[trshman]

Someone I met recently suggested that cars like my '66 and his '67 Mustang ran cooler on the leaded gas available when those cars were new.
I never heard anything like that before, could that be possible ??

Thanks for the input.

Bob G.

[trshman]

Someone I met recently suggested that cars like my '66 and his '67 Mustang ran cooler on the leaded gas available when those cars were new.
I never heard anything like that before, could that be possible ??

Thanks for the input.

Bob G.

[sleepy]

don't know about runing cooler temp But unleaded will toast the exhaust valves without harded seats or lead substitute.

[78 GHOST]

Bob, there is actually some truth to what you've been told, although probably not for the reasons we might initially think.

The primary difference between leaded and unleaded fuel is the presence of Tetraethyl Lead. This Lead was added as a property to aid in increasing Octane. Octane is the resistance to Preignition, or Detonation. Losing this lead results in lower emissions, but also poorer resistance to "Pinging".

On a high compression engine (of any vintage) the engine was designed with a particular grade of suitable fuel to be used. On older vintage HP engines, we manually adjusted the timing using a timing light and setting the initial. When we had to use low octane fuel in our 10.25:1 engines, we had to set the initial at a lower value than optimum. This "Retarding" of the initial timing setting results in a spark starting later in the compression cycle. As fuel burns at a (relatively) constant rate, this need to "retard" the timing results in the action of causing the "burn" of fuel/air to occur closer to when the exhaust valve is opening. As we know, overly retarded timing can cause an engine to overheat just like too far advanced. The retarded timing and "burn" is occuring when the exhaust valve is opening and heating the exhaust valve beyond it's design temperature, transmitting more heat to the head, engine, coolant, radiator, etc.

This same phenomenon will quickly waste an exhaust valve(s) if left unchecked. I believe the late combustion is more detrimental to exhaust valves than the lack of lead. I have one HP 455 in the fleet now that I manage to run on high octane pump gas at the factory timing specs, by close attention to other factors affecting preignition such as spark quality/intensity, cooling system health, Fuel delivery, etc.

Modern cars have some computerized timing adjustment to accommodate differing fuel octanes. Cars such as the M Roadster, Z8, Vette, 745i, etc. are required by the factory to operate on the highest available octane. I suspect that the designers may have been anticipating the need for more timing adjustment than was available if you tried to run on 87 vs. 93, as opposed to 91 vs. 93. This is a guess on my part. IMHO, if you drive any of these cars and try to run 87, you should be shot for being stupid.

____________________________________
"Gee, what's that goat doing way up here in a cloud?" -Gary Larson

TO THE MILITARY FORCES CURRENTLY ACTIVE, AND THOSE THAT HAVE SERVED; THANK YOU FOR YOUR SACRIFICE FOR FREEDOM. PLEASE COME HOME SAFE.

[trshman]

Sleepy: Thanks
Ghost- I appreciate the detailed explanation. It's an important addition to one I had with my mechanic last week about that very topic while doing a tune up on my car.

If I remember correctly his simplified explanation was that higher octane fuel burns slower (?) which is necessary to compensate for fuels tendency to burn faster the more it's compressed, as it obviously is in a high compr engine.
He also mentioned, as you did, the ability of modern ignition/fuel systems to vary the timing as specific conditons vary, something we're unable to do with our vintage cars.

However the reason we discussed this was because he said my engine might run cooler after the tuneup. A few days later I ran into the fellow with the Mustang and had the conversation that was the reason for my post here.

Whether or not leaded gas would make the original engines run cooler is a question I'd like to verify one way or the other, just out of curiousity. The guy I bought my car from put heads from a '78 TransAm on the '70 / 400 block which results in roughly 8 1/2 to 1 compression and allows me to run on regular with no pinging or other noticable side effects. Since I drive it every day I prefer it this way because even detuned it sucks up the gas pretty good. I'm just curious if the setup I have runs hotter than the original did and if the gas is the reason.

Thanks for the info.

Bob G.

[78 GHOST]

Bob, this is a good topic. If I recall correctly, the wives tale of higher octane burning slower was taught to me in High School ROP Auto Shop. Unless I'm mistaken, there might be a small difference in the burn rate from say 87 to 91. The biggest difference is in how much heat the fuels can withstand prior to spontaneous combustion. Higher octane raises this tolerance, allowing us to start the fire earlier in the compression stroke because the fuel will not explode, it will burn. Up to a point, the more advanced you can get your timing, the more efficiently the burning fuel will act upon the piston.

The key here is having a fuel capable of resisting combustion caused by the heat of compression. Lead is added to fuel to raise that tolerance (octane) to preignition.

