Excellent daily driver
Estimates for entertainment and general information only. Verify everything with a qualified mechanic before buying parts or building. Full disclaimer
Feasibility and notes
- Torque is within this gearbox's comfort zone (55% of the rough limit).
- The most common K-to-RWD route, with adapter kits from several vendors. Direct-drive 5th means cruise RPM depends heavily on axle ratio.
- Cruise RPM is high (3,514 at 70 mph). A numerically lower axle or a taller tire will calm the highway drone.
- A 2.93 axle would score 97% with this setup (cruise 2,761 rpm).
RPM vs speed by gear
Estimated engine output
Gear breakdown
| Gear | Ratio | Overall | mph @ shift | RPM after upshift | RPM @ 70 mph | Peak thrust |
|---|---|---|---|---|---|---|
| 1 | 4.210 | 15.70 | 33 | 4,140 | over redline | 0.83 g |
| 2 | 2.490 | 9.29 | 56 | 4,667 | over redline | 0.49 g |
| 3 | 1.660 | 6.19 | 84 | 5,229 | 5,834 | 0.33 g |
| 4 | 1.240 | 4.63 | 112 | 5,645 | 4,358 | 0.25 g |
| 5 | 1.000 | 3.73 | 139 | n/a | 3,514 | 0.20 g |
All five transmissions with your car
Same weight, tire, engine and turbo settings. "Best axle" is the common ratio that scores highest for that gearbox.
| Transmission | Score | Cruise RPM | Best axle | Best score | Torque load |
|---|---|---|---|---|---|
| BMW ZF S5D-320Z 5-speed | 92% | 3,514 | 2.93 | 97% | 55% |
| BMW ZF GS6-37BZ 6-speed | 97% | 2,991 | 3.15 | 97% | 51% |
| BMW Getrag 260 5-speed | 97% | 2,847 | 3.38 | 97% | 75% |
| Nissan CD009 6-speed | 97% | 2,790 | 3.38 | 97% | 36% |
| Mazda Miata NB 6-speed | 97% | 2,963 | 3.15 | 97% | 86% |
Turbo size vs estimated power
K24A2 at 12 psi (turn on the turbo to use these in the score). Stock output ~205 hp.
| Inducer | Compressor limit | Est. peak hp | Gain over stock | Max potential* | Full boost | Drivability | Stock block |
|---|---|---|---|---|---|---|---|
| 20 mm | 72 hp | 205 | none (restriction) | n/a | 1,800 rpm | 87% | OK |
| 25 mm | 113 hp | 205 | none (restriction) | n/a | 1,800 rpm | 84% | OK |
| 30 mm | 162 hp | 205 | none (restriction) | n/a | 1,800 rpm | 78% | OK |
| 35 mm | 221 hp | 221 | +15 | 221 hp @ ~1 psi | 1,800 rpm | 79% | OK |
| 40 mm | 288 hp | 288 | +83 | 288 hp @ ~7 psi | 2,170 rpm | 80% | OK |
| 45 mm | 365 hp | 348 | +142 | 365 hp @ ~13 psi | 2,645 rpm | 81% | OK |
| 50 mm | 450 hp | 348 | +142 | 450 hp @ ~21 psi | 3,120 rpm | 81% | OK |
| 55 mm | 545 hp | 348 | +142 | 545 hp @ ~29 psi | 3,595 rpm | 84% | OK |
| 60 mm | 648 hp | 348 | +142 | 648 hp @ ~37 psi | 4,070 rpm | 89% | OK |
| 65 mm | 761 hp | 348 | +142 | 680 hp @ ~40 psi | 4,545 rpm | 88% | OK |
| 70 mm | 882 hp | 348 | +142 | 680 hp @ ~40 psi | 5,020 rpm | 86% | OK |
| 75 mm | 1,013 hp | 348 | +142 | 680 hp @ ~40 psi | 5,495 rpm | 80% | OK |
| 80 mm | 1,152 hp | 348 | +142 | 680 hp @ ~40 psi | 5,970 rpm | 74% | OK |
| Max potential: the lower of the compressor flow limit or ~40 psi, and it needs a built engine, E85-class fuel and supporting mods well before the top of the table. Drivability uses your current transmission, axle, tire and weight at the selected boost. | |||||||
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How the estimate works
Drivability is a weighted blend of six gearing factors: highway cruise RPM (25%), launch and first gear (20%), gear spacing (20%), around-town gearing (15%), power-to-weight (10%), and response and traction (10%). Turbo builds shift the targets: cruise can sit lower, and gear spacing is judged against staying above spool.
