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Why do people rely so heavily on compression braking?
Posted 16 July 2005 - 04:06 PM
Posted 16 July 2005 - 04:14 PM
Posted 16 July 2005 - 04:32 PM
I'll show you.
When the engine is accelerating, there are forces that cause the engine internals to move relative to the block. In this case, specifically, we'll talk about the rings being forced against the cylinder walls.
If the engine is accelerating, then the side of the rings that corresponds to the clockwise-direction of the engine will have the force on them as such:
The lines are the side of the cylinders that are getting the force (this is looking at the car... left is pass, right is driver), so the rings are wearing on those sides of the cylinder walls more.
Now, if you are using engine braking, that transfers the force to the OTHER side like so:
Now the rings are wearing into the other side of the cylinder walls more.
So, if you are breaking in a new engine, it would be 100% ideal to brake with the engine as much as you accelerate with the engine. This is pretty much impossible in the real world, but the best way to do this in the real world is to vary your engine speed, so that should explain how that phrase made its way into break-in instructions. That will cause the rings to break-in on all 360º instead of only 180º that would be used if you were to be accelerating or sitting at the same engine speed the whole time.
Even if your engine is old, I feel that it helps you wear your engine down evenly. We all know an engine will not last forever, even if it is a Subaru , so by doing this, you're wearing it down evenly and keeping your compression healthy.
This is the way I see it, but I'm no engine builder. So if someone who has a lot more experience disagrees with something, feel free to correct me.
Posted 16 July 2005 - 05:24 PM
I have no way of judging whether that statement is valid or not, but I have been driving manual transmission vehicles for about 35 years, and have always downshifted to use the engine to help stop. In all that time, the only problem I encountered was when a friend's Ford Falcon(!) blew a headgasket during a high speed downshift. My 91 Subaru has 190K on it, all of which I put on there, downshifting at stoplights daily all of those years, and the engine is still in excellent shape.
Don't know if it really wastes a noticable amount of gas - with the throttle closed, it shouldn't really add up to a lot. My gas mileage is around 33, but I never compared it without compression braking.
Based only on my experience, I would offer the opinion that combining a downshift with braking can get you stopped a LOT faster and safer than brakes alone if done properly, at least on these older non ABS vehicles.
Posted 16 July 2005 - 06:12 PM
Posted 16 July 2005 - 06:53 PM
Posted 16 July 2005 - 09:27 PM
This is another ongoing debate that's just as bad as engine oil choices!
Posted 16 July 2005 - 09:43 PM
Posted 16 July 2005 - 09:49 PM
.Brake fixing easy,transmission fixing hard,got it?Love XXXXXOOOOO,Ed
190K, original transmission, compression braking. I've never popped a clutch to slow down using the transmission, you have use a little feel, unless it's an emergency. Shouldn't be much if any harder on a transmission than upshifting, just pushing the gears the other way.
Posted 16 July 2005 - 09:56 PM
also, if you're going down a long hill you'll want to use compression braking to avoid heating up the brakes and not having them when you need them for the sharp curve at thew bottom...
Posted 16 July 2005 - 10:43 PM
I tested out the effects of compression breaking on my way back from La Porte california, 30 mile downhill. For about 5 miles of the road I just used brakes alone, that was all the subi could handle. Brakeing power went to zero. Finally got it stopped thanks to compression brakeing and really standing on the pedel and my brake pads where smokeing.
Now I know this is an extreme case. I really have nothing against you non-compression brakers. As long as you can stop your vehicle good for you.
One form of braking that I feel is totally wrong is holding the clutch in while coasting this could wear out your throwout brearing prematurly (just a theory)
One one more thing. While your subi is coasting the injectors are off. So no extra fuel is going into your engine. Only works on fuel injected cars.
Maybe I am a sadist but I love rowing the gears. Downshifting into the corner and letting that thing sissle on out of there, there is nothing better.
Posted 17 July 2005 - 02:45 AM
right now i'm getting 25.7 mpg in mixed driving conditions, mostly around town... I'll use engine braking for a while and see how it effects the gas mileage. If it doesn't.... then I'll do that from now on and save some brakes. I'll take about a week to experiment, get gas, and get back to you guys.
in my 94 legacy i get 23 mpg,that's mainly city driving with maybe about an hour total of highway driving during the week.i use engine braking to slow the car abit then reg brakes go on.
on the highway last week i did prob a big NO-NO!! was at about 65mph an coming up to the off-ramp i needed to take.realised i was going a bit to fast for it so what do i do?downshifted it from fifth to third without braking first.tach shot up to about 5000rpm (was at about 2500rpms) i did slow down though,quicker then my reg brakes would have done.(everything seems ok with those)there doesn't seem to be any effects from the downshift boo-boo
compression braking can't be that bad,18 wheelers do it
Posted 17 July 2005 - 03:50 AM
Posted 17 July 2005 - 09:18 AM
I usually shift to neutral and coast a while in anticipation of a stop (stop sign / traffic signal / brake lights ahead) and even on long, long downhills (only hitting the brakes hard occassionally to slow to the limit). I don't tailgate so this works even on freeways.
