[DirtyDan]

Originally Posted by robinson.s

Guess its a quite lot of information flowing on. If I just have to understand in simple words;
The current Gear ratio in my jeep(invader) is {Low 2.46 and the axle ratio is 4.27}. What would be the best ratio for low gear and axle to get a breaking of 1.5 mts per/scnd.

That sounds like the stock ratios for just a few years back. These are not bad differential ratios at all. There is a thread on gear ratios around here somewhere. I must warn you, as an Invader owner myself, the Invader is not the world's best off-roader. Are you sure you want to put money into changing the gears in this vehicle that alrady has decent ratios? There are other issues to contend with, e.g. the rather low ground clearance, or your wheels and tyres. I don't want to curb your enthusiasm but I would attend to other issues first. Just my 2 rupees worth here.

Headers?....you only got a snub nosed .38?

[headers]

Originally Posted by DirtyDan

Headers?....you only got a snub nosed .38?

. Well, whats your POV on the above discussions?

[DirtyDan]

Originally Posted by headers

. Well, whats your POV on the above discussions?

When I find it, I will post a photo! Horse pistol is: 1.) a cavalry pistol 2.) my meaning...a pistol to euthanize animals, big ones, that are terminally ill and suffering greatly....moderator's disease.

I would invest my money in skid plates and recovery stuff before I spent money to alter gears on a vehicle that already had 4.27 crown gears in it. Just my personal choice.

Hmmm...I wonder if I get an insurance break if I am the sole operator of my POV.

[Sutripta]

Originally Posted by DKG

This is exactly what I implied in my earlier post that a FI does not have a venturi whereas a carb engine does. This adds to the vacuum generation in a carb engine and hence I guess there just might be an added advantage in a carb when it comes to vacuum generation. As to whether the venturi in a carb system has a considerable/negligible effect is something I don't know for a fact.

What exactly is your problem with that premise? Its so obvious that engine braking has everything to do with the inlet blocked off !!

How else is vacuum generated if not for the inlet blocked off when you let off the throttle?

My initial ramblings were to say that for the purpose of this discussion, there should be no difference between carb and FI.

Question is not of the vacuum being generated, but of its significance.
Assuming that a petrol engine works with a BMEP of 11-12 bar, the maximum retarding torque (theoretically ideal conditions) we can generate because of the closed throttle (BMEP= -1) is 8 - 9% of the peak torque of the engine. Is this significant?

Originally Posted by Sankar

>>Sir, air suspension in a bus? Airsprings? Air suspension in a Merc S Class? If still not convinced i have posted a simple syringe experiment before.

>>IDI is a closed chamber during powerstroke, right? Compressed air = air spring. Doesn't matter if the IDI has a precombustion chamber.


Yes sir. I rest my case.

A high quality steel spring has very little hysteresis loss. Not so with air. Squish bands, turbulence, and above all adiabatic compression in a system designed to remove heat results in a pretty lossy system.
A suspension system, by its very design needs a dissipative element. This is normally provided by the damper. Extra dissipation in the springing media is not a problem. And the range of pressure variations is not as extreme as in an engine.

The pumping loss in an IDI is significant, and is one of two major contributors to its lower efficiency compared to the DI. (The excess surface area is not the only reason).

Just another random thought. It is fairly trivial to detect an overrun condition in an engine. It would be extremely simple to detect this condition and close a flap in the inlet in the interests of engine braking. Has any manufacturer done this? Would be a far cheaper and simpler alternative to the Jake/ exhaust brake.

There are other ways of looking at the problem, but will only further complicate matters.

Regards
Sutripta

[DKG]

Originally Posted by Sutripta

Question is not of the vacuum being generated, but of its significance. Assuming that a petrol engine works with a BMEP of 11-12 bar, the maximum retarding torque (theoretically ideal conditions) we can generate because of the closed throttle (BMEP= -1) is 8 - 9% of the peak torque of the engine. Is this significant?

I am not an automobile engineer to carry on a meaningful discussion with you on techinical aspects of measuring optimal engine performance.

All I know is I change down gears in a petrol engined car or bike to effectively utilise the vacuum in the engine when the throttle is closed to engine brake. I won't be able to explain in lab values what the forces are, but I do know that engine braking on account of vacuum is like hitting a mountain of foam when you change down. Its the most effective way to slow down a vehicle.

I would be surprised if any automobile engineer here will tell me that vacuum has a negligible role to play in engine braking !!

