FAQ: What is spark knock? What is KR (knock retard)?

Engine knock, also known as "ping", spark knock, or detonation, is a phenomenon that occurs inside the combustion chambers when a portion of the air/fuel mix is ignited under extreme heat and pressure, burning more rapidly than it should for creating optimum power on the piston's downstroke. The result is a harsh detonation as the air/fuel mix explodes, colliding violently with parts of the engine such as the head, piston, cylinder walls, etc., as well as the original portion of the air/fuel mix that was ignited by the spark plug.

Knock can occur at different intensities depending on conditions inside the combustion chambers. It can be caused by various things, including low octane fuel, lean air/fuel mixture, higher boost (smaller SC pulleys), increased engine load, and restricted engine air flow, and all of these things may act to influence each other. Perhaps the two greatest direct causes of knock are: A) excess heat and B) a lean air/fuel condition. Warmer outside air temperatures, "hot" air intake designs, higher engine/transmission temps, incorrect spark plugs, and simply racing the engine at high RPMs can also contribute to heat build-up and knock.

KR stands for knock retard. Knock retard is the PCM's response to detected knock, which results in retarding of ignition timing. KR can be measured using an OBDII scan device. KR is measured in degrees depending on its level of intensity. The more knock that is detected, the greater the response by the PCM in dealing with it. As timing is pulled, power output is reduced. For obvious reasons, zero degrees KR is optimal, but this is not always realistic due to the nature of the engine's operation. Because our cars are supercharged, the effective compression inside the cylinders is much higher than with normally aspirated engines under increased throttle, thus we are prone to knock much more than NA engines when under loads. In fact, it is normal for a SC 3800 V6 to be on the verge of knocking during WOT (wide open throttle), or under heavy load conditions, such as pulling up a grade in overdrive. In order to take advantage of the extra air from forced induction (supercharging), knock is a sort of necessary evil, but you don't want very much. Ideally, you want to get the most power possible before engine knock ensues.

Even a stock vehicle can exhibit knock during WOT at times. If your engine is knocking slightly, say 1-3 degrees, you probably won't notice it, because the PCM uses microphones to listen for these sounds and then takes quick action to correct with KR. A small amount of power will be lost, but it is very small, probably in the area of 5 hp. Higher levels of KR above 4 or 5 deg may require extra response from the PCM to control the knock. When this happens, the engine could lose a noticeable amount of power, and a "ping" sound may actually be audible is extreme cases. When you use premium fuel, cooler plugs, a colder thermostat, or a cold air intake (CAI), you may notice restoration of lost power due to the reduction of existing engine knock (the only way to verify is to monitor KR using a scan tool).

Modifying the engine to run at higher levels of boost (swapping to smaller SC pulleys) can yield considerably more power output, but requires added effort to keep knock and KR under control. Running extra boost will increase heat and test the PCM's ability to deliver the needed fuel. Further lowering intake temps (IC), maximizing engine air flow (rockers, cam, exhaust), and tuning the PCM's fuel curves (PowrTuner) are just a few ways to help keep knock levels in check.

Knock Retard

What is Knock Retard?

Knock Retard (hereafter referred to as KR) is the response from the PCM to cylinder detonation. KR is the measure of the number of degrees of overall ignition timing advance that must be removed from the engine to prevent detonation from continuing, thus protecting the engine from damage.

What is detonation?

KR is a result of detonation. To have 'real' (more on 'real' vs 'false' KR later) KR, you MUST have detonation. Detonation is the uncontrolled combustion of the intake charge. "Uncontrolled" means that the mixture ignites via a means other than the spark from the spark plug. In most cases, the uncontrolled ignition is due to a 'hot spot' in the cylinder. Hot spots can be caused by uneven combustion, spark plugs that are rated too 'hot', lean fuel conditions, breathing restrictions (exhaust / intake), bad gas and so forth. One problem in particular that came to light for me was the head gaskets. During one of my engine teardowns, Zooomer from ZZP pointed out that, while my cylinder bores are perfectly round, the head gaskets are NOT made perfectly round. Some of the gasket material actually protrudes slightly into the combustion chamber. Since the head gasket bore linings are made of metal, that little bit that protrudes into the cylinder glows red hot, thus creating the potential for a nasty 'hot spot'. This is a good area to check and perhaps replace with an aftermarket head gasket. In other cases, the 'hot spot' is due to unreasonably high cylinder compression. Either way, the 'pinging' or 'rattling' sound you hear is the result of the actual collision of the flame front produced by the 'hot spot' and the normal flame front produced by the spark plug. Typically, these two flame fronts are opposing fronts, meaning that they are expanding, or propogating toward each other, thus the collision. Real KR does NOT occur without detonation occurring FIRST.

