curious about engine breaking 1

[rwelk]

I don't know if anyone can answer this question. But I was just curious if engine braking a car (manual transmission) has any negative effect on the motor, or gearbox?

 

[GregLocock]

Incidentally Nick, you'll get the same load transfer so long as the retarding force is present at the contact patch.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SolderingGunslinger]

For the past few months, I have been teaching my 15 year old daughter to drive a manual transmission. She has taken quite nicely to catching a lower gear going into a corner (and at least partially using engine braking)and accellerating through the turn...much to her mother's chagrine. In a month or two, I plan to graduate her to something a little hotter than the DOHC 4-banger in the '02 Ford Focus...like the tri-power 390 in the '64 Galaxie Rag-Top...Pray for me

I remain,

The Ol' SGS

 

[NickE]

"evelrod - I still race, but no longer use the engine for anything but acceleration. Braking is solely by the quite capable brakes.

NickE - I find it interesting that no one here mentioned the use of engine braking in performance driving.

Actually, someone specifically mentioned NOT using engine braking in performance driving. Judging by the video footage, I'd say he's got a bit of experience too...

Oopss... I'll just go back to lurking.

Greg- That may be true, it just doesnt feel the same, that may be the way my brakes are biased (pad cpd, disk size). I tend to see much more dive with (?foot?)braking than with engine braking.

Thanks for pointing out the errors...
nick

 

[evelrod]

Nick, you see more "dive" with the brakes possibly because of geometry of the suspension bits and the fact that the brakes are MUCH more effecient at decelerating the vehicle.

SGS, steer clear of the high hp street machine for your kid. BTDT and it aint pretty in the long run. Instead, why not pick up a little used Miata and trick it out with the Spec Miata kit and take her to an SCCA approved professional school? Cost in the long run will be MUCH less, trust me on that one!!! Keep the high performance and racing on the track and leave the freeways to the rest of us dorks. (Being a competant and experienced race car pilot don't mean doodly on the street, but racing on the street is sure death)!!!!!!!!

Rod

 

[GregLocock]

Nick, Evelrod is pointing you in the right direction. By using engine braking you are using the anti squat geometry of the suspension for that axle, whereas when you use the wheel brakes you are using the anti (or pro) -dive characteristic of both axles. antisquat and antidive are similar in concept, but different geometrical constructions are required because the brake torque is resolved locally, whereas the drive torque is resolved at the diff in an IRS or IFS.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

Thanks, and of course with every answer new questions are raised.

My car is a Subie, WRX (and like every other young american rally fan it's blue too....). Does the fact that its AWD with viscous diff's in both the rear and center affect this?

 

[NormPeterson]

NickE - the AWD does, but the viscous diffs don't (as long as all of the wheels are rotating at the same speed).

Your engine is providing some braking at both axles, so you're invoking axle-height-based amounts of anti-dive up front and anti-lift out back. Your outboard brakes work with ground-level-based amounts of A-D and A-L (which are greater than the axle-height-based effects, hence braking with the brakes gives you less pitch than does an equal deceleration using only the powertrain).


All - in slow-moving and stop-and-go traffic I use engine braking to as great an extent as is practical, though that's as much a function of all of my cars having manual transmissions as anything else. All else equal, more engine braking is available, brakes don't get glazed, throwout bearings last longer, and I'm more relaxed for not having to keep a tap-dance going on all the pedals (save for having to occasionally alert some moron behind me who watches for brake lights but not closing speed).

It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine. With all the braking out back,

As far as the usefulness of engine braking in performance driving goes, there is some limited applicability at least in autocross with front-engine rear-drive cars, which normally understeer. You're planting the front tires for better grip and unloading the rears for less grip (directly) while simultaneously applying longitudinal load to the rears (indirectly leaving still less available grip for lateral purposes). The overall effect is for the fronts to turn in better and the rear to drift a little wide for greater yaw response. In my case with 5.7 liters of 10.2:1 engine and ~10 mph/1000 rpm gearing this effect can vary from "clearly noticeable" up to "more than I need or even want".

