Blow By Blow Description of What's Happening in the Video:
The car is running on the standard radial street tires, which is why you hear so much tire squeal in the video. When you understand how brisk this goes, you should have a grander appreciation for how well the fat front sway bar works. Click on the picture to view the video in a separate window, and you can refer to these notes at the same time. Turn up the volume and enjoy the experience.
- The power launch starts at about 4000-4500 rpm constant engine speed to wind up the flywheel inertial energy, then throttle goes almost full down while clutch is feathered a little. The clock starts when front tires trip the light beam a few feet after first motion, so this is a drag race style all out launch. There will be some tire spin and some clutch slippage. See notes below for more intimate details of this process. In the video the power launch eases off a bit early due to a slow slalom and hard left before full on throttle again, all in 1st gear under 25 mph.
- The first 1-2 shift comes around 12 seconds, just after the 180 to the left. A quick poke at the clutch pedal along with a strong pull on the shift lever is about all it takes to do the 1-2 shift really fast. Momentary lift of the throttle is optional. 2nd gear synchronizer absolutely has to work. This results in a momentary punch in the back with a bit of tire spin until inertia of the flywheel is spent and engine speed slows down to ground speed (still at full throttle). See notes below for more intimate details of this process.
- A few seconds later, braking for one second (mostly in straight line), turn right, then onto momentary throttle, then a short touch on braking and another hard right, in and out of the box near the big sand piles. Then hard on the throttle heading out.
- When I see daylight straight ahead, then the hard throttle is accompanied by a power downshift 2-1 at 20 seconds. Yes I did say power downshift into 1st gear (non-synchronized). See a detailed description of that slightly scary and more involved process below. The next 1-2 upshift comes just 2 seconds later at 22 seconds, because that's all the time it takes to go from say 3000 to 5500 rpm in 1st gear.
- A second 2-1 (power downshift) happens coming out of the box (garage) at 27 seconds, followed by 1-2 at 29 seconds.
- The third 2-1 happens coming out of the second garage at 36 seconds followed by 1-2 at 38 seconds.
- Notice the car is in 1st gear for only 2 seconds at a time to get it back up to speed (near red line). If I left it in 2nd gear after the slow turns the engine would bog down at too slow speed, and acceleration is lost (along with increased lap time).
- 2nd gear then carries through a series of faster broad turns, some at partial throttle near constant speed at the limit of lateral tire grip.
- At 44 seconds the left side tires run through a touch of rain water on the pavement while turning right at speed. This upsets the grip enough to cause a slight drift to the left. Subsequently pulling it back to the right results in a slight overshoot of the next left turn, then a bit of oversteer with rear end out and rubber dust from the rear tire at 49 seconds.
- Then it goes hard on throttle, curve to the left increasing in radius commensurate with increasing speed as the tires work for best grip without too much drift.
- Then off throttle at 54 seconds, followed by a couple seconds of hard braking (55-57 seconds) to scrub off speed (maybe 50 to 15 mph real quick in about 50 feet travel distance) before a constant speed 180 to the right.
- The one second pause to coast down from 54 to 55 seconds is used to let the suspension settle and select just the right instant to start braking. The rule is, "Stand on it until you see God, and then brake". This one second was time for reality check to be sure I wasn't going to pass God before braking.
- Next it may look like a big overshoot past the apex cone on the hairpin turn, but this is the intentional correct line for approaching entry angle into the following slalom (going back the other direction). In short, there is no apex cone, so I get to pick my own turning line for the 180.
- The fourth and last magic 2-1 power downshift happens at 58 seconds with notable tire squeal on launch after the hairpin. You notice this very well as it is quite close approaching the camera and microphone.
- Two seconds later this is followed by a half-second momentary missed 1-2 shift right at the 1:00 mark. Oops, bummer, that almost hurt (but at least it wasn't a missed 2-1 shift). When I finally get it into 2nd gear (on the second stab) there is notable tire squeal as the high spun flywheel and MGB clutch make more torque than the tires can contain (until the engine slows down to match ground speed).
- The rest is 2nd gear accelerating, then sneaking through tightening switchbacks designed to slow us down before the finish trap at the BIG orange cones at 1:09.
- End is casual straight line braking from 1:09 to 1:13 where there is a little radio sitting on top of the last big orange cone to announce the lap time.
