Aviation "Racing" fuel dangers to the engine?

[William F.]

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

I've mentioned this on several other occasions when questions about octane requirements for the higher compression engines has come up, but have never gotten a reply:In an older "Corvette News",which, as you know, was written by GM and sent to Corvette owners, warned against setting timing "by ear , which some such as Duke suggest, since there is a form of detonation that can be detected only by special equipment and leads to significantly increased ring and cylinder wall wear .Wonder if some who say they have no problems with just pump gas do very regular pulls to the redline like these cars were meant to be driven when new or are just out for a cruise? Would like to hear a response based on engineering fact about this inaudible detonation with too low octane and too much ignition advance.In view of this article (I can give exact reference if desired) don't think it's prudent to say, "If you don't hear detonation,' forgetaboutit'."

[Michael H.]

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

I've mentioned this on several other occasions when questions about octane requirements for the higher compression engines has come up, but have never gotten a reply:In an older "Corvette News",which, as you know, was written by GM and sent to Corvette owners, warned against setting timing "by ear , which some such as Duke suggest, since there is a form of detonation that can be detected only by special equipment and leads to significantly increased ring and cylinder wall wear .Wonder if some who say they have no problems with just pump gas do very regular pulls to the redline like these cars were meant to be driven when new or are just out for a cruise? Would like to hear a response based on engineering fact about this inaudible detonation with too low octane and too much ignition advance.In view of this article (I can give exact reference if desired) don't think it's prudent to say, "If you don't hear detonation,' forgetaboutit'."

William,

That's a very interesting point that most seem to ignore, or do not understand.

[John S.]

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

I can give first hand knowledge of California 91 octane in my original 35k mile 65 Fuelie. Car has original .018 head gaskets and original intake gaskets between manifold and heads. Duke here can attest to the fact that it does not detonate/ping even in 4th gear going up a hill starting at around 1500 rpm. Car is tuned correctly with what I believe is the original vac advance. This car also "runs like Forrest Gump" when you put the peddle down. I think Duke will also attest to that.
Here is where it gets funky though. At ambient temps above 90 degrees it will tend to percolate in the spider at idle pretty consistently. I do believe this is due to the ethanol and is a common problem. Have to keep the idle up with my right foot to keep it running. I put VP 110 octane race gas in it recently and it did take care of the percolation problem. Funny thing is the car actually runs better overall with the pump premium in it. I can only attribute this to the fact that the 380 unit has been worked on a bit and tuned up to the pump gas.
On another note, welcome back Duke!

[Duke W.]

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

Glad you chimed in. There was a discussion on 'that other board' regarding the association of RVP with percolation and vapour lock. It was this same discussion that first advanced the increased usage of butane as a possible culprit. What say you?

Butane has traditionally been added, especially to winter blend fuel, and especially in very cold climates, to aid cold starting. I'm not sure how much is used nowadays versus the sixties, but given butane's low boiling temperature of 30F, likey less can be tolerated and meet today's low RVP requirements, but I'm sure you have a lot more in your Canadian winter blend than Florida's winter blend.

Modern port injection engines exhibit quicker cold starts with a lower RVP/higher 0-10 percent boiling point temperature fuel than vintage carbureted engines. Since most of us don't drive our vintage cars in temperatures less than about 40F, we rarely hear complaints about slow cold starting with modern gasoline. Direct injection engines can tolerate even lower RVP/higher 0-10 percent boiling point fuel and still have excellent cold starting characteristics.

RVP has been lowered over the years to reduce evaporative emissions in modern vehicles, and since RVP is measured at 100F, this implies higher temperature 0-10 percent boiling points on the distillation curve. Simultaneously, the E10 20-50 percent boiling points are lower with the greatest reduction at the 40 percent point - down to about 160F from 190F for conventional summer blend straight gasoline. (Ethanol boils at 170F.) The boiling points of 100LL avgas and most racegas in this range are higher than mogas, which is why they are much more resistant to vapor lock and percolation flooding than modern mogas.

