FAQ - Frequently Asked Questions

Air and Fuel System Questions

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1. I want more power and the engine is detonating!  Do I need to run richer to eliminate the knock?
   
   What is the optimum air-fuel ratio to run for best power?
   
   I've heard racers say that we need to run a bit lean for best power - is this true?
    
2. I've looked at my scan tool and I see the AF ratio is 10.9:1.  What's this mean?
    
3. What does the honeycomb screen in the throttle body do?  Should I take it out?
    
4.  So should I use an adjustable fuel pressure regulator or not?
    
5. I want more power!  Should I put bigger injectors on my car?
    
6. What is the injector size on the L67 engines, and how much HP can they support?
    
7. You've mentioned injector on time, or BPW (Base Pulse Width).  What's the max number?
    
8. So what's going to happen if I stick bigger injectors on my GTP right now?
    
9. But I've heard / seen this GTP owner that swears he's running faster with larger injectors.  How's he doing this?
    
10. I hear a lot about knock or detonation or pre-ignition.  What is this, and why is it so bad?
     
11. So how is knock or detonation controlled?  What is Knock Retard?
    
12. Why can't I just run smaller and smaller pulleys and keep increasing boost, and therefore, power?  

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1. I want more power and the engine is detonating!  Do I need to run richer to eliminate the knock?

What is the optimum air-fuel ratio to run for best power?

I've heard racers say that we need to run a bit lean for best power - is this true?

Maybe.  It's somewhat true that running a tad bit richer will help quench detonation (knock), but only to a certain extent.  The stochiometric AF ratio for gasoline is 14.7:1.  Stochiometric means that at this point, the fuel is completely burned with no unburned hydrocarbons left, giving you best emissions.  This is the AF ratio the PCM is shooting for under idle and cruise.

However, best power is achieved for gasoline at around 12.5:1, leaving excess unburned hydrocarbons, but no unused oxygen molecules!  (This is why 12.5:1 is typically the best power point.)  So under ideal circumstances, the best power AF ratio to shoot for is around 12:1 to 12.5:1.  However, running a tad bit richer will throw excess fuel (gasoline) into the combustion chamber, and the extra unburnt fuel actually acts as a heat sink, absorbing heat and cooling the combustion temperature, hence reducing the tendency for detonation to develop.  However, this only helps to a certain extent.  If you're knocking severely, you can throw all the raw fuel you want into the mix and it won't quench the detonation.

When you run richer than 12:1 on gasoline, you're losing power.  Between 11:1 and 12:1, the power curve is fairly flat and you're not losing very much power, much richer than about 10.5:1 and you're starting to lose power drastically, and probably not helping your detonation problem either.

So the answer is to achieve around 11:1 AF ratio for your best compromise, not going richer than about 10.5:1 at most.  You won't be inherently down very much on power due to the AF ratio, but you're probably delaying the onset of detonation enough so that your spark advance isn't being severely retarded, thus helping your power - end result, you get the optimum power / knock control balance.

When racers speak of running on the "rich" side they're really referring to running around 10.5:1 area, whereas when they speak of running on the "lean" side, they're really referring to the 12.5:1 area, whether they know it or not.  They're not actually running "lean" (which would be something greater than 14.7:1), they're just running on the "lean" side of best power.

2.  I've looked at my scan tool and I see the AF ratio is 10.9:1.  What's this mean?

This is the COMMANDED or DESIRED AF ratio, not the actual AF ratio that is achieved.  If everything is accurate (i.e. the MAF reading is accurate, the fuel pressure is exactly at the factory calibrated level, and the injector flow rate is as calibrated) then this commanded or desired AF ratio will be exactly what is actually achieved.  As you can see, the PCM is already commanding the AF ratio to be on the rich side for the best compromise between power and detonation avoidance as explained in question 4 above.

