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  1. #11
    Quote Originally Posted by UselessPickles View Post
    My 27% drivetrain loss assumption
    Pickles. Is power loss in the drivetrain always a percentage of engine hp/torque that scales with the engine or does it take a fixed amount of effort to turn the drivetrain at a given rpm/gear? In other words would a 1000 hp engine give up 250 hp to turn the driveline based on a 25% loss factor; a 500/125; 100/25 and so on or do all engines start with a -X loss due to the hp/torque required to turn the drivetrain?

  2. #12
    Quote Originally Posted by gbaumann View Post
    Pickles. Is power loss in the drivetrain always a percentage of engine hp/torque that scales with the engine or does it take a fixed amount of effort to turn the drivetrain at a given rpm/gear? In other words would a 1000 hp engine give up 250 hp to turn the driveline based on a 25% loss factor; a 500/125; 100/25 and so on or do all engines start with a -X loss due to the hp/torque required to turn the drivetrain?
    All depends on the efficiency of the transmission, differentials, number of axles being driven. 27% seems a little high to me. I've had my truck dynoed when I first bought it-- it has about 140hp at the flywheel at 2600 rpm and it showed about 120-125hp at 2600rpm redlined at the rear wheels or about 14.29% loss. Which is pretty good.

  3. #13
    Let's get all four power mods to the dyno for JeepLap Spring Dyno Day 2015. Then get them on the trail! As much as I'm dying to see the numbers I'm also looking forward to meeting some of you guys and getting a day out in the woods.

    Comparing the SC Pentastar to the 6.4L hemi is easy for me in certain respects. I ran the Mag unit for almost 25,000 miles. It was a struggle to say the least due to the auto trans issues. But, of course, I know what forced induction does for the Pentastar because I could manually make conditions right and get her on boost when I wanted. I'm now driving the same truck with the 6.4L. So I'm experiencing it in real time. I'm not sure how to describe the difference. Every time I get in the truck convinced that I'm going to really study the differences I step on the gas and the logic center in my brain dumps all operations. The only result I get as output is a juvenile giggle that keeps looping until I turn the truck off.

  4. #14
    Quote Originally Posted by Timmy View Post
    I would love to see your 0-60 time Pickles and compare it to your calculations. 4.81s is just insane!
    I will refer you to this statement in my previous post

    Quote Originally Posted by UselessPickles View Post
    NOTE: I'm not claiming that my simulation will perfectly simulate the real world and make perfect predictions. Real world results depend on so many variable factors, human imperfection, etc. The goal of the simulation is not to accurately predict real-world results, but to compare relative real-world *potential* between different vehicles/configurations, using computerized perfection to remove the uncontrollable variables of the real world. This allows us to objectively/relatively compare stuff without arguing about whether the results were simply due to difference in driver skill, a bad launch, a poorly timed shift, bad weather conditions for traction, etc.
    However, I do purposely use input (launch rpm, gear shift time) that seems reasonably attainable to get results that are reasonably close to what should be possibly in the real world. I definitely need to improve my launching skills before I can hope to approach the predicted results


    Quote Originally Posted by Timmy View Post
    Part of me is sitting here thinking, no way on earth are you going to see that, then the other part says, well... maybe so. I really would be shocked if you are below 5 seconds. An Audi S4 has a 0-60 of 5.2 seconds. That's a 6cyl with a supercharger on it producing 333hp that is geared for speed, weighs 3985 lbs (close to your weight) and has a drag coefficient of 0.30 compared to a Jeep somewhere north of 0.50 (stock.)
    A couple things...

    * Drag coefficient is not the full story of the vehicle's aerodynamic drag. A drag coefficient (Cd) only describes the shape, but not the size. The drag coefficient must be multiplied by the frontal area to get the drag-area coefficient (CdA), which is what is actually used in calculations to determine the force of aerodynamic drag. So the Wrangler has even a bigger disadvantage than you would expect from drag coefficients alone (compared to typical cars) due to its larger frontal area.

    * Aerodynamic drag doesn't have a very big impact on 0-60 mph times. To illustrate this, I simulated the turbo Wrangler's 0-60 run as if it had the same aerodynamic drag as a 2006 WRX STi. The STi's CdA value I have is 0.694. That's less than half of the Wrangler's CdA of 1.762. The end result for a turbo Wrangler that is more than twice as aerodynamic than reality is 0-60 mph in 4.78s. That's only 0.03s faster. Power:weight ratio has a much larger impact on 0-60 times than aerodynamics, just because so much of the 0-60 mph run is spent at low speeds where aerodynamic drag is minimal.

    * My Wrangler is almost exactly the same weight as the Audi S4 example, but with about 380 whp, compared to the S4's 333 whp. Given that power:weight ratio is the dominant factor in 0-60 mph times, the turbo Wrangler's simulated 4.81s vs the S4's 5.2s seems pretty reasonable.
    Last edited by UselessPickles; 11-23-2015 at 12:12 PM.

  5. #15
    Quote Originally Posted by gbaumann View Post
    Pickles. Is power loss in the drivetrain always a percentage of engine hp/torque that scales with the engine or does it take a fixed amount of effort to turn the drivetrain at a given rpm/gear?
    Both

    My brother (grad student in physics) and I actually explored this question (and many others) by working through the relationships between things from the engine, through the drivetrain, all gear reductions, through the tires, to the dyno drum, as a complete system being accelerated in a power pull sweep on a dynojet style dyno. I'll spare you the math and try to just describe the findings relevant to your question.

