700r4 upgrades

[HotRodPC]

I see the point you're making Christian but many times lower rpms doesn't always mean the best efficiency. Keep in mind, the taller the rear gear is, the more strain you're putting on the rest of the drivetrain too. The more strain you put on the drivetrain does not always net best efficency.

About the best example of this I can think of is using a 10 speed bike. If you put the bike in a lower gear like 4.10s, you pedal faster but it pedals easy and you don't go near as far since you're actually moving at a lower speed. Then using a high gear, like 2.73s, you pedal much slower keeping your rpms down, but its hard to pedal and more strain on yourelf. This is the same exact principle as a vehicle rear gear and this is why lower rpm is NOT most efficient, and its also more strain on a 700r4 in both the hard parts and the clutch packs especially the 3-4 clutch packs that are notorious for burning up. In most cases, due to to tall of a gear ratio, or big tires.
I like to go low gears and have an OD to compensate the low gears to be able to cruise 75-80 at a reasonable rpm.

 

[Christian Nelson]

I see the point you're making Christian but many times lower rpms doesn't always mean the best efficiency. Keep in mind, the taller the rear gear is, the more strain you're putting on the rest of the drivetrain too. The more strain you put on the drivetrain does not always net best efficency.

About the best example of this I can think of is using a 10 speed bike. If you put the bike in a lower gear like 4.10s, you pedal faster but it pedals easy and you don't go near as far since you're actually moving at a lower speed. Then using a high gear, like 2.73s, you pedal much slower keeping your rpms down, but its hard to pedal and more strain on yourelf. This is the same exact principle as a vehicle rear gear and this is why lower rpm is NOT most efficient, and its also more strain on a 700r4 in both the hard parts and the clutch packs especially the 3-4 clutch packs that are notorious for burning up. In most cases, due to to tall of a gear ratio, or big tires.
I like to go low gears and have an OD to compensate the low gears to be able to cruise 75-80 at a reasonable rpm.

Ok, so, what is the most efficiency? I am thinking, the efficiency would come from what your engine's powerband, which is something you design as you build your engine.

Most HP is kinda peaked at like 3500, unless you've made your engine for high reving via cam destroke, etc..

If you have pretty tall gears (differential), you compensate by using more gears in your tranny (in my mind this should enable HIGHER gear sets with a 4speed than what you had with a 3 speed, if yu did not have OD) ..

An engine turning more RPM's will generally use more fuel, even if there isn't much load, simply by the matter of the fact that you are filling each cylinder with a fuel air mix and detonating it more often than you would be at lower RPM's...

Therefore, in my mind, a vehicle which would handle no problem 3.73's with a 3 speed should be able to handle the same tires/gearset (all other things being equal) with the same setup and increase it's fuel mileage with an overdrive transmission.

Now, here comes the if's but's, and and's..

If you have a hilly location, and find that the machine is constantly loading down in OD, and having to down shift, and the poor thing seems to be hunting for the right gear to be in, you might wanna reconsider raising the the ratio (thus lowering the final gear) at the differential.

Riding a Harley, an old one at that, where it originally came with a 4 speed transmission, one could lower RPM's on the freeway by decreasing the teeth on the rear sprocket, but you might end up lugging it, and burning the clutch a bit starting off, or you could get a wide gear set ratio, and then you have to wind out each gear more, or you can get a 5 speed, which the final drive ratio is the same 1:1, and it works really well, increasing your gas mileage, etc, etc. Or, you can do like I did, and get the 6 speed, with overdrive, and use the final drive ratio that I would have put on for a 5 speed, or lugging with the 4 speed in it, and enjoy even BETTER gas mileage, after making sure my engine (shovelhead) was set up to make it's best power in the lower end of the band, I now get about what some of the guys brag on getting in thier sportsters (50mpg) on a bigtwin wide glide machine. My bike is pretty good off the line, I never feel like I have to ride the clutch to take off.

If you can set your engine to make good power down low, I would think things would be better, if you aren't trying to go up hills etc.. Now, this doesn't take into account how many rpm's are needed to keep the pump pressure in the tranny up, you've got lock up, but you need to maintain a certain pressure to keep that from slipping, oil pump needs to be pumping, etcc, so you can't go TOO low. But cams can be set up to get your power down low, and I would think you could set up your tranny pump and regulator to get enough pressure, etc. Key is, driving style, and designing the whole vehicle around this.

What rpm do you say is ideal for say 70mph HRPC?

 

[Christian Nelson]

One could also question why on earth a guy would be putting 36" tires on a lifted truck, and trying to do a bunch of highway driving too though.....

 

[HotRodPC]

You're right Christian. The ideal rpm all depends on the vehicle, the mods etc. Sure you want to be in your power band, so cams, carbs, electronics, timing, weight of vehicle, wind drag, etc etc, all play their part in determining all this, so it hard to say, but lowest possible is NOT always best.