I drive daily a 7.7:1 CR 400 CI '78 T/A, with the anemic CR I get away with 87 octane camel piss. On the hottest day, stopped in traffic with the A/C going wide open, the car might see 200F. If I'm moving at 10-20 MPH or better, temps drop to 180F or lower. This engine sees a LOT of timing. Its been over a year since I checked, but I seem to remember seeing something like a total of 40-45 degrees of timing in by 2500 or so. Initial is pretty high at maybe 15, mechanical adds quite a bit, and vacuum brings it up to what would appear to me to be suicidal levels. However, the car likes all this timing, no hint of ping, easy starting, crisp power (tire spin with 2.56 gears), cool running, and good mileage (22.5 MPG documented at 65 MPH no A/C with Cruise on).

Then there is the GTO. '70 455 running 10.25:1 CR. This car sees initial at around 11 degrees, and only about 35 degrees in by 3000. It too behaves well and gets around 14-16 MPG on the freeway at 65. The difference is this engine WILL NOT tolerate anything less than the highest octane available. This car runs right at 180 pretty much all the time. The only times I've gotten it warm were when I had the timing too far retarded.

The point here is: Each engine performs most efficiently when supplied with the fuel grade envisioned by the designers. By grade I mean octane. The engine doesn't care about anything except octane.

When comparing different grades of pump gas, you will not find a significant difference in temperatures between the two. Running cooler is permitted by getting the timing back to where the designers intended it to be.

____________________________________
"Gee, what's that goat doing way up here in a cloud?" -Gary Larson

TO THE MILITARY FORCES CURRENTLY ACTIVE, AND THOSE THAT HAVE SERVED; THANK YOU FOR YOUR SACRIFICE FOR FREEDOM. PLEASE COME HOME SAFE.

[78 GHOST]

Bob, this is a good topic. If I recall correctly, the wives tale of higher octane burning slower was taught to me in High School ROP Auto Shop. Unless I'm mistaken, there might be a small difference in the burn rate from say 87 to 91. The biggest difference is in how much heat the fuels can withstand prior to spontaneous combustion. Higher octane raises this tolerance, allowing us to start the fire earlier in the compression stroke because the fuel will not explode, it will burn. Up to a point, the more advanced you can get your timing, the more efficiently the burning fuel will act upon the piston.

The key here is having a fuel capable of resisting combustion caused by the heat of compression. Lead is added to fuel to raise that tolerance (octane) to preignition.

I drive daily a 7.7:1 CR 400 CI '78 T/A, with the anemic CR I get away with 87 octane camel piss. On the hottest day, stopped in traffic with the A/C going wide open, the car might see 200F. If I'm moving at 10-20 MPH or better, temps drop to 180F or lower. This engine sees a LOT of timing. Its been over a year since I checked, but I seem to remember seeing something like a total of 40-45 degrees of timing in by 2500 or so. Initial is pretty high at maybe 15, mechanical adds quite a bit, and vacuum brings it up to what would appear to me to be suicidal levels. However, the car likes all this timing, no hint of ping, easy starting, crisp power (tire spin with 2.56 gears), cool running, and good mileage (22.5 MPG documented at 65 MPH no A/C with Cruise on).

Then there is the GTO. '70 455 running 10.25:1 CR. This car sees initial at around 11 degrees, and only about 35 degrees in by 3000. It too behaves well and gets around 14-16 MPG on the freeway at 65. The difference is this engine WILL NOT tolerate anything less than the highest octane available. This car runs right at 180 pretty much all the time. The only times I've gotten it warm were when I had the timing too far retarded.

The point here is: Each engine performs most efficiently when supplied with the fuel grade envisioned by the designers. By grade I mean octane. The engine doesn't care about anything except octane.

When comparing different grades of pump gas, you will not find a significant difference in temperatures between the two. Running cooler is permitted by getting the timing back to where the designers intended it to be.

____________________________________
"Gee, what's that goat doing way up here in a cloud?" -Gary Larson

TO THE MILITARY FORCES CURRENTLY ACTIVE, AND THOSE THAT HAVE SERVED; THANK YOU FOR YOUR SACRIFICE FOR FREEDOM. PLEASE COME HOME SAFE.

[mike nixon]

good explaination ghost.

i think you're right on with the comment regarding the exhaust valves and the unleaded fuel. one other thing to consider on this matter is the grade of casting steel used for the heads. i've noticed that chevy engines dont take the abuse as long as a pontiac,cadillac or olds will.

i'll also agree with the explanation on the temps related to fuel quality / timing issues.

exhaust restrictions also play a big part in this scenario. the better an engine breathes the quicker it gets rid of the heat generated during the combustion process and the cooler it runs.

mike

[trshman]

Ghost,

Thanks very much. Very informative. I'm sure I didn't do my mechanic justice when I attempted to restate his explanation to me but frankly it's because it was my first discussion on the topic and I hadn't totally wrapped my brain around his explanation. You've cleared up things I didn't understand before. I appreciate it.

Your timing settings are substantially different from where mine are. I'd be interested in seeing how my car reacts to settings closer to yours, especially since we both share some pretty low compression ratios.

I'm beat. Again thanks. You'all have a nice weekend.

Bob G.