85 Octane V.S. 90+ Octane?

[6219_Rules]

This question came up yet again this week when my son accused me of 'ruining' my engine in the 1956 Cadillac by putting in the basic Regular at the pump. My engine is the 365 CID bored out 331 modification originally put into the car, except with hardened valve seats. I thought that meant I could use basic unleaded fuel and was told that I could use regular in the car since that was basically what was used in the past. Jonathan begged to differ... he said that a high compression engine like that (8.25:1 compression) requires the higher octane fuel.

So the question is... who is right? Is the 365 CID a high compression engine?

Does it require the higher octane? And what about the higher amounts of Ethanol included in that higher octane fuel?

Thank you in advance for Your advice!

[Guest BJM]

You are bound to get a million different answers. As a general rule I prefer no less then 87 octane and find 89-90 to give the best overall performance in vehicles with 9.0 to 1 compression or less.

Octane rating is a related to delay in combustion, measured in milliseconds I suppose but the reason "high octane" is preferred is to allow maximum compression of the fuel/air mixture subject to expansion.

The lower octane lends to concerns of preignition. 85 is pretty low, it might even effect timing. Too high of an octane means "maybe" unburned gas, but that's in the 93 to 97 octane range so not much of a concern.

The reason hardened exhaust seats are needed is for additional heat not aided by lead "lubricant" in the gas. You can probably use 85 octane as long as the ambient temperatures don't place an added burden on your Cadillacs cooling system, and ability to wick heat away at stops.

Safer, in my opinion to give yourself the added security of 87 to 90 octane typically afforded by a 10 per cent ethanol blended gasoline.

[R W Burgess]

Randall, if it doesn't ping/labor knock, don't worry about it. Even if it does, you can retard the timing somewhat to allow your Caddy to run on most any kind of gas. I don't consider your Caddy a high compression engine.

I have a mildly modified 327 Chevy engine with high compression that requires high test (93). She'll run on 87 octane, just don't run her at "wide open throttle". With this particular engine I have more trouble with the new oxygenated fuel in the local big cities (Richmond) than I do any other kind. She'll ping, knock, and throw out black smoke like you ain't ever seen.

The only thing I'll say about the higher octane gas used today is that it may last a little longer in storage (maybe), but I'd still put a treatment in it if your car is setting all winter.

Happy driving! <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" />

Wayne

[6219_Rules]

Thanks, Jake and Wayne.

Our 'Regular' in Denver is 87. I could easily add a tank of 92 every third tank just to be sure. My son felt that the engine is high compression due to its 8.x:1 but I always heard that it was not, and would not be considered 'high' compression until 9:1 or better.

Good answers.

Now what is considered a 'high' compression engine over a 'low' compression engine, and why?

[ply33]

see:

http://www.faqs.org/faqs/autos/gasoline-faq/part3/section-1.html

[Owen_Dyneto]

I think your car will be borderline for regular 87 octane, so i'd proceed this way. Put in regular 87 octane, time the ignition to specification, and drive it, listening carefully for preigntion knock. Any more than almost imperceptible knock on hard acceleration would indicate you should step up the octane.

[Packard enthus.]

FOR PLY 33

Thank you for referring us to an excellent, concise, well-writen article! Randall - everything we laymen need to know about the "octane" issue is right there - I have printed out a copy for my reference library.

PLY33 - Can you help me - I am not good with computers - would like to know WHO this excellent reference source is (sure looks like an ASTM or SAE tech. paper, but would like to know for sure !).

Folks, those of you who know computers and how to get at LEGIT tech. reference sources like what PLY 33 obtained for us, really should contribute more. Of course the "back-yard" mechanics with their "my buddy says" or "we did it that way" mean well. But there are too many examples where this can leave a hobbyist astray, cause him financial loss or even put him or her in physical danger, and often result in them losing interest in the old car hobby.

Thanks again PLY33

[Guest BJM]

6686L,

Technical letters and scientific papers are nice, and add to the discussion from that angle, however, members on this forum who experience real world evidence of the gasolines in question should not be debunked.

In fact, we used pure Octane as a solvent when I was employed as a BioChemist with a research lab in 1995-97. Knowing the technical aspects of octane as a component of modern gasolines though, won't help a particular reader understand real world applications of octane rated gasolines.

