leaftye

This article got me looking for more information about combustion chamber design, and how it affects the combustion cycle. While this post may seem unrelated, I do believe understanding the dynamics of combustion contributes to a better understand of timing.

WAHUSKER, this should help you understand how more air and rpms doesn't require an earlier spark. If I understand it, more air and more pressure creates a faster burn, which means less timing, or no timing at all needs to be added.
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Fire in the Hole - How combustion chamber design relates to engine performance

By Ray T. Bohacz

Most aspects of a cylinder head are relatively straightforward and easily grasped. Valve size, deck thickness and combustion chamber volume are often topics of discussion and qualify the design's ability to be a player. But there is much more to be considered. The characteristics of the combustion chamber will dictate the engine's power, octane tolerance and brake specific fuel consumption (BSFC). So if you thought the shape of the combustion chamber had little to do with a cylinder head's performance, you will be surprised.

Author's note: Due to the nature of this story, references to cylinder heads from manufacturers other than Pontiac are used to demonstrate different combustion chamber technologies.

Basic Combustion

The flame from a candle, a simple form, has a key element of combustion related to that of an engine. But taking place in the atmosphere, it differs from that of an engine where the gas exchange process occurs internally under higher than atmospheric pressure. A flame can have two distinct regions: pre-mixed and diffusion. A burning candle experiences a diffusion flame because it occurs at the interface between the fuel and the oxidant. With a candle, the fuel is melted and evaporated by the radiant heat of the flame and then oxidized by the air.

A more complex example of combustion is a Bunsen burner which has both pre-mixed and diffusion flames. It consists of an air regulator, fuel source and a cylindrical tube. The flame generated nearest the base is identified as pre-mixed. The air entering at the base of the Bunsen burner is not sufficient for complete combustion. Consequently, a second flame front above that point is established at the interface where the air is diffusing into the unburned fuel. This is responsible for the Bunsen burner's flame-within-a-flame appearance.

Even though the combustion event inside an engine is quite a bit more complicated, the basics still hold true. Gasoline, a hydrocarbon-based fuel, needs to be atomized and emulsified (broken down into small particles and mixed with air) to burn. It will not burn by itself in liquid form. When atomized, gasoline has a laminar burning velocity of approximately 0.5 meter/second (m/s) or 1.64 feet/second. As a comparison, acetylene mixed with air burns at a rate of 1.58 m/s or 5.18 feet/second. The slow laminar burning speed of gasoline poses an interesting problem when used as a fuel for an internal combustion engine.

Since this is best represented using metric measurements, ignore the dimensions but accept the concept. Given a cylinder with a 100 mm diameter and an ideal central location for ignition, the time for a gasoline-fueled flame to travel this distance is 100 milliseconds. The problem is that when an engine of this dimension is running at 3000 rpm, there is only a window of 10 milliseconds for the combustion event to take place. Obviously another force must be at work because we all know that a gasoline engine can operate at speeds substantially higher than 3000 rpm. The key is to increase the burn velocity.

It has been established that the flame in an engine travels across the bore at a rate of 10-25 m/s. This is substantially faster than the velocity stated earlier, but it is the reason why gasoline can be used as a motor fuel. To increase burn velocity, turbulence needs to be introduced to the combustion event. In an engine, this is accomplished by the induction and compression process along with the design of the combustion chamber. During pre-mixed combustion, the effect of the turbulence is to break up or wrinkle the flame front, creating burnt gases in the unburned region, and vice versa. This effectively increases the flame front area and speeds up combustion. Though diffusion is usually associated with a compression ignition engine, better known as the diesel, it can also occur in a spark ignition engine when stratified charged. The fuel would be injected in a fine spray and the turbulent air motion would sweep away the vaporized fuel and combustion products from the fuel droplets, speeding up the burn velocity.

