Pump Gas And Detonation

Eliminate Engine Detonation Without Losing Power and Throttle Response
Knock, Knock, Go Away
Understanding the Causes of Detonation to Better Help You Eliminate It In Your Engine
From the April, 2009 issue of High Performance Pontiac
All contributors: Ray T. Bohacz
 
The demon of engine knock is something an owner of a traditional high-performance Pontiac knows all too well. An engine designed when 102-octane high test was at almost every fuel station in the country being forced to run on today's gas means that ping has become an all too common companion, and what if your engine was rebuilt by a previous owner? You may not even know the compression ratio. If octane boost or race gas is not in the budget, the typical frustrated hobbyist's response is a twist of the distributor to retard the ignition timing.

With today's pump-gasoline prices, the thought of filling up a big Pontiac fuel tank is enough to wreck the budget, even without the expense of high-octane supplements. The problem with retarding the ignition timing to eliminate detonation is power, throttle response, and fuel economy also go away with the knock. Less ignition lead, though necessary in some circumstances, isn't the cure for every engine with an appetite for octane--Pontiac, corporate, what have you.

When fighting ping, retarding the timing should be the last resort, not the first approach. There are, of course, instances when even retarding the ignition timing and adding octane doesn't eradicate pinging. This is not often the case with a properly built, tuned, and maintained Pontiac engine unless you're trying to run a very high compression ratio on the street. It proves there is more going on to create detonation than just fuel quality and ignition lead. The battle must be fought systematically as opposed to randomly.

What is Octane?
Tuning a Pontiac engine begins with an understanding of gas octane ratings. The calibration of the fuel and ignition system should be set for the octane level of the car. Thus, if you want to burn 87-octane fuel, the tune-up is calculated for that grade of gasoline.

By definition, octane is a fuel's ability to resist auto ignition from pressure and heat. Thus, the fuel doesn't ignite, instead waiting for the arcing of the spark plug. The higher the octane rating of the gasoline, the more pressure and heat it can withstand before self-igniting. Conversely, lower octane fuel ignites independently of the arcing of the spark plug through either pressure or heat or a combination.

The chemical isooctane is considered a hydrocarbon 2, 2, 4 (a trimethylpentane), which has eight carbon atoms. It's used as a primary reference fuel with an assigned octane rating of 100 for both RON and MON, and n-heptane has an assigned value of zero. The octane number of a fuel is the percentage of isooctane in a blend with n-heptane that gives the same knock intensity as the fuel when evaluated under test conditions in a standard engine. Oil companies use what is called a CFR (Cooperative Fuel Research) engine to determine octane. It's a single-cylinder, overhead valve, variable compression-ratio engine designed for the purpose of testing gasoline.

During the past few years, the octane rating of street gasoline that is posted on the pump is an average of the RON and MON. That is why you see the octane listed as R+M/2. Many years back, there was only one octane rating on the pump, and it was not an average of the two test methods.

RON stands for Research Octane Number and measures the antiknock quality of a gasoline as determined by the ASTM D 2699 method (ASTM stands for the American Society of Test Methods). It is a reference to the antiknock performance of a fuel when the vehicle is operated under mild conditions such as low speeds and light loads.

The Motor Octane Number (MON) measures the antiknock quality of a fuel as determined by the ASTM D 2700 method. It is a guide to the antiknock performance of a fuel under relatively severe driving conditions such as at wide-open throttle when the incoming air temperature and engine speed are both high.

Due to the way fuel is rated today, many experts in the petroleum industry are quick to note the octane rating is not as low as one would think when making a comparison to the old days. Many believe that today's 93 octane fuel calculated as R+M/2 is equal in the antiknock quality of the 97-98 octane-fuel years ago.

But keep in mind that during the 60s, fuel of this antiknock rating was considered regular grade. Most, if not all, gas pumps list the octane rating but don't list the RON or MON independently, only the average. This information would be helpful when fighting a pinging problem. For example, if your Pontiac only knocked at light load, then a fuel with a higher MON is needed. A higher RON value most likely wouldn't do anything to help the problem. But since gasoline is what it is and the retailer will not list the octane ratings separately unless mandated by law, we need to accept the facts and learn to work around them.

