Slant Six Forum :: Why do you compression brake? (if you do)

To slow a stick-shift car, should you brake or downshift?
27-Feb-1998

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Dear Cecil:

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

Dear Gearheads:

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 (b) 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.
Besides, we have to ask ourselves, how much harm can downshifting do? Sure, you're putting twice the wear on your throwout bearing and other critical clutch parts. But it's not like they make these things out of plastic. Why, I've been downshifting my Toyota since the day I drove out of the new-car showroom and . . . well, come to think of it, I had to get the clutch rebuilt at 85,000 miles. But everybody knows '87 Corollas had clutch problems.

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.

--CECIL ADAMS

Author:	markjs [ Sat Jul 16, 2005 2:22 pm ]
Post subject:	Why do you compression brake? (if you do)
I drive a manual transmission and when I brake I use the brakes (In neutral or clutch out). It seems to me that its much more sensible to put the wear and tear on your brake pads than to use the clutch to slow the engine down and end up with a very expensive and difficult repair job. I have heard the opinion that compression breaking does not wear the clutch but I am 99% certain that that is hogwash, not to mention that it puts extra stress on engine components. Sure I compression brake on long hills, but not when coming to an expedient stop. Have any studies on wear been done about this subject?....Just curious (and bored today).

Author:	emsvitil [ Sat Jul 16, 2005 4:20 pm ]
Post subject:
I do it with my truck (my valiant is an auto)..... 1. Virtually no clutch wear if you match revs on downshift (I double clutch) and then there's no slipping. 1a. Occasionally to stay in practice (for when the clutch master or slave goes out (been there, done that) I'll shift without the clutch by matching revs........) 2. What stress on engine???? Engine is under more stress when you accelerate... 3. You're in correct gear when you need to accelerate again 4. Good for gradual slow downs... 5. Really good for turns, incorporates 3 & 4 above, you gradually slow down, turn, accelerate.............. 6. No unnecessary brake wear. 7. The BIG trucks do it.......... Now if I really need to slow down, I'll use the brakes and pop it into neutral.........

Author:	markjs [ Sun Jul 17, 2005 11:30 am ]
Post subject:
Quote: To slow a stick-shift car, should you brake or downshift? 27-Feb-1998 -------------------------------------------------------------------------------- Dear Cecil: 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 Dear Gearheads: 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 (b) 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. Besides, we have to ask ourselves, how much harm can downshifting do? Sure, you're putting twice the wear on your throwout bearing and other critical clutch parts. But it's not like they make these things out of plastic. Why, I've been downshifting my Toyota since the day I drove out of the new-car showroom and . . . well, come to think of it, I had to get the clutch rebuilt at 85,000 miles. But everybody knows '87 Corollas had clutch problems. 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. --CECIL ADAMS http://www.straightdope.com/classics/a980227.html

Author:	kesteb [ Sun Jul 17, 2005 4:01 pm ]
Post subject:
I don't down shift when coming to a stop, but I also don't let the engine free wheel. I push the clutch in, right before I come to a stop.

Author:	Dennis Weaver [ Mon Jul 18, 2005 7:56 am ]
Post subject:
Downshift. I downshift everything I own, including the automatics (into "2" when stopping or slowing, but not usually "1"), for the reasons Cecil Adams states above. No ill effects that I'm aware of to date after 25 years of driving this way. D/W

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Author:	Jeb [ Mon Jul 18, 2005 3:37 pm ]
Post subject:
I something as big and heavy as my dad's Mack B-35, downshifting is almost neccesary, especially when your brakes don't hardly work worth a crap. I don't hit my brakes until I've hit 3rd or 2nd gear. All of your truckers do it. Whenever I pull a trailer in my pickup, I do the same thing as Dennis to slow down in order to save my brakes. The owners manual even recommends engine braking when going down steep grades or trying to slow a heavy load.