The power launch starts with about 4000-4500 rpm constant engine speed to wind up the flywheel while sitting still in 1st gear. Then throttle goes almost full down while clutch is feathered a little. The clock starts when front tires trip the light beam a few feet after first motion, so this is a drag race style all out launch. If you are timid on the throttle or too fast letting the clutch up, the engine bogs down, and you lose. Power is a function of torque and speed, so if you want power (and you do) then you have to keep engine speed up.
A bit of intended clutch slip limits tire spin, while a bit less than full throttle inhibits engine over-revving. This transition period lasts maybe 1-1/2 to 2 seconds until ground speed is up enough to let the clutch full up and put the hammer full down. That might span about 20 feet of travel distance, after which it hooks up with (almost) no tire spin (around 15 mph) and the tach never drops below 3000 rpm in the process. Then just stand on it a couple more seconds until approaching yellow line on the tach before time for the 1-2 upshift at 23-25 mph and 5000-5500 rpm.
A couple of considerations here: If you have too much power applied it will either smoke the tires excessively (not the quickest way off the line) or burn the clutch up while it is slipping excessively. Feathering the clutch pedal is the means to modulate propshaft torque. The correct clutch torque will have the tires slipping just a tiny bit, most likely only the right rear slipping while the left rear is giving full grip. This is due to torque on the propshaft applying twisting force to the rear axle, lifting the right rear tire slightly while pressing down slightly on the left rear tire. Also a LHD car has weight of the driver sitting more to the left side. When the tires slip just a little, that's how you know you have the right propshaft torque.
Correct engine torque is just slightly more than enough to slip the clutch (with the selected clutch engagement pressure). Correct engine speed is slightly higher than the peak of the engine torque curve (assuming you might know where the peak falls on the chart). You don't have to have one eye on the torque curve and one eye on the tach to do this, but you need to listen to the tires and engine, and you can also feel the tire grip and vehicle acceleration. If the tires slip too much, you have let the clutch up too fast. If the tires don't slip, you're either smoking the clutch too much or killing the engine. If the engine revs too high, you have too much throttle and need to ease up on the loud pedal to avoid smoking the clutch. If engine speed is allowed to drop below the peak of the torque curve, then the slightest increase of torque demand downstream will suddenly kill the engine (or bog it down to very low speed).
During this process you have some discretion on whether you spin the tires more (or less) or slip the clutch more (or less). Since you have to keep the engine speed up, something is going to be slipping with a lot of torque being applied. High torque and high relative slip rate multiplies to equal high power dissipation. Power dissipation equals heat and wear. A pertinent question is, would you rather heat and wear the tire rubber or the clutch disc? Tires are easier to replace than a clutch disc, so maybe excessive tire spin is not as bad as excessive clutch slip. Following that logic, I tend to select what many people may think to be excessive tire spin for a couple of seconds on initial launch. In any case, clutch modulation limits tire spin, while throttle modulation limits engine speed, and your toes need to learn this little balancing act. That requires some seat time (practice), but when you understand how it's supposed to work you can get the hang of it sooner.
The power up shift action commences when engine is near red line and accelerating. A quick poke at the clutch pedal along with a strong pull on the shift lever is about all it takes to do the 1-2 shift really fast. Momentary lift of the throttle is optional, depending on how far you can bear to see the tach needle swing upward (and how light is your flywheel?). 2nd gear synchronizer absolutely has to work and work well. This results in a momentary punch in the back with a bit of tire spin until inertia of the flywheel is spent and engine slows down to match ground speed (still at full throttle).
If you do this with maximum vigor while in a turn, the rear end may move sideways rather dramatically as the tires spin. You may anticipate the side slip and ease off throttle to keep it under control, but less throttle equates to less power throughput and less acceleration, so this goes best in a spot with nearly straight line travel. As such, you get to pick the spot where you do the power up shift. If necessary you might up shift a little early (called short shifting) to get it done before entering a turn. On the flip side, if you are accelerating and approaching red line while in a turn, you might wait a bit longer before up shifting to allow doing it more in the straight line travel. This is driver's choice, and it may depend on far you feel like letting the tachometer needle rise near or into the red zone.
The power downshift will happen starting with engine speed low enough that it would be bogging down in 2nd gear (maybe 2000 rpm), and engine speed in 1st gear after the shift will not be anywhere near red line (commonly commencing at about 15 mph). Step hard down on the loud pedal and the clutch at the same time while the engine spools up as rapidly as possible. Shift from 2nd to neutral as soon as the clutch is down. Time things right and ham fist the lever into 1st gear at the moment the engine speed is up to matching ground speed in 1st gear, then off the clutch as quickly as possible. Get it right and the car will be under maximum acceleration in 1st gear the instant the clutch is re-engaged. If the clutch comes up a tad early, before engine is up to speed, you get a momentary engine braking effect, not good for lap time. If the clutch comes up a tad late, when engine is turning faster than necessary, you get a momentary kick in the back with a touch of tire spin while the engine speed is pulled down a bit or until ground speed catches up with the engine. If you have to miss on timing, better slightly late than early.
