Octane - Power - Change - Very Odd

[James_Douglas]

This is going to be a rather theoretical thread. 

 

Ever since last year when I had the head of my '47 Desoto I noted that at high RMP (direct drive) pulls up a hill with a high slope that I was getting a little bit of a ping.  If I climbed up with the overdrive, at less RPM I did not notice it as much.

 

So, a few weeks ago I decided to fill up the tank with 91 octane to see if the ping would stop.  It did.

 

I of course can roll out a little bit more timing and get the light ping to stop.

 

I suspect that the gasket may be just enough thinner that the compression is up at the regular octane tipping point.

 

What is interesting is that in the big Desoto, with it weight and relatively low horsepower, when climbing hills on the freeway is very sensitive to any change in horsepower. So, what I noted was that when  I am climbing the same hills I do every week in and around San Francisco (freeway driving) and I have been able to climb those hills without kicking out the overdrive.

 

Clearly the engine is generating more power. But How?

 

I am well versed on the subject that an increase in octane, in and of itself, does nothing to increase engine power output.  I have not changed a thing on the car.  Yet, there is clearly something going on.

 

I have even accounting for the air temperature as is has not floated more then 15 degree's over the last month while this is going on.  I have also accounted for winter blend and summer blend as this phenomenon has been repeatable across both blends.

 

I have racked my head trying to come up with a explanation. I do not think it is just my perception as the couple of places were the extra power really counts is a static run and the car is producing more power with the higher octane.

 

I have one possible theoretical idea.  But before I post it, I want to hear if anyone has any idea that could explain the power increase.

 

Interesting issue for me to say the least.

 

James

[dpollo]

The only explanation I can think of is that while an engine is pinging, it cannot possibly be putting out full power.

Full power has to be just at the point before detonation occurs.  Great subject to ponder on a sleepless night.

[martybose]

Flatheads in general are notorious for pinging due to poor combustion chamber design.  When we built my motor my mechanic said to make it 9 to 1 and expect to run high octane gas.  We set the initial timing by running the car up a local hill at about 2000 RPM, then added timing until it started to ping, then backed it off a little.

 

Marty

[Plymouthy Adams]

pinging usually occurs under light throttle ie subject of pulling a small grade or overpass etc due to too much distributor curve timing not basic engine timing set by distributor position at idle speed.  Only by measuring your advance at X rpm's will determine where in the curve the advance is too high.  Loss of vacuum during engine load should drop your advance curve and prevent this pinging..if not then you may have to add a washer or so to adjust this in these old vacuum advance assemblies.  If dropping under loss of vacuum then your distributor spring tension may be the culprit allowing too much mechanical advance.  The reason the pinging is going away at the higher octane level is exactly the very design of the higher octane to begin with,  You are now altering the flash point of the fuel by retarding its burn rate thus it is less susceptible to burn till closer to TDC..

 

added

 

...you mentioned ever since you had the head but left us hanging as to what was attributed to the head.  I am assuming that you may have had it shaved only to the point to true it.  This will alter the compression even if ever so slightly.  Remember even a stock running engine with some coke deposits can and very often does cause pinging.  Your problem could simply be a combination of a number of small insignificant facts that when combined is the cause and result.  Do you have access to a timing light where you can actually dial back and read the timing advance.  This in absence of a distributor curve machine is the next best way to determine and separate the mechanical and vacuum advance in the distributor.  Without known test, you are back to guessing.

Edited April 14, 2016 by Plymouthy Adams

[James_Douglas]

Hi Guys,

 

1. The distributor timing is dead on as I own a Sun Machine and the curves are correct.

2. The head was not shaved when it was off.

3. Item #2 assumes that the replacement gaskets (Best) is the same thickness as the one that came off, I did not check it. It could be thinner and thus raised the compression.

 

I think that dpollo is onto the only explanation that I can come up with. Everything I have read about early detonation suggests that it happens before one hears it.  Then it gets worse and you hear it.  If the charge is igniting before top dead center then the piston has to "plow through" the flame front until it hits TDC.  That power is then removed from the output of the crankshaft.

 

I think this is what is happening.  Given the weight of the car, a very small change in power output is noticeable at the margins like up a long hill at freeway speed. 

 

In my lighter convertible, I am sure one would never notice the difference as there is enough reserve power for such situations. I will just sue 91 octane going forward.

 

James.

[greg g]

Do you guys have winter blend fuels in your area?  Here they add oxiginator beginning around October, and then remove it in March/April.  The oxiginator suposidly makes for easier ingnition in the cold but it saps BTU's from the fuel air mix. My daily driver drops about 3 to 4 mpg and feel like it has dropped several HP.as per butt dyno. 

[Adam H P15 D30]

I have heard the premium fuel here does not contain ethanol while the regular does. Could be the answer??

Ethanol fuel has less BTU's

Adam

[Powerhouse]

See if you can get some 100 octane from shell. That ought to show you a big difference. Ethanol in gas is a problem when it comes to not wanting pinging/detonation due to less octane. From what I've found out, ethanol allows for easy pinging. I have had a similar issue. I tried some shell 100 and it was much nicer. But since it runs about $9 a gallon I think I'll change the head back to a stock one. Just have to find one without a hump. I have two of those.

Edited April 17, 2016 by Powerhouse

[Niel Hoback]

Not 100% sure, but I think you can use an internal bypass head on an external bypass block. The gasket seals the hole in the head that doesn't have a matching hole in the block.  Is that true?  Anybody?