How is knock detected?

Since detonation results in noise (the rattling or pinging sound of the two colliding flame fronts), it can easily be detected through the use of microphones attached to the engine in key locations. On both the L36 and L67 3800 engines, there are two microphones. Each one is located immediately beneath a cylinder bank and are mounted in the block of the engine directly into the cylinder water jacket. As the sound of detonation occurs, the noise is 'heard' by the microphones and the signal is carried to the PCM where it is analyzed. The PCM determines whether or not the signal provided by the microphones is knock or just normal engine noise. Knock is detected by the frequency of the signal. The severity of the knock is determined by the voltage level of the signal. Another way to say it is the voltage level of the signal will determine the level of KR. The PCM is tuned to responded ONLY to those signal frequencies that it has been programmed to recognize as knock. Anything else is engine noise.

How does the PCM respond to knock ?

Engineers designed into our engines a safety mechanism for protecting our engines from KR. To do so, the PCM must respond electronically somehow to the knock signal. To electronically eliminate KR, and thus detonation, it is necessary to reduce the heat in the cylinders. Heat is a byproduct of power, so to reduce heat … power must be reduced. The PCM can reduce power electronically by retarding the overall ignition timing. The PCM converts the voltage level to a corresponding spark timing degree (KR) by which the engine should be retarded so that the detonation is naturally eliminated. The higher the voltage, the higher the KR. By doing this, the spark ignition of the combustion mixture occurs much later in the cycle of the piston compression stroke, thus reducing the effort the piston undergoes in compressing an explosion that has occurred ~15 degrees prior to TDC (top dead center). The later the ignition occurs, the less combustion that is compressed, and the less work the engine has to do. The effect of this is to cause the engine to lose power …. a noticeable amount of power. The other effect of this is reduced cylinder temperatures which immediately dissipates cylinder 'hot spots'. With temperatures down and 'hot spots' gone, detonation has been eliminated. The KR response by the PCM is limited to not exceed 25.5 degrees.


What does the PCM do immediately after the detonation levels begin to fall?

Once the PCM has retarded timing sufficiently to reduce knock below the currently detected peak level, a changeable parameter in the PCM governs how quickly the overall ignition timing can be restored to normal levels (more on this later). The engine could see a peak of 15 degrees of KR from which the originating detonation may immediately disappear. However, the PCM will not instantly restore timing to pre-detonation levels. Instead, the PCM cautiously and conservatively restores ignition timing at a rate of 0.8 degrees per second. In the event of a 15 degree KR event, it would take nearly 19 seconds for the ignition timing to be restored to pre-KR levels. By the time your car sees full power again, the race is already over. This 'time' that the PCM takes to restore the ignition timing is called the Recovery Rate (more on this later). The Recovery Rate will continue in this slow fashion until KR reaches zero, KR increases back above the current recovery value, or the throttle is released.

How much horsepower do I actually lose with KR?

Approximately 2 hp per degree. At 15 degrees of KR, you are subject to lose 30 hp. At 25 degrees of KR, you lose approximately 50 hp. Yes, it is VERY substantial and VERY noticeable. Please note that this is not EXACT hp lost … it is approximate.

Why do I NOT want to have KR (why is it bad)?

Due to the retardation of the ignition timing, KR causes the vehicle to lose substantial power. More importantly, though, the flame front collisions are EXTREMELY harmful to the pistons. These highly volatile areas in the cylinder can cause stress cracks in your piston, which will eventually give way causing an entire CHUNK of your piston to lift right off and begin banging around inside the cylinder. This is why when the spark plug is removed after such an event, the plug end is bent all the way over. The broken piston can be VERY expensive to fix if you are not capable of doing the work yourself. DON'T EVER DISABLE YOUR KNOCK SENSORS. It takes less than 3ms to damage your engine due to knock.

How do I know if I have KR?

