Auxiliary hydraulic flow is the oil the machine sends to a powered attachment, measured in gallons per minute.
It's the single most important spec for anyone running augers, trenchers, mulchers, breakers, grinders, or brooms — and it's the one most buyers never look at, because they're comparing horsepower and lift capacity instead.
Why Horsepower Isn't the Answer
Engine horsepower drives everything on the machine: the tracks, the loader arms, the hydraulic pump. What reaches an attachment's motor is only part of it, and that part is determined by flow and pressure, not by the number on the engine badge.
Two machines with the same engine can deliver very different performance to the same attachment, depending on pump sizing and system pressure.
The Formula
Flow alone doesn't tell you the whole story either — you need pressure too.
Hydraulic horsepower = (GPM × PSI) ÷ 1,714
That's the power actually available at the attachment.
Worked examples from published specs:
| Machine | Flow | Pressure | Hydraulic HP |
|---|---|---|---|
| Bobcat MT85 / MT100 | 12 GPM | 2,900 PSI | 20.3 hp |
| Kubota SCL1000 | 15 GPM | 2,800 PSI | 24.5 hp |
| Toro Dingo TX 1000 | 15.2 GPM | 3,000 PSI | 26.6 hp |
| Ditch Witch SK1050 | 15 GPM | 3,625 PSI | 31.7 hp |
Note that the Bobcat and the Ditch Witch are both well-regarded machines, and the difference in power delivered to an attachment is more than 50%. That shows up directly in whether a mulching head cuts or bogs.
What Happens If Flow Is Too Low
Every hydraulic attachment publishes a minimum flow requirement. Run below it and:
Performance drops. An auger stalls in clay. A mulcher bogs in material. A breaker delivers weak blows.
Heat builds. This is the part that causes damage. Under-fed hydraulic systems generate heat, and heat degrades oil, seals, and components on both the attachment and the machine.
Wear accelerates. Sustained under-feeding shortens the life of the attachment motor and the machine's hydraulic system.
It's the most common and most expensive attachment mistake in this category.
What If Flow Is Too High?
Less common but real. Attachments also publish a maximum flow, and exceeding it can damage the motor. Check both ends of the range.
Attachments by Demand
No flow required: buckets, forks, bale spears, levelers, pushers, boom poles
Low flow (intermittent): grapple clamping, blade angling, hopper dumping, plow positioning
Moderate flow (continuous): augers, angle brooms, plate compactors, post drivers, salt spreaders
High flow (sustained): trenchers, tillers, harley rakes, breakers, brush cutters, stump grinders, snow blowers
Highest flow: forestry mulchers, rock saws
The higher the demand, the more carefully you need to match machine to attachment.
The Breaker Exception
Hydraulic breakers have a second requirement: a carrier weight rating. The breaker has to be matched to the operating weight of the machine carrying it, or the machine gets pushed around instead of the concrete getting broken — and impact loads travel back into the loader arms.
Check flow and carrier weight on breakers.
How to Check Before You Buy
- Find the attachment's minimum and maximum flow requirement, and required pressure
- Find the machine's published auxiliary flow and relief pressure
- Confirm the machine falls inside the attachment's range
- Calculate hydraulic HP for both if you're comparing machines
If a manufacturer won't publish flow and pressure for their machine, that's information in itself.
FAQ
Where do I find a machine's auxiliary flow?
On the spec sheet, usually listed as auxiliary hydraulic flow in GPM and system relief pressure in PSI. Both numbers should be there.
What's standard flow vs. high flow?
In full-size skid steers, "high flow" is a specific option package. In the mini category the terminology is looser — compare actual GPM and PSI figures rather than relying on the label.
Can I increase my machine's flow?
Not practically. Flow is determined by pump sizing and engine speed. Buy the machine that matches the attachments you'll run.
Can I run two attachments at once?
No — one at a time on a single auxiliary circuit. Some machines offer dual circuits for specific applications.
Does flow affect non-hydraulic attachments?
No. Buckets, forks, and blades don't care.
Why does my attachment run hot?
Frequently under-feeding, sometimes a restriction or a coupler issue. Heat is the warning sign that something is mismatched.
What about coupler compatibility?
Flat-face and pioneer-style couplers both exist in the category. Confirm compatibility or plan on adapters.