Hypothetical overdrive question

[willowbilly3]

Maybe we could convince someone with a little land and sea dyno to test some rear ends with a fixed load and see how much difference in drag there is from say 4.11 to a 2.84. And also do the same test between 1:1 and .62 overdrive in the same gear box. Or better yet test the trannys and rear ends as a unit. Would they cancel out? At this point I would still stick with my original thought, transferring the power path through the overdrive gearset would have more parasitic drag than the lower numerically rear end would add. I guess until someone comes up with some sort of real test data it will just be an opinion on any front.
I did read an interesting dyno article on parasitic loss a few years back, in CarCraft I believe, and I was intrigued by the fact the the very thing that makes a 9 inch strong (low pinion placement) also makes it one of the most parasitic rear ends. They made no mention of taller gears having more parasitic loss though.

[Hawkrod]

Hawk,
I think you are missing that we are talking about maintaining the same final drive ratios, but using taller rear gear ratios and shorter gears in the transmission. We all know that you need lower (numerically higher) ratios for performance. The article talks about changing rear end gears, but not changing the transmission gears at the same time. In the article they are changing the final drive ratio. Willowbilly is talking about keeping the same final drive ratio, but using higher (lower numeric) gears in the rear and lower (higher numeric) gears in the transmission, thus maintaining the same final drive ratio, but not having an overdrive top gear, but a 1:1 top gear.

No, I did not miss anything, I am talking about the same thing. I think you missed the point of that specific post. willowbilly3 and I are discussing the issue of the fact that by changing rear ratio to a numerically lower gear requires the use of more power and that is why the automotive manufacturers do not do it. Modern drivelines behave much better than the Model T, but manufacturers did go to numerically lower ratios in the 70's and 2.50 or lower was not unusual. The problem is that a car with a 2.75 final ratio and a 1-1 trans will not do as well as if that car had a 3.23 and .85-1 overdrive for nearly the same final ratio. The car with the 3.23 and overdrive will usually perform better on fuel consumption, emissions and performance. That is the point of the discussion and what is truly important when you get to the bottom line. Hawkrod

[FORDification]

Excellent discussion and information!

[willowbilly3]

Hawk,
I think you are missing that we are talking about maintaining the same final drive ratios, but using taller rear gear ratios and shorter gears in the transmission. We all know that you need lower (numerically higher) ratios for performance. The article talks about changing rear end gears, but not changing the transmission gears at the same time. In the article they are changing the final drive ratio. Willowbilly is talking about keeping the same final drive ratio, but using higher (lower numeric) gears in the rear and lower (higher numeric) gears in the transmission, thus maintaining the same final drive ratio, but not having an overdrive top gear, but a 1:1 top gear.

No, I did not miss anything, I am talking about the same thing. I think you missed the point of that specific post. willowbilly3 and I are discussing the issue of the fact that by changing rear ratio to a numerically lower gear requires the use of more power and that is why the automotive manufacturers do not do it. Modern drivelines behave much better than the Model T, but manufacturers did go to numerically lower ratios in the 70's and 2.50 or lower was not unusual. The problem is that a car with a 2.75 final ratio and a 1-1 trans will not do as well as if that car had a 3.23 and .85-1 overdrive for nearly the same final ratio. The car with the 3.23 and overdrive will usually perform better on fuel consumption, emissions and performance. That is the point of the discussion and what is truly important when you get to the bottom line. Hawkrod

Hawkrod, the last half of that statement, exactly why is that true? I'm not saying I don't believe it but inquiring minds need to know how the same top gear overall ratio would effect emissions and fuel consumption by arriving at it in a slightly different way? What are the mechanical principles involved?

[Hawkrod]

Hawk,
I think you are missing that we are talking about maintaining the same final drive ratios, but using taller rear gear ratios and shorter gears in the transmission. We all know that you need lower (numerically higher) ratios for performance. The article talks about changing rear end gears, but not changing the transmission gears at the same time. In the article they are changing the final drive ratio. Willowbilly is talking about keeping the same final drive ratio, but using higher (lower numeric) gears in the rear and lower (higher numeric) gears in the transmission, thus maintaining the same final drive ratio, but not having an overdrive top gear, but a 1:1 top gear.