Author:	ceej [ Mon Jul 18, 2005 7:10 pm ]
Post subject:
I do and I don't. That doesn't sound right. Been driving a Diesel pickup with no engine/exhaust brake for some time. Downshifting just makes it wind-up with a load. Exhaust brakes are available, but not recommended by the manufacturer. Brakes pads last about 30k, then they're gone. On the 2 1/2 Ton Chevy, I down shift and let the TH400 eat the heat. Lots of cooling capacity, so coming off the pass in second keeps the speed manageable. 17 quarts for a filter change. At least it has a tranny drain. It's got discs front and rear, but they get pretty hot on the long grades. The Trans cooler is about 1/4 the size of my Valiant's coolant radiator. (Just a bit thicker.) I used to brake my 83 3/4 Ton Dodge Conversion van by shifting into second on the 904 lockup. Third transmission, and I stopped that. Kept burning 2nd out of it. That was fun hitting an on-ramp in Sacramento at rush hour... Not. On the old Flatbed, I use 3rd to slow down, when I can find it. Really need to rebuild that transmission. Got tired of not having brakes, so converted the front end to disc. Doesn't really need the help now. At least the sound of me jamming gears keeps the local kids amused. CJ

Author:	Jeb [ Mon Jul 18, 2005 7:22 pm ]
Post subject:
I have only driven one diesel that doesn't compression brake very effectively when you downshift and that is a 1990 Ford Dump truck. It has a 5.9 Cummins and a 6-speed Eaton trans. One of them grind em' and find em' units. No wondr why this thing requires constant brake work. I have never driven a diesel pickup so I guess I'm missing out on something. You think your trans is messed up, half the time the roadranger on the Mack won't come out of high range. You just have to keep flipping the lever and eventually it will pull back down into lowrange. I guess I might need to fix that someday.

Author:	steponmebbbboom [ Thu Jul 21, 2005 7:44 pm ]
Post subject:
Count one vote for NO on downshifting. It's hard on not only the clutch but also the syncros. Double clutch and that is another matter.

Author:	69a100 [ Sat Jul 30, 2005 8:14 am ]
Post subject:	Compression Brakes
I guess I know how to do it! I got 130K outta my clutch before it needed replacing. And thats doing it 85-90% of every stop I did. OH, and I got 90K outta the original brakes before they needed replacing.

Author:	emsvitil [ Sat Jul 30, 2005 1:13 pm ]
Post subject:	Re: Compression Brakes
Quote: I guess I know how to do it! I got 130K outta my clutch before it needed replacing. And thats doing it 85-90% of every stop I did. OH, and I got 90K outta the original brakes before they needed replacing. You must use the Flintstone method....... How long do your shoes last???????

Author:	argentina-slantsixer [ Sat Jul 30, 2005 2:25 pm ]
Post subject:
I'm with dennis in this one... I always downshift even on my 904. downshifting keeps the car "wake" on your maneuvers, increases handling, and wears nothing. The clutch is under more stress when you go off line slipping it than when you downshift. Bigger chances to wear clutch assy if you drive slow and easy and it kinda slips than if you're a vanishing point kinda driver, is the same that happens with auto trans: people like their shifts silky but that wear all the clutches inside the trans. Firmer and harsher is good and don't puts under stress nuttin but rear end maybe... but face it: if you have a /6 and a crappy 7 1/4 rear end, that rear end is already bigger than stock needs. If you own a pepped /6 and it has a 8 1/4 ain't gonna suffer, much less if you have an 8 3/4 but if you go bigger than real needs, is kinda dragging, like comparing a 727 and a 904 behind a slant.... unless that slant puts out 250 hp at the crank and I'd abuse it a lot, I'd stay with my 904. Me, personally, with all stick shift cars I ever had (99% of my cars, since auto trans are scarce down here) always downshifted and abused the transmission and never could relate clutch wear to downshifting. I even use no clutch between gears after 1st, and I downshift w/o clutch also. Of course, that requires a top notch "ear" and steel nerves so you won't be hesitating about when or where.

Author:	62savoywagon [ Sat Jul 30, 2005 3:34 pm ]
Post subject:
I don't downshift with a juice box...but always with a manual...actually I do a combination of both in a manual...downshifting and braking. I do a lot of driving at highway speeds and often have to make left turns across highway traffic. So I have to make sure my speed matches what gear I'm in for a quick left turn or from a dead stop depending on oncoming traffic. I also live on dirt roads in a rual area....brakes are pretty usless on dirt roads here especially when rain turns 'em to snot. Even dry they're slippery. besides, downshifting is a lot of fun!

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