The reduction of the mid range boiling points due to ethanol has little to no effect on modern vehicle exhaust emissions or operating characterisitics, and apparently no thought was given to the considerable increase in evaporative emissions and degradation in hot weather operating behavior of vintage vehicles when the all-knowing US Congress passed the "renewable energy" legislation a few years ago that effectively forced refiners to use ethanol in virtually all commercially available gasoline.

I started a thread a few months ago asking for IR temperature measurements on fuel system components beginning with the forward end of the fuel supply pipe that runs along the frame rail, then onto the fuel pump, pump to carb line, and carb bowls. The temperatures were higher than I would have guessed, and when the temperature of the forward end of the frame pipe is already near the 40 percent boiling point of current E10 gasoline, it's no wonder that vapor lock and hot soak percolation flooding are an issue, especially in hot climates and low speed traffic.

Several owners saw meaningful drops in fuel system component temperatures and better operating characteristics by fitting sleeves of insulating material (like on the wire from the coolant temperature sensor) to the pipes and you should start with the end of the frame pipe since this area is already very hot - probably due to passing close to the exhaust pipe/manifold.

As far as what you hear on the Corvette Myth and Misinformation Forum, whenever the discussion gets remotely technical, I would take 99 percent of it with a grain of salt. With a few exceptions, most of the participants are clueless when indepth technical understanding is required to sort out an issue. Even simple questions can get dozens of responses with about as many answers.

If you want to understand modern motor gasoline technology, downloading and studying the following pdf will be a lot more productive than spending time on internet forums.

http://www.chevronwithtechron.com/products/documents/69083_MotorGas_Tech_Review.pdf

You don't have to get beyond the distillation profiles on page 2 to understand why vapor lock and hot soak percolation flooding is a problem for many vintage car owners.

Duke

[Duke W.]

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

Ethanol does not boil at 125F. The correct boiling point of ethanol is stated in post #27, paragraph 3.

NO GASOLINE BLEND HAS A SPECIFIC "BOILING POINT."

Gasoline contains hundreds of hydrocarbon species that have boiling points ranging from less than 100F to greater than 350F. Download the referenced pdf and get up to speed on gasoline distillation curves. Then go educate bubba the mechanic.

Inaudible detonation is not an issue in high powered light weight vehicles like Corvettes because they don't operate under significant load for more than a few seconds at a time in normal road driving. It could be an issue with heavy gasoline powered vehicles that spend a lot to time climbing mountain grades like motor homes or medium duty trucks, so, just to be on the safe side, they should have less aggressive spark advance maps than the same configuration engine in a Corvette.

As I said in my San Diego presentation (see post #16), the ragged edge of detonation, and I'll expand this to say audible detonation, is the most efficient operating point for most speed/load conditions assuming fuel octane is slightly marginal for the engine configuration, and a little audible transient detonation is okay. If worse case conditions - high ambient or coolant temperature and high ambient pressure cause more than just a little transient detonation on upshifts or upon an increase in throttle, I suggested ways to "drive around" it.

I've never seen any more than light-non lethal detonation damage - like tiny erosion craters in piston crowns - in engines other that those that had clear, audible, sustained detonation over a period of time. There's not much engine noise isolation in vintage Corvettes, so even if you're old and your hearing acuity has normal age degradation it's mostly above 10kHz, and detonation is in a much lower frequency range. So if you can't hear sustained detonation, which would be more than about a second of transient detonation, you don't have a problem. If you wear a hearing aid, get a young buck to ride with you and listen for detonation.

There's no smoke here, so don't try to fan a fire. Worry about something more important like the last time your Corvette got a coolant, brake fluid, transmission, axle oil change, and distributor overhaul and lubrication. Many of you probably don't know, especially if your vintage Corvette is a recent acquisition.

Duke

P. S. My San Diego presentation last year was classified Top Secret, NoForn to all but those in attendence, but Wikileaks posted it to the Web, so now anybody has access to the secrets.

