Gearboxes for AC Cobra replicas: a comparative technical analysis (small block / big block)
AC Cobra replicas offer a wide range of engine configurations, from Ford small blocks (302 / 347 / 351W) to FE big blocks (390 / 427 / 428) and even the 460.
The choice of gearbox is therefore crucial: it directly affects reliability, driving character, cruising speed, fuel consumption, and even overall road behavior.
Four main gearbox families can be identified:
Historic 4-speed manual gearboxes
Modern 5-speed manual gearboxes
6-speed manual gearboxes
Automatic transmissions
Examples: Ford Toploader RUG, Toploader Close Ratio (2.32:1), Wide Ratio (2.78:1)
Original 289 FIA and 427 S/C Cobras were predominantly equipped with Ford Toploader 4-speed gearboxes. This transmission is renowned for its exceptional strength, thanks to its extremely rigid one-piece cast iron case and simple yet very durable synchronizers.
Very high torque capacity: up to 600–650 lb-ft in reinforced versions. Ideal for a 427 FE Side Oiler.
Mechanical simplicity: fewer moving parts, better heat dissipation, reduced maintenance.
Pure mechanical feel: firm engagement and precise gear locking, especially when paired with a Hurst Competition Plus shifter.
Gear ratios well suited to high-torque engines: particularly for big blocks, where the broad torque curve compensates for the limited number of gears.
No overdrive:
engine speed of 3,000–3,500 rpm at 120 km/h (75 mph) depending on rear axle ratio (3.31 or 3.54),
increased noise, fuel consumption, and heat buildup in summer (big block).
Faster clutch wear on twisty roads: no intermediate ratio between 2nd and 3rd gear.
Not ideal for small blocks: a 302 or 351W, which delivers its power higher in the rev range, benefits from closer gear spacing.
427 / 428 FE big block: excellent choice, both historically authentic and mechanically appropriate.
Small block: functional but sub-optimal; lacks fine gear spacing.
Examples: Tremec T5, TKO500 / TKO600, TKX
Most modern Cobra replicas opt for a 5-speed manual gearbox, mainly to benefit from an overdrive, which significantly reduces engine speed during highway cruising.
With small block engines (302, 347, 351W)
These engines produce moderate torque but are capable of higher revs.
A shorter 1st gear and well-spaced 5 ratios help keep the engine within its optimal power band.
A reinforced T5 or TKX is ideal for around 90% of small block Cobras.
With big block engines (FE 390 / 427 / 428, 460)
The T5 must be avoided (insufficient torque capacity); a TKO600 or TKX is the minimum requirement.
The massive torque of a 427 FE is handled far better thanks to the larger input shaft and reinforced synchronizers.
The overdrive greatly reduces engine stress:
427 FE + TKX + 3.31 rear axle = ~2,200 rpm at 120 km/h (75 mph)
(vs ~3,400 rpm with a 4-speed).
Overdrive (0.64–0.82) → lower temperatures, reduced noise, improved cruising range.
Better gear spacing:
smoother, more linear acceleration,
ability to keep the engine in its ideal torque range.
Excellent parts availability: bellhousings, shafts, crossmembers, and Cobra-specific kits.
Faster gear changes.
Installation adaptations required: shifter position, driveshaft length, crossmember.
Less historically authentic: more modern driving feel.
Older-generation TKO units can feel stiff when cold (issue resolved with the TKX).
Small block: TKX = best overall choice.
Big block: TKO600 or TKX mandatory.
Examples: Tremec T56 Magnum, TR6060
6-speed manual transmissions are typically used in high-performance restomod Cobras.
Dual overdrive (0.80 + 0.63): extremely low engine speed during highway cruising.
Close ratio spacing from 1st to 4th gear: ideal for high-revving small blocks or highly tuned big blocks.
Very high torque capacity:
T56 Magnum: 700+ lb-ft
TR6060: up to 750+ lb-ft
High efficiency and very fast gear changes.
Significant weight (55 to 65 kg / 120–145 lb).
Very large physical size: often requires cutting and reshaping the transmission tunnel.
Low historical authenticity.
High cost: often €5,000+ installed.
Highly built small block engines (347 stroker, 408 cu in): excellent choice.