I still maintain don't go below 87, but perhaps someone else, "shadetree" or otherwise, has knowledge to share not from a technical paper.

[Loyd Smith]

Randall

My '55 Packard's (352 engine) runs well on mid-grade (in this area, Florida) fuel which is rated at 89 octane but detonates (pings) badly under hard acceleration using the 87 octane, "regular," with all timing/ignition factors set to factory specifications. This engine's compression ratio is 8.5 to 1. According to the owner's manual, premium fuel was recommended by the manufacturer and this would compute as, according to my research, average premium fuel available in 1955 was rated at 88 research octane. During the period from 1955 through 1960 I drove a '54 Pontiac Chieftan which would run on anything, a '57 Pontiac Starchief which would run on nothing but high test, a '55 Ford which would run on regular if you didn't get on it too hard and a '59 Chrysler Imperial which, like the Pontiac, would, "ping," badly on anything but high-test fuel. I have come to the conclusion that, all things taken into consideration (hardened valves/seats, etc to make up for lack of tetra-ethyl lead anti-knock compound and timing/ignition set to original factory specs) one should use as close to the manufacturer's recommended fuel grade as possible. There are service stations (not really but, I remember them), in this area that only carry, "regular," (87 octane) and, "premium," (93 Octane). When I run across one of these and have to fuel, I use the, "premium," grade. Unburned fuel is less harmful than detonation.

[ted sweet]

WHEN i cant get mid grade 89 i use 50/50blend of 87 and 92

[Guest bkazmer]

As noted,octane requirement is related to pre-ignition and pinging will tell you if your gas is too low in octane. It's relatively easy to boost octane (or retard timing).

Tetra-ethyl lead also protected valve seats. This dual purpose seems to lead to the "higher octane gas protects valves" comment - it isn't necessarily so. It's a separate concern.

[Packard enthus.]

you say you "used pure octane as a solvent"...eh. Could I trouble you to "scan" the label it came in...? Did this "octane" you used, come in jars, bottles...? tubes...? Pills....? I would REALLY like to know....

( Posts like 3Jakes suggest to me why more contributions in the form of legit technical info, would be helpful to us auto hobbyists.....!)

[Matt Harwood]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">WHEN i cant get mid grade 89 i use 50/50blend of 87 and 92 </div></div>

That's exactly what mid-grade is. There are only two storage tanks under your gas station's pumps: regular and high octane. Most mid-grades are mixed at the pump using a combination of the two.

As far as what octane to use, it can vary from car to car, even if they're identical in every other way. The 360 in my Dodge Ram 2500 is designed to use 87 octane, and a vast majority of them run fine on 87. But mine pings lightly so I use 89. No big deal--even if I fill the tank from empty, it's only an extra $3.50 compared to 87.

Randall, the advice here is sound--literally. If it's knocking, go up a step and see if it stops. If it isn't knocking on the low octane stuff, you don't need more. There may be knocking that you can't hear, but it's unlikely.

As an enthusiast, your ear is more finely tuned to your car's operation than most. If something is amiss, you'll probably hear it. At cruising speeds, step into the throttle aggressively, but not so aggressively that the transmission downshifts, and listen for knocking. It is exactly at loads such as this that detonation is most likely, and you'll definitely hear it.

Running a tank of high octane every few fill-ups won't give you any benefit during those tanks when you're not running high octane. High octane doesn't "clean the gunk out" better than regular gas, and it doesn't make a hotter explosion or more horsepower. If your engine isn't knocking, you do not need it, because that's all high octane gas is designed to do: prevent pre-detonation knocking.

Hope this helps.

[Packard8]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Thanks, Jake and Wayne.

Our 'Regular' in Denver is 87. I could easily add a tank of 92 every third tank just to be sure. My son felt that the engine is high compression due to its 8.x:1 but I always heard that it was not, and would not be considered 'high' compression until 9:1 or better.

Good answers.

Now what is considered a 'high' compression engine over a 'low' compression engine, and why? </div></div>

The GM LT1 V8's (1994-1996 Camaro, Firebird, Impala SS) are 10.5 CR and run perfectly well on 87 octane as recommended in the owner's manual. Many other factors can contribute to pre-ignition ping, such as carbon deposits, combustion chamber design, cooling system efficiency etc. If your Caddy doesn't complain with 87 octane and you have hardened valve seats you are most likely okay running it.