KR is an electronically determined value based upon signal input from the knock sensors. As such, the best way to determine whether or not you have KR, and if so how much, is to use a scan tool to actually read that parameter ID (PID) from the PCM. There are three tools readily available …. Autotap, Scan Master, and a Tech 2 that can show you your KR value.

What is REAL KR and what is FALSE KR?

Real KR is KR that grows with engine RPM and engine load. It depends entirely on detonation, which is dependant upon throttle position, MAF, MAP, engine load, engine temperature, and RPM. As RPM and engine load increase, the chance for KR (or higher KR) increases. As the vehicle shifts to the next gear, KR will usually make a small jump up as well due to the higher engine load.

False knock is characterized by a sharp spike to an immediately high value of KR followed instantly by the KR Recovery Rate. It doesn't grow with engine RPM or load, it jumps to a high value on throttle input and then recovers to a low value, or zero perhaps, as engine RPM continues to increase. Note that this is exactly opposite to the characterization of REAL KR. Remember, knock is simply specific noise detected by engine microphones. Because it happens to fall with in the frequency of real KR does not necessarily mean that it IS real KR.

What can cause FALSE KR?

Outlined below is a list of things that can cause false knock.

1. Sway bar hitting exhaust downpipe - This happens typically with the downpipe of headers because that configuration puts the downpipe in very close proximity to the sway bar … much closer than the stock downpipe. The banging noise from the two metal objects hitting may resonate through the frequency band that the PCM detects as knock through the knock sensors. The solution to this is to flip the swap bar over. Because of the curvature of the sway bar near the downpipe, flipping it will allow the sway bar to curve AWAY from the downpipe rather than toward it.
2. Transmission oil stick hitting exhaust crossover pipe - This typically happens with the crossover pipe of headers due to their large size and proximity as opposed to the stock crossover. The banging noise from the two metal objects hitting may resonate through the frequency band that the PCM detects as knock through the knock sensors. The solution to this is to carefully bend the trans oil stick away from the crossover pipe so that the two do not touch.
3. Anything loose in the engine or outside the engine may cause noises that drift through the frequency range that the PCM detects as KR. Carefully check your engine! This is very vague and is intended to be vague because just about anything loose in or out of your engine that is making noise could cause this. This includes loose or noisy components in your transmission as well.
4. Loose knock sensors, or knock sensors that are too tight. Double check that your knock sensors are torqued to spec (14 lbft).

How do I FIGHT KR?

The simple and most basic answer is in one of the following (in no particular order):

1. Reduce boost - Boost is a direct reason for increased cylinder temperatures and thus detonation.
2. Reduce timing (if added) - Timing advance is another direct reason for increased cylinder temperatures.
3. Install an intercooler - This is the best solution of all … no doubt. This will reduce the intake charge temperature by approximately 100 degrees F (results vary among intercooler manufacturers).
4. Add water injection - While much harder to tune, this is still an option for reducing charge temps.
5. Run race gas (or at least the highest available octane gas) - Always a solution, high octane gas slows the burn rate of the combustion, thus acting inherently as a cooling agent.
6. Keep your engine running cool - A cool engine helps to reduce the chance for 'hot spots'. Things like lower temperature thermostats, larger radiators, etc will help.
7. Free intake and exhaust restrictions - i.e cold air intakes, cams, headers, cat-back exhausts, larger throttle bodies, etc.
8. Prevent parts from hitting (i.e. header downpipe with front swaybar)
9. Add more fuel (to a point).

But there is much more to many of these and MANY necessary explanations and modifications that can help reduce KR … but most of them fall under one of these categories. What none of these address is what the PCM can do to help reduce or eliminate knock altogether or help to decrease the affect KR has on vehicle performance. Before we get into the PCM, let's talk about each of the solutions in the list individually.

I will NOT talk about how good or bad a particular product is or compare them to other similar products across manufacturers. That is not the purpose of this document. The purpose is to talk about KR and how to reduce or eliminate it.

1. Reduce Boost - As you increase boost, cylinder pressures will increase because more air is being forced into the engine. As pressures increase, the temperatures will naturally increase as well and will lead to detonation. By lowering boost, you lower the cylinder pressure and temperature and thus deter the likelihood of detonation. The bottomline to this solution is … since we are all enthusiasts and want more power, this will be our LAST solution.

