MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Downshifting vs. braking

This may be apocryphal but I've always been led to believe that it is better for a car to use downshifting to slow the car than heavy break pressure.

Is this true?

Is it true for an old car?
Tysen

engine braking was certainly very common in my Dad's day before hydraulic brakes. Going down steep hills there was so much give in the old cable operated brakes that the large capacity long stroke engines provided a very necessary additional stopping capability.

Today, we all still engine brake to some extent - just lifting your foot off the throttle could be defined as engine braking, but it is all a question of balance between engine braking and wheel braking to get the optimum performance out of your car. The important thing is to have the car under your control down that steep hill and not it controlling you.

I always find it very entertaining watching the brake lights of other drivers down hills and round corners. There is an amazing variety of techniques out there still.

Steve
Steve Gyles

Well, would you rather wear out your brakes or your engine?

That said, I do use the engine to control my speed on decents as well. I routinely watch other drivers brake lights as well. Many times they are on the brakes while I'm actually on the gas. People just have no control of their cars.
Steve Simmons

Well, just my .02 cents worth here, but this very topic came up on the TD-TF board a couple of weeks ago and some interesting info was given that has made me think twice about downshifting vs. braking to slow my car.
A post there mentioned that when downshifting, tremendous amounts of vaccuum are created in the cylinders and pistons, enough that under an incline at speed to literally pull gudeon pins loose and seperate pistons from con rods. I thought this sounded extreme, but it has made me change my habits until I can get more info to substantiate or disprove it.

A side thought, Jackie Stewart the great Formula one driver used to say...."It is cheaper to reline brakes than to fix transmissions."

Clayton
C. Merchant

I would also think that the answer has to do with the length and severity of the downhill run. A few weeks ago I was test driving (in modern iron) the route of a planned Fall excursion for my local MG club. It will include a drive to the top of Brasstown Bald Mountain in North Georgia (4,800++ feet). That drive's downhill portion will have several miles of steep descents and switchbacks. Frankly, I am going to recommend using downshifting and engine braking to avoid brake fade. (I am also going to urge each club paticipant take part in a tech session: a very thorough braking system examination prior to departing Tallahassee.)
Frank Nocera

While I use engine braking personally at times, it is much less expensive to replace the brakes than it is the clutch.

YOMV

rick
rickingram

I agree Frank and I can tell you from experience that if I had to rely strictly on the drum brakes on my 1500 on some of the descents from 10k to 12k feet here in Utah, there would have been nothing left before I even got halfway down.
I can't imagine these engines creating that kind of vacuum. I could see it being a problem if there was a preexising issue in the engine, but it seems unlikely to me.

C
C. Merchant


And so much easier to get to!
Frank Nocera

Actually I wasn't thinking about down hill runs, but rather using motor (in tandem with the brakes) to slow you down as you enter a turn or coming to a stop sign.
Tysen

For coming to a stop brakes are cheaper, when slowing for a turn you may want to use the engine if you need to downshift to power out of the turn, depends on how "brisk" you like to drive. One issue with engine braking on long down hills is that with the throttles closed the engine vacuum will draw excess fuel into the engine and may dilute your oil. Later modle SU's had a spring loaded valve in the throttle plate the pulls open under heavy vacuum to relieve this issue. Smog control did have some small benefits.
John H

I used to drive my MGA-1600 and later 1275 midget with a Vacuum Gauge mounted on the dashboard for years and NEVER saw more than 21" Hg under ANY conditions! For any particular gear the highest vacuum reading gave you the best gas mileage. When the vacuum dropped below 18" Hg, it meant you were accelerating too rapidly for maximum gas mileage, or if it was ALWAYS LOW your car needed some work on carburetors, ignition, and/or valve tappet clearance adjustments.
wattswh1

