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Crankcase Delivery Ratio

Calculate two-stroke bottom-end primary compression ratio and swept delivery flow volume to dictate high-RPM cylinder scavenging.

Bottom-End Squish Dynamics

Crankcase Measured Volumes

🔧 SCAVENGING PERFORMANCE TARGET: Standard high-performance 2-strokes target a primary compression ratio strictly between 1.3:1 and 1.5:1. If it's far too high, vicious mechanical pumping losses sap 10% of engine power spinning the crank. If it's far too low, the delivery ratio dies and the cylinder terminally fails to scavenge fresh air.

Primary Comp. Ratio

1.42 : 1
PCR Bottom-end squish block.

Delivery Ratio

1.000
Swept vs Displ.

Pumped Flow

250.0 cc
Violent CC sweep.
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What is The Physics of CrankcaseDelivery?

The technical foundation and mathematics behind the CrankcaseDeliveryCalc tool.

Mathematical Foundation

Crankcase Swept Volume Delivery
Delivery_Ratio=VolumeTDC−VolumeBDCDisplacementEngineDelivery\_Ratio = \frac{Volume_{TDC} - Volume_{BDC}}{Displacement_{Engine}}
Delivery_RatioDelivery\_Ratio= The mathematical efficiency metric expressing how effectively the bottom-end successfully pumps a fresh charge relative to the engine's total displacement size.
VolumeTDCVolume_{TDC}= Maximum internal crankcase volume when the piston is fully at Top Dead Center (cc).
VolumeBDCVolume_{BDC}= Minimum internal crankcase volume when the piston is fully at Bottom Dead Center (cc).
DisplacementEngineDisplacement_{Engine}= The advertized cc size of the cylinder.

Laws & Principles

  • The Downstroke Pumping Loss: As a 2-stroke piston fires downward, it acts as a massive mechanical air compressor, squashing the fuel/air mixture trapped inside the crankcase until it reaches the physical volume constraint of BDC. The total cc difference between TDC and BDC is the brute-force volume of mixture shoved up the transfer ports.
  • The Delivery Efficiency Target: A high-performance karting engine relies heavily on a high Delivery Ratio. If the engine is 250cc, but the crankcase only manages to physically pump 200cc of mixed gas on the downstroke (Delivery Ratio = 0.8), the cylinder will severely under-scavenge, leaving dead exhaust gas behind and killing horsepower.

Step-by-Step Example Walkthrough

" A race mechanic is analyzing a 250cc 2-stroke. They use a buret to measure crankcase volume at Top Dead Center (850cc) and again at Bottom Dead Center (600cc). "

  1. 1. Calculate Swept Pumping Volume: 850cc (TDC) - 600cc (BDC) = 250cc of air physically displaced by the crankcase stroke.
  2. 2. Calculate Primary Compression Ratio (PCR constraint limit): 850cc / 600cc = 1.41:1.
  3. 3. Determine Engine Top-End Size: 250cc cylinder displacement.
  4. 4. Calculate Final Delivery Ratio: 250cc (Total Pumped Volume) / 250cc (Total Cylinder Size) = 1.000.
Final Result: The crankcase has perfectly balanced volumetric efficiency (1.000 ratio). It physically pumps an exact 1:1 match of the cylinder's displacement through the transfer ports, resulting in exceptional scavenging at operating RPM.

Quick Answer: What is a Crankcase Delivery Ratio?

The Crankcase Delivery Ratio is a high-performance 2-stroke tuning metric that compares the volume of air your crankcase physically pumps against the actual displacement of your cylinder. If you have a 250cc engine, but the crankcase only manages to displace 200cc of mixture on the downstroke, your delivery ratio is 0.8:1 — meaning the cylinder is starving for fuel. Ideally, a perfectly tuned bottom-end delivers a 1.0:1 or slightly higher ratio to ensure furious high-RPM scavenging. Use the Crankcase Delivery Ratio Calculator above to instantly compare your measured Top Dead Center and Bottom Dead Center casing volumes against your total swept engine displacement.

Delivery Ratio Catastrophes

The "Big Bore" Starvation

An amateur mechanic buys an aggressive +4mm over-bore big bore kit for their Yamaha YZ125, pushing the top-end displacement up to 144cc. They bolt the new massive top end directly onto the stock crankcase without any bottom-end modifications. Instantly, the bike feels incredibly sluggish at high RPM and refuses to reach its top speed. The problem? The stock crankcase was engineered to pump 125cc of mixture per stroke (a 1.0 ratio). Now, the massive 144cc cylinder descends over the same bottom end, causing the delivery ratio to drop to a suffocating 0.86:1. The crankcase literally cannot pump enough physical volume to fill the new massive cylinder.

