Aviation "Racing" fuel dangers to the engine?

[Duke W.]

Re: Aviation "Racing" fuel dangers to the engine?

Mike is correct. Sixties RON for premium fuel was as low as 95-96 in a cold winter climates or at high altitude. Actual 100 RON or higher was probably rare unless you bought "super premium." If you lived at sea level in a warm climate premium was probably 97-99 RON, and maybe some were actually 100 RON.

Higher octane increased production cost, so the oil companies blended the lowest octane they could get away with for the ambient conditions and not get too many customer complaints about detonation, and name brand stations probably had one or two points higher octane premium than no-name discount stations.

Modern PON rated premiums are in at least the lower range of sixties premiums as far as detonation resistance is concerned.

All this has been discussed a zillion times on the TDB, and I thoroughly reviewed it in my San Diego presentation - RON, MON, PON/AKI, and Aviation Method, and how to make an approximate conversion of one to any other.

If you made it through the fourth grade and can add 2+2 it should be understandable, but maybe some guys didn't get that far.

Here's a question to test your knowlege of fuel octane. What does it take to increase a batch of straight hydrocarbon 100 RON to 101 without changing the HC blend.

Anybody...?

Duke

[Michael H.]

Re: Aviation "Racing" fuel dangers to the engine?

If you made it through the fourth grade and can add 2+2 it should be understandable, but maybe some guys didn't get that far.


Duke

That was uncalled for. I don't know who that was directed to/at but I don't think we need that here. I'm tired of your superiority attitude.

[William F.]

Re: Aviation "Racing" fuel dangers to the engine?

"Ford' not only read it but printed it for reference. "Williams" just won't address the admonition against setting timing by ear issued in Corvette news or why back yard timing experiments with irreplaceable vintage Corvette engines is better than what GM recommended.

[Duke W.]

Re: Aviation "Racing" fuel dangers to the engine?

Okay, what issue and page of Corvette News. Also, type in exactly what was said in case I don't have that issue in my collection.

In the presentation I described why most OE spark advance maps are conservative and also called out some engine configurations where GM actually made mistakes in setting up the spark advance map. Yes, they do make mistakes except back then it rarely resulted in recalls or service campaigns.

Also, "timing by ear" traditionally refers to advancing the timing to the detonation point without the use of a timing light. That's NOT what I recommend, nor have I ever recommended this.

My spark advance map recommendations are based on published GM recommendations (Chevrolet Power Manuals) and decades of experimentation by me and others. Then it's a matter of running simple detonation tests to ensure that significant detonation is not present with commonly available pump gas.

I have recently run across a couple of cases where excess detonation was a problem without running at least a 50 percent blend of avgas or race gas with pump premium. One, a big block, had been significantly decked and disassembly and measurements proved that it had higher compression than any engine that ever left Tonawanda other than L-88s and ZL-1s. The other, a 327/365 had been rebuilt some time ago with larger domed pistons than OE. In these cases the only choice is to deal with the cost and inconvience of running whatever blend of race gas or avgas it takes to quell the detonation or rebuild the engine with no more than typical OE compression ratio, which averages several tenths of a point less than the advertised value

Find a thread where someone had to rebuild an engine due to detonation damage. I can't. Most engine rebuilds are initiated due to high oil consumption or poor performance, both of which are usually due to plain old wear, and a good number require major rework again due to improper parts selection or poor quality workmanship by "engine builders".

If you want to rebuild your L-79 with 8:1 compression, keep the lazy OE centrifugal advance curve and use 100LL avgas or racegas that's your choice, but your obsession with engine damage due to "inaudible detonation" based on a blurb published over three decades ago and for which is there is no meaningful evidence since that time borders on pathalogical. It's in the same category as installing exhaust valve seats to keep the valves from pounding in due to "lack of lead".

Duke

[Don H.]

Re: Aviation "Racing" fuel dangers to the engine?

64 posts and this topic has been run into the ground. Can we all agree to disagree and call a truce on octane ratings and detonation for now?

OK, 65 posts and now the reference info is here. There will likely be some rebuttal to this, so let's see where it goes, hopefully with respect for all viewpoints.

this thread is re-opened.