My Subies are all retrofitted with 4w discs so they do stop in a hurry if need be! The rear discs are really not noticably different (than drums) for routine driving. It's only when you want to stop in a hurry and get on them hard that you can appreciate the difference.
That said, the cars will stop even faster when in gear.
Brake pads are cheap (if you remember to check them and change them before trashing your rotors or drums), mechanical repairs are expensive (even if done myself - it takes a lot of time), and gas is expensive (and getting more so every day).
BTW - I've been driving like this for over 850K with various subies and maybe another 1M miles on earlier vehicles and have never had any problems because of it.
Well, maybe one. I used to put my autotranny Jeep cherokee in neutral for a down hill coasts and then match rpms and shift back into drive when i was done coasting. The physical direction to shift to go from neutral to drive is towards the back of the Jeep. In my Subies and my 5 spd Jeep, from neutral to 5th is towards the FRONT of the vehicle. Anyone care to guess what I did to the Jeep??
Right, I put the Jeep into REVERSE at about 60 mph! It killed the engine and I almost lost control of the Jeep. Lucky for me I was on a wide freeway and no one was in the next lane. Oh, the Jeep? Still driving it - it didn't seem to hurt it!!!!! But I don't coast with it in neutral any more. Maybe if it was my only vehicle and I wasn't so conditioned to the neutral to 5th shift I would do it, but it's really scary to put a car in reverse at that speed!
You've just got to pay attention out there!!
Posted 17 July 2005 - 10:00 AM
Posted 17 July 2005 - 12:24 PM
Posted 17 July 2005 - 01:27 PM
<H2 align=center>To slow a stick-shift car, should you brake or downshift?
My wife and I are at somewhat of an impasse. We debate (intermittently) whether or not braking a manual-shift car should be done solely with the brakes (meanwhile putting the car into neutral) or by gradually slowing down the car by downshifting--that is, shifting from fifth to fourth to third, etc., and allowing engine compression to slow the car. Which is better for the car? Downshifting and braking, or braking while in neutral? I guess what I am trying to determine is, which will need to be replaced first, the brakes or the clutch? --Gearheads, East Brookfield, Massachusetts
We have a problem here. On the one hand, the pretty much unanimous opinion of people in the automotive business is that downshifting to slow the car is a completely stupid and pointless practice that increases the risk of a premature (and expensive) clutch job. On the other hand, I always downshift myself. So you have to ask yourself who you're going to believe: (a) the massed weight of expert authority, or ( those automotive schmucks. I'll merely lay out the facts, and you can decide for yourself.
Among the more vocal exponents of the don't-downshift school are Tom and Ray Magliozzi, hosts of the radio show Car Talk. These guys admit you should downshift when driving down a long hill; otherwise your brakes heat up so much that the brake fluid boils and you lose your ability to stop the car. But on all other occasions, they argue, downshifting does nothing but wear out your clutch faster. A clutch job is expensive; a brake job is cheap by comparison. The proper way to stop is to rely solely on the brakes. Don't put the car into neutral right away, though. Wait till you get down to 10 or 15 miles per hour or just before the engine starts to lug, then throw in the clutch and shift into second in case you need to accelerate. When you come to a full stop, shift into neutral and release the clutch.
Numerous mechanics, auto engineers, and auto buffs echo this view. They say downshifting may have made sense in the 60s and earlier, when many cars had manual drum brakes. These were much less effective than today's power disk brakes and you needed all the help you could get stopping the car. But not any more.
OK, fine. But let's consider the advantages of downshifting:
- It's fun. Face it, this is the main reason anybody drives a stick-shift car in the first place, as opposed to an automatic like a normal person. You get to shift gears like Al Unser Jr. and tame a hurtling hunk of steel, even if all you're doing is going to the corner for a box of baby wipes.
