GM 8L transmission

The main objective in replacing the predecessor model was to improve vehicle fuel economy with extra speeds and a wider gear span to allow the engine speed level to be lowered (downspeeding). The layout brings the ability to shift in a non-sequential manner – going from gear 8 to gear 2 in extreme situations simply by changing one shift element (actuating break B and releasing clutch D).^[8]

In order to increase the number of ratios, ZF and consequently GM have abandoned the conventional design method of limiting themselves to pure in-line epicyclic gearing and extended it to a combination with parallel epicyclic gearing. This was only possible thanks to computer-aided design and has resulted in a globally patented gearset concept. The resulting progress is reflected in a better ratio of the number of gears to the number of components used compared to existing layouts. The ZF 8HP has become the new reference standard (benchmark) for automatic transmissions.

Innovative Strength Analysis

With Asessment	Output: Gear Ratios	Innovation Elasticity[a] Δ Output : Δ Input	Input: Main Components
			Total	Gearsets	Brakes	Clutches
8L Ref. Object	${\displaystyle n_{O1}}$  ${\displaystyle n_{O2}}$	Topic[a]	${\displaystyle n_{I}=n_{G}+}$  ${\displaystyle n_{B}+n_{C}}$	${\displaystyle n_{G1}}$  ${\displaystyle n_{G2}}$	${\displaystyle n_{B1}}$  ${\displaystyle n_{B2}}$	${\displaystyle n_{C1}}$  ${\displaystyle n_{C2}}$
Δ Number	${\displaystyle n_{O1}-n_{O2}}$		${\displaystyle n_{I1}-n_{I2}}$	${\displaystyle n_{G1}-n_{G2}}$	${\displaystyle n_{B1}-n_{B2}}$	${\displaystyle n_{C1}-n_{C2}}$
Relative Δ	Δ Output ${\displaystyle {\tfrac {n_{O1}-n_{O2}}{n_{O2}}}}$	${\displaystyle {\tfrac {n_{O1}-n_{O2}}{n_{O2}}}:{\tfrac {n_{I1}-n_{I2}}{n_{I2}}}}$  ${\displaystyle ={\tfrac {n_{O1}-n_{O2}}{n_{O2}}}}$ ·${\displaystyle {\tfrac {n_{I2}}{n_{I1}-n_{I2}}}}$	Δ Input ${\displaystyle {\tfrac {n_{I1}-n_{I2}}{n_{I2}}}}$	${\displaystyle {\tfrac {n_{G1}-n_{G2}}{n_{G2}}}}$	${\displaystyle {\tfrac {n_{B1}-n_{B2}}{n_{B2}}}}$	${\displaystyle {\tfrac {n_{C1}-n_{C2}}{n_{C2}}}}$
8L 6L[b]	8[c] 6[c]	Progress[a]	9 8	4 3[d]	2 2	3 3
Δ Number	2		1	1	0	0
Relative Δ	0.333 ${\displaystyle {\tfrac {1}{3}}}$	2.667[a] ${\displaystyle {\tfrac {1}{3}}:{\tfrac {1}{8}}={\tfrac {1}{3}}}$ ·${\displaystyle {\tfrac {8}{1}}={\tfrac {8}{3}}}$	0.125 ${\displaystyle {\tfrac {1}{8}}}$	0.333 ${\displaystyle {\tfrac {1}{3}}}$	0.000 ${\displaystyle {\tfrac {0}{2}}}$	0.000 ${\displaystyle {\tfrac {0}{3}}}$
8L 3-Speed[e]	8[c] 3[c]	Market Position[a]	9 7	4 2	2 3	3 2
Δ Number	5		2	2	-1	1
Relative Δ	1.667 ${\displaystyle {\tfrac {5}{3}}}$	5.833[a] ${\displaystyle {\tfrac {5}{3}}:{\tfrac {2}{7}}={\tfrac {5}{3}}}$ ·${\displaystyle {\tfrac {7}{2}}={\tfrac {35}{6}}}$	0.286 ${\displaystyle {\tfrac {2}{7}}}$	1.000 ${\displaystyle {\tfrac {1}{1}}}$	−0.333 ${\displaystyle {\tfrac {-1}{3}}}$	0.500 ${\displaystyle {\tfrac {1}{2}}}$
^ a b c d e f Innovation Elasticity Classifies Progress And Market Position Automobile manufacturers drive forward technical developments primarily in order to remain competitive or to achieve or defend technological leadership. This technical progress has therefore always been subject to economic constraints Only innovations whose relative additional benefit is greater than the relative additional resource input, i.e. whose economic elasticity is greater than 1, are considered for realization The required innovation elasticity of an automobile manufacturer depends on its expected return on investment. The basic assumption that the relative additional benefit must be at least twice as high as the relative additional resource input helps with orientation negative, if the output increases and the input decreases, is perfect 2 or above is good 1 or above is acceptable (red) below this is unsatisfactory (bold) ^ Direct Predecessor To reflect the progress of the specific model change ^ a b c d plus 1 reverse gear ^ of which 2 gearsets are combined as a compound Ravigneaux gearset ^ Reference Standard (Benchmark) 3-speed transmissions with torque converters have established the modern market for automatic transmissions and thus made it possible in the first place, as this design proved to be a particularly successful compromise between cost and performance It became the archetype and dominated the world market for around 3 decades, setting the standard for automatic transmissions. It was only when fuel consumption became the focus of interest that this design reached its limits, which is why it has now completely disappeared from the market What has remained is the orientation that it offers as a reference standard (point of reference, benchmark) for this market for determining progressiveness and thus the market position of all other, later designs All transmission variants consist of 7 main components Typical examples are Turbo-Hydramatic from GM Cruise-O-Matic from Ford TorqueFlite from Chrysler Detroit Gear from BorgWarner for Studebaker BW-35 from BorgWarner and as T35 from Aisin 3N 71 from Nissan/Jatco 3 HP from ZF Friedrichshafen W3A 040 and W3B 050 from Mercedes-Benz

