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Fuel Recommendations for 14:1 or more compression

General engine tech -- Drag Racing to Circle Track

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Postby David Redszus » Sun Dec 02, 2007 1:26 am

Ed-vancedEngines wrote:Additional fuel octane is not additional power ingredients. It is the opposite. Ed

Actually, excessive fuel octane does not increase power nor does it diminish power. Octane is totally unrelated to power; only to destruction.
Higher octane will not prevent pre-ignition and may in fact, mask it.

Using compression ratios as a basis for fuel octane selection can be misleading. A much better indicator would be compression pressure which is the result of Trapped Compression Ratio, Inlet Temperature and Inlet Temperature, compared to the MON octane value of the considered fuel. Other factors such as fuel evaporation, residual gases, and squish velocity also contribute to the octane performance of a fuel.
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Postby jmarkaudio » Sun Dec 02, 2007 2:12 am

Ralph,
As you can tell there is no simple answer. Almost any engine can be made to run with almost any gas. It's a matter of proper timing, and jetting to make it work. It may not make as much power, but it can be made to work. When it was more available we ran Aviation fuel years ago against race fuel and both ran the same ET. We had to run it a little richer than race fuel, but both worked. You can take one engine with two different cams, and change what fuel makes the most power. Like anything else, it's all in the combination. There are a lot more choices lately with fuel, confusing the issue even more. My suggestion is this. If you are bracket racing, 110 to 112 would cover up to 14 to 1. There or above, you would go at least 112 to 114. Pick a brand that is readily available and tune your car to work with that fuel. Keep the fuel as cool as you can and sealed between races, and buy as little as you can to use the freshest fuel. Once you get a baseline, then you can experiment with other brands and octanes. Be ready to go conservative on the timing and a tad rich on the jets when changing. Some fuels like the new oxygenated fuels require major fuel changes to work correctly.
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Postby MadBill » Sun Dec 02, 2007 4:29 pm

[quote="David Redszus...Actually, excessive fuel octane does not increase power nor does it diminish power. Octane is totally unrelated to power; only to destruction...[/quote]

This would be true if it was possible to just vary the Octane rating with no change in chemistry, but is an oversimplification in the real world, where it is achieved via who-knows-what different compounds, bringing their full array of physical and chemical characteristics along with them.
A number of experienced builders here (bigJoe1 comes to mind) have reported that high Octane racing fuels often cause significant power loss Vs. good grade pump fuels when the engine can operate satisfactorily on the latter.

That actual trapped compression ratio is an important factor is a good point, and has been discussed extensively here, mostly under the description DCR or dynamic compression ratio.
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Postby David Redszus » Sun Dec 02, 2007 9:41 pm

MadBill wrote:[quote="David Redszus...Actually, excessive fuel octane does not increase power nor does it diminish power. Octane is totally unrelated to power; only to destruction...


This would be true if it was possible to just vary the Octane rating with no change in chemistry, but is an oversimplification in the real world, where it is achieved via who-knows-what different compounds, bringing their full array of physical and chemical characteristics along with them.
quote]

It is possible in the real world to just vary the octane rating without other changes in chemistry. We do it all the time. The problem is that even very good engine builders do not take the time to learn about fuel chemistry.

If we took a certain fuel with enough octane to prevent detonation, and simply increased the TEL content, we would raise the octane value without changing any other property of the fuel. It would run exactly the same.

If however, we simply specifiy fuel octane, without regard to other properties of the fuel such as distillation curve, vapor pressure, specific gravity, stoichiometric value, (and a few others) then we have selected an entirely different type of fuel and should not ever expect it to run the same. It may run better, it may run worse, but it is not due to the fuel octane value.

Why are racers (and sanctioning bodies) so afraid of learning something about fuel chemistry? The ignorance and folk lore surronding fuels is absolutely amazing and very stupid. The power we make comes from the fuel, why wouldn't we want to get it right?
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Postby rskrause » Sun Dec 02, 2007 9:59 pm

It would help, a lot, if the people who make race gas provided more than minimal information about what they are selling. Also, to let the racing community know when they change the formula but keep the same name. Very frustrating. If there is any way to use methanol, use it and forget gasoline. Just about everything is better with some alcohol ;) Among the many, many reasons to prefer it, at this point it is cheap compared to race gas. Last few drums I bought were a little over $2/gallon delivered. Even accounting for using ~2.5 times as much as gas, the equivalent mixture would be like $5/gallon gas. So, there is considerable savings as well as more hp.