- You have more control over the vehicle. This is the fallback contention of most downshifters once they realize how feeble the conserve-the-brakes argument is. By downshifting you're always in the appropriate gear for the speed you're traveling. Suppose you were a brakes-only type of guy doing 40 miles per hour in fourth gear. You see a red light ahead and brake down to 20. Suddenly in the rearview mirror you notice a runaway concrete truck bearing down on you. You want to accelerate out of harm's way, but you lose precious time shifting from fourth to second and get creamed. Whereas if you'd been downshifting and were in second already, you could accelerate immediately and plow into the car in front of you. All right, so maybe this isn't the ideal illustration. But you see what I'm getting at.
OK, I'm not claiming I've got an airtight case. Tell you what: I won't hassle you about tapping the top of your Coke can to keep the fizz from exploding, and you don't hassle me about my idiosyncrasies with the clutch.
Posted 17 July 2005 - 02:16 PM
It also seems to my feeble thinking, that if you downshift correctly, and don't burn the clutch, that the clutch will last a long time anyways.
Another thought I've had about the people who say they can stop quicker with compression vs. brakes... The tires only have so much friction or grip on the road. If you overcome that amount with either compression or brakes, or a combination of both, the tires will lock up. Albeit it's probably harder to lock up the tires with solely compression than brakes.
Posted 17 July 2005 - 07:17 PM
I'm driving a jeep right now that my dad bought brand new off the showroom floor, and it's got 220k miles on it. Dad drove it 110 miles a day to work and back.. and he's a "downshifter"
then about a year or two ago, dad bought an 80's audi, and let my older brother drive the jeep for a year.. he is also a "downshifter"
and, when I got in an accident in my xt6 last fall, the jeep was passed to me.. ALSO A DOWNSHIFTER...
well, the clutch has never been replaced. and the brakes have only been replaced ONCE in the jeep's 220k mile life. (they need to be replaced again soon, but I downshift, so I don't need them very much )
Posted 17 July 2005 - 09:30 PM
Brake pads are more expensive than nothing!
Coasting in Neutral is no good for your gearbox.
Posted 17 July 2005 - 10:22 PM
As long as you double clutch properly when downshifting (you should anyway), there is no extra wear on either gearbox or clutch.
Brake pads are more expensive than nothing!
Coasting in Neutral is no good for your gearbox.
Bingo. All this discussion normally assumes that you're wearing the brakes down *OR* the clutch down. However, there's an option that involves wearing neither down, and that's rev matching/double clutching.
If you downshift, and slip the clutch to get the engine back up to speed, then it is a "wear the brake pads or wear the clutch" issue, and using the brakes is better.
If you clutch in, downshift, rev the engine to match the new RPM, and clutch out, there's no wear on the clutch (if you do it right), because the clutch & flywheel are already at the correct speed. There is, however, wear on the synchros, and this may take some time if you have weak synchros.
The better (and IMHO more fun) option is to double clutch. Clutch in, shift to neutral, clutch out, rev engine to new speed, clutch in, shift to new gear, clutch out. If you're really good, you don't even need to use the clutch for this (unload the engine, match RPMs perfectly in neutral, slide into new gear). This involves basically no wear at all of anything other than the throwout bearings (which aren't even really wear items).
I compression brake in all my cars. I drive an RX-7 and a Subaru GL. Neither one has any torque down low, so sitting in 4th or 5th till a stop means I can't go if I want to - I hit the gas at 1500 RPM, and it just makes a bit more noise and I go basically nowhere.
I also have no issues with high RPM in either car. I'll double clutch up to 5000 RPM coming into a corner in the Subaru, and up to 6000+ RPM in the RX-7 if I want to get somewhere in a hurry. Coming down off the highway, I'll double clutch down through the gears, and get onto the brakes at sub-15mph. Can't say I've noticed much clutch wear.
Finally, there's no question as to which is a faster method: Using the brakes is MUCH faster, or at least should be - once you lock up all 4 tires (actually, slightly before that point), you're at maximum braking. I don't know of any engine that can lock up tires on dry pavement unless you badly misshift, and the brakes should be able to (if they can't, fix your brakes and avoid driving behind me, k?).
Posted 17 July 2005 - 11:42 PM
Posted 17 July 2005 - 11:54 PM
and about crashing my car ot involved a racing clutch and 700 rpm at about 40 but later experances failed to convert me
Posted 18 July 2005 - 12:43 AM
This may sound kinda dumb and noob-ish, but I have only been driving a standard for 6 or 7 months and still don't know all the ins and outs of it. Russ, when you talk about matching RPMs and double clutching, thats all cool but it doesn't slow the car down any correct? This is just to downshift to a new gear, will you be still traveling at (relatively) the same speed?
compression braking is whats slowing the car down when youre downshifting. at the exact moment youre downshifting and rev-matching, you arent dropping speed, but as soon as you let the clutch out the deceleration of the engine will slow the car.
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