3.  What does the honeycomb screen in the throttle body do?  Should I take it out?

The function of the honeycomb is to create a laminar flow across the entire surface of the throttle body, so that air flow is uniform and homogenous.  The MAF sensing element is "spot checking" the airflow in one spot inside this area, and for accurate airflow measurement, the airflow in this spot should not be high or low relative to the rest of the area.  Without the honeycomb, turbulence could make that spot reading inaccurate.  What you don't know is whether it's skewing the reading higher or lower.  If higher, it'll make the engine run rich.  Lower, and she'll run lean.  

4.  So should I use an adjustable fuel pressure regulator (AFPR) or not?

Again, that depends.  You've understood from the previous questions the importance of getting the AF ratio right, and that the factory commanded 10.9:1 is already optimum.  So, why would you ever want to adjust the fuel pressure?  In a properly functioning system where all parameters and measurements are accurate, you don't!  It all depends on whether or not the MAF (Mass Airflow Sensor) is accurately measuring the amount of air the engine is using!  From the factory, the MAF system is deadly accurate.  However, if you've a altered throttle body / MAF unit, it probably isn't accurate anymore. Also, if you've taken the honeycomb screen out of the throttle body, you may have made the system inaccurate as well as stated above.  So how does a AFPR help?  Since your MAF may be reading the airflow inaccurately, your engine is either flowing more or less air than the MAF reads.  The fuel delivery is assuming accurate airflow measurement, so if the MAF is wrong, you'll  need to "trick" the system and adjust your fuel delivery to match your airflow.  The simplest way to do this is to adjust the fuel pressure.  

Bottom line, if you've messed around with the throttle body / MAF, you MAY need an adjustable fuel pressure regulator.  However, NOBODY can give you a recommendation on what pressure to run!  This all depends on your car, and how the MAF reading is skewed.  Remember, you don't necessarily have to increase the pressure at all!  If the MAF is skewed to read too much air (i.e. too rich), you'll need to decrease the pressure for optimum performance.

Of course, changing the fuel pressure is at best, a band-aid to the problem.  The best way to address the problem is to correct the root cause, which is the accuracy of the MAF.  Any MAF inaccuracies will be non-linear, meaning it may be 10% off at a lower RPM, but at higher RPMs it may be 20% off, just for example.  By tweaking the fuel pressure, you can only optimize the system for one RPM, not the whole range.  Given a correct  MAF calibration across the entire RPM range, there is no need for tweaking the fuel pressure.

5.  I want more power!  Should I put bigger injectors on my car?

No!  Definitely not!  Not unless your engine is making power in the 400+ HP range!  See our follow-up questions below for more details.  This is the most commonly made mistake in hot-rodding!  It's just like the old carburetor days when misinformed hot-rodders thought "bigger was better" and a they all thought bigger carb was just what they needed!  Naturally, a bigger carb or bigger jets than the engine required only made them lose power and pour black smoke from the unburnt fuel out their tailpipes!  Of course, all the retailers loved to sell more carbs, and that's the same thing all over again with injectors.  To understand this, you really have to understand some fundamentals of how fuel injection works.

Very simply, a fuel injector is like the nozzle on your water hose.  This injector, like your water nozzle, has a certain flow rate which depends on the fuel (water) pressure behind it.  More pressure makes more flow of course, up to a certain point, beyond which the flow just becomes erratic.  This will be explained in more detail in the next question.  

The amount of fuel an injector will deliver depends on three things:  The flow rate of the injector (i.e. the "size" of the injector), the amount of pressure behind it, and the length of time the injector is turned on.  This injector "on time" is referred to as BPW (Base Pulse Width), measured in ms (milliseconds).  In a production car, the first two items are set.  Those of you with adjustable fuel pressure regulators can obviously tweak the pressure.  The PCM (computer) is calibrated with a set injector flow rate in mind based on the production fuel pressure.  The MAF is measuring the amount of air flowing into the engine in terms of mass (in grams per second), and if it has not been altered, this measurement is very accurate.  The PCM determines the optimum AF (air-fuel) ratio to deliver.  (This AF ratio will be discussed in more detail in another question too.)  So these are the known variables: the amount of air being ingested into the engine, the commanded AF ratio (which as we've previously discussed, is optimized for the conditions.).  Also known is the flow rate of the injectors under the production factory fuel pressure.  This is a calibrated variable inside the PCM, and no, it has not been altered from the factory setting even if you've got one of those "off the shelf" recalibrated PCMs.

