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Why do people rely so heavily on compression braking?


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75 replies to this topic

#26 MilesFox

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Posted 18 July 2005 - 02:18 AM

you cant just cram 65 miles per hour into 2ng gear ithout doing something in between

i drive by when i need to slow down, i let off the gas. that easy. this would be the real "downshifting" we would like to refer to because its the "negative tourque" that creeps you down, coasting with the engine engaged to the tranny so to speak. why would we need to shift into neautral. neutral is for when youre not moving:)

if you need to slow down use the brake. if you are still within ghe gear's rpm you can go again with the gas, since you dont have to use the clutch

so much as you slow down more than the gears, rpm, of course, downshift. my rer drum shoes are about to fall apart so im easy on the wheel cylinder, i downshift whilt riding on the brakes. not cramming the brakes all at once, not dragging them, but just cousting withy clutch out and making "jabs" at the brake although not all hard and such. brake to stop from slowing

what i will do is let off the gas and use the brake, shif tinto 2nd as i roll up to a stop, neutral waitong for green

slowing down form an exit ramp i will just coast as i merge off, in gear to start losing speed. as the speed limin reduces around the ramp i will downshif to 4th and touch the brake here and there. coming to a stop 2nd gear in or out of the clutch as necessary, brake to stop

simple dynamics, "let off the gas"

#27 MorganM

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Posted 18 July 2005 - 08:39 AM

Re: Why do people rely so heavily on compression braking?

A: Why not? I'm not worried for one single second about wearing out my clutch any faster. The only time you are wearing the clutch is the action of pressing the clutch pedal down OR letting it out; the clutch is not slipping when the pedal is all the way in or out. It's upto the driver on how long they care to 'slip' the clutch and thus how much 'wear' they put on it. Clutch friction plates wear out prematurely from ppl slipping them too much; not from properly shifting... up or down.

#28 Tin Soldier

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Posted 18 July 2005 - 08:52 AM

All I have heard is arguments for brake and clutch wear, which in my humble opinion, who gives a crap? You will have to replace those things eventually anyhow and you probably won't still have the car when it's time to do it. I have only ever had to replace one clutch in my ten + years of driving.

Now here is my unasked for three cents worth. :)

I live in the mountains of Pennsylvania. I have driving sticks and autos in the snow and rain and crap, as has my wife.

We would both take a stick that can be downshifted over an auto any day of the week. She once wrecked a car because the auto wouldn't down shift and the brakes locked up.

I always downshift and I still have to replace brake pads. I have never had to replace a clutch on a sube, but if I ever do it will be from offroading and not from downshifting.

Again with downshifting I am always in the right gear to go again. And I would thinking that running your engine in the optimal rpm range would get you better gas mileage then not.

I get 24 mpg city driving on a 92 loyale with 26.5 tires on it. 2" exhaust and Cone air intake. Downshifting always. Doubt gas mileage is effected by downshifting. :)

Just don't be drunk and go from 5th to 1st like my uncle. Bad things happen. :drunk:

#29 Gravityman

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Posted 18 July 2005 - 09:13 AM

Hey just to throw a wrench in the mix.

I use compression braking all the time in my AUTOMATIC SPORT SHIFT 05 Legacy 2.5GT. No clutch to wear out no brakes wear out. I have the shifter on the steering wheel and just hit the down shift to drop the gear and it works great. NO wearing anything, smooth power transfer and everything.:brow:



#30 TahoeFerrari

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Posted 18 July 2005 - 11:14 PM

I think neutral coasting is illigal. Reason being you have no way to speed the car up in an emergency. Not sure what that emergency could be. Maybe the motorhome behind you burned up his brakes?


From the California Motor Vehicle Code:

"Coasting Prohibited
21710. The driver of a motor vehicle when traveling on down grade upon any highway shall not coast with the gears of such vehicle in neutral."