The ratios of the 8 gears are relatively unevenly distributed in all versions. Particularly noticeable are the too small step between 3rd and 4th gear and the too large one between 7th and 8th gear. This cannot be eliminated without affecting all other gear ratios. On the other hand the selected gearset concept offers 2 to 3 gears more than conventional transmissions of comparable manufacturing costs, which more than compensates for the weaknesses.

Gear Ratio Analysis

With Assessment			Planetary Gearset: Teeth[a][b]				Count	Total[c] Center[d]	Avg.[e]
Model Type	Version First Delivery		S1[f] R1[g]	S2[h] R2[i]	S3[j] R3[k]	S4[l] R4[m]	Brakes Clutches	Ratio Span	Gear Step[n]
Gear Ratio	R ${\displaystyle {i_{R}}}$	1 ${\displaystyle {i_{1}}}$	2 ${\displaystyle {i_{2}}}$	3 ${\displaystyle {i_{3}}}$	4 ${\displaystyle {i_{4}}}$	5 ${\displaystyle {i_{5}}}$	6 ${\displaystyle {i_{6}}}$	7 ${\displaystyle {i_{7}}}$	8 ${\displaystyle {i_{8}}}$
Step[n]	${\displaystyle -{\tfrac {i_{R}}{i_{1}}}}$ [o]	${\displaystyle {\tfrac {i_{1}}{i_{1}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{2}}}}$ [p]	${\displaystyle {\tfrac {i_{2}}{i_{3}}}}$	${\displaystyle {\tfrac {i_{3}}{i_{4}}}}$	${\displaystyle {\tfrac {i_{4}}{i_{5}}}}$	${\displaystyle {\tfrac {i_{5}}{i_{6}}}}$	${\displaystyle {\tfrac {i_{6}}{i_{7}}}}$	${\displaystyle {\tfrac {i_{7}}{i_{8}}}}$
Δ Step[q][r]			${\displaystyle {\tfrac {i_{1}}{i_{2}}}:{\tfrac {i_{2}}{i_{3}}}}$	${\displaystyle {\tfrac {i_{2}}{i_{3}}}:{\tfrac {i_{3}}{i_{4}}}}$	${\displaystyle {\tfrac {i_{3}}{i_{4}}}:{\tfrac {i_{4}}{i_{5}}}}$	${\displaystyle {\tfrac {i_{4}}{i_{5}}}:{\tfrac {i_{5}}{i_{6}}}}$	${\displaystyle {\tfrac {i_{5}}{i_{6}}}:{\tfrac {i_{6}}{i_{7}}}}$	${\displaystyle {\tfrac {i_{6}}{i_{7}}}:{\tfrac {i_{7}}{i_{8}}}}$
Shaft Speed	${\displaystyle {\tfrac {i_{1}}{i_{R}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{1}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{2}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{3}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{4}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{5}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{6}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{7}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{8}}}}$
Δ Shaft Speed[s]	${\displaystyle 0-{\tfrac {i_{1}}{i_{R}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{1}}}-0}$	${\displaystyle {\tfrac {i_{1}}{i_{2}}}-{\tfrac {i_{1}}{i_{1}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{3}}}-{\tfrac {i_{1}}{i_{2}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{4}}}-{\tfrac {i_{1}}{i_{3}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{5}}}-{\tfrac {i_{1}}{i_{4}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{6}}}-{\tfrac {i_{1}}{i_{5}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{7}}}-{\tfrac {i_{1}}{i_{6}}}}$	${\displaystyle {\tfrac {i_{1}}{i_{8}}}-{\tfrac {i_{1}}{i_{7}}}}$