The actual combustion event that causes a flame front to be established and expand against the piston is very complex. At this level a full grasp of the chemistry involved is not required, but the laws of thermodynamics, the study of energy and its transformations need to be touched upon briefly. Consisting of two statements deemed laws, the first says that energy cannot be consumed or destroyed; only its state can change. In simpler terms this can be applied to an engine and how energy is turned into heat, then motion, and back to heat. The second law is more complex but can be summarized thus: energy follows certain guidelines and never deviates. For example, heat will travel only from hot to cold without an external energy force being present. The laws of thermodynamics apply to a combustion chamber directly due to thermal transfer into the casting and engine coolant, along with the effect that the compression ratio has on thermal efficiency.

A common analogy that compares an engine to an air pump establishes the fact that the more air pumped, the greater the output. This cannot be denied but it's a one-dimensional statement ignoring the fact that without an efficient combustion event, the air by itself can do nothing. For this reason we need to examine the impact the combustion chamber has on an engine.

hondo

iTrader: (2)

wow that was a great thread ...thank you

leaftye

The Combustion Chamber

In 1673, Christian Huygen, King Louis XIV's water keeper, invented the first engine. It was developed as a better means to transport water from the Seine River to the grounds and gardens of the Palace of Versailles. This gunpowder-consuming single-cylinder external-combustion behemoth was welcomed by the peasants and oxen that were used as water carriers prior to its existence. As the internal combustion engine slowly progressed from these humble roots it was discovered that efficiency and power could be increased with a controlled process in a closed environment. The early combustion chambers were simply little more than covers for the cylinder. A major breakthrough in combustion chamber design was realized by Ricardo, who invented the turbulent cylinder head for a side-valve motor. Taking place in the early 1900s, it set new standards for compression ratio, running at 6.00:1. At that time, the fuel had an octane rating of only 60 to 70. Over the next decades the impact the combustion chamber had on the character of the engine was accepted and explored. A major breakthrough occurred in 1951 when the Chrysler Corporation introduced its hemispherical combustion chamber on its 331-cubic-inch V8. Today, combustion chamber design and technology are constantly evolving and producing smaller, higher specific-output, fuel-efficient engines.

Criteria that drive combustion chamber development involve many facets. The distance that the flame front needs to travel should be minimized. It can be accomplished by reducing the distance from the spark plug electrode to the incoming charge, called the end gas. This allows for higher potential engine speeds, which will produce more power. In addition, there is less time for something to go wrong. Abnormal combustion, better known as detonation, is more likely with a slow combustion process since it allows time for an additional flame front to start.

Each spark plug should be centrally located in the bore and nearest the exhaust valve because these are the most turbulent and hottest parts of the combustion chamber, respectively. Additionally, the exhaust valve should be as far from the intake valve as possible, limiting thermal transfer to the fresh incoming charge.

There needs to be sufficient turbulence to promote rapid combustion, but too much can create an issue, transferring heat away from the chamber and promoting noisy combustion. This turbulence is generated by design and can be induced either externally in the intake port, or internally using squish pads. The clearance between the deck of the cylinder head and the piston is identified as the squish region. It acts to cool the intake valve and is best located near it.

Valvetrain design and the number of valves impact the combustion chamber with concerns for valve placement, size and actuation. As you gain knowledge in this area you will see that many Pontiac engines have very poorly designed combustion chambers due to economic pressures. Another case of greed by corporations and the power of the bean counters and stockholders. No traditional domestic pushrod V8 engine with inline valve placement other than the old Chrysler Hemi allows for a central spark plug. What is often done though is to design the chamber to use a long-reach plug that places the electrode tip near the center, even though the entry point is at the perimeter. The General Motors LS1 and L31 Vortec castings are excellent examples of this method. Disappointingly, many cylinder heads place the spark plug electrode at the perimeter of the bore, and some early Pontiac V8s actually had a bias toward the intake valve, allowing function to take a back seat to ease of manufacture.

