Turbos, Valvetrain & the Combustion Chamber
Every turbo configuration explained, plus an animated 4-stroke diesel cycle and labeled valvetrain cross-sections of the Cummins 5.9 and the 7.3 Power Stroke family.
Turbo Configurations
Single, twin (parallel), compound (sequential), triple, and one-per-cylinder — what each one is, and what would actually happen if you ran it.
Characteristics
- • One turbocharger handles all of the engine's airflow
- • Simplest, cheapest, lightest, and most common setup
- • Turbo size is a compromise: small = fast spool but chokes at high RPM; large = big top-end but laggy low-end
- • Stock on the Cummins 12-valve (Holset HX35), most 7.3 Power Strokes (Garrett), and the vast majority of factory diesels
What would happen
A single turbo is sized to the engine's airflow window. Pick too big and you get lag — the engine feels dead until the turbo spools, then hits hard. Pick too small and it spools instantly but runs out of breath up top, raising exhaust gas temperature (EGT) and choking power. The sweet spot is a turbo that spools by ~1,800 RPM and flows enough air to keep EGT safe at peak torque. The Cummins HX35 is the textbook single — good low-end, enough top-end for stock fuel. Go bigger (HX40, HE351) and you trade low-end response for top-end power.
Characteristics
- • Two identical turbos in parallel, each fed by half the cylinders (one per bank on a V8)
- • Each turbo is smaller than one big single for the same total airflow, so each spools faster
- • Common on V-engines for packaging (one turbo per cylinder bank, short exhaust runs)
- • NOT the same as compound — both turbos feed the same intake plenum at the same pressure
What would happen
Parallel twins split the work: each turbo sees half the exhaust and feeds half the air. Because each is smaller, both spool quicker than a single big turbo of equivalent total flow, giving better low-end response with the same top-end — the best of both worlds for a V-engine. The trade-off is complexity (two turbos, two wastegates, more plumbing) and balance: if one turbo fails or one bank runs lean, that bank overheats. The Duramax and many Power Stroke builds use parallel twins. People often call these 'twin turbos' and confuse them with compound — they are not staged, they are side-by-side.
Characteristics
- • Two turbos in SERIES (staged), not parallel — the output of one feeds the input of the next
- • Small high-pressure (HP) turbo spools first for instant low-end; large low-pressure (LP) turbo compounds the boost for huge top-end
- • Boost multiplies: HP turbo adds pressure on top of the LP turbo's output (compound = multiply, not add)
- • The Cummins community's signature big-power setup — 50-100+ psi is common
- • Complex plumbing: air goes LP→HP→engine; exhaust goes engine→HP turbine→LP turbine→tailpipe
What would happen
Compounds give you a fat, broad torque curve with almost no lag and enormous peak boost. The small HP turbo spools almost instantly off idle (it's small, and it's being fed pressurized air once the LP wakes), giving great low-end. As RPM and exhaust flow climb, the big LP turbo comes alive and compounds the boost — the HP turbo keeps compressing air the LP already compressed, multiplying pressure. The result is boost that climbs with RPM to 50, 80, even 100+ psi on serious builds, with EGT kept manageable because there's always enough air. This is how 500-1,000+ hp Cummins builds are made. The cost is complexity, head studs (the cylinder pressure is enormous), and tuning — you must gate the HP turbo so it doesn't over-speed, and manage the LP's gate too. Compounds are the king of big reliable diesel power.
Characteristics
- • Three turbos in series (compounded three deep) — small, medium, large
- • Each stage multiplies the last; the smallest spools first, the largest makes the big top-end boost
- • Very rare, custom-built, almost exclusively competition (sled pull, drag, land speed)
- • Insane complexity, tuning, and cost — but the broadest, highest boost curve possible
What would happen
Triple compounds take the compound idea one stage further. A tiny turbo spools almost at idle, feeding a mid turbo, feeding a huge turbo — so boost builds smoothly from idle to redline with no single 'spool' step and enormous peak pressure. The result is the most linear, highest-boost curve you can build, capable of 1,000+ hp on a Cummins. The cost is extreme: three turbos, three sets of plumbing, three gates to manage, and tuning that borders on black magic. One stage out of sync and the whole stack surges or over-speeds. Triples are for competition trucks where the builder wants every last CFM and has the bottom end (O-ringed block, billet rods) to hold it. For anything street-driven, compounds are more than enough.
