low octane vs high octane in your 1971& older vette

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Peter,
2 seconds is HUGE.

Actually the vette was heavy and the small tin cars did the best in the 1/4 like the chevy II .

We followed the tow trucks to the wrecking yards when a vette was wrecked and got in line to buy the engine and trans.

Mostly we ran them in 55,56,&57 chevy cars because they were lighter and had a stiff rear axle.

I didn't read road tests as we were drag racers.

I am TOTALLY amazed that we have a few here that think they are smarter than the General Motors engineers.

It is obvious that they never strayed away from pump gas or felt the difference.

Even after hearing the guys that run the CORRECT octane say how good it works, they just don't get it and push what they read or paper formulas.

you can lead a horse to water but

DOM

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Peter,
What I do remember was loosing by a few hundredths of a second.

There were guys that went dow the 1/4 mile just to say they did it, then there were guys that did it to win.

We paid attention to the winners not the magzine tests as the winners usually drove better and had a few secrets.

We used the best Sunoco had, contrary to what a few think is wrong. AND the 4 speed was the best then if you didn't miss a shift.

Then they went to automatics because they were consistent and you didn't loose because you couldn't shift.

4 door cars were daddys and no one paid attention to them.

If you Shifted a 4 speed correctly you held the gas pedal to the floor and never let it up during a shift (power shifting)

The magazine times were much slower than the actual times.

My friend put a junkyard 1966 327 out of a chevy II in a Austin Healy and made the high 10's. He made it a 365 HP by putting the 30/30 solid lifter cam in it. The car was not street drivable.

I bought the engine and used it in my vette because at the time I didn't like big blocks and the 435 HP that was in my vette was beat to death with headers and the 327 was fresh.

When I saw the casting mod only used on chevy II engines I took it out amongst other reasons and went back to the BB.

The stock 365 HP was remarkable and showed a few big blocks that too.

DOM

[Duke W.]

Re: low octane vs high octane in your 1971& older vette

There is no formula to compute the octane requirement for a given compression ratio. Valve timing, design features of the combustion chamber, jacket temperature, and inlet air temperature and pressure are all significant variables, but, working with owners who rebuilt their engines I have established a reliable body of evidence for several OE engine configurations. See the Fall 2010 Corvette Restorers for one example of this research.

Most OE high compression vintage engines will run satisfactorily on modern premium fuel, but some may require total WOT timing to be reduced 2 to 4 degrees from optimum, especially if they have a very aggressive OE centrifugal advance curves which is NOT the case with most OE configurations. A very few may require additional octane to avoid an excessively retarded spark advance map.

As a rule of thumb, if actual total WOT spark advance is 3 degrees less than optimum, the power loss is about one percent, which is equivalent to a 10 degree increase in inlet temperature and is only detectable with well controlled dyno tests since this is within the range of dyno test repeatability. Dyno test data I have (and that I reported and analyzed on the internet and The Corvette Restorer) shows that vintage high compression small blocks produce best torque/power with 38 deg. total WOT advance, and the later the inlet valve closes, the faster it can be brought in.

Typical as-built OE compression ratios are a few tenths to half a point lower than advertised, so a typical 327/365 is closer to 10.5 than 11:1. The advertised CR can usually only be achieved by decking the block since most blocks are .005-.015" higher than the nominal deck height that is used to compute the advertised CR, and the head gasket must be no thicker than the OE type thin shim gasket; however, most rebuilt OE engines are typically assembled with thick composition gaskets that knock another half point off the CR, so a rebuilt 327/365 with OE replacement pistons typically ends up closer to 10:1 than 11:1.

The bottom line is that most OE configurations can run the factory spec spark advance map (which often don't achieve the best total WOT advance for best power) and many can run more than the factory recommended maximum spark advance to achieve the total WOT advance that yields best torque/power.

Few owners of reasonably "stock" vintage engines report detonation with the OE centrifugal curve and OE recommended initial timing. Big blocks do have more octane appetite than small blocks, so that's were most of the complaints come from, and retarding initial timing no more than 4 degrees from OE spec will usually tame the beast. If not, then the owner may be forced to blend high octane race gas or avgas with pump premium, usually no more than 25 percent.

There is usually no need to lower OE compression ratios during a rebuild, but the measurements should be taken to compute the CR on ALL cylinders, which is not hard to do. If the engine didn't detonate prior to the rebuild, it can be assembled with the same CR.

Various "blueprinting" procedures such as grinding cylinder chambers to equalize CR on all cylinders also helps. CR variation between cylinders can be up to 0.5 point, so knocking down the highest with a little chamber grinding will keep those cylinders out of detonation.

I listed my recommended maximum CRs for various OE engine configurations in the past, and most are based on engines where the actual CR was carefully measured and followed up with road testing using different fuels (such as premium mogas and avgas) to determine the maximum total WOT spark advance that the engine would tolerate without detonation in warm to hot weather.