With the laws of thermodynamics presented, the ideal engine would have a high compression ratio for thermal efficiency and throttle response but would need to work in unison with a combustion chamber that has a fast burn rate. This is essential to increase the engine's octane tolerance and limit the production of the emission oxides of nitrogen (NOx). This poisonous gas is responsible for photochemical smog and has been the impetus for enhanced emissions testing such as I/M 240. It requires three elements to be produced: heat, pressure and exposure time. A high compression ratio increases the production of NOx by its elevated cylinder pressure and the heating of the charge as it is forced into a smaller region. This phenomenon can be cheated by the implementation of a fast burn rate, eliminating the third element, exposure time, in the recipe for NOx. The best production example to date for a balance between octane tolerance and high compression ratio with quick burn rates is the May Fireball combustion chamber, produced by Jaguar in 1982, which allowed 11.0:1 compression ratio on 87-octane fuel.

Other factors that come into play are the material used and the spark plug location. As mentioned previously, the start of the flame in the center of the bore allows for a quicker more even burn that translates into higher cylinder pressure in less degrees of the crankshaft's rotation past TDC. Looking beyond emissions and octane tolerance to produce power, it's necessary to have the cylinder pressure rise as quickly as possible, allowing it to be used to expand against the piston for as much of the stroke as possible.

Most performance aftermarket cylinder heads are aluminum castings due to their light weight and ease of porting and manufacturing, along with the ability to dissipate more heat and allow a higher compression ratio. But it is often overlooked that it's easier to produce power with a cast-iron head if all factors of design are the same, due to its superior thermal efficiency. When switching from iron to aluminum the engine will require about one additional point in compression ratio to maintain the same thermal efficiency. This is due to the cast iron's ability to hold heat and use it to expand against the piston.

Of great concern to the combustion engineer but never mentioned in the aftermarket is the surface-to-volume ratio. This minimizes heat loss into the casting and water jacket of the cylinder head along with reducing hydrocarbon production. It is desirable to have a surface area as small as possible, relative to the volume occupied by the chamber. It can be derived with the following calculation:

surface-to-volume ratio =surface area/volume of chamber

Hydrocarbon emissions are created due to the outer layers of the mixture being cooled in the region of the chamber walls of designs with high numeric ratios. The flame cools as it approaches the chamber wall, extinguishing and leaving a layer of hydrocarbons behind. The hemispherical combustion chamber offers the best surface-to-volume ratio and tests conducted by Chrysler in 1950 showed that to match the thermal efficiency of a 7.00:1 compression ratio Hemi engine, its previous combustion chamber would need 10.0:1 at 1200 rpm, 9.4:1 at 2000 rpm, 8.9:1 at 2800 rpm and 8.5:1 at 3600 rpm. The required compression ratio drops as engine speed increases due to gains in volumetric efficiency at higher piston speeds.

leaftye

Types of Combustion Chambers

Most of us are familiar with the terms open and closed when referring to a combustion chamber, a term popularized by Chevrolet with its big-block engine series. In fact, most times Pontiac cylinder heads are classified as "closed" chamber up to 1967 and "open" from 1968 forward. There has been much confusion over these designations. In fact, they are not even engineering jargon to identify a combustion chamber, and are arbitrarily used to describe the squish-to-bore-area relationship. A combustion chamber is nothing more than a cavity in the cylinder head casting, with the exception being the bowl-in-piston designs used in many diesel engines. The relationship to the area of the bore that is consumed by the combustion chamber quantifies whether a chamber is opened or closed. The easiest method to determine this is to place the proper head gasket on the deck of the cylinder head to orient the bore position. If a large amount of the deck surface of the head is exposed to the bore, the chamber can be considered closed. The portion of the head's deck that is outside the combustion chamber but exposed to the bore is used as a squish region. Its function is to create internal charge acceleration that stimulates the end gas and increases the burn velocity as it rushes to escape this area as the piston sweeps toward TDC. It is considered internal charge acceleration because it's created in the bore.

To properly identify a combustion chamber, all its aspects including shape need to be considered. For our purposes we limit the discussion to those found on most production engines in America.

