350 Bored 30 Over HP: Unleashing Maximum Performance for Your Engine

Exploring the world of engine modifications, the term ‘350 bored 30 over’ often surfaces among car enthusiasts. This article delves into what this modification entails and its impact on horsepower.

While the concept of boring out a 350 engine to increase its displacement to 355 cubic inches seems straightforward, the actual benefits, particularly in horsepower gains, are more nuanced and deserve a closer examination.

Join me as I unravel the mysteries and realities of enhancing your engine’s performance with this popular modification.

350 Bored 30 Over HP: Unleashing Maximum Performance for Your Engine

Understanding 350 Bored 30 Over HP

I recently came across an engine term called “350 bored 30 over” and wanted to better understand what it means and its impact on horsepower.

A 350 bored 30 over engine is one that has its cylinders bored out by 0.030 inches. This results in increased displacement, more power output, and improved performance.

For example,

  • A standard 350 engine typically has a displacement of 350 cubic inches
  • A 350 bored 30 over, also called a 355 engine, has a slightly larger displacement of 355 cubic inches

I also learned that the increased horsepower gain from boring a 350 engine by 30 over is not significant. In fact, some people argue there might not be any power gain at all1.

This is because the heavier pistons and larger circumference piston rings used after boring can create more friction, which offsets the potential benefits.

Pros of a 350 bored 30 over:

  • Increased engine displacement
  • Improved performance, in some cases

Cons of a 350 bored 30 over:

  • Additional friction from heavier pistons and larger piston rings
  • No notable horsepower gain, in some cases

To summarize, a 350 bored 30 over refers to the engine process where the cylinders are increased by 0.030 inches in size. While not a significant increase in horsepower, it’s still appreciated by some car enthusiasts for improved engine displacement.

Technical Specifications of 350 Bored 30 Over Hp

Cylinder Bore and Stroke

When we talk about a 350 bored 30 over engine, we’re referring to a Chevy 350 engine that has been bored .030″ over its stock size. This increases the engine’s displacement slightly, resulting in a new size of 355 cubic inches.

The process of boring involves removing material from the cylinder walls to increase the cylinder diameter. By doing this, you get a bit more displacement and potentially more power.

However, it’s important to highlight that this alone won’t result in a considerable horsepower increase. For that, we need to consider other factors, such as engine components and configurations.

Horsepower and Torque

When you bore a 350 engine 30 over, the horsepower gain is typically not too significant. For example, in a stock 250 hp 350 engine, a 0.030 bore could result in just a 3.5 hp increase.

Yet, the actual change in horsepower depends on several factors, such as the quality of the bore job, as well as the supporting modifications made to the engine.

For instance, let’s consider a 355 small block engine that has been bored 30 over and comes with upgrades like a Competition 292Hyd cam, Pro-Action cast-iron cylinder heads with 2.02-inch intake, 1.60-inch exhaust valves, and 64cc chambers.

With the right combination of parts and tuning, your engine could see a boost in horsepower and torque.

In conclusion, merely boring a 350 engine 30 over doesn’t guarantee a significant increase in horsepower. The supporting modifications made to the engine play a crucial role in determining the actual power gains one can achieve.

Benefits of Boring Over a 350 Engine

Increased Horsepower

When you bore a 350 engine 30 over, you’re increasing its displacement by 0.030 inches, which results in a boost in horsepower.

For example, a stock 250hp 350 might increase to 253.5hp with this modification1. While this isn’t a huge gain, it’s still a noteworthy improvement that can make a difference in your driving experience.

Improved Torque

Along with horsepower, you’ll also experience improvements in torque. This is because increasing the engine displacement, as a result of boring, allows the engine to generate more force.

A better torque directly affects acceleration and overall performance of the vehicle2. So, if you’re looking for a little extra power on the road, boring your 350 engine 30 over could be the perfect solution.

Drawbacks of Boring Over a 350 Engine

Potential Engine Failures

When you bore a 350 engine 30 over, you increase its displacement and potentially improve its performance, but it comes with some risks.

One of these risks is potential engine failure. For example, if the bore is done poorly, you could lose horsepower instead of gaining it.

A poorly done bore job can also lead to problems like overheating and excessive oil consumption. These issues put a strain on the engine and, if not addressed properly, can cause premature wear and tear and even catastrophic engine failure.

Increased Fuel Consumption

Another drawback of boring a 350 engine 30 over is the potential for increased fuel consumption. Since the engine displacement is increased, it can lead to an increase in the amount of fuel needed to run the engine.

This can result in reduced fuel efficiency, which translates to higher fuel costs over time.

