D or OD??

[Dominic]

The question ends up at this:

Is the engine more efficient at 2500RPM or 1500RPM?

Once you solve that problem you know which gear to use... because regardless of whether you're doing 40mph in 4th or 40mph in 3rd, the horsepower required is IDENTICAL.

[HAZ-Matt]

<blockquote>quote:</font><hr>Originally posted by Dominic:
The question ends up at this:

Is the engine more efficient at 2500RPM or 1500RPM?

Once you solve that problem you know which gear to use... because regardless of whether you're doing 40mph in 4th or 40mph in 3rd, the horsepower required is IDENTICAL.<hr></blockquote>

But luckily the easiest way to determine that is the amount of throttle you have to apply to maintain that speed in each gear. It's is like no throttle in 4th, and a little more in 3rd.

[Camarorulz]

<blockquote>quote:</font><hr>Originally posted by Dominic:
Once you solve that problem you know which gear to use... because regardless of whether you're doing 40mph in 4th or 40mph in 3rd, the horsepower required is IDENTICAL.

<hr></blockquote>

Let me give the example of a bicycle to simplify things: say you're trying to maintain a speed of 15mph on your bicycle in a high gear. You then switch to a low gear and try to maintain the same speed. Obviously you will be pedalling faster; the question is: are you putting out more power? Well, the same amount of work is being done, because the effect is the same: you maintain a speed of 15mph. However, you are exerting more effort to maintain the exact same speed, you are wearing your legs out more. The same concept applies to an engine. Regardless of how much hp an engine puts out at any rpm, the higher the rpm, the more gas it burns, and the more horsepower it makes to a certain point. If you're doing 40mph in 4th as opposed to 40mph in 3rd, the horsepower required is not the exact same, as Dominic said, because a higher rpm is required. Saying the hp requirement is the same in this example is like saying that the engine produces the exact same amount of horsepower at 2500 rpm as it does at 3500 rpm, which is not necessarily true. That is not the way many power curves work. The effect is the same, but the power required to achieve a given speed is different in varying gears. That's why gears help our cars so much in particular. A certain car may have an advantage over me in power/weight ratio, but if his gearing is extremely high, then the power that is required for him to accelerate is much less than mine. As a result, I win in a 1/4 mile race.

To answer the original question: in general, my car does not shift into 4th until 45mph in OD. This means that for my car, it really doesn't matter which one I have it in under 45mph. When I'm on the highway cruising at 70 the rpm is around 3300 in D, but 2300 in OD. Throttle response is therefore better in D on the highway because the rpm are already up, closer to peak torque and horsepower.

[ March 05, 2003: Message edited by: Camarorulz ]</p>

[HAZ-Matt]

<blockquote>quote:</font><hr>Originally posted by Camarorulz:
If you're doing 40mph in 4th as opposed to 40mph in 3rd, the horsepower required is not the exact same, as Dominic said, because a higher rpm is required. Saying the hp requirement is the same in this example is like saying that the engine produces the exact same amount of horsepower at 2500 rpm as it does at 3500 rpm, which is not necessarily true. That is not the way many power curves work. The effect is the same, but the power required to achieve a given speed is different in varying gears.<hr></blockquote>

The power required to maintain 45 mph is the same in 3rd or 4th gear... power at the wheels that is. [img]smile.gif[/img]

When you are in 3rd gear, you will need less power (torque really) applied from the engine to the transmission in order to maintain 45 due to the extra torque multiplication you get from 3rd being the lower gear. However, it takes a little more power to keep the pistons and valves moving in 3rd gear at 45 than it does in 4th gear, because the components are moving faster.

EDIT: Forgot the important part. The sum of the two amounts of power needed to drive the transmission and the engine tells you which gear to use. That power should proportional to throttle. Choose the gear in which it is lower.

[ March 05, 2003: Message edited by: HAZ-Matt ]</p>

[Camarorulz]

<blockquote>quote:</font><hr>Originally posted by HAZ-Matt:
The power required to maintain 45 mph is the same in 3rd or 4th gear... power at the wheels that is.

When you are in 3rd gear, you will need less power (torque really) applied from the engine to the transmission in order to maintain 45 due to the extra torque multiplication you get from 3rd being the lower gear. However, it takes a little more power to keep the pistons and valves moving in 3rd gear at 45 than it does in 4th gear, because the components are moving faster.

EDIT: Forgot the important part. The sum of the two amounts of power needed to drive the transmission and the engine tells you which gear to use. That power should proportional to throttle. Choose the gear in which it is lower.

<hr></blockquote>

You make an important point about the difference between power at the wheels and power at the flywheel. I was thinking specifically about flywheel horsepower related to gas mileage, efficiency and engine speed.

You're saying to choose the gear in which throttle position is lower, or the amount of power needed to keep the car at a given speed is lower?

[HAZ-Matt]

I know what you meant, and I agreed, but I just wanted to state that technically you and Dominic were both correct, but about power at different locations [img]smile.gif[/img]

I'm saying both... they should be proportional. I'm assuming throttle is a useful approximation of the power, since all the power comes from the amount of fuel you burn. But if I am wrong about that relationship, you want the one where the power is lowest.

