What is a percentage range of horsepower difference between SAE gross horsepower and SAE net?
Executive summary
The difference between SAE gross and SAE net horsepower is not a fixed number but typically falls in the range of about 10–25% lower for SAE net compared with SAE gross; many authoritative and enthusiast sources cite roughly 15% as a common gap and a 20% “rule of thumb,” while other reporting warns there is no exact conversion because test conditions varied historically [1][2][3][4]. Those variations reflect differences in test accessories, exhaust treatment and manufacturer practices, so any single percentage should be treated as an approximation rather than a strict engineering conversion [5][6].
1. Why two numbers existed and what they measure
SAE gross horsepower was the pre‑1972 convention that measured an engine on a dyno without accessories, air cleaner, or production exhaust, producing an optimistic “bare engine” figure, whereas SAE net (phased in around 1972 under SAE J1349) measures the engine in an as‑installed state with belts, water pump, alternator, air cleaner and full exhaust attached, so net figures are lower and closer to real installed output; both are measured at the crank/flywheel level and do not include drivetrain losses to the wheels [5][7][8].
2. Typical percentage differences reported
Multiple technical histories and enthusiast analyses converge on a typical range: Ate Up With Motor and Hagerty note differences “around 15%” as a common historical gap between gross and net [1][9]; several forums and practical writeups quote a roughly 20% rule of thumb and describe SAE net as about 80% of SAE gross [4][2][3]. Summarizing these signals, a reasonable working range for the percentage reduction when moving from SAE gross to SAE net is approximately 10–25%, with ~15%–20% being the most frequently cited single‑value estimates [1][4][2].
3. Why the range can widen — real‑world and historical caveats
There is no universal conversion because manufacturers sometimes used different “idealized” conditions for gross testing (e.g., long‑tube headers, removal of accessories, optimistic atmospheric corrections), and some makers published both gross and net figures inconsistently by model year and market; as a result, comparisons of unchanged engines across the transition period show absolute gaps that can vary dramatically (sources document examples from a few tens of hp up to differences of 40–150 hp in some comparisons), so percentage differences depend on the specific engine, its original testing configuration, and whether compression and emissions tuning also changed between model years [5][6][10].
4. Practical guidance for interpreting old vs. modern ratings
When comparing a vintage “gross” rating to a modern “net” rating, treat any single conversion as an estimate: use ~15% as a conservative baseline reduction and ~20% if the engine lacked accessories or used aggressive headers in gross testing, but be ready to find exceptions where the gap is smaller or larger depending on engine plumbing and manufacturer idiosyncrasies [1][4][2]. For enthusiasts seeking greater rigor, the reporting notes that the only precise way to compare is to dyno the same engine under both protocols or rely on documented third‑party dyno tests rather than a blanket percentage [6][7].
5. Bottom line and where reporting diverges
Most technical histories and specialist outlets converge on a typical 10–25% reduction (with 15% commonly cited and 20% a frequent rule of thumb) when moving from SAE gross to SAE net, but multiple sources explicitly warn that “there is no precise conversion” and that manufacturer practices and test details drive outliers, so the single best answer is a percentage range coupled with the caveat that precise conversion requires engine‑specific data or dyno measurement [1][2][6].