PS: I ride a sports bike for leisure and can swear the most effective way to safely slow it down while applying brakes (without locking) is to change down in quick succession, effectively utilising engine braking. Its the only way I know I can slow down without losing traction

[robinson.s]

I have been using CJ3b with peugeot engine and 3 speed transmission for all my off road trips. The kind of engine breaking I get in that is, amazing.
The tracks I normally go for OTR is heavy, and without having high torque on low, it is practically difficult to drive through such stretches. With respect to the clearance, I plan to put high profile tyres. (Bighorn 275/70/16) and then increase the height of leaf springs.
For Invader there are two types of transmission
1. BA-10V with a low gear ratio of 2.48.1 and axle ratio of 4.88 and
2. KMT90 with low gear ratio of 2.46 and axle ratio of 4.27.
Which one would be better in terms of generating high torque? Is the casing of both same?

[ex670c]

Originally Posted by robinson.s

I have been using CJ3b with peugeot engine and 3 speed transmission for all my off road trips. The kind of engine breaking I get in that is, amazing.
The tracks I normally go for OTR is heavy, and without having high torque on low, it is practically difficult to drive through such stretches. With respect to the clearance, I plan to put high profile tyres. (Bighorn 275/70/16) and then increase the height of leaf springs.
For Invader there are two types of transmission
1. BA-10V with a low gear ratio of 2.48.1 and axle ratio of 4.88 and
2. KMT90 with low gear ratio of 2.46 and axle ratio of 4.27.
Which one would be better in terms of generating high torque? Is the casing of both same?

Hi Robinson,

Before DD starts of with A Horse, A Horse, My kingdom for a Horse!!!!!

Regarding your CJ3B
1) What is your Differential/Crown-Pinion Ratio? 47/11, 44/9, 43/8
3 speed 1st Gear = 2.78:1
T-Case =2.46:1

2) BA10 came with the XD3P and corresponding Diff-Ratio is 4.88 (44/9)
Crawl Ratio = 4.3 X 2.46 X 4.88 = 51.6:1

3) KMT90 came with some XD3Ps, mostly MDI3200 and MDI3200TC with 4.27 (47/11)
Crawl Ratio = 3.986 X 2.48 X 4.27 = 40.87:1

Regards,

Arka

[siddartha]

@sankar

"In a Petrol engine with bad worn out valve guide the engine smokes when its idling, but as the throttle gets opened the smoking becomes less. This is due to engine oil being sucked in through the valve guide in the closed throttle partial vacuum phase (not applicable for engines with stem seals), once the vacuum is relieved by opening the throttle, engine sucks in air instead of oil. "


i remember reading the exact same wordings somewhere else, can you please give me the link or mention the source.

[Samurai]

If I recall correctly, the conclusion of this thread was diesel engine has lot less engine braking than petrol engine. However, the low end gearing of the Jeeps being much lower than Gypsy makes it feel like Diesel engine has more engine braking.

In other words, what makes No-ABC technique possible in Jeeps in 4L is not engine braking, but the crawl ratio. That was a major shock to know, but an excellent thing to learn.

Here is a related question.

We all know that when we go down the slope in regular gears without pressing A-pedal, the cars/jeeps still tend to gain speed. However, if you go down a slope in 4L gears (say 1st or 2nd), you don't tend to accelerate, in fact you can feel something holding you back, keeping you to the crawl ratio. In fact, it feels like engine braking.

Why is that?

[headers]

Originally Posted by Samurai

We all know that when we go down the slope in regular gears without pressing A-pedal, the cars/jeeps still tend to gain speed. However, if you go down a slope in 4L gears (say 1st or 2nd), you don't tend to accelerate, in fact you can feel something holding you back, keeping you to the crawl ratio. In fact, it feels like engine braking.

Why is that?

It is ENGINE holding the vehicle's momentum which many call by different names!!

Just try pressing the clutch to realise the difference - But I did NOT tell you this

[Samurai]

Why it gains speed in H while it doesn't in L?

[Red Liner]

The engine feeds power into the prop-shaft which in turn feeds it into the ring/pinion - which in turn feeds it into the axles.

If you de-clutch, there is a power loss at the source. So whatever gear you are in, you will free-fall.

What these gears do is they just multiply the torque by a number of times and thus restrict engine power to be driven fully into the axles at any rev-range. Thus, this is why your engine RPM shoots up and you feel the engine working a lot harder (what in effect its doing is "fighting against the crawler gears" to supply all its power to the wheels - when you're going downhill. Your engine is trying to force power into the axles, but the ring/pinion/Gbox is distributing that power in minute quantities. Which is why, when you brake in a decline, your engine stalls.

