Cruising regularly at 4000 rpm

[ImAdopted]

Originally Posted by ermax

Just ran the math... even though your 90mph trip would take less time, the total number of revolutions is slightly higher on your 90mph trip.

300/90*60*4000=787500 revolutions
300/80*60*3500=800000 revolutions

Seriously splitting hairs. But as you can see we are on the same page.

But just to clarify, my advice was never really intended to be applied to top gear cruising. It applied to times where people cruise in 4th gear when they could be cruising in 6th.

Here is the main post I was referring too. He said at 60 he is typically cruising in 3rd and at 80 he is typically in 4th. That is 5200RPM in the first example and 5400 in the second example. 60 in 6th is 2600rpm and 80 in 6th is 3450. That is where I feel wear isn't as negligible.
http://www.ft86club.com/forums/showp...3&postcount=66

im no math wizard and i assure youre better at it than me but i believe you may have mixed up the answers?

[Brink]

Originally Posted by ermax

Just ran the math... even though your 90mph trip would take less time, the total number of revolutions is slightly higher on your 90mph trip.

300/90*60*4000=787500 revolutions
300/80*60*3500=800000 revolutions

Seriously splitting hairs. But as you can see we are on the same page.

But just to clarify, my advice was never really intended to be applied to top gear cruising. It applied to times where people cruise in 4th gear when they could be cruising in 6th.

Here is the main post I was referring too. He said at 60 he is typically cruising in 3rd and at 80 he is typically in 4th. That is 5200RPM in the first example and 5400 in the second example. 60 in 6th is 2600rpm and 80 in 6th is 3450. That is where I feel wear isn't as negligible.
http://www.ft86club.com/forums/showp...3&postcount=66

I think you got your final numbers flipped around there.

[ermax]

Originally Posted by ImAdopted

im no math wizard and i assure youre better at it than me but i believe you may have mixed up the answers?

Nice find, fixed it. Did a bad cut and paste from the calculator.

[Capt Spaulding]

In reality both should generate the same number of revolutions assuming the clutch isn't slipping and shifts are all made at the same shift points. I suspect your difference is due to my lack of precision in proposing the rpm/mph relationship. Revs per mile in any specific gear are the same regardless of road/engine speed.

edit: ignoring tire growth.

[ermax]

Originally Posted by Capt Spaulding

In reality both should generate the same number of revolutions assuming the clutch isn't slipping and shifts are all made at the same shift points. I suspect your difference is due to my lack of precision in proposing the rpm/mph relationship. Revs per mile in any specific gear are the same regardless of road/engine speed.

edit: ignoring tire growth.

I actually didn’t validate the ratios. I just took your numbers and did the math. Technically 90 is more like 3900 and 80 is closer to 3450. But I was under the impression you were throwing out numbers simply to validate that you and I were under the understanding that it’s total revolutions that you would be interested in.

Edit: revs per mile on the wheels would be the same (excluding tire growth) regardless of speed or gear but we aren’t discussing the revs at the wheels. We are talking about revs of the engine. Or maybe again we are on the same page but confusing each other. Hahaha

[Vic4uf]

I need Engineering Explained to make sense of this argument on a YouTube video for me

[Capt Spaulding]

Originally Posted by ermax

I actually didn’t validate the ratios. I just took your numbers and did the math. Technically 90 is more like 3900 and 80 is closer to 3450. But I was under the impression you were throwing out numbers simply to validate that you and I were under the understanding that it’s total revolutions that you would be interested in.

Edit: revs per mile on the wheels would be the same (excluding tire growth) regardless of speed or gear but we aren’t discussing the revs at the wheels. We are talking about revs of the engine. Or maybe again we are on the same page but confusing each other. Hahaha

Probably. You're correct about the numbers. Just my rough recollection from looking at the tach. It seems to me that on an MT car the linkage between the engine and wheels is pretty close to direct.

[Teseo]

Originally Posted by Vic4uf

I need Engineering Explained to make sense of this argument on a YouTube video for me

What about this

[Grady]

This is a crazy thread, I have not read all of it but think everyone is missing a lot of what goes on in an engine. An engine spinning twice as fast with the same power output will have lower internal cylinder pressures. This will cause less wear on rings and bearings. The statement that an engine spinning 2X the RPM will see 2X the wear is not a true statement.

[ermax]

Originally Posted by Grady

This is a crazy thread, I have not read all of it but think everyone is missing a lot of what goes on in an engine. An engine spinning twice as fast with the same power output will have lower internal cylinder pressures. This will cause less wear on rings and bearings. The statement that an engine spinning 2X the RPM will see 2X the wear is not a true statement.

And if you read the thread you would see that no one said it doubles. But what makes you say the pressures drop? We don’t have a dynamic compression ratio.

[humfrz]

Originally Posted by ermax

And if you read the thread you would see that no one said it doubles. But what makes you say the pressures drop? We don’t have a dynamic compression ratio.

Yes, I'm curious about that too .......??


humfrz

[humfrz]

Originally Posted by Grady

This is a crazy thread, I have not read all of it but think everyone is missing a lot of what goes on in an engine. An engine spinning twice as fast with the same power output will have lower internal cylinder pressures. This will cause less wear on rings and bearings. The statement that an engine spinning 2X the RPM will see 2X the wear is not a true statement.

I will agree that this is a crazy thread .....

About "everyone is missing a lot of what goes on in an engine" ....... I would agree with IF you changed "everyone" to "some people".

About your saying "lower internal cylinder pressures" ........ you totally lost me there (with this engine) .......

I believe that your last bolded statement, shows that maybe you should slow down your speed reading ...... you ain't as young as you used to be .......


humfrz

[Jordanwolf]

Originally Posted by ermax

And if you read the thread you would see that no one said it doubles. But what makes you say the pressures drop? We don’t have a dynamic compression ratio.

Aren't variable compression ratio engines like brand new?

[Ultramaroon]

Originally Posted by Grady

This is a crazy thread, I have not read all of it but think everyone is missing a lot of what goes on in an engine. An engine spinning twice as fast with the same power output will have lower internal cylinder pressures. This will cause less wear on rings and bearings. The statement that an engine spinning 2X the RPM will see 2X the wear is not a true statement.

While that statement fits intuitively, it doesn't fit from a thermodynamic perspective.

All other things being equal including power to the wheels, oil temperature follows engine speed. That means the engine is operating less efficiently. There must be real world consequences to dumping that much more heat through the engine to extract the same mechanical power. I just don't know how to correlate a higher operating temperature to a higher rate of engine wear.