What is the maximum safe octane rating to use in pre-war cars?

[37PackardMan]

When considering the fact that the octane rating was much lower and the metallurgy  was much different, is there a concern  for damage to an engine if a high octane rating fuel is used in the old cars?

I see on the internet that the average octane rating in 1937 was 71.

Curious to hear from the experts....

 

Bob

[ply33]

The octane rating is just a measure of how resistant the fuel is to pre-ignition (pinging). Putting fuel with too low an octane rating into an engine can damage it.

 

Putting fuel with a higher octane rating than needed simply removes some extra money from your wallet. That said, the flame propagation speed may be slightly different and you might want to adjust your spark timing a little to get a bit more power if your fuel octane is higher than available when the car was new.

[Bloo]

37 minutes ago, 37PackardMan said:

is there a concern  for damage to an engine if a high octane rating fuel is used in the old cars?

 

Some people seem to think so. I'm not ready to jump on the bandwagon. I think you'll do more damage with octane too low. High octane fuel isn't more energy-packed or hotter or anything like that, it just burns slower. It's kind of like having the timing a little retarded. The exhaust valves and exhaust may run hotter than they need to.

 

To that end, it is probably advantageous to advance your timing a little. Just don't advance it so much that you get preignition or detonation or an apparent random misfire that wasn't there before you advanced the timing. Also not so much that it chugs or tries to kick back while cranking. If any of these things happen, back it off a little.

 

Your mileage may vary.

 

Edited February 2, 2022 by Bloo (see edit history)

[Str8-8-Dave]

11 hours ago, Bloo said:

it just burns slower

And, in very severe cases, does pose some risk. First of all, everything else being equal, the correct fuel for any engine is the cheapest, lowest octane fuel that you can buy at the pump that will not cause pre-ignition detonation in the cylinder at the worst load conditions. With today's cars, spark knock sensors and the engine control module take care of that.  I pulled out Bloo's phrase because in severe cases the slow burn rate of too-high octane fuel put's exhaust valves at risk.  If the combustion cycle completes before the exhaust valve opens, we're good.  If, because the octane is way too high, the combustion cycle continues after the exhaust valves open, the exhaust valves are going to run very hot, hot enough to damage them over time.  When the exhaust valve is closed and sitting on its seat it can transfer the high temperatures of combustion to the cylinder head by conduction.  Once it lifts off the seat the valve has to absorb all the heat which can cause them to pit or warp.  

 

Once upon a time I owned a 63 Dodge with a 426 max wedge engine that was not designed to run on unleaded fuels.  I used to keep the tank full of a mixture of premium unleaded and some Torco 110 octane race fuel.  At least once I had too much Torco fuel mixed in.  In the correct mixed proportions the car would accelerate hard and rev smoothly to 6,500 rpm.  When it had too much Torco it was louder out the exhaust pipes and very lazy on acceleration and would not rev to redline.  That was a sure sign to back off the Torco.   Fortunately that only happened once to the car.  A steady diet of that would have had me pulling heads...

 

 

[37PackardMan]

17 minutes ago, Str8-8-Dave said:

And, in very severe cases, does pose some risk. First of all, everything else being equal, the correct fuel for any engine is the cheapest, lowest octane fuel that you can buy at the pump that will not cause pre-ignition detonation in the cylinder at the worst load conditions. With today's cars, spark knock sensors and the engine control module take care of that.  I pulled out Bloo's phrase because in severe cases the slow burn rate of too-high octane fuel put's exhaust valves at risk.  If the combustion cycle completes before the exhaust valve opens, we're good.  If, because the octane is way too high, the combustion cycle continues after the exhaust valves open, the exhaust valves are going to run very hot, hot enough to damage them over time.  When the exhaust valve is closed and sitting on its seat it can transfer the high temperatures of combustion to the cylinder head by conduction.  Once it lifts off the seat the valve has to absorb all the heat which can cause them to pit or warp.  