[William F.]

"The Wise and Wherefore Selection of Gasoline"

Title of this thread is title of the article I referred from a Corvette News-Volume 7, number 4, 1964.Duke wanted reference and exact wording; so, I'm providing it so some wouldn't say I'm just blowing smoke. "I quote:It has been determined that this knocking condition also subjects the piston rings to abnormal wear. For example, this "trace knock" (determined by special instrumentation) will accelerate piston wear up to three times normal; a noticeable light knock will accelerate wear up to six times normal. Because of this, a seat of the pants tuneup should never include road-timing (where spark is advanced until the engine begins to ping on acceleration).This is what has caused me concern about the idea of advancing timing until you get audible pinging under certain conditions. Thought it was reasonable to let others know about this article.Also thread on this subject was closed by administrator even though it was getting a lot of response and no "F bombs" were being thrown-(but one did suggest if you didn''t agree with him, you must be a 4th grade dropout.)By the way my L-79 has stock setup-at least close to 11:1 compression.

[Duke W.]

Re: "The Wise and Wherefore Selection of Gasoline"

I have that volume and have just finished reading the article, which is about gasoline. It's a very good article and does not at all conflict with the material I presented in San Diego last year.

The discussion about "trace knock", which may not be audible to the driver is not specific to Corvettes, but it can be an issue in heavily loaded spark ignition engines such as trailer hauling and medium duty trucks. It's less likely to be an issue in a Corvette where the engine only sees moderate to heavy load while accelerating or climbing a grade. Even climbing the steepest interstate grade - about 6 percent - at 70 MPH/2500-3500 RPM in top gear does not require more than about half the available power at the specific RPM, which varies depending on axle ratio.

Since six percent interstate grades are not always readily accessible, I can usually find a six percent or steeper surface street grade and my test is to pull the grade in top gear starting at 1000-1500 revs and accelerating up to as high a speed as I can achieve before the top of the grade. This is a tough detonation test, and most drivers, including me would use third or even second gear to climb such a grade at 25-35 MPH in normal driving. One doesn't need to do a detonation test at every opportunity, but only until one is confident that the spark advance map doesn't cause sustained detonation under average conditions.

The article refers to a habit of "timing by ear" - they call it "road timing", which was common back in the fifties and sixties when timing lights were relatively more expensive than today. Shade tree mechanics would sometimes advance the initial timing until the engine began to detonate under acceleration, and at least the smart ones would then back it off until the detonation stopped, but some just accepted the detonation. Try this technique and then check the timing and you will likely find that the total WOT advance is well into the mid-forties range, which is well above the Chevrolet Power Manual's recommended range of 36-40 for both small and big blocks.

As I said previously, I have never used this "timing by ear" or "road timing" method, and do not recommend it. In my San Diego presentation I recommended starting point spark advance maps for various engine families based on my understanding of design factors that affect detonation, four decades of experimentation with my and other Corvette engines, and Chevrolet Product Performance engineers' recommendations, and all require the use of a dial-back timing light to properly verify the setup. I then discussed detonation testing. I further stated that brief TRANSIENT detonation of no more than about one second duration on upshifts or upon sudden throttle increases IN WORST CASE CONDITONS such as hot weather and high coolant temperatures was okay, and I further explained how to "drive around" this problem, if it occurs.

This is a far cry from the the shade tree mechanic method that you refer to as "timing by ear" and the article refers to as "road timing" that you imply that I am using.

The article also discusses owner detonation problems with recent Corvette engines, and remember, this article was written in 1964... but didn't everyone have "100 octane" gas available back then? Apparently not because the article discusses the wide variation in actual octane rating depending on geography and climate. I also discussed this in a previous post in this thread. We know that in mid-'62 Flint began double gasketing 340/360 HP engines due to customer detonation complaints; 60-61 cc head chambers and 5.3 cc dome pistons even with a conservative spark advance map went beyond the limits of many available fuel octanes. The larger '64 SHP/FI chambers along with the 30-30 cam, which has a much later inlet valve closing point than the Duntov cam cut SCR and DCR with a single .018" gasket enough to quell detonation even with a much more aggressive spark advance map than than the antecedent '62/'63 SHP/FI engines.