So in effect - you're running the same engine power (BHP does not increase). Torque increases (the engine power does not get wasted in wheel-spin), you go slower, and thus you have more time to make corrections.

Sorry, this aint a text book response with numbers and figures. I will leave that to ArkaJEEP.

[The Wolf]

Lovely topic, love the technical ambiguity that is being created here...hehe!

To start, there are two things in this whole deal that is being dealt as one hence the confusion. The surprise is that there is a difference between Engine braking and Gear braking.. but both, when applied together, causes retardation of kinetic energy transfer to the axle or drive shaft through the gears causing the vehicle to slow down

Let me explain the first phenomenon;
Engine braking: This is when you make the piston run against or toward the engine head with the valves/throttle body valves shut causing subtle vacuum inside the cylinder. Simpler example, in your right hand, take an empty injection syringe without the needle in it, shut the needle side orifice with the fore finger of your left hand and now try pressing the plunger into the barrel with your right hand thumb. What happens? Either the air inside the barrel tries to escape forcefully through the gap it creates between the orifice and your left fore finger skin or simply the plunger wouldnt budge. Repeat the process inversely, now try pulling the plunger from the bottom most point with the orifice shut, the plunger wouldnt budge again. Well this is what happens exactly during engine braking. The piston is made to pull or push against the presence of free available air or combusted incylinder gas respectively.

Note: In case of diesel engines where there are no throttlebodies which in our context work as secondary intake valves the vacuum created is relatively lower that the petrol engines which necessarily has one. The space in between the throttlebody valve and the intake valve works as secondary vacuum chamber when the piston is made to work against keeping them shut.

Gear braking: We all know that the torque produced at the crankshaft, due to the piston travel, is transfered to the wheels through gears used in the gearbox or torque converter/multiplier as we call it and the transfercase via an axle or driveshaft. Each gear has a certain size in diameter and the corresponding number of teeth or cogs right? Now, why is one gear different from another? Simply coz of their difference in running diameter and teeth count. For every power stroke there is a certain amount of torque/rotational force produced at the crank shaft, now when this torque requires to be transmitted to the wheels appropriate gears are applied starting with the one that can produce the maximum among all the gears, usually the 1st gear...then the 2nd gear and so on.

Now the important bit, each gear has its organic limitation as explained above due to which that can rotate only up to a certain speed for the given input hence one gear allows you to go upto one certain speed. The Low gearbox has a higher final reduction/final drive ratio than the High gearbox hence they allow only lower speeds for the given rpms at the flywheel.

@Samurai: Does the above answer you question "Why it gains speed in H while it doesn't in L"?

When Engine braking and gear braking both are applied at tandem or together one can imagine the retardation that can occur at the wheels. Hence the 2kmph descends at 45deg without throttle, almost antigravity phenomenon

[tsk1979]

Its simple. Imagine a gentle slope.
In third gear you will have sufficient braking.
Now make the slope steeper.
you will need second gear
Make it even more steep.
you will need first gear.
In the Himalayas there are slopes so steep, that even in 1st gear, you need gentle brake tapping.
Now make it even more steeper. You will need an even shorter gear.
In 4L, you will need such a steep slope, that if its untarred, the vehicle will skid down.

[DirtyDan]

Originally Posted by tsk1979

Its simple. Imagine a gentle slope.
In third gear you will have sufficient braking.
Now make the slope steeper.
you will need second gear
Make it even more steep.
you will need first gear.
In the Himalayas there are slopes so steep, that even in 1st gear, you need gentle brake tapping.
Now make it even more steeper. You will need an even shorter gear.
In 4L, you will need such a steep slope, that if its untarred, the vehicle will skid down.

This is the effect, not the cause. In LOW range and in lower gears you are forcing a Mahindra diesel motor to rotate (RPM) more upon descent per distance travelled downward than most petrol motor set ups and their ratios. In other words, Mahindra's older gear ratios (some changed recently because of emissions) make the engine turn faster on descent and this creates more drag, more braking. This makes up for the difference between diesel and petrol, I think. This agrees with Mr. Wolf above.

I changed my Invader differential crown gears from the OEM stock 3:73 to 4:88 and the difference is amazing. I have roughly 20% more low end power for starting from a stop and also 20% more engine braking. By the way, I still get about 12.2 kpl fuel average around town and 14.6 on the hwy at speeds around 60-90 with the 4:88 crown gears. And, it really helps the Invader climb the big hills around here. The difference is like forcing all your tranny gears down one gear. I am very happy with this mod.

And, no, it did not magically make my Invader into a Pajero or a MM550. It still has a crappy rear suspension and the paint and finish is still laughably chintzy.