diff --git a/core/src/consensus.rs b/core/src/consensus.rs
index 81623c71f..44d6712d1 100644
--- a/core/src/consensus.rs
+++ b/core/src/consensus.rs
@@ -18,7 +18,7 @@
 //! enough, consensus-relevant constants and short functions should be kept
 //! here.
 
-use std::cmp::max;
+use std::cmp::{max, min};
 use std::fmt;
 
 use global;
@@ -293,8 +293,10 @@ where
 	HeaderInfo::from_diff_scaling(Difficulty::from_num(difficulty), sec_pow_scaling)
 }
 
+pub const MAX_SECONDARY_SCALING: u64 = (::std::u32::MAX / 70) as u64;
+
 /// Factor by which the secondary proof of work difficulty will be adjusted
-fn secondary_pow_scaling(height: u64, diff_data: &Vec<HeaderInfo>) -> u32 {
+pub fn secondary_pow_scaling(height: u64, diff_data: &Vec<HeaderInfo>) -> u32 {
 	// median of past scaling factors, scaling is 1 if none found
 	let mut scalings = diff_data
 		.iter()
@@ -305,18 +307,30 @@ fn secondary_pow_scaling(height: u64, diff_data: &Vec<HeaderInfo>) -> u32 {
 	}
 	scalings.sort();
 	let scaling_median = scalings[scalings.len() / 2] as u64;
-	let secondary_count = diff_data.iter().filter(|n| n.is_secondary).count() as u64;
+	let secondary_count = max(diff_data.iter().filter(|n| n.is_secondary).count(), 1) as u64;
 
 	// what's the ideal ratio at the current height
 	let ratio = secondary_pow_ratio(height);
 
+	println!(
+		"-- {} {} {} {}",
+		scaling_median,
+		secondary_count,
+		diff_data.len(),
+		ratio
+	);
 	// adjust the past median based on ideal ratio vs actual ratio
-	let scaling = scaling_median * secondary_count * 100 / ratio / diff_data.len() as u64;
-	if scaling == 0 {
-		1
+	let scaling = scaling_median * diff_data.len() as u64 * ratio / 100 / secondary_count as u64;
+
+	// various bounds
+	let bounded_scaling = if scaling < scaling_median / 4 || scaling == 0 {
+		max(scaling_median / 4, 1)
+	} else if scaling > MAX_SECONDARY_SCALING || scaling > scaling_median * 4 {
+		min(MAX_SECONDARY_SCALING, scaling_median * 4)
 	} else {
-		scaling as u32
-	}
+		scaling
+	};
+	bounded_scaling as u32
 }
 
 /// Median timestamp within the time window starting at `from` with the
diff --git a/core/tests/consensus.rs b/core/tests/consensus.rs
index b06cf82c8..aee015de9 100644
--- a/core/tests/consensus.rs
+++ b/core/tests/consensus.rs
@@ -17,10 +17,7 @@ extern crate grin_core as core;
 extern crate chrono;
 
 use chrono::prelude::Utc;
-use core::consensus::{
-	next_difficulty, valid_header_version, HeaderInfo, BLOCK_TIME_WINDOW, DAMP_FACTOR,
-	DIFFICULTY_ADJUST_WINDOW, MEDIAN_TIME_INDEX, MEDIAN_TIME_WINDOW, UPPER_TIME_BOUND,
-};
+use core::consensus::*;
 use core::global;
 use core::pow::Difficulty;
 use std::fmt::{self, Display};
@@ -514,6 +511,71 @@ fn next_target_adjustment() {
 	);
 }
 
+#[test]
+fn secondary_pow_scale() {
+	let window = DIFFICULTY_ADJUST_WINDOW + MEDIAN_TIME_WINDOW;
+	let mut hi = HeaderInfo::from_diff_scaling(Difficulty::from_num(10), 100);
+
+	// all primary, factor should be multiplied by 4 (max adjustment) so it
+	// becomes easier to find a high difficulty block
+	assert_eq!(
+		secondary_pow_scaling(1, &(0..window).map(|_| hi.clone()).collect()),
+		400
+	);
+	// all secondary on 90%, factor should lose 10%
+	hi.is_secondary = true;
+	assert_eq!(
+		secondary_pow_scaling(1, &(0..window).map(|_| hi.clone()).collect()),
+		90
+	);
+	// all secondary on 1%, should be divided by 4 (max adjustment)
+	assert_eq!(
+		secondary_pow_scaling(890_000, &(0..window).map(|_| hi.clone()).collect()),
+		25
+	);
+	// same as above, testing lowest bound
+	let mut low_hi = HeaderInfo::from_diff_scaling(Difficulty::from_num(10), 3);
+	low_hi.is_secondary = true;
+	assert_eq!(
+		secondary_pow_scaling(890_000, &(0..window).map(|_| low_hi.clone()).collect()),
+		1
+	);
+	// just about the right ratio, also playing with median
+	let primary_hi = HeaderInfo::from_diff_scaling(Difficulty::from_num(10), 50);
+	assert_eq!(
+		secondary_pow_scaling(
+			1,
+			&(0..(window / 10))
+				.map(|_| primary_hi.clone())
+				.chain((0..(window * 9 / 10)).map(|_| hi.clone()))
+				.collect()
+		),
+		100
+	);
+	// 95% secondary, should come down
+	assert_eq!(
+		secondary_pow_scaling(
+			1,
+			&(0..(window / 20))
+				.map(|_| primary_hi.clone())
+				.chain((0..(window * 95 / 100)).map(|_| hi.clone()))
+				.collect()
+		),
+		94
+	);
+	// 40% secondary, should come up
+	assert_eq!(
+		secondary_pow_scaling(
+			1,
+			&(0..(window * 6 / 10))
+				.map(|_| primary_hi.clone())
+				.chain((0..(window * 4 / 10)).map(|_| hi.clone()))
+				.collect()
+		),
+		112
+	);
+}
+
 #[test]
 fn hard_forks() {
 	assert!(valid_header_version(0, 1));