[Michael W.]

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

Download the referenced pdf and get up to speed on gasoline distillation curves.

+1 (as the kids say). The paper deals with far more than distillation curves and sets the story straight on many of the myths and misconceptions. Not sure if the OP is still around or has run for the hills, but it answers pretty much all the questions he asked in the original post.

124 pages of technical discussion and data is not the easiest of reads but it sure is better than hearing another round of the sky is falling.

[Duke W.]

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

Ah, gee, you probably scared everyone off with "124 pages". A lot of those pages have easy to read charts and graphs that take up a good deal of the space. It's a "technical discussion", but geared to the general public that has an interest in this issue.

Most of the answers and information relative to questions about fuel for vintage vehicles are answered in the first chapter. Anyone who can change a flat tire on the road without calling AAA should get a lot out of the first chapter - or one can just keep wasteing their time searching for answers on car guy forums. Maybe that's more entertaining for some.

Anyway, kudos to Chevron for publishing this work and to Mike Ingahm, our Western Regional Representative, retired Chevron engineer and a fellow graduate of the University of Wisconsin Engine Research Center, who is one of the principal authors.

Maybe you can save some of the lost souls on the other forum by posting the link over there.

Duke

[Michael H.]

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

I can give first hand knowledge of California 91 octane in my original 35k mile 65 Fuelie. Car has original .018 head gaskets and original intake gaskets between manifold and heads. Duke here can attest to the fact that it does not detonate/ping even in 4th gear going up a hill starting at around 1500 rpm. Car is tuned correctly with what I believe is the original vac advance. This car also "runs like Forrest Gump" when you put the peddle down. I think Duke will also attest to that.
Here is where it gets funky though. At ambient temps above 90 degrees it will tend to percolate in the spider at idle pretty consistently. I do believe this is due to the ethanol and is a common problem. Have to keep the idle up with my right foot to keep it running. I put VP 110 octane race gas in it recently and it did take care of the percolation problem. Funny thing is the car actually runs better overall with the pump premium in it. I can only attribute this to the fact that the 380 unit has been worked on a bit and tuned up to the pump gas.
On another note, welcome back Duke!

John,

I agree with what you are saying. My 63 FI car that's never been apart seems to run very well on E10 fuel. I don't think I've ever heard a ping/knock, even at low RPM with a load. I do have the hot idle and hot restart issues that you mention though. All FI cars had hot restart issues, even when new with straight gasoline.

My 66 425 HP is an entirely different story. When fuel changed from 5% to 10% ethanol several years ago, that's when the problems began. Poor idle was only the beginning. Hot restarts, fuel boil over when shut off hot and completely empty carburetor by the next morning.
I fought this problem for two years until ethanol free fuel became available here. Now the car runs great, just like it did years ago.

[Mike E.]

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

Mike,

"Quench", or the volume left between the flat part of the piston crown and the flat part of the head (i.e.: outside of the combustion chamber) is a very important factor in determining detonation resistance. Vintage engines built during the 50's and 60's had wide variations in quench compared to modern engines, on the order of .020" or more. The nominal quench was specified at .050" (nominal piston to deck clearance) plus .018" (compressed steel head gasket thickness), or .068". Keep in mind that the piston to deck clearance could have varied by as much as about .020" within one engine and between engines, which yields quench anywhere between .058" and .078" . The tighter the quench, the less the tendency to detonate, and conservative minimum "safe" quench (before piston to head contact occurs) is about .040". Factors such as piston material, piston to bore clearance, connecting rod strength and modulus of elasticity and maximum RPM affect minimum safe quench.

If an engine is very carefully designed and built, safe quench has been achieved at as low as .025"! Obviously, an engine whose quench is .025" will have less tendency to detonate than one whose quench is .068", all else being equal. The engine that was mentioned in post #15, above, was designed and built with quench of .032" (plus/minus .0005"), as well as with special cooling capability and very low intake manifold temperature.