    The context of what I say here is limited to a vehicle on a dynojet style dyno, going through an rpm sweep power pull with a given transmission gear, axle ratio and tire size. Things change as you change transmission gear, axle ratios and tire size. I do not attempt to cover this here.

    There's two main components to drivetrain loss:
    1) Loss due to friction.
    2) loss due to moment of inertia of all the drivetrain components.

    For a given drivetrain, transmission gear, axle ratio, etc., the amount of the engine's torque that is "consumed" by friction is practically constant. Doesn't matter how fast the drivetrain is spinning or how quickly it is accelerating. So this component takes a "fixed amount of effort" as you worded it.

    The loss due to moment of inertia is directly proportional to how quickly the drivetrain is being accelerated. On a dyno (at least, on a dynojet dyno in a power pull sweep), the drivetrain is accelerated at a rate directly proportional to the amount of torque the engine produces (minus the the amount of torque required to overcome drivetrain friction). This component is essentially a "percentage of the engine hp/torque".

    Due to the way dyno power pulls are done (on Dynojet and similar dynos, at least) with a fast sweep through the RPM range (quick acceleration) with a fixed dyno drum mass, the loss due to moment of inertia is much more significant than loss due to friction. So for a given drivetrain, the amount of drivetrain loss will be close to the same percentage, but that percentage of loss will go down some as you increase the engine's power because there is the friction component of the loss that is constant. That frictional loss becomes a smaller percentage of the engine's total output as you increase the engine's output. The specifics of how much the overall percentage will change depends on the specifics of the particular vehicle's drivetrain friction and moment of inertia.

    There's lots more details I could try to get into about what/how a dyno technically measures, differences between different types of dynos, etc., but I think what I've said so far is good enough for the context of this discussion (measured drivetrain loss on a dynojet dyno).

    One interesting note worth mentioning, though, is that a steady dyno pull (where the dyno applies torque to hold the RPMs steady, records the amount of torque, moves up to another RPM, rinse and repeat) completely removes the drivetrain's moment of inertia loss from teh equation because the drivetrain is not accelerating while taking the measurement. This would result in higher dyno number, and a completely constant amount (not percentage) of drivetrain loss, because friction is now the only source of loss. This does not produce realistic results, though, because most people want dyno results to gain an understanding of how the vehicle will accelerate... not how much work it can do at a steady speed.


    Quote Originally Posted by KaiserBill View Post
    27% seems a little high to me. I've had my truck dynoed when I first bought it-- it has about 140hp at the flywheel at 2600 rpm and it showed about 120-125hp at 2600rpm redlined at the rear wheels or about 14.29% loss. Which is pretty good.
    Your truck is not a Wrangler. Stock Wranglers typically show about 30-33% drivetrain loss. JeepLab's dyno results for the stock Wrangler show a 33% loss. So 27% loss with the Hemi is not unreasonable. Since the 27% loss came from a 5.7 Hemi dyno chart, I actually expect that the 6.4's drivetrain loss will be a bit less than 27% (see above explanations), but nowhere near something like your truck's 14%.

    Also keep in mind that there's a difference in transmission with the Hemi Wranglers, so that could be a significant factor in the difference in drivetrain loss between stock and 5.7 Hemi.

    Basically, like I said at the beginning, we don't have any exact data fro the 6.4 Hemi Wrangler, so I'm just doing the best I can with the data I can find to create a close approximation. Don't take the exact simulated numbers/times as absolute truth. Just look at overall general trends as an approximated relative comparison.

  6. #16
    If you haven't actually removed your engine to dyno to figure out bhp then you really cannot tell how much you've lost due to the drivetrain. It is all guess work.

  7. #17
    It's reasonable estimation based on known drivetrain loss of a 5.7 Hemi Wrangler, comparing dyno charts to crank hp ratings. Again, read all the things I already stated about this being a best estimate with the available data, to be used for comparing general trends and relative differences. More specifically, for comparing large relative differences. Any area where the differences are very small could be well withing a margin of error of the assumptions.

    The data I have so far is enough to see that the 6.4 Hemi will clearly destroy the superchargers throughout the RPM range, will destroy the turbo at low RPMs, but the turbo will likely have some level of advantage in the upper RPM range.

    I'll gladly update everything as soon as someone produces an actual dyno chart of a 6.4 Hemi Wrangler. The amount of drivetrain loss becomes irrelevant then, because we'll have an actual measured torque curve at the wheels.

  8. #18
    Looking at this from the view of a guy that wants more power for primarily trail runs and the occasional rock crawl, prodigy seems to be the best answer dollar for dollar. The 6.4L is out of the question simply due to price, but Magnuson, RiPP and Prodigy are all in the same ball park.

    Reading up on the hot side of the turbos and pipes as well as the future problems of the prodigy though, I'm more swayed towards RIPP with a high boost pulley.
    Last edited by HahaJK; 04-05-2015 at 12:56 PM.

  9. #19
    Pickles, the 6.4L and 5.7L hemis since 2012 use the original trans. I have the same WA580E/NAG-1 that was bolted to my Pentastar. So your assumptions should be pretty good if you're figuring in the original trans dynamics.

  10. #20
    I know we're focused here on power but if Pickles has any spare computing time can we get a chart showing where the dollars go for these mods as they relate to power? We know what Mag, Ripp and Prodigy cost. I spent $28k on the 6.4. Which of the FI kits puts the torque/dollar in the most efficient spot? I'm assuming there's nothing efficient about the cost of the hemi. Assume daily driving with a typical commute and weekend trail time.

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