As far as your question is concerned, I know a lot less than some who have answered. But I have seen lots of drag motors with 14:1 running well on 110-112 octane.

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Postby Metrobilly » Sun Dec 02, 2007 10:27 pm

I am at times impressed at the compression ratio used by some of the engines described in this forum relative to the fuel octane used. I have a pretty good understanding of the variables that determine the octane needs, but I have a question. How much of a factor is the application a a given engine's octane requirement?

I would imagine that a drag race motor that runs at full power for only a few seconds would be less prone to detonation then a road race or circle track motor that runs for many minutes and every thing gets heat soaked. I would also expect that a drag race engine would survive a little detonation for a few seconds while an engine that runs for a longer time would fail.

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Postby David Redszus » Sun Dec 02, 2007 10:45 pm

Metrobilly wrote: How much of a factor is the application a a given engine's octane requirement?

I would imagine that a drag race motor that runs at full power for only a few seconds would be less prone to detonation then a road race or circle track motor that runs for many minutes and every thing gets heat soaked. I would also expect that a drag race engine would survive a little detonation for a few seconds while an engine that runs for a longer time would fail.

Charlie


You are absolutely correct. A drag race engine can survive all sorts of abuse for a few seconds of operation.

Any engine in continous operation is far more demanding of proper design and execution. The same holds for road racing bikes vs motorcross bikes, drag snowmobiles vs circle track vs snocross sleds.

The reason is, as you suggested, due to heat soak of critical components.

The heat range of Bosch spark plugs were once depicted by a three digit number that indicated how many seconds the plug would survive at wide open throttle. If we apply the same heat saturation logic to critical engine parts, we can either push to higher limits or restrain ourselves to save a lot of money due to engine failure.
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Postby ralph85 » Sun Dec 02, 2007 10:51 pm

Thanks for all the replies. This cam seems a little on the small side for that much compression and my simulator shows about 245 psi cranking compression and i guess that scared me a bit. Didn;t want to have to buy 116 octane to run it.

This is an EFI engine, drag race application, 650-700 hp pushing a 3300lb car. We're currently just above 12.5:1, running 110 octane and thinking about increasing the CR.
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Postby MadBill » Sun Dec 02, 2007 11:03 pm

[quote="David Redszus...It is possible in the real world to just vary the octane rating without other changes in chemistry. We do it all the time. The problem is that even very good engine builders do not take the time to learn about fuel chemistry...[/quote]

I guess I should re-phrase my statement. It is possible for a supplier to formulate and retail a fuel with increased Octane and no other significant chemical changes. However, as rskrause alludes, it's by no means certain or even likely that they will do so.

With a myriad of molecules at their disposal, the possibilities are endless and the permutations made available for the varied needs of race engines (even from a single supplier) are vast. For competitive reasons, only limited information is usually made available to the customer, so outside of broad descriptions of intended usage, class legality and a bit of basic data such as specific gravity and the TEL, MMT or oxygenate content, he is on his own in determining what works best for his application.

Given the significant variations required in jetting alone, few have the resources to thoroughly evaluate the perhaps 20 or so available formulations that might appear promising for a particular application.

To summarize, I think that, absent specific power-boosting compounds, a polling of the performance engine builders here would show more reporting power loss, Vs. break-even or gain, from using unnecessarily-high Octane race fuels for low compression applications.

(Help me out here. Joe? Someone??)
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Postby Ed-vancedEngines » Sun Dec 02, 2007 11:25 pm

David Redszus
You seem to be very sharp, partucuraily on this subject of Fuel requirements.l I will be wanting to pick your head severely about a current project we are working on, if possible.

I am not arguing, but in a way I guess I am.

Most all of us who have been around the block a few times in years do understand that TEL does not produce or enhnace power. It is a known anti-knock agent only. Am I not correct?

To introduce additional content of TEL into a gallon of fuel mixture by volume, the volume displaced by the additional TEL would leave need to remove other power making ingredients out by the same volume. Am I corrrect?

If I am correct then that same measured gallon of the new imporoved higher octane fuel with the additional TEL would have less power making content by volume that it did previous. Or if I am wrong please educate me. I need it. ;)

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Postby David Redszus » Mon Dec 03, 2007 12:47 am

Ed-vancedEngines wrote:

Most all of us who have been around the block a few times in years do understand that TEL does not produce or enhnace power. It is a known anti-knock agent only. Am I not correct?

To introduce additional content of TEL into a gallon of fuel mixture by volume, the volume displaced by the additional TEL would leave need to remove other power making ingredients out by the same volume. Am I corrrect?