Hemispherical or Pentroof

A chamber of this design is considered to offer the least amount of compromise for the efficiency gained. The valves are placed at the bore perimeter and, in the instances of the original Chrysler Hemi, at an included angle of 58.5* from the crankshaft centerline. This position also allows for huge airflow gains since it moves the valve away from the wall and unshrouds quickly. This creates a more efficient cross-flow movement of the charge during overlap and limits thermal transfer from the exhaust valve to the fresh charge. As mentioned previously, this design offers the best surface-to-volume ratio and also creates a very short direct exhaust port, essential in limiting heat rejection into the coolant. Having a central spark plug, the Hemi offers excellent octane tolerance. At the perimeter of the bore across from the valves are small squish pads to help move the end gas over to the spark plug and increase burn speeds. With pushrod designs, the valve placement requires dual rocker shafts but lends itself very well to dual OHC configurations. An additional benefit is the distance between the intake and exhaust valves, which further limits heat transfer. The incoming charge also generates a high rate of tumble.

Mickey Thompson experimented with Hemi heads on Pontiacs in the 1960s and you'll recall that the division designed an experimental aluminum Pontiac Hemi engine, reported on in the March 2002 issue of HPP.

Wedge

Used over the years by almost every manufacturer including Pontiac, this chamber resembles an inclined bathtub recessed into the deck of the head. Inline valves are normally tilted to accommodate the sloping roof of this design. The spark plug is located on the thick side of the wedge and is usually positioned midway between the valves. The inherent steep walls work to mask the air/fuel flow path and deflect and force it to move in a downward spiral around the cylinder axis. During the compression stroke, the squish area reduces to such an extent that the trapped mixture is violently thrust from the thin to the thick end of the chamber.

Bathtub or Heart-shaped

The bathtub designation is generally reserved for any chamber that's not a wedge or hemispherical. Most domestic engines of pushrod design have used it in varying forms. In some instances the shape of the combustion chamber was almost oval, with the latest trends being the efficient heart shape. An example of this would be the current L-31 Vortec, LT1, LT4 and LS1, all by Chevrolet. The deck of the cylinder head that overlaps the piston forms two squish regions: a large area across from the spark plug and a smaller region on the opposite side. Its crescent shape has nicknamed it the heart chamber. The valves are inline and are partially masked by the chamber wall being more exposed on the plug side. The area across from the major squish region is generally tapered and does not have the steep wall of a wedge style. Spark plug location is maximized by biasing toward the exhaust valve and as central as possible, working under these limitations. Heat transfer from the close proximity of the valves limits volumetric efficiency and octane tolerance.

Bowl in Piston

To the best of HPP's knowledge, this style has not been utilized by Detroit on a gasoline engine in the last fifty years but is common in Europe. It consists of a flat cylinder head deck with a single row of valves facing a circular cavity cast into the piston. An annular squish region is created around the piston perimeter. Known for very turbulent combustion, it works well for diesel engines but was deemed excessively noisy for American standards.

Making Sense of It All

Since we don't have the means to create our own cylinder head, we're forced to work with what is available. The theory of combustion chamber design and function was touched on only briefly here; many have spent their entire lives studying this with new discoveries each day. Our reasoning was to establish that more than flow numbers need to be considered when choosing a Pontiac cylinder head. How the combustion chamber uses the airflow is just as important as the flow value itself. Even the worst combustion chamber design can be improved upon by smoothing the walls and surface of the chamber to increase flame speeds, reducing the volume of the squish region with a zero deck or thinner head gasket, and indexing the spark plug. The worth of these simple tricks is diminished, as the design of the chamber becomes better, but should not be forgotten.

Airflow numbers are easily obtained on a test bench but a trained eye is needed to identify a more efficient combustion chamber. A good rule is to query the manufacturer on the amount of spark advance his cylinder head would require with your combination. The more lead it needs, the greater the propensity for detonation and the slower the burn speed.