Knock, Ping, and Other Noises
In a spark ignition (SI) engine, the combustion event should be initiated by the arcing of the plug electrode, and the air/fuel mixture should burn smoothly and evenly across the bore. When this occurs, it is known as normal combustion. Anything else is identified as abnormal combustion. On the hobbyist level, we refer to any abnormal combustion event as ping, knock, or detonation. Within the engineering community, where the abnormal ignition event occurs in relation to the piston's position in the bore identifies whether it is pre-ignition, post-ignition, knock, or ping.

Some abnormal combustion is the result of two colliding flame fronts: the first started by the arcing of the spark plug and a separate, rouge flame that was the result of auto-ignition. For this reason, before making any tuning changes to your Pontiac, you should experiment with different brands of gasoline. The RON and MON values can be manipulated to achieve the same average and will impact the way the engine runs. You may find your Pontiac runs perfectly on a certain brand of fuel and may require no further tuning.

In an SI engine, auto-ignition of the end gas in the combustion chamber causes a knocking or pinging sound. When this occurs, engine damage is created by the sharp pressure rise and roughcombustion taking place. Localized pressure peaks and the explosion, rather than the burn of the fuel, causes the piston, rings, and rod bearings to shake and make a sound similar to a ball-peen hammer hitting a metal surface. Over time, an engine that constantly pings will show excessive wear in the rings, cylinder walls, and rod bearings. If the detonation becomes severe, the possibility of burning a hole in the piston or lifting a ring land away is very high. This usually occurs when the abnormal ignition event occurs during high engine loads.

For our discussion, we aren't concerned with when the pressure spike and uneven burn occurs, so we're quantifying all abnormal combustion under detonation.

The Causes of Abnormal Combustion
Eliminating the octane rating of the fuel, one word can describe the cause of detonation: heat. Excessive heating of the combustion chamber, incoming charge, piston crown, and so on, through either external causes or from high pressure, increases the temperature in the bore. As this value escalates, the fuel enters a point where auto-ignition can occur. Many times the fuel may be on the verge of detonation, and an engine may be sensitive to a difference of one octane point.

Once detonation occurs, both the piston crown and combustion chamber become super-heated, which then makes further abnormal combustion events much more likely. For this reason, once an engine starts to ping during a particular driving or load scenario, it usually doesn't stop until the previously mentioned parts cool. Pontiacs equipped with electronic engine-management systems that incorporate a knock sensor acknowledge this and identify it as the hysteresis of knock. Once knock is identified, the engine controller retards the ignition timing an excessive amount, allowing the piston and combustion chamber to cool so the detonation can be controlled. GTP owners who drag race know all about this.

As with any driveability related issue, the problem is often the culmination of many different things, not a single item. A Pontiac that is prone to detonation can be suffering from many different issues that, when put together, result in poor performance or a need for high-octane fuel. A common mistake made during diagnosis, by both the professional and hobbyist, is not acknowledging how the little areas of error can create a bigger problem--a perfect example of the whole being greater than the sum of the parts.

When tackling detonation, always confirm and eliminate each area of concern first. Don't take the mindset that an area is close enough or not out of tune enough to cause the problem. From the author's experience, this mentality often results in failure and then forces drastic steps to cure a problem that could have been repaired easily.

Keep in mind that the goal of this primer is not to get a 500 hp 455 to run on 85-octane fuel. Certain internal design elements drive the required octane of an engine. Factors include the compression ratio, combustion-chamber design, spark-plug location in reference to the cylinder-bore center, cylinder-head material, length of the connecting rod, and camshaft profile, to only name a few. The goal of this article is to show our readers how to possibly either use a lower-octane fuel without a loss in performance or how to make sure your engine is producing the most power and best driveability for the octane you are feeding it.

With premium-grade fuel costing approximately twenty cents more per gallon, savings are quickly realized if you can safely switch to regular without a performance or durability penalty. It's all too easy to add octane to the problem by purchasing more expensive gasoline. It's analogous to keeping the windows open in your house and turning the thermostat up higher to keep warm. None of us would do that, but we think nothing of spending too much at the fuel pump for the results delivered.