8L 90 M5U	900 N⋅m (664 lb⋅ft)[9] 2014[t]		41[10] 79	46 86	37 73	25 89	2 3	6.9991 1.7236	1.3204[n]
Gear Ratio	−3.8176[o] ${\displaystyle -{\tfrac {43,043}{11,275}}}$	4.5600 ${\displaystyle {\tfrac {114}{25}}}$	2.9709[r] ${\displaystyle {\tfrac {817}{275}}}$	2.0751 ${\displaystyle {\tfrac {12,540}{6,043}}}$	1.6876[n][r][s] ${\displaystyle {\tfrac {4,121}{2,442}}}$	1.2700[r] ${\displaystyle {\tfrac {28,817,100}{22,690,429}}}$	1.0000 ${\displaystyle {\tfrac {1}{1}}}$	0.8455[r][s] ${\displaystyle {\tfrac {5,160}{6,103}}}$	0.6515 ${\displaystyle {\tfrac {43}{66}}}$
Step	0.8372[o]	1.0000	1.5349	1.4317	1.2297[n]	1.3288	1.2700	1.1828	1.2977
Δ Step[q]			1.0721[r]	1.1643	0.9254[r]	1.0463[r]	1.0738	0.9114[r]
Speed	-1.1945	1.0000	1.5349	2.1975	2.7021	3.5905	4.56	5.3933	6.9991
Δ Speed	1.1945	1.0000	0.5349	0.6626	0.5047[s]	0.8884	0.9695	0.8333[s]	1.6057
8L 45 M5N	550 N⋅m (406 lb⋅ft) 2015[u]		41 79	41 79	41 79	26 94	2 3	7.0107 1.7431	1.3208[n]
Gear Ratio	−3.9278[o] ${\displaystyle -{\tfrac {4,187}{1,066}}}$	4.6154 ${\displaystyle {\tfrac {60}{13}}}$	3.0385[r] ${\displaystyle {\tfrac {79}{26}}}$	2.0648 ${\displaystyle {\tfrac {7,200}{3,487}}}$	1.6583[n][r][s] ${\displaystyle {\tfrac {199}{120}}}$	1.2587[r] ${\displaystyle {\tfrac {740,760}{588,527}}}$	1.0000 ${\displaystyle {\tfrac {1}{1}}}$	0.8494[r][s] ${\displaystyle {\tfrac {9,480}{11,161}}}$	0.6583 ${\displaystyle {\tfrac {79}{120}}}$
Step	0.8510[o]	1.0000	1.5190	1.4715	1.2451[n]	1.3175	1.2587	1.1773	1.2902
Δ Step[q]			1.0322[r]	1.1819	0.9450[r]	1.0468[r]	1.0691	0.9125[r]
Speed	-1.1751	1.0000	1.5190	2.2353	2.7831	3.6669	4.6154	5.4338	7.0107
Δ Speed	1.1751	1.0000	0.5190	0.7163	0.5479[s]	0.8837	0.9485	0.8184[s]	1.5769
Ratio[v] R & even	${\displaystyle i_{R}={\tfrac {R_{2}(S_{1}S_{4}-R_{1}R_{4})}{S_{1}S_{4}(S_{2}+R_{2})}}}$		${\displaystyle i_{2}={\tfrac {R_{2}(S_{4}+R_{4})}{(S_{2}+R_{2})S_{4}}}}$		${\displaystyle i_{4}=1+{\tfrac {S_{2}R_{3}}{S_{3}(S_{2}+R_{2})}}}$		${\displaystyle i_{6}={\tfrac {1}{1}}}$	${\displaystyle i_{8}={\tfrac {R_{2}}{S_{2}+R_{2}}}}$
Ratio[v] odd	${\displaystyle i_{1}={\tfrac {S_{4}+R_{4}}{S_{4}}}}$		${\displaystyle {\tfrac {(S_{3}+R_{3})(S_{4}+R_{4})}{S_{4}R_{3}+S_{3}(S_{4}+R_{4})}}}$		${\displaystyle {\tfrac {S_{3}R_{2}R_{4}(S_{1}+R_{1})+S_{2}S_{1}(S_{3}+R_{3})(S_{4}+R_{4})}{S_{3}R_{4}(S_{1}(S_{2}+R_{2})+R_{1}R_{2})+S_{1}S_{2}S_{4}(S_{3}+R_{3})}}}$			${\displaystyle {\tfrac {R_{2}(S_{1}+R_{1})}{R_{2}(S_{1}+R_{1})+S_{1}S_{2}}}}$
Algebra And Actuated Shift Elements[w]
Brake A[x]	❶	❶	❶					❶	❶
Brake B[y]	❶	❶	❶	❶	❶	❶
Clutch C[z]		❶		❶		❶	❶	❶
Clutch D[aa]	❶				❶	❶	❶	❶	❶
Clutch E[ab]			❶	❶	❶		❶		❶
8HP 70 [ac][ad]	700 N⋅m (516 lb⋅ft) 2008		48[11] 96	48[11] 96	69[2] 111	23[2] 85	2 3	7.0435 1.7693	1.3216[n]
