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#1
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Best time to shift?....
hello,
I've been wondering this for quite a while. I've got a 99 SL1, manual trans, and was wondering: when is the best time to shift? Especially concerning gas mileage, is it better to shift around 2,000 rpm's if accelerating slowly? How about the best shift points to save on engine/trans wear? Is there any harm that can be done from shifting early, like around 1,700 - 2000 rpm? what do you think? just wondering...... |
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#2
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Best time to shift?....
I would try to stay in the range where there is plenty of torque. You
reach 90% torque at 1100rpm, and it peaks out at 100% torque at 2400rpm, and decreases after that. These figures are for the 1995 single cam, but your 1999 should be very similar. John Cowart |
#3
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Best time to shift?....
"oh123" > wrote in message oups.com... > hello, > > > I've been wondering this for quite a while. I've got a 99 SL1, manual > trans, and was wondering: when is the best time to shift? Especially > concerning gas mileage, is it better to shift around 2,000 rpm's if > accelerating slowly? How about the best shift points to save on > engine/trans wear? Is there any harm that can be done from shifting > early, like around 1,700 - 2000 rpm? what do you think? > > just wondering...... > IMHO the S1 engine is one of the best examples of a small gas engine optimized for high torque output. It does not need to run fast to make torque which is the force which turns the wheels. When operated at high RPM, any engine will make more power only because power is a function of torque and time or speed, it will actually make less force. Piston speed is the real killer of reciprocating engines and high piston speeds will cause higher piston, ring and cylinder wear and also very increased oil and fuel consumption. In my experience the S1 engine does not really like high loads like hill climbing below about 1700 rpm (depending on weight and grade). At light weight when on the level or when going slightly downhill the S1 will be comfortable and make smooth torque at lower engine speeds. Your engine will tell you if the RPM is too slow by decreasing speed (even with full throttle) and not smooth production of power. It is really a function of power available vs. power required. In general if your engine speed is increasing then the power available is greater than the power required. Excessive low speed operation at high load is often called 'lugging' and can cause damaging loads to the engine lower end. Engine roughness and lugging at too low rpm should be avoided. Since power required increases with speed, the proper minimum shift point also increases with each higher gear. Always start in first gear but shift to second as soon as the vehicle is rolling. Shift to third as soon as the vehicle is accelerating. I can usually get into third gear before crossing an average intersection after starting first in line from a green light. The important thing is the rpm AFTER shifting, it should be slightly higher in each higher gear. This technique is called 'progressive shifting' and was developed with the introduction of high torque diesel engines in heavy trucks. http://en.wikipedia.org/wiki/Progressive_shifting http://www.roadstaronline.com/2005/07/068a0507.asp more at Google search "progressive shifting" http://www.google.ca/search?hl=en&q=...fting%22&meta= Use the lowest engine speed possible depending on load and grade. Listen and feel what your engine is telling you. You can accelerate at very low engine speed when going downhill. When climbing hills allow the engine to slow to 2400 rpm as this is the speed where the engine is making peak torque. It is also the speed of best volumetric efficiency and best fuel economy/unit of power. On a steep grade you want the engine speed to be about 25-2600 rpm AFTER downshift. It takes a while to become comfortable with the techniques required to get maximum efficiency and performance from a modern high torque engine. When computerized fuel injection was introduced in the 1985 Corvette it took a while to convince the racers that they would get lower lap times by operating their engines at a lower rpm where they were producing more torque. Happy landings, |
#4
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Best time to shift?....
O just short shift and forget about all those other gears if your
worried about a couple cents worth of gas. oh123 wrote: > hello, > > > I've been wondering this for quite a while. I've got a 99 SL1, manual > trans, and was wondering: when is the best time to shift? Especially > concerning gas mileage, is it better to shift around 2,000 rpm's if > accelerating slowly? How about the best shift points to save on > engine/trans wear? Is there any harm that can be done from shifting > early, like around 1,700 - 2000 rpm? what do you think? > > just wondering...... > |
#5
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Best time to shift?....