[bhclark]

I find it interesting that there has been no mention of immitation lead addtives in this discussion. My engine was recently rebuilt without hardened valve seats and my mechanic specified that I should use premium and a lead additive.

I have been doing so, typically using a bottle of Gunk Lead Additive anytime I give here a complete fill up and skipping it if I'm just topping off.

I have been tempted to use the cheaper CD2 lead additive(listed on the bottle I believe "not for automotive use") I figured that was for emissions purposes moreso that the health of the vehicle though.

I plan on trying a lower grade fuel shortly, but am I correct that I should still use the lead additive to "protect" the valve seats?

Also, aren't there concerns about the ethanol damaging older rubber seals in engines that haven't been updated?

Does anyone know of a chain that still uses 100% gasoline or have the corn farmers lobbied to get that eliminated? (I used to have a Texaco station close by that advertised 100%, but they have closed in favor of a BP superstation)

[Guest imported_DaveZZZ]

I sense a little confusion here, so if you don't mind I will try to calrify:

Gasoline is made of a mixture of hydrocarbons, ranging in length (# of carbon atoms) from 7 (heptane) to 12 (I forget-ane). Strictly speaking, octane is an 8 carbon chain. This chain has a certain characteristic with regards to its ability to resist knocking, which is essentially related to its auto-ignite under pressure and temperature. Octane is arbitrarily assigned a value of 100, so a tank of pure octane (iso-octane, to be specific) would in fact be expected to have an octane of 100. However, this does not mean that octane is necessarily the most knock resistant species in gasoline. They just decided one day long ago that they would use octane as the basis. I think n-heptane, which is present in all gasoline, has an octane of zero. This is why octanes of 110 are possible.

SO, while it is possible to buy pure octane (look at websites for places like Sigma-Aldrich, Fisher Scientific, VWR, JT Baker, some of the major chemical supply houses), it will be astonishingly expensive, and not really what you want anyway. The process of taking gasoline from the refinery and separating out individual species is going to be very expensive. Besides, even if you had pure isooctane available, and you combined it with regular (87) at 1:1, you'd only get 93.5- might as well just go to Shell and buy the 94.

There are additives available which interact with the gasoline to produce much higher octanes than one would achieve by adding straight octane. Tetraethyl Lead is one. And it can be purchased under the brand "lead supreme." This is much more effective because just a few ounces will bring your tank up several points, and it is the actual additive that your cars were built to run on. It isn't exactly cheap, and one must handle with care, as lead really is that bad for you.

Since we are on the subject, I would also like to point out that to convert the gas at the pump TODAY to what the numbers were BACK THEN, add 3. So today's 87 is 1968's 90. 93 = 96, etc. The reason is that they used to use one method of measuring the octane in gas, but there was debate over whether a different method was superior, so now they average the two methods. This is why you always see Methd: RON + MON / 2 on the pumps. This means they took the Research Octane Method (RON) and the Motor Octane Method (MON) and averaged them. So while the old day's 103 octane is gone, the best of the new stuff is equivelant to 97 (not 94).

Personally, my biggest concern is that the refineries use MTBE and ethanol as octane boosters today, and I don't know how safe these are for the old fuel lines, seals, etc. I am pretty sure they won't hurt the engines.

I hope that wasn't dry, and I hope it wasn't too lng winded, and I know nobody really asked, but I also know there is a lot of confusion about this stuff, so I hope it was helpful.

[6219_Rules]

DaveZZZ, that was an excellent and concise discussion of modern fuel. Thank you. I know for my own understanding, it was of great help.

I would like to add, if you use the Lead Supreme Tetraethal additive, you most likely will not pass emission tests in those States that still require it. However, I have found the Non-lead additives to work well. But the point has been made, and correctly so, that to buy Regular, then adding an additive, you might as well buy Supreme, and may still have to add a Non-lead additive.

My understanding of the Hardened Valve Seats in older engine rebuilds was to prevent damage from pre-ignition knock. Tetraethal was added to fuel in the 1940s to prevent that annoying 'ping' and it worked. It was also cheap so the oil companies like Standard Oil had no problem adding it following the commercial success of the Ethal Oil. Most fuels were non-lead fuels at base until the companies began experimenting with fuel additives to boost performance, and silent running of engines. According to older family members, it was normal for engines to 'ping', stall, and overheat causing vapor lock. Summer and vacations to the mountains were often exciting adventures as the old Merc or DeSoto would chug and stall its way across the Badlands to the Rocky Mountains.