Duke

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Duke,
Thanks for the agreement.

I think you said it best when you said they will run satisfactory on modern fuel, retarding the timing.

My whole point is to have it run GREAT and leave the GM enginered timing alone.

We deserve a treat once in a while.

DOM

[Kurt G.]

Re: low octane vs high octane in your 1971& older vette

Don,

If you're running 93 octane pump gas now and go to 100 racing fuel you will see a slight increase in performance and fuel efficiency, but only slight. Although I am not a petroleum engineer, several years ago one of the engineers at a refinery owned by a company I used to fly for gave me a formula for mixing 93 octane pump gas and 100 octane low lead avgas for an outboard I had that was bored 05 over. 1 gal of avgas to 6 gals of 100LL would give you an octane equivalent of about 105 R+M/S. Don't ask me how the numbers worked. But, long story short, the knocking went away and the engine performed beautifully for several years.

[Duke W.]

Re: low octane vs high octane in your 1971& older vette

Duke,
I think you said it best when you said they will run satisfactory on modern fuel, retarding the timing.


DOM

That's NOT WHAT IT SAID - not even close! This is what I said:

"The bottom line is that most OE configurations can run the factory spec spark advance map (which often don't achieve the best total WOT advance for best power) and many can run more than the factory recommended maximum spark advance to achieve the total WOT advance that yields best torque/power."

Duke

[Michael W.]

Re: low octane vs high octane in your 1971& older vette

Duke,
Thanks for the agreement.

I think you said it best when you said they will run satisfactory on modern fuel, retarding the timing.

Not quite DOM -here's what Duke actually said, the bolding is mine:

T

Most OE high compression vintage engines will run satisfactorily on modern premium fuel, but some may require total WOT timing to be reduced 2 to 4 degrees from optimum, especially if they have a very aggressive OE centrifugal advance curves which is NOT the case with most OE configurations. A very few may require additional octane to avoid an excessively retarded spark advance map.

Big difference! What Duke said mirrors what's been said on this subject by myself and others.

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

For my base 69 427, the listed compression was 10.25:1; so targeting 10.0:1 with some head work should be a good target.

Another question: If I'm currently running on regular pump premium and want to see how a tank full of 100 octane feels; will I need to adjust timing and advance; or just fill the tank and drive it?

thanks,

Don

Don,
Most of these cars end up getting the heads milled every time they go into a shop because they don't know how to use a straight edge.

So it's my belief that the compression ratio will be closer to 11:1 because of that.

I find some heads that have been milled a number of times and We have to do work on the intake to get them to work. Some about .090 and more.

We used to mill the heads to get 1/2 point of compression and use the steel shim head gasket the factory used.

When drag races were stock class the limit was .030 I believe or you were not concidered stock, you were modified stock.


DOM

[Michael W.]

Re: low octane vs high octane in your 1971& older vette

Don,

If you're running 93 octane pump gas now and go to 100 racing fuel you will see a slight increase in performance and fuel efficiency, but only slight. Although I am not a petroleum engineer, several years ago one of the engineers at a refinery owned by a company I used to fly for gave me a formula for mixing 93 octane pump gas and 100 octane low lead avgas for an outboard I had that was bored 05 over. 1 gal of avgas to 6 gals of 100LL would give you an octane equivalent of about 105 R+M/S. Don't ask me how the numbers worked. But, long story short, the knocking went away and the engine performed beautifully for several years.

Kurt-

Apples and oranges. Your engine had a knocking (detonation) condition, for reasons not disclosed. Don's does not. Your engine consumed less fuel and made more power by addressing the detonation issue, not because of the increased octane.

A 6:1 ratio of 93 to avgas would yield an AKI of about 95, not 105.

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Don,

If you're running 93 octane pump gas now and go to 100 racing fuel you will see a slight increase in performance and fuel efficiency, but only slight. Although I am not a petroleum engineer, several years ago one of the engineers at a refinery owned by a company I used to fly for gave me a formula for mixing 93 octane pump gas and 100 octane low lead avgas for an outboard I had that was bored 05 over. 1 gal of avgas to 6 gals of 100LL would give you an octane equivalent of about 105 R+M/S. Don't ask me how the numbers worked. But, long story short, the knocking went away and the engine performed beautifully for several years.

Kurt,
I'm not sure if this was for don or Dom?

Here is my answer, Yes.

It's like you turbo charged your engine.

I mainly did this with 11:5 compression small blocks and found if I ran av-gas straight the performance seemed to get worse so I never used more than 50%.

My guage in performance was my tires. They would smoke by stepping on the gas with a 50/50 mixture. With the pump gas using water injection they would not.

My distributor cap was polished by my hand turning it every time I bought gas.

I would retard about 5 degrees with pump gas and water injection.