Head Games

Here is the evolution of the combustion chamber for high performance Pontiac engines. Special thanks to Jim Taylor and Mark Erney for their assistance in obtaining this photography.

As you can see, the "basic" design of Pontiac's fully machined combustion chamber was little changed over its history. It is actually a combination of the wedge and bathtub style. However, its size and the valve placement were modified as needed. The early 716 Tri-Power heads shown feature 1.92/1.66 valves and are referred to as "closed chamber" in the hobby. In 1967, as shown on the 670 heads, the "closed chamber" remained but the valve inclination was changed from 20* to 14*, which provided space for larger 2.11/1.77 valves. The chamber was also relieved on the intake side.

For 1968, the chambers were opened up, reducing the shrouding of the valves. In mid 1968, the round-port Ram Air II heads debuted and as you can see, the chamber shape was subtly changed as well as compared to D-port heads. It was done by opening up the area around the valves on the spark plug side and adding a small scallop above the plug hole. The same holds true for the 1969-70 Ram Air-IV heads and the HO and SDs of the '70s.

A special treat is to see a Ram Air V head. The chamber is exclusive to the Ram Air V and closely resembles that of a Tunnel Port Ford head of the era. Valve sizes would be a whopping 2.19/1.73.

Chamber size varied by year and application, with early 400 heads using a 71-72 cc chamber in most cases; some chambers were as small as 67 ccs. The 1971 and later heads exhibit a much larger chamber to reduce compression with the 96 400 head featuring a 96 cc volume and the 455 HO heads featuring large 111 cc chambers. The 1976 6X 400 head reveals the smaller 1.66 exhaust valve that returned in 1973 on all D-Port 400 and 455 4-barrel heads. Its chamber size can range from 95 ccs to 101 depending upon application, to provide a compression ratio as low as 7.6:1 in 350s and 400s.

The LS6 head displays the current thinking at GM regarding combustion chambers. Note the differences between this chamber and the vintage Pontiacs, as discussed in the text.--Thomas A. DeMauro
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There are pictures on their High Performance Pontiac website.

leaftye

Don't forget the laminar layer on the top of the piston. So long as air velocities are not strong enough to destroy the layer, thermal efficiency would remain high.

I think you're confusing "could" and "should". Yes, higher pressures will force the fuel mix to burn faster, and force the LPP to occur earlier. It "can" occur at any point, even during BTDC, but to produce the most power it "should" occur at 14 degrees ATDC.

I understand where you're coming from, and you may be right. I'd rather go for the ideal and coat the valve faces, and install a colder plug that protrudes less into the chamber like the Brisk plugs.

Grant B

Good point, didn't think about that.

I think you misunderstood me. I was saying for best power, IMO, peak pressure should occur at different angles depending on the engine geometery. We all know rod ratio effects piston speed (albiet very slightly). With a lower CR, cylinder pressure drops less rapidly as the piston moves down the bore, right? So the point where crank angle times cylinder pressure is greatest should be farther down the bore. IMO.

I read that in a book at our University's library, and I've heard it mentioned other places as well. Someone did a study with the spark plug in different positions of a 2v head, and found better knock resistance if the plug was closer to the exhaust side (of course this also hurts combustion time). In theory it just keeps the end-gasses cooler. Look at the Yamaha R1 heads, some 5V Audi heads, and even the LS1 heads. They all try to bias the burn towards the exhaust. The LS1's plug looks like its angled slightly towards the exhaust valve.

Coating the valves helps? Are you saying end gases are heated significantly by the exhaust valve(s)? Also, why do a more recested spark plug? Although I haven't been around that long, it seems like pre-ignition due to plugs is very rare (without some det to heat the plug up in the first place, of course).