2. Reduce Timing - If you are experiencing KR and you want to get rid of some (perhaps all) of it, AND you have the ability to add/retard timing to the engine (via a MAF Translator Plus, a DHP PCM, a ZZP Mini-AFC, or the ZZP ICCU), then start by reducing to 0 (or until KR is gone) the added ignition timing. If you still have some KR, you can then start RETARDING the timing some until KR has been eliminated. Unfortunately, if you are on the edge of having KR then adding timing is the FASTEST way to get KR … and a LOT of it.

It is common knowledge that KR is not exactly a one-to-one ratio to the amount of corresponding ignition timing that is pulled. In my experience, for 5 degrees of KR … 6-7 degrees of ignition timing is pulled. The bottomline to this solution is … since we are all enthusiasts and want more power, this will be our second to last solution.

Maximum timing advance allowed by the stock cals is 17 degrees. Typical values seen at WOT is 15 degrees. Adding timing on top of this will only improve power, assuming you have no KR. This is one good reason why it is important to have something like an intercooler to control KR. With KR under control, a substantial amount of hp can be added by spark advance. The amount possible is under some scrutiny, but everyone agrees that it is not less than 1hp per degree, and not more than 3 hp per degree of added timing. This means that by adding a maximum of 10 degrees of timing, 10-30 hp will be seen by nearly everyone who is successful at implementing it WITHOUT KR.

3. Install Intercooler - This is the most reliable and recommended solution to KR in a force fed application. The available intercoolers for the L67 (Thrasher, ZZP) will drop about 100 degrees off the lower intake manifold air temperature. There is some consequence though. You will drop some boost across the intercooler (either intercooler) as it IS a restriction. The gains you will see are instantaneous if you had KR previously. If you did not have KR previously, you may not see any initial gains. The REAL gains to the intercooler are what it ALLOWS you to do as later mods. For instance, with an intercooler KR is MUCH less an issue, therefore running smaller pullies is MUCH easier. Additionally, timing can now be added to the engine, again without as much worry for KR. Intercoolers are self-contained so you don't have to worry about depleting or adding anything to it later, unlike water injection. The bottomline to this solution is … this is the ideal way to go!

4. Install Water Injection - Not as many Grand Prix owners have done this. While some have been successful, others have struggled to tune it such that the car runs well. The idea is to inject a small, very fine mist of water into the air as it passes through the intake, or into the lower intake manifold after it has already passed through the supercharger. The water will absorb the heat in the air, thus cooling it, and then become vaporized in the cylinder and pass harmlessly out the exhaust as steam. The amount of water we are talking about here is very small. A negative effect of running any liquid in the air stream prior to the supercharger is its affect on the SC rotors. They are teflon coated in the Series Two engine and some have experienced delamination of the teflon from the rotor and ultimately damaged their SC … so BE CAREFUL. The bottomline to this solution is … it is viable and can work, but can take time to tune and should only be used in the air stream AFTER the SC.

5. Run Race Gas - This is a GREAT solution that has EXCELLENT results in fighting KR. High octane gas slows the burn rate of the combustion mixture, thus reducing the rate of heat build up which helps to cause 'hot spots'. The downfall to this is cost. At an average price of $4.00 per gallon, it is not a realistic choice for everyday use. Bottomline to this solution is … very good results against KR but NOT cost effective. Save it for the track.

6. Keep Your Engine Running Cool - Since detonation is typically caused by 'hot spots' in one or more of the engine's cylinders, running the engine cooler can help reduce the chance for a 'hot spot' to occur. Simple ways to do this is to run a lower temperature thermostat (more on this is a second), a larger radiator, an intercooler, water injection, and to lesser extents headers, cold air intakes, less restrictive exhausts and so forth.

Lowering the thermostat temperature to a 160 or 180 can help some. As I remember, a local club member tested this mod and found an average gain of 2.5 hp by lowering the temperature. This is roughly equivalent to a little over one degree of KR. Of course, even with a lower temp thermostat, it is likely that your engine temp will STILL go well above that temperature simply because the radiator does not have the capacity and heat dissipation ability to keep the coolant THAT cool. You may not see that problem during the winter months, but during the summer, and in a lot of stop and go traffic, that temperature is going to climb regardless. So don't expect it to stay at 160 degrees just because you install a 160 thermostat. Bottomline on this mod is it is so inexpensive and easy to do, it is well worth it to save a degree of KR.