Norm

 

[NormPeterson]

It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine. With all the braking out back,

should be edited/completed to read


It's definitely possible to have too much engine braking in slippery conditions is not the hot tip if you're driving a RWD vehicle, especially one with a fairly large-displacement engine. With a disproportionate amount of the total braking out back, a spin is entirely possible if you try to use the additional engine braking effects of lower gears. Let's just say that a car with a rear end that starts making threats about leading the parade for a change has a way of getting your attention. I can thank one of my folks' 6 cylinder mid-1950's cars for that little bit of (fortunately) no-cost education.

Norm

 

[MikeHalloran]

Engine braking is _way_ more dangerous in a FWD vehicle.

The slightest drag on the front wheels applies a retarding force, _ahead_of_ the center of gravity.

Result? Lift, and the car turns around, really fast.

It happens before you can apply the brakes.

I figure it's the real reason why old Saabs had overrunning clutches.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[SolderingGunslinger]

I have no intentions of letting my daughter drive the galaxie on a regular basis, however one of these days she'll get behind the wheel of a car with more horsepower than the Focus she's been learning to drive in. I want to take her somewhere where she can discover that all cars are not created equal.

Gee, I wish I knew someone at the Ford proving ground so I could get her some time on the skid pad.

I, on the other hand, learned on the road Ice of a Nebraska Winter. Talk about the school of hard knocks.

--SGS

 

[MikeHalloran]

Many winters ago, as instructed by my DriverEd teacher, I was burning donuts on a freshly plowed shopping center parking lot. Think 'skid pad with cushions'.

Someone in an adjacent residence must have been awake, at two a.m. Along comes a cop car, so I slide to a stop and roll my window down.

"Son, what are you doing?"

"I'm practicing skidding, Sir."

Cop turns to his partner and says,

"NOW, I've heard everything."

... and drives off.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[ivymike]

I had almost the same experience - except when I got pulled over, they tried to pin some prior act of vandalism on me. Apparently weeks earlier someone had run over the sprinkler system at the high school while spinning donuts on the lawn, and they figured "hey, this guy likes donuts, maybe it was him!"

 

[DaveKillens]

I would strongly recommend attempting to teach a relatively inexperienced driver to downshift as entering a corner. Even expereinced, skilled racers can get into trouble doing that.
Use the brakes to decelerate, and the engine to accelerate. They were designed for those functions. Brakes make poor engines, and vice versa.

 

[BarneyD]

To enlighten some of you on the effects of engine braking, let me give you some of my thoughts on this.
I have replaced the flywheel on my Integra with an 8.8 lb. flywheel. The factory flywheel is an 18 lb. boat anchor. The reason for this was to make the car more roadcoarse friendly and get more acceleration out of the engine. The whole idea was to lighten the rotational mass of the rotating assy of the motor. It made a tremendous differece.
Now, to get to the point of this thread. Upon taking the car to Gingerman raceway in Michigan, the benefits of the flywheel were very obvious. I was able to use more engine braking while downshifting, (even though I heel toe while blipping the throttle), and it helped me slow the car much quicker due to the lessened rotational mass. The engine was able to slow down quicker which in turn slowed the car faster under engine braking. The acceleration of the car was incredible. The engine really came to life on acceleration. I've seen this happen on an NSX I replaced the clutch in also. I replaced the factory flywheel in it with a 12 lb. It really came to life.
I'd personaly rather buy brakes than a motor, but I enjoy driving too much to do that! Plus, I can rebuild the motor if need be. At the present, the engine has 40,000 miles on it.....HARD miles, with no issues. I say drive like you want to. Enjoy life while you can.
Have a great weekend guys,
Barney

Barney Demonbreun
Technician for Gary Force Acura, and owner of;
Ashland Motorsports
1304 Big Marrowbone Rd.
Ashland City, Tn. 37015

 

[JayMaechtlen]

"go down the hill in the same gear they went up it"-
they do...