Turbo power uses rule-of-thumb math: naturally aspirated output scaled by boost pressure at about 85% efficiency, capped by a compressor flow limit of roughly 0.18 × inducer² (mm) crank horsepower. Spool is estimated from inducer size on a 2.4L. Real results depend on the exact turbine, A/R, manifold, fuel, tune and intercooling.
Transmission torque ratings are rough community figures, not manufacturer guarantees; clutch, driveshaft and differential limits aren't modeled. Gear ratios should be confirmed against your specific transmission code before buying parts. This is a planning tool, not an engineering analysis.
K24 RWD transmission gear ratios
Factory gear ratios for the five transmissions in this calculator. Ratios can differ by transmission code and year, so confirm yours before buying parts. Torque figures are rough estimates, not guarantees.
| Transmission | 1st | 2nd | 3rd | 4th | 5th | 6th | Torque capacity | Notes |
|---|---|---|---|---|---|---|---|---|
| BMW ZF S5D-320Z E36 328i / M3, E46 328i/330i | 4.210 | 2.490 | 1.660 | 1.240 | 1.000 | n/a | Rated 320 Nm (~236 lb-ft); est. ~300 lb-ft practical | Most common K-to-RWD route; direct-drive top gear |
| BMW ZF GS6-37BZ E46 330i (2003+) / E60 530i | 4.350 | 2.496 | 1.665 | 1.234 | 1.000 | 0.851 | Rated 370 Nm (~273 lb-ft); est. ~320 lb-ft practical | Mild overdrive 6th; about 105 mm longer than the ZF 5-speed |
| BMW Getrag 260 E30 325i | 3.830 | 2.200 | 1.400 | 1.000 | 0.810 | n/a | Est. ~220 lb-ft | Compact and light; popular in Miatas |
| Nissan CD009 350Z / G35 (2004.5+) | 3.794 | 2.324 | 1.624 | 1.271 | 1.000 | 0.794 | Est. ~450 lb-ft | Strongest common option; large and heavy |
| Mazda Miata NB 6-speed 1999–2005 Miata | 3.760 | 2.269 | 1.645 | 1.257 | 1.000 | 0.843 | Est. ~190 lb-ft | Bolts up with K-to-Miata adapter plates; fragile under boost |
Quick cruise-RPM formula: RPM = mph × top gear × axle ratio × 336 ÷ tire diameter (inches).
K24 swap FAQ
What is the best transmission for a K24 RWD swap?
There is no single best choice. The BMW ZF S5D-320Z 5-speed is the most common and affordable route, with adapter kits from several vendors. The Nissan CD009 6-speed is the strongest of the common options and the usual pick for turbo builds, but it is large and heavy. The BMW Getrag 260 and Mazda Miata 6-speed are compact and light for small cars but are not suited to big turbo torque. The comparison table on this page scores all five against your car.
What rear axle ratio should I run with a K24?
For a naturally aspirated K24, aim for roughly 2,500 to 3,300 rpm at 70 mph in top gear; turbo builds can cruise a little lower. Engine RPM = mph × top gear ratio × axle ratio × 336 ÷ tire diameter in inches. For example, a ZF 5-speed (1.00 top gear) with a 3.73 axle and a 225/45R17 tire (about 25 inches tall) turns about 3,500 rpm at 70 mph, so a taller 2.93 to 3.15 axle is usually a better highway match for that gearbox.