Another way to run your engine cooler is to install a larger radiator. With additional capacity for cooling, this can go a long way toward controlling your engine temperatures. This combined with the thermostat mod is worth the effort. The radiator will help to keep engine temperatures down near the thermostat temperature during those times when it wants to creap well past the thermostat temperature. Bottomline for this solution is that it is definitely a worth while effort, but the radiator does take a fair amount of time to install.

Additionally, running a cooler spark plug will help. Autolite 103's, for example, are three heat ranges colder and make an excellent choice for highly modified 3800 engines. These colder spark plugs have less area exposed to the combustion chamber and do not heat up nearly as quickly as hotter plugs.

7. Free Intake and Exhaust Restrictions - Allowing your engine to breathe more easily will help. Installing headers, removing the U-bend, removing the resonator, installing a cat-back exhaust system, cam shafts, cold air intake, and ported & polished throttle bodies or larger throttle bodies from other vehicles like the Corvette (75 mm LS1 TB) can help. All have the same effect, but to varying degrees. The engine has to work LESS to breath in MORE air and pump out MORE exhaust. Less work equals less heat over the same period. A local club member ran dyno tests regarding the U-bend (installed and then removed) and found that removed, 5 hp was gained across most of the RPM band along with 2.5 lbft of torque!!! Bottomline is these are ALL excellent mods to do and are the kind you should be considering.

8. Prevent Parts From Hitting - This has already been discussed in the "What can cause FALSE KR" section. See that section for more detail.

9. Add More Fuel - The best way to tune your vehicle when adjusting your air/fuel ratio is on a dyno. Most dynos have a wideband O2 sensor that can reliably measure your engine's actual air fuel ratio across your entire dynoed RPM band and displays it on the computer for your analysis.

Fuel Background
The stoichiometric ratio for any internal combustion four stroke gas engine is 14.7:1. That means 14.7 units of air to one unit of gas. This is the perfect combination of air and gas AT IDLE. The PCM will command this combination. Due to inherent inefficiencies in the engine, the PCM can't simply command 14.7:1 and leave it at that. The engine will naturally drift a little in one direction (more rich or more lean) based on the last commanded a/f value. To control this drift, the PCM actually needs to MONITOR the oxygen content of the exhaust gas so that it knows when the engine drifts off of 14.7:1 and by how much. This information is used by the PCM to counter those drifts by commanding more or less gas depending on the direction of drift. This whole procedure is indicated by the PCM parameter ID (PID) called LTFT or Long Term Fuel Trim (and STFT or Short Term Fuel Trim). This parameter indicates how much the PCM is adding or deleting fuel to/from the engine over the long term (and short term for STFT). A value of 0% indicates that the PCM does not have to make any adjustments. A positive value indicates that the PCM is adding fuel because it is running lean, and a negative value means the PCM is removing fuel because of a rich condition. The PCM IS limited, however and can only adjust up to 16% additional fuel or 23% less fuel.

At WOT (wide open throttle), the story is completely different. The PCM relies on static fuel tables to determine what a/f ratio to command. The PCM never uses the oxygen sensor under WOT conditions. As a result also, the LTFT is never used under WOT. This is when it is necessary to use a wideband O2 to determine your true a/f ratio and tune accordingly. The O2 sensors used by the engine are in their nonlinear region at those O2 voltage levels which is why they are not used. However, as a RELATIVE value for YOUR car, YOU CAN use them to get an idea of where you are at RELATIVE to previously known GOOD values that you may have correlated to a dyno.

As such, adding fuel under the right circumstances can have a positive impact on KR. There is no clear cut formula for the do-it-yourselfer because of the unique conditions that everyone's vehicle is running under. Fuel can be added through several methods such as the MAF Translator, a DHP PCM, the ZZP Mini-AFC, the ZZP ICCU, or simply increasing the fuel pressure at the rail through an adjustable fuel pressure regulator.