[ March 06, 2003: Message edited by: HAZ-Matt ]</p>

[Takumi Fujiwara]

In the words of Jim morrison, I like the way i said it better. You guys are 2 funny. More power less power, I dont know about ur little cars, But My Torque curve Is flat around 1800rpm-5000rpm not much change. The RPM doesnt mean much of anything when it comes to fuel milage. I like the Bike reference though, but I think ur looking at it wrong. On a bike, think of it this way. Your flying @ 40mph on a bike and u put it in 1st- U peddle and look stupid but if u arnt above your legs redline ( as in going so fast that your gonna fall off the bike ) It takes literally no effort to move the flywheel. just inertia. but think of it this way, u are not moving and u put ur bike in 30th gear and u try to move now we think physics, Work = ForceXDistance ( thats right yeah ? ) You Are putting a TON of force on the peddle- but getting no distance. So your force is wasted: Now back to what I was saying about how much accelorator you give it, if your car is in 4th or a high gear 3rd or even 2nd ( lets say u are going a speed to which u can be in a higher gear ) and u press the pedal down, if the car doesnt downshift u are going to be doing to be wasting your Force. Which is your gas. Pressing the engine to burn gas with so much resistance that u dont go ( this is extreme but think about it ) Dont think of it as RPM, Because ur throttle is cable driven, if u floor it the throttle opens and gas goes in even if you cant use it. =) So just think about how much u are pushin on the Gas when ur driving and try backing out of it and up shifting into higher gear, less gas same accell usually as long as the car stays 1:1 ( in 3rd ) or drive for u camaro kids. And even if Im wrong which I think I am, I still think that my FireBird doesnt have an OD. 123D 4 Life! [img]graemlins/burnout.gif[/img]

[Camarorulz]

I think the Firebirds may have automatic overdrive: you don't have the option to put it in regular drive or overdrive. I could be wrong, it may not have overdrive at all, but I know many GM cars do have automatic OD.

More rpm = higher engine speed = more gas being burnt. Plain and simple. I was using the bike analogy just to simplify it a bit (its a lot simpler than our cars, granted, but the same principle still applies.) Maybe on a bike I can put it in too high of a gear to be able to pedal efficiently, but our cars are designed to choose the rpm that is most efficient for speed and throttle position. If the car shifted into a gear that is too high, the engine would bog down and die. Obviously you can do this with a manual transmission, but not an auto. That's why overdrive is better. It shifts to the most efficient rpm to maintain your speed and give you good gas mileage. The GM engineers say your gas mileage is improved in overdrive, and you can believe them if not anyone else. Contrary to popular opinion, engineers at General Motors are not just a collection of dumb shmucks that barely managed to graduate from Country Hick University.

[HAZ-Matt]

<blockquote>quote:</font><hr>Originally posted by Camarorulz:
I think the Firebirds may have automatic overdrive: you don't have the option to put it in regular drive or overdrive. I could be wrong, it may not have overdrive at all, but I know many GM cars do have automatic OD.<hr></blockquote>

Way back in the day, cars had overdrives that were separate from the forward gears in the transmission. Now they are simply incorporated into the transmission. 4th gear is overdrive in all automatic F-bodies. It is overdrive because the ratio is less than one (0.70). The T-5 has an overdrive as well... 5th gear (0.74). The T-56 has two overdrives... 5th (0.75) and 6th (0.50).

[ March 08, 2003: Message edited by: HAZ-Matt ]</p>

[Camarorulz]

I always thought it was a 4-spd with an extra overdrive gear as well. Hmm, learn something new every day [img]graemlins/thumbsup.gif[/img]

[Texas4ever]

D or OD....?? Dramatic (D) or Overly Dramatic (OD)...!!! Lots of tying, very little info... I say lock this one and let's move on to something with some substance....!!!

[img]graemlins/omg.gif[/img] [img]graemlins/omg.gif[/img] [img]graemlins/omg.gif[/img]

[Camarorulz]

<blockquote>quote:</font><hr>Originally posted by Texas4ever:
D or OD....?? Dramatic (D) or Overly Dramatic (OD)...!!! Lots of tying, very little info... I say lock this one and let's move on to something with some substance....!!!<hr></blockquote>


Ummm, yeah whatever, this thread basically ended like a week ago.

[3.4 slow to go]

<blockquote>quote:</font><hr>I always thought it was a 4-spd with an extra overdrive gear as well. Hmm, learn something new every day <hr></blockquote>

same difference as coke, pop, & soda.
what does the owners manual say?

My 85 owners manual says (for the 700-R4):

OD: is the overdrive gear range position, used for most normal driving. This position lets te trany choose the appropriate gear for load & driving conditions. Also, the tranny is designed to shift automatically into overdrive (from drive) when car reaches a steady cruising spped of 40mph or faster.

D: (third gear)same as D on cars w/o overdrive feature & should be used when increased performance is needed, such as hilly roads or towing. It prevents the engine from shifting into OD& provides more engine braking than OD.....Also shift into D if you notice what feels like excesive shifting between ranges.
Shifting from OD toD will improve this.

[ March 10, 2003: Message edited by: 3.4 slow to go ]</p>