Based on these known parameters, the PCM can calculate how long to open the injectors (the BPW) to spray exactly the right amount of fuel, given the airflow, to achieve the target AF ratio.  So, your enemy here is TIME.  The amount of time you have to spray fuel is limited to the time between consecutive intake valve openings, so at higher RPMs, you have less time to spray fuel.

The ONLY time you need to step up the injector size is when you're out of time, where the engine is ingesting more air than the injectors can handle for a given fuel pressure.  If the engine is truly ingesting enough air so that the injector cannot turn on long enough to spray the necessary amount of fuel, you need higher flow injectors!

Check out the following questions and answers for more details concerning injector sizing.

6.  What is the injector size on the L67 engines, and how much HP can they support?

The production injector for all L67 engines is calibrated from the factory as a 36 lb/hour (4.6 g/sec) injector.  A very rough formula to determine the power level a injector can support is simply to divide the total flowrate of all the injectors (6 in this case) by the BSFC (Brake Specific Fuel Consumption).  If we conservatively estimate our BSFC at an inefficient 0.6 lb/hr/HP, six 36 lb/hr injectors will provide 216 lb/hr of total flow, so this indicates they can support up to 360 HP.  A more realistic BSFC is 0.5 lb/hr/HP (our engines are a bit more efficient than 0.6!), which indicates our stock injectors can support up to 432 HP!  Actual engine dyno testing has shown that the BSFC of our engines is between 0.35 and 0.4, meaning the stock 36 lb/hr (4.6 g/sec) injectors can theoretically support between 540 to 617 HP!

As an aside, our GSX engine which dynoed a conservative 309 at the wheels (that's 370 at the engine taking into account a 20% loss through the transmission) is still using the stock injectors.  Other stock components include: fuel pump, fuel pressure regulator, throttle body, spark plug wires and ignition coils.

Here is an actual dyno run demonstrating the low BSFC numbers these 3800 engines operate at.  Note the stock fuel injectors & BSFC is as low as 0.35! Engine: 3800 Series II Block, Crank, Pistons, Rods, fuel injectors, spark plug cables, ignition system: Stock Production L67 3800 Series II Supercharged Cam Grind: Experimental Heads: Ported with large valves Intercooled

7.  You've mentioned injector on time, or BPW (Base Pulse Width).  What's the max number?

As mentioned before, BPW is the time in msec the injector is turned on.  Obviously, this time is limited to the time between consecutive intake valve openings, or combustion events, on the same cylinder, and varies with engine RPM.  This is easily calculated, and a table is shown below:
 

What does this mean?  If you're seeing BPW less than any of those max numbers at those RPMs, guess what - your injectors have some headroom left.  That is, if more fuel was required, the PCM is able to command additional on-time out of your injectors to get more fuel.  If you're seeing BPW reaching or exceeding those max numbers, and you've increased fuel pressure up to the maximum reasonable level, and all other indications are that you're still not able to run the correct AF ratio (see question #4 above), you need larger injectors.

Now, there is a general rule of thumb that you size the injectors so that they're large enough so that they only have to work at up to 80 or 90% capacity at most.  This is true, because if you size the injector to it's limits so that it's turned on all the time (that's referred to as being "static", meaning they never get a chance to turn off because the next cylinder event comes around where the injector is commanded open again before they ever are commanded to turn off) it can eventually overheat the injector and cause durability concerns.  But, this does not mean that you can not command a injector to have more than a 80% duty cycle - you can, and you will get more fuel above 80% duty cycle.  In fact, the production 86-87 Buick Grand National ran the injectors static at WOT (wide open throttle).  It's just not recommended if you can help it.

So does this mean you should put bigger injectors on your L67 just to be on the safe side to avoid running the duty cycle above 80%?  Well, only if you can get a custom PCM calibration to provide the correct injector flow value!  Read on to the next question for more details.

