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Old 01-07-2009, 01:53 AM   #1
sc00ter
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Default Safe cruising RPM's

I would like to get some opinions on this.

I live in Colorado, and every day I have to drive from downtown Denver up the mountain where I live. It is about a 30 mile drive mostly uphill on the highway. The speed limit is 65mph and the minimum speed is 55mph (they have a minimum speed because it's pretty steep and actually hard to stay at 65mph). The elevation up in this area is over a mile above sea level. So the air is quite thin up here. Because of this there is little power under 3k RPM's.

Anyway, when I drive up the mountain in 5th gear, my RPM's range from 2,700-3,000 RPM with the pedal to the floor, trying to stay at 65mph. But when climbing the mountain in 5th gear, in this RPM range, the car slowly loses speed at a constant rate. So the obvious solution to this is to downshift into 4th. However, in 4th gear I am right about 4,200RPM when staying at a constant speed between 65-70MPH.

What I have found myself doing is dropping into 4th and staying in it until about 75mph then shift to 5th and stay there for as long as I can before I drop below 60mph. Then I shift into 4th and repeat the whole process. I am also afraid all this shifting is gonna be bad for the clutch/tranny.

My question: if I stay in 4th gear at 4,200RPM for 20-30 minutes on my way home from work 5 nights a week, is this going to be really terrible for the engine?


***I use 5w-30 full Mobile 1 synthetic oil and I change it myself religiously every 3k miles.***


Sorry for the novel, just trying to get every detail I can think of out there to help with any educated solutions to this issue.

Thanks
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Old 01-07-2009, 01:58 AM   #2
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As long as the engine is well maintained it should be fine so long as you're not constantly red lining it. The only problem I see with sitting in 4th gear so much is you loose gas mileage. On steep hills I usually avoid 5th gear since it doesn't provide much torque.
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Old 01-07-2009, 02:26 AM   #3
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Your engine probably won't care, but your wallet sure as hell will. Upshift a gear and you'll get better gas mileage. The engine is plenty happy pulling from 2k on up as long as you're not looking to accelerate in a hurry. I select my cruising gear to be as close to 2k as I can get.

You're probably going to want to look at other options than that M1 though.
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Old 01-07-2009, 02:37 AM   #4
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leave it in 4th. The car circulates more oil and coolant that way to keep the motor cool. Additionally if you're closer to peak torque the engine is operating at a higher VE and probably gets better economy at higher rpm.

I have 4.11 gears so I spent most of four days cruising at around 4k rpm when I drove to montana and back recently.
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Old 01-07-2009, 11:38 AM   #5
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You'll likely see an increase in MPG if you just leave it in 4th and stay at 65mph. Cuz you won't be having to go WOT in 5th just to maintain speed. 4th gear and 4k RPM won't hurt your car. Sure.. being able to go into 5th would be ideal, but your situation is different from most of us.
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Old 01-07-2009, 12:20 PM   #6
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Quote:
Originally Posted by williaty View Post
The engine is plenty happy pulling from 2k on up as long as you're not looking to accelerate in a hurry.
Move somewhere with some hills. There are a couple roads i drive regularly which my car will not climb in anything above 3rd gear. As in, the car will stall.



Leave it in fourth, and change your oil more often (since you're running more revs per mile than you would be otherwise. 3k intervals are probably a good idea. Also, i'd probably just slow down. 55mph is still fast if you're climbing a steep hill.

EDIT: i've found that downshifting on steep hills is usually better for mileage. My guess as to why is that if you're at or near WOT at low RPM, the piston is literally not traveling as fast as it should be to take advantage of the combustion (bogging, i guess, but not enough to make noise or rumble). That is to say, the combustion is over and the gasses are cooling down before the piston has moved enough to take advantage of it, reducing the motor's efficiency.

Last edited by sniper1rfa; 01-07-2009 at 12:38 PM.
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Old 01-07-2009, 12:52 PM   #7
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with the 4.11 FD in my car, 4200rpm is around 82mph - right around cruising speed for the highways here (ie I'm regularly in that rpm band for long periods of time). I don't see any problem riding 4th up the hill, it's probably worse lugging the engine in 5th while trying to climb IMO.
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Old 01-07-2009, 02:17 PM   #8
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Quote:
Originally Posted by williaty View Post
Your engine probably won't care, but your wallet sure as hell will. Upshift a gear and you'll get better gas mileage. The engine is plenty happy pulling from 2k on up as long as you're not looking to accelerate in a hurry. I select my cruising gear to be as close to 2k as I can get.

