Octane

[cuba_pete]

In the old gasoline burning engines the octace rating is significantly (?) lower. My '52 lists 72 octane on the data plate.

1) Is there a way to reduce the octane rating safely for both man and truck?

2) If you don't reduce the octane rating, what timing setting do you use (specifically for the 302 I6) to make up for the difference?

3) Is it necessary to reduce the octane rating at all (I cannot really hear pinging and knocking--go figure)?

4) What methods have people used to increase the compression to make up for the difference?

If people go to such great lengths to make up the difference between 87 and 92 (5 points), then I would imagine that the difference between 72 and 87 (15 points) would be pretty significant.

 

[Bob H]

no need to do anything

spark knock would come from too low of an octane or to much spark advance, not from too high of octane.

pump 87 in it

 

[ODdave]

Truck will be fine with the higher octane. Octane is the resistance to ignite, It was increased in the 70's to make up for the higher compression ratios. Adding a can or 2 of lead substitute would be a wiser idea as the modern fuels are hard on the valve seats.

 

[therealsundance]

Octane prevents fuel pre-detonation, cylinder compression is how you figure out what octane you need.

You can use higher octane rating than needsd, but lower can cause catastrophic failure.

 

[M813A1]

Octane is used to help the fuel burn more efficiently !! For the older vehicles the lead subtitute would be best because the newer fuels do not have the lead for lubrication and newer engine have hardened valve seats !! As was said by ODdave above !!!

 

[cuba_pete]

I understand the octane ratings, but I don't see how this has no effect.

Essentially, with the higher octane the fuel doesn't burn until it is at a higher compression and temperature. This is how it withstands knock and ping. By putting in a higher octane fuel we change the burn characteristics of the fuel, allowing higher compression before ignition. In our older engines there is even lower compression due to age to contend with.

Does the increased octane just prevent pre-ignition and detonation? If so, then once the spark ignites the fuel does the flame-front spread at the same rate no matter what the octane rating is? It doesn't seem so, since Shell and BP mfr pages state that the higher octane rating also affects burn rate. Therefore, our older engines were designed with a faster burn rate in mind. I could see this considering the mass of the engine we need to rotate. Back in those days there was no replacement for displacement and we burned gas as fast as we could pump it out of the ground.

Car manufacturers (for new cars) state that by putting in a higher than necessary octane that the ignition curve is automatically set by the vehicle's computer to compensate. We should need to do this manually for our vehicles, right?

The mfrs also state that there is a loss of power due to not selecting the proper grade of fuel which the engine is designed for, including putting in a higher than required octane.

If this really does make a difference (to my truck), is it only a difference of improving mileage from 6 up to 6.5 mpg? If so...it's more trouble than it's worth to compute the necessary changes, isn't it?

 

[M215]

fuel

I have been running regular gas through my M215 GMC 302 for almost 10 years with no problems. I do use a lead additive in the fuel. Biggest problem I have is how fast today's cheap gas goes bad, maybe 2 months. My M221 GMC would not even start this year because the cheap gas clogged up the carb. I usually run the M221 several times a year until I'm ready to start it's resto.
My future plans are to run some avgas through the vehicles in the fall to keep the carbs from gumming up over winter. Aviation gas has 100 octane, lead and will keep for more then a year. When our halftrack's fuel tanks are installed, I will only use avgas in it.
Hope this is helpful.

 

[dozer1]

I don't think that using 87 octane regular unleaded is going to hurt anything and a lead substitute for the soft valve seats would be a good idea. Here is what I would consider doing. Pull the head and plane it to increase the compression ratio. Get hardened seats installed. Just a top shelf valve job from a shop that knows what it is doing. Make the engine need the 87 octane fuel. Just a thought.

 

[BBoomerBootCamp]

M215, How available is Avgas? I had also heard it had a much longer shelf life than 1 year.

 

[wreckerman893]

M215, How available is Avgas? I had also heard it had a much longer shelf life than 1 year.

You can get it at any airport that supports piston driven planes.

Be prepared for sticker shock..........it is not cheap.

It also may be considered "off road fuel" so there may be a tax issue using it in a vehicle.

 

[BBoomerBootCamp]

Yea it's $5.25+ at the local airport, they said it can hold it's juice for over 2 years.

 

[ODdave]

Aviation fuel is also higher octane. 100+.

 

[M813A1]

The only thing I would watch out for is using Ethanol base gas in your older vehicles because the Ethanol will dry out the rubber parts in the carbs and fuel pump and fuel system and that will lead to problems !!

 

[M215]

Avgas

M215, How available is Avgas? I had also heard it had a much longer shelf life than 1 year.

Avgas is expensive and available at any local airport. I may have a line on purchasing avgas that has been pumped out of airplanes for maintenance. Once the fuel has been removed form the air craft it can not be reused for aviation.

One large MV collector I know has stopped using regular gas in his collection. The cheap gas ruins the fuel pumps and carbs. He only uses avgas now, cheaper in the long run and the vehicles actually start after sitting for months.

 

[smoke]

I would say you can use the 87 octane put some lead sub in it and I would put stabuilt fuel treatment in it to help with keeping it fresh. I even think they have one out now spec for ethanol problems. You could also be able to run a little more spark timing do to higher octane but I leave that to oldtimers to advise you on that.

 

[Keith_J]

Running higher anti-knock index gasoline (AKI) does no damage. All gasoline ignites easily by a spark, lower AKI gasoline will ignite easier from just heat of compression.

This is why high AKI aviation gasoline is not recommended in the multifuel LDS series as these engines require auto-ignition.