What should I monitor with a scan tool?
The following values, at a minimum, should be monitored with an Autotap, Tech 2, or Scan Master (some parameters may not be available with the Scan Master) when tuning your car for spark or fuel:

1. Engine RPM - Useful for monitoring your ICCU various fuel and spark segmentations if you have a ZZP ICCU, or if KR is occurring at only certain RPMS and so forth.
2. B1S1 O2 Sensor - For WOT adjustment. Typical accepted values range between 0.88 - 0.93 volts. Higher values indicate a richer mixture, while lower values indicate a leaner mixture.
3. Injector Pulse Width - Must be less than 23ms at 5200 rpm, 21.4 ms at 5600 rpm, and 20ms at 6000rpm. Anything equaling or greater than these values at these RPMs indicate that your stock injectors have gone static (i.e. always on at WOT).
4. Spark Advance - Look for values from 15-17 with no knock on a stock PCM. Values below 15 will likely have knock associated.
5. Knock Retard - Best is obviously 0. Most authorities agree that approximately 2 hp per degree of knock retard is lost. An intercooler is the best choice to take care of this.
6. Long Term Fuel Trim - Used to determine if your engine is running within the adjustable limits of the PCM. LTFT should never read as low as -23%, nor as high as +16%. Anything between means that the PCM is able to correctly adjust for engine input/output variations.
7. Throttle Position - Used to see when you have gone to WOT, at idle, or at cruise. Range should run from 0 to 100%

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Found this over at regalgs. A member there found it somewhere else I thought it was a pretty good read. Message was too long to post in one post.

I believe this is definately something that should go on the Riv FAQ site.

Yes, a classic written by Bill Hooper for ZZP in Jan of 2003. Some of my first true understandings of modding came from reading that link:

http://74.57.209.83:9000/maint/kr.htm

very informative. i had no idea. but my 95 rivi, is throwing up a knock sensor code. according to the gm dealer, but they want a fortune to replace it, so i think i can tackle the problem myself ,but i cant locate a repair manual for my 95, helmsinc. want $ 158.00 for theirs,and right now i cant afford it. can someone send me into the right direction as to replace it? i kn ow its under the front passenger side, but after crawing under there ,i could not even see it! help please!

update on knock sensor problem. i found it! there it was poking its little snout out! it was no real problem to change out either. just a few problems getting a tourque wrench into place , because apparently the right tourque setting is critical.....on my way to cali today to spend new year with family ,,,hope she runs well.........

So are only SC motors prone to knocking? Ive noticed the vast majority of people talking about knock and KR had SC vehicles...

no this could apply to N/A , but highly unlikely or very very little... our engines are the same therefore the N/A have knock sensors just like ours.... if you wanna learn about knock go to zzp under tech articles an theres a whole sheet about knock retard...

Or scroll up - lots of info.

It's all about compression.

Some NA engines are prone to knocking, but the 3800 isn't one of them. Engines like the Northstar, with greater than 10:1 compression ratio can see some knock, but cutting timing can fix the problem.

Our engines have a static comp ratio of 8.5:1, but after boost we can see 13:1 or higher effective compression, so a little knock is common place with our engine. Sustained knock is also more violent in our higher compression engines, so we need to take it more seriously to avoid chipping pistons.

Ive had a few friends who owned Cadillacs or Auroras with Northstar motors and I'd hear them occassionally say something about the motor knocking. I assumed it was from using cheap gas and not premium, poor oil, and basically that the motor was about to blow lol. You see alot of those two cars in particular in the Detroit area with SHOT motors because of poor maintenance, and I always thought thats what knock was- the pistons knocking because it was about to blow.

Knock is when the air/fuel burn is ignited at the wrong time - usually before the spark. The result is a quick explosion called detonation, ping, or spark knock.

Sometimes you hear pre-ignition, or dieseling, in older NA cars (with dirty valves & pistons), where the engine sort of does a "clank-clank", especially when it's hot or when shutting off. This isn't as harmful in a NA engine as it is in a SC one. If our engines see pre-ignition even for a couple of revolutions, usually the result is a hole burned through the piston.

ok so what do i do to ELIMINATE KR from a stock 98 rivi. i do want to run mods but i want to make sure its safe i guess

Before you can eliminate it, you need to detect it. Scanning is the only way. read Modding 101 for ideas on how to get started with a mod plan:

https://rivperformance.editboard.com/t23-faq-modding-101-how-to-increase-acceleration-performance

If I suddenly lay into it from a stop or roll, I can see up to around 5 degrees of knock. Maybe a second or two after, the knock disappears and I am straight 0s. What is that initial spike? Is it real? Normal? Anything to worry about? Do people with mods have consistent knock and live with it? Is 0 the only way to drive? What is the max degree of knock before you know you're doomed?

