Practical Transmission Tuning: Torque Convertor & Clutch Lock Up Tuning

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Torque Convertor & Clutch Lock Up Tuning

09.42

00:00	With accurate tire size and final drive ratio in hand, next we'll look at the process of tuning torque converter and clutch lockup.
00:08	Let's start by discussing when it's necessary to unlock the torque converter clutch and let the torque converter slip.
00:16	First, anytime wheel speed is zero or very low.
00:20	When the vehicle is stationary but the engine is running, we have to decouple the engine from the drivetrain to avoid stalling the engine and the torque converter achieves that for us.
00:31	Remember, if the engine speed suddenly equaled a transmission and vehicle speed of zero, the engine would stall and come to a stop.
00:39	When engine speed is low, there's less fluid pumping into the torque converter and it slips a great deal.
00:46	This avoids wasting lots of engine output on the drivetrain while we aren't trying to drive forward.
00:53	While in park or neutral, only input side transmission components are moving, keeping the engine output required to a minimum.
01:02	Once the vehicle is in gear, then due to partial coupling of the engine to a stationary drivetrain, we need a bit more engine output to avoid stalling.
01:12	You've probably noticed the sound of an engine laboring a bit more when you shift it from park or neutral into reverse or drive and this is due to the partial coupling of the engine through the torque converter to the stationary drivetrain on the other side of the converter.
01:28	If the driver needs to move the vehicle slowly, they can gently release the brakes and the torque which was required to avoid stalling during partial converter coupling to a stationary drivetrain now allows the vehicle to move slowly.
01:43	As the driver starts moving more quickly, we don't want the torque converter or the converter clutch to sync up engine and transmission speed until the vehicle is moving quickly enough that the coupled engine speed is sufficient.
01:57	So, let's walk through an example.
01:59	If we're working with a rear drive vehicle that uses a GM 4L80E transmission with a 2 .482 first gear ratio, a 3.7 final drive ratio, and we need to keep engine speed over 800 rpm to maintain stability, here's how we get from engine rpm to wheel rpm.
02:20	First, we multiply our first gear ratio of 2.482 with our 3.73 rear end ratio which gives us a total 9.233 drivetrain ratio.
02:33	Now, we divide our 800 rpm by the 9.233 total drivetrain ratio giving us 86.646 wheel rpm.
02:45	To get from wheel rpm to wheel speed, we simply need to know the tire size.
02:51	We work it out by taking wheel rpm times tire diameter times pi times the number of units per mile which must match the units for our tire diameter.
03:04	So, in this case, wheel speed equals 86.646 wheel rpm times 28 inch tire times pi times 60 minutes in an hour divided by the number of inches in a mile since I chose to use inches for tire size.
03:25	There are 5,280 feet in a mile, 12 inches in a foot, 5,280 times 12 equals 63,360 inches in a mile.
03:38	Now, we have the full equation 86.646 wheel rpm times 28 inch diameter tire times pi at 3.14159 times 60 divided by 63,360 equals a wheel speed of 7.22 miles per hour.
04:04	Feel free to use a calculator or set up a spreadsheet as you'll likely do this again in the future.
04:10	Now, that we've done the math, we know we definitely don't want to lock up our converter before 7 miles per hour because doing so would make the engine speed drop below the 800 rpm we decided was the minimum speed required for stable engine operation while driving.
04:28	The reality is we want to do more than avoid shutting the engine down.
04:32	We want to allow the engine to rev up to a point where the vehicle can pull away from a stop well.
04:39	This will require more torque than many engines can produce at idle speed, so we'll want to allow the converter to stall up well beyond idle speed.
04:49	As engine speed increases further to perhaps 1,500 rpm, the engine can now produce more torque output and we're no longer in danger of stalling the engine if the torque converter isn't slipping.
05:02	Beyond the point of stable engine operation while driving, how much we want the torque converter to slip depends on our goals.
05:10	Remembering back to the torque converter upgrade module, we ran through some pros and cons of tight versus loose converters and now we're going to bring the torque converter clutch into the mix.
05:20	In situations where torque converter slip is not desired, such as highway cruising where maximum efficiency is preferred, an electronic clock up clutch can activate to create direct coupling of the input and output sides of the torque converter causing all engine output to be transferred directly to the transmission and linking engine and transmission input speeds.
05:45	While some older automatic transmissions only use the lockup clutch in overdrive conditions while cruising, modern automatics enable the lockup clutch quite often to reduce engine torque loss during torque converter slip.
05:58	This improves fuel economy and emissions and with proper tuning there can be performance benefits as well.
06:05	Modern torque converter clutches are electronically controlled via PWM signal passed from the transmission control module to one or more solenoids.
06:16	PWM stands for pulse width modulated and that allows for more precise control than simply flicking it on or off.
06:24	Torque converter clutch PWM solenoids work like clutch or holding device solenoids within an automatic transmission.
06:30	By applying various duty cycle percentages to the solenoid, more or less fluid is allowed to flow creating more or less pressure in the system which actuates the converter lockup clutch.
06:42	At zero duty cycle, the clutch is fully disengaged.
06:46	At a hundred percent duty cycle, maximum pressure is being applied.
06:50	Depending on the engine torque output at a given time and lockup clutch capability, the torque converter clutch may be able to fully lock up equalizing torque converter input and output speeds without a hundred percent duty cycle being applied.
07:06	Generally speaking, we'll want to use a high duty cycle when we're after full lockup so I only mention this as a reminder that lockup may occur as the duty cycle on the lockup clutch solenoid increases between zero and a hundred at perhaps sixty percent under certain conditions.
07:24	Once the converter clutch duty is high enough, the torque converter can be considered locked up when the engine crankshaft speed matches the torque converter output speed which is the transmission input speed as well.
07:39	If the torque converter pump and turbine speeds are far apart, engaging the lockup clutch at that time can excessively wear the clutch and cause unwanted drivability behavior such as shuddering or a harsh jolt.
07:53	While there may be a desire to run a loose converter to get lots of torque multiplication in some conditions, like when throttle is rapidly applied, that could in theory use the lockup clutch to tame drivability and increase efficiency some of the time, but torque converter clutches aren't really meant to suddenly equalize a massive differential between converter input and output speeds.
08:18	In a racing environment, there's a lot of debate about the pros and cons of locking the converter and this is a case where one size certainly does not fit all.
08:27	So, executing your own testing in a safe and controlled environment will help determine which conditions are ideal to lock the converter and which conditions are best suited to allow the converter to slip.
08:39	With that covered, let's recap what we've learned in this module before moving on to drive mode tuning.
08:45	While the engine is at idle or below the engine speed associated with the current vehicle speed, the lockup clutch has to remain off to let the engine continue operating without stalling or shuddering.
08:58	With a few pieces of data in hand, we can calculate the minimum engine speed for converter lockup as the vehicle gets moving.
09:07	As fuel economy and emissions have become increasingly important, lockup clutches are more commonly fitted and are more often engaged while driving down the road.
09:17	We should avoid locking up the converter clutch during periods of high torque converter slip to avoid excessive clutch wear and harsh drivability.
09:26	In a racing environment, the best way to determine optimal converter lockup clutch choices is by testing, data logging, reviewing, and then taking a data-driven approach to optimizing performance.