Factory 440 Blocks: Real HP Limits, Common Failures, and Lessons Learned.
Craig Schober
To add some context for this article, all my data and research was done with a 505 cubic inch RB block (4.350" bore x 4.250" stroke). I’ll sprinkle some specs throughout the article — even though I always ran the same rotating assembly size; The cylinder heads, cams, and intake did change throughout this time.
History
If you’ve been around Mopar guys, you’ve heard the argument: How much power can a stock 440 RB block really take? Some will tell you it’s done at 600 horsepower, others brag about pushing past 700 without issue — but we rarely get to see what happens weeks, months, or even years down the road.
After 15 years of building, racing, and breaking four different engines, I’ve seen firsthand where the stock block holds strong and where it gives up. This isn’t internet hearsay. It’s data backed by a sportsman racer who spent every spare dollar and weekend testing the limits.
The RB/440 block itself, built from 1966 to 1978, powered everything from Coronets and Chargers to Road Runners and even Winnebago RVs. Factory ratings ranged anywhere from 225 to 390 horsepower, and the “440 Wedge” quickly became a favorite for street/strip builds and dedicated race cars.
When the 2000s rolled in, aftermarket support exploded — stroker kits became the norm, and new aluminum cylinder heads easily outperformed any ported iron head out there. However, with more airflow and bigger cubes came a new challenge… Now that we have modern "go fast" parts, modern machining, and modern horsepower goals, can these blocks designed back in the ’60s hold up to the task?
Yes. Just not for long. Let me explain.
Talking with older Mopar racers over the years, I’ve noticed one common theme: block failures were everywhere back in the day. Before today’s aftermarket parts and tech, information was mostly word of mouth. Many of those guys spun their 440s past 7,000 RPM on the regular - and that, in my opinion, is mistake #1. These RB blocks do not like RPM. Because of that, I’ve always kept my redline around 6,800 RPM, and even then, I rarely push it that far. (I see why B-block 400 strokers were so common, but that’s a conversation for another time.)
The major issue that plagues big blocks is the two-bolt mains and the all-too-common “cap walk.” Cap walk happens when the main caps shift under load, allowing the crankshaft to move around, unfavorably. If it gets bad enough, the block begins to develop some core shift — and when that happens, cracks follow. Once a cast-iron block cracks, it’s usually game over. Leaks, compromised integrity, and other issues pile up, and off to the scrap yard it goes!
I’ve tried many upgrades to help stock blocks survive under higher-horsepower applications (700+ HP): steel girdles, main studs, billet caps, block fill — I’ve even talked with people who experimented with aluminum main caps and connecting rods, all in an effort to strengthen the weakest link in a high-HP build.
Throughout the last 15 years, I always ran the same rotating assembly size — 4.350" bore x 4.250" stroke — for a 505 cubic inch RB stroker motor. I chose this setup specifically for the long rod and shorter piston. Stock 440 pistons are massive, and less weight equals less strain on the block. I nicknamed the project “Operation 505.”
The Research
Vehicle: 1967 Coronet – 3,450 lbs with driver
Operation 505 – Block #1
Performance: 6.80s 1/8 mile
Specs: 12:1 compression, 264/274° @ .050, ~.620" lift, iron 452 heads
Upgrades: Girdle with main studs, no block fill
Estimated Output: ~640 HP
Racer Notes:
This one was my favorite. It lasted about three years and made hundreds of passes. It never saw above 6,700 RPM and I shifted at 6,000 RPM. Ran on 110 race fuel, extremely reliable, and had zero issues. The block originally came from a 1976 RV. It showed minimal signs of cap walk but eventually developed a crack on the driver side between the freeze plugs. A machinist suspected it was an old freeze crack from the block’s time in Chicago. Either way, I’m not a fan of racing with a cracked water jacket — so off to the scrap yard it went.
Final Thoughts:
If someone asked me for a reliable RB combo that will just let them race without worrying, this is it. I consider this 650 horsepower level the “safe zone” for stock RB blocks.
Operation 505 – Block #2
Performance: Lasted only a few passes
Specs: 12:1 compression, 264/274° @ .050, ~.620" lift, iron 452 heads
Upgrades: Girdle with main studs, old block fill
Estimated Output: ~640 HP
Racer Notes:
This one cracked at the bottom of a cylinder after only four or five passes. It already had old block fill in it that was deteriorating and likely caused more problems than it solved.
Final Thoughts:
Garbage block. Just a bad pick from the stash. Scrap yard.
Operation 505 – Block #3
Performance: 6.40s 1/8 mile, lasted 2–3 years
Specs: 12.5:1 compression, 280/284° @ .050, ~.650" lift, Trick Flow 270 heads
Upgrades: Girdle with main studs, billet caps, new block fill
Estimated Output: ~700 HP
Racer Notes:
This build picked up .04 in the 1/8 mile after switching to aluminum heads. Bigger camshaft, higher compression, and Max Wedge ports helped it breathe. It lasted a good while, but eventually the engine lost its “snap.” A crack developed on the side of a cylinder wall, and oil mixed with water. That was the end of that block.
Final Thoughts:
Could have been detonation, but it’s hard to say. I run very conservative timing (32° total locked out) with aluminum heads, always on 110/112 race fuel, and with an overbuilt ignition system. Regardless, scrap yard.
Operation 505 – Block #4 (Current)
Performance: 6.15 @ 111 MPH (1/8 mile), 9.70 @ 136 MPH (1/4 mile)
Specs: 13:1 compression, 280/284° @ .050, ~.650" lift, Trick Flow 270 heads
Upgrades: Girdle with main studs, billet caps, block fill
Estimated Output: ~725 HP
Racer Notes:
Currently about 250 passes on this engine with minimal signs of cap walk. I’m shifting at 6,500 RPM now and crossing the line around 6,000–6,200 RPM. It’s been solid so far, but recently the engine lost its “snap” again. Thankfully, this time it wasn’t the block — it was a cracked piston ring. Replacement parts are on the way, and I’ll update once it’s back together.
Final Thoughts:
I’m blaming this one on faulty parts. I’ve found an upgraded piston ring design I’m excited to try once it’s reassembled... We will see!
Conclusion
To put it bluntly, 650 horsepower is the max I trust from a factory RB block. Anything beyond that is a gamble. Maybe it lasts one pass. Maybe it lasts a few seasons. Who knows. But keeping RPM under control is the #1 thing you can do to help these blocks live.
I’m excited to see more aftermarket blocks starting to come to market. That’s the next step toward regaining the reliability I had with my first build. As of this writing, Callies and BMP both offer aftermarket Wedge blocks, and 440 Source is actively developing one with production underway. I’m eager to try one of the 440 Source blocks — they’re priced very fairly compared to what blocks have cost in the past.
I hope you found this article helpful. I know it was a long read, but I had a ton of information to share from over a decade of real-world testing. Thanks for reading — and for visiting SchoberMotorsports.com!
– Craig Schober
P.S.: I started a thread back in 2017 with even more info and discussion from Mopar racers!
https://www.forbbodiesonly.com/moparforum/threads/stock-440-block-horsepower-limit.141165/