So I guess the answer is to just bite the bullet, and buy the top grade. Buying the middle grade is pointless, and Regular may cause damage or at least poor performance. Coupled with modern oils that have reduced ZDDP, and you are looking at potential problems with lifters and valves, if they are not of the Roller Type. And, Dave's point about old rubber seals is a good point. They should be replaced with modern rubber or its equivalent that is made for the modern fuels.

Good thread. Thanks, Guys.

[Guest bkazmer]

The C7-C12(it's dodecane) mix also has non-paraffins. The confusion is between "octane" the chemical, and "octane number" related to the volatility of the gasoline, expressed as explained by comparison to octane. I seem to recall xylenes and toluene have octane numbers in the 120-130 range. Again, "high octane" is easy to get and your car tells you if it wants more. It's the other things additives do that are harder to keep in control.

[ted sweet]

if it isnt ping using super unleaded is pointless, it also as no more additives to help with valve recesiion

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">DaveZZZ, that was an excellent and concise discussion of modern fuel. Thank you. I know for my own understanding, it was of great help.

I would like to add, if you use the Lead Supreme Tetraethal additive, you most likely will not pass emission tests in those States that still require it. However, I have found the Non-lead additives to work well. But the point has been made, and correctly so, that to buy Regular, then adding an additive, you might as well buy Supreme, and may still have to add a Non-lead additive.

My understanding of the Hardened Valve Seats in older engine rebuilds was to prevent damage from pre-ignition knock. Tetraethal was added to fuel in the 1940s to prevent that annoying 'ping' and it worked. It was also cheap so the oil companies like Standard Oil had no problem adding it following the commercial success of the Ethal Oil. Most fuels were non-lead fuels at base until the companies began experimenting with fuel additives to boost performance, and silent running of engines. According to older family members, it was normal for engines to 'ping', stall, and overheat causing vapor lock. Summer and vacations to the mountains were often exciting adventures as the old Merc or DeSoto would chug and stall its way across the Badlands to the Rocky Mountains.

So I guess the answer is to just bite the bullet, and buy the top grade. Buying the middle grade is pointless, and Regular may cause damage or at least poor performance. Coupled with modern oils that have reduced ZDDP, and you are looking at potential problems with lifters and valves, if they are not of the Roller Type. And, Dave's point about old rubber seals is a good point. They should be replaced with modern rubber or its equivalent that is made for the modern fuels.

Good thread. Thanks, Guys. </div></div>

[6219_Rules]

I am still unclear as to why, but it does seem as though the higher octane fuel available does make a difference in performance. The Mustang's 289 V-8 (rebuilt by Jaspers) makes all sorts of sounds if you use 87, but not with 93. I do not understand the difference. Some say there are more additives, others say less; some say there is a higher amount of Ethanol in the 93, while others say not. So I would hazzard a guess that if a tank of 93 makes a noticable difference, in comparison to 87, then it seems reasonable to use 93 most of the time. Ipso facto.

[Pontiac59]

Engines are unpredictable anyways. My '60 Pontiac always ran fine on 87, even though the stock compression rating was 10.0:1 (and it tested good when I did a compression test on it, too). In the '70s they lowered compression to 8.0:1 and then 7.6:1 in these engines to help them run on the cheaper fuel. The '70 Pontiac I drove years ago would give a lot of knock if you ran it full bore up a hill, on cheap gas. That was another 10.0:1 motor. And we're talking all basically the same engine, a 400 is just +.055 over on a 389. The 455's I ran, 8.0:1 motors, the 2bbl was fine on 87, the 4bbl liked it better if I ran some mid-grade in it.

If it runs well on the gas I wouldn't worry either. Even if it's not rebuilt, if an engine was run on leaded gas for years, you can get away with unleaded for quite some time in it with no lead additive - the lead builds up in there, it doesn't just go through with the gas and get burned. If you have a valve job done, though, then you need the hardened seats.