 

Once upon a time I owned a 63 Dodge with a 426 max wedge engine that was not designed to run on unleaded fuels.  I used to keep the tank full of a mixture of premium unleaded and some Torco 110 octane race fuel.  At least once I had too much Torco fuel mixed in.  In the correct mixed proportions the car would accelerate hard and rev smoothly to 6,500 rpm.  When it had too much Torco it was louder out the exhaust pipes and very lazy on acceleration and would not rev to redline.  That was a sure sign to back off the Torco.   Fortunately that only happened once to the car.  A steady diet of that would have had me pulling heads...

 

 

 

[37PackardMan]

 "way too high" and "over time" are disturbing variables.

My initial query was 'What is the max safe octane...".

I was hoping for some statements like...

"Never use 93 octane if you are going to drive 50mph for an hour."

 

A related question is....if you mix (equal parts) 80 octane and 100 octane do you get 90 octane?

 

 

[alsancle]

I can tell you that we’ve run high octane gas (over 93) in our pre-war cars for 30 years or more and never had an issue. Now the car is are driven each hundreds of miles a year not thousands, but they’re driven. 

[Matt Harwood]

I don't think you'll do any damage using high octane gas, beyond the damage to your bank account. However, I've had cars that didn't run as well on high octane gas simply because their ancient ignition systems and combustion chambers aren't really designed for it. Skipping the ethanol discussion, it's worth noting that there's nothing in high octane fuel that isn't in the lower grades, so you're not cleaning any extra gunk out or treating the car to "better" gas. High octane fuel is just more resistant to pre-ignition, that's all it does. Run the cheapest, lowest octane fuel you can find and your pre-war car will never complain. Again, this is ignoring the ethanol factor, which is a separate issue.

 

20 minutes ago, 37PackardMan said:

A related question is....if you mix (equal parts) 80 octane and 100 octane do you get 90 octane?

 

Yes, more or less. That's how mid-range fuel grades are made--it's mixed at the pump.

[Bloo]

28 minutes ago, 37PackardMan said:

My initial query was 'What is the max safe octane...".

I was hoping for some statements like...

"Never use 93 octane if you are going to drive 50mph for an hour."

 

I don't have a good answer to your question. If you happen to live in the US, 87 octane is the lowest octane you can buy nationally, with 85.5 available in a very few areas. The calculation is done differently than it was in the 30s, so the numbers aren't directly comparable.

 

Some guys mix diesel in. My gut feeling is it just makes crappy fuel, never mind the octane. It might not be too far off from 1920's fuel when the refineries couldn't keep up, gas was full of "heavier portions" (kerosene), and quite a few new cars in the 20s had extreme amounts of carburetor heat just to get that fuel to sort of work right. By the late 30s that was all ancient history. You could even get leaded gas if you could afford to pay more.

 

While @Str8-8-Dave is not wrong, I don't think you will run into the issue he did on any flathead or other normal passenger car engine of the 1930s. Hey @edinmass are you reading this? Flatheads in particular have a lot of turbulence in the cylinder and tend to have a shorter burn time.

 

Sorry for the non-answer. I don't know where to get anything less than 87 octane. I use 87 octane 10% ethanol. I has not yet caused any burned valves in my 36 Pontiac, even when I crested the continental divide recently unaware that the timing had become retarded.

 

 

Edited February 2, 2022 by Bloo (see edit history)

[edinmass]

 

 

OK.....NOT A EASY ANSWER. In the collection we have our own fueling station, and I run VP Fuel C-9.......here it is.

 

We formulate VP C9 to complement our popular C10 fuel. It's an economical alternative that offers more resistance to detonation. Additionally, it provides substantial performance improvements over premium pump gas. In fact, crate engine tests showed gains of 3-4 HP and more torque than a competitor’s standard 110 leaded race gas! VP C9 burns very clean and won’t harm catalytic converters or oxygen sensors. C9 fuel was chosen as spec fuel for many racing series in North America. C9 is ethanol-free. As a result, it is an excellent long-term storage fuel.

When results matter, try VP racing motor oil and engine lubricants. Our extensive line affords exceptional durability and delivers unmatched wear protection under extreme conditions for vital components.

 

It's 98 octane.