Joe,
Thanks for that info! In another thread I saw you mentioned David Visard's methods of working heads so I picked up one of his books on the topic. I actually bought one of his other books too on How to build Max-Performance. I've only gotten through a couple of chapters but I must say he makes a somewhat dull subject very interesting. He really pushes the idea of a flow bench and describes how to build a do-it-yourself flow bench using a shop vac. This you can actually measure flow improvements. I'm just getting to the part where the talks about wet benches and "Quench pad" and frankly I'll probably have to read it several times before I start to grasp it.

I have a set of virgin 461 casting I would like to start with & would like the engine to retain a stock appearance. Do you feel these casting are a good starting point?

Currently I'm just doing a bunch of reading and accumulating some parts before I start any type of build, and believe me, I've go a lot to learn.



Mike

[Michael G.]

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

As I've mentioned here a few times and as Duke referenced, a fuel injected car can easily be altered so that it runs without percolation, on on any gas, so long as its pump octane is 91 or higher. The key is to keep the fuel lines as cool as possible. I altered the fuel lines on my stock 63 L84 black car such that it does not percolate, even when using 10% ethanol fuel. This is because the fuel temperature as it enters the fuel meter is now approximately 30 degrees cooler than it used to be. It has had no problem with stalling, variable idle, or difficult re-start since I made the changes, (over a year ago) no matter what fuel I use or what the ambient temperature.

My blue car just finished its Duntov, so I am about to alter its fuel lines also, in order to allow me to drive it more often without percolation problems. I don't intend to use ethanol fuel in it, but my changes also eliminate the high temperature percolation historically seen with non-ethanol, leaded gas (as Michael Hanson just noted). As I make the changes I will be writing an article for the restorer. If anyone wants help making the necessary changes to their car (they are simple, cheap, and easily reversible) before the article gets published, just PM me and I will help you.

One other note regarding use of ethanol in a Rochester FI unit. While the percolation issue may be solvable, I try not to use 10% ethanol pump gas in either of my cars. There are several problems with using it, I'd suggest you all read Jerry Bramlett's web site "Ramjets That Run" for insight into the issues. One particular concern is that the water held in solution by the ethanol component of the fuel mix will cause fairly severe corrosion of the fuel bowl, if it remains there long. As such, if I need to refuel on he road and can't find racing gas or 91 octane non-ethanol fuel, I fill up with 10% ethanol premium, but make sure I burn it all as soon as possible.

[Domenic T.]

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

I am not looking for a argument but I would like to see the Dyno results when you switch to pump gas on a car that is already running on the av gas mixture.

I hear a lot say PERFECT on pump gas!! I would safely bet that pump gas supporters have NEVER tried av gas mixed with pump.

I have a 425HP in my Chevelle and had a 365HP in a vette and when tuned correctly to FACTORY spec they ran better than PERFECT, untill I filled the tank with pump gas. The engines continued to run (diesel) then blow thru the carb and die. I could solve that by retarding my timing and giving up the HP the FACTORY engineers built into the engine that was designed fgor our pump gas at the time.

My shop is on the airport so I can get away with a burn out or two and my burn outs are over double when using a better than perfect av gas mixture.

Some one step up and do a test. Mine are all judged on the lack of performance using straight 91 pump.

I am also licenced as a aviation mechanic and there has been auto fuel approvals (STC's) for the lower compression aircraft engines (about 6.0:1 to 7.5:1) but with problems. The engine timing is from 26 to 28 on most and the 8.0:1 and higher need the slow/even burn rate of the 100LL.

Some that raced in the days these engines were designed would remember using SONOCO high octane to get MAX performance on the strip.

What about NITRO?? Why did we use it to increase HP & performance??

Actually you will " knock it if you don't try it".

DOM

[Joe C.]

