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My thoughts - 19 psi on pump gas / detonation / octane etc

Jack

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Dec 31, 1969
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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…
 

gnxs

Electron Powered
Apr 26, 2004
8,930
256
Cobra Jack said:
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…

Good Post. :headbang: And you hit a few key reasons why I think many Terminators should not run 93 octane and 19psi.

- Too many people running too much timing AND high boost
- No knock sensor to help "bail you out" if detonation occurs
- Even with low timing, 93 octane is on the edge of safety and one marginal batch of gas could spell disaster (also, Winterized gas is an issue that I have direct experience with)
- Alot of people don't address all the cooling issues they should to help reduce the chance of detonation
- The tight piston clearances on these motors (not yours Jack ;) ) leaves very little room for the expansion that results from the elevated cylinder temperatures of high boost

I know alot of people don't agree with me on this (including yourself I think), but I don't see why this is any different than spending extra money on something like say synthetic oil to promote engine longevity. I may be wrong, but it seems to me that most of the "surviving" high-boost, high-mileage Terminators here all supplement their octane in one way or another.

Like you, I'll qualify this with the same disclaimer:

"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."
 

Jack

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Staff member
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Dec 31, 1969
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583
gnxs said:
I know alot of people don't agree with me on this (including yourself I think), but I don't see why this is any different than spending extra money on something like say synthetic oil to promote engine longevity.

I don't disagree with you... I'm not atking one side or the other on the 19psi issue.. everyone has to make their own choice

I'm just trying to share some insight and dispell a lot of misinformation
 

gnxs

Electron Powered
Apr 26, 2004
8,930
256
Cobra Jack said:
I don't disagree with you... I'm not atking one side or the other on the 19psi issue.. everyone has to make their own choice

I'm just trying to share some insight and dispell a lot of misinformation

Which you certainly did with your very informative post.

Unlike me and you, for many here these are the first F.I. cars they've owned. At the very least, you've helped people understand some of the key things needed to keep their cars alive with high-boost/93 octane and what's important to support that combination.

Again, nice synopsis. :headbang:
 

Mystic Terminator

TCG Elite Member
Oct 6, 2004
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Cobra Jack said:
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.

It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.

Jack,

What about mixing E85 w/ Premium? Is this something that will safely raise the octane rating? Say 25% E85 and 75% Premium?
 

Jack

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Mystic Terminator said:
http://en.wikipedia.org/wiki/Octane_rating
What about mixing E85 w/ Premium? Is this something that will safely raise the octane rating? Say 25% E85 and 75% Premium?


Here's a good article on E85 from Wiki:
http://en.wikipedia.org/wiki/E85

E85 has an octane rating of 105, which is higher than typical commerical gasoline mixtures (octane ratings of 85 to 95); however, it does not burn as efficiently in traditionally-manufactured internal-combustion engines. Additionally, E85 contains less energy per volume as compared to gasoline. Although E85 contains only 72% of the energy on a gallon-for-gallon basis compared to gasoline, experimenters have seen slightly better fuel mileage than the 28% this difference in energy content implies. For example, recent tests by the National Renewable Energy Lab on fleet vehicles owned by the state of Ohio showed about a 25% reduction in mpg [1] (see table on pg 5) comparing E85 operation to reformulated gasoline in the same flexible fuel vehicle. Results compared against a gasoline-only vehicle were essentially the same, about a 25% reduction in volumetric fuel economy with E85.

Use of E85 in non-FFV vehicles is generally experimental, with some users recommending light blends as low as 20%, while others have successfully run 100% E85. The main attraction of burning E85, of course, is the lower price per gallon at the pump of E85 versus gasoline. Other advantages include the common benefits of renewable energy sources, such as less environmental impact and less reliance on foreign energy.

Modern cars (i.e., most cars built after 1988) have fuel-injection engines with oxygen sensors that will attempt to adjust the air-fuel mixture for the extra oxygenation of E85, with variable effects on performance. All such cars can burn small amounts of E85 with no ill effects.

Operating fuel-injected non-FFVs on more than 50% E85 will generally cause the check engine light (CEL) to illuminate, indicating that the electronic control unit (ECU) can no longer maintain closed-loop control of the internal combustion process due to the presence of more oxygen in E85 than in gasoline. Once the CEL illuminates, adding more E85 to the fuel tank becomes rather inefficient. For example, running 90% E85 in a non-FFV will reduce fuel economy by 33% or more relative to what would be achieved running 100% gasoline. Even more importantly, continuing to operate the non-FFV with the check engine light (CEL) illuminated may also cause damage to the catalytic converter as well as to the engine pistons if allowed to persist. To run a non-FFV with amounts of E85 high enough to cause the CEL to illuminate risks severe damage to the vehicle, that may outweigh any economic benefit of E85.
 

rdsnake

formerly RD SNAKE
Mar 5, 2006
5,733
401
very good topic guys and I am one of the people that are runny 19 on 93 and pretty much....after that I had to give a call into VT engines...so do what you want...Also N20 will help lower intake temps a bit (20-40 shot) Also someone was talking about Winterized gas, I have HEARD not proven that they are doing away from that starting this winter because of emisson realaited problems. take it for what its worth 19 on 93 will make 600+ HP but will also come at a cost of a engine sooner or later....
 

1badass96gt

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Mar 22, 2004
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0
Good post. You can run almost any amount of boost with 93 octance (within reason) if you can make up for it somewhere else, i.e. spark, compression, IATS, etc. I think people try to generalize combos too much. Because you have an 04 cobra, and I have one and both have been built up does not mean we live within the same limitations. As I get more and more exposure to motors I've learned they are all mathicatical equations. It doesn't matter if its a+b=C or z+x=C as long as the end result is C. Think out of the box, and you'll be able to do things nobody thought safe or possible. Thats why cars are so cool. Good post, it put me in deep thought.


p.s I love the part about octane making power. People always say "I put race gas in it today to make it a little faster." They always have the same look when I tell them that gas actually makes them slower.
 
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