Clayton's mention of the TD/TF BBS posting on the effect of downshifting to slow down was probably mine. I still have the remenants of the block from our TD in the basement as a reminder of what happens when a gudgeon pin pinch bolt brakes. Downshifting at the start of a hill to help brake and keep the brakes from fading if not going to be much of a problem. Downshifting while entering a turn at speed in order to bring the speed of the car down is a very definite no-no in any car, but especially ones with engines that have spit little ends on the rods. This kind of driving risks over reving the engine and bending push rods, but on an engine like the XPAG/XPEGs of the T series and the early B series engines used in the MGA and early MGBs, there is the very real risk of developing sufficient force from vacuum to break a gudgeon pin pinch bolt. Often times when someone makes a habit of downshifting to slow the car radically, if they are lucky the vacuum developed will suck a valve open and the damage is limited to a bent push rod. However, when the luck gives out and the pinch bolt breaks at high rpm (as would be the case when downshifting to slow the car), the engine simply "granades" - not a pretty thing to witness, particularly if it your engine. Downshifting while braking for a turn so the car is in a lower gear to power out of the turn is fine so long as the engine speed in the lower gear will match the speed of the car at the time the clutch is released (a good reason to learn to heal and toe).

I was lucky to learn auto mechanics from the owner of a local, independent parts house and machine shop. He told me a story about the young man he rebuilt a Chevy small block V8 for to go in his car for the race track. Several weeks later the man came back, with the engine and was furious at Dell. After the first race he ran, the engine quit. Obviously, Dell had screwed the rebuild up in some way. Dell just smiled and invited the man back into the shop while he removed the intake manafold, which revealed every single push rod laying loose in the valley. Dell turned to the man, smiled and said, "let me tell you what you did and you tell me if I am right". He went on to say, "after you crossed the finish line at speed, you lifted your foot off the accelerator and the car, still at a good speed cause the engine to rev and really rap - sounded real good didn't it". The man smiled and noded afermatively and Dell continued, "It sounded so good in fact that you stepped on it and got a real head of steam built up then released the accelerator with a snap". Again, he was rewarded with a smile and a nod. Dell again continued with, "the car really sound god this time for a very short while, followed by dead silence, am I correct?" This time no smile, just a nod and a quick retort, "I told you you screwed up"! Dell, as he was so good about doing, calmly explained about the forces of vacuum tha is developed in an engine when the throttle is closed ha high speed and explained that the vacuum was sufficient in his "well rebuilt" engine that it simply lifted all the valves and allowed the push rods to fall into the valley. This was followed with a rather curt, "learn how to drive"! Enough - Dave
David DuBois

I was curious about this thread and wondered what everyone had concerns about!
Then I remembered that most cars in the USA are automatic, therefore the normal american has a totally different style of driving to those of us in Europe.
Like Steve Gyles says we always drive the same, we lift of the throttle when we approach a hazard and the engine slows us down. We also change gear as the road speed decreases to allow the engine to do more braking. I have never seen an occassion where this has caused problems with any engine in Europe.
Going down a very steep hill is different. If you do not assist the engine braking force with the brakes then the engine is likely to over rev with consequences described by David.
Bob (robert)

It makes a really good story, but I really can't buy this theory about super high vacuum sucking valves open and breaking gudgeon pin bolts. If the vacuum is caused by a closed throttle plate, the vacuum would be the same on both sides of the intake valve, and it would not be sucked open. If the exhaust valve is sucked open, it would pull in air from the exhaust and the vacuum would be broken. And I haven't seen too many throttle plates or springs that were stronger than the springs on an exhaust valve or stronger than a gudgeon pin pinch bolt. If the vacuum were strong enough to suck a valve open, or break a pinch bolt, it would surely force open or bend or break the throttle plate.

I agree that you have to be careful that you don't over-rev the engine when down shifting. And shifting way down and droping the clutch with a closed throttle at speed, is just as bad as reving the engine and droping the clutch when stopped. And though it may cause some slight additional wear on the engine, clutch and gearbox, judicious use of engine braking is not going to cause a catastrophic failure in an otherwise sound engine and gearbox.

My answer to the question "would you rather wear out your brakes or your engine?" would definately depend on whether or not I was in motion at the time.