Gear Ratio	−3.2968[o] ${\displaystyle -{\tfrac {1,744}{529}}}$	4.6957 ${\displaystyle {\tfrac {108}{23}}}$	3.1304[r] ${\displaystyle {\tfrac {72}{23}}}$	2.1039 ${\displaystyle {\tfrac {162}{77}}}$	1.6667[n][r][s] ${\displaystyle {\tfrac {5}{3}}}$	1.2845[r] ${\displaystyle {\tfrac {8,826}{6,871}}}$	1.0000 ${\displaystyle {\tfrac {1}{1}}}$	0.8392[r][s] ${\displaystyle {\tfrac {120}{143}}}$	0.6667 ${\displaystyle {\tfrac {2}{3}}}$
Step	0.7021[o]	1.0000	1.5000	1.4879	1.2623[n]	1.2975	1.2845	1.1917	1.2587
Δ Step[q]			1.0081[r]	1.1787	0.9729[r]	1.0101[r]	1.0779	0.9467[r]
Speed	-1.4243	1.0000	1.5000	2.2319	2.8174	3.6555	4.6957	5.5965	7.0435
Δ Speed	1.4243	1.0000	0.5000	0.7319	0.5855[s]	0.8382	1.0401	0.9000[s]	1.4478
Ratio[ae] R & Even	${\displaystyle i_{R}={\tfrac {R_{2}(S_{3}S_{4}-R_{3}R_{4})}{S_{3}S_{4}(S_{2}+R_{2})}}}$		${\displaystyle i_{2}={\tfrac {R_{2}(S_{4}+R_{4})}{(S_{2}+R_{2})S_{4}}}}$		${\displaystyle i_{4}=1+{\tfrac {S_{2}R_{1}}{S_{1}(S_{2}+R_{2})}}}$		${\displaystyle i_{6}={\tfrac {1}{1}}}$	${\displaystyle i_{8}={\tfrac {R_{2}}{S_{2}+R_{2}}}}$
Ratio[ae] Odd	${\displaystyle i_{1}={\tfrac {S_{4}+R_{4}}{S_{4}}}}$		${\displaystyle {\tfrac {(S_{1}+R_{1})(S_{4}+R_{4})}{S_{4}R_{1}+S_{1}(S_{4}+R_{4})}}}$		${\displaystyle {\tfrac {S_{1}R_{2}R_{4}(S_{3}+R_{3})+S_{2}S_{3}(S_{1}+R_{1})(S_{4}+R_{4})}{S_{1}R_{4}(S_{3}(S_{2}+R_{2})+R_{2}R_{3})+S_{2}S_{3}S_{4}(S_{1}+R_{1})}}}$			${\displaystyle {\tfrac {R_{2}(S_{3}+R_{3})}{R_{2}(S_{3}+R_{3})+S_{2}S_{3}}}}$
^ All 8L-transmissions are based on a dedicated 8-speed layout, first realized in the ZF 8HP 70 gearbox ^ Layout Input and output are on opposite sides Planetary gearset 1 is on the input (turbine) side Input shafts are C2 and, if actuated, R1 and S4 Output shaft is C4 (planetary gear carrier of gearset 4) ^ Total Ratio Span (Total Ratio Spread · Total Gear Ratio) ${\displaystyle {\tfrac {i_{n}}{i_{1}}}}$  A wider span enables the downspeeding when driving outside the city limits increase the climbing ability when driving over mountain passes or off-road or when towing a trailer ^ Ratio Span's Center ${\displaystyle (i_{n}i_{1})^{\tfrac {1}{2}}}$  The center indicates the speed level of the transmission Together with the final drive ratio it gives the shaft speed level of the vehicle ^ Average Gear Step ${\displaystyle ({\tfrac {i_{n}}{i_{1}}})^{\tfrac {1}{n-1}}}$  With decreasing step width the gears connect better to each other shifting comfort increases ^ Sun 1: sun gear of gearset 1 ^ Ring 1: ring gear of gearset 1 ^ Sun 2: sun gear of gearset 2 ^ Ring 2: ring gear of gearset 2 ^ Sun 3: sun gear of gearset 3 ^ Ring 3: ring gear of gearset 3 ^ Sun 4: sun gear of gearset 4 ^ Ring 4: ring gear of gearset 4 ^ a b c d e f g h i j k Standard 50:50 — 50 % Is Above And 50 % Is Below The Average Gear Step — With steadily decreasing gear steps (yellow highlighted line Step) and a particularly large step from 1st to 2nd gear the lower half of the gear steps (between the small gears; rounded down, here the first 3) is always larger and the upper half of the gear steps (between the large gears; rounded up, here the last 4) is