 

So....this fuel is great, but EXPENSIVE. It has a six year shelf life and no additives are need for long term storage. We run and drive all our cars extensively..........so fuel seldom is in a tank longer than 9 months. All of our collection cars are individually tuned to run on C9. I actually have other carburetors for many of our cars set up for E10 when we are on tour. Yes, I have spare carburetors for all our cars.......if you want your car right, it's not easy.  95 Percent of the time I carry enough fuel in the car and car hauler that for most events under 300 miles we run only VP C9. On the long tours........and we do lots of them....... I swap out the carbs and they are set up for E10. When we return home, I drain the systems and flush. I also adjust timing when we go to E10 on most of the cars........

 

 

On my own cars......they are all set up for E10 across the boards. 

 

 

Below are two Strombergs that sit on display in our shop.......They are set up for E10. On the left is a DV-32 Stutz unit, on the right is a Duesenberg JN unit. 

 

Edited February 2, 2022 by edinmass (see edit history)

[carbking]

I do not know the answer to your questions, but will offer the following:

 

Current octane numbers at the pump are AKI (anti-knock index).

 

For years, the USA used RON (research octane number) as the standard, whereas much of the rest of the world used MON (motor octane method).

 

Some years ago, I don't know the exact year, and too lazy to try to find it, but the octane requirements in the operator's manual for my 1964 were RON; the USA, probably to stifle complaints from both sides, changed from RON to AKI. While both RON and MON are done by testing using specific methods, AKI is the simple average of the two for a given product. So AKI = (RON + MON) / 2.

 

According to a number of charts found by Google; for a certain product RON was 99, MON was 91, thus the AKI was 95.

 

I consistently ran 93 AKI in the 1964 GTO (owners manual asking for at least 100) with no issues.

 

Would GUESS the 71 mentioned as being average in 1937 would be 67~68 by today's rating system.

 

Jon

[edinmass]

 

I agree with Jon above.......

[hidden_hunter]

3 hours ago, carbking said:

For years, the USA used RON (research octane number) as the standard, whereas much of the rest of the world used MON (motor octane method).

I think the US/Canada are the only ones using AKI, everywhere else in my travels uses RON at the pump

[jdome]

Speaking from my pre-war car experience, I think it depends on if we are talking about combustion or carburation issues. I may have the octane numbers bass-ackwards by todays standards but as I understood it decades ago when ethanol & vapor lock (lose term) became a problem for me. The higher the octane # the higher the boiling point of fuel, or if you prefer, less volatile. However, the addition of ethanol lowers the boiling point. So as I understand it, hi octane # of pure gas would be less likely to have "vapor lock issues" than lower # E10. Having said all that, E10 is not necessarily a problem and use of high octane is not necessarily the best solution. From my experience, a brass era in-line engine with gravity feed tank, updraft carb hanging on the side below the hot exhaust manifold and next to open side vents on the hood; can handle just about any octane fuel, with/without Ethanol. OTOH, all of the pre-49 Cadillac V8's are probably the most vulnerable to fuel "vapor lock." I use that term loosely as a catch all because there are more heat related fuel issues than the technical term.  Gas will boil or "percolate" in the carb bowl if it gets hot enough and actually boil out.  The Cad V8's have up or down draft carbs in the middle of the V with exhausts that exit inside the V with a cross over pipe that surrounds the carb, just a couple inches away on 3 sides. if that is not bad enough, the fuel line has to run parallel to & across the manifold from pump to carb. The hood acts as a dome to retain all that heat over the carb. But gas doesn't have to actually boil to create a problem. As gas in the bowl heats up it expands and becomes less dense which at some hot temp. can cause buoyancy problems with the float as well as a lean, low density fuel to air mixture.  I've played with most year V8's but 39-42 seem to tolerate E10 the least but even these V8's can be made to run fine on low octane E10.  Apparently Cadillac recognized the vapor lock issues with these pre war cars and upgraded the post-war engine with "Anti-percolate" carbs.  