Jeff Schultz

Having used engine braking for something over 35 years, I have to agree that it is a good technique when used properly. When I attended the Bondurant school, it was taught along with "double clutching" on the downshift. Yes, if you downshift into a lower gear at too high an engine rpm, the operating limits of an engine can be exceeded. But, knowing what the engine rpm limits in various gears is a part of learning how to drive properly. There were factory warnings, on overdrive equipped cars, that the driver should not shift out of overdrive, and into direct drive, at speeds which would cause the engine to redline.

Simply taking the foot off the gas, or causing the throttle to be open a lesser amount, to allow the engine to become part of the slowing process should never be a problem. Downshifting, on the other hand, can be a problem if it is done too soon and causes the engine to over-rev.

The purpose of downshifting is only partly to assist in slowing the vehicle. The primary reason for downshifting when slowing is to keep the engine operating within it power band and to ensure maximum car control. If the transmission is in the incorrect gear and the engine goes outside of the best power band, you cannot accelerate your car out of trouble if that should be necessary.

An example would be taking a relatively slow right hand corner at an intersection. You would slow your vehicle, using the brakes, then, downshift to third gear to provide additional braking effect from the engine. As you reach the corner, you have downshifted into second gear before entering and have your car set to make the turn and accelerate out of the turn. However, Soccer Mom, on her cell phone while making a left hand turn in her SUV, is about to swing wide, into your lane, and prang the little car you are driving. Since you are in second gear, and your engine is well within the power band, you simply punch it and get out of her way. If, however, you had elected to stay in 4th gear (or had only gone into 3rd gear), when you punched it your engine would be outside of the power band, would have stumbled and not reved up quickly and you would have been hit.

Thus, we are actually discussing two different things here. The use of engine braking, normally in top gear, for long stretches and the requirement to keep the engine in the proper power band, through the proper selection of gear ratios, when turning. Both areas produce an engine braking effect as part of their use, and, if performed properly, neither will damage the engine. At least within my experience.

Les
Les Bengtson

The load on the gudgeon pin pinch bolt during engine braking is vastly less than the load during the inlet stroke.
The load due to vacuum is measured in lbs, but the load due to the reciprocating forces is measured in tons.
On an MGA 1600 with a bore of 2.968 inches the area of the piston is 6.921 square inches. If you had a complete vacuum during braking the load on the piston would be 15 pounds a square inch multiplied by 6.921 square inches. That is a total load on the piston of 104 lbs.
I do not have the weight (mass) of a piston with gudgeon, nor do I have the acceleration of the piston at the point of maximum acceleration.
If anyone has those values could they do the calculation F=MA to get the force being applied to the piston?

Mick
Mick Anderson

Use the car in front! Thought I'd get this one in before Steve does
Terry Drinkwater

You have just made my day Terry. However, I think I was in neutral when you arrived. Mind you, I think you were too, so not only did you save on engine wear, you also saved on brake wear!! That must be the optimum solution in this thread.

Steve
Steve Gyles

Steve

I thought you were accelerating hard with your new found power, obviously my Coupe is a lot quicker.
Anyway, I didn't think A55 sports didn't go fast enough to worry about brakes etc

Terry
Terry Drinkwater

The story about "vacuum" Is the same story as spark plugs for a diesel. Sounds like a new "urban legend" Vacuum can't hurt any engine component. That said downshifting into to low a gear and over revving can destroy the engine. I had a customer bring an Infinity G-20 in on a hook. He had been cut off in traffic and then panicked and stuffed it in 1st gear. The car was going so fast that the engine overreved so high that it broke most of the rocker arms and bent the valves and damaged the pistons. That said normal downshifting causes no problems for the engine. The wear on the clutch and the transmission syncros is more expensive to repair that replacing brake pads. To save the car the only time you should down shift for braking is to keep from over heatihg the brakes on long downhills. Unless you are driving for the fun of it and frankly its more fun to drive in a spirited manner.
Randy
R J Brown