Given the amount of free real-estate these large-bore 2v motors have, I wonder why they don't try dual spark plugs? From the literature I have read, it does slightly decrease combustion time if the plugs are positioned at opposite ends of the CC.

leaftye

Moving the spark plug closer to the exhaust valve(s) moves the heat (and transfer of it) away from the intake charge. It is that way in the LS1 heads, or at least it is in the pics I remember seeing. I have a set of 241's and plugs sitting around I need to take a better look at. The plug next to the exhaust is a good thing to do with the way these heads are designed, any may always be necessary. Ideally the plugs would be at a point in the chamber where the distance to the edges are as equalized as possible.

Coating is a little confusing. It can reflect heat back into the chamber and prevent heat from being transferred out of the chamber and into the heads. Both of those are good properties. One possible problem is that the coating itself can heat up and remain hot. Minimizing the thickness of the coating reduces the amount of heat the layer of coating can retain. Coating the combustion chamber and polishing the piston tops should minimize hot spots, which minimizes the chances of detonation & pre-ignition, which in turn allows timing to be reduced.

So let me now try to answer your question about coating the valves. It's not the end gases that really matter, although they heat up the exhaust valve which in turn heats the intake charge. Actually anything in the chamber can heat the charge, but exhaust ports do tend to run hot. Coating the exhaust port also helps minimize the heat transfer to the intake port. I'm still not totally sure if coating the exhaust valve has negative effects.

A piston is cooled by oil being squirted on the back side of it, and some pistons are designed to contact the cylinder liners to get extra cooling. An exhaust valve is cooled from time on the seat, the valve guides and the oil squirted on the top of the piston. While the coating may still get hot and cause problems, I think the exhaust valve gets enough cooling to be okay, and do remember that the exhaust valve is going to get hot without the coating too. 1st answer: Yes, I think coating the valves helps. 2nd answer: A small part of the end gases is heated, and given enough heating, detonation will occur, but it won't heat the charge enough to cause pre-ignition unless something is SERIOUSLY wrong.

Obviously we can't move the plug, so I'll do what little I can do by coating the combustion chambers and exhaust ports.

I kind of like the dual plug idea, but the idea of having to change twice as many plugs makes me shudder! I already have one tough plug, and being the pessimist I often am, I bet those extra 8 plugs would all be next to impossible to R&R.

As for the Brisk plug, I'm thinking that since it doesn't have any part of it hanging way out, it should be more effectively cooled by the cylinder head, and thus have less problems with detonation/pre-ignition. Honestly tho, I'm just bringing it up because I think it's a cool kind of different, and I want more input about them!

Grant B

For the fastest burn, yeah. But that may not lead to the best detonation resistance.

I see we were talking about different things... I was considering the detonation resistance of the chamber, and you were talking about its thermal effeciency. Of course they are related, but not the same.

Do you think polishing the combustion chamber helps? I have heard some engine builders tell me it doesn't. They say it could be the thinner carbon layer on a polished chamber retains less heat than a rougher chamber. Then again, if you need more heat in the chamber, why not first try a hotter thermostat?

According to the book, the advantages in burn time are very small. I can see why it wouldn't be worth the trouble. BTW, I had great luck with denso iridium plugs on my Supra, 2 years of daily abuse at 19 psi of boost and they never missed a beat. Some turbo guys are still chaning copper plugs every 3000 miles Yeah I tried copper, and I'm sure it is less likely to pre-ignite because of copper's high thermal conductivity, but I never let the car knock enough to pre-ignite and I hate loosing a race because I put 4000 miles on a set of plugs and they started to miss at WOT.

OK, but who really has pre-ignition problems that aren't caused by detonation? We've all seen plugs melted before, but every time I've seen that its been caused by rampant detonation. Seems like a cooler plug isn't treating the cause.

From what I've read if a plug, piece of carbon, or part of a coating heats up and becomes a hot spot that causes pre-ignition, its because detonation heated it up. And detonation starts with the end-gases exploding about 20 degrees ATDC. If this is correct, it makes sense to fix the causes of detonation and not one of the causes of pre-ignition.

