratio.

 

In ANY of these cases the car will STILL be DRAG limited to 125 mph, so gearing to accelerate to 125 the quickest way possible is the 'performance oriented approach'...

 

There is nothing wrong with the cruise speed you mention. This is not a Chevy V8 we're talking about here, it's an engine HALF the size.

 

Logically, TWICE the cruise rpms would be EXPECTED!

[Tony D]

BTW:

Standard offerings for the Z(S) model was a 3.7 gearset with the four speed in the early cars. A five speed came with an early box and a 3.9.

 

Deluxe versions Z(L) came with more weight, and accoringly were fitted with 4.11 and 4.38 gearsets. A Z(L)2/2 would have a 4.38 gearset, as would a five speed Z(L) Coupe FairladyZ.

The four speed Z(L) would have a 4.11, and a Z(S) would go back to the 3.7.

 

Only in the barcalolunger driving US of A did we get 3.36's and 3.54.

 

In Europe, the 3.36 came in the Turbo Cars...for a 260KPH top speed potential with the 200BHP Non-Catalyst Version...

 

FYI.

[Guest Rolling Parts]

... This is not a Chevy V8 we're talking about here, it's an engine HALF the size. Logically, TWICE the cruise rpms would be EXPECTED!

 

That would only be be TRUE if you installed the L6 in the Chevy.

This is not a Chevy car we are talking about here, it's HALF the size/weight of a Chevy Caprice. Logically TWICE the power is needed to shove a ponderous 2 1/4 ton American land yacht down the road than a JDM Zcar.

 

The problem with ancient cars is that they have ancient drivelines.

It's obvious by just looking at the old 170SAE HP Vs. 125 rwhp confusion.

At the 4500 range the engine LOOSES 45HP documented HP to all the driveline and associated frictional losses! Reduce RPM's in half and gross frictional losses greatly reduce as well.

 

Displacement is also less relevant in these size cars. A 2800 pounds Corolla is easily moved along with 1.8 liter engines turning 2200rpm all day long. Why cant a 2500 pound Z with a 2.8 liter 6 be capable of doing so at 2200rpm as well?

 

As with all cars going for higher MPG the secret is TUNING for that lower RPM. If you take the time to select the right cam and a tunable EFI/ignition system then the L6 engine will deliver power and economy at 2200-2500rpm. ONLY problem is when people slap in a trans/diffy without addressing tuning that reducing engine/driveline RPM's would not deliver maximum benefit.

 

YMMV.

[Repiv]

There is no concern for a car turning 4Krpms at 80 mph, or even at any desired cruising speed. The design range for cruise in a Nissan L is anywhere from 3000 to 4000 rpms.

I'm at 60 at 4k

[Iceman]

You are getting what you should be expecting if you have 4.11 gears. If you have anything in the 3.xx range, then you may have a problem with your clutch slipping.

As far as I know it is a stock rearend. My speedo is all out of whack though which may lead me to believe it is not stock.

 

I used a GPS to get the speed I was going.

 

[edit]

Actually, putting 4.11 into the calculator makes everything close to how it is. 4k around 70mph and 2.3k around 40mph. I don't know why someone would add such a high gearing without changing the transmission though

 

You should be taching 3501 rpm.

I'm taching 2393 with 215-50-17 3.7 ratio but with .7 overdrive tranny.

What does it tach at in 4th gear? In 4th, it's 1:1 just like his.

 

 

There is nothing wrong with the cruise speed you mention. This is not a Chevy V8 we're talking about here, it's an engine HALF the size.

 

Logically, TWICE the cruise rpms would be EXPECTED!

Half the engine doesnt mean that it should run at twice the RPM. There is no correlation.

[Tony D]

I stand by it, in either case, with either weight, and with either engine, with either car it will take approximately 19HP to propel a car down the road at 65mph.

 

Where does this occur at on a 350 Chevrolet Engine?

Where does this occur at on a 168CID Datsun L6?