Mick - your query about the acceleration forces. At 5000 rpm, assuming a stroke of 3" (not sure of exact figure), the acceleration is 1064 x gravity. So, if the piston and pin together weigh 8 oz (?) the force at the top of the stroke for the rod to pull the two downwards would be 532 lb. Subtract from that the compression force and you have the small end load at TDC. the force goes up as the square of the speed and in direct proportion to the stroke. The reverse of all the above happens at BDC.
Art

Art,
That is some very good calculating. The maximum stretching force on the rod should come at TDC on the exhaust stroke where there would be little compression force to be subtracted from the acceleration forces. I found this page http://2.6liter.com/Calc2.htm which has lots of online calculators relating to engines and gear ratios. I don't know the numbers for the TC, but entering the numbers for the MGA(stroke 3.5", rod length 6.5") it comes out to 931g's at 5000rpm, very close to your number.

Jeff

Jeff Schultz

I am not an MGA driver but I am an MGB driver, I am also an ex race driver. I have applied my race training to my road driving for the last 20 years.

You slow down using brakes, when you are going slow enough you change down as many gears as is necessary - 4th into 1st is just fine on the race track - you do not use the engine to slow down, it upsets the balance of the car and you cannot corner as fast.

I realise that the track and the road are different but I do find that 90% of the time I use the same mothod on the road as I did on the track. Yes, if someone pulls out 100 yards in front of you, you use the engine to slow down enough you don't have to touch the brakes but if you are having some serious fun you use the brake and accelerator pedal almost as much as each other.

The Wiz

Jeff,

Using the calculator on the website you gave, on the mga twin cam.
Stroke 3.5 inches
Rod length 6.5 inches (center to center)
RPM 7800 at valve crash
1.125 lbs Piston weight with rings and gudgeon
Calculated G is 2265
Force on piston is 2548 lbs,or 1.1 tons.

All Twin Cam figures are from factory published data.
Maybe this is why the Twin Cam does not have a pinch bolt.

Mick
Mick Anderson

David,
I was reading your comments about learning auto mechanics from a owner of a local parts house. I wondered if that was the same fellow who taught auto mechanics at Olympic College. I couldn't recall his name, then later in your comment you mentioned Dell, that was the same fellow. I took two classes from him, he let me work on my 65 Chevy II during class. One terrific person, always seemed to give the simple version of things. I recall the class on the ignition system for a car, not sure if at the time I got it correct, but that class sticks in my mind. I can still remember his drawings on the board about the wiring of the coil and the points.
He had me bring my car into the shop since the car had a alternator and he wanted to demonstrate the differences of a generator versus a alternator. For some reason we pulled the plugs and I checked the compression and he commented I must have had the head shaved since the compression was above what it should have been. He was right, I rebuilt the engine a couple of years before and had the head shaved.
I rebuilt my carburetor in that class. It worked for about a week and quit. It would not run unless I choked the carb. I rebuilt it again and the carb worked again about a week and the darn thing gave out. Only thing that save me was to manually wire the choke so it was half choked. I caused quite lot of noise at my girlfriends apartment building trying to get it right. Eventually I bought a used carburetor, though to this day I wish I knew what I did wrong.
That was back in 71-73. I knew Dell had a parts store,I went there a few times when I needed items. I have passed by the store many times since then and wondered what ever happened to him. I suppose by now he has passed along.
During my college days a few people had sports cars, one had a red MGA, he was short fellow that we all called Short Sh*t. His "A" was pretty much a tattered old car that I recall he had a hard time shutting the drivers door.
That red MGA was the start of my interest in MGs.
What American iron had a four speed on the floor, factory tachometer and only two seats for you and your gal?


Ray
Ray Ammeter

Ray - That is the same Dell I told the story about. He owned Bremerton Auto Parts at the corner of 11th and Callow. He retired probably 15 years ago and there was not an amature mechanic in town that didn't morn that day. He just passed away last year. Cheers - Dave
David DuBois

This thread was discussed between 05/08/2005 and 08/08/2005

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.