It is most likely real, and whether you should be concerned depends on how often you engage in this type of driving. Once in a while, its not a big deal. Repetitively, such as a racing type activity, then I would be more concerned. If you drive "reasonably" as 99% of folks on the road, you'll rarely see even 5° KR.

The spike is caused by a sudden increase in load presented to the engine. The combination of high RPM and demand for boost allows lots of air into the engine. Fuel delivery can't keep up, so detonation occurs. Very quickly the PCM sees what's going on and corrects the situation by retarding timing the term KR. Also, once your rolling speed increases, the is less torque required to push the car, so knock naturally lessons, and timing advance can recover. Not a huge deal.

Cold air intakes: CAI and FWI designs can help lower KR in certain situations (lower IAT), but fact remains a larger intake lets in more air, which = higher levels of boost. This can cause KR to increase without additional mode to the engine.

Ways to fight KR with breathing mods are discussed here:

https://rivperformance.editboard.com/t194-faq-why-is-air-flow-breathing-mods-important

Another way is to tune the PCM to deliver more fuel to mix with the additional air induction. You can also decrease timing if needed. There are a few threads on this.

At the end of the day, a little KR (2-5°) under a rolling throttle application isn't going to kill the engine during normal driving. If you decide to do something extreme, like drag or road racing, or driving in extreme conditions for long periods of time, a little bit of KR can become bigger KR with some heat + time. Sustained 8-12° is a warning flag that means you're close to damaging a piston. Obviously, if you tune for zero KR, it's better insurance you'll never reach this point.

Hope this helps.

5 deg is not that bad in a small spike. The setting you are looking for is "Power Enrichment". It's adjustable with a tuner. In the old days on a carb it would have been the "Accelerator Pump"...

A little knock or light detonation is nothing to worry about, especially if it only happens for a short time under high load. Heavy detonation, that is sustained for a relatively long amount of time at or near wide open throttle, will destroy any engine very quickly. Modern engines have knock sensors, and the PCM will pull timing in an attempt to protect the engine, and they do a great job by keeping the engine right on the edge for optimal power and efficiency. Things that contribute to the tendency for detonation are high engine temperatures, lean fuel mixture, and insufficient octane. Eliminating detonation is EXTREMELY important when drag racing a car. You may not hear an engine detonating at full throttle, especially if you have a loud exhaust system, or are running with open pipes. When I am at the track with my 70 GS, I run race gas. I always put in 10 gallons of 110 or 112. Sure, that octane may slow me down a little, but it is cheap insurance for a 10,000.00 motor. If you get a little too aggressive with the timing, or the engine leans out big time, it will slow down, or misfire, not detonate, and that is really important.

I recently put a new engine in my 70. It made 602 HP, and 589 Ft lbs. at the crank. The car weighs about 4100 lbs with me in it. I went 11.67 @ 115.49 MPH with traction problems. I was happy to get the car into the 11's for the first time, but disappointed with the trap speed. It should have trapped up around 119-120. I had to rush to get the engine into the car for the last meet of the season. The engine has a Nascar style mechanical fuel pump (CV products), that is capable of pumping 90 gallons PER MINUTE (-10 supply, -8 to carburetor), so I didn't bother to check my fuel pressure. When I got home, I rigged up a temporary gauge, and found the fuel pressure was low. It was probably low enough at the track, to limit my top end power, but not low enough to nose the car over. In this case, I'm confidant that the race gas saved me from possible detonation. Turns out this new pump is very sensitive to particulate contamination. I need to remove the pump, and clean it, and install a filter before the pump. That should remedy the problem, and the car should go even faster next year.

If you race any car, get the highest octane fuel available at the track. I think that is 100 octane no lead for our Rivs.

deekster_caddy wrote:

5 deg is not that bad in a small spike. The setting you are looking for is "Power Enrichment". It's adjustable with a tuner. In the old days on a carb it would have been the "Accelerator Pump"...