That's it. The complete section 21710. It doesn't say a anything about coasting on the flats or uphill - only downhill. Doesn't say why.

and from the California Commercial drivers license handbook:

"Turns
As you approach the turn:
* Use turn signals to warn others of your turn.
* Slow down smoothly, change gears as needed to keep power, but do not coast unsafely. Unsafe coasting occurs when your vehicle is out of gear for more than the length of your vehicle."

Doesn't reference any section of he motor vehicle code, but does define "unsafe coasting"

I think most states have something like this in the Vehicle Codes. Originally I kind of thought - like some others have mentioned - that the prohibition on coasting was a relic of some bygone era when vehicles had band brakes and later on had undersized drum brakes.

I still think that when it comes to personal transportation vehicles - that is motorcycles, cars, and light trucks. But I think we all would agree that it would not be a good idea for busses and heavy trucks to be screaming down a 5 or 6 percent freeway grade at 70+ mph in neutral! (Although I've been passed while going downhill at 75 by semis on interstate 80 in the Sierra Mountains - usually in the middle of the night. Pretty scary).

Also this:

A friend of mines son was recently failed on his drivers test for putting the car in neutral while waiting at a traffic light. It happened somewhere in the San Francisco area. The DMV examiner didn't say why you shouldn't do this - only that the clutch should be held in and the car left in gear. I don't know about where the rest of you live, but around some parts of California there are some pretty long lights - long enough to give you a cramp if you keep holding the clutch in (even in a Subie, not to mention something with a really beefy clutch). I certainly don't suscribe to the no nuetral at a traffic light - but it's in the back of my mind to pass on to kids, nieces, nephews, etc. at the right time.

I would also like to ammend my earlier statment about only using compression braking on long, steep downhills and in city traffic. In the winter I live by compression braking. Best thing you can do when driving in lots of snow and ice (we get 20+ FEET of snow here in the winter - sometimes 3 or more feet at a time) is pretend you don't have brakes. Never get going too fast and always use the engine compression to slow down.

"Sorry officer. I just don't know why this darned think keeps jumping out of gear. Only seems to do it when going downhill or when the transmission is lightly loaded!"

#31 hooziewhatsit

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Posted 18 July 2005 - 11:43 PM

Also this:

A friend of mines son was recently failed on his drivers test for putting the car in neutral while waiting at a traffic light. It happened somewhere in the San Francisco area. The DMV examiner didn't say why you shouldn't do this - only that the clutch should be held in and the car left in gear. I don't know about where the rest of you live, but around some parts of California there are some pretty long lights - long enough to give you a cramp if you keep holding the clutch in (even in a Subie, not to mention something with a really beefy clutch). I certainly don't suscribe to the no nuetral at a traffic light - but it's in the back of my mind to pass on to kids, nieces, nephews, etc. at the right time.


only thing I can come up with is... you're sitting there in neutral, and something big comes up fast behind you, not able to stop. If you're in gear, you can hopefully punch it and get out of there (provided the intersection is clear). If you're not in gear, you'll probably get crunched. This is also probably why you're supposed to park far enough behind the car infront of you to see their tires - so you have room to move around them if needed without backing up.

#32 TahoeFerrari

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Posted 19 July 2005 - 08:35 AM

I think someone back in this thread also said - or at least implied - that coasting can damage transmissions but I can't find it right now to quote, so here goes anyway:

I seriously doubt that this is true in all, if not most cases.

Specifically in the case of manual 2wd and manual non-FT 4wd transmissions. As I said in my first post on this tread, I've coasted many, many miles in neutral with this type of transmission in my lifetime - Subies, Fords, Chevies, Triumph, MG, and probably a few others. I'd estimate at least 100k miles and maybe as much as 200k miles - often for distances of 10 miles or more at a time. Even have done some coasting with auto trannys, but I don't do that any more - see the first post.

You also would not see so many manual transmissoned 2wd and non-FT 4wd cars and trucks being used as toads if it was trashing the transmissions.

Even with 2wd and non-FT 4wd auto trannies, I believe that most can be safely coasted with the condition that the engine is left running. The reason is that when the engine is running, the input shaft to the transmission is being driven and thus the internal pumps are
running and transmission fluid is being circulated and able to do its job lubricating and keeping internal valves in the right postition to enable/disable internal workings of the tranny. With the engine off, all bets are off.

