Heel & Toe, Trail Braking, Double Clutching
> Is 'heel & toe' the same as 'speed shifting'?
> Essentially shifting without the clutch...
> Or is this one of those, 'if you have to ask,
> i'm not going to tell you'
> types of question?!? ;)
Heel and toe is NOT the same as speed shifting. The former will help preserve your gearbox, the latter will help to turn it into scrap metal.
The concept behind heel and toe is to match engine speed with driveline speed.
Picture yourself entering a turn for which you will have to brake. You want to do the majority of your braking in a straight line, so you get heavy on the brakes before entering the turn. Just before you start to turn in, your foot is still on the brake, but you begin to lessen brake pressure as you begin to turn the wheel. While still continuing to brake, you depress the clutch pedal and use the other side of your right foot to blip the throttle to match the engine speed to the driveline speed that will be appropriate after making a downshift.
In a simple example where you only downshift one gear, you want that gear to be the one you want to be in when you exit the turn. If done properly, the tranny will glide smoothly into the lower gear because the revs are matched, at which point you release the clutch and roll your foot back on the brake to finish trail braking. That's what's involved in heel and toe and the whole process will take just a few seconds. To carry on a bit further, by the apex of the turn, your car should be in a neutral state - no braking and just maintaining balance by feathering the gas. After the apex, you ease back on the gas to exit the turn.
Since I brought it up in a previous post, I'll mention the technique for double clutching. Here, instead of depressing the clutch and going directly from a high gear to a lower one, you do your braking and depress the clutch to go from the high gear to neutral and then let off the clutch. Next, you do the heel and toe technique to match the revs so that when you depress the clutch again to go from neutral to
the lower gear, you'll get the smooth downshift transition. Given the synchros in modern boxes, this technique really isn't used much any more. I believe it was developed in the days of "rock-crusher" boxes that weren't synchronized.
'83 928 S 5-speed (U.S. spec)
Hill Country Region PCA (Austin)
>On a related note, it saves the engine which is primarily designed to "GO" and should
>not be used to "SLOW" the car down. That's what the brakes are for. - Ed Ruiz
I used to know a guy that built engines for a living, and he would cringe every time he heard someone using the engine braking to slow the car down. He claimed that the connecting rods were designed for the compressing forces put on them during engine operation, and not the pulling forces they experience during engine braking. The engine braking would result in the connecting rods stretching. "That's what the brakes are for" were the exact same words he'd use too.
'88 928 S4 Black/Black Linen
I hope this engine guy you knew was better at building engines than understanding them. Rods are under tremendous tension and compression during each and every stroke, it has nothing to do with whether the engine is accelerating or decelerating. The stresses are greatest at max engine speed, and fall off rapidly as engine speed drops. Engine braking doesn't harm the engine in any way...it's just that brakes do a far better job of slowing a car down than engine compression can ever hope to accomplish.
I built engines for over ten years, and attended two factory engine schools. This same discussion came up in class. The end result was, absolutely, the engine was built to "Go," but the problem was not that simple. I won't go into all the fancy math that our instructor (Rolland Pike) used to make his point, which I probably couldn't remember any way. We got into connecting rod design, centrifugal force, etc. The stresses forced on the connecting rods at full throttle acceleration, are at least hundreds of times greater than while slowing down, using the engine. We all know that Porsches are over designed. Does any one really think that this isn't taken into consideration while designing the engine?
Anyway, just my opinion. I could be wrong.
Exactly how is slowing with the engine different from accelerating with the engine? It seems to me that the rods can't possibly know the difference. There is a small suction pressure on the head of the piston as the charge is drawn in but the rod is designed to withstand the positive pressure (compression) of combustion. Gotta be orders of magnitude between the two. How could there be a large pull on the rod? I think what we got here is an urban legend.
The best argument I have heard against not using the engine for braking is that your brakes are ALOT cheaper to repair than your engine and clutch!
Jay F. Kempf
79 US 5 speed silbermetallische
One more additional thought on careful examination of my last post. Over speeding your engine could do more damage than anything else regarding this subject. I omitted this from my previous answer. Large tensile forces are generated by radial acceleration of the piston and rod assembly and this is all contained by the rod cap fasteners. I wonder how big a factor of safety those Teutonic designers put in on those. Probably huge! I still think that the engine doesn't know the difference between accel and decel though. I would worry more about high RPM oiling than rod strength. Am I missing something here?
Suction?? Valve timing provides the same compression at the same times during a complete two revolutions when accelerating or decelerating. There is certainly less horsepower being transmitted but there is still combustion going on. There is still a compression stroke on decel. For what you are saying to be true you would have to interrupt spark and disconnect your cams so that you weren't getting a compression stroke. The only way you could get the effect you are talking about is to weld your valves shut and unplug your coil. The braking effect of which you speak has got to be the load of the car trying to speed the engine up without increasing the throttle setting. That means that all that potential energy is now fighting the compression of the engine, not suction on the downstroke of the piston.