One thing I REALLY want to know (back more to the topic of this thread), is which LSx heads run well with less timing? Seems like the LS1 and LQ9 heads are missing a quench pad. The LS6 and AFR heads have 2. If I had to guess, I would say the later two heads need less timing and are more effecient overall. Is this correct? If so, they should be able to take more CR/Boost as well. I always found it odd that most people seem to consider 11.5 to 12:1 the limit for 93 octane. Plenty of 4-valve heads with much less quench area run more compression than that.

FASTONE

Great topic J-rod,pretty deep but very informative.I always noticed that the Gen 3 needed less advance then most smallblocks,say around 28 degrees while a typical sb likes 32 to 36 degrees.Just to back up what you said about chamber design take the sb Vortec head,it works great with about 2-4 degrees less advance then a older design head.Adding more advance would either: not make any more power or power would drop off.I"ve heard of guys running 42 degees on the old 062 fuelie head,wow!!I think piston design has a lot to do with flame travel too,as you know the ls1 piston has a flat top with no relieves,this has got to be great for a faster burn rate.I always thought a single valve trough relieves would work better then 2 separate relieves,this may not be the case as 2 relieves may promote more swirl to the air\fuel mixture therefore more hp.Anyway good food for thought as always!!

Ed Curtis

You mean things like THIS

Ed

leaftye

Do you have any idea of the controller for the new MSD coil packs will be able to do the same things?

J-Rod

That would be it... That box is has more neat features than most folks realize. Slew rate rev limiter, high speed retard... Very, very cool stuff. And it works....

Ed Curtis

I know!

`

What's funny is some sanctioning bodies allow this unit yet frown upon data aq systems and traction control... Hello????

Ed

zamboxl

iTrader: (1)

ok people please take it easy on my if my analogy is totally off iam just a humble boy trying to make sense out of all this hight tech stuff. Ok so detonation is ignition of burnt gasses, so lets say if we where to put a cam in the car with a longer exhaust duration that would help with detonation since it clears out the gases better leaving less in the chamber to detonate??? Also if lets say you had bad rings and oil was blowing by and getting burnt up creating hot spots in the chamber because oil doesnt burnt nearly as easily as gas, well how would pulling timing help prevent detonation in this case since it is being cause by a heat source in the chamber.

I am just asking couse i think i got detonation ( i knew i had detonation but now after this article i am going to go with think untill the experts verifie) well the car fells like a machigun under medium acceleration at around 1.5k rmp to abut 2.2k rpm it soulds like trtrtrtrtrtrtrtrtrtrt then after wot it goes away. The other day however i was city driving for a while during rush hour and i notice that under acceleration it still would detonate a litte after wot, but only after driving for a long while it did this, i also notice that my engine runs just above the 210 temperature line andi mean just above like a milimiter above. So i was wondering if gettign a 160 thermostat, and pulling some time would help my situation out, o also would msd help this out at all since they got that multiple fire system that more tourghly burns the mix, or does our stoch ignition does a good enough job with that already??????

o and J-rod you the man awesome stuff dude

zamboxl

iTrader: (1)

anybody??