Example being, my dad has 94 Cab and Chassis 1 ton with a flat bed on it. He has the Granny 5 speed NV4500 tans. He put 3.23 gears in the rear for mpg thinking he has the granny low to cmpensate. He was complaiming about having to downshift alot. I told him, don't use 5th gear on highway and see what happens. Although very little, he gets better mpg without using the 5th gear. The good news is, he can drive 80 in 5th and does about the same as 70 in 4th.
My newer trucks seemed to do well at around 2300-2500 rpm using OD, but it also had 3.73 rear gears. If it were 3.08 gears, I doubt it woudl do as good at the same rpm because of strain, but speed would have been faster of course. That is another thing to consider too, the faster you are going, the more wind drag you are putting on the situaion and that will also lower efficiency.
My box van with a biggo wind drag box with 4.10 and OD, get the same mileage loaded or empty, city or hwy. The only thing that makes a difference is wind. I can go West empty and get 8mpg. I can go East loaded and get 10-11. Go figure that. Bottom line, its wind drag and the way the Jet stream flows. I've owned the truck long enough now I've got calculating fuel down to an art and I am rarely wrong on my guesstimates. But it does cost me alot more to drive West than it does East.

 

[dhamp]

Why in the world would anyone drive West? Just come as far East as you can then stay here!

 

[HotRodPC]

You're nuckin futs !!! I have driven west to pick up computer parts and made trips to west Texasss to see my daughter at the AFB. The truck has always gone west and come back to OK. Gone North a time or 2 but not many at all. Someday I'll be transplanted in AZ close to the CA and/or NV borders.

 

[dhamp]

Once you drive South & East, you never go North or West again. LOL!

 

[dhamp]

Seriously though, I've got the equivalent of a 29" tall tire with the original 2.73s. The 3.08s aren't better by much. Even with stock tires I can sometimes hear a little lifter tick down low in the RPMs. I definitely wouldn't go 33s & up with 3.08s, no matter what the tranny. It's good as a highway truck where you're not doing much low RPM acceleration, but not many use their square as a highway vehicle much. Once's I get the more pressing stuff done, I'm planning on going 3.42s just so it's not working so hard to get going. Like others have indicated, there's no magical combination that's perfect for everyone. It's all dependant on how you plan use your truck.

 

[HotRodPC]

You're right, there is no perfect combo for anyone really unless its just an of farm use truck only then you can 5.13's with a Th350 or Th400 and be done but that is rarely the case. I personally like 3.73 as the ultimate safe middle that can go either way. Still low enough to run some taller tire, but not to low to where your rpms will be hell on the freeway. That's JMO though. I like 3.73 for a nice universal well rounded ratio for some fun, some light to medium towing, and still drive 75 - 80 on the interstate. For my play truck, I intend to go with 4.10 or 4.56, just depends on what axle I find for the front since I have both set up in rear ends already to go. 3.42 or 3.54 (depnding on gear availability) is the tallest I'd ever consider for a truck. 2.73 and 3.08 should have been outlawed. Just dumb as hell period IMO. Its a damn truck for god sakes, not a friggin station wagon. And 3.21s in a 3.4 ton like I got?? WTF was someone thinking???? I only know of one other member of another forum with 3.21's in a 14b FF, his is also OEM.

 

[dhamp]

Well, you must remember....the gas crisis of the 80's had the car companies scrambling to lower consumption. My dad used to drive my truck to work daily and it was like 45 min each way. Doing that on the highway would yield some advantage to 3.08s or 2.73s. I believe that was a way they could get another 1 or 2 mpg of hwy mileage out of 'em for the marketing folks.

 

[HotRodPC]

Well, you must remember....the gas crisis of the 80's had the car companies scrambling to lower consumption. My dad used to drive my truck to work daily and it was like 45 min each way. Doing that on the highway would yield some advantage to 3.08s or 2.73s. I believe that was a way they could get another 1 or 2 mpg of hwy mileage out of 'em for the marketing folks.

You're right about a crisis causing this to happen in the 80's. But, lets get the crisis on the gear ratios correct. The Federal Gubment was the crisis and I'll leave the politics out of it. So moving on, the tall gear ratios were not necessarily to improve mpg. Even the auto manufactuers aren't stupid enough to beleive tall gear saves fuel. The reason for the tall gears and to run LOWER RPM, was to meet Gubment regulation on HC, CO and NOx readings, which are Hydro Carbrons, Carbon Monoxide and Oxides of Nitrogen which pollute the air. So with vehicles running a lower rpm and they polluting less and the manufactuers were able to have an AVERAGE fleet that met requiremnts. And for all those that say all the "SMOG ****" is bad. I reasonablly disagree. The smog **** isn't that bad and thanks for computer controlled autos, we don't know "SMOG ****" as we used to know it in the 80's. My only issues with smog devices are that they are in my way when working on the vehicle, and some devices hamper me wanting to put high performacne parts on, past that, most of the smog devices are no big deal and well worth having IMO. Maybe is we were all CA smog techs at one time and really understood smog devices, then we'd all agree, but I don't expect everyone to agree with me.