While the performance gains from boring a 350 engine 30 over might be appealing, it’s essential to weigh these benefits against the potential drawbacks like engine failure and increased fuel consumption.

Before making any decisions, consider consulting with an experienced automotive professional to ensure you make the best choice for your specific needs and the longevity of your engine.

The Process of Boring Over a 350 Engine

Measurement and Machining

When I’m boring over a 350 engine, the first step is to measure the current bore size and determine how much material needs to be removed.

This is done using precision measuring tools to ensure accuracy. After the measurements are taken, we go ahead and bore out the cylinders by 0.030 inches.

Boring over an engine requires specialized equipment and expertise. This process increases the engine displacement, resulting in improved power output and performance.

Assembly and Tuning

Once the boring is complete, the engine components need to be reassembled. This process often involves replacing new piston rings and pistons that are designed for the larger bore size.

After reassembling the engine, it’s important to ensure that everything is properly aligned and adjusted. This may involve tuning the engine to optimize its performance with the new bore size.

As a result, a 350 bored 30 over engine will see an increase in horsepower, although the increase is often nominal.

Case Studies of High-Performance 350 Bored 30 Over Hps

I came across an interesting discussion on SVTPerformance.com, where a user explained that a motor bored 30 over essentially adds about 2 cubic inches of displacement.

This results in a slight increase in horsepower. However, the improvement may not be too significant, as the same user noted that the power is mainly in the heads.

On Hot Rod Forum, I found a thread that mentions that the improvements of boring 30 over could be as little as 3.5 horsepower. This isn’t a huge boost, but it’s essential not to underestimate it.

Nevertheless, it’s important to understand that the quality of the bore work could impact the power gain, as poorly done work could lead to a loss in horsepower.

In my research, I also came across Automotive Simple’s article about 350 bored 30 over engines.

Their explanation highlighted that the main benefit of such an engine is the increased power and torque output, making it ideal for high-performance applications.

Though the actual amount of horsepower increase can be variable, the improved performance and potential power gain make 350 bored 30 over engines a popular choice among enthusiasts.

In conclusion, 350 bored 30 over engines can offer increased power and torque output. But the actual amount of horsepower gain is often minor and can depend on the quality of the bore work and other factors.

Frequently Asked Questions

What is the horsepower of a 350 engine bored 30 over?

A 350 engine bored 30 over sees a small increase in horsepower, typically around 1.4% more power. For example, a stock 250 hp 350 engine would produce approximately 253.5 hp when bored 30 over 1.

What are the benefits of boring a 350 engine 30 over?

When boring a 350 engine 30 over, the main benefits include:

  • Increased engine displacement
  • Improved power output
  • Enhanced performance

Bigger bores can increase horsepower and torque, resulting in better acceleration and overall performance2.

How does a 30 over bore affect a 350’s performance?

A 30 over bore increases the engine’s original bore size and displacement, leading to higher horsepower and torque. This means that acceleration and overall performance would be improved3.

What is the difference between a 350 bored 30 over and a 355 engine?

A 350 bored 30 over is essentially a 355 cubic inch engine4. The main difference between the two is that the 355 engine has a slightly larger bore, resulting in a higher overall horsepower and torque potential.

What is the maximum overbore for a 350 Chevy engine?

The maximum overbore for a 350 Chevy engine depends on factors such as the specific engine block and the condition of the cylinder walls. In most cases, a maximum overbore of 0.060 inches is considered safe. However, it is best to consult a skilled machinist or engine builder to ensure the proper limits are followed for a specific engine.

Are there any drawbacks to boring a 350 engine 30 over?

There are some drawbacks to boring a 350 engine 30 over, such as:

  • Potential for decreased engine lifespan due to thinner cylinder walls
  • Increased engine heat
  • Potential for decreased performance if the bore is not done correctly5

It is essential to consult a skilled machinist or engine builder when considering an engine bore to avoid these potential issues.


In conclusion, while the horsepower gains from such a bore increase may not be monumental, they contribute to a more robust and efficient engine.

This modification, popular among car enthusiasts, underscores the delicate balance between engine displacement and performance.

Whether you’re a seasoned mechanic or a curious car lover, the insights gained from this exploration can guide your decisions in engine enhancements and performance tuning.


  1. Discussion about HP gains from overboring 2 3

  2. https://automotivesimple.com/what-is-a-350-bored-30-over/ 2

  3. https://www.svtperformance.com/threads/will-i-get-more-hp-if-i-bore-30-over.651617/

  4. https://www.timesmojo.com/does-boring-increase-hp/

  5. https://automotivesimple.com/what-is-a-350-bored-40-over/