 

I found that it isn't one simple fix, such as eliminate E or change octane to make prewar cars run on modern fuels, if it is a problem. My experience has been with problems before the fuel gets to the combustion chamber and switching octanes will not likely solve the problem. Wrapping insulation around the pump to carb line certainly prevents fuel from pre-heating before it gets to the carb. I suppose if you put enough cloths pins on the supply line it may provide some insulation. I also use one of those insulating fiber manifold to carb gaskets to prevent heat conduction to the carb. Some owners have put a heat shield around the carb - I have not found that necessary.  I also find that many Cadillac V8s are driven with incomplete fan shrouds in particular the 37-48s.  The inner fenders and upper radiator shroud and/or hood form 3/4th of the shroud but I often see the flimsy lower panel that attaches to the underside of the chassis and fills the void below the fan, between engine and bottom radiator. If this is missing it draws air from under the car instead of through the radiator. Also Cadillac wrapped the exhaust down pip with asbestos insolation (as many other manufacturers did) to lower under hood heat. That needs to be replaced with a good substitute material.  OK - I do cheat some. I replace the standard 4-blade fan on my small series Cadillacs with 5-blade fans from the series 72 & 75 to move a larger volume of air thru the radiator and blow hot air out of the car.  I can drive & start my Cadillac V8s with 6-volt systems, mechanical fuel pumps and E10 all day in 90+ degrees without problems. 

[hidden_hunter]

My 1922 Cadillac I blanked off the preheat to the carb and haven’t had any issues, I’ve also wrapped my exhaust in a wrap that’s supposedly as good as the asbestos (it was awful to install) but there is a tremendous amount of heat that comes off the motor.

[jdome]

Pre-war cars all seem to have some way to heat the fuel/air mixture between the carb and the combustion chamber. Some use exhaust manifold heat and some plumb (hot) coolant around the intake manifold or the throat of the carb itself. Some engines you can eliminate carburation heat and some you can't. My experience in dealing with modern fuels has been to minimize excessive heat under the hood and fuel supply system.  

[hidden_hunter]

4 hours ago, jdome said:

Pre-war cars all seem to have some way to heat the fuel/air mixture between the carb and the combustion chamber. Some use exhaust manifold heat and some plumb (hot) coolant around the intake manifold or the throat of the carb itself. Some engines you can eliminate carburation heat and some you can't. My experience in dealing with modern fuels has been to minimize excessive heat under the hood and fuel supply system.  

The first cadillac v8's used water from the radiator so it could easily be plumbed out. I just measured the thickness of the gasket and made a blanking plate out of copper which works well enough

[Str8-8-Dave]

21 hours ago, Bloo said:

If you happen to live in the US, 87 octane is the lowest octane you can buy nationally, with 85.5 available in a very few areas.

Another problem with the low octane stuff in the US is it all contains ethanol.  You have to buy 93-94 octane if you want ethanol free.  

[37PackardMan]

I don't know about MI, but we are 26 west of Houston. Our community has it's own pumps and we get our fuel from Columbus, TX ....50 miles west of us.... and it does not have all the junk that the cities have.

Edited February 3, 2022 by 37PackardMan (see edit history)

[Fox39olds]

I buy the higher octane because it has a lower amount of ethanol in.  

[Ben Bruce aka First Born]

41 minutes ago, Fox39olds said:

I buy the higher octane because it has a lower amount of ethanol in.  

 I thought ethanol was added as an octane booster.

 

  Ben

[Matt Harwood]

Ethanol does offer higher octane but its purpose isn't to raise the octane of today's fuels. It's purely to give the corn farmers something to grow. They also thought it would help cut down on our demand for oil by reducing our overall consumption by 10% but that didn't really change anything.

 

Higher octane fuels do not necessarily have less ethanol unless specifically stated on the pump. Here in our area, it's almost impossible to find non-ethanol fuel of any grade unless you go with Turbo Blue or something like that @ $15/gallon.

[Bloo]

It's sole purpose might not be to raise octane, but it does raise octane. Circa 1980s, it was one of three things commonly used for that purpose.

 

I can buy 3 different octane grades of gasoline (87. 89, 92) here in central Washington State. I can buy all three with or without 10% ethanol, depending on the vendor, making a total of 6 different types. Non-ethanol is harder to find, but it is here.

