Gravity or pump is the first real decision in buying a soft serve machine, and it is the one operators most often make on price alone. The pump machine is more expensive. It is also the one that changes your cost per serving.
What overrun actually means
Overrun is the air worked into the mix as it freezes. More air means more finished volume from the same gallon, a lighter texture, and more servings sold per gallon bought. A gravity machine takes in whatever air the design allows. A pump machine injects it deliberately and lets you set how much — Spaceman's pump system offers six overrun settings.
That is the whole argument. Everything else follows from it.
The manufacturer's own figures
These pairs are the same machine in every respect except the feed system, which makes them a clean comparison:
- 6236-C vs 6236A-C — both a 3.4 qt cylinder. Gravity pours 350/hr from a 20 qt hopper. Pump pours 400/hr from a 13.8 qt hopper.
- 6250-C vs 6250A-C — both twin 1.8 qt cylinders. Gravity pours 350/hr from twin 12.7 qt hoppers. Pump pours 450/hr from twin 8.5 qt hoppers.
- 6378-C vs 6378A-C — both twin 3.4 qt cylinders. Gravity pours 700/hr from twin 20 qt hoppers. Pump pours 720/hr from twin 13.8 qt hoppers.
Notice the hoppers shrink as the output rises. That is the clearest evidence of what overrun does: the pump machine needs to hold less mix because it gets more servings out of every quart. On the 6250 pair, output rises 29% while hopper capacity drops by a third.
When gravity is the right buy
Pump is not automatically correct. Choose gravity when:
- You want a dense product. Gelato and premium custard are supposed to be heavy. Overrun works against you.
- Cleaning time is your constraint. A pump is another assembly to strip, wash and rebuild every day. If closing is already tight, that matters more than a margin point.
- Volume is genuinely low. Below a few hundred servings a week, the extra servings per gallon will take a long time to repay the price difference.
When pump is the only option
If you intend to pour frozen cocktails — the soft serve margarita that bars are adding right now — you need a pump machine. The drink will not hold its structure out of a gravity cylinder. Spaceman builds two models for it, the 6235A-C and the 6378A-C.
See both soft serve margarita machines.
The short version
Buy gravity if you want simplicity, a denser product, or the fastest close. Buy pump if you are selling volume, watching food cost per serving, or putting frozen cocktails on the menu. At high volume the pump machine repays its price difference in mix you did not have to buy.
