'New' roots style B&M supercharged motor going into '76. What distributor do I use and how do I prevent 'knock', detonation? The motor I am removing had HEI...



|UPDATED|3/10/2004 5:57:27 AM (AZT)|/UPDATED|

Owen,
I would consult B&M. They can tell you how much room you will have and recommend a distributor. I doubt you will be able to use the large HEI, pending the size of the supercharger you have purchased. MSD and Accel make small diameter HEI's for this application. As for detonation, it will depend on compression, amount of boost, and cam/timing. Crane & Comp Cams make camshafts for supercharger applications. They or B&M should be able to help you estimate your timing settings for your motor. There should be someone in the Houston area that can help you setup your supercharger. The guys at Clear Lake Speed Center can probably help you or point you to someone who can. Phone: 281-488-0371. Good Luck! Let us know how the installation goes.

Thanks for the reply! This is a learning matter for me, and judging from the lack of responses, for some others also. I have been doing some research and here are some excerpts that helped to explain things to me. Perhaps they will be interesting to others also...


As you probably have already figured out, detonation (aka "knock") is a big issue in the world of forced induction. You probably know that detonation is a bad thing, and that by adding a supercharger (or any forced induction power adder), you must take additional measures to avoid detonation, especially if your engine has other modifications. Normally the simple solution to stop detonation is to run higher octane fuel... but before we get ahead of ourselves, let's start from the beginning.

What is detonation / knock?

Under normal conditions, the combusting air and fuel mixture inside the combustion chamber ignites in a controlled manner. The mixture is ignited by the spark, normally in the center of the cylinder, and a flame front moves from the spark towards the outside of the cylinder in a contolled burn. Detonation occurs when air and fuel that is ahead of the flame front ignites before the flame front arrives because it becomes overheated. Under these conditions, the combustion becomes uncontrolled and sporadic and often produces a pinging noise, or a "knock" noise when the conditions become worse.

So far, detonation sounds cool... why is it bad?

Detonation is definitely not cool. Detonation causes sudden pressure changes in the cylinder, and extreme temperature spikes that can be very damaging on engine pistons, rings, rods, gaskets, bearings, and even the cylinder heads. Even the best engine components cannot withstand severe detonation for more than a few seconds at a time. More severe detonation obviously leads to more severe forms of engine damage. If there is enough heat and pressure in the combustion chamber, detonation can begin to occur before the spark plug even fires, which would normally initiate the combustion. Under these circumstances, known as "pre-ignition", the piston may be travelling up towards a wave of compressed, exploding gas. These are the worst kinds of detonation conditions, and can bend con-rods and destroy pistons.

What causes detonation?

Detonation occurs when several conditions / factors inside the combustion chamber exist at the same time. Increased compression, high temperatures, lean fuel/air mixture, advanced ignition timing, and lower octane fuels are all factors that PROMOTE detonation conditions. The good news is that, because there are so many factors in play, you can always find a way to eliminate detonation if it exists.

So, where do superchargers fit in?

A supercharger increases the amount of air inside the combustion chamber, which in turn increases the compression inside the combustion chamber. Along with increased compression comes higher temperatures and higher pressures, which as we know, tend to increase the chances that some form of detonation will occur. In order to compensate for the increased compression and heat, we must change one or more of the other factors / conditions to move us away from our detonation threshhold. Tuning the supercharger system to the engine in this way for maximum performance without detonation is something that supercharger manufactuers do so, chances are, you won't have to worry about it unless you do other modifications to your engine that place you closer to your detonation threshhold.

How do I get rid of it?

The two most common tricks used by supercharger manufactuers and engine tuners looking to obtain maximum performance without detonation is 1. use higher octane fuel, and 2. retard the ignition timing.

Higher octane fuel burns more controllably and is not as likely to combust before the flame front. This is why racing engines use 100+ octane gasoline. The ONLY benefit of racing gasoline is that it moves you away from the detonation threshhold, which allows you to be more aggressive with power producing factors - i.e. raise compression, advance timing, etc. This is why you'll be disappointed if you put racing gasoline in your mom's bone-stock '82 Toyota Cressida thinking you'll turn it into a race car. If you don't have detonation, the increased octane will do you no good. For cars designed for daily street driving, you obviously won't want to fill up with 100+ octane fuel every week at the tune of 5 bucks a gallon. This is why supercharger manufactuers tune their supercharger systems to run properly without detonation on 91 octane fuel - aka "premium" at your local gas station (in some states premium gasoline is around 93 octane).

Retarding the ignition timing will delay the timing of the spark, which also moves you away from your detonation threshhold. Most popular "power programmers" or "chips" increase engine power by advancing the ignition timing, and requiring you to run a higher octane fuel to avoid detonation. These work great, except the advanced ignition timing is NOT compatible with most superchargers, unless you're happy to run 100 octane fuel. In fact, many supercharger systems include an "ignition boost retard" that retards the ignition timing when it senses boost from the supercharger. This allows you to maintain stock performance while not under boost, yet still remain safe while the supercharger is making its boost (and power).

Another way to avoid detonation is to cool the incoming air charge to lower the temperature inside the combustion chamber. On a supercharged application, this task can be handled by an intercooler or by a water injection system (less common). The intercooler takes the incoming air charge and passes it over a series of air-cooled or water-cooled fins and ducts, thus cooling the air in the same way that a radiator cools your engine's coolant. Intercoolers are thus very popular in higher output supercharger systems, where detonation becomes more of a problem. Often times, the intercooler allows you to run more boost and also allows you to eliminate the ignition boost retard, meaning you'll notice increased performance, and still experience no detonation. Another way to lower the temperature of the combusting air and fuel is to run cooler heat range spark plugs. Many supercharger manufacturers will recommend cooler plugs for you supercharged engine.