Ed


You are correct, TEL is an anti-knock agent only. It serves no other purpose. When lead is added to fuel, it forms nasty deposits that fouls plugs, builds deposits on valves, and in general is a nasty character. To avoid the problems of lead deposits, a bromide scavenger is added as well.

The amount of TEL that is added is very, very small; typically about 4.0 to 6.0 grams per gallon (a gallon is about 3000 grams). While the addition of TEL (and the bromide scavengers) does take up a very small amount of space, this is almost unmeasurable.

A far more important issue that is lurking below these waters is that a fuel with a higher octane may have changes in many other components. Based on the feedstock available to the blender, he may be forced to use inferior components and then add additional lead in order to raise the octane value of the fuel. In other cases, the fuel octane from lead may be the same but the components used may be completely different.

Octane numbers do not represent the composition of the fuel. The composition of the fuel determines its performance, not the octane.

A somewhat larger problem is the use of unleaded fuels. In order to increase the octane value of unleaded fuels, it was necessary to add an oxygenate such as ether or alcohol. Since both ether and alcohol contain oxygen, the stoichiometric value of the fuel is lowered. Now it is necessary to run more fuel (run richer) in order to deliver the same fuel energy as before.

With the exception of nitro components, there are no trick fuels, no silver bullets that will enhance power. But each fuel will run slightly differently, even if from the same supplier. The real trick is to determine just what the engine wants and then feed it what it wants. When an engine really does not like the fuel you feed it, it will puke parts all over you.

Another very large problem is that engine builders hardly ever really know what air/fuel ratios they are running. When numbers are quoted, they are typically very inaccurate. Others have determined to tune an engine to a specific a/f number, which is a very big mistake.
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Re: Fuel Recommendations for 14:1 or more compression

Postby AdioSS » Mon Dec 03, 2007 2:17 am

ralph85 wrote:This is an alum headed, 6 inch rod, SBC 434 w/ a relatively small cam (260/270 @ .050). Assuming gas, what octane?


rskrause wrote:If there is any way to use methanol, use it and forget gasoline. Just about everything is better with some alcohol ;)


ralph85 wrote:This is an EFI engine, drag race application, 650-700 hp pushing a 3300lb car. We're currently just above 12.5:1, running 110 octane and thinking about increasing the CR.


Do you folks think E85 or straight denatured Ethanol could be used in an engine such as this? I know that it has lower octane than the other fuels mentioned, but it also has the cooling benefits of Alcohol. Not to mention, it can be found at some regular fueling stations.

I know that E85 can allow 12:1 CR with MUCH shorter duration camshaft.

BTW- I didn't see what kind of cylinder head was being used? What kind of pistons (flat top with a tiny chamber in the head, or big chambers and a dome)? Wouldn't the design of the combustion chamber make a difference to what octane is needed?
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Postby JCR » Mon Dec 03, 2007 8:39 am

David Redszus wrote:A somewhat larger problem is the use of unleaded fuels. In order to increase the octane value of unleaded fuels, it was necessary to add an oxygenate such as ether or alcohol. Since both ether and alcohol contain oxygen, the stoichiometric value of the fuel is lowered. Now it is necessary to run more fuel (run richer) in order to deliver the same fuel energy as before.


The oxygenates MTBE (now banned) and ethanol were added for EPA compliance. That they raised octane, was just a side benefit. In the refinery biz, octane=money. That's why at the pump you didn't see octane go up when they added those 10% ethanol stickers. They just used a cheaper lower octane base gasoline to blend and get the target number.
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Postby ralph85 » Mon Dec 03, 2007 9:29 am

Adio.......it's a 60cc chamber and we'll need a very small dome to get over 14:1 CR.

I was just trying to get a recommendation on an octane starting point, but this turned into a much more in depth analysis. Very interesting.
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Postby David Redszus » Mon Dec 03, 2007 2:48 pm

That they raised octane, was just a side benefit. That's why at the pump you didn't see octane go up when they added those 10% ethanol stickers. They just used a cheaper lower octane base gasoline to blend and get the target number.


Actually, oxygenates were added only to raise octane after lead was banned. It cost the fuel industry more money since the oxygenates are not found in crude oil but had to be obtained elsewhere.

The octane numbers did not go up because there was no improvement in octane values.

All pump gasolines are made of the cheapest possible components that will burn and meet EPA mandates. They are inconsistent and unpredictable. Quality fuel components are sold as specialty chemicals at a much higher price than common fuels.
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