Many (most or all?) owners manuals have a prohibition against towing with all four or even 2 wheels on the ground. They do not say anything about coasting in neutral. This has become much more prevalent since the advent of FT 4WD / AWD.

From the owners manual for an '88 Jeep Cherokee:

"Commerical Tow Truck Hookup. Always tow with the front wheels turned all the way to the right and lifted off the ground." (I don't understand why - see the following)

NOTE: Selec-trac is FT 4wd auto tranny only and Command-trac is PT 4wd

"Recreational Towing (Behind Motorhome, etc.):

Selec-Trac: 1) trans in N(eutral), transfer case in N, shift trans to D(rive) and make sure there is no vehicle movement, shift trans into P(ark) Warning: With the transfer case in N, the vehicle could roll unexpectedly..."

Command-Trac: xfer case to 2H and make sure front axle is unlocked. Then trans to N, xfer case to N, and finally shift manual trans into gear or auto trans into park. Warning: same as for Selec-Trac).

and finally

"Towing a disabled vehicle: If towed with front or rear wheels off the ground, limit speed to 30mph and distance to 15 miles"

So you can tow it forever at any speed behind an RV with all four wheels on the ground but cannot tow it with two wheels off the ground over 30mph for longer than 15 miles????????
(I'll grant you that this is a bit of apples and oranges here - with the transfer cases in neutral, the transmission is completely disconnected from the rest of the drive train. I don't think I'd want do do that while coasting down a hill. BTW: I think most of you know there is a spot with the d/r 5spd - and maybe 4s -shifter in Subies between 4H and 4L that disconnects the trans from the diffs - just no detent or any way of keeping it there)

From owners manuals for '87 & '89 Subies: (all you EA82 owners know this, right?) No quotes here - it's pretty wordy but boils down to the following.

FWD - put in N and don't tow auto trannies over 20 mph or longer than 6 miles without raising front wheels. No restrictions on manual tranies other than to put in neutral.

selective 4WD - put manual tranny in FWD and N - no speed/distance restriction. Auto tranny, put in FWD with tranny in N. Never exceed 20mph/6 miles without raising front wheels.

FT 4WD precautions: manual tranny with all 4 wheels on the ground - make sure diff lock is off and the diff is actually unlocked (gives procedure). "When towing with a rope or with 2 wheels raised, unlock the diff" and don't exceed 20mph/30 miles. Auto tranny - with all 4 wheels on the ground, trans in N, put a spare fuse in FWD connector, and never exceed 20 mph/6 miles.

I couldn't find anything about use as a toad. But from the preceding, it looks ok except for auto trannies.

And, finally (right??), from '82 Subie owner's manual:

Well, it pretty much says the same as '87/'89s except oviously no mention of FT 4WD.

I'd say this stuff supports my opinion that rolling down the pike in neutral with a manual ranny is OK from a mechanical stand point, but not with automatic trannies. It even looks like it would be OK with a manual tranny FT 4wd Subie as long as the diff is not locked. Don't know about newer AWD though - but this is the Old Gen. forum, right?

I can't believe I'm doing this much research - I've got waaaayyyy too much time on my hands. I need to find a job and get back to work.

#33 PopsicleMud

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Posted 19 July 2005 - 01:15 PM

The only time you are wearing the clutch is the action of pressing the clutch pedal down OR letting it out; the clutch is not slipping when the pedal is all the way in or out.

This is exactly the point I wanted to make. The clutch only wears for a second or two for each shift, whereas the breaks are wearing the entire time you use them.

From my '91 Loyale manual*:

Page 6-7: "Never coast in neutral."
Page 6-8: "On steep downgrades, downshift the transmission to '4th,' '3rd,' or '2nd,' as necessary; this helps to maintain a safe speed and to extend brake pad life. In this way, the engine provides a braking effect. Remember, if you "ride" (over use) the brakes while descending a hill, they may overheat and not work properly."