Due to the eclectic mix of areas that either create or contribute to detonation in a Pontiac engine, they are listed and explained separately in no particular order, so review all of them before tackling a knocking Pontiac. Some of this information is applicable not only to vintage Pontiacs but corporate V-8 engines and even V-6 and four-cylinder powerplants.

Heat Riser
Older Pontiacs (before early fuel evaporation with CCC feedback carburetor systems) used a heat riser passage that brought hot exhaust gas under the carburetor during cold engine operation. This improved the vaporization rate of the gasoline in a cold engine. If the heat-riser valve fails and remains open, the exhaust gas heats the incoming charge even after the engine has fully warmed. A stuck heat riser creates the need for higher octane fuel.

Air Filter Snorkle
From the late '60s on up, carburetor-equipped Pontiacs have a hot-air stove that connects to the air-cleaner snorkle along with a vacuum motor. It's task is to close a flap in the snorkle and allow the engine to breathe heated air from the exhaust-manifold stove. It was designed to aid vaporization and hasten choke engagement. As with the heat riser, if the vacuum motor doesn't open the snorkle to feed cool air, the engine is prone to ping. This can be confirmed visually or by removing the air-filter top so fresh air is inducted. If the detonation is reduced, your engine is probably breathing preheated air.

Vacuum Leaks
Lean mixtures create higher combustion temperatures and increase the propensity toward detonation. But often neglected is vacuum leaks that are unique to a single cylinder, as with a poor or loose intake-manifold to cylinder-head connection. Often detonation occurs on only one cylinder and can be traced to a leaky intake gasket. Always check for individual cylinder vacuum leaks, even small ones that cause a light ping, usually at part throttle.

Fuel Distribution
Poor fuel distribution from a misadjusted/mistuned carburetor or injector flow variation with port EFI are a sure way of creating detonation. Again, lean cylinders want to detonate while the richer bores are happy since the fuel is acting as a coolant. If your engine is dyno tuned before it's bolted into the car, monitor exhaust-gas temperature in each header pipe to determine fuel distribution. Don't check only EGT at full throttle; be concerned with light load operating conditions when pinging on the street may occur.

Exhaust Gas Recirculation
From 1973 forward, most Pontiacs were equipped with EGR valves. The purpose of this emissions-control device is to introduce inert exhaust gas at part throttle and light load to take the place of combustable mixture. Its effect is lower cylinder-operating temperatures and reduced oxides of nitrogen emissions. Various Pontiacs produced from 1977 to the late '80s are prone to severe detonation, especially at light load, if the EGR system isn't functioning properly. It's common for either the valve to fail or the passage in the manifold to plug up. Since there are many styles of valves, always reference the proper shop manual for the correct test protocol. Regardless of the valve design, a quick test is to lift the diaphragm with your fingers from underneath the valve while the engine is idling. This should cause rough running or a complete stall. If there is little or no change, the passage is carbon blocked and not feeding exhaust gas. Just make sure you wear insulated gloves before doing this, or you'll have toasted fingers!

Cooling System
The radiator cools the liquid while the coolant removes heat from the cylinder heads. Metal-surface temperatures of the combustion chamber have a huge impact on the engine's ability to detonate. Advanced, waterless coolants, such as the Evans NPG+, create lower metal-surface temperatures and limit detonation in Pontiac engines. If traditional anti-freeze is used, employ a pressure cap of at least 15 psi, and use a proper 50/50 mix of coolant with water. Higher concentrations of EG-based antifreeze increase combustion chamber metal temperature.

Cooler thermostat opening ratings, such as a 160 or 180 degrees F, help but not as much as one would think. It may make the temperature gauge reading look better, but it may have minimal impact on the metal-surface temperature of the cylinder head.

Rate Of Ignition Advance
The amount of ignition lead at a given load and engine speed are crucial in eliminating detonation. This is in contrast to twisting back the distributor. The rate of ignition advance in a Pontiac engine is a two-dimensional graph. Too much advance at a given load and speed creates excessive heat and a rouge flame front. Too little advance allows potential energy from the end gas to exit the exhaust port unburned. An adjustable vacuum advance and distributor recurve is the proper approach for an efficient advance curve while maintaining octane tolerance.