 

Acceleration will be the prime canidate for consideration of final gearing, and in any of the mentioned cases, none of the normally supplied ratios changed the top speed of the car one whit....but it DID markedly influence the characteristic of how fast it attained those speeds.

 

"At the 4500 range the engine LOOSES 45HP documented HP to all the driveline and associated frictional losses!"

What document can I reference to read up on this claim?

 

As for fuel economy, there are so many misconceptions about that it's not funny. The fuel consumption curves are shown in the Nissan Documentation, and if you follow them in your gearing most people find they overgear their car quite a bit and get worse fuel economy than they would if less gear were run and the engine speed was slightly higher.

[Iceman]

I stand by it, in either case, with either weight, and with either engine, with either car it will take approximately 19HP to propel a car down the road at 65mph.

 

Where does this occur at on a 350 Chevrolet Engine?

Where does this occur at on a 168CID Datsun L6?

 

If you can leg-press 300lbs and your buddy can only leg-press 100lbs and you peddle a bicycle in the same gear at the same RPM, you go the same speed. Your comparison does hold some value in the case of an automatic transmission, or a manual transmission with a slipping clutch, however, as the engine will strain more to make the same speed. With a properly functioning clutch or a lock-up converter, the gear ratio is static so it doesnt matter how much HP the engine can make. When lack of HP becomes a factor, the car just simply wont reach a higher speed.

[Tony D]

I think you miss the point at what rpm the engine will make the power.

 

The 350 chevy will make 19hp at off-idle conditions, probably before 1100 rpms.

The Datsun will take that to close to 2000 rpms.

 

For cruising, the cars will need to be geared to have 19 HP available to propel the car at 65mph.

 

Hence my statement that 2X the RPM of a Chevy going the same speed is to be expected.

 

You have agreed with my earlier statement in your Leg-Press Analogy, you just don't realize it yet.

 

The contention that it takes 2X the power to propel a big american car down the road also plays into the same logic, but it's an incorrect assumption anyway. Most cars only require around 19-30 HP to go 65mph. Weight has VERY LITTLE to do with it in static state conditions. It becomes important in varying conditions such as acceleration and climbing hills, be let's not even go there...

[Xnke]

No, Iceman, you are wrong.

 

IF I can legpress 300lbs, and you can legpress 100lbs, and we are going along at the same RPM, in the same gear, yes, we are both doing the same amount of work. Now, make the ground not level. Increase the incline to the point where you can no longer pedal the same RPM in the same gear. I still can. I have three times more incline ability than you would, for the purposes of this example.

 

In the same way, a 350CID V8 will produce more torque at a given RPM than the 168CID L6.

 

Tony, you beat me to the post. Maybe mine will help clear things up.

[Repiv]

I just counted the drive shaft turns and it is somewhere closer to the ~3.364-3.545 range than 4. It does have a new clutch so maybe that is it.

[Daeron]

I just counted the drive shaft turns and it is somewhere closer to the ~3.364-3.545 range than 4. It does have a new clutch so maybe that is it.

 

A new clutch would grip solidly, and you would have virtually no chance at slipping, and increasing RPM without increasing speed. In other words, a new clutch would PREVENT this problem.

 

 

There is no 3.365-3.545 "range." Get a paint marker and mark a bright spot on the driveshaft if you have to, and start it with that mark at 6 o'clock dead low. If it is a 3.364, then you will get three full revolutions and then it will stay between 9 and 10 o'clock when you complete one revolution of the halfshaft. If it is a 3.545, then the mark you made will be just PAST the high noon position. That is almost a quarter turn difference. Apologies for spelling it out, but 9 o'clock and 12 o'clock are pretty difficult to mix up.

 

 

 

What Size Tires Do You Have?? I asked for specifics, not "I think" followed by an extremely rare and hard to find tire size. (In other words, you probably don't have tires the size you said you did!!!) For all we know, you're running 13" tires to boot.

[Repiv]

Now that I have access to my car I can say for sure I have 205/60R14 tires/rims.

 

I meant range in the sense that I couldn't see my mark where it stopped at.