always smaller than the average gear step (cell highlighted yellow two rows above on the far right) lower half: smaller gear steps are a waste of possible ratios (red bold) upper half: larger gear steps are unsatisfactory (red bold) ^ a b c d e f g Standard R:1 — Reverse And 1st Gear Have The Same Ratio — The ideal reverse gear has the same transmission ratio as 1st gear no impairment when maneuvering especially when towing a trailer a torque converter can only partially compensate for this deficiency Plus 11.11 % minus 10 % compared to 1st gear is good Plus 25 % minus 20 % is acceptable (red) Above this is unsatisfactory (bold) ^ Standard 1:2 — Gear Step 1st To 2nd Gear As Small As Possible — With continuously decreasing gear steps (yellow marked line Step) the largest gear step is the one from 1st to 2nd gear, which for a good speed connection and a smooth gear shift must be as small as possible A gear ratio of up to 1.6667:1 (5:3) is good Up to 1.7500:1 (7:4) is acceptable (red) Above is unsatisfactory (bold) ^ a b c d From large to small gears (from right to left) ^ a b c d e f g h i j k l m n o p q r s t u v w x y Standard STEP — From Large To Small Gears: Steady And Progressive Increase In Gear Steps — Gear steps should increase: Δ Step (first green highlighted line Δ Step) is always greater than 1 As progressive as possible: Δ Step is always greater than the previous step Not progressively increasing is acceptable (red) Not increasing is unsatisfactory (bold) ^ a b c d e f g h i j k l m Standard SPEED — From Small To Large Gears: Steady Increase In Shaft Speed Difference — Shaft speed differences should increase: Δ Shaft Speed (second line marked in green Δ (Shaft) Speed) is always greater than the previous one 1 difference smaller than the previous one is acceptable (red) 2 consecutive ones are a waste of possible ratios (bold) ^ Model year 2015 ^ Model year 2016 ^ a b Adjusted formulas with gearset 1 and 3 swapped ^ Permanently coupled elements S1 and R2 C1 (carrier 1) and C4 (carrier 4) S2 and S3 C3 (carrier 3) and R4 ^ Blocks S2 and S3 ^ Blocks R3 ^ Couples C2 and S4 with the turbine ^ Couples R1 with S4 ^ Couples S1 and R2 with S4 ^ without generation designation ^ First transmission on the market to use the dedicated 8-speed layout gearset 1 and 3 not swapped for comparison purposes only ^ a b Original formulas gearset 1 and 3 not swapped for comparison purposes only

The 8L 90 is the subject of a class-action lawsuit filed in December 2018 that alleges the transmission suffers from persistent "shudder" issues and that GM has known about the problems since its introduction and has failed to provide a solution, instead choosing to wait until the unit is out of warranty.^[16]

• List of GM transmissions

• General Motors Powertrain Division
• https://gmauthority.com/blog/gm/gm-transmissions/m5n/
• https://gmauthority.com/blog/gm/gm-transmissions/m5u/