Why are we even talking about this... "there is no replacement for displacement"
I have been to many web pages and forums and one question I keep seeing is "What can I do or add to my motor to get more horsepower?"  I have seen many answers to this question, mostly in the form of lists of parts.   Before we go and spend our whole car budget on parts, it is a good idea to understand how the part is improving horsepower.  That way you can pick the combination of parts that will best suit your car's application, ie you wouldn't want to put a blower on a car that you use every day for a 60 mile commute. There are three general ways to increase horsepower at the flywheel.

The first way is to increase the fuel air volume in the chamber, which correspondingly produces more energy during combustion.  This is the most straight forward way to increase horsepower, more CI or fuel/air volume per cylinder = more power.  However, it comes at a price (literally), more CI= less MPG.

The second way to increase horsepower is to improve the efficiency of the combustion so as to burn a greater % of the air fuel mix.  Other than parts, there is no added cost to doing this; however  the economic "law of deminishing returns" applies:  as more parts are added, the % of increase gets smaller and smaller as we move toward 100%.  The fuel is already in the chamber, we are just burning a greater % of it.  The average stock motor only burns 60% - 80% of the fuel that passes through it.  Higher % Burn = more HP.

To understand the third and final way to increase horsepower, you need to know that a motor produces more horsepower than what  can be measured at the flywheel.  The problem is the motor uses those horses to run. in other words it takes HP to turn the cooling fan, draw in fuel/air mix, compress the mix and then push out exhaust gasses.  Less effort from the motor = more HP at the flywheel. Like before, other than the price of parts, there is no extra cost to freeing up these horses.  There are, however, only a limited number of horses available to be harnesed.
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The following chart lists performance parts and how they contribute to  HP increases.   Keep in mind, some parts fit in more than one category.
I generated most of these numbers using "Digital Dino 2000".  I took a Ford 302 and built it on the computer completely stock.  It bench tested at 167 HP.  I then added each part by itself and recorded the gain in HP.  The next step was to build a 302 with all of the performance parts listed above, except the forced induction and NOS and it bench tested at 417 HP.  I then changed each part back to stock by itself and recorded the loss in HP.  Some of the items listed did not have an option on my computer, such as performance ignition and ram air.  I found these % on line form manufacturers and took the highs and lows.  Keep in mind that parts work together, in other words, you will not get the full benefit of a performance motor if you are only running a 350 cfm carb.  All percentages should be taken into consideration with the following disclaimer:

THESE NUMBERS ARE  THE EXTREME  HIGHS AND LOWS AND WHEN PUTTING PERFORMANCE PARTS ON A MOTOR, THERE ARE A LOT OF VARIABLES THAT CAN INCREASE OR DECREASE THESE NUMBERS. 

Use this chart as a jumping off point or reference to potential gains.  Don't add up the % to 100, get those parts and expect to double your HP.  It's probably not going to happen.  The last thing to consider is cost.  Sure, if you want to drop $2,000 or more, you can go buy a blower and see some huge performance gains, but by adding the right combination of parts, you can get almost as much power for a lot less money.

Efficiency

Higher % Burn = more HP
Free Horses

Less effort from the motor = more HP
Volume

More Fuel/Air per Cylinder  = more HP



Hedders / Free Flowing Pipes & Mufflers

A free flowing exhaust makes for less effort on the part of the motor in pushing out  exhaust gasses.

18% to 21% HP increase


Performance Heads / Port and Polish

Longer smoother runners allow for  better air flow and air/fuel atomization, or a more complete mix.   Also, many performance heads have a smaller combustion chamber which  = a higher compression ratio. (see below)

16% to 33% HP increase

Performance Heads / Port and Polish

A larger exhaust valve makes for less effort on the part of the motor in pushing out exhaust gasses. 





16% to 33% HP increase

Performance Heads / Port and Polish

Longer smoother and larger runners coupled with larger valves allow for more air flow of air/fuel mix into the chamber. 