Because lean condition (fuel starvation) also contributes to detonation, it is important to make sure that the fuel system (pump, injectors, etc.) is capable of delivering the increased fuel requirements of the supercharged engine. Often times, an otherwise perfectly tuned engine will experience detonation just because the fuel pump can't deliver enough fuel to the engine. Upgrading certain fuel components is almost always necessary when supercharging an engine. Most supercharger systems normally include the upgraded fuel components if they are necessary. If you are installing a supercharger on an engine with other modifications, make sure you consider the additional fuel requirements and compensate with larger injectors and / or a bigger fuel pump.

Some modern vehicles come with "knock sensors" that listen for detonation, and automatically retard the ignition timing to eliminate detonation. Although these devices are effective in preventing engine damage, they are not tuned for performance, so you should not rely on the knock sensors and expect your engine to run its best.

Conclusion

Altough detonation can be potentially damaging to an engine, a simple understanding of what it is, and what causes it, will help you stay away from your detonation threshhold. Pay attention to "knock" and pinging noises that come from your engine becuase they could indicate detonation inside the combustion chamber and should be dealt with immediately. If you're looking for a new supercharger system, don't worry too much about detonation - the manufacturers have designed the system for use on your stock engine, and if you follow the manufactuer's fuel recommendations, you will not have a detonation problem. If you ever do notice detonation, perhaps from bad (low octane) gasoline or extremely high air temperatures, just drive with a light foot until you are able to resolve the cause of the problem.

Great post

Owen,
Looks like you have spent alot of time on line looking into potential problems. Good post! Good information for everyone.
What model supercharger are you getting from B&M and how much boost are your planning to run?

All the information above is on line in various places. That was the best write up I could find that put it all together. The motor is a 350 with a 144 B&M huffer. The set up was in his '67 vet. He has purchased a 327 to put in his. The set up is pretty strong and the builder estimated 480hp and 560ft/lbs. Stock compression is 8:1. Boost is set at 6psi. I did not want to use the dual point ignition that was with it so that's what started me looking for the best one. The motor is in great shape and has fairly low mileage on it. The motor I am pulling was a crate motor installed by the previous owner. It has an over heating problem I have been unable to cure. I suspect a blocked water jacket or too much cam. I will tear it down and fix it. I have started the removal and hope to finish the install this month.

You might be able to fit a HEI behind a B&M 144. I would measure first.

Very good post. But here are a few additions to the information.

The point of detonation and knock occours at about 2500 degrees F. This does not vary much. Any thing that causes excessive heat will cause it. This includes cooling system problems and restricted exhaust systems, or hot spots in the cylinder from carbon deposits. You may try to fix knock by retarding timing or changing fuel when the cause is somewhere else.

Octane is the resistance to burning. High octane fuel is more controlled only in the fact that it burns slower. The slower burn produces less temp, but due to the fact the flame exist longer, it produces more heat. Heat and temp are two different things. Heat is volume, measured in BTUs. Temp is intensity, measured in degrees. A 9000 deg torch won't heat your house, a 140 degree furnace won't cut steel. SO, high octane produces more heat, but at a lower temp.

If the octane rating is too high the cylinder temp is too low to create a complete burn, then we lose heat. If the octane rating is too low, combustion is over too soon, and we lose heat. Too low of octane will burn the tips off plugs, melt pistons, and still not power the car. Of course then you get knock. Too high can foul plugs and waste fuel. Both create poor performance and damage.

Air and fuel is pulled into the cylinder. We burn the fuel and oxygen, but that leaves most of the cylinder full of air, about 70 percent Nitrogen, an inert gas. We do get some pressure on the piston due to combustion pressure, but combustion is over at about 10 to 20 degrees after top dead center. From that point the heat created was absorbed in the remaining air. The superheated air expandes and creates most of the power to push the piston down. We can have too high of temp, but we can't really have too much heat. Blowers and Superchargers and Turbos all supply more air into the cylinders. The more air in there, the more expansion, and the more pressure, which is more power. The extra heat from compression pressure alone is enough to make the engine knock. That combined with the heat volume absorbed into the air, combined with the extra fuel to keep the extra air from making us lean, and thus produces even more heat, can create a lot of knock, which bring us back to your question, which you answered well.

To expand a bit more for those of you with EGR valves. EGR adds exhaust back into the intake manifold, and into the cylinders. You can't reburn exahust gas. It's already burnt. This makes it an inert gas. It does not respond to the combustion process.
If we make a fuel mix leaner, the temp goes up. Knock get worse and we can damage the engine.
If we make the fuel mix rich, the temp goes down but we waste fuel and pollute. While most of us don't worry about that, the EPA does, so we have EGR.
By adding inert exhaust gas we dilute the cylinder mixture without changing the air fuel ratio. For pratical purposes with a properly running engine, there is no fuel or oxygen in the exhaust. It's just now the fuel mix is diluted so the molocules are further apart. This slows down the burn, and lowers the combustion temp, stopping knock.

Keep up the good work

Ken Styer

|UPDATED|3/10/2004 7:37:24 PM (AZT)|/UPDATED|

Ken,
Great information! Thank you.
Seems the more I learn, the less I know .

Quite welcome. And believe me, I know that feeling.

Ken Styer