Characteristics
- • One small turbo per cylinder — each exhaust port drives its own compressor
- • Each turbo is tiny, so it spools almost instantly off idle — effectively zero lag
- • Each cylinder's airflow is independent — perfectly matched to that cylinder's demand
- • Extremely rare; insane complexity, cost, and packaging; mostly theoretical or industrial/racing experiments
What would happen
One turbo per cylinder eliminates the fundamental turbo problem — lag — because each turbo is so small it spools almost the instant its cylinder exhales. Every cylinder gets exactly the air it needs, when it needs it, with no shared plenum to fill. The theoretical result is diesel response like a naturally-aspirated engine with turbo-level torque. The reality is why almost nobody does it: you need as many turbos as cylinders (6 turbos on an inline-6), each with its own oil feed, its own compressor outlet plumbed to the intake, and a nightmare of packaging and heat. The cost, weight, and failure points multiply with every cylinder. It has been tried in a few industrial and racing experiments, but for any real engine, compounds give you 95% of the benefit at 10% of the complexity. One-per-cylinder is the fascinating extreme that proves why staged compounds won.
The 4-Stroke Cycle, Animated
Watch the piston rise and fall, the valves open and close, the pushrods and camshaft work, and the combustion bowl in the piston crown light up on the power stroke.
4-Stroke Diesel Cycle — Cummins 5.9 Style
Pushrod OHV valvetrain • combustion bowl in the piston crown (direct injection)
Stroke 1: Intake
Piston descends, intake valve opens — fresh air drawn in
Key visual: the combustion chamber is the bowl in the piston crown, not a prechamber in the head. This is direct injection — the Cummins 5.9 and the 7.3 Power Stroke both put the combustion bowl in the piston. The pushrods run from the camshaft (in the block) up to rocker arms (in the head), which open the valves.
Valvetrain Pictorials
Labeled cross-sections showing how the pushrod valvetrain works on the Cummins 5.9, and how the 7.3 Power Stroke (and its 4-valve 6.0 evolution) put the combustion chamber in the piston.
Cummins 5.9L 12-Valve
2 valves per cylinder • pushrod OHV • Bosch pump
What to see
- • 2 valves per cylinder (12 total on the I6) — one intake, one exhaust
- • Pushrod OHV: camshaft in the block, pushrods up to rocker arms in the head
- • Direct injection: the combustion chamber is a bowl in the piston crown, not a prechamber in the head
- • Bosch VE (early) or P7100 (1994-98) mechanical injection pump — no electronics
7.3L Power Stroke (T444E)
2 valves per cylinder (16V) • pushrod OHV • HEUI injection
What to see
- • 2 valves per cylinder (16 total on the V8) — same valvetrain layout as the 5.9, in a V8
- • Pushrod OHV with cam-in-block, just like the Cummins
- • Direct injection: combustion bowl in the piston crown (NOT the 7.3 IDI's prechamber-in-head)
- • HEUI injectors fired by high-pressure engine oil (the 5.9 uses a mechanical pump instead)
6.0L Power Stroke — the 4-Valve Evolution
4 valves per cylinder (32V) • pushrod OHV • HEUI + VGT
What to see
- • 4 valves per cylinder (32 total) — 2 intake + 2 exhaust, the layout you asked about
- • The 7.3 Power Stroke is 2-valve; the 4-valve-per-cylinder design arrived with the 6.0 Power Stroke (2003)
- • Still pushrod OHV (cam-in-block) — the 4 valves are actuated by the same pushrod/rocker system
- • More valves = better breathing at high RPM, which is why the 6.0 revs and flows more than the 7.3
The big picture: the Cummins 5.9 and the 7.3 Power Stroke share the same fundamental recipe — pushrod OHV (cam in the block), and a combustion bowl in the piston crown (direct injection). The difference is the fuel system (mechanical pump vs HEUI) and cylinder count (I6 vs V8). The 4-valve-per-cylinder layout is the 6.0 Power Stroke's evolution — more valves breathe better, but it's still the same pushrod family.