Do not be timid about ham fisting it into 1st gear at 15 mph while the engine is spooling up. Shifting quickly from 2nd to 1st, the gearbox laygear and input shaft and clutch disk will be turning too slowly. There is not a lot of change of inertia involved while increasing speed of these parts, so a sudden slam into 1st gear (with clutch disengaged) will kick the input parts up to speed instantly. Figure the input shaft will suddenly go from 2000 to 3000 rpm, but those straight cut gear teeth are pretty strong so nothing breaks. It may sometimes make a few tiny chips on the entry ends of the straight cut gear teeth, but the gears tolerate this quite well over long periods of aggressive abuse (so to speak). What will give you problems is shifting too slowly, in which case the entry ends of the gear teeth will skip over each other with a nasty grinding sound until the mating gear teeth synchronize in speed. That process will chew up the gear teeth faster and worse than the ham fisted slam directly into full engagement.
Power braking is the primary way of scrubbing off speed in a hurry. The general rule is, "Stand on it until you see God, and then brake". Longest time possible on full throttle gives best speed and least lap time. Going heavy on brakes gives shortest time (and shortest distance) possible for required deceleration. That short distance for braking means the preceding straightaway was longer, and longer straight means higher speed and shorter lap time.
Moving at speed in a turn induces side loading (and some body roll). Center of gravity of the vehicle being higher than ground level results in a torque lever effect that transfers weight from the inside tires to the outside tires. More weight gives increased grip for the tires while less weight gives reduced grip. Braking effect on the wheels remains matched between left and right sides. Result is that the inside tire will give up the grip while the outside tire still has grip to spare. To prevent locking up an inside wheel under hard braking you have to ease off on the pedal pressure. That in turn reduces overall braking effect, increases distance and time for deceleration, and increases lap time. Bottom line is picking a good spot to do maximum power braking in a straight line just before the turn can reduce lap time.
The power slide, commonly called Four Wheel Drift, is an alternate means for scrubbing off speed while using little or no brakes. This requires a lot of finesse or practice to keep it under control and prevent losing it all together, and it may have only rare useful applications. In essence it involves turning or rotating the car intentionally too far sideways at speed (intentional oversteer) so the tires lose grip and go into side slip mode. Friction between ground and tires as they slip generates heat and dissipates energy. This turns the tires into braking surface rather than heating up the brake linings. Lateral braking effect of the tires slipping sideways (while still rotating) will be proportional to weight on the tires. Therefore the outside tires bearing more load will provide greater braking effect while inside tires are less effective.
This may be useful if you need to slow down in a turn and don't want to lock up an inside tire with excess braking. Bad news is, this is hard on tire rubber. Good news is, it works without locking up a tire regardless of the weight discrepancy from left to right sides. If you overshoot a turn slightly and end up late braking in the turn, and you have to ease up on the brakes to avoid locking up an inside tire, then a small amount of power slide can help make up for the missing braking effect. This is exactly what happened in the video at 49 seconds when you see the tail end hang out a bit with a substantial amount of visible smoke or rubber dust coming from the rear tire. In this case it was only two wheel drift as the back end got a little loose while the front end was going exactly where I pointed it.
Bottom Line: Both in the video and in the car, this Autocross process seems to be going like a bat out of hell, and it is, sort of. The impression comes mostly from sudden steering transitions that tend to throw a passenger around the cockpit a bit (lap belts only if you recall). It also has a lot to do with heavy throttle and loud exhaust at high engine speed, along with mildly squealing tires up to 1/3 of the travel time (if you get the ground speed and mild drift just right). But the actual speed for this run does not exceed 45 mph max, which would be 6000 rpm in 2nd gear. With slowest turns at about 15 mph, and max speed near 45 mph, average speed for the run is likely in the 34-35 mph range, and the lap distance is about 2/3rds of a mile. Cooking around at 45 mph tops is not particularly fast or dangerous, as long as you don't hit anything solid. If you should miss the intended line a little, just keep in mind that "Pylons are designed to be run over by automobiles".