Power Bands?
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Laughing Gas
Joined: Oct 2012
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From: Rockport MA
Power Bands?
I've been doing a lot of research on drive trains, and how I can get good MPG out of my options. I am strongly leaning toward a zf5, with a 3 speed brownie, all in front of my transfer case. However I have heard the term 'power band' enough times that I want to do some more research in this area before I go to far.
To my understanding I can over do my gear and end up in to low of an RPM range and kill my mileage, is that right?
I currently have 4.10 gears, and am looking for a good RPM range for cruising at 65ish. What do I need to know?
Thanks Again,
Devin
To my understanding I can over do my gear and end up in to low of an RPM range and kill my mileage, is that right?
I currently have 4.10 gears, and am looking for a good RPM range for cruising at 65ish. What do I need to know?
Thanks Again,
Devin
Lead Driver
Joined: Apr 2008
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Im going to say that is very application dependant, and speed relative. At one point i did find a chart that listed the brake specific fuel consumption for the 6.9 as placed in medium duty apps. Want to say 1700 was the sweet spot. Past that, wind resistance increases exponentially the faster you go. Given that 90% of fleet trucks are governed at 62mph, im guessing thats the best compromise between speed and wind resistance. I imagine the dynamic timing in your fuel pump plays a big role in this too. My grandpa always goes on about this guy... give him $50 and he would almost double your fuel mileage, no matter what you drove (this was in the 70s / 80s). All it was, was basically re-curving the distributor, and tuning the carb.
Postmaster
Joined: Aug 2010
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From: Northern Arizona
The text book answer to this is keep your rpm's a couple hundred above the torque peak for your engine. In real life driving my IDI the sweet spot for mpg has been 1800-2000 in my experience. For highway driving 3.55's and ZF-5 is the best stock option unless you want to run huge tires (if you are after mpg I assume you're not). That is 2.73 final drive ratio. I am going to be running 4.10's ZF-5 and a Gear Vendor overdrive. My final drive ratio will be 2.43 and that is about the highest you want to go. Justin had 4.10's ZF-5 and a DNE-2 overdrive in Smogie. But he also runs 35" tires. I talked to him about it and he said in double overdrive it was a tad sluggish and even a slight rise in the road or a headwind would raise his EGT's and there was no mpg gain over driving in regular 5th gear. So yes, you can over gear it. 4.10's with 35" wheels is almost exactly equivalent to 3.55's with stock 31" tires, the combination I mentioned earlier as a good highway gear with a ZF5.
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Laughing Gas
Joined: Oct 2012
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From: Rockport MA
I wonder if watching your EGT's could help you sniff out clues would work?
Also is it naive to think that RPMs alone could not be used to find your power band? If not the only other thing I can think of is resistance, so what would that be final drive ratio, everything else in between is just cloudiness? (IE tire size, axle ratio, overdrives, gears, etc.)
Devin
Also is it naive to think that RPMs alone could not be used to find your power band? If not the only other thing I can think of is resistance, so what would that be final drive ratio, everything else in between is just cloudiness? (IE tire size, axle ratio, overdrives, gears, etc.)
Devin
Laughing Gas
Joined: Dec 2013
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Depends what driving you will be doing, if you want peak mileage at 60-70mph then zf5 and 3.55 gears. If you want to cruise on the interstate then a double overdrive could be beneficial. My truck tops out at about 85 or 90 so 80mph interstate is not conducive to decent mileage. If you could cruise 80 at 2000-2200rpm I think it would be in the powerband enough to be happy.
Postmaster
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I'm pretty happy with my 4.10s, stock tires and GVOD. This gives me about 60MPH at 1600, 65 at like 1720 or so. This is right at the bottom of the 'power band' with a turbo; I wouldn't want to go any lower.
It makes for nice cruising at 75, too -- RPMs are right around 2K, giving some decent boost from the turbo and extra power needed for pushing through the air at that speed.
It makes for nice cruising at 75, too -- RPMs are right around 2K, giving some decent boost from the turbo and extra power needed for pushing through the air at that speed.
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These IDI's seem to like low rpm. If you are gonna hassle with gears, I'd put 3.55's at the top of the list and run a zf5 for the OD.
The brownie box will not help with fuel efficiency, and if you need that kind of gear splitting to haul a load you simply have the wrong vehicle and running gear for what you are trying to do. But if that's the direction you are leaning, the ZF6 would probably fit the bill since the gears are closer together. Less hassle than integrating a splitter, but you still have the length/driveshaft type issues.
Your best fuel efficiency will be at the lowest rpm that the engine makes adequate power, but must be balanced to prevent "lugging". Driving at torque peak or the like is an erroneous notion that wastes fuel.
The brownie box will not help with fuel efficiency, and if you need that kind of gear splitting to haul a load you simply have the wrong vehicle and running gear for what you are trying to do. But if that's the direction you are leaning, the ZF6 would probably fit the bill since the gears are closer together. Less hassle than integrating a splitter, but you still have the length/driveshaft type issues.
Your best fuel efficiency will be at the lowest rpm that the engine makes adequate power, but must be balanced to prevent "lugging". Driving at torque peak or the like is an erroneous notion that wastes fuel.
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Postmaster
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From: Northern Arizona
The ZF-6 does not have closer gears than the narrow ratio diesel ZF-5. It basically adds a granny low gear which is missing on the close ratio ZF. The GearVendor overdrive is designed to work as a gear splitter, but on the close ratio transmission it is just redundancy. The gears are already that close together. I ran the close ratio T19 and GVOD, never used it for gear splitting. I pulled some monster loads and never once wished for closer gears. In my opinion they are close enough for heavy hauling. My complaint is driving it unloaded or lightly loaded. Then the gears feel obnoxiously short. For my rebuild I am going with a wide ratio 460 ZF-5.
Postmaster
Joined: Aug 2010
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From: Northern Arizona
Is it? Care to share some information that supports this? Because there is a holy crap ton of lab tests and road tests that support the torque peak "notion". The published torque peak of the 6.9 is 1,400 rpm. Go much lower than that and it's going to lug.... which puts you right around the torque peak again...
i ain't rite
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From: Marlboro Mental Hospital.
250,000 miles with a ZF5 speed told me just the opposite. best fuel economy was achieved at 1900 rpm in 5th gear with 4.10 gears and 35 inch tires on my 88 7.3 4X4 truck
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Laughing Gas
Joined: Oct 2012
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From: Rockport MA
Devin
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Joined: Aug 2010
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From: Northern Arizona
tjc was running 35" tires which is almost identical gearing/rpm-wise to 3.55's with stock 31" tires. At 1900 rpm that would have put him right around 65 mph in 5th gear.