I am a mechanical engineer. Although I possibly buy your theory about oil sucking past piston rings, the idea about connecting rods/bearings being in tension rather than compression during engine braking is
probably inconsequential. I did some calculations a few years ago on another engine. At high rpms the centripetal forces on the conecting rods vastly out weighed gas pressure forces on the piston crown. There will be large tensile forces in the connecting rod at the top of every stroke, when the piston has to stop travelling up and start travelling down again. These forces are proportional to the square of engine speed. So are very large at 6000 rpm.
My 2c worth.
928 5sp 1979
The way I have always approached heel and toe braking, and the reason that I try to use it is:
When you finally get off the brake, you are going to need to be a gear or two or three, or four lower, in order to get the right drive out of the corner. If you just brake till it is time to put your foot back into it, then you have to waste some time downshifting before you can get going again. I use heel and toe to downshift to the proper gear while I am braking, and to relieve the stress on the syncros and transmission by matching the rpm of the engine to the next lower gear. If you are always(through the whole corner) in the proper gear, you can use the throttle to catch the car, pass someone who has made a boo boo, etc. The amount of time you are actually engine-braking is short, if you are really on the binders, sometimes you don't even have time to let out the clutch.
The same techniques are used on roadrace motorcycles, but there you use two fingers on the front brake lever, and two to blip the twist grip which is in the same hand as the brake. If you don't, the rear wheel gets quite weird. (don't ask me how I know)
Double-clutched rev-matched downshifts always. Heel and toe quite a bit.
While even rev-matched double-clutched compression braking may put some wear on the back sides of gears there ain't a sound in the world like an RMB'd Shark coming to a stop in second gear.
-- David Chamberland
91 928GT (Amazonagrunmetallic: Green. NO! Blue! Ahhhhhhhhhhh!)
I've been following the thread regarding heel & toeing, and engine braking, and it's possible negative effects on rods, clutches, etc. As an ex engineer myself, I think that sometimes guys in that profession have a tendency to look for excessively arcane and esoteric solutions, and/or explanations, to relatively simple problems. Not that there is anything simple about 928s unless its' the addiction to them that many of us share.
I think that possibly some of the technical discourse on the relative merits of heel & toeing, engine braking, etc. misses a pretty fundamental point, and that is the difference between smooth driving and placing sudden and often unnecessary traumatic loads on the car's drive train. In almost 400,000 miles in eight 928s, I've had everything go wrong with them BUT clutches, connecting rods, and other components that supposedly suffer from engine braking. Same goes for all the bikes I've had and raced. I think that much of it boils down to simply developing over time, the instinctive reflexes to automatically match the speed of the different drivetrain components regardless of the momentary situation without conscious thought. Doesn't make a hell of a lot of difference whether you're on the track or highway, or what kind of a machine you're driving or riding.
Case in point; Look at the driving records of most of the great drivers in any of the different branches of racing. A few; Fangio, Moss, Mansell, Mears, Wolleck, Clark, Hill, even Jeff Gordon. They all had or have one thing in common. They all drove/drive smoothly, without huge amounts of sturm und drang, ala the Andrettis, Tracy, Irvan, etc. etc. who seem to spend as much time going sideways and backwards than anything else. They either win or DNF for mechanical reasons, and often take good drivers out with them because they're brutalizing the machinery.
Makes for pretty exciting TV fare, but doesn't compare with the smooth and almost unobtrusive manner that Emmo, Nigel, or Rick used to win countless races and championships while keeping their expensive machinery out of the junkyard.
Another analogy that comes to mind is jockeys who are known for their "soft hands" like Arcaro or Shoemaker who won a zillion races without killing their mounts.
By the wildest stretch of the imagination I'm not trying to compare my driving skills with any of the above, but the only clutch I ever ruined downshifting or any other way was on the Thompson, Connecticut racecourse in my brand new TR-3, and that was because the clutch had been gnawed out of a block of oak in the Sherwood Forest by Triumph gnomes.
87S4 5sp almost 100k miles, original clutch and con rods.
FYI: In "Porsche Panorama", you'll see ads for a heel&toe device from Wings Eng'g. I've got one, modified to get rid of the lower left extension. It attaches perfectly to a '79 pedal (they're all the same??), adding height so you can blip with the side of your foot.
The design shown in the ad is for 911's, which I guess are suited to using the heel. BTW, don't order the extra width version unless you have very narrow feet.
'79 Euro 5-speed 5.0/2-v/CIS track car
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