A few owners have detonation problems, today and a few had them when these cars were new. How about that! It appears not much has changed, other than using different octane test methods and a different octane scale that a lot of guys don't understand even though it's been around for three decades and it's as simple as fourth grade math... well maybe seventh grade math because when you run the (R+M)/2 calculation through your calculator you will usually end up with a decimal fraction. Ouch! Those are tough.

So the question remains. In the last 40 years has anyone ever seen excessive piston ring wear that can be attributed to detonation on an OE or slightly modified (like spark advance and fuel flow map changes) OE engine? I haven't, but I'm all ears!

Likewise, I haven't seen, nor have I ever heard of excessive valve seat erosion on vintage Corvette engines due to the use of unleaded fuel. This was also published by GM, but it was only an issue on highly loaded engines like trailer towing and heavy trucks, not Corvettes or even full-sized passenger cars. Sound familiar?

So I consider any hysteria about "inaudible detonation" to be a non-issue just as the valve seat erosion hysteria is a non-issue on vintage Corvette engines. There's no evidence to justify any concern about either issue in normal driving. If anyone uses their vintage Corvette to pull a double horse trailer, may I suggest that you consider buying a heavy duty pickup truck for the task.

Duke

[William F.]

Re: "The Wise and Wherefore Selection of Gasoline"

Duke,Thanks for taking the time for reading the old" Corvette News" article and giving your views. I understand that while you accept minimal ping under a very hard detonation trial, you do keep total WOT degrees of advance within an accepted range and verified by a timing light.I do refer to my off print of your tuning seminar frequently, have found it useful, and agree with most of it. By the way are the Chevrolet Power books published by GM still available? I had one years ago but can no longer find it. Best regards

[Domenic T.]

Re: "The Wise and Wherefore Selection of Gasoline"

Michael,
You asked about the dangers of av gas and Yes, there were dangers.
We don't have to worry about them as the 2 higher octanes were eliminated, the green & purple that were mega octane.
The old timers in the motor pool (WWII) said that the green & purple would find its way in the jeeps when the lower octane was not available and the old flat heads would burn the exhaust valves if the jeep was driven hard.
I only found one WWII mechanic that explained it to me and he tought the Aviation Mechanics Class at Long Beach City.
His explanation was that the higher octane burned slower and more even and was not completly burned as it was passing the exhaust valve causing the valve to warp & burn.
The av gas octanes were all colored differently and would turn CLEAR if mixed. That was part of the pilots pre-flight check.
In school they made it easier to remember the (4) different octane ratings as each had one more letter in the color.
RED
BLUE
GREEN
PURPLE............. being the highest and usually burned in a radial that would cruise around 1500 RPM's.

The only one found now is the BLUE and the dangers are gone for the most part.

DOM

[Duke W.]

Re: "The Wise and Wherefore Selection of Gasoline"

Duke,Thanks for taking the time for reading the old" Corvette News" article and giving your views. I understand that while you accept minimal ping under a very hard detonation trial, you do keep total WOT degrees of advance within an accepted range and verified by a timing light.I do refer to my off print of your tuning seminar frequently, have found it useful, and agree with most of it. By the way are the Chevrolet Power books published by GM still available? I had one years ago but can no longer find it. Best regards

Just to clarify, my allowable detonation limit is brief (about one second) transient detonation on upshifts or sudden throttle increases under worst case conditions (high ambient temperature and high coolant temperature). These conditions should be rare, but if they do occur, I explained easy ways to "drive around" the problem until weather conditions return to something more normal for where you live. All my cars are set up this way.

I doubt if the Chev. Power Manuals are still in publication, but I'm sure they show up on ebay. Another good engine restoration reference is H. P. Books' "How to Hot Rod Small Block Chevys". Ignore "hot rod" in the title. It has very good information on blueprinting and head massaging. Head massaging, in particular, will yield about 10 percent more top end power and 500-1000 more useable revs on any OE configuartion, and it's undetectable in NCRS judging.

I'm sure a Google search on either title will get you some links.

Duke