Cummins Compound Turbo Sizing — Tow vs Sled-Pull

September 4, 2026

Cummins Compound Turbo Sizing — Tow vs Sled-Pull

A compound turbo system that hangs a 1,200 lb-ft tow rig on a grade all day and a compound system that makes a 2,000 HP sled-pull run for nine seconds are built on the same idea and almost nothing else. The architecture is identical — a small turbo feeding a big turbo in series. The sizing is close to opposite. Get this backwards and you either build a tow truck that melts pistons at 1,400°F on a long pull, or a pulling truck that won't light the tires until half-track.

This is the article that the published Cummins platform guide flagged but didn't have room for: how to actually pick the two turbos in a compound setup, and why the answer changes completely depending on whether the truck's job is sustained load or one violent run. If you're spec'ing a compound kit for a 5.9 or 6.7, read this before you buy anything.

How a Compound System Actually Works

A compound (or "twin," but series-compound is the correct term) turbo setup runs two turbochargers in series, not parallel:

The high-pressure turbo (also called the high-side, primary, or manifold charger) bolts to the exhaust manifold. It's the small one. Exhaust hits it first, so it spools fast and provides low-RPM response and throttle.
The low-pressure turbo (the atmosphere charger, secondary, or "big" turbo) breathes ambient air and sits downstream of the exhaust flow. It's the large one. It does the heavy lifting at high RPM and sets the airflow ceiling.

The atmosphere charger draws ambient air, compresses it a little, and stuffs it into the inlet of the high-side charger. The high-side then compresses that already-pressurized air a second time before it goes to the intercooler. The two pressure ratios multiply, which is how a compound setup makes 60+ psi of manifold boost while neither turbo individually runs a brutal pressure ratio. That multiplication is also why compounds run so much cooler than a single big charger straining for the same boost — each turbo is working in an efficient part of its map instead of one turbo redlined off the edge of its.

The whole game is matching those two chargers to the duty cycle. That's where tow and pull split apart.

The Tow Compound — Spool, Drivability, and EGT Control

A tow truck's enemy is heat over time. You are not making one run. You are holding 15,000–25,000 lbs on a 6% grade for twenty minutes, and exhaust gas temperature (EGT) is the number that decides whether the pistons survive. A tow compound is sized to move enough air to keep EGTs down at sustained, moderate load — not to make a dyno number.

That dictates the sizing:

High-side: small and quick. You want the manifold charger to light early and hard so the truck pulls a loaded trailer away from a stop without a hole in the powerband. Think a quick-spooling S300-class or stock-frame-ish high-side (often a 62–66mm-class compressor on the atmosphere side of the high charger). Take the specific high-side sizing from the kit maker's recommendation for the truck's weight and fuel level — the ranges here are representative.
Atmosphere charger: moderate, not huge. An S400-class charger in the 75–80mm range is the classic tow atmosphere turbo. Big enough to feed real airflow and crush EGT at cruise-and-climb load, small enough that it isn't constantly surging at light throttle — the kit maker sizes the frame and compressor per application.
Priority: low drive pressure and a flat, early torque curve. A tow setup should make peak torque low — often by 1,800–2,200 RPM — and hold EGT under roughly 1,250–1,300°F on a sustained pull. Treat any single EGT number as a guideline, not a limit, and monitor pre-turbo where possible.

The failure mode of an over-sized tow compound is surge and lazy spool. Put a sled-pull atmosphere charger (a 480+) on a tow truck and at light cruise load the big turbo has nothing to do — it stalls, barks, and chatters (compressor surge), which is hard on the turbo and miserable to live with. It also won't light when you need grunt at 1,800 RPM with a trailer. Bigger is not safer here. Bigger is hotter at low load and slower to respond, which is exactly wrong for towing.

The tow compound's whole reason to exist is that a single fixed-geometry charger big enough to control EGT at high load won't spool for towing, and one small enough to spool runs the EGT up. The compound solves both at once — if you size it for the sustained-load job.

The Sled-Pull Compound — Maximum Airflow for One Run

Now invert every priority. A sled-pull or competition truck makes one pass. Drivability at 1,800 RPM is irrelevant. Cruise EGT is irrelevant. Surge at light load is irrelevant because there is no light load — the truck is at the load stop from the moment the sled hooks. The only thing that matters is mass airflow at high RPM to support the fuel, and surviving the run.

That dictates the opposite sizing:

Atmosphere charger: as big as the class and the engine will support. Sled-pull atmosphere chargers run S480, S488, S491, and larger — and serious classes go to triples (and quads on the biggest builds) to stage three pressure ratios together. The published platform tiers put a sled/race Cummins at 1,500–3,000+ HP precisely because of this airflow stacking. Check your class rules — most pulling associations cap inducer diameter by class, so the rulebook sizes the turbo, not the dyno.
High-side: larger than a tow high-side. Because the engine lives at high RPM during the run, the high-side can be bigger (it doesn't need to light at 1,800 RPM), and it needs to be bigger so it doesn't become a flow restriction choking the atmosphere charger up top. A high-side that's perfect for towing will be a cork at 4,000 RPM and 100 psi.
Priority: peak airflow and staging, with spool a distant concern. A pulling truck can tolerate spooling at higher RPM, more drive pressure, and a powerband that lives between, say, 3,000 and 4,500 RPM. Methanol and nitrous are common to help the big chargers light and to add charge cooling.