16% to 33% HP increase

Performance Carb

A performance carb creates a more balanced mix of fuel and air, as well as better atomization of the mix. 

1% to 25% HP increase


Performance Carb

A performance carb provides more fuel/air mixture than a stock one.


1% to 25% HP increase

Increased Compression Ratio

The higher the CR, the more volatile the fuel/air mix becomes, making it burn better.

0.5% to 1% HP increase per 1:1 increase




Roller Cam

A roller cam reduces the friction in the valve trane, reducing motor effort.  Many roller cams are performance oriented (see performance cam)

1% to 14% HP increase



Performance Cam

A performance cam keepes the exhaust valve open longer allowing easier removal of exhaust gasses.

1% to 25% HP increase

Performance Cam

A performance cam keepes the intake valve open longer, allowing more fuel/air mix per cylinder. 

1% to 25% HP increase

Hotter, Mutiple Spark, Ignition

The hotter and longer a spark fires, the more fuel is burned.

1% to 5% HP increase



Performance Intake

The longer larger runners allow for better atomization of the fuel/air mix .

1% to 12% HP increase

Performance Intake

The longer larger runners allow for less effort on the part of the motor to draw in fuel/air mix.

1% to 12% HP increase

Performance Intake

The larger runners allow more fuel/air mix to enter each cylinder.

1% to 12% HP increase


Electric Fan / Flex Fan / Clutched Fan

There is a considerable amount of drag created by a fan, but reducing the drag to only what is needed to effectively cool, or eliminiating the drag all together, we  reduce motor effort.

2% to 15% HP increase



Electric Waterpump / Oilpump/ Fuelpump

Mechanical parts are driven by the motor.  By converting these parts to electric, we reduce motor effort.

1% to 15% HP increase



True Ram Air (Thunderbolt style)

Air pressure builds up in the front of the car.  By running tubes from the front of the car to a sealed carb box, when the car is moving, fuel/air mix is pushed into the motor rather than making the motor go through the effort of pulling it in.









5% to 10% HP increase

True Ram Air (Thunderbolt style)

Air pressure builds up in the front of the car.  By running tubes from the front of the  car to a sealed carb box, when the car is moving, fuel/air mix is pushed into the motor rather than making the motor go through the effort of pulling it in. This pressure increase allows for a larger volume of fuel/air mixture to enter each cylinder.  Also, the air from the front of the car is cooler than the air under the hood.  You can fit more cool air and fuel in a cylinder than you can warm air and fuel

5% to 10% HP increase

NOS

NOS  oxigenates,  making the burn  more complete.

12% to 30% HP increase  2# per min N2O flow


NOS

NOS super cools, allowing for more mix volume per cylinder

12% to 30% HP increase  2# per min N2O flow



Turbos / Blowers

Turbos and blowers use the same idea as ram air, however, they are mechanically driven and add a huge increase in fuel/air mix volume per cylinder. 

26% to 40% HP increase

Synthetic Oil

Synthetic oil coats cylinder walls better and makes a better seal on the compression stroke.  This improves compression (see above)

1% to 3% HP increase

Synthetic Oil

Synthetic oil lubricates better, reducing friction on ALL moving parts.  This frees up more power.


1% to 3% HP increase


EFI

Electronic Fuel Injection delivers the perfect mixture of fuel and air at the most optimum time, creating a more efficient burn.

1% to 26% HP increase





Stroke the Motor

By increasing bore and stroke, we increase volume per cylinder.

You do the math

Disclaimer on Daze Tech Tips
      I am not an expert in this field. I have performed these modifications myself with very good results. I am passing along restoration and performance tips for the purpose of education.  If you are concerned about reliability or safety issues, I do not recommend that you or any other individual perform these changes or attempt to modify your cars from stock configuration except under your own volition.  I do not assume nor accept any liability for the use of this information or how it is applied.

















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