Drivetrain options · 1 of 4 · Allison Transmission
Allison eGen Flex and eGen Power
Allison makes the six-speed automatic in most classic diesel pushers, including the MH 4000 in a 2002 Country Coach Magna. Its electrified line comes in two very different forms: a hybrid unit that replaces a bus transmission, and a complete electric axle for battery-electric and series-hybrid vehicles.
System
Allison eGen Flex
Type
Hybrid unit in place of transmission
Keeps C12 + MH 4000?
Engine yes, transmission no
Battery (supplier)
26 kWh LTO (Allison)
High voltage
450–756 V
Sold for retrofit?
No (bus makers)
Coach fit
Low
Tested against a 2002 Country Coach Magna: 40 ft, tag axle, Cat C12 (about 425–505 hp, up to about 1,550 lb-ft) and an Allison MH 4000.
In plain terms
What Allison offers, and why it matters for a coach
The name is familiar to every coach owner, which makes Allison the obvious first question. The answer is more complicated than swapping one Allison for another. eGen Flex keeps a diesel engine but replaces the automatic transmission with a two-motor hybrid unit and a small, high-power lithium-titanate battery. It is built and sold for transit buses, and its torque rating is well below what a Cat C12 produces.
eGen Power goes further. It is an electric rear axle with the motors, a two-speed gearbox and the differential in one housing. There is no engine, transmission or driveshaft in that layout, so the vehicle builder has to supply the high-voltage battery, power electronics and controls around it.
For a Magna-type coach, neither product is a bolt-in. eGen Power is the more interesting part for a ground-up series-hybrid rebuild, because the axle ratings are in the right range for a heavy rear axle, but Allison sells these systems to vehicle manufacturers rather than to converters.
01 · How it works
How it works
Two different products: eGen Flex and eGen Power
eGen Flex is a two-motor hybrid unit that takes the place of a transit bus’s automatic transmission. The diesel and the motors share the work, braking energy goes into an Allison-supplied 26 kWh lithium-titanate pack (450–756 V), and the bus can run engine-off in mapped zones. Ratings: 280 hp / 910 lb-ft or 330 hp / 1,050 lb-ft input.
eGen Power is a complete electric rear axle with motor(s), a 2-speed gearbox and the differential in one housing, for battery-electric trucks and buses.
eGen Power
Continuous / peak
Wheel torque
Axle rating
85S
225 / 325 kW
12,000 N·m
18,739 lb
100S · 130S
225 / 325 kW
26,000 N·m
23,000–28,660 lb
100D · 130D
450 / 650 kW
47,000 N·m
23,000–28,660 lb
Recommended battery voltage 550–850 V. Maker claim 100% regenerative braking torque capability.
Allison EP 50, the earlier hybrid unit that eGen Flex succeeds. Photo: Resaleking, Wikimedia Commons, public domain.
Where it sits among the four architectures
2 · Hybrid transmission Allison eGen Flex
Motors replace the automatic transmission and share work with the engine. Sold for transit buses.
The other architectures are compared on the overview.
Proven brand, but sold only to bus and truck makers
Advantages
Maker claim eGen Flex: up to 25% better fuel economy and up to 50% engine-off operation (duty-cycle dependent).
Independent NREL’s King County Metro study of the earlier Allison hybrid: 27% better fuel economy and 15% lower total operating cost per mile than diesel.
eGen Power packs motors, gearbox and differential into one axle; Allison’s existing distributor and service network.
Disadvantages
eGen Flex is rated to 1,050 lb-ft, well below a C12’s ~1,550 lb-ft, and is offered only through transit-bus builders.
Independent IndyGo’s eGen Flex buses ran engine-off 19–40% of the time, below the 50% claim.
eGen Power needs a full high-voltage battery-electric (or series-hybrid) design; an eGen Power axle weighs about 929 kg (2,048 lb).
Nothing from Allison replaces an MH 4000 in a motorhome. eGen Power could only serve as the drive axle in a ground-up electric or series-hybrid rebuild, and Allison sells to vehicle manufacturers.
The practical takeaway: an existing MH 4000 stays where it is unless the whole drivetrain is being replaced. If it is, an eGen Power-class axle is one candidate drive axle, subject to Allison agreeing to supply it and to a full weight, axle-rating and brake review of the coach.