The failure mode of an under-sized pull compound is running out of air — the atmosphere charger maxes out, EGT skyrockets, drive pressure climbs into reversion territory, and power falls on its face at the top of the run. On a pulling truck, "too small" kills the pass; "too big" just means it spools a little later, which the driver manages with the clutch and the converter.

Drive Pressure — The Number That Decides Both

Here is the single most important concept and the one most often ignored: drive pressure (exhaust manifold backpressure ahead of the high-side turbine) versus boost (intake manifold pressure). The ratio between them — the backpressure ratio — tells you whether your compound is healthy or eating itself.

A good compound runs drive pressure close to or below boost. A backpressure ratio near 1:1, or even below 1:1 (more boost than drive pressure), is the hallmark of a well-matched, efficient compound — and is one of the main reasons to run compounds at all.
When drive pressure climbs well above boost — 1.5:1, 2:1 or worse — exhaust is being forced backward past the valves (reversion), in-cylinder temperatures spike, EGT readings lie low while the head is actually cooking, and head-gasket and piston life collapse. The exact ratio a given engine tolerates varies — take thresholds from your builder — but the principle holds.

This is why the high-side choice matters so much. Too small a high-side raises drive pressure — it becomes a restriction that the engine has to push exhaust through. A tow truck can get away with a smaller high-side because its airflow demand is moderate; push that same small high-side to pull-truck airflow and drive pressure goes vertical. The whole art of sizing a compound is finding the high-side that spools for the job without choking drive pressure at the top.

You cannot size a compound by boost number alone. Two systems making "60 psi" can have completely different drive-pressure realities and completely different EGTs. A boost gauge and an EGT gauge are not enough — a drive-pressure (exhaust backpressure) gauge is the instrument that actually tells you if the system is matched.

You're Not Just Buying Turbos — Size the Supporting Parts to Match

A compound system is an airflow upgrade, and air without fuel, fastening, and instrumentation is wasted at best and destructive at worst. Every compound build is really a system:

Head studs are mandatory, not optional. Stock head bolts will not hold a compound's cylinder pressure. The category answer is ARP 425 head studs — non-negotiable on any 12V or 24V running compounds, on both tow and pull builds. Confirm the current ARP kit number for your specific engine.
Fuel has to match the air. More air with stock fueling just leans it out and makes heat. Compounds pair with an upgraded CP3 (or a CP3 conversion on a CP4-equipped 6.7) and larger injectors sized to the power target — modest on a tow truck, large on a pull truck. Air and fuel are sized together, never separately.
Gauges are part of the kit, not an accessory. At minimum: boost, EGT (pre-turbo), and drive pressure. On a compound you are flying on instruments — the drive-pressure gauge in particular is what keeps a mismatched system from quietly destroying a head gasket.
The rest of the bottom end and driveline. At tow power levels the stock rods and trans may survive with tuning discipline; at pull-truck power they will not. Valve springs, a built transmission or clutch, and on the upper tiers billet rods and a girdle all enter the conversation. Match the whole driveline to the airflow you're now capable of.

Quick Reference — Tow vs Pull at a Glance

Variable	Tow compound	Sled-pull compound
Duty cycle	Sustained load, long pulls	One short, violent run
#1 enemy	EGT over time	Running out of air
High-side	Small, quick-spooling	Larger (lives at high RPM)
Atmosphere charger	Moderate (S400 ~75–80mm)	Huge (S480+, often triples)
Spool priority	High — must light ~1,800 RPM	Low — clutch/converter manages it
Drive pressure goal	Near or below 1:1	As low as the airflow allows
Power band	Low and flat	High-RPM, narrow
Fueling	Modest injectors + CP3	Large injectors, dual/race CP3, often meth

Sizing figures are representative starting points, not prescriptions — every compound must be matched to the truck's weight, fuel, transmission, and (for competition) the class rulebook. Confirm specific turbo frames and inducer sizes with the kit manufacturer.

What to Do Next

Define the job honestly. Is this truck 90% tow and 10% fun, or is it a dedicated pull truck that also drives to the track? The duty cycle picks the turbos — not the other way around.
Pick the atmosphere charger to your airflow ceiling, then size the high-side to spool for your duty cycle without choking drive pressure.
Buy the head studs and gauges in the same order as the turbos. ARP 425 studs and a drive-pressure gauge are not a later upgrade — they're part of the compound system.
Match fuel to air. Spec the CP3 and injectors to the power target at the same time you spec the chargers.
Tune for drive pressure and EGT, not just a boost number. A matched compound is defined by what the exhaust side is doing, not the peak on the boost gauge.