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Old 03-13-2013, 08:25 PM   #1
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anyone know anything about FLYWHEELS?

Hey everyone,

2012 JKUR 6spd. My last car was a vette, and when i downshifted it, it would engine break like i was running into a sand dune.

My jeep dosent do that, I try to downshift to slow down, but it dosent seem to slow down. I end up using the brakes.

Im told the reason is the fly wheel. Anyone know whats really going on?

How do I get the truck to engine brake better?

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Old 03-13-2013, 09:08 PM   #2
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Flywheel? Really?

I thought it was purely a matter of gear ratios and engine compression. Period.

That's why it's called compression or engine braking.

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Old 03-13-2013, 09:26 PM   #3
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Ive seen a lot of talk on forums talking about weights of flywheels, cam pulleys.etc. affecting engine braking but those where sports cars.
IMO and I guess the easiest answer is I would get anything and everything out of your head about getting your Jeep to do anything close to what the vette could do and vise versa. just my .02
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Old 03-13-2013, 10:42 PM   #4
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I guess maybe if you tell us more about the 'vette we can offer more informed conjecture, but basically I agree with both uphill bill and bengen.
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Old 03-13-2013, 11:07 PM   #5
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I don't get a sense that the flywheel is very heavy in our Jeeps, but the heavier it is, the less engine braking you will feel because it carries a lot of energy or momentum.

Also, the vehicle weight of the Jeep vs the Vette might be roughly similar, but the Jeep engine is a lot smaller, so it can't hold back as hard, and in a similar weight vehicle, the effect would be less.

I wish our flywheels were a lot heavier for four wheeling.
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Old 03-14-2013, 06:57 AM   #6
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I've found that with the 6 speed the ratios are pretty close together which negates the engine braking effect. I have found that if I drop it from 6th to 4th I get the engine braking I need.

There are times I also short shift on the way up through the gears also, depending on traffic conditions. After I pull away from a light and get into 2nd gear, if I run it up to 3000 rpm, I can shift to 4th and then 6th get if traffic allows.

This is a JK Sport with 3.73 gears.
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Old 03-14-2013, 07:02 AM   #7
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Quote:
Originally Posted by Up Hill Bill View Post
Flywheel? Really?

I thought it was purely a matter of gear ratios and engine compression. Period.

That's why it's called compression or engine braking.
x2
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Old 03-15-2013, 01:44 AM   #8
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Flywheel? Really?

I thought it was purely a matter of gear ratios and engine compression. Period.

That's why it's called compression or engine braking.
It's not about engine "compression". it's about gasoline engine manifold vacuum. That's why diesels don't hold back like gas engines do. If it was all about gear ratios, the Jeep would hold back much more than a Vette because the Jeep is geared much lower.
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Old 03-15-2013, 08:33 AM   #9
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Originally Posted by Raspy View Post

It's not about engine "compression". it's about gasoline engine manifold vacuum. That's why diesels don't hold back like gas engines do. If it was all about gear ratios, the Jeep would hold back much more than a Vette because the Jeep is geared much lower.
Really? My 'vette had 3.73s just like my jku. And my old 7.3 super duty would about lock up it's rears when I down shifted a couple gears. Coulda just been cranky I guess...
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Old 03-15-2013, 09:33 AM   #10
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Quote:
Originally Posted by PieFace View Post
Hey everyone,

2012 JKUR 6spd. My last car was a vette, and when i downshifted it, it would engine break like i was running into a sand dune.

My jeep dosent do that, I try to downshift to slow down, but it dosent seem to slow down. I end up using the brakes.

Im told the reason is the fly wheel. Anyone know whats really going on?

How do I get the truck to engine brake better?
Your going from sports car to a 285 hp 4x4 box ! What can you ask for
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Old 03-15-2013, 12:37 PM   #11
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Really? My 'vette had 3.73s just like my jku. And my old 7.3 super duty would about lock up it's rears when I down shifted a couple gears. Coulda just been cranky I guess...
Well, OK, but your not downshifting to high gear where the only reduction is the differential. Your downshifting to a lower gear. Are you saying your lower gears in the vette were as low as your Jeep?

When you first let out the clutch in a downshift and you haven't matched the RPM, of course you would "about lock up the rears" because you are spinning up the mass of the flywheel and the crank, putting energy into the flywheel. The 7.3 has a lot of mass to spin up. The difference I'm referring to is when you take your foot off the loud pedal and feel the deceleration caused by the engine only. The, so called, compression breaking. Diesels don't hold back very well because they have a lot more rotating mass and very little manifold vacuum. They have much higher compression, but still don't hold back as well.

My experience is the same as what PieFace talked about, but I think it's because the Jeep engine is so small and light. It doesn't seem like the Jeep has a heavy flywheel because it's so easy to kill it.
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Old 03-15-2013, 12:53 PM   #12
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Your going from sports car to a 285 hp 4x4 box ! What can you ask for
I think PieFace asked a perfectly good question. And the answer is not that one is a sports car and one is a box. That's true, but their purpose or shape has little to do with it. They are both gas powered cars with manual transmissions. The Vette is probably 800 lbs lighter, but it has a much larger engine. The answer involves displacement, gearing and rotating mass, among other things. A larger engine in a lighter car is bound to hold back more effectively and he really noticed the difference.
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Old 03-15-2013, 12:53 PM   #13
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The op was talking about downshifting, and in 2wd not off road so I'm thinking the gearing isn't all that different, unless i read that wrong and he's got a unique 'vette.
Really doesn't matter. I like my jku more than I liked my,'vette. Hope the op does also.
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Old 03-15-2013, 02:31 PM   #14
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I think PieFace asked a perfectly good question. And the answer is not that one is a sports car and one is a box. That's true, but their purpose or shape has little to do with it. They are both gas powered cars with manual transmissions. The Vette is probably 800 lbs lighter, but it has a much larger engine. The answer involves displacement, gearing and rotating mass, among other things. A larger engine in a lighter car is bound to hold back more effectively and he really noticed the difference.
I understand what the op is asking but he's comparing apples to oranges.
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Old 03-15-2013, 02:49 PM   #15
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It really has to do with the gear ratios in the transmission. The lower gears have a much closer ratio and are "taller gears" in relation to the higher gears so downshifting is not going to be as effective. The taller gears are used to help preserve fuel consumption.

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