Spark Plug Heat Range
The Aug. '06 issue of HPP had a complete primer on spark plugs and should be referenced before attacking a detonation problem. Keep in mind that a glowing spark-plug electrode is a sure way of starting and maintaining abnormal combustion.

Carbon
Gasoline is a hydrocarbon-based fuel, and the end result of combustion is carbon deposits. An increase in required octane occurs in an engine over the first several thousand miles of its life due to the build up of carbon and other deposits in the combustion chamber and piston crown. These deposits not only retain heat, but slightly increase the static compression ratio by decreasing the volume at TDC. "Performance with Efficiency," Part 1 in the June '06 issue provides a full explanation on how to keep your engine carbon free. The rate at which deposits form is influenced by driving mode, gasoline composition, and the presence of other additives such as those used in the engine oil.

Determine What Is Happening
The first step in solving a Pontiac's appetite for octane is to get a handle on how and when the detonation occurs. Does it happen only at part throttle or all the time? Is the ping only there at the top of the gears at full power? Does air temperature and humidity affect the condition ("Weather or not you go faster ..." HPP April '06). Do different brands and grades of gasoline help any?

As stated in the beginning, there does come a time when the distributor needs to be moved and the total timing retarded, but only after all of the areas that impact abnormal combustion are addressed. Many hobbyists believe Pontiacs will always ping on today's fuel. There is nothing further from the truth. They ran great years ago, and with a little attention to detail, they can run perfectly on today's modern fuel. So put this magazine down, get out in the garage, and eliminate the knock from your engine!



Click on Link Below, to Read more:
http://www.highperformancepontiac.com/tech/hppp_0609_automotive_engine_detonation/viewall.html#ixzz2GMHPE7b7

 
When The Flag Drops,,,



The Bull Chit Stops,,,


P. Engineer, Engine Builder

Good post TN.
Yep, seen it too many times were people try to fix a detonation problem by retarding the timing only to make things worse.
Not to mention some engines will even over heat do to the retarded timing.

Imo the best way to not have detonation is to build a engine with a conservative CR and run the proper octane of fuel the engine requires.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races

Again Tuff,
thanks for posting the actual article. I just posted the link in Rustyskills thread..  
Jim,

Good read Tuff, and true! Things have changed a lot with cars and fuels. New and stock, mundane passenger cars are running 11.5:1 compression and more (yikes!) on the dishwater they sell at the gasoline pumps these days, thanks to direct injection and other modern features. They are able to safely use a variety of fuel qualities by merely retarding spark on the fly to control combustion chamber temps, and the engines go hundreds of thousands of miles, some of them even with boost, and never hurt a piston. All the while being amazingly efficient and powerful.

But the trusty Pontiac V8 has no such technology available that compares to this, that I am aware of. Even the aftermarket EFI doesn't sense knock. While Pontiac V8s were in production a knock sensor was never developed for them. So we are left in a state of constant guessing and de-tuning (by de-tuning I don't mean JUST cranking the distributor back but rather addressing the design issues that affect an engine's fuel quality requirement) to safely operate our engines and compromise to match declining available fuel quality. I doubt we will ever be blessed with a multi-port or direct-injected setup for Pontiac that adjusts for fuel quality because R&D would be too costly, territory where only OEMs dare tread. I speculate that if a Pontiac engine were outfitted like that, it would produce results as remarkable as the rest of the world.

In the meantime- To anybody going to the gas station all the way across town for ethanol-free "91" premium (thats how it is in my area) and tuning to match it ... you ought to give "91" with ethanol a try. I find its MORE knock resistant. The drawback is you can't leave the same gas in your tank for months or corrosion can be an issue. All just thoughts from 'junk.  

Great Post Tuff. That would have taken me a week to type.