EdmontonSS

Ok, detonation is "uncontrolled" ignition of UNburnt fuel/air. Burnt exhaust gas present in your intake mixture will actually cool combustion chamber temperatures, hence EGR is used to reduce NOX emmisions (a product of higher-temperature combustion). A cooler thermostat (combined with fans programmed to come on at a cooler temperature) will help detonation, as the coolant can take more heat away from the chamber in a given amount of time. However, this can be overdone as well, just as ignition timing, there is an optimum coolant temperature for emmisions (factory settings) and there is an optimum temperature for performance. Optimizing your coolant temperature alone will not generally give you much extra power, however it MAY allow you to run more timing, which MAY or MAY NOT allow you to make more power. It WILL make you less prone to detonate, all else being equal. If you are in fact detonating now, you may be running too lean, may have too much spark advance, too high a heat range on your plug, or too high of a coolant temperature, or even a mechanical cause such as a poorly seating valve or carbon buildup in the chambers. Because your detonation only occurs primarily in one RPM range, I would tend to think the cause is in tuning, or lack of tuning if you have modified the engine (such as EGR removal, aftermarket MAF, timing tricker, etc.) I would hope any ring seal problems you may have would be far too minor to cause detonation... A multiple spark, or hotter spark is NOT the solution to detonation. A hotter spark MAY establish a better flame front and may allow the burn to occur in a shorter time frame, not allowing the UNburnt gasses enough time to asorb enough heat energy for auto-ignition.... BUT, the LS1 engine have a very good ignition, and I imagine you are VERY unlikely to make that much better flame front from any other ignition out there, at least on a near-stock engine. One degree of ignition timing would make ten times the difference a hotter spark could, in my opinion. I think a cheap and easy solution for you may be a MAF-translator, I would try to richen her up a bit in the mid-range if i were you, just enough to stay out of knock. And please take off any timing tricker, or aftermarket MAF you may have put on there... Unless you're willing to tune for the new MAF of course

zamboxl

iTrader: (1)

cool thanks for the info man, yea the car is due for a tunning iam just waiting to get the new heads on so i could do it all at once, the only reason i was a litle worrie is becasue what i said about the engine detonating more once heated and runnig righ above the 210 mark. After reading the write up i guess the tunnin or lack there off could be casuing the detonation wich casues the engine to heat up making it worse after running in sitty traffic for a while. Sorry bout the hijack guys, handing thread back to the experts 3, 2, 1, now

J-Rod

Sorry I haven't responded in this thread, I've been busy. I'd like to point out what I'm talking about. I see folks adding a bunch of timing to a motor, and now there are folks selling a water injection kit to help combat the knock retard, and make more power in a N/A car. This confuses me since I suspect if the car had less timing, ther would be no need for a water injection kit, and as efficent as the ls1 is, it wouldn't nee the extra timing to begin with.

Let me cite some examples.

Ross Batrick (rbatrick) and McRat did EXTENSIVE testing with Z06's. Someone else (and I forget who it was) all did tesating with the car strapped to the dyno. They tested the stock Z06 @ 22 degrees (stock timing) and then began to add timing all the way to 28 degrees. Guess what. The car made no more power @ 28 than it did @ 22.

In an Ls1 you don't always need tons of timing to make good power. Unfortunately some folks seem to think that you keep giving it timing till it pings, and call it a day.

As an example...


Supporting threads

http://www.z06vette.com/forums/showt...threadid=20789
http://www.z06vette.com/forums/showt...threadid=20818
http://www.z06vette.com/forums/showt...threadid=20948

When the car doesn't make any more power don't keep adding timing. The biggest gain will be from leaning the car out a bit. Once that is done, then check for how much timing is required. The Ls1/Ls6 heads are efficenient, and don't needs gobs of ignition timing...

marc_w

This is really interesting stuff.

I've always been on the wrong train of thought when it comes to timing. You hear people say, "I'm running 28* at 6,300rpm", as if it's a goal to run as much as you can up top.

I went and build anoter HO timing table for myself this weekend. I took one of the stock graphs for a particular g/cyl range that was about four to five g-cyl ranges down from my WOT logged values. It had a 27* hump in the middle (2800-3200) that tapered off to 23* at my 6,000rpm shift. It just looked like a nice smooth set of numbers to run. I've never run that much spark in the midrange and I was real carefull about KRl as I started some little test-runs.

I can't give you any hard numbers, but SOTP, I've never felt the truck pull as nice as that in the midrange, and rev out so cleanly and effortlessly.

The whole time I've been trying to dial in the timging with a competely wrong shaped graph in mind.

I was previously running 24* in the midrange, climbing to 27* at the top. I was fighting with frequent bursts of KR during heavy acceleration in the 4,000 range.

FWIW, Stock I was seeing a peak of 17* at 3,600, tapering down to 6* at 6,000. Needless to say, it was a dog.

J-Rod

Here is the stock Z06 high octane table....