It was actually easy doing it because of the way the hood opens, I held it with my left hand and turned the distributor wit my right , all behind the drivers door.

DOM

[Duke W.]

Re: low octane vs high octane in your 1971& older vette

Don,

If you're running 93 octane pump gas now and go to 100 racing fuel you will see a slight increase in performance and fuel efficiency, but only slight. Although I am not a petroleum engineer, several years ago one of the engineers at a refinery owned by a company I used to fly for gave me a formula for mixing 93 octane pump gas and 100 octane low lead avgas for an outboard I had that was bored 05 over. 1 gal of avgas to 6 gals of 100LL would give you an octane equivalent of about 105 R+M/S. Don't ask me how the numbers worked. But, long story short, the knocking went away and the engine performed beautifully for several years.

Your formula makes no sense, but here is how you compute the approximate octane number of a blend, keeping track of the fact that there are three different ways to test/measure octane and a fourth that is derived from two of the tests/measurements.

Let's use a blend of 25 percent 100LL avgas and 75 percent 93 PON mogas. Avgas uses a different octane rating method than mogas, but it is usually very close to the MON if the avgas is tested using the MON test, so we can reasonably assume that 100 aviation octane method is equivalent to a 100 MON mogas in terms of detonation resistance.

Further, understand that a fuel's "sensitivity" - the difference between MON and PON - is usually in the range of 8-10, and PON = (MON + RON)/2. I'll use a sensitivity of 10.

The approximate MON of the blend is:

.25(100) + .75(88) = 91

The corresponding approximate PON is 96 and approximate RON is 101.

There is no practical way to compute the additional octane required to eliminate detonation. It's a matter or finding, through experimentation, the minimum octane blend that will tame the detonation.

Once this is found, one can do the above computation to determine the octane of the blend using whatever octane method you want.

I'll further add than most internet discussions about octane are worthless, if not downright misleading, because the discussions rarely distinguish between the different octane measurement methods, so they usually end up being apples and oranges discussions because few understand the simple difference between vintage gasoline (RON) modern gasoline (PON) and avgas (aviation method) octane numbers and their relationships, but it's not rocket science - just seventh grade arithmetic!

Duke

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Hey Don,
when you learned to fly do you remember the fuel issue that they pounded in our heads.

Quote
Using a lesser octane will cause detonation and a los of power possibly causing engine failure.

Also that you cannot go from 100 to 80 but you can go from 80 to 100 limited to 1/2 tank or valve damage may occure.

Believe it or not the majority of those engines have compression in the 6's & 7's

Don, I think part 43:13 addresses this also in the FAR's.

Your performance section in your POH (pilot operating handbook) was only accurate using the correct OCTANE fuel.

I suppose they were wrong also Thank God these gas experts don't fly.

DOM

[Pat M.]

Re: low octane vs high octane in your 1971& older vette

Your engine consumed less fuel and made more power by addressing the detonation issue, not because of the increased octane.

Michael, I understand what you and Duke have generally said in this and the other octane thread and your assertions make the most sense to me.

But in Kurt's detonation case, didn't the increased octane address the detonation issue, by being more resistant to detonation, and thus in a roundabout way "made" more power because the engine now operates correctly with combustion, not detonation?

I thought you all had said that you normally don't need the higher octane unless you have detonation, like Kurt did. Did I misunderstand?

[Domenic T.]

Re: low octane vs high octane in your 1971& older vette

Hey Don,
when you learned to fly do you remember the fuel issue that they pounded in our heads.

Quote
Using a lesser octane will cause detonation and a los of power possibly causing engine failure.

Also that you cannot go from 100 to 80 but you can go from 80 to 100 limited to 1/2 tank or valve damage may occure.

Believe it or not the majority of those engines have compression in the 6's & 7's

Don, I think part 43:13 addresses this also in the FAR's.

Your performance section in your POH (pilot operating handbook) was only accurate using the correct OCTANE fuel.

I suppose they were wrong also Thank God these gas experts don't fly.

DOM

OK,
I goofed this was for the pilot.

DOM

[Mark K.]

Re: low octane vs high octane in your 1971& older vette

Don,
Most of these cars end up getting the heads milled every time they go into a shop because they don't know how to use a straight edge.

So it's my belief that the compression ratio will be closer to 11:1 because of that.

I find some heads that have been milled a number of times and We have to do work on the intake to get them to work. Some about .090 and more.

We used to mill the heads to get 1/2 point of compression and use the steel shim head gasket the factory used.

When drag races were stock class the limit was .030 I believe or you were not concidered stock, you were modified stock.


DOM

What do these "modified" engines have to do with stock engines and their performance with various octane rated fuels? My stock 67 L71 does just fine with 93 unleaded that I get from the local Exxon/Mobil stations. No detonation, strong acceleration right up to 6500 RPM. My car runs stock settings for everything including timing.