"D"

I learned on the junk, but haven't had anything vrey hot yet...
But the 3.8 V6 Corvar-thing may qualify...
Looks like it will be about 2200 pounds all up!

cheers
Jay

Jay Maechtlen

http://home.covad.net/~jmaechtlen/

 

[Rob45]

First:RMW, that word is "hygroscopic"

Second, if it were cheaper to use the wheelbrakes than the engine brake, heavy trucks wouldn't all be equipped with "jake" brakes as they are in the U.S.
The Jacobs and similar brake devices enable an engine to develop as much braking horsepower as when accelerating, and this can be up to 600 HP or so.

Particularly with trucks in the heaviest classes, it is almoost impossible to fit them wtih wheelbrakes that will enable them to stop as quickly as the average car, and the fade resistance is not so great either.

Probably 99%+ of the trucks we build today have "jakes."

 

[ntsqd]

Ever calculated the effective Horsepower of your brakes? It IS enlightening. Make it easy on yourself, use .5 for the pad Cf.

 

[Rob45]

Or make it easier still, and assume a .9 G stop.
----------------------
And for Greenracer, who's "never heard of a motorcycle racer using engine braking" - dare I suggest that your knowledge of motorcycle racing is limited to just the past few years?

Hints: Stirling Moss, Mike Hailwood,...

 

[evelrod]

Hi Rob.
I have spoken to Mr. Moss, years ago at the Long Beach GP and since I was on course and on my motorcycle at the time I am sure we may have spoken about motorcycles. However, to my knowledge, Mr. Moss never raced motorcycles.

Perhaps you ment John Surtees and Mike Hailwood.

As to the question of engine braking on a motorcycle---I still ride the 1948 Norton that belonged to the 4 time World Trials champion Bill Young---I may slow a LOT more than he would have, but engine braking is really dodgy.

Mike Hailwood did not start racing until he was 17 and by that time brakes on most all bikes were quite good. He won his first championship in 1961 on a 250 Honda.

Engine braking on race cars of old may have been, indeed, probably was a necessary evil. You must finish to win and you cannot finish if you relied solely on the drum brakes of some/most of the older race cars, e.g., Jag XK 120 or, in my case, a 50 MG. You just have no idea how thrilling it is to approach the same corner at the same high speed that you did just one lap before and reach for the whoa pedal and find it as hard as a rock with no apparent effect on diminishing your velocity! Early drivers often had large laundry bills. ;-)

One last comment---On many of the F-1 and Indy/Cart cars, G loadings greatly exceed 1G. I saw a accelerometer reading of 4.0 on one Al Unser Jr. car after just a short jab at the brakes. Since I have no firsthand idea how many G's those cars pull in a corner, and my experience was in the early 80's, I can only assume the magazines and TV commentators are fairly accurate. I do know firsthand that standing at the apex of T-1 on the Long Beach GP circuit and watching an F-1 car slow from near 200 to a bit over 75 is indeed one of the most awsome sights I have ever witnessed.

Rod

 

[JayMaechtlen]

Current truck brakes can't dissipate enough heat to handle long hills, heavy loads, and reasonable speeds all at the same time, I think. That's why they are required to descend a lot of steeper roads slower than cars are.
The engine and exhaust can dissipate quite a bit of energy, and do it for a sustained period. I suspect that it is cheaper overall to accept a bit higher engine wear and avoid the weight and other costs of a brake system that could really do a complete job. No reason, for example, that brakes couldn't use active cooling via liqud coolant and a air-liquiid heat exchanger.
The system could dissipate arbitrarily large amounts of heat and be completely fadeproof.
Of course, it would cost money, complexity, weight, etc.
I'll bet that forcing large volumes of air through each brake assembly could increase capacity greatly. Still costs something- what's the tradeoff? Of course, disk brakes probably cost more than drums, but they are on nearly(?) every car today...
Comments?

Jay Maechtlen

http://home.covad.net/~jmaechtlen/