Re: cruise rpm: how low is too low?
[Re: Twostick]
#819374
10/09/10 05:01 PM
10/09/10 05:01 PM
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Joined: Jan 2005
Posts: 12,423 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,423
Kalispell Mt.
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Quote:
The vast majority of pumping loss is caused by compression followed by friction. To even consider vacuum resisting the piston going down during intake as a significant loss factor just defies logic and common sense IMHO. I'm not saying its not part of the loss, just not a significant part.
Kevin
Ever down shift into low and see how fast you car slows down? That is vaccume right there and that should give you an idea how much work the engine is doing to overcome that vaccume. Remember that a vaccume gauge plugged in the bottom of the carb is gonna show a lot less vaccume than than the top of the piston sees.
Any piston driven engine is very inefficent because of the great loss due to vaccume, compression, and friction. Only a small part of the energy the combustion process produces ends up at the end of the crank and an even smaller amount at the wheels. You can not do a lot about those 3 things, you can run a vaccume pump to increase vaccume under the piston to minimize the effects of vaccume above the piston but a vaccume pump still takes enery to operate, you can run all kinds of friction reduceing tricks like thin rings, narrow bearings, lightweight synthetic oil... but there is still gonna be a lot of friction loss, and to combat compression, well there ain't a lot you can do because reduceing the compression reduces the efficency of the tightly packed fuel burning and reduces the expansion ratio, a high expansion ratio can achived by increasing the compression ratio and by opening the ex valve later, you can reduce the dynamic compression ratio by closeing the intake valve later and this will reduce the compression loss and keep the octane requirment lower but you lose some energy pumping the mix back up the intake tract, you also lose TQ but some of that is gained back by the higher compression/expansion ratios. There are just too many losses of energy in a piston engine to make it very efficent and vaccume is definately high on the list.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819376
10/10/10 01:27 AM
10/10/10 01:27 AM
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Joined: Jan 2005
Posts: 12,423 Kalispell Mt.
HotRodDave
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I Live Here
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Kalispell Mt.
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If the throttle is closed, or mostly closed like it is at cruise then there is nothing or very little to compress, therefor it is working against the vaccume. A gasoline engine has vaccume brakeing and diesel engines have compression brakeing.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819380
10/10/10 02:05 PM
10/10/10 02:05 PM
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Joined: Jan 2005
Posts: 12,423 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,423
Kalispell Mt.
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How can you have compression if there is virtually nothing going into the cylinders? Ever do a compression test and see how much differance there is with the throttle open VS closed? And that is only mabey 50 RPM not 2000 or so.
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819382
10/10/10 02:33 PM
10/10/10 02:33 PM
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Joined: Jan 2005
Posts: 12,423 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
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Kalispell Mt.
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Again I say, hook up a vaccume gauge, go for a drive then report back what your vaccume was at cruise. My 318 held 17 inches at 70 mph cruise. A piston area of 10.65716 pulling 17 inches of vaccume is 181.1717 inches of vaccume on each cylinder X 8 almost 1500 inches vaccume being pulled every other revolution of the crank, that is far from nothing. Again I am not saying the fight against compression is insignificant but the vaccume loss is huge also. Compressing air that is under 17plus inches vaccume into an area 1/10 the size it was is not hard when you have 14 PSI (atmospheric preasure)or so pushing the bottom of the piston up. Someone a little more mathmatically gifted than me can convert it to LBS for you so you can see how much it really takes to pull those pistons down, you WILL be suprised. I know when someone showed me I was totally shocked
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: stumpy]
#819384
10/20/10 02:31 AM
10/20/10 02:31 AM
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Joined: Jan 2005
Posts: 12,423 Kalispell Mt.
HotRodDave
I Live Here
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I Live Here
Joined: Jan 2005
Posts: 12,423
Kalispell Mt.
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Even if the compressed mix did not light off that preassure that resisted the pistons upward movement will still push it down on the power stroke, this is why a Diesel engine will not slow you down much going down a hill unless you have a jake brake, a diesel has very high compression and NO SIGNIFICANT VACCUME in the intake like a gasoline engine. I came up with some experiments for you to do stumpy. Get in your car, prefferably manual trans, go to the top of a big hill and let your car start coasting down the hill, engine off with the clutch in and the trans in 2nd or 3rd then once you get rolling real good let the clutch out, you will feel the engine rev up and the car will begin slowing drastically, now with the engine still off stomp the gas so the vaccume goes away and full compression can now be achieved, you will notice the car will not be brakeing near as hard as it did with the throttle closed and very little compression but high vaccume. That should prove beyond a shadow of doubt to you that vaccume has a much larger loss of energy than compression. Hope this helps
I am not causing global warming, I am just trying to hold off a impending Ice Age!
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Re: cruise rpm: how low is too low?
[Re: feets]
#819385
10/20/10 05:28 AM
10/20/10 05:28 AM
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Joined: Aug 2005
Posts: 403 30 miles west of EuroDisney
fbernard
mopar
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mopar
Joined: Aug 2005
Posts: 403
30 miles west of EuroDisney
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I regularly drive between 1600 and 2000 RPM now. Used to be hard to maintain anything under 1800 in 5th gear before (I have a tremec with .64 5th gear and 3.55 gears), but now I have a properly curved distributor, lots of initial advance [24°], and absolutely no knocking in the driveline down to almost idle (I can coast to 1200 and slowly ramp up again). Before this distributor, I could not maintain 90 km/h (56mph, the posted limit on backroads here) in 5th, I had to stay in 4th.
Of course, my car is much lighter, around 3000 pounds.
I drive to and from racetracks, usually between 2000 and 2500 RPM, at which point I'm slightly above the highway speed limit. I get 14 to 16 mpg.
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Re: cruise rpm: how low is too low?
[Re: 360view]
#819388
10/20/10 08:16 AM
10/20/10 08:16 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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You can get HP and MPG estimations for the Imperial at different speeds on the road from here: http://www.bgsoflex.com/mpg.htmlI would guess an Imperial would have a Cd of approximately 0.50 and a frontal area of approximately 25 square feet. Taylor is using lbs of fuel divided by horsepower-hours in his graph, getting a best of about 0.5 lbs/hp-hr in the little circle in the upper left hand side. Bowling and Grippo want to use a slightly different BSFC in the units of gallons of fuel divided by horsepower-hours. Most gasoline blends are between 6.5 and 7.5 lbs per gallon density, so maybe use 7 lbs/gallon as a guess unless you want to weigh your local gasoline.
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Re: cruise rpm: how low is too low?
[Re: 360view]
#819389
10/21/10 09:12 AM
10/21/10 09:12 AM
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Joined: Jan 2006
Posts: 8,162 USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Possible Correction: a Sept 1981 article by Jim Dunne in Popular Science magazine titled 'Low Drag Cars' lists the Chrysler Imperial of that year as having a Aerodynamic Cd of 0.41 pretty good... considering that the Porsche 924 of that year was 0.34 and a Corvette 0.45 The Eldorado of 1981 is listed as a whopping 0.55 shape at: http://www.oldcarmemories.com/content/view/51/114/sample quote "Interesting to note the aerodynamics on the 1981 Imperial was so good for its era that some NASCAR teams used the Imperial as race cars."
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