How does this change with the DOHC S2 engines? Should the shift points be
higher due to the higher rpm that max. torque is reached at? Is that just for power or for fuel economy as well? JB "Private" > wrote in message news:OECUf.175254$H%4.63800@pd7tw2no... > > "oh123" > wrote in message > oups.com... >> hello, >> >> >> I've been wondering this for quite a while. I've got a 99 SL1, manual >> trans, and was wondering: when is the best time to shift? Especially >> concerning gas mileage, is it better to shift around 2,000 rpm's if >> accelerating slowly? How about the best shift points to save on >> engine/trans wear? Is there any harm that can be done from shifting >> early, like around 1,700 - 2000 rpm? what do you think? >> >> just wondering...... >> > > IMHO the S1 engine is one of the best examples of a small gas engine > optimized for high torque output. It does not need to run fast to make > torque which is the force which turns the wheels. When operated at high > RPM, any engine will make more power only because power is a function of > torque and time or speed, it will actually make less force. > > Piston speed is the real killer of reciprocating engines and high piston > speeds will cause higher piston, ring and cylinder wear and also very > increased oil and fuel consumption. > > In my experience the S1 engine does not really like high loads like hill > climbing below about 1700 rpm (depending on weight and grade). At light > weight when on the level or when going slightly downhill the S1 will be > comfortable and make smooth torque at lower engine speeds. Your engine > will tell you if the RPM is too slow by decreasing speed (even with full > throttle) and not smooth production of power. It is really a function of > power available vs. power required. In general if your engine speed is > increasing then the power available is greater than the power required. > Excessive low speed operation at high load is often called 'lugging' and > can cause damaging loads to the engine lower end. Engine roughness and > lugging at too low rpm should be avoided. > > Since power required increases with speed, the proper minimum shift point > also increases with each higher gear. Always start in first gear but > shift to second as soon as the vehicle is rolling. Shift to third as soon > as the vehicle is accelerating. I can usually get into third gear before > crossing an average intersection after starting first in line from a green > light. The important thing is the rpm AFTER shifting, it should be > slightly higher in each higher gear. This technique is called > 'progressive shifting' and was developed with the introduction of high > torque diesel engines in heavy trucks. > http://en.wikipedia.org/wiki/Progressive_shifting > http://www.roadstaronline.com/2005/07/068a0507.asp > more at Google search "progressive shifting" > http://www.google.ca/search?hl=en&q=...fting%22&meta= > > Use the lowest engine speed possible depending on load and grade. Listen > and feel what your engine is telling you. You can accelerate at very low > engine speed when going downhill. When climbing hills allow the engine to > slow to 2400 rpm as this is the speed where the engine is making peak > torque. It is also the speed of best volumetric efficiency and best fuel > economy/unit of power. On a steep grade you want the engine speed to be > about 25-2600 rpm AFTER downshift. > > It takes a while to become comfortable with the techniques required to get > maximum efficiency and performance from a modern high torque engine. When > computerized fuel injection was introduced in the 1985 Corvette it took a > while to convince the racers that they would get lower lap times by > operating their engines at a lower rpm where they were producing more > torque. > > Happy landings, > |
#6
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Best time to shift?....
"Jocko" > wrote in message ... > "Private" > wrote in message > news:OECUf.175254$H%4.63800@pd7tw2no... >> >> "oh123" > wrote in message >> oups.com... >>> hello, >>> >>> >>> I've been wondering this for quite a while. I've got a 99 SL1, manual >>> trans, and was wondering: when is the best time to shift? Especially >>> concerning gas mileage, is it better to shift around 2,000 rpm's if >>> accelerating slowly? How about the best shift points to save on >>> engine/trans wear? Is there any harm that can be done from shifting >>> early, like around 1,700 - 2000 rpm? what do you think? >>> >>> just wondering...... >>> >> >> IMHO the S1 engine is one of the best examples of a small gas engine >> optimized for high torque output. It does not need to run fast to make >> torque which is the force which turns the wheels. When operated at high >> RPM, any engine will make more power only because power is a function of >> torque and time or speed, it will actually make less force. >> >> Piston speed is the real killer of reciprocating engines and high piston >> speeds will cause higher piston, ring and cylinder wear and also very >> increased oil and fuel consumption. >> >> In my experience the S1 engine does not really like high loads like hill >> climbing below about 1700 rpm (depending on weight and grade). At light >> weight when on the level or when going slightly downhill the S1 will be >> comfortable and make smooth torque at lower engine speeds. Your engine >> will tell you if the RPM is too slow by decreasing speed (even with full >> throttle) and not smooth production of power. It is really a function of >> power available vs. power required. In general if your engine speed is >> increasing then the power available is greater than the power required. >> Excessive low speed operation at high load is often called 'lugging' and >> can cause damaging loads to the engine lower end. Engine roughness and >> lugging at too low rpm should be avoided. >> >> Since power required increases with speed, the proper minimum shift point >> also increases with each higher gear. Always start in first gear but >> shift to second as soon as the vehicle is rolling. Shift to third as >> soon as the vehicle is accelerating. I can usually get into third gear >> before crossing an average intersection after starting first in line from >> a green light. The important thing is the rpm AFTER shifting, it should >> be slightly higher in each higher gear. This technique is called >> 'progressive shifting' and was developed with the introduction of high >> torque diesel engines in heavy trucks. >> http://en.wikipedia.org/wiki/Progressive_shifting >> http://www.roadstaronline.com/2005/07/068a0507.asp >> more at Google search "progressive shifting" >> http://www.google.ca/search?hl=en&q=...fting%22&meta= >> >> Use the lowest engine speed possible depending on load and grade. Listen >> and feel what your engine is telling you. You can accelerate at very low >> engine speed when going downhill. When climbing hills allow the engine >> to slow to 2400 rpm as this is the speed where the engine is making peak >> torque. It is also the speed of best volumetric efficiency and best fuel >> economy/unit of power. On a steep grade you want the engine speed to be >> about 25-2600 rpm AFTER downshift. >> >> It takes a while to become comfortable with the techniques required to >> get maximum efficiency and performance from a modern high torque engine. >> When computerized fuel injection was introduced in the 1985 Corvette it >> took a while to convince the racers that they would get lower lap times >> by operating their engines at a lower rpm where they were producing more >> torque. >> >> Happy landings, >> > How does this change with the DOHC S2 engines? Should the shift points be > higher due to the higher rpm that max. torque is reached at? Is that just > for power or for fuel economy as well? > > JB > I have little personal knowledge of the Saturn DOHC engines, but have studied their specs and have read the reports of many owners in this NG. I would like to see the graphs of torque, HP and BSFC? (fuel consumption/HP) for both Saturn 1 & 2 cam engines. I have never requested them from Saturn and have not found them on the web. If anyone has a URL, please post it here. Without complete graphs we are limited to the published specs. Usually, mfgs post and advertise the highest peak torque and HP amounts and the RPM where these occur. Saturn reports 114 ft/lbs @ 2400 RPM for both the S1 & S2, they also report 122 ft/lbs @ 4800 RPM for the S2. This is a 7% increase in torque for a doubling of engine speed. A twin cam 4 valve engine will usually have better breathing ability which may result in the best volumetric efficiency occurring at a higher engine speed, the speed of best torque is usually also the speed of best volumetric efficiency. Compared with the 1 cam/2valve engine, we would expect the 2 cam/4 valve engine to maintain higher volumetric efficiency as the engine speed increases. After doing the math this will result in the S2 having more HP than the S1 and for the peak HP to occur at a higher speed than the S1. The lighter valves and lack of rocker arms will also raise the red line engine speed. The S1 has a HP peak of 100HP @ 5000 RPM, the better breathing of the S2 results in a peak HP of 124 HP @ 5600 RPM. When you are racing you may find the best performance by keeping the engine close to its power peak and since the S2 reports peak torque @ 4800 RPM and peak HP at 5600 I would expect the best performance to be found by operating close to the 4800-5600 RPM range. However, I would also expect engine wear (particularly ring and timing chain) to be ~4x as great at this speed as compared with operation at 2400 RPM. I suspect that this is the cause of the many reports of Saturn oil burning and timing chain problems. I would also expect the engine to operate hotter and burn 2x as much fuel. High speed operation is also harder on the transmission and clutch and will also increase wear on engine driven accessories like coolant and power steering pumps and alternators and idler pulleys and serpentine belt and tensioner. This kind of 'spirited' driving also usually results in decreased tire and brake life. The S1 is also 70 lbs lighter and has a much higher final drive ratio which lowers engine speed at cruise. The S1 cruises 65 mph (110 kph) @ 2400 RPM which IMHO is the ideal and also great engineering. I do not know what the S2 RPM would be at this speed (one of the S2 drivers here could post the answer) but based on math (without consideration of tire size), I am going to estimate 2900 RPM. This higher cruise RPM means higher wear and fuel consumption /mi. Readers here may note that I greatly prefer the S1 and am not a big fan of the S2. The most important measure of an engine is the amount of torque it can produce. The easiest way for a designer to increase torque is to increase displacement. I am old enough to think that 'the only substitute for cubic inches is cubic money.' Superchargers and turbochargers are often used to increase effective displacement and volumetric efficiency, but also often reduce engine life and increase maintenance costs. The most cost effective way to increase performance is to decrease weight and increase displacement. The easiest way to increase engine life and decrease maintenance is to decrease engine speed and load. A modern Corvette cruises 60 mph at ~1700 RPM and IMHO gets very good fuel economy. Just my .02, YMMV |
#7
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Best time to shift?....
Best Time to shift: Simple! Use the Light, Luke!
But it depends on what you are doing. I have an SC2 and I usually shift between 2 or 3 thousand. I also try to drive like I have an egg glued on my gas pedal and I'm trying not to break it! I use my car for work - and I try to get as good milage as I can. If you drive like this your engine will probably last a lot longer. But shifting at lower than 2000 is probably "lugging" your engine - which is actually bad for it. On the other hand. If I'm pulling out into traffic I'll occasionally wind it up until it screams. The redline for the SC2 is around 6,500 rpm. It really starts to create loads of power above 4000 rpm and that's where having 4 valves per cylinder really starts to have an advantage. Someone sated that the single cam engine weighs 70lbs less than the twin cam.... I cannot see how that could be. Maybe 20 or 30 lbs at the most but the cam only weighs about 10 lbs and valves, springs, and rocker arms just dont wiegh all that much. The bottom end on these engines is just the same I think. |
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