Fuji Heavy Industries said it, I believe it, and that's that. ;)

It also says, "When it is necessary to reduce vehicle speed due to slow traffic, turning corners, or driving up steep hills, downshift to a lower gear before the engine starts to labor," and "When driving speed is reduced, downshift through the gears to avoid placing unnecessary strain on the engine." I think these two are really talking about matching engine speed as opposed to engine braking, though.

*For some reason the previous owner of my '92 Loyale had a '91 owners manual.

#34 TheSubaruJunkie

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Posted 19 July 2005 - 01:37 PM

Reasons to compression brake rather than slamming on your brakes...

1) Its illegal in some states
Thats right, in Anchorage Alaska where I took my drivers license test... the ONLY thing I got docked on was the fact that I coasted to a stop by putting the car in neutral and braking from 35mph to a stop. When I asked why, i was explained this by the DMV personel, which brings me to...

2) You have more control while braking
While engine braking, your tires are guaranteed to still have traction. Kinda like ABS before ABS was invented, if you press the clutch and brake you disconnect your drivetrain from the engine powering it... and you risk the chance of locking up your tires or losing control of your vehicle. If your car is in gear, you still (or should) have complete control of the vehicle while stopping.

3) To save my brakes
As Morgan said... you only cause wear on your clutch when you are slipping it. When the clutch is fully depressed there is no slipping and no wear being created. You create more wear and tear on your clutch by accelerating than decelerating could ever cause (unless you dont know what your doing). Ride your brakes on a mountain decent, and then do it again with the clutch. Tell me which you smell 1st... clutch or brake.

4) It sounds cool!
Thats right... nothing sounds cooler than a un-muffled subaru compression braking. I do it for fun even if I dont intend to stop.... just downshift to hear the engine decell.... its great fun to scare the ricers.

-Brian

#35 TahoeFerrari

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Posted 20 July 2005 - 04:38 AM

From my '91 Loyale manual*:

Page 6-7: "Never coast in neutral."
Page 6-8: "On steep downgrades, downshift the transmission to '4th,' '3rd,' or '2nd,' as necessary; this helps to maintain a safe speed and to extend brake pad life. In this way, the engine provides a braking effect. Remember, if you "ride" (over use) the brakes while descending a hill, they may overheat and not work properly."

Fuji Heavy Industries said it, I believe it, and that's that. ;)


Sounds like the attorneys at Fuji Heavy Industries are just making sure their butts are covered to me.

#36 azsubaru

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Posted 20 July 2005 - 08:40 AM

Did I understand that right? - that coasting a manual transmission in neutral could cause damage? That makes no sense at all. Some auto manufacturers have always claimed that coasting with an automatic might cause damage, but I've never heard of it happening. Anyone on the board ever done it?

I have noticed that it's illegal to coast, at least in many states, but no one ever explained exactly why from the legal point of view.

#37 subyrally

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Posted 20 July 2005 - 09:41 AM

in my impreza, i rarely actually use the brakes.i just let off the gas and let it slow down. i only use the brakes to keep the car from idling through the intersection on red. my impreza will idle uphill:drunk:

but when i am driving around town, you can hear me coming to the intersections due to the not so stock intake and exhaust....basically i get an exagerated boxer rumble when coming to the intersections.

but using the engine to stop and only using brakes when i actually have to, i manages to get 50k outta worn out brakes on my car, mainly cause at first i wasnt aware they were worn and then cause i couldnt afford new brakes, well, i still cant really.


in the mountains when i am diving, well, spiritedly, i only have to actually use the brakes on one section due to the fact that its incredably steep, tigh, and there are a few hairy corners. thats also the road i love to drive cause i can get airborn at 40mph if i wanted to. any other time, it usally takes about 80mph in the impreza. then again, i drive way too fast on most back roads.



either way, ive been using compression brakig as long as ive owned my car, it still runs nearly perfectly, it would run better with a tune up though, its at 125k and i was supposed to have a 90k service, that never happened.

