Some of the talk in one of the threads here in the SBB section got into detail of the pistons and what would be optimum for the 350. I am starting this thread so that we can look further into what would be the ideal piston for this engine. Being that we have an open chamber head design that makes me think that a flat top piston design would be far less off an advantage in our 350s compared to in a closed chamber head like a typical Small Block Chevy. Here is my thinking: With a closed chamber engine there is a distinctive "squish" or quench area between the top of the piston and the flat part of the head which is commonly called the "quench pad" of the head. With this type of engine it is a huge disadvantage to have either a piston sitting down below the deck of the block or having a dish. The piston sitting down below the deck height or the dish both give a lack of squish area and therefore a lot of performance lost. With an open chamber head like the Buick 350 has there is no squish area and thus the engine is less sensitive to the piston to deck height. It seams far less critical that the piston sit at zero deck on the Buick 350 so it got me thinking about optimal piston design for these engines. Possibly a piston with a spherical dish like this one would make for a better match to the Buick 350 design being an open chamber: https://www.google.ca/search?q=sphe...Fmain_page%3Dindex%26cPath%3D8_9_15;1440;1440 To simplify this lets just say that we are shooting for a 10.5:1 static compression and we could use any length rod and compression height. Money is no object in this discussion, we can have any piston design we can think up. Something like this is about the best I have seen for a Buick 350 and it would need a longer than stock rod but it is really light, has enough dish to allow zero deck of the piston, has valve reliefs. This piston has a compression height of 1.335" and dish of 6 CC. I do think that a Spherical dish would be the best option based on some of the stuff I have read... This is also a good place to discuss rod length and how it relates to the piston heights and maybe even discuss aftermarket rod options for something that would work well in a stroker or non stroker build. Any other ideas?
I think You have the Piston right here Sean. Light, Dished and with Valve reliefs. Allowing for a longer rod also. But what about us who would rather use a stock rod. Money IS an object for me. I'd rather just buy a set of pistons OR a maybe a set of Rods. When you get into both you get into some large cash that a lot of us don't have. But with cash no object, I guess this combo would be great. I'd love it in my car!! :TU: PONCH
You make a good point! I was reading some articles yesterday about piston selection and looking at used pistons to determine the quality of the "burn". The article says that the used piston should have an even coating of carbon on the top of the piston if the engine is burning the air and fuel completely. Well last night I was looking at one of the pistons that came out of the 35,000 mile 73 engine I tore down and the piston top looks evenly carbon covered and the carbon layer is so thin it can be scraped off with a fingernail. I have always said that if an engine is running well then it is a good idea to leave the bottom end together and increase power with head porting and a different camshaft. Maybe Buick did a nice job with the stock pistons? They seem to burn well and they have proven to be reliable with nitrous and turbos. I don't have a pic of the actual piston I am talking about but here is a pic showing the design of the stock piston:
I'd like to point out a couple things regarding this subject. First, the ultimate goal is to have a durable piston that can withstand detonation (forged), yet have it so it doesn't expand too much with heat (higher silicon content) for use with milder engines and street use, without the relief notches (for better piston balance/weight distribution and no potential hot spots), has a spherical dish (as opposed to a 'stepped' dish) for better flame travel (which results in more detonation resistance and better fuel/air mixture), yet have this dish be shallower on the outer edges and deeper in the center, which would permit it to be smaller and have a better 'swirl' effect, effectively making the spherical shape with a gentler slope from the outer edge to the center. Second, give the piston a higher compression height so less needs to be removed from the deck to achieve desired compression, along with the smaller dish size, would permit it sitting further below the deck and reducing the need for piston notches, unless extreme lift cams (such as roller grinds) would warrant the need for such. Even then, if the notches are ground in, create their edges to mimic a similar slope as the dish, so they roll downward and eliminate any hot spots (if notches are even needed with this design). The Buick 350's narrow bore is going in its favor here, as the smaller combustion chamber size fights detonation by virtue of the fact that there is less room for the fuel/air to detonate before being burned by the spark-induced flame. It therefore is less dependent on quench than larger bore engines. In summary, creating a forged piston with higher silicon content (say around 11-12% silicon) so it expands similar to a stock cast piston, shaping it to better reduce detonation while making it durable to withstand said detonation. This is of course the 'ideal' vision, and there's already other pistons out there that do the job quite well that don't fit these criteria, though if they did, would make them even better, IMO. This would permit a person to simply buy a set of pistons and do little else to the rest of the block in order to accommodate these pistons. Gary ---------- Post added at 11:03 AM ---------- Previous post was at 10:58 AM ---------- The Buick engineers did a fine job at everything. The 'nipple' in the center of OEM pistons created a type of 'lightning rod' effect, so that if any detonation DID occur, it would be centralized in the middle of the piston. Might not be a terrible idea to create something similar on those forged pistons I was talking about. :grin: Gary
Sean, here's the piston you showed with potential hot spot areas that I circled using MSPaint. Anywhere that's thin or 'sharp' has the potential for detonation risk starting in that area, including the 'stepped' dish area that doesn't have round contours, as well as the abrupt, yet somewhat smooth, ridges on the valve reliefs. This is because thinner areas can't dissipate the heat as fast, and so will increase in temperature (hence the 'hot spot'), in addition to creating abrupt ridges for turbulence on the fuel charge swirl. Gary Also, notice here the black circle. This is supposed to be the 'quench area' that is raised to meet a combustion chamber that has a corresponding area on the head size for creating a thin quench zone. This does not exist on a Buick 350's head chamber, unless special machine work is done to create it. This makes this 'quench zone' moot and unnecessary for this piston. I can understand why the manufacturer did this though, because most performance engines have it: Chevy, Buick big blocks, etc. so it's only natural to go with what you're most familiar with, unless otherwise new information is introduced to indicate a better design is possible. Gary