You're probably going to want to look at other options than that M1 though.
I see where you are coming from here. I also like keeping the RPM's closer to 2k as well. But in this case, staying in 5th gear doesn't allow me to stay at a constant speed. I'm quite positive that if I left it in 5th the whole time I would constantly lose speed until I stalled. So I find myself switching into 4th quite often, even though being up at 4k+ RPM's for long periods of time scares me.


I was beginning to consider something besides M1 because the engine eats it sooooo fast. Any other recommendations?

Quote:
Originally Posted by rougeben83 View Post
with the 4.11 FD in my car, 4200rpm is around 82mph - right around cruising speed for the highways here (ie I'm regularly in that rpm band for long periods of time). I don't see any problem riding 4th up the hill, it's probably worse lugging the engine in 5th while trying to climb IMO.
I am beginning to think the same thing. I don't see how staying WOT in 5th gear at 2k RPM's (while constantly losing speed) could possibly be any better for the engine than staying at 4200RPM. In this case, if I stay in 4th I can maintain speed and keep torque for passing when necessary. All while barely touching the pedal.

It looks like for now most of you are agreeing that staying in 4th is my better option here. And those who brought up the fact that I might get worse gas mileage staying in 4th: I would MUCH rather spend more money on gas than kill my engine/clutch/transmission ahead of its time.
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Old 01-07-2009, 02:36 PM   #9
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I'd keep it in 4th.

Lugging in 5th at wide open throttle is putting max pressure on the piston rings and bearing surfaces. Your also probably increasing blow-by of unburnt fuel into the crankcase and eating up your oil's detergent. It is good that your change intervals are 3k miles. Normally that is a waste of money with a synthetic oil. Most high quality synthetics can last 7500 miles no problem.

Now on the flip side, constantly higher than normal revs while keeping it in 4th will stress the oil in a different way -- viscosity breakdown. For that reason a slightly higher than normal viscosity should be appropriate for your application.

I'd recommend using Shell Rotella T 5w-40 fully synthetic or Chevron Delo 400 5w-40. With these you could possibly even stretch your intervals to 5k miles.

I know your probably going to realize, "Hey wait, that is a diesel oil?" Well, yea it is, but if you look closely at the specifications on the back you will see that they have SL specs for gas powered engines. Because diesels produce a lot of soot, diesel oil has a higher level of detergent than straight SL gasoline powered engine oil. So much so it almost smells like gear oil. The detergents will not only absorb the byproducts that unburnt fuel contain that turn into acids. It will also absorb the broken down viscosity modifiers that result from a high revving motor.

I ran Rotella T 5w-40 in my 350z with an 8k mile change interval. I certainly didn't drive that car slow by any stretch of the imagination. I could usually still see through the oil somewhat as it dropped down into the drain pan. I also didn't have nearly as bad oil consumption like I did with the Mobil 1 fully synthetic. And the VQ series motors are notorious for drinking down synthetic oil by the quart every 5k miles.

If your really wondering what a particular style of driving is doing to your motor then you should just get your motor oil analysis done. Analysis of the oil will tell you if metals from the engine are appearing in the oil. This is an indication of excessive wear if that so happens to be the case.

Search around the web to see who does a good engine oil analysis.
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Old 01-07-2009, 03:31 PM   #10
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Two separate things are bothering me here. First, 70mph is just over 3kRPM. My car has absolutely no problem maintaining speed up even very steep inclines (ones that get truck lanes uphill and runaway truck ramps downhill) at 3kRPM. If I have to slow down to 50mph, then 5th won't cut it on even moderate hills, but at 70mph it just chuggs right up them. Prior to getting EL headers, the car pulled hard from about 2300 on up. With EL headers, it pulls hard from about 1550 on up.

Second, something about everyone's comments about fuel economy was bothering me but I couldn't really figure out what it was. It finally came to me while I was sitting in the car today. For maximum cruising efficiency, you're not looking for maximum VE, you're looking for minimum pumping losses. Pumping losses increase with RPM and increase as the throttle closes. To run a given speed in a higher gear, you reduce your RPM (which reduces pumping losses) and you open the throttle more (which reduces pumping losses).