 

The fact that ethanol raises octane, yet the octane number on the pump can be the same speaks loudly about how crappy the petroleum components must be that they use in the ethanol mixes.

 

Edited February 3, 2022 by Bloo (see edit history)

[37PackardMan]

In 1920 Packard  brought out the 'Fuelizer' on the Twin-Six.  We have it on our Third Series SIX (1925). It is a seventh spark plug that is mounted on a chimney on top of the carburetor to pre-heat the fuel before it got to the intake manifold.  Rational was that warm dry vapors combust better than cold wet vapors.  My understanding is that when fuel octane increased, this technology was no longer necessary. The seventh spark  (not the same kind as the other six) ran like a glow plug off the back end of the coil.  On top of the chimney was an isinglass window so you could see if the plug was functioning properly.

[37PackardMan]

The illustration is from the 1920 brochure for the Twin-Six.

[Fox39olds]

Depending on the brand here there is 0-5% ethanol in premium, regular and mid grade is 10-15%. I have an 70’s outboard that hates ethanol too. So I’m not sure if it matters with my 39 but I try to run as little ethanol through it as possible.

[Stude Light]

I have used 100 octane aviation fuel in many of my engines for years with no problems. It still contains tetraethyl lead. I'm not advocating using it in your daily driver as it is not taxed for road use so would be illegal to use. It works great for engines that sit for extended periods as it has at least a 7 year shelf life and will never varnish out. Works great for generators and museum cars. I know of a 1923 and 1939 car that run fine using it with a little advance on the ignition 🙂.

 

So, to answer your question, no concern for damage from high octane, just more expensive.

 

 

[Crazyred]

On 2/3/2022 at 8:43 AM, 37PackardMan said:

I don't know about MI, but we are 26 west of Houston. Our community has it's own pumps and we get our fuel from Columbus, TX ....50 miles west of us.... and it does not have all the junk that the cities have.

@37PackardMan i live in NW Houston, where is this your getting fuel from?

[37PackardMan]

Our source is:  Tricountry Petroleum  (979) 732-8333.

It is my understanding that the Buc-ees on I-10 in Katy, TX has ethanol=free gas, but I have not verified that.

This gas station is reported to be the largest in Texas...maybe the world. It has 100 pumps...yes one hundred.

 

 

[Crazyred]

Thanks for the info, I'll have to check them out. 

[cgerman1]

Has anyone used this type of high octane gas in their nail head?

[Rusty_OToole]

To answer the original question. Running high octane fuel in a motor that does not need it, will not hurt anything. But, an engine will run best, develop most power on the fuel it was designed for.

Today's fuel is made for modern fuel injection engines. Vapor lock is not an issue as the fuel is under pressure at all times. This can cause problems on some older, carbureted engines especially in hot weather.

Also, today's fuel is dry. If you get some on your hands it leaves them white and dry when it evaporates. Old gas from the fifties and sixties was more yellow and oily. This is connected to the dryness mentioned above.

Therefore, some think it is a good idea to add a little upper cylinder lubricant to the gas. Marvel Mystery Oil, Redex, Seafoam, or 2 stroke oil (ashless or clean burning). This will lubricate your rings and valves, prevent them sticking and extend their life. On the other hand most people don't bother and nothing very bad happens.

 

Now how do you know what octane is correct for your car? As a rule of thumb the octane should look like the compression ratio. For example a prewar car with 6.5:1 compression should run well on 65 octane. A typical fifties car with 8.5:1 can use 85 octane. A high performance car from the fifties or sixties with 10:1 compression needs 100 octane. And a twenties car with 5:1 or less will run on 50 octane, next thing to lamp oil.

This is not a hard and fast rule, just a general guide. Some motors require higher octane, some not so much depending on the efficiency of the combustion chamber design, and other factors such as altitude, and how the motor is being used.

Oil companies sell a special low octane "tractor gas" to farmers for low compression tractors, trucks, and industrial engines. This may be the thing for prewar, low compression motors.

I know this question is a year old but thought the info might be of interest to others.

