Disclaimer: I am not an engine builder, or mechanic, or tuner supreme…. I’m just a car enthusiast that has owned a lot of forced induction cars.
Everything written below is from my own head based on what I’ve read, and not copied from some website, as a result I’m probably wrong on a few finer points and welcome any corrections.
Well, here are my thoughts on running 19 psi on pump gas:
Ask anyone if you can run 19psi on pump gas and most will say no.. but relatively few will tell you why, for one simple reason… they don’t know why, it’s just what they’ve heard.
The simple answer why you shouldn’t/can’t run 19psi on pump gas is heat and the resultant detonation. Increasing boost, results in an increase of temperature. But, why does the temp increase?
Our superchargers work by mechanically compressing the air, they create pressure (boost) by reducing the volume of the air. Compression of any gas adheres to “Charles’s Law” which states that “at constant pressure, the volume of a given mass of a gas increases or decreases by the same factor as its temperature (in kelvins) increases or decreases”. That basically means compression of a gas naturally increases its temperature..
So what is detonation, also referred to as spark knock.. Detonation is the unwanted, erratic autoignition of fuel in the cylinder… basically the air fuel mixture ignites by itself before the spark plug fires, this causes tremendous additional strain on the piston and the rest of the drive train. Detonation can be caused too high of a compression ratio (remember compressions byproduct is heat), too much timing, to high of a cylinder air temp, low octane gas, or spark plugs that are too hot of a heat range.
To prevent detonation most car manufactures use knock sensors, (like on my old ’87 GN and ’98 GTP).. the knock sensor “listens” for the signs of detonation, which are typically a certain frequency on each motor.. when the sensor hears the slightest hint of knock/detonation, it commands the engine computer to pull timing and the result is the elimination or the decrease of detonation. Unfortunately our Cobras don’t have a knock sensor; the theory is that the knock sensor created too many false alerts due to engine noise. Without a knock sensor detonation is a huge concern.
As far as Octane goes. Many people have the incorrect assumption that higher octane provides more performance, the opposite is true.. The octane rating in fuel is a measure of the fuel’s ability to burn, an 87 octane gas has more energy than 104 octane gas.. The benefit of higher octane fuel is the prevention of detonation.. a car designed to run on 87 octane should perform better using 87 octane rather than 93…
Most cars that require premium gas have knock sensors which allow you to safely run 87 octane with reduced performance, this is because the knock sensor will pull timing is there is a hint of detonation.. our cars however must have 93 octane in them because of the lacking knock sensor.
Sorry for going off tangent.
Back to running 19psi on pump gas, as I’ve alluded to, 19psi can be safely run on pump gas if you prevent detonation..
The ways in which detonation can be prevented:
- Running the appropriate spark plug heat range, you’ll need a colder spark plug than stock.. the NGK TR7’s should be good
- Use the best gas and the highest octane available, if you can get 93 octane, don’t settle for 91 or 92.. I like to use BP, Shell, or Mobile. Stay away from off brand gas
- Lower your intake air temps as much as possible
- Keep your engine as cool as possible, any heat source contributes to intake temps
- You must use an aftermarket heat exchanger and preferably use a larger reservoir to increase the amount of fluid in the system.
- Set your max timing accordingly, 23 degrees of timing is too much for 19-21 psi, I’d probably go with 18-19 degrees, but definitely not over 21
Ok I’m tired of typing…
Everything written below is from my own head based on what I’ve read, and not copied from some website, as a result I’m probably wrong on a few finer points and welcome any corrections.
Well, here are my thoughts on running 19 psi on pump gas:
Ask anyone if you can run 19psi on pump gas and most will say no.. but relatively few will tell you why, for one simple reason… they don’t know why, it’s just what they’ve heard.
The simple answer why you shouldn’t/can’t run 19psi on pump gas is heat and the resultant detonation. Increasing boost, results in an increase of temperature. But, why does the temp increase?
Our superchargers work by mechanically compressing the air, they create pressure (boost) by reducing the volume of the air. Compression of any gas adheres to “Charles’s Law” which states that “at constant pressure, the volume of a given mass of a gas increases or decreases by the same factor as its temperature (in kelvins) increases or decreases”. That basically means compression of a gas naturally increases its temperature..
So what is detonation, also referred to as spark knock.. Detonation is the unwanted, erratic autoignition of fuel in the cylinder… basically the air fuel mixture ignites by itself before the spark plug fires, this causes tremendous additional strain on the piston and the rest of the drive train. Detonation can be caused too high of a compression ratio (remember compressions byproduct is heat), too much timing, to high of a cylinder air temp, low octane gas, or spark plugs that are too hot of a heat range.
To prevent detonation most car manufactures use knock sensors, (like on my old ’87 GN and ’98 GTP).. the knock sensor “listens” for the signs of detonation, which are typically a certain frequency on each motor.. when the sensor hears the slightest hint of knock/detonation, it commands the engine computer to pull timing and the result is the elimination or the decrease of detonation. Unfortunately our Cobras don’t have a knock sensor; the theory is that the knock sensor created too many false alerts due to engine noise. Without a knock sensor detonation is a huge concern.
As far as Octane goes. Many people have the incorrect assumption that higher octane provides more performance, the opposite is true.. The octane rating in fuel is a measure of the fuel’s ability to burn, an 87 octane gas has more energy than 104 octane gas.. The benefit of higher octane fuel is the prevention of detonation.. a car designed to run on 87 octane should perform better using 87 octane rather than 93…
Most cars that require premium gas have knock sensors which allow you to safely run 87 octane with reduced performance, this is because the knock sensor will pull timing is there is a hint of detonation.. our cars however must have 93 octane in them because of the lacking knock sensor.
Sorry for going off tangent.
Back to running 19psi on pump gas, as I’ve alluded to, 19psi can be safely run on pump gas if you prevent detonation..
The ways in which detonation can be prevented:
- Running the appropriate spark plug heat range, you’ll need a colder spark plug than stock.. the NGK TR7’s should be good
- Use the best gas and the highest octane available, if you can get 93 octane, don’t settle for 91 or 92.. I like to use BP, Shell, or Mobile. Stay away from off brand gas
- Lower your intake air temps as much as possible
- Keep your engine as cool as possible, any heat source contributes to intake temps
- You must use an aftermarket heat exchanger and preferably use a larger reservoir to increase the amount of fluid in the system.
- Set your max timing accordingly, 23 degrees of timing is too much for 19-21 psi, I’d probably go with 18-19 degrees, but definitely not over 21
Ok I’m tired of typing…