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

Joe,
Thanks for that info! In another thread I saw you mentioned David Visard's methods of working heads so I picked up one of his books on the topic. I actually bought one of his other books too on How to build Max-Performance. I've only gotten through a couple of chapters but I must say he makes a somewhat dull subject very interesting. He really pushes the idea of a flow bench and describes how to build a do-it-yourself flow bench using a shop vac. This you can actually measure flow improvements. I'm just getting to the part where the talks about wet benches and "Quench pad" and frankly I'll probably have to read it several times before I start to grasp it.

I have a set of virgin 461 casting I would like to start with & would like the engine to retain a stock appearance. Do you feel these casting are a good starting point?

Currently I'm just doing a bunch of reading and accumulating some parts before I start any type of build, and believe me, I've go a lot to learn.



Mike

Some credit belongs to M Williams, who pointed me toward Mr. Vizard. Further research caused me to push the envelope by combining Mr Vizard's old principles with current technology, especially within the combustion chambers. The results were better than Mr. V predicted and very remarkable as far as real world power output. The 461 is an excellent head to begin with, with 3 caveats:
1. DO NOT have hardened seats installed
2. Have them pressure and crack tested before investing time and energy into them
3. The use of the homemade thickness measuring tool as described by Mr Vizard is essential

When carefully ported, the intake ports can safely be enlarged to almost 185 cc.

Once the heads are properly ported, then the use of a modern camshaft with high lift and fast dynamics is warranted, which makes elongation of pushrod slots, screw in rocker studs with hardened guides, and probable enlargement of the spring pockets necessary. They will flow better than many new, expensive aluminum heads, and you will have bragging rights as to the prodigious power output using double humps!

[Joe L.]

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

Yes, another advantage of unleaded.

Mike-----


As I've mentioned before, the late John Lingenfelter (who reportedly knew something about high performance) once said that the best thing that ever happened for performance was unleaded gasoline.

[Joe L.]

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

Michael W- help me understand your statement. I have a 64 and 67, both with original 11:1 compression. I'm not too worried about valve beat in from unleaded fuel but I have always been told that you will have detonation (you can't always hear it so car may appear to run great) if you don't have higher octane than our local 93. There is a very well regarded engine builder in our area ( built a 427/425 for me several years ago) who refuses to built a motor with anything higher than 9.75:1 CR. I drove the 67 to B Gold last month (300+ miles). It ran fine on pump gas but again, I am not sure I would be aware of detonation. Is there some resource you can direct me to that can give more detail/ test results etc. I'm going to start a frame off on the 67 next month and will go through the engine as a part of that. I would love to go back with domed pistons and retain 11:1 but have assumed I would have to do flat tops. Regards, Jim B

Jim-----

Here's a "clue": all current GM big block crate engines with cast iron heads have a compression ratio of 8.75:1 or lower AND require 91 minimum octane fuel.

Most current GM big block crate engines with aluminum heads have a compression ratio of 9.6:1 and require 91 minimum octane fuel.

Big bore engines are more susceptible to detonation than smaller bore engines.

Notwithstanding the above, if, regardless of bore size and compression ratio, you have an engine which experiences no detonation on, say, 91 octane pump gas, then you don't need higher octane. Period.

[John D.]

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

Didn't really want to get involved with this subject but here is how I handle it at the Shows and on the phones when customers say bad stuff about 100LL.
AT the fleas I just smile and nod my head and listen to the negative comments. I am not one to argue. Then when the customer walks away I completely ignore his/her comments about 100LL.
When my 63, the LWC (little white car) was judged at Marlborough and went thru the ops one judge said JD just what are burning in that thing?. Smells sweet. You know the answer.
Biggest reason I use it in everything I own that has a spark plug is it can set for an eon and never gum up. Once you start cutting it with crap gas though that really takes away from the effects of 100LL.
Anyhow I didn't read all your posts and typically don't.
You old guys remember Sunoco 260. Our Corvettes loved that stuff. My Dad only used Amoco White gas as he called it. No lead in it. That's all he would use in the old days. Later as back to work. CC is in 18 or 19 days. Whew!!!!