  So many times we forget about the top of the pistons getting carboned up. I use Techron every oil change. For the injectors.  When I did work for the Chevy dealer I was in touch with GM engineers.
  I used the Top End Cleaner GM has. That conviened me about how well that stuff works. GM had a few high performance 454 SS trucks. The dealer had one that could not stop pinging. Light throttle it sounded like someone dropped marbles into a metal pan. I suggested pulling the heads, GM didn't want to do that? Maybe the flame travel was a problem. Noone ever pulled the plugs out of it either. MY thinking with all the detonation maybe the plugs were damaged. I've seen them arc and fire off the cylinder head, or piston top.
  I told the engineer we used water back in the day to clean the piston tops. But you better know what your doing. Too much water and you could hydraulic the engine. We did it on Jet engines also. First we used ground up walnut shells, to remove what was called coking. Or the turbine blades coated with fuel residue. We watched the fire go out the back of the engine. It smelled nice too. Then we used high pressure water, and they ran ran the Turbine up and we put water in the intake.
  That was an art you didn't want to drown it, if you put the flame out you were in trouble. But the point was to get the HP or thrust back. Sometime a problem Jet engine if shutting it down and restarting it caused problems. It like letting your car idle all day while your at work.
  If you read some of the Henry Ford's engineers original procedures. They used finely crushed walnut shells, in the induction and checnged the RPM    
  The engineer told me to use this stuff,BUT make sure I didn't use it near any cars. I could not believe the stuff that came out of the exhaust. It stopped the ping.    
 
  Back in the 70's there was a small auto parts store that was dealing with OTC Cylinder Heads.
  Well a few years later when the push was on to get the compression from 10:5 to 8:5 was over. One day the owner said do you want some cylinder heads.I went to his storage room. The cylinder head company didn't want the High Compression heads. He said $5.00 a head or $8.00 a pair. I was like a kid in a candy store.
 
  I pulled  10  pair of SBC heads, 2 sets 291, 2 sets 461, 2 sets 461Xs, 2 sets 462. 1 041 casting.  almost all were sold. 1 set was stolen.

2 heads for the Cosworths. My neighbor bought both of them

2 sets BBC heads, 2 sets aluminum Crane Fireballs. Crane Fireball heads where a hot set-up back in the day.

Pontiac heads there were only 4 heads. Sold them

Chrysler heads  there were 3 sets. Sold them I remember there was a set of Chrysler heads that had burnt valves so bad they burnt the seats and head.

2 sets BBF FE heads. I heard Ford Guys were installing thicker head and intake gaskets, and some guys took some cam away to cut down on overlap.

Bob  

idrivejunk:
Good read Tuff, and true! Things have changed a lot with cars and fuels. New and stock, mundane passenger cars are running 11.5:1 compression and more (yikes!) on the dishwater they sell at the gasoline pumps these days, thanks to direct injection and other modern features. They are able to safely use a variety of fuel qualities by merely retarding spark on the fly to control combustion chamber temps, and the engines go hundreds of thousands of miles, some of them even with boost, and never hurt a piston. All the while being amazingly efficient and powerful.

But the trusty Pontiac V8 has no such technology available that compares to this, that I am aware of. Even the aftermarket EFI doesn't sense knock. While Pontiac V8s were in production a knock sensor was never developed for them. So we are left in a state of constant guessing and de-tuning (by de-tuning I don't mean JUST cranking the distributor back but rather addressing the design issues that affect an engine's fuel quality requirement) to safely operate our engines and compromise to match declining available fuel quality. I doubt we will ever be blessed with a multi-port or direct-injected setup for Pontiac that adjusts for fuel quality because R&D would be too costly, territory where only OEMs dare tread. I speculate that if a Pontiac engine were outfitted like that, it would produce results as remarkable as the rest of the world.

In the meantime- To anybody going to the gas station all the way across town for ethanol-free "91" premium (thats how it is in my area) and tuning to match it ... you ought to give "91" with ethanol a try. I find its MORE knock resistant. The drawback is you can't leave the same gas in your tank for months or corrosion can be an issue. All just thoughts from 'junk.

Yes, great post.

Idj-some aftermarket EFI's do offer knock sensors.

But it's not just spark timing that effects detonation as stated. But also one thing that I didn't see mentioned specifically was proper air/fuel ratios at any given load and rpm. Because once detonation starts it raises combustion temps. You can't just retard it a few degrees. You need to go past the previous "best" setting in order to cool the temps down to non-detonating conditions.

Case in point, my 9.7:1 iron headed '72 Olds wouldn't run well on anything less than 93 octane when it had a conventional distributor and 750 Edelbrock carb on it. I changed jets, metering rods, timing curves etc. It was what it was.