No, I did not miss anything, I am talking about the same thing. I think you missed the point of that specific post. willowbilly3 and I are discussing the issue of the fact that by changing rear ratio to a numerically lower gear requires the use of more power and that is why the automotive manufacturers do not do it. Modern drivelines behave much better than the Model T, but manufacturers did go to numerically lower ratios in the 70's and 2.50 or lower was not unusual. The problem is that a car with a 2.75 final ratio and a 1-1 trans will not do as well as if that car had a 3.23 and .85-1 overdrive for nearly the same final ratio. The car with the 3.23 and overdrive will usually perform better on fuel consumption, emissions and performance. That is the point of the discussion and what is truly important when you get to the bottom line. Hawkrod

Hawkrod, the last half of that statement, exactly why is that true? I'm not saying I don't believe it but inquiring minds need to know how the same top gear overall ratio would effect emissions and fuel consumption by arriving at it in a slightly different way? What are the mechanical principles involved?

Torque multiplication. Too much is useless and breaks stuff and too little uses power. Hawkrod

[FLATBEDFORD]

So, how does the big truck two speed rear fall into this. In this case the overdrive is in the rear gear, not in the transmission. Isn't that what we are talking about here?

[willowbilly3]

No, I did not miss anything, I am talking about the same thing. I think you missed the point of that specific post. willowbilly3 and I are discussing the issue of the fact that by changing rear ratio to a numerically lower gear requires the use of more power and that is why the automotive manufacturers do not do it. Modern drivelines behave much better than the Model T, but manufacturers did go to numerically lower ratios in the 70's and 2.50 or lower was not unusual. The problem is that a car with a 2.75 final ratio and a 1-1 trans will not do as well as if that car had a 3.23 and .85-1 overdrive for nearly the same final ratio. The car with the 3.23 and overdrive will usually perform better on fuel consumption, emissions and performance. That is the point of the discussion and what is truly important when you get to the bottom line. Hawkrod[/quote]


Hawkrod, the last half of that statement, exactly why is that true? I'm not saying I don't believe it but inquiring minds need to know how the same top gear overall ratio would effect emissions and fuel consumption by arriving at it in a slightly different way? What are the mechanical principles involved?[/quote]
Torque multiplication. Too much is useless and breaks stuff and too little uses power. Hawkrod[/quote]

I have typed and retyped a response to that. I don't want to be rude but that reply just doesn't answer any of the questions I asked you to address. Please tell us all how having the same combined transmission and rear end ratios in top gear but a achieved in the slightly different manner ,as we have discussed thoroughly, would have any affect at all on mileage or emissions. What would most likely break due to the torque multiplication? And to keep it simple lets assume a 6000 pound pickup with 300 horse V8 that can produce a maximum of 325 ft lbs of torque, all at the flywheel for arguments sake. You can plug in your own rpm torque curve as a constant. One combination is a .80 overdrive and a 4.11 and the other is a direct 1:1 with a 3.28 gear. As previously established the other gears would each have the same combined ratio in every given gear for both hypothetical transmissions, but first let's just address the mileage and emissions in the top gear. What would be the forces at play that would cause the mileage to change? And what exactly would cause the emissions to change?

[willowbilly3]

I have had this thread going on 3 different forums and today I got the first answer with the kind of substance I had hoped for.



A typical helical gearset is 98-99% efficient. In a direct drive top gear there are none, in an OD top gear there are two. For the sake of discussion, assume 99% for the input shaft gearset and 98% for the OD gearset. These assumptions are supportable given that the input set is typically quite robust and has a relatively low reduction whereas the OD gearset is typically rather weak and is being used inefficiently as a speed increaser.

A hypoid bevel gearset is 92-96% efficient; efficiency decreases as ratio increases. For the sake of discussion assume 95% for a 3.08 gearset and 94% for a 3.55. These are supportable assumptions given the range of 1.5:1 to 6:1 normally encountered.

A Ford 300 at 2000 RPM produces approx. 130 HP under full load and WOT at a BSFC of approx. 0.5 lbs/HP-hour. At cruise under some unspecified but significant load assume for arguments sake that it produces half the HP (65HP) and that the BSFC is 10% worse or 0.55 lbs/HP-hour. Assume further that the gearing is such that this occurs exactly at 60 MPH.

If this engine were to be coupled to the direct drive transmission with the numerically lower rear end, it would deliver:

65 x 0.95 = 61.75 HP to the rear wheels.

It would consume:

65 x 0.55 = 35.75 lbs of fuel per hour which is also 35.75 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.72 gallons per 60 miles or 10.49 MPG, arguably what one could expect with a heavy load.

