Replacing a QSK38 1000HP @1800 with a Mitsubishi S12R-Y3 1100 HP @ 1800 RPM

Replacing a Cummins QSK38 with a Mitsubishi S12R

Replacing a QSK38 with an S12R requires changes to exhaust piping, air intake sizing, cooling capacity, and bellhousing due to differences in exhaust flow, heat rejection needs, and overall engine length. The S12R produces 1100 horsepower at the same 1800 RPM while using a larger displacement platform, which increases durability but requires larger exhaust silencers, intake diameters of 8 to 10 inches, and upgraded jacket water and aftercooler cooling systems.

Engine length increases by approximately 10 inches, requiring checks for clearance in front of the engine and potential relocation of nearby components. Exhaust flow rises from 6,425 to 8,368, resulting in a shift from 10-inch to 12-inch piping, and intake size will depend on the location of the existing filter elements. On the cooling side, both the jacket water cooler and aftercooler will have to be replaced, with most repowers requiring higher-capacity solutions, stacked configurations, or alternative layouts depending on available hull space.

Power & Configuration Difference

Metric	QSK 38	Mitsubishi S12R-Y3
HP Rating	1000 HP @ 1800 RPM	1100 HP @ 1800 RPM
Configuration	V-12	V-12
Displacement	38 L	49.4 L
Weight	10692 lbs	11729 lbs
Fuel System	High Pressure Common Rail	Mechanical
Bell Housing	Single 0	Double 00
HP/Liter	26.3	22.44
LBS./HP	10.6	10.66

Dimensional Delta: QSK38 vs Mitsubishi S12R-Y3

Dimension	QSK38	Mitsubishi S12R	Delta
Length	89.86 in	94.13 in	+4.27 in
Width	61.95 in	59.54 in	-2.41 in
Height	88.26 in	68.78 in	-19.48 in

Performance Characteristics

Metric	QSK 38	Mitsubishi S12R-Y3
Exhaust Flow (CFM)	6425	8368
Exhaust Size (Typical)	10	12
Intake Air Flow (CFM)	2952	3178
Intake Pipe Size (Typical)	8	8-10
Jacket Water Heat Rejection	17,181 BTU/min	23,375 BTU/min
Jacket Water Flow Rate (GPM)	292	454
Heat Rejection to After Cooler	8812	28,825
After Cooler Flow Rate (GPM)	151	53
Fuel Consumption @ 1000 HP	50.6	49.2
Fuel System	High Pressure Common Rail	Mechanical

Key Notes

The S12R requires more jacket water and aftercooler capacity than the QSK38, and both coolers will need to be replaced during a repower.

Heat rejection and flow rate data must be confirmed for each vessel, as available information from QSK installations can vary and may not reflect the exact configuration on board.

Cooler sizing cannot be standardized across vessels; determining compatibility requires serial numbers or model data from the existing units to verify performance with the S12R.

Stacked cooler configurations or outboard mounting may be necessary to fit the larger cooling hardware within the vessel’s available hull profile.

Some operators prefer separate jacket water and aftercooler circuits rather than combined layouts, citing simplicity and serviceability despite the additional space required.

Case-by-case evaluation is required, especially on vessels with channel-type coolers, where verifying cooling capacity is difficult without physical dimensions and flow information.

Common Modifications to Plan For

Cooling System

QSK38 vessels use a single jacket water cooler and aftercooler setup sized for 1,000 HP at 1800 RPM.

The S12R requires significantly higher cooling capacity:

Both the jacket water cooler and aftercooler must be replaced
Additional capacity is required beyond what is installed on the QSK38

Minimum required modifications:

Install larger coolers for both jacket water and aftercooler circuits
Increase cooling hardware size and footprint within the hull

Installation considerations:

Cooling layout varies by vessel, so sizing must be verified case-by-case
Existing data from drawings may be incomplete or inaccurate
Model/serial numbers from cooler manufacturers (e.g., Duramax, Fernstrum) are required to confirm capacity
In some cases, stacked cooling arrangements or outboard mounting are needed to fit larger coolers in the hull profile
Combined circuit designs are generally viewed as less desirable, with operators preferring separate circuits for serviceability

Exhaust & Air Inlet Piping

QSK38 commonly installed with:

10″ exhaust piping
10″ muffler configuration
8″ air intake (location varies)

S12R requires:

12″ exhaust piping and silencer modifications
8–10″ air intake piping depending on filter location
(inside engine room vs outside on deck)

Reasons for change:

S12R has higher exhaust flow (8,368 vs 6,425 on QSK38)
Larger airflow and exhaust volumes require oversized piping to maintain performance

Bellhousing & Gear

QSK38 uses a different bellhousing configuration than the S12R.

Modifications required:

Bellhousing adapter is needed to align with existing gear interface
The adapter adds additional length to the package
Total length increase is approximately 4 inches, plus the adapter

Installation considerations:

Check clearance at the front of the engine
Relocate forward-mounted components if needed
(e.g., Racor filters or other equipment blocking the increased engine length)

RPM match is identical:

- QSK38: 1800 RPM
- S12R: 1800 RPM
  → Gear ratio can typically be retained

Pedestals & Front-End Clearance

Most QSK38 repowers require pedestal modifications to align the S12R footprint and mount locations. The mounting points shift slightly, so existing pedestals are typically cut, extended, or re-built.

Common changes:
• Modify or replace existing pedestals
• Adjust height/spacing to match S12R mounts
• Install updated alignment hardware

Space at the front of the engine can impact pedestal work. On some vessels, generators or filtration equipment sit close to the engine face and limit access. In these cases, equipment may be shifted forward to create room.

Layout considerations:
• Check front clearance around generators and filters
• Confirm access to front-mounted components after installation

Pedestal work is expected on about 90% of repowers based on field experience.

Repower Planning Summary

Primary Required Changes

New coolers needed (JW + AC)
New engine mounting pedestals
Bellhousing adapter required
New mufflers and 12″ exhaust piping
New air filters and 8–10″ intake piping
S12R is longer — confirm front-end clearance

General Observations

Most QSK38 vessels have adequate width and height for an S12R repower, with the primary difference being added engine length.
Cooling capacity is the most consistent upgrade—both the jacket water cooler and aftercooler typically need to be replaced.
Exhaust systems usually require upsizing to 12″, and intake routing may change if filter elements are moved outside the engine room.
Pedestal modifications are common, as mounting geometry differs between the QSK38 and S12R.
Engine room layouts vary significantly, so front-end clearance and generator placement must be confirmed case-by-case.