[Str8-8-Dave]

On 2/2/2022 at 12:20 PM, Bloo said:

While @Str8-8-Dave is not wrong, I don't think you will run into the issue he did on any flathead or other normal passenger car engine of the 1930s. Hey @edinmass are you reading this? Flatheads in particular have a lot of turbulence in the cylinder and tend to have a shorter burn time.

Just to clarify, with the 426 Dodge I used in my example, you had to mix a little 110 Torco leaded race fuel with the then available 93octane unleaded to keep it from pinging.  I accumulated too much Torco 110 race fuel in the mix one time and the engine lost it's ability to rev and you could hear it popping out the tail pipes.  I backed off the race fuel and things returned to normal, pulling like gangbusters to the 6500 rpm shift point and no funny exhaust noise.  With too much Torco on board it was clearly not completing the combustion cycle in the cylinder before the exhaust valve opened.  Too much of that could have become a very expensive burned exhaust valve issue.

Edited April 3, 2023 by Str8-8-Dave (see edit history)

[Stude Light]

You may find this interesting. Detroit, MI fuels in 1939.

 

[37PackardMan]

Thanks!

I like those prices!!!

[Rusty_OToole]

1 hour ago, Stude Light said:

You may find this interesting. Detroit, MI fuels in 1939.

 

Thanks Stude, this is excellent information for owners of prewar cars. I notice the Regular gas is all around 70 octane, higher than I would have expected for the time. Ethyl or leaded gas, up to 80.5. Especially interesting is the third grade, or low grade gas, meant for trucks, tractors, donkey engines and Model Ts. Older cars with low compression engines would love this stuff.

[Stude Light]

The octane averages from most of those same gas stations in 1932 were: 

Regular - 62

Premium - 72

Third - 53

 

So in those 7 years from 1932 to 1939 there was a significant increase in octane ratings due to compression ratio increases.

[Bloo]

On 4/3/2023 at 4:48 PM, Stude Light said:

You may find this interesting. Detroit, MI fuels in 1939.

 

Check out the vapor pressure in that chart. They are higher numbers than current fuels. If that is true vapor pressure (TVP) rather than Reid Vapor Pressure (RVP) as used today, 10psia TVP is approximately the same as 9psi RVP at 100F (a common test pressure), according to the charts in the link below.

 

http://www.jmcampbell.com/tip-of-the-month/2016/02/correlations-for-conversion-between-true-and-reid-vapor-pressures-tvp-and-rvp/

 

And in the US, according to this EPA source, you can expect about 9.0psi from summer fuels, while some areas must meet 7.8psi RVP. There is information going back to the early 90s, and it's all about the same numbers as now, the differences being who had to meet which standard where. That and there used to be a 1.0psi RVP allowance for ethanol. Scroll down to the bottom for the 2020 changes (current rules).

 

https://www.epa.gov/gasoline-standards/gasoline-reid-vapor-pressure

 

Maybe I'm missing something, but it sure seems to blow up the idea that modern fuel is highly volatile compared fuel from the mid to late 1930s. I have been suspicious of that claim for a long time.

 

 

Edited April 5, 2023 by Bloo (see edit history)

[jp1gt]

has anyone had problems with the alcohol eating up the carburetors?  I use aviation fuel for my small engines. High octane and no alcohol.

[Stude Light]

@Bloo …… FYI - The samples were all taken in February 1932/1939. There is no documentation on the test methods in the report. We would have to assume the standard method of testing used by the automotive industry at the time.

 

I was also reading a test report from 1920 and there was a lot of discussion at the time about using ethanol to increase the availability of a fuel supply along with fuels from coal and oil shale.

 

The understanding of pre-ignition, fuel chemistry, global production and processing, different crudes and alternative extraction techniques, other options for fuels was surprising well understood.

According to a report from 1939, referenced articles on pre-ignition and fuel chemistry goes back to 1896 and the ability of alcohol to withstand high compression pressures without pre-ignition was discussed as early as 1902. By 1913, the antiknock qualities of benzoyl and cracked gasolines was known. Exhaust gas analysis of carburetor performance was established by 1908. According to the report, the understanding between knock and fuel composition really took off in the mid to late 1920s and was very well understood by 1930.