If this same engine were to be coupled to an OD tranny with a numerically higher rear end, to produce the same 61.75 HP at the rear wheels, it would need to produce:

61.75 HP / (.94 x .98 x .99) = 67.7HP at the flywheel

That would result in a consumption of:

67.7 x 0.55 = 37.23 lbs of fuel per hour which is also 37.23 lbs of fuel per 60 miles

At 6.25 lbs per gallon this would be:

5.96 gallons per 60 miles or 10.06 MPG.

So running at highway speeds under a significant load, the difference would be 0.43 MPG. At lighter loads it would be proportionally less, meaning about half as much difference at cruise running empty or about .20-25 MPG.

This treats only the effect of transmission losses and ignores all the other sources of inefficiencies such as bearing losses, seal drag aero and rolling resistance, etc. assuming that they are equal in both cases.

Obviously YMMV both literally and figuratively.

[Fordnatic]

It may not have much relevance here-but has been said before, the torque multiplication adds stress to components down the line. For example, many tractors, military vehicles and other heavy equipment have final drives. These are a gearbox that the wheel mounts to, thereby applying the extra torque multiplication to the wheel/tire assembly only. With a numerically low gear in the diff, the torque multiplication is applied to everything behind the transmission, with a numerically high diff it is only applied to the axles out.

[willowbilly3]

It may not have much relevance here-but has been said before, the torque multiplication adds stress to components down the line. For example, many tractors, military vehicles and other heavy equipment have final drives. These are a gearbox that the wheel mounts to, thereby applying the extra torque multiplication to the wheel/tire assembly only. With a numerically low gear in the diff, the torque multiplication is applied to everything behind the transmission, with a numerically high diff it is only applied to the axles out.

They also do that to get more reduction with a smaller center section. When you start getting into really deep differential gears the diff gets huge and really close to the ground.

I also asked the same poster this question and his answer.


willowbilly3 wrote:

Was it a typo that one combination used flywheel hp and the other rear wheel hp?


No, it was intentional. In the first case I start with 65 flywheel HP and after factoring in driveline inefficiencies, I ended up with 61.75 RWHP. In the second case I started with the same 61.75 RWHP and factoring for the inefficiencies ended up with 67.7 HP at the flywheel, meaning that the OD tranny and higher reduction rear end consume 2.7 more HP under the load conditions stated

Quote:
I have this discussion going elsewhere and have a guy who claims that lower numerical differentials are less efficient that higher numerically ones. Can you elaborate on that a little more? Thank you


In general for any gearset, the less the reduction the greater the efficiency. He may have a less technical understanding of efficiency (e = work out/work in) or he may simply be confusing it with torque multiplication which is what numerically higher ratios are all about.

[Hawkrod]

willowbilly3, I think you are over thinking this. At a given constant speed on flat level ground there would be no significant difference between the two combinations we are talking about once the vehicle is up to speed. The problem is a vehicle is virtually never at a constant. The vehicle has to start and stop in traffic and has to overcome obstacles such as hills and other traffic. Because of this a vehicle is not designed for a constant., it is designed with all the variables. The Torque multiplication comes in to play every time the vehicle experiences a load change such as accelerating or climbing a hill. It is easier for the engine to turn the 3.25 in my example than it is to turn the 2.75. If the engine has enough power the change is not noticeable but on smaller and lighter fuel efficient designs it can be a significant issue.

FLATBEDFORD, a two speed truck rear is just like a transmission, there is still only one set of ring and pinnion gears but the case also contains a set of straight through drive gears. This basically is the same as having the trans in one gear or another and has nothing to do with the rear end except for the fact that the gears happen to be placed there rather than in the trans (although on some the ratio change is done in the driveshaft plane while in others it is done in the axle plane, either way it is still teh same result).

Hawkrod

[willowbilly3]

Hawk, now you are back to comparing 2 different ratios again and we already established over and over that we would be comparing THE SAME ratios arrived at with different trans/diff combos. You keep dancing and side stepping around the direct questions.At this point I have to believe that you are in over your head on this one and really don't have any data to back up your position. So for now I will have to go with the guy who provided me with numbers, drag percentages and some examples of how the differences were measured. He might be wrong but was the only person on three different forums to give any real solid information. You know, it's ok to be wrong, everone is once in a while.