#38 NorthWet

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Posted 20 July 2005 - 12:37 PM

There are situations where compression braking is very appropriate, and others where it is just not needed. But people are creatures of habit, and they will either tend to do it all of the time or never do it at all. Given the choice between the two, I personally would overuse compression braking. (Plus, I like the "boy racer" feel of it.)

Anyone who has smoked their brakes coming down a long incline understands the importance of compression braking. I did this in a 1-ton truck, loaded and pulling a loaded car trailer, down a mountain pass in the summer. I was behind a low-boy hauler with a humongous excavator on it, through a construction zone that was down to 1 lane. Over 10 miles of incline at 12 miles per hour. I was in first gear and wishing I had a granny-gear. My truck's huge front vented disks were smoking when I slowly drifted to a stop at the bottom of the hill.

That being said, WJM's comment about 4 wheel disks and Subarus is pretty much right on.

Compression braking is also sort of a side effect of being in the right gear. If your car is in gear and your engine isn't above idle, there is no practical way that you can accelerate if the need arises without taking that possibly crucial time to recognize that you need to change gears, figure out which gear you need to be in, and engage the proper gear. It can make the difference between being in an accident and avoiding one. Which leads to:

...I have noticed that it's illegal to coast, at least in many states, but no one ever explained exactly why from the legal point of view.

If you are in neutral you do not have proper control of your moving vehicle. The current control issues are based on the above avoidance scenario, but it used to be something more serious and more common. Prior to modern transmissions and the Highway System, it was not uncommon for a car/truck descending a grade to get into trouble because its engine quit while the car was going downhill in neutral. Without synchromesh transmissions, it was not possible to get the car into gear without the engine turning, and if the engine quit it was unlikely that it would restart easily. CRASH!

So, legally, you do not have proper control of your moving vehicle if it is in neutral.

#39 DerFahrer

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Posted 20 July 2005 - 12:46 PM

2) You have more control while braking
While engine braking, your tires are guaranteed to still have traction. Kinda like ABS before ABS was invented, if you press the clutch and brake you disconnect your drivetrain from the engine powering it... and you risk the chance of locking up your tires or losing control of your vehicle. If your car is in gear, you still (or should) have complete control of the vehicle while stopping.


This is an excellent point. I have used this premise to stop quickly several times in my non-ABS cars.

However, do understand that your brakes are MUCH more powerful than your engine, and you can actually stall your engine and lock up your wheels not only with the brakes, but with the drivetrain too.

Been there, done that, got the T-shirt :-\

#40 Syonyk

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Posted 21 July 2005 - 12:24 AM

I was going to comment on that too... more than once I've been slowing down HARD and stopped the entire drivetrain, engine included.

With regards to braking power & such:

Your wheel brakes should be able to lock up the tires on dry pavement. Period. If they can't, get them fixed (or, more likely around here, fix them).

For an emergency maximum deceleration braking event, you shouldn't bother leaving the engine engaged to the rest of the drivetrain. You're braking so hard that the effect of compression braking is negligible, and stalling out the engine & jolting it back into motion probably isn't terribly good for the engine or transmission. Plus, if you do end up locking the tires and coming to a stop, you now have an engine that's not turning. Not a terribly useful state for an engine to be in.

I engine brake constantly, usually with a well executed double clutch (I've been driving a stick just over a year, so it's quite possible to learn "advanced" shifting quickly, you just have to do it). However, if I need to come to a stop in a hurry, I push the clutch in and use the brakes. I have better modulation of the brake pedal, and more control over what's going on. Note that this is for emergency braking (maximum effect, just short of wheels locked) only.

-=Russ=-

#41 bushbasher

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Posted 21 July 2005 - 02:35 AM

There should be no noticeable difference in gas mileage with downshifting. The reason the motor is slowing you down is becuase it is going through all the work of compressing the air in the cylinders then venting it without getting any explosions to make up for that work. If the engine was burning anything but a negligeable amount of fuel you it wouldnt be slowing the car down. If you want to test this, go downhill with the engine holding you at a constant speed through compression braking, then shut the ignition off then back on. You wont notice a difference in the behaviour of the car.

