Cruising rpm

[Zmanco]

You've confirmed that you have either a 3.36 or 3.54 diff, your tires are pretty close to stock diameter and you have a stock 4 speed. You've measured your speed with a GPS (eliminating the speedo gear from this discussion).

 

I think all that leaves is the tach, no?

 

BTW, just remembered that my dwell meter can also measure rpm up to around 2k rpm IIRC. While that's probably not high enough to do a proper calibration, it would let you get a rough idea if your tach is even close to reading correctly.

[Tony D]

Z Tachs can drift, and they are "calibratable"...

Autometer gauge in my car is ATROCIOUS below 1500 rpm. Shows 1100 when it's really 850-900. It skims like that then around 1300, it starts getting closer, at 1500 its nearly spot on, and above 1700 it's dead nuts accurate to over 7000 rpms.

 

Usually they go 'dead' in the bottom and top of the range. Using a known good tach, you can verify. There is an aftermarket tach I picked up on clearance when Autobacs closed, that used an inductive pickup to trigger the tach, pretty neat for a 'check tach'...though my Sun Meter has a digital rpm pickup off the tach terminal of the ignition system, and I use that a lot more. Digital is sooo nice! No guess, it's 1475 rpms, man! (think Dennis Hopper voice for that last phrase...)

[Iceman]

If you have a new clutch, it is possible that it isnt fully engaging. This would cause slipping when the load is heavier. This would be the only way that your RPM could be higher than it should be for a given speed. Your car wont spin a faster RPM just because it's a smaller engine. If it did, then my commuter CRX with a 1.4L would blow up before it got to 60mph,

 

If we assume that your clutch is NOT slipping, then only one of two scenarios could be occuring..

1: Your tach is off like folks are suggesting, or

2: Your engine is revving according to the gear ratio.

RPMs and gears are part of the mathematical equation that can tell you your speed that you see in the calculators from the links that have been posted. There are only 2 places that slipping can occur.. clutch, and of course tires. If nothing is slipping then your RPM is where it should be.

 

I have really low gears in my Z. It has been off the road for a while, but if memory serves correctly, in 4th gear the RPMs were pretty high at 65mph. I know my tach wasn't very accurate, and I never paid much attention to it, but the engine at 65mph sounded like the bottom end of shifting range (probably ~3500).

 

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.

[Guest Rolling Parts]

"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?

.

 

That's the reference to the little plaque in the 240Z engine bay that says 150hp. That document is stamped into steel as measured w/o any driveline. If you measure RWHP on a 240z then it's down around 110. the difference in that instance is 40hp that's caused buy SPINNING the transmission, u-joints, differential, accessories, half shafts, wheels, etc at PEAK rpm for power. Increase rpm's to 6400 and at least 45hp are wasted.

 

What I'm saying (and car makers today are doing) is reducing all those frictional losses by lowering rpm on all the components. Obviously even small 1.8 litre engines run at low rpm's just fine. There is NO TECHNICAL REASON why a person can't make a 2.8 liter six cylinder run happily at lower rpm's. Yes there are compromises and yes this is for mileage and not performance, buy it's completely doable if that's what an owner wants to do.

 

I have a 3.39 and a late 5 speed in a 240z with megaquirt and a turbo that happily loafs along at 2400rpm and 16.0 A/F in highway cruise BUT has damn good performance when the turbo kicks in when not cruising. My performance AND mileage are better than a stock 240z. That's what this site is great for ("when stock just won't cut it")!

[Repiv]

Just to follow up, it ended up being the tacho. Dropped in a 3wire from a 280z and my RPMs dropped 300 just at idle. You can imagine how much it was off at the higher RPMs.

[Tony D]

That's a generous and flawed assumption to make based on an engine output plaque.

 

Manufacturing difference can mean as much horsepower is lost in transmission as from production tolerances.

 

Taking a factory HP rating from an era when there were no standards for measuring it, and then comparing the results to current dyno comparisons is foolish.

 

What if the rear wheel said 127HP, then this car only lost 23HP---then how do you account for a 45 HP loss on one vehicle, and another identical vehicle in similar trim makes 127 on the same dyno, or on a different dyno for that matter? What of a power peak different than claimed on the factory documentation, frictional losses building as speeds climb skewing it downward (if I'm reading your post correctly)?

 

The rating on the plate is useless. And certianly is not a basis for making a claim of 45 HP lost through the driveline. That is just foolishness.

 

Good to see the OP got the thing dialed in, looks like it was a Tachometer Error.

 

As for loafing along at 2800 in a turbocharged car, hope your oil pump is up to the challenge. Most people are looking for a 2000 rpms cruise speed, and that just doesnt' supply required oil flows for longevity IMO. You can do what you want, but oil pump performance curves and bearing loads are very definite science. And lugging an engine is lugging an engine.

[Guest Rolling Parts]

As for loafing along at 2800 in a turbocharged car, hope your oil pump is up to the challenge. Most people are looking for a 2000 rpms cruise speed, and that just doesnt' supply required oil flows for longevity IMO. You can do what you want, but oil pump performance curves and bearing loads are very definite science. And lugging an engine is lugging an engine.

 

Well, I would assume that most turboZX street car engines spend a lot of there life at or below 2800 rpm. They seem to have held up pretty well over time...

[Tony D]

It all depends on what speed and load they are generating.

 

Do a dyno pull from 1000 rpms in 4th gear and look at the power generation of that engine. Plot 25HP.

 

Then extrapolate 5th gear and 25hp. That's as low as you can go and likely keep forward momentum on a level highway at 65-70mph.

 

Now, put a load like a 5% grade. Just add boost. Right?

 

It's an extreme, but demonstrates that while you might be able to do some things, it may not be best advisable.

 

Shifting to 5th gear at below 45mph is lugging the engine. But you can do it. Now climb a hill with it. Just add boost, right?

[Guest Rolling Parts]

It all depends on what speed and load they are generating....Shifting to 5th gear at below 45mph is lugging the engine. But you can do it. Now climb a hill with it. Just add boost, right?

 

True. I really don't think that most people would refer to hill climbing as "cruising" or drive all day 45mph in 5th gear. Yes, that is an extreme. But even with such moron drivers like that who bought Nissan 280ZXT's I've never heard of any oil starvation problems with the L28et design itself. Great engine.