If you want to see this borne out, go drive a car with a CVT. The CVT will put the car up into peak VE range to accelerate (peak VE is most efficient for acceleration) and then immediately drop the engine to a much lower RPM for cruise.
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Old 01-07-2009, 04:27 PM   #11
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The fuel thing is more a case of
Lug in 5th -> re-gain in 4th -> lug in 5th -> re-gain in 4th.
Seems to me like it would cost more fuel than
maintain in 4th.
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Old 01-07-2009, 04:30 PM   #12
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Quote:
Originally Posted by williaty View Post
Two separate things are bothering me here. First, 70mph is just over 3kRPM. My car has absolutely no problem maintaining speed up even very steep inclines (ones that get truck lanes uphill and runaway truck ramps downhill) at 3kRPM. If I have to slow down to 50mph, then 5th won't cut it on even moderate hills, but at 70mph it just chuggs right up them. Prior to getting EL headers, the car pulled hard from about 2300 on up. With EL headers, it pulls hard from about 1550 on up.

I know what you are saying here. And I have experienced the exact same thing when I used to live in Michigan. If you remember, I had you and Sean (Master2192) help me tune my RS when I lived there. And in Michigan 5th gear still has a fair amount of torque, even at lower RPM's. But I don't think you really understand the power decrease that is experienced when moving to an elevation such as where I am at, which is just about 5,500 feet above sea level. As soon as I drove into Colorado when I moved here last year, I instantly felt my car lose A LOT of torque. Climbing up a steep incline hill at 5,500+ feet above sea level is completely different than doing the same climb at sea level (which is basically what you are at being in Ohio). The air up here is much much less dense that it noticeably hurts performance .
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Old 01-07-2009, 04:39 PM   #13
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That's a good point, I hadn't considered elevation.
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Old 01-07-2009, 04:50 PM   #14
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HeftyJo,

That's definitely something to think about. I will look more into the oils you mentioned and with some research this may be just what I need to keep my mind at ease when I'm staying at a constant 4,200 RPM for 20-30 minutes every day.
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Old 01-07-2009, 08:21 PM   #15
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Use 4th gear. Your are using more gas with the pedal to the floor in 5th, as well as putting undo stress on the transmission and engine. Engines run just fine when they are at a constant rpm, around 4,000 just cruising might actually help clear it out a little bit. This is why airplane engines last so long, they are cruising at a constant power setting most of the time with little rpm change.
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Old 01-07-2009, 08:49 PM   #16
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^^ Never thought of it that way.
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Old 01-07-2009, 11:00 PM   #17
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1) Airplane engines do tend to run at a steady cruising RPM. But it tends to be lower. As in mid-2kRPM. Hmmmmm...

2) To run in a higher gear, you do have to open the throttle. However, if you multiply the injector on time (which will be higher in a higher gear) by the number of injections per mile (which is lower in a higher gear), you'll get a lower number (less total fuel consumed) in the higher gear.

Last edited by williaty; 01-08-2009 at 09:38 PM.
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Old 01-08-2009, 09:36 PM   #18
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Quote:
Originally Posted by williaty View Post
1) Airplane engines do tend to run at a steady cruising RPM. But it tends to be lower. As in mid-2kRPM. Hmmmmm...

2) To run in a lower gear, you do have to open the throttle. However, if you multiply the injector on time (which will be higher in a lower gear) by the number of injections per mile (which is lower in a lower gear), you'll get a lower number (less total fuel consumed) in the lower gear.
Do you mean "taller" or "larger" instead of "lower"? wouldnt the # of injections be higher in a lower gear, i.e. higher rpm in lower gear? My tiny brain cant seem to get what your describing.
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Old 01-08-2009, 09:37 PM   #19
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Err, yeah. I meant a higher gear. I'll edit that to make it less wrong
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Old 03-14-2009, 12:37 AM   #20
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Get a better engine management map.
You need more power, thats all.
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Old 03-14-2009, 12:42 AM   #21
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Quote:
Originally Posted by r.i.pm3 View Post
Get a better engine management map.
You need more power, thats all.
1) thanks for bumping a dead thread

2) thanks for bumping a dead thread with a completely, utterly, hopelessly wrong answer
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Old 03-14-2009, 12:57 AM   #22
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I crossed over the Rockies on I-70 back in 2002, and have experienced exactly what the OP was asking about. Even with Cobb heads and cams and full MRT header-back exhaust I couldn't maintain speed in 5th gear making that climb. To me the right answer is to just stay in 4th.

With the RA/Outback 5th gear I was cruising at 4000rpm at 80mph on the highway, no issues at all, so I wouldn't worry about that.

The other option would be to get about 2-1/2 times the power and torque. My CTS-V crossed over the Rockies on cruise control in 6th gear at 82mph without any issues.

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