The Case-Trenching Solution

A professional engine builder receives a badly mismatched drag-racing quad. The previous owner had massively over-stuffed the crankcase pockets with metallic epoxy in an attempt to build low-end torque. The builder calculates the delivery ratio and finds it is choking at 0.75:1 at 9,000 RPM. The builder takes a die-grinder and literally cuts deep "trenches" into the aluminum engine cases directly below the reed cage. This mathematically increases the internal cc volume at Top Dead Center, restoring the swept pumping volume necessary to force exactly 1.02 times the cylinder volume through the transfer ports on every downstroke. The top-end power immediately returns.

Optimal Delivery Ratio Target Guide

Engine Tune Type Target Delivery Ratio Primary Sensation / Symptom
High-RPM Road Racing (e.g., Moto3)1.05:1 — 1.15:1Massive over-rev capabilities, weak bottom end.
Standard Motocross (MX)0.95:1 — 1.05:1Perfect balance; pumps exactly what the cylinder holds.
Low-RPM Trials / Enduro0.85:1 — 0.95:1Emphasis relies on pure primary compression pressure, not volume.
Poorly Designed Big-Bore Kit< 0.85:1Chokes out early; spark plug reads lean at wide-open throttle.
Massive Stroker Fails> 1.25:1Wasted energy; the exhaust port dumps fresh fuel out unburnt.

Note: The Delivery Ratio interacts intimately with exhaust pipe design. A massive Delivery Ratio (pumping way too much fuel) can sometimes be "saved" by a highly tuned expansion chamber that uses a hyper-aggressive sonic return wave to stuff the fresh, escaping fuel back into the combustion chamber just before the piston closes the exhaust window.

Pro Tips for Case Diagnostics

Do This

  • ✓Use a burette for physical measurement. Because crankcases are cast with complex internal webbing, irregular pockets, and transmission wall curves, you cannot calculate the internal volume using math. You must install the crankshaft, seal the edges, set the piston to exactly TDC, and drip measure fluid in via a graduated glass burette to find the true starting CC volume.
  • ✓Check delivery ratio before running a stroked crank. A stroker crank increases the swept pumping volume drastically by pushing the piston further up and pulling it further down. This will radically alter your delivery ratio overnight. Always crunch the numbers before firing the engine; you will likely need to adjust port timing to compensate.

Avoid This

  • ✗Don't confuse Delivery Ratio with Primary Compression Ratio. Primary compression tells you how hard the bottom end "squeezes" the charge. Delivery Ratio tells you how much total physical volume (CCs) actually gets shoved upward. You can have a very high primary squeeze, but terrible actual volume delivery (e.g., heavily stuffed cases on a large displacement engine).
  • ✗Don't ignore the rod length. Changing connecting-rod length does not change total engine displacement, but it explicitly alters where the piston stops at TDC and BDC relative to the crankcase base. This fundamentally alters your resting TDC/BDC CC cavity volumes, thereby changing your delivery ratio without ever boring the cylinder.

Frequently Asked Questions

What does a 1.0 Delivery Ratio mean?

It means the crankcase physically pumped the exact same volume of mixed gas as the engine's stated displacement size. If you have a 125cc engine, the bottom end successfully shoved exactly 125cc of fresh fuel up the transfer ports. This is a highly efficient 1:1 ratio. Lower ratios mean the top-end is starving. Higher ratios mean you are forcing excess fuel through.

Can a Delivery Ratio be greater than 1.0?

Yes. If you have a 125cc engine, but the crankcase geometry forces 140cc of air up the transfer ports, your ratio is roughly 1.12. This is common in high-RPM road racing engines that require massive amounts of raw mixture to breathe at 14,000+ RPM. However, if your exhaust pipe isn't perfectly tuned, much of that excess 15cc of fuel will simply blow straight out the exhaust port unburnt.

Why did my Big Bore kit make my bike slower on top end?

Because you dropped your Delivery Ratio. The massive new cylinder requires more raw cubic volume to fill correctly. The stock crankcase casing underneath it hasn't changed. The crankcase is now physically incapable of pumping enough volume to satisfy the new larger displacement. The top-end quite literally runs out of fresh air-fuel mix at high RPM and chokes.

How do I measure TDC Volume at home?

You must assemble the engine block (split cases bolted together), install the crankshaft and connecting rod, seal all weeping points (like bearing journals or reed block mounts) with heavy marine grease, and position the crankshaft so the connecting rod small-end is perfectly dead center at the absolute top of its stroke. Then, use a graduated burette to slowly drip measurement fluid (like light machine oil masked with rubbing alcohol) into the chamber until it is perfectly flush with the cylinder deck base.

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