#42 1-3-2-4

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Posted 01 June 2013 - 06:00 PM

one thing I noticed between the EJ22 and EJ25 is the Ej25 I don't have as good compression braking because the flywheel is a bit heavier, I think it's around 22-25 Lb vs whatever the EJ22 flywheel is, it's noticeable when shifting between gears how the rpms are being held.

 

yes I know this is an old thread but found it when searching for a rear view mirror thread :P



#43 maozebong

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Posted 02 June 2013 - 12:11 PM

There should be no noticeable difference in gas mileage with downshifting. The reason the motor is slowing you down is becuase it is going through all the work of compressing the air in the cylinders then venting it without getting any explosions to make up for that work. If the engine was burning anything but a negligeable amount of fuel you it wouldnt be slowing the car down. If you want to test this, go downhill with the engine holding you at a constant speed through compression braking, then shut the ignition off then back on. You wont notice a difference in the behaviour of the car.

 

actually, the fuel is completely cut during decel. pretty much zero fuel is used during engine braking, and if it is, you probably have an engine that pops and backfires on decel. 

i am 100% sure about this, i have a wideband o2 gauge/sensor installed in my car. during decel, the meter reads 99.99 (full lean, zero fuel present), and while coasting in neutral, the AFR meter reads 14-16:1.... same as idle. 

 

Edit: crap, this thread is 8 years old :\


Edited by maozebong, 02 June 2013 - 12:12 PM.


#44 skishop69

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Posted 02 June 2013 - 12:47 PM

It's all dependent on doing it correctly as has been previously stated. My 84 Brat... 10 years on the same clutch, roughly 80K miles and I'm just about to change my SECOND set of pads and the rear shoes are still on my first set. My 95 3/4 ton, BB, king cab Chev... 60K on the clutch I installed when I bought it, and just into my second set of pads and this truck is used for my camper, boat, sleds or hauling. I don't care what the *experts* (coughing noise *bullsh*t*) say, no where I can find has anyone actually ever done a real world study on this. I've got enough proof sitting in my driveway to know there's nothing wrong with it. Yes, there is a little extra wear on your TO bearing, but if you bought good parts, it shouldn't matter. Synchro's.... Negligeable wear if you shift properly and your fluid is up to snuff. Gas mileage.... No effect what so ever. Just because your RPM's come up does NOT mean more fuel is being added. If you don't step on the gas pedal, the ECU does not see an input from the TPS and will not add fuel. In fact, if you are decelerating, depending on the car, foot off the gas, the ECU will do one of two things: Take the injectors down to bare minimum, or shut them down all together, IE, deceleration cutoff. In the case of a carb, unless you step on the throttle, the throttle plate stays closed and vacuum at the carb does not increase, and therefore does not draw in any more fuel than idling. I've got a 60 mile round trip commute each day on the freeway in stop & go traffic. I may actually use my brakes to stop 12-15 times on the trip. It makes people pay attention, and the stay the hell back when they see you've slowed down and there were no brake lights. I do pay attention to the schmucks behind me to make sure they are paying attention, but then again I always do. Reality trumps theory every time.....



#45 l75eya

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Posted 02 June 2013 - 01:04 PM

75 in 5th vrmmmmmmmmmmmmmmmmmmmmmmm
Coming to a corner! Heel toe to 3rd for the 40 mph followhtrough. va-roommmmmmmmmmmmmmmmmmmrrrrrrrrraaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhhhhh 4th! VIRoommmmmmmmmmmmmmmmmmmmm..


Gotta lay off the caffeine...... :huh:


Edited by l75eya, 02 June 2013 - 01:05 PM.


#46 1-3-2-4

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Posted 02 June 2013 - 02:06 PM

I I've got a 60 mile round trip commute each day on the freeway in stop & go traffic. I may actually use my brakes to stop 12-15 times on the trip. It makes people pay attention, and the stay the hell back when they see you've slowed down and there were no brake lights. I do pay attention to the schmucks behind me to make sure they are paying attention, but then again I always do. Reality trumps theory every time.....