[Hawkrod]

Hawk, now you are back to comparing 2 different ratios again and we already established over and over that we would be comparing THE SAME ratios arrived at with different trans/diff combos. You keep dancing and side stepping around the direct questions.At this point I have to believe that you are in over your head on this one and really don't have any data to back up your position. So for now I will have to go with the guy who provided me with numbers, drag percentages and some examples of how the differences were measured. He might be wrong but was the only person on three different forums to give any real solid information. You know, it's ok to be wrong, everone is once in a while.

I am sorry, I did not realize I was side stepping the question. I THOUGHT I had answered the questions. I thought the important question was "why do we use a high numerical rear ratio and an overdrive transmission versus a low numerical rear ratio and a direct drive transmission?". Correct me if I am wrong but here is the question again in your own words:

Snip My query is why the automakers have overdrive. Please read on before you respond. First of all I want to make sure you understand when I say "overall" ratio I mean the combined transmission and rear end ratio in any given gear. Lets use a manual 5 speed with .80 overdrive with a 3.55 rear end. So with my improved transmission you would have the same overall ratio in each of the 5 speeds but 5th would be direct or 1:1 and we would use a 2.84 rear end (do the math, that's the same final drive rpm as the .80 overdrive and 3.55) Would it not be more efficient to have a direct power path in top gear instead of the overdrive that has to route the power through 2 gearsets? And also isn't the higher rear end ratio more efficient with less parasitic drag? In an age where automakers will trade their offspring for ounces off the weight or minuscule reductions in drag it would seem they missed something here.

If that was not what this discussion was about then I am wrong as all of my responses were aimed at answering those questions. We use an overdrive and a high numerical rear ratio as torque multiplication allows the engine to operate more efficiently and thus reduces emissions and fuel consumption and the vehicle is more "powerful". I also noted that drag in the trans is negligible due to the direction of power flow versus load related rear end ratio. You are assuming that drag in the rear end and trans are the issues and they really are not but load is and a high numerical ratio sees less load due to torque multiplication. After you asked this you started citing irrelevant facts regarding constants but because vehicle does not operate that way it was not an issue I felt had any value to discuss. The responses you got from the other person are certainly interesting but they were not really relevant to the questions I was answering because they are based on a constant and vehicles do not operate that way and are not designed based on that. I guess I missed the point of this discussion completely as what I read in your post and what you seem to be looking for are very different. In the end, if your vehicle always operates at a specific speed with a specific load and you never have to accelerate or stop and wind, terrain, and other traffic do not exist then an overdrive is senseless and using a higher numerical ratio and a 1-1 trans makes more sense (but then, why bother having a transmission at all?). If you expect to drive a vehicle efficiently on planet Earth with everybody else then I think I did answer the questions. Sorry if I am wrong, I will bow out as it is obvious, as you note, I missed the boat and did not understand what you asked, Hawkrod

[willowbilly3]

Well, since nobody else has anything to say and it's obvious that getting a straight answer from you Hawk, isn't going to happen. I am just going to let the whole thing go. I don't know why you have to turn a simple discussion into an exercise in futility.
I think the post from a different forum is the one I will go with. It was in a format that directly addresses the question asked and leaves no confusion as to what he said. ttfn

[Hawkrod]

Well, since nobody else has anything to say and it's obvious that getting a straight answer from you Hawk, isn't going to happen. I am just going to let the whole thing go. I don't know why you have to turn a simple discussion into an exercise in futility.
I think the post from a different forum is the one I will go with. It was in a format that directly addresses the question asked and leaves no confusion as to what he said. ttfn

You are correct, I cannot possibly give a straight answer to an unstated question that is different than the one asked. I apologize for not having any idea about what you "meant" when you asked the question and for specifically answering the question as YOU asked. It is hard to give a straight answer for what you "meant" rather than what you wrote. It actually seems as though you were determined to get the answer you wanted even if it did not fit the question you asked. Perhaps I am wrong but that is how I am seeing it. After review, I am also somewhat disappointed that you do not feel that I gave a straight answer to the question you asked as it does appear to me that I did respond to what you asked. Please feel to point out what part of the original question I did not answer. The sad truth is you do not seem to think I answered the question as asked and I think you were intentionally finding fault with the correct answer because it was mine not because it was wrong. You seem quite comfortable in calling my response an issue when I see it as complete and reasonably correct but your responses have been disjointed and out of context. It really is hard to offer help when you clearly seem to want to find fault and attack where it is not appropriate. Was this entire thread posted just to draw me in to try and attack me? It sure looks like that in retrospect. Hawkrod