8.  So what's going to happen if I stick bigger injectors on my GTP right now?

Well, if you just stick them in, and don't adjust anything else, you'll probably pour black smoke out the tailpipe and run slower.  Remember, the PCM has an injector flow rate variable inside it that tells it what the flow rate of the injector is, and that needs to be changed if you put in larger injectors.

Here are the scenarios of all the possibilities when you put in larger injectors:

1.  Your MAF was reading way too little air, so the PCM was calculating a small BPW and you were running lean.  You stick in larger injectors, the BPW calculation remains the same, but the injectors flow more fuel, so you end up running ok.  Note that you could have accomplished the same thing if you just increased the fuel pressure on the stock injectors.

2.  Your MAF was reading accurately or maybe too much air.  In this case, the BPW remains the same with the larger injectors, and now you're flooding the engine with too much fuel.  In this case, your only choice is to reduce the fuel pressure and bring the fuel delivery back to normal, or remove the bigger injectors and go back to the correct stock ones.

3.  You really are putting out more than 400 HP, your MAF is accurate but you're flowing so much air that the injectors are running static and still not supplying enough fuel, and you've increased the fuel pressure as much as reasonable.  In this case, sticking in larger injectors will give you more fuel for the same BPW.

4.  You've stuck in larger injectors, but the fuel trim variables have compensated for the larger injectors and brought everything back to normal, negating the additional fuel flow from the injectors by decreasing the time they are turned on to deliver fuel. (decrease in BPW)

In all cases, except for #4, we're ignoring one very important thing, and that is this:  the PCM was never correctly recalibrated for the larger injectors.  This means that under part throttle cruising operation, the injectors will be delivering much more fuel than normal under the same BPW, and therefore the fuel trim will quickly learn this and decrease the BPW to compensate, thus negating any benefits of having a larger injector!

The only correct way to even try to run larger injectors is to change the injector flow rate constant parameter in the PCM.  Otherwise, your results will be random and the fuel trim will simply compensate for the larger injectors.

Again, he'll fall into one of the above 4 cases or some combination, and more than likely, he'll probably run just as good or even better if stock injectors were put back in.

Our best advice is to have a correctly calibrated MAF that accurately measures airflow.  Once you have this, there is absolutely no need for adjusting the fuel pressure from the factory setting, or increasing the injector size (as long as you're making less than 400 HP).  

Again, we should point out that our GSX engine which dynoed 309 at the wheels (that's 370 at the engine taking into account a 20% loss through the transmission) is still using the stock injectors, stock fuel pump, fuel pressure regulator, throttle body, spark plug wires and ignition coils.

10.  I hear a lot about knock or detonation or pre-ignition.  What is this, and why is it so bad?

Knock, can be extremely damaging to an engine if uncontrolled.  It should not be confused with pre-ignition, which is a different phenomenon.  Most current automotive engines are 4 cycle spark-ignition (Otto) engines, so we'll refer to this type of engine exclusively.  We'll also assume the reader is familiar with the main components of a internal combustion engine and how the 4 cycles operate, and what spark advance means.

The rate at which an air/fuel mixture burns is a combination of flame speed and combustion chamber geometry.  For best efficiency and torque, peak cylinder pressure/greatest heat release should occur 5-10 degrees ATDC, so spark timing must be adjusted to compensate for different burn rates at different engine speeds and loads.

A fundamental means of increasing engine performance and efficiency is increasing static compression ratio (CR), or using a blower and artificially forcing the compression ratio higher.  One obstacle to further increases in boost or CR is engine knock.  Even today the details of knock are not well defined - it is an extremely complicated combination of chemical and physical abnormal combustion phenomena.  

The textbook definition of knock is this:  “The explosive spontaneous ignition of fuel-air mixture ahead of the normal propagating flame and the subsequent cylinder pressure oscillations in homogeneous-charge spark ignition engines.” 

Under normal combustion, the flame front propagates very smoothly in a uniform manner.