How much horsepower does a stock K24 make?
The K24A2 from the 2004–08 Acura TSX makes about 200 to 205 hp, the K24Z7 from the 2012–15 Civic Si makes about 201 to 205 hp, and the K24A1 from the 2002–06 CR-V makes about 160 hp. All are 2.4-liter engines with Honda i-VTEC.
What is a K24/K20 Frankenstein build?
A Frankenstein or hybrid build pairs a K24 bottom end with a K20 cylinder head, usually an RSX Type-S style head and intake manifold. It keeps the 2.4-liter torque while breathing and revving better up top. Builders commonly keep redline around 7,800 rpm on stock internals and about 8,500 rpm with forged internals. It needs a K24 timing chain, an oil restrictor for the K20 head, and a proper tune.
How much boost can a stock K24 handle?
A common community rule of thumb is roughly 350 to 400 crank horsepower on stock K24 rods and pistons with a careful tune and good fuel. Beyond that, forged internals are strongly recommended. The calculator flags builds above about 400 crank horsepower on stock internals. Results vary with tune quality, fuel and engine condition.
What size turbo should I use on a K24?
Compressor size sets the power ceiling and affects spool. A rough rule is maximum crank horsepower ≈ 0.18 × compressor inducer diameter in millimeters squared, so a 54 mm compressor supports about 525 hp and a 62 mm about 690 hp. Smaller compressors reach full boost sooner on a 2.4-liter, which makes a street car more pleasant; very large compressors trade low-RPM response for top-end power. Compressors below about 35 mm are too small to add power to a 2.4-liter engine.
How is the drivability score calculated?
The score is a weighted blend of six gearing factors: highway cruise RPM (25%), launch and first gear (20%), gear spacing (20%), around-town gearing (15%), power-to-weight (10%), and response and traction (10%). Builds that exceed a transmission's estimated torque limit lose points. 100% means a very pleasant daily-driver setup.
Are the calculator's results accurate?
They are estimates for planning and entertainment, based on published gear ratios and rule-of-thumb engine and turbo models. Real results depend on the exact transmission code, tune, fuel, turbo turbine and A/R, and driveline. Confirm gear ratios and part fitment for your specific vehicle and have a qualified mechanic review your build.
K24 RWD community & resources
Where swap builders compare gearboxes, gearing and hybrid heads. Links open in a new tab.
Transmission & gearing threads
- Best transmission for K20 to RWDk20a.org: Miata vs BMW vs Nissan boxes, and which survive boost
- K series RWD trans optionsk20a.org: TH400, S2000, 350Z and BMW ZF options
- RWD Turbo K24 Helpk20a.org: planning a 450 hp turbo K24A2 in an E30
- K24ranger swapk20a.org: RX-8 and 350Z adapter kits and flywheel notes
K20 head / Frankenstein builds
- 400hp Frankenstein Build K24/K20k20a.org: realistic hybrid power, redline and boost
- K20/K24 Hybrid Engine Build GuideHybrid Racing: step-by-step hybrid parts and machining
Chassis communities
- K24A2 swap parts listS2KI: full S2000 K24 swap build list
- KPower Industries K24 swapFT86Club: K24 in the GR86, FR-S and BRZ
- Best RWD platform for NA K24Honda-Tech: RX-7, Miata and S13 chassis debate
Build series
- Speed Academy RWD K24 swapMulti-part K-swap S14 240SX series
Know a thread or site that belongs here? Send it in.
Disclaimer
For entertainment and general information only. The scores, horsepower figures, torque limits, spool estimates and other results on this page come from simplified rule-of-thumb models and publicly available information. They are not engineering calculations, professional advice or a guarantee of how any vehicle, engine or part will perform.
Use at your own risk. Engine swaps, turbocharging and drivetrain modifications can cause mechanical failure, property damage, serious injury or death. Always have your plans and work reviewed by a qualified mechanic or engineer, follow manufacturer specifications, and verify gear ratios, part numbers and fitment for your specific vehicle.
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