Transmissions for AC Cobra Replicas

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

1️⃣ 4-speed manual gearboxes – the historical legacy

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.

🔹 Technical advantages

• 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.

🔹 Drawbacks

• 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.

🔹 Recommendation

• 427 / 428 FE big block: excellent choice, both historically authentic and mechanically appropriate.

• Small block: functional but sub-optimal; lacks fine gear spacing.

2️⃣ 5-speed manual gearboxes – the best modern compromise

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.

🔹 Comparison by engine type

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).

🔹 Technical advantages

• 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.

🔹 Drawbacks

• 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).

🔹 Recommendation

• Small block: TKX = best overall choice.

• Big block: TKO600 or TKX mandatory.

3️⃣ 6-speed manual gearboxes – extreme modern performance

Examples: Tremec T56 Magnum, TR6060

6-speed manual transmissions are typically used in high-performance restomod Cobras.

🔹 Technical advantages

• 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.

🔹 Drawbacks

• 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.

🔹 Recommendation

• 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.

4️⃣ Automatic transmissions – comfort and smooth power delivery

Examples: Ford C4, C6, AOD, AODE

🔹 Advantages

• 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).

🔹 Drawbacks

• 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).

🔹 Recommendation

• Drivers seeking comfort: AOD / AODE.

• Very high-torque big blocks: C6.

⚠️ Fairly comprehensive list of gearboxes for AC Cobra replicas

1️⃣ Historic “Cobra / US muscle car” gearboxes

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

2️⃣ Today’s replica favorites: TREMEC

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)

3️⃣ Aftermarket performance / racing gearboxes

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)

4️⃣ European or “repurposed OEM” gearboxes

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

5️⃣ Automatic transmissions used in Cobras

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)

6️⃣ Small manufacturers and gearbox specialists

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