FE big block engines: outstanding performance potential, but major space constraints → not ideal for ERA, better suited to Superformance or Backdraft chassis.
Examples: Ford C4, C6, AOD, AODE
• Superior comfort, no driver fatigue in urban driving.
• Exceptional durability:
• C4: up to 500 lb-ft
• C6: 700 lb-ft (ideal for big block engines)
• Very consistent on track when properly built.
• Possibility of semi-manual operation (modified valve body).
• Reduced driving involvement compared to a manual transmission.
• Lower efficiency due to the torque converter.
• Less consistent with the Cobra spirit.
• Additional heat management required (auxiliary transmission cooler mandatory on big blocks).
• Drivers seeking comfort: AOD / AODE.
• Very high-torque big blocks: C6.
⚠️ Fairly comprehensive list of gearboxes for AC Cobra replicas
These are the transmissions found in original Cobras and in many “authentic-style” replicas:
Ford / FoMoCo
Ford Toploader 4-speed (the classic 427 transmission, derived from 1960s Ford performance cars)
Ford 3-speed (rarer in Cobra replicas)
Borg-Warner
T-10 4-speed (used in most original 260/289 Cobras)
Super T-10 variants
Muncie (GM: M20, M21, M22) – sometimes used with Chevy engines in kit cars
Saginaw (GM) – entry/mid-level GM gearboxes, found in some GM-powered Cobras
The modern benchmark supplier for Cobras, Mustangs, and hot rods:
TREMEC T-5 (including “Cobra-spec” versions)
TREMEC TKX (new-generation compact 5-speed, widely used in Backdraft, Superformance, Factory Five, etc.)
TREMEC TKO-500 / TKO-600 (previous generation, still very common; e.g. Superformance MKIII with 427 + TKO600)
TREMEC T-56 / T-56 Magnum (6-speed, mainly for very powerful or modern replicas; also seen in Superformance Coupe)
TREMEC TR-6060 (evolution of the T-56, rarer but adaptable)
Less common, but clearly used in highly aggressive Cobra builds:
Richmond Gear (formerly Doug Nash
4-, 5-, and 6-speed “street / strip” gearboxes, sometimes chosen for 427 engines with short final drives
Jerico Performance (dog-box racing transmissions, widely used in drag racing and road racing)
G-Force (G-Force T5, GF5R, etc.)
Liberty’s Gears (modified and complete gearboxes, often based on Toploader or T-56)
Tex Racing / Andrews Products (NASCAR-style transmissions)
Hewland (mainly racing transaxles; rare in “classic” front-engine Cobras)
Some European-built or “exotic engine” replicas use:
Getrag (BMW / modern Ford gearboxes – e.g. BMW engines in some European Backdraft chassis)
ZF
ZF 5-speed (mostly seen in GT40s but occasionally fitted to Cobras)
Audi / VW
01E / 01X, etc. (common in GT40s, occasionally seen in mid-engine Cobras)
Renault UN1 (primarily GT40, but adapted to some special Cobra builds)
Toyota
R154, W58 (used in a few Toyota / Lexus engine kit cars)
Lexus / Aisin
Aisin 6-speed gearboxes paired with Lexus V8s (1UZ / 3UZ); Backdraft has referenced use of the 1UZ-FE in its chassis
BMW
ZF / Getrag units branded BMW, used in Coyote or BMW engine swaps
Although the classic image is a 4- or 5-speed manual, many replicas are built with automatics:
Ford
C4, C6
AOD / AODE / 4R70W (very common in Factory Five and Backdraft kits)
GM
TH350, TH400, 700R4 / 4L60E
Chrysler
Torqueflite 727 / 904 (for rare Mopar-powered Cobras)
Modern automatics
6-, 8-, and 10-speed Ford / GM / ZF units
(marginal, used mainly in very recent replicas with full electronic integration)
These names sometimes appear in Cobra specifications:
David Kee Toploaders, Dan Williams, Gear Vendors (add-on overdrive units)
Builders / remanufacturers: Modern Driveline, American Powertrain, McLeod, etc., supplying Tremec or Toploader gearboxes specifically prepared for Cobra, Superformance, Factory Five, and Backdraft builds