So happens differently when detonation occurs?

As with normal combustion, abnormal combustion begins in a similar fashion.  Ignition occurs at the spark plug, causing combustion, and cylinder pressure and temperature increases as the piston moves up and the air-fuel mixture is burned.  Under certain conditions, the unburned mixture ahead of the flame front (end gas) is auto-ignited due to rising pressure and temperature.  This auto-ignited end gas explodes before the flamefront arrives, with a resulting violent rise in pressure.  These propagating waves then exceed the speed of sound, and a characteristic audible “knock” or “ping” is heard.  If uncontrolled, excessive knock is extremely damaging to internal engine components, such as pistons.

Spark-induced knock should not be confused with Pre-ignition.  Pre-ignition is ignition of the fuel-air mixture before the timed spark event, usually caused by surface ignition emanating from hot spots.  Detonation occurs when the fuel-air mixture burns too fast, independent of the time it is ignited.  Therefore Detonation will generally accompany pre-ignition, although the reverse is not necessarily true.  Knock is the sonic concussion resulting from either pre-ignition or detonation.

11.  So how is knock or detonation controlled?  What is Knock Retard?

There are many methods of suppressing knock - the knock control system in the L36 and L67  accomplishes this by retarding spark timing, the most practical and effective dynamic method.  Spark knock occurs at specific frequencies determined by the combustion chamber geometry, and always occurs in a specific time window after the peak cylinder pressure point.  The knock control system must correctly differentiate between normal engine noise and true knock, and correctly retard spark timing only enough to eliminate this knock 

Every engine/trans combo in a particular platform will have a certain characteristic sonic signature. The general engine noise of the pistons and valvetrain have their own characteristics, and the sonic signature of knock is very distinctive as well. The engine is mapped out via accelerometers and spectrum analyzers to determine the best practical location to locate a knock sensor, where the signal to noise (SN) ratio of knock to base engine noise is best. A particular knock sensor (Piezoelectric resonant in the case of these engines) of the right characteristics is chosen, generally one whose resonant frequency is compatible with the center frequency of that engine’s knock. In the case of these 3800s, two 6kHz sensor is used. This knock sensor hears knock and engine vibrations, and provides a voltage input to the knock control system.  The knock control system inside the PCM then processes this signal, determines whether it's real detonation or noise, and retards the ignition timing the proper amount to suppress this knock.  This is the knock retard (in degrees) that is being referred to on the scan tool.

This knock control system also learns, much like the fuel trim does.  So if you've run some cheap gas or switched to a small pulley and your engine has been knocking quite a bit, the system will learn the spark advance lower to protect the engine, giving you decreased performance.  This of course, can affect your track times.  How can you "unlearn" this spark retard?  Disconnect the battery power for an hour or so, or put in some good gas which won't knock, and drive it fairly hard for 10 minutes or so, spending as much time in a high load state, but without any knock being detected.  If any knock is being detected, you're wasting your time as it's probably just learning deeper into the retard!

12.  Why can't I just run smaller and smaller pulleys and keep increasing boost, and therefore, power?  

Pulleys less than 3 inches in diameter has not proven beneficial in testing.  The high terminal speed reached by the blower as the engine spins to redline (6000 rpm) really start beating the air and generating excessive heat, enough heat that even an air to liquid intercooler is not capable of reducing the heat to an acceptable level.  Another major problem at these extreme engine (and therefore, blower) speeds is belt slippage, as it is nearly impossible to prevent the belt from slipping at these speeds.  The smaller the pulley, the more the torque band is shifted towards the lower engine speeds, giving you incredible low end torque, but sacrificing the HP at the upper engine speeds.  Small pulleys are great for doing smoky burnouts, but don't win races due to the loss in upper RPM power.

These are dyno runs made with three pulley sizes.  Notice the decrease in boost as the engine speeds increase, indicating belt slippage.  Also note the excessively high air temperatures before the intercooler (T1).
2.4 inch pulley	2.6 inch pulley	2.8 inch pulley

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