High RPM's

Hi, I asked this question last week and got one or two responses. I'm looking for more. My 91 190e runs at 3500rpm while traveling at 70mph. Is this normal? It seems high. Thanks

[Duke2.6]

849 revs per mile tires times 3.27 axle ratio and add a couple of percent for converter slippage.

You do the math.

Duke

[boneheaddoctor]

Originally Posted by james54

Hi, I asked this question last week and got one or two responses. I'm looking for more. My 91 190e runs at 3500rpm while traveling at 70mph. Is this normal? It seems high. Thanks

seems a bit revy....my honda does 4,000 at 70 and thats from low gears...but I never drove a 190E...just surprised a benz is geared that low.

[joseph c]

my 84 190e is about 3500 rpm at 70 mph in 4th gear.........yes this is normal.
joe

Originally Posted by Duke2.6

849 revs per mile tires times 3.27 axle ratio and add a couple of percent for converter slippage.

You do the math.

Duke

Hi thanks for your reply. However, I'm confused. Would you please explain this a little differently? Thanks again.

[t walgamuth]

revs per mile x 3.27 = about 3000 + at 60 mph.

so i suppose duke means 849 revolutions of the tire per mile, then after getting that number divide by 6 and multiply by 7 to get rpm at 70 then add 2 to 3% to that figure for slippage.

clear as mud?

tom w

[Duke2.6]

Yeah, that's the deal.

Tire revs/mile times axle ratio equals revs at 60 MPH in direct drive. Revs at any other speed can be ascertained by linear proportioning, plus add a fudge factor if the car has an auto trans with a non-locking torque converter.

In the case of my five speed, since fifth is indirect, I have to multiply by the gear ratio:

(849 x 3.27)0.80 = 2221 revs at 60 or (2221) 7/6 = 2591 revs at 70

Duke

[geordie43]

Originally Posted by james54

Hi thanks for your reply. However, I'm confused. Would you please explain this a little differently? Thanks again.

Hi. you'll need to know the Circumference measurement of your tyres.

Then divide that number into 5,280 (#of feet in mile) to get the number of times or revs. the wheels turn in a mile x the axle ratio to achieve rpms.

[Duke2.6]

Duh!!!

Use the tire revs per mile specified by the tire manufacturer. Most tires of a given size are usually within one percent, so you can use nearly any manufacturer's data for a given size.

Computing the revs per mile from static dimensions will not yield accurate results because tires are not rigid bodies.

Actual loaded revs per mile figures are usually about 2-3 percent greater than the rigid body calculation using inflated OD.

Why use the inflated OD to calculate a revs per mile figure that will be inaccurate when manufacturers specify actual loaded revs per mile based on testing?

Did I say Duhhhhhhhhh!!!

Pneumatic car tires do not behave the same as steel railroad wheels.

And just in case you didn't notice, this thread is six months old.

How about another Du...!

Duke

[softconsult]

Talk about an arrogant set of posts. Duke's tone is, unfortunately, one of, "I understand this, how could you readers be so stupid as to not understand this!". When I read his first post, I wondered, not what revs per mile meant, but rather where did he get this number? I'm sure others had the same question.

If your goal is to communicate your thoughts clearly to a widely varied audience, then you ought make sure you explain all the assumptions clearly and with as much simplicity as possible.

As I was reading the posts, I kept thinking of the computer techo geeks that I have to deal with occasionally in course of my management consulting business. I often have said to them, "Slow down. Remember that the people you are talking to do not know what your abbreviations stand for and they mostly can't keep up because you are talking too fast. So if possible, slow down explain it in layman's terms." I see this over and over again. They are generally introverts who are in their own little world, and have difficulty dealing with people. Not necessarily placing Duke in this category, just making an observation.

Apparently, you are some sort of automotive engineer. That's fine. My experience is that engineers often have a holier than though attitude because they think they are smarter than everyone else (they often are, in their area of expertise), and think in terms of black and white, true or false. Psychologists will tell you this is true. Makes perfect sense. Gray areas could be dangerous in engineering.

Anyway, no offense meant, simply stating my opinion. I'm sure this is a useful formula to someone. Personally to me, it's a simple as this. My 300E is a high rev small six. The factory optimized the gearing to cause it to run at around 3,000 rpm at 70 mph. Works fine. I don't need to know the details.

Steve

[blueeagle289]

Engineers, including computer types, do tend to have a narrow viewpoint -- that their description or solution is the only possible one. They also love to use acronymic terms, partially because they are used to them and sometimes to confuse general management people. In my consulting days, I always told management clients to deal with engineers and computer people the same way they do all others; ask them if they can do a specific thing and how much it will cost in time and money. Actually, anything you can do with paper and pencil can be done by the computer -- but sometimes you may not want to spend that extra high dollar amount to achieve a minor result.

All four of my cars, with minor variation, run 2700 to 2800 rpms at 70 MPH --
on a fairly level interstate highway ... two of them are sixes (the 230S and the ML320) and two are V8's (420SEL and Corvette). I do wonder why a 190E runs at such high rpms at that speed, but am not familiar with the car and engine.

[Matt L]

Originally Posted by softconsult

As I was reading the posts, I kept thinking of the computer techo geeks that I have to deal with occasionally in course of my management consulting business. I often have said to them, "Slow down. Remember that the people you are talking to do not know what your abbreviations stand for and they mostly can't keep up because you are talking too fast. So if possible, slow down explain it in layman's terms." I see this over and over again. They are generally introverts who are in their own little world, and have difficulty dealing with people. Not necessarily placing Duke in this category, just making an observation.

I have a similar observation. It seems that management and marketing types think that if engineers would just slow down and expand on all terms, that they would fully understand both the problem and the solution.

In effect, it's thinking that all those years in college are completely unnecessary, if only people would talk slower. Sorry, but this is a misconception. Going to first principles only gets in the way, and doesn't solve the problem, nor even explain it.

[lkchris]

This is absolutely normal.

If you've changed tire size from specification, you'll continue to know how fast your engine is running but not your road speed.

[Jim Anderson]

management and marketing types should just think a little faster

[mctwin2kman]

Originally Posted by blueeagle289

Engineers, including computer types, do tend to have a narrow viewpoint -- that their description or solution is the only possible one. They also love to use acronymic terms, partially because they are used to them and sometimes to confuse general management people. In my consulting days, I always told management clients to deal with engineers and computer people the same way they do all others; ask them if they can do a specific thing and how much it will cost in time and money. Actually, anything you can do with paper and pencil can be done by the computer -- but sometimes you may not want to spend that extra high dollar amount to achieve a minor result.

All four of my cars, with minor variation, run 2700 to 2800 rpms at 70 MPH --
on a fairly level interstate highway ... two of them are sixes (the 230S and the ML320) and two are V8's (420SEL and Corvette). I do wonder why a 190E runs at such high rpms at that speed, but am not familiar with the car and engine.

The 190E has a very underpowered engine compared to of course todays cars. It has high gearing to compensate for low power. Also starts off in second gear and has a very short and almost useless first gear. The Automatic is a 4 speed so without that nice 5th gear that is in the cars you mention it needs to rev that high at highway speed to be able to still move when needed. Plus that rev is right in the middle of the power band at 70MPH. Get a 5 speed stick and you get lower revs and of course better gas mileage. Sorry laymans terms as I am an IT/Engineer.