Actually, power enrichment in a carburetor is controlled by vacuum, either via a power piston, or a power valve. In the case of your 73 455, the stock Q-jet has two primary jets, and 2 primary metering rods. There is a power piston that controls the primary rod's position inside the jets. The rods have a thin tip, and taper thicker higher up the rod. The power piston has a spring below it that forces it up, pulling the primary rods up, out of the jets to the thinnest portion of the rod. When the engine is at idle, and at small throttle openings, high engine vacuum holds the piston down against spring pressure, and the thickest part of the rods is held down in the jets for leaner mixtures. Under high load, and wide throttle openings, vacuum drops, and the power piston spring lifts the piston, and rods out of the jets for richer fuel mixtures.

Of course in modern engines the PCM commands the injector pulse width to adjust fuel delivery to the cylinders. Now back to your regularly scheduled programming

LARRY70GS wrote:

deekster_caddy wrote:

5 deg is not that bad in a small spike. The setting you are looking for is "Power Enrichment". It's adjustable with a tuner. In the old days on a carb it would have been the "Accelerator Pump"...

Actually, power enrichment in a carburetor is controlled by vacuum, either via a power piston, or a power valve. In the case of your 73 455, the stock Q-jet has two primary jets, and 2 primary metering rods. There is a power piston that controls the primary rod's position inside the jets. The rods have a thin tip, and taper thicker higher up the rod. The power piston has a spring below it that forces it up, pulling the primary rods up, out of the jets to the thinnest portion of the rod. When the engine is at idle, and at small throttle openings, high engine vacuum holds the piston down against spring pressure, and the thickest part of the rods is held down in the jets for leaner mixtures. Under high load, and wide throttle openings, vacuum drops, and the power piston spring lifts the piston, and rods out of the jets for richer fuel mixtures.

Of course in modern engines the PCM commands the injector pulse width to adjust fuel delivery to the cylinders. Now back to your regularly scheduled programming

Power Enrichment in a carb, but what was an Accelerator Pump on a carb is the "PE" table in the fuel injection system, as best as I can tell.
The needles you are describing do the same thing the VE and MAF tables do to fueling as airflow increases.
There is also a 'fuel enrichment over time' setting, to help manage cylinder temps for extended time spent at WOT - something you can't do on a carb!

The accelerator pump mainly covers the hole in fuel delivery as you open the throttle rapidly. When the throttle is opened rapidly, air flow changes almost instantaneously. The fuel which is heavier, lags behind causing a momentary leanness. The accelerator pump is used to provide the extra fuel necessary for smooth operation during this time. You also have idle discharge, and off idle transfer slots that contribute fuel.

Yes, fuel injection is much more sophisticated and precise than any carburetor. No argument there.

Thanks for the knowledge, guys. I made a little trip to Chicago and back this weekend. I noticed if I slowwly lay into it, I see knock start around 3.5psi. Knock is sensed until about 4.5-5psi and then drops off. I would see about ~1-2 degrees on a gradual acceleration like this.

I also noticed that if I give it a quick love tap from a roll, I should say push, that I could smell a rich mixture of fuel. Is that normal? I wasn't watching fuel trim stats but I my thought is that because I would suddenly punch it, that the ignition couldnt keep up with the injection.
Thoughts?

charlieRobinson wrote:

Thanks for the knowledge, guys. I made a little trip to Chicago and back this weekend. I noticed if I slowwly lay into it, I see knock start around 3.5psi. Knock is sensed until about 4.5-5psi and then drops off. I would see about ~1-2 degrees on a gradual acceleration like this.

I also noticed that if I give it a quick love tap from a roll, I should say push, that I could smell a rich mixture of fuel. Is that normal? I wasn't watching fuel trim stats but I my thought is that because I would suddenly punch it, that the ignition couldnt keep up with the injection.
Thoughts?

sounds pretty normal to me. Some massaging of the timing table may eliminate that light/med throttle KR.

Knock is more likely with increased load. If you slowly lay into it, load goes up quickly until the converter unlocks, and then the transmission downshifts. Both of those things reduce load and knock. The PCM should react pretty quickly to the O2 sensor input, but if you punch it, and quickly close the throttle, the mixture will enrich, especially with a carburetor. Older cars used to have a device called a dashpot that allowed the throttle to close slower to prevent stalling when the throttle was released suddenly.