Good to see I'm not the only one that does this.. It's true it works but I've had people cut in front of me because I'm slowing down because I'm looking 4 car lenths AHEAD with the brake lights coming on, it's amazing some people only look as far as the front hood of the car..

 

About 4 months ago here in town from a friken block away I saw the road was blocked off for party.. It was amazing the same car that was beside me got all the way up to it then had to turn around and go the same way I went.. I just had to shake my head.

 

But yes you have to watch for others when slowing down with no brakes.



#47 subarurx yo

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Posted 02 June 2013 - 03:58 PM

I engine brake all the time, I just put er in neutral, rev match for the gear I want, let the clutch out and its probably wearing less on  the clutch than normal upshifting, plus you know that you will have vaccum for the brake booster, since for sure the engine is turning, unlike if you slap it in neutral and coast to a stop with brakes their is sometimes that possibility of stalling and loosing a lot of the brake force, which could be sketchy



#48 Idasho

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Posted 02 June 2013 - 04:26 PM

This thread needs to be renamed.  You cannot effectively "compression brake" with any gas engine.  You can however "engine brake"    There is a huge difference.

 

Compression braking is a mechanical method of slowing a diesel engine by releasing the compression on the up stroke.  This generates a vacuum on the down stroke, slowing things down.  

 It is required due to diesel engines not having a throttle plate. 

 

 

There should be no noticeable difference in gas mileage with downshifting. The reason the motor is slowing you down is becuase it is going through all the work of compressing the air in the cylinders then venting it without getting any explosions to make up for that work. If the engine was burning anything but a negligeable amount of fuel you it wouldnt be slowing the car down. If you want to test this, go downhill with the engine holding you at a constant speed through compression braking, then shut the ignition off then back on. You wont notice a difference in the behaviour of the car.

 

Actually on a gas engine it has little to do with combustion, and much more to do with the throttle plate,

 

An engine is nothing more than an air pump.  Keep turning the crank with a cork stuck in the intake (throttle plate shut) and you will create resistance in the form of a vacuum.  This slows you down.

 

 

Point a vehicle downhill with the engine off and your foot off the throttle and it may or may not move.  Apply the throttle and it will start rolling.


Edited by Idasho, 02 June 2013 - 04:27 PM.


#49 MilesFox

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Posted 02 June 2013 - 05:06 PM

Edit: I realize i already posted here...


Edited by MilesFox, 02 June 2013 - 05:07 PM.


#50 Gloyale

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Posted 02 June 2013 - 10:22 PM

I've recently started using heel-toe shifting (I think that's what it's called) for downshifting. Since then, I've had much smoother downshifts. (for those that don't know, right foot on brake & gas. While braking, you rev the engine up to the new gear you're downshifting to. Then when you let the clutch out again, the clutch should grab without slipping since the engine speed is already where it should be. This prevents wear on the clutch, and change in speed while the engine catches up)

 

 

This is the key here.  Rev matching....

 

And of course just like any clutch action it should be in or out, with not alot of slipping at all.  many people ride the clutch both on up and downshifts to keep everything all smooth...as a substitute for proper throttle technique.

 

The biggest wear with downshifting is actually on the Ring and pinion in the diffs, and in CV axles which normally have all their load in one direction.  Heavy compression braking loads thier opposite wear faces. 

 

But whatever.......It's not a big wear issue if properly Rev matching and clutching..... AND braking at the same time 

 

Heel n' Toe

 

The biggest thing for me is more control, espescially with an AWD car.  Braking you can lock up whereas a downshift will not. Braking affects mostly rear wheels whereas a downshift affects all wheels evenly and keeps the engine engaged with the roll action/balance of the car.  Properly done it's best to downshift and brake before or just entering a corner, and accelerate out.  Coasting during cornering and or braking just leaves you WAAAY less in control.






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