[Guest BJM]

Higher rated octane gasoline slows the burn, allowing for maximum compression of the gasoline before the hot gasses force the piston down. Lower octane gasoline, in reverse - can casue pre-ignition or uncontrolled flame fronts, reducing power and playing havoc with the smooth operation of the engine, so whether it's the negative aspects of pre-ignition or the (slight) delaying of using high octane fuel, that is probably 'why' you are experiencing increased power with higher octane gas.

[Guest bkazmer]

Right! and if your engine does not have high enough compression to be sensitive to this difference, higher octane gasoline will make no difference. This is also why you can alter the timing to run lower octane gas in a higher compression engine, but then you lose power by firing at a suboptimal point.

[Packard enthus.]

STRONGLY recommend that you do NOT use "ipso facto" in your car. Suggest you use gasoline. What kind of gasoline ? ALL GASOLINE PROVIDES THE SAME ENERGY. THERE IS NO DIFFERENCE IN THE AMOUNT OF ENERGY. The only difference is "octane rating" which simply means "flame speed". The higher the octane, the slower the gas burns. It is that simple.

If you are hearing "pinging", you need a slower-burning gasoline, meaning, again, higher octane.

It is that simple. As for "additive claims" - ALL gasolines sold for consumer use in the United States MUST be compatible. They all do pretty much the same thing, which is to prevent "gumming". It dosnt make any difference WHAT the "octane rating" (again, "flame speed") of the gasolin is, as far as additives go. Again, by law they MUST be compatible with any other gasoline lawfully sold in the United States.

Because of a change in the way "octane rating" is calculated (see a comment about this in another contributor's "post" elsewhere) todays 91 + octane has a flame speed (octane rating) roughly equiv. to what 96 octane of years past had.

TUNE YOUR CAR RIGHT - set it to the timing specs. of the mfg. With the possible exception of one or two REALLY high compression ( meaning over 10.5 to one ratio) "factory hot rods" of the late 60's,. today's "premium" fuel will be fine for any car. Today's "regular" will be fine for ANY car...ANY car...unless, of course, WHEN SET TO FACTORY SPECS...it "pings".

This is not rocket science, folks.

[Matt Harwood]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">Higher rated octane gasoline slows the burn, allowing for maximum compression of the gasoline before the hot gasses force the piston down. Lower octane gasoline, in reverse - can casue pre-ignition or uncontrolled flame fronts, reducing power and playing havoc with the smooth operation of the engine, so whether it's the negative aspects of pre-ignition or the (slight) delaying of using high octane fuel, that is probably 'why' you are experiencing increased power with higher octane gas. </div></div>

This is all correct. However, I would like to point out that high octane gas doesn't make extra power--it only allows the engine to run optimally and produce as much power as it was designed to make. If your engine was designed for low octane and is rated at 200 horsepower, and it does not ping on regular, using high octane will still only make 200 horsepower.

In some applications, higher octane may actually hurt performance for the same reasons above. While I don't know why, my 1993 Mustang gets about 50 more miles per tank when I use mid-grade instead of premium. I can't discern any difference in performance. The engine is a 342 cubic inch stroker with 9.7:1 compression that I built about 13 years ago. I thought it would need premium when I built it, but it seems perfectly happy on the 89 octane stuff and pulls like a freight train without knocking.

Hope this helps to further not confuse the issue. <img src="http://forums.aaca.org/images/graemlins/grin.gif" alt="" />

[Guest imported_DaveZZZ]

What you are saying has the gist correct, but to nitpick just a bit:

The increased octane does not slow the burn. This is an oversimplification. Any flammable vapor has, assuming a correct air:fuel ratio, an autoignition point. This is the temperature at which the mixture will burn without any spark or other ignition point (i.e., a match). I believe that for gasoline this is around 475F. However, for ANY combustible vapor, as you increase the pressure, the autoignition point drops. A higher octane results in a higher autoignition point. A higher compression engine results in a lower autoignition point. So, if you run low octane fuel in a high compression engine, you can end up in a situation where the pressure on the compression stroke brings the autoignition point of the air/fuel mix down to the "ambient" temperature inside the cylinder before it is time to fire the plug. This results in the fuel going off too son and knocking/pinging. Use a higher octane fuel, raise the autoignition point of the fuel, and you can run the higher compression ratio.

-Dave