Then came my multiport EFI conversion. Same intake, same cam, same compression ratio.

It has now been running flawlessly on 89 octane for 8 years. No ping, knock etc. And has more power than ever before. And there are points in my spark map in the mid 40's. No knock sensor and no issues.

My point? A good engine management system will go along way to maximize any combination, whether it's an 11.0:1 aluminum headed modern engine or an iron headed V8 muscle car. There are a lot of other variables that will dictate octane requirements.

Ray did a tuning class for Accel some years ago. While he is a very knowledgeable man his EFI class instruction was less than stellar. He ultimately left out a lot of info.  
Mark
Engine builder, located in Central Florida

"Idj-some aftermarket EFI's do offer knock sensors."


fastoneefi- Is a street EFI system for the Pontiac V8 that senses knock and alters the tune accordingly available? Please tell me who makes it, I would like to research it. Oldsmobiles probably stayed around long enough to get one but I thought nobody had developed a knock sensor for the Pontiac V8 block, and I don't think one for another engine could be as accurate as if GM had designed it. I was aware that when knock is heard, the amount of KR is an awful lot on some vehicles. Enlighten me.

As far as I know at this time, current OEM engine management systems only alter spark advance to compensate for knock with the exception of "flex-fueled" vehicles. Of course fuel mixture (or valve timing if the design permits) is the next variable down the list to alter, when a tank of crummy gas confronts the engine controller. Its always going to administer the least fuel possible for smooth operation at the moment, regardless of the fuel quality. The atomization multiple injectors offer is of course far superior to what carbs can do. Not at WOT at operating temp, but the rest of the time. That of course is why EFI excels in daily use under a variety of conditions, it sniffs the exhaust and adjusts fuel mixture on the fly with digital precision, allowing more power with less fuel overall than carbs.

My point is that engine management systems of an OEM quality aren't available for Pontiacs and probably never will be.

I hope I'm wrong!  

My71:
Again Tuff,
thanks for posting the actual article. I just posted the link in Rustyskills thread..

You're welcome and I thank you.,. it's archived now and should be read.
A lot of good info in that article.

Results of DETONATION



 
When The Flag Drops,,,



The Bull Chit Stops,,,


P. Engineer, Engine Builder

Idj- You're wrong to some degree, and right as well.
True a "Pontiac" knock sensor hasn't ever been produced. But an aftermarket one can be calibrated to work in some cases.

But here's something just as important. I can run the advance I run partially because I can pull timing out as both the intake charge and/or engine coolant temps rise. That's huge. Conversely I add timing during cold start and warm up. That helps as well.

And you're right, the O2 will have final say as to regulating the proper air/fuel ratios. But the input in that table has to be what the engine NEEDS, not what you think or hope it should have/run. Two different things. That only comes after much trial and error. Too many guys are hell bent on running stock iron headed V8's real lean for better mileage etc. That doesn't always result in the best situation.  
Mark
Engine builder, located in Central Florida

Agreed! The ability to reference ambient, intake, and exhaust temps and adjust to suit via electronic means is gigantic in controlling combustion temps (and detonation / preignition). The OEs had plenty of castings to destroy in sensor R&D and we sure don't. Part right and part wrong describes me most of the time! I'll get there with help.

Your mention of aftermarket knock sensors is interesting, it sounds like I should research more. I would feel a lot better leaving for a big road trip knowing that my engine controller had the ability to make an effort to keep my pistons safe if poor fuel is all thats available. Such as maybe a "limp mode" tune for the ECU to use if exhaust temps climb too high. I would feel even safer if it could do so immediately because it heard a knock.

With a traditional carb and distributor (and headers-loud exhaust) its tough to listen for clatter. Even harder to pull over and look at your plugs to see if you need to turn around or proceed up a mountain road. But its still amazing how well carbs and HEIs do the job without all the wires and sensors, and its always a pleasure to fire up and drive one that all works right!  

If you get really bad gas and have a detonation issue then you could always just retard the timing a bit. The ECM won't know the difference and will still just follow the preprogrammed curve at that point with less overall per your change.  
Mark
Engine builder, located in Central Florida

bump

 
When The Flag Drops,,,



The Bull Chit Stops,,,


P. Engineer, Engine Builder