SC vs. HEMI - Let the TRASH talk begin!

[Yoinkers]

Ok, Ok, i know this is not one of those forums were people get flamed for asking a dumb question, and the atmosphere around here is all very friendly and educational. And thank god for that. So forgive my thread title. I was trying to get attention.

I dont know who really knows who around here so we may all be strangers, i know JL and Snarf hang out, and gbaumann and JL live near one another and all the jeeps have different gear and gearing.

Here is what Im getting to.... Black Betty, Vs. Cherry Bomb (did that name stick? I liked it) gbaumann's hemi. Maybe the turbo makes an appearance.

Not so much a pissing contest, but the actual facts of what we think would happen if we raced them..... And maybe provoke these two guys to find a stretch of pavement to answer all the questions.

[Yoinkers]

FIRST - Power / Weight ratios

Both jeeps make over 300 whp. Understood. Given. Both are rockets. Maybe they will both make a about 330, (bb's chart is around here somewhere)

My point, The SC makes that much power, but the HEMI has to make it earlier in the power band. BUT then we have to factor weight. Is 400 lbs extra weight enough to neutralize the extra early power?

So in a test tube, with everything else excluded, how much does the HEMI's weight hold it back?, Or the SC's weight make up for its steeper power band?

[UselessPickles]

The 6.4 Hemi is the setup that we have the least info about: I have not yet seen a dyno chart of a 6.4 Hemi Wrangler.

However, I did find a dyno chart of a 5.7 Hemi Wranger, and it shows about 27% drivetrain loss compared to the crank power ratings: http://www.jk-forum.com/forums/jk-sh...sults-89983/#b

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It's also easy to find dyno charts of 6.4 Challenger SRTs:

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If we make a big (but reasonable) assumption that the 6.4 Hemi Wrangler will also have about 27% drivetrain loss also, it's easy to take that 6.4 Hemi torque curve and scale it down accordingly. This is really the best estimate we could possibly come up with for the torque at the wheels of a 6.4 Hemi Wrangler right now.


So I've also taken data from JeepLab's dyno charts for stock, RIPP and Magnuson. I got torque curve data for the Prodigy turbo from Prodigy's dyno charts. Here they are all together in one chart for comparison:

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The 6.4 Hemi definitely has the all-around nicest torque curve. In the upper RPMs, the turbo has a significant advantage, even before taking into consideration the extra 400 lbs of the Hemi.


UPDATE: Here's a power chart, for those that like to think in terms of power rather than torque.

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[2k13jk]

Ripp did a dyno pass with a 392 in awd and make 325 wheel

[UselessPickles]

Ripp did a dyno pass with a 392 in awd and make 325 wheel

That was, unfortunately, in 4HI because of some technical issue about the traction/stability control systems not allowing a rear-wheel-only dyno run.

My 27% drivetrain loss assumption gives about 345 hp and 350 ft-lbs of torque at the rear wheels for the 6.4 Hemi. This seems like a reasonable difference between 4WD and RWD numbers. Maybe? No idea, actually

[gbaumann]

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?

[KaiserBill]

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.

[UselessPickles]

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.

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.

[Gunner]

https://www.youtube.com/watch?v=p1A64GjXe6o

[UselessPickles]

So what does this all mean for what really matters: acceleration?

Luckily, I've written some code to process torque curve data, along with other key specs about a vehicle (weight, gear ratios, tire size, aerodynamic drag, etc.) and simulate how they would accelerate under full throttle.

For all my acceleration simulations, I assume the following (based mostly on my Jeep):

• Base model 2-door Wrangler with 3.21 axles and stock 29" tires.
• Weight for 6.4 Hemi: 4249 lbs = 3849 lbs (base model curb weight) + 400 lbs (AEV's claim of added weight)
• Weight for all others: 3960 lbs (my actual weight; assuming all bolt-on kits are close enough in weight to not have a big impact on results)
• Weight of driver: 140 lbs (my weight)
• Stock 6-speed manual transmission. THIS IS NOT CORRECT FOR THE HEMI. Unfortunately, my code can only simulate a manual transmission. This is the best I can do for this comparison. It's still at least an interesting comparison of the differences in torque curves (and weight), with ALL ELSE being equal.
• For simulated races, launch at 2500 rpms, perfectly slipping the clutch (holding 2500 rpm during the entire launch) at full throttle, and executing gear changes in 0.6s each.
• 11 inches of "roll-out" before the race officially begins for the 1/4 mile races. This simulates how cars are "staged" for a drag race, and how they trigger the timing system to start.

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.


Now on to some fun results...

First up is a chart showing for any given road speed, what is the fastest the Jeep could accelerate at that speed if you're in the best transmission gear for max acceleration. Due to the torque curve shapes, the best gear for acceleration in this case is always the lowest gear you can possibly be in at that speed (upshift as close to the rev limiter as you can), but this is not true of all vehicles/engines.

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Where the end of each line hits the bottom of the chart (0 acceleration) represents the predicted top speed.

That's interesting data, but it doesn't really clearly show how quickly each Jeep will be able to accelerate up to speed over time. For that, I can simulate a 1/4 mile drag race for each and graph speed vs time:

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The end of each line indicates the time and speed at which the Jeep passes the finish line. Here's the numeric results:

Stock:
15.89s @ 85.79 mph

Magnuson:
14.05s @ 97.87 mph

RIPP:
13.89s @ 98.05 mph

6.4 Hemi:
13.27s @ 101.53 mph

Prodigy:
13.18s @ 106.62 mph


Another interesting aspect is how quickly does each Jeep actually move down the track? The most interesting and clearest way of presenting this that I found was to show how much of a lead each modded Jeep has over a stock Jeep throughout the race. This gives a pretty good visual of how much they all leave the stock Jeep in the dust, and also how they "jockey for position" with each other throughout the race:

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I think this is the best visualization of how the different power mods actually compare to each other in acceleration. The 6.4 Hemi clearly destroys both superchargers. The turbo clearly has an early disadvantage due to its minimal low RPM gains, but more than makes up for it by the end with his upper RPM power.

Keep in mind, though, that this is a race from a complete stop to a 1/4 mile distance. Not quite a typical real-world scenario. For example, cross-reference the two drag race charts to compare how each Jeep would perform when accelerating up to various legal road speeds. You may find that one Jeep (Prodigy) will reach a target speed in a shorter amount of time, but a different Jeep (6.4 Hemi) is actually further ahead by distance, despite requiring more time to get up to speed.

Also keep in mind that a 2500 rpm launch was arbitrarily chosen because it seems reasonable and minimally abusive to the clutch. Launching at higher rpms would produce very different results for the turbo, while having relatively limited impact on the results of all the other Jeeps, but is not a very good representation of how you would most likely drive it on the streets.

On the other hand, if you begin the acceleration from a rolling start at some higher speed, like executing a pass on a highway for example, differences in low end torque won't matter because you can downshift for maximum acceleration (see the first chart in this post).

The actual results of any "race" between these different Jeeps will vary quite a bit depending on the starting speed, and whether it is a race to a particular speed, a race to see who covers a given amount of distance first, or a race to see who is furthest ahead after a given amount of time.


I think it is safe to say, though, that the 6.4 Hemi should be a rocket with the best balance of torque/acceleration available at any time.


(if you want to skip past a couple pages of bickering and see more analysis, click here)