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Laces Out
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i know a few load dyno's around here who run all trucks with overdrive off, not sure about dynojet's, but i've always been told that because the ratio is above 1:1 in overdrive, most wont run like that. does anyone know the truth?
 

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On our Superflow it really doesn't matter a bunch whether you run the truck in 1:1 or overdrive. As long as we keep the rate of acceleration equal, both gears will pull very similiar numbers.

I usually always run 1:1 simply so we don't have to run the truck 120+ mph to get into the higher rpms.
 

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Bigfoot is an OBS
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I don't know about the 5Rwhateveritis, but the 4R100 and E4OD have the coast clutch engaged when OD is disengaged. This could in theory help to reduce slipping in the transmission during the run.

You may dyno slightly less in OD simply due to spinning the driveline and tires that much faster (speed takes power), but unless the truck could produce several back to back runs within 2-3hp of each other at the exact same engine RPMs, you wouldn't be able to notice it.
 

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when running on inertia dynos ive always ran in OD to help load the engine up. Im fairly certain that when floor it diesel has ran my truck on their load cell they've always used direct.
 

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6Leaker
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Only times I have run was on a dynojet and all runs were done in OD, even 7.3 trucks where you could lock OD out. I think for the 5R's its just a lot easier to run it in OD than to try and keep it in direct without it shifting.

Certainly fun waiting for the speedo to come off the pin. They couldn't get a tach signal off my truck so it read mph insted and showed about 145mph at the top of the run.
 

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Wheelie Dude
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FOr us it all depends on the build and truck. For the most part 1:1 will do the same as OD but some of the bigger built cummins motors have to run OD to get the best #.

For instance mine evein on the street you will not feel the hp until you get into OD. Goin through the gears its ho hum-whatever, get to 1:1 gear and ya thats nice, go into OD and HOLY CRAP. It shows the exact same thing on the dyno even though its loaded. I can pick up over 150hp going to OD compared to 1:1. But with other trucks we have tried both and it puts down pretty much the same either way.
 

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Laces Out
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Discussion Starter #7
FOr us it all depends on the build and truck. For the most part 1:1 will do the same as OD but some of the bigger built cummins motors have to run OD to get the best #.

For instance mine evein on the street you will not feel the hp until you get into OD. Goin through the gears its ho hum-whatever, get to 1:1 gear and ya thats nice, go into OD and HOLY CRAP. It shows the exact same thing on the dyno even though its loaded. I can pick up over 150hp going to OD compared to 1:1. But with other trucks we have tried both and it puts down pretty much the same either way.
yeah i tried it both ways today... 571 in 1:1 and 577 in overdrive.... but the truck seemed to like overdrive better.
 

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I think the big difference comes from rate of acceleration changes. On our dyno we specify how long the run will take and the dyno won't let it progress any faster. If we do a ten second run in either od or 1:1, the numbers will be very similar as long as we test the same rpm range. For example, if the tests both start at 2000 rpm and end at 3800 and we run the same length of test, the numbers will be very similar.

Let's say our test vehicle has a test range of 55 mph to 130 mph in 1:1. There will be a wheel speed acceleration of 75 mph. If we ran that vehicle in OD that is overdriven 20%, to keep our rpms in the same range for the test, we would have to run the vehicle from 66 mph to 156. In OD we will have a wheel speed acceleration of 90 mph. So, the vehicle is doing more work in OD than in 1:1 if we have no way to account for time.

On a Superflow, if we have our time frames for our tests set up properly and equally, wheel speed means very little and there is an equal load on the engine. The engine rpms will increase at the same rate of time and load no matter the gear (to a point). For both tests above, we could specify an equal acceleration rate so you could lay both graphs over each other when mapping time vs rpm.

Where I have noticed a change in hp from running a vehicle in OD instead of 1:1, we don't have our test time frames right. By lengthing the 1:1 test time so the same load is applied for the same length of time, we can duplicate the OD numbers.
 

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On the Dynojet, the numbers will be very close. 1:1 is the "proper" gear and will give you closer to real world results. If you dyno in lower gears, like 1st and 2nd, the numbers will be lower. It's all about the gearing and the acceleration rate. Power readings recorded in OD are generally higher than those performed in whichever gear is 1:1. The higher the transmission gear, the greater the load and you'll see more power gain. Basically, the higher gear ratio slows the engine’s acceleration rate, which in turn rquires less power to accelerate the rotating parts. Also, a rear gear ratio, such as a 3.55:1 will show a higher power reading than a 3.73:1.


We usually dyno the big turbo, lower rpm trucks in OD. This gives you a longer pull and it spools harder. A 6.4L, or any close to stock truck, should be dyno'd 1:1 for more realistic numbers.
 
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