04 · Maintenance
Maintenance
Maker claim eGen Power is filled for life with no oil changes; Allison says it will validate that over the next 5–10 years. TES 668 synthetic fluid, integrated oil cooler and pump.
Field action 2025FA13 (Dec 2, 2025, MY2024–25 eGen Flex buses): software to stop drive-unit oil expulsion through the breather (oil level recalibrated about 1.66 L lower), added cooling-fault protection for the inverter, DC-DC converter and battery, and fixes for false battery-relay fault codes.
2025FA15: software update for eGen Power on refuse-truck units.
Service through Allison’s distributor network; anyone opening the 450–850 V system needs high-voltage safety training.
Allison 4000 Series six-speed, the transmission family in many diesel pushers (the Magna has an MH 4000). Photo: Spielvogel, Wikimedia Commons, CC BY-SA 4.0.
What a conversion would involve in a 40-ft, tag-axle diesel pusher such as a 2002 Country Coach Magna (Cat C12, Allison MH 4000). Figures are only given where a source publishes them.
Space: eGen Power replaces the rear drive axle; engine, MH 4000 and driveshaft come out; the pack goes in the engine bay and basement bays.
Weight: eGen Power axle about 929 kg (2,048 lb). Pack weight depends on size: Estimate about 20 lb per kWh installed, from BYD’s published 8TT curb weights (422 vs 563 kWh: +2,873 lb).
Electrical: 550–850 V pack, inverters, HV DC-DC for 12/24 V, cooling loops, charging inlet.
Labor:Not public. A one-off engineering program with brake, axle-rating and weight review, not a shop bolt-in.
06 · 12 V house power & coach batteries
12 V house power & coach batteries
eGen Flex buses already make 24 V from the hybrid pack
12/24 V from high voltage
eGen Flex systems use a Vanner Hybrid Beltless Alternator, a DC-DC converter that replaces the belt alternator: 500–780 V in, 28 V out, 300 A per unit (about 8.4 kW), 600 A dual. It is 24 V; a 12 V coach would need a 12 V-output converter or a 24→12 V stage. eGen Power: no DC-DC from Allison; the builder supplies it.
Traction pack as house bank?
Not something Allison offers. The 26 kWh LTO pack is sized for hybrid power bursts, not overnight camping, and it is a 450–756 V system. A coach should keep a separate 12/24 V battery for starting, controls and contactors. Using a larger eGen Power-style pack for house loads would be the builder’s design (see BYD/Edison slides for pros and cons).
AC power & inverters
Vanner’s IAP2 package for eGen Flex includes an inverter (VEPI) that turns 600 V DC into 230 V three-phase for electric A/C and air compressors. A coach needs 120/240 V split-phase, so a different inverter would be required. Estimate typical coach peaks: 3 roof A/Cs at ~1.2–1.8 kW each running, plus induction at 1.8–3.7 kW, point to a 6–10 kW inverter.
Independent DeKalb, Illinois approved $972,239 for a 35-ft hybrid bus against $606,629 for diesel (about $366K more). NREL found maintenance roughly even with diesel on the older system. eGen Power price Not public.
08 · Battery & supplier
Battery & supplier
eGen Flex: 26 kWh lithium-titanate, supplied by Allison (cell maker Not public). eGen Power: no battery from Allison; the vehicle builder chooses one, e.g. Anadolu Isuzu’s Novo Volt bus pairs an 85S with 176 kWh LFP.
09 · How it compares
How it compares
Allison in the side-by-side table. The full table, including the parallel PTO add-on, is on the overview.
System
Type
Keeps C12 + MH 4000?
Battery (supplier)
HV
12/24 V & AC power
Sold for retrofit?
Coach fit
Allison eGen Flex
Hybrid unit in place of transmission
Engine yes, transmission no
26 kWh LTO (Allison)
450–756 V
Vanner DC-DC 28 V, 300–600 A; 230 V 3-phase export
No (bus makers)
Low
Allison eGen Power
Electric axle
No
Builder’s choice, e.g. 176 kWh LFP
550–850 V
Builder supplies
No (truck/bus makers)
Low
10 · Sources & credits
Sources & credits
Every figure on this page links back to one of these. Labels:
Maker claim what the manufacturer says
Independent government test, public record or trade press
Estimate typical engineering range, not a measured figure
Diagrams are original and simplified, based on the manufacturers’ published specifications. Photo credits and licenses appear under each image. Research compiled October 23, 2026; specifications, prices and availability change.