Using these images as reference, notice the black circles. This is to point out where the curve is gentle without any ridges. This is how pistons should look that will be resistant to detonation. Decrease the depth of the dish, make the slope even shallower, so on the outer part of the dish it's shallow, then gradually increases in depth toward the center. The red circles indicate where the 'nipples' should be. Centralizing these raised portions of the dish, creating a place for detonation to occur, in the event that it does. This creates a centralized, more downward thrust so as to make the detonation practically unnoticeable and certainly less harmful. Make the slopes transition from the outer part of the piston dish to their maximum depth, then upward again to create a circular central 'plane' which is to be smooth and ridge-free. This will serve as the 'nipple' or 'lightning rod' for detonation, in the unlikely event that it occurs. Gary Additionally, here is an image of a set of cast pistons 'hi comp' with the shallow dish and spherical shape, sans the 'nipple' made by Speed Pro I think. A set of 8 can be had for 300 bucks, and come in sizes of .030 and .040, if I remember correctly. Dish size is 3.050 wide x .0850 deep, which comes to around 10.18 cc. CH is 1.835, which comes to about .040 below deck on a standard 10.187 DH. With 58cc heads, .040 gasket, and .030 overbore, SCR comes to 9.73:1 with no machine work done to the deck. Would be ideal for mild-moderate street machines with cam IVC points of 62*-68* (approximation) that can benefit from an ideal SCR of 9.75:1-10.25:1. Gary
Remember with a sbb 350 cam clearance can be an issue if the rods get longer than the factory rod. Even an aftermarket rod that is factory length can have cam interference,if it isn't built like the factory one with the built in cam clearance design. Jim Blackwood mentioned this on his 340 build when he used 7.00" Flat Head Ford aftermarket H-beam rods,that he had to do some clearance work on the cam side of the rods. The longer the rod the less it will swing back and forth and the closer it will be to the cam when it is moving up and down. Of coarse piston design will depend on how TA decides to make the head chamber on the al. sbb 350 heads. But if we're talking the factory design then I would say the spherical design would be optimal with the right cc's for the desired compression ratio starting from about 3/8" in from the outer diameter without any sharp corners. As Paul has mentioned,the flame travel would be concentated in the center of the piston at the lowest point of the sphere moving up to the outer flat ring at the top of the piston pushing it down more evenly with less scuffing of the cylinder walls and piston skirts.Less rocking of the piston will be less resistance on the power stroke unleashing a little extra HP that would be lost from a piston that the skirt would drag on the cylinder walls. This would be especially helpful with a forged piston that needs more piston to wall clearance. If valve notches are needed,then they should be machined on symmetrically(on both sides and the same size) so they don't unbalance the flame travel in the spherical dish. And if $$ were no object,then the specialty piston coatings would be a great compliment also. On top a thermo coating and on the skirts a anti-friction coating to make it more detonation resistant and to move up and down with less friction. Derek
I bought a set of 8 Hyper V6 pistons from Postons back when they were Viable. They had relief areas on both sides of the piston then a gentle slope to a shallow tub. These have worked well for my 350. I used the same design on my 455 but I have quit working on it. I didn't deck the block or heads, just used a .02 Steel Shim Gasket. I use premium gas, 93 Octane pump gas and have never had a problem with detonation. Even under a heavy load, as in my size 13 Foot. Maybe I can take a head off my 455 and get a picture of the pistons. PONCH Good to hear from you Sean. I like your Truck. Rope ladder for entry and all.
Actually that "nipple" that is left on the factory pistons were not how the engineers intended them to be. The bean counters of the day cancelled the removal process of it to save pennies per piston. If you notice in the center of that "nipple" it is center drilled so the piston can be stabilized on the lathe with a live center when the dish is being machined. But thats a nice therory that they were engineered to be "lightning rods" for detonation. When detonation accures the entire air/fuel mixture ignites before TDC,so the flame is in the entire combustion chamber,not just at the "nipple". Actually that "nipple" can cause a hot spot to make the A/F mixture to ignite if the octane of the fuel is to low for the compression ratio. And when detonation accurs even with pistons with the raised center,it will hammer the rod bearings and still can crack that style of piston. The pistons would be better off without that center raised portion,and thats why the aftermarket manufacturers take the extra machining step to remove it. Derek
That may very well be, but it would take more machining motion to 'leave them in' rather than to just keep the machining arm at the same level and go inward to shave them off. It would take more effort to create the 'nipple' than to leave it out. The idea is that should detonation occur, it would occur centralized and therefore be less detrimental. It would certainly work. (The ping sound you hear is the piston slapping the side of the cylinder wall, not a downward thrust, which is what the piston is SUPPOSED to do.) The upward curve toward the center just before the 'nipple' would also enhance combustion swirl, instead of just sending it all to the other side of the piston. Pistons should be made with this feature to enhance the combustion along with creating a 'fail safe' zone for detonation should it occur. As far as causing detonation, the 'nipple' is thick enough and round enough so it wouldn't encourage detonation at all, and certainly no more so than the valve relief edges...but would facilitate a place for it to occur should it do so. I'm not unfamiliar with people disagreeing with my ideas, especially you Derek. This is a good thing though, since it encourages thought, and thought can certainly encourage newer and better ideas, should there be an opportunity. I've discovered that I'm correct much more often than I'm wrong, even with 'theories'...this isn't to brag, but to assure people that my mind's always in the right place and is often a precursor to invention should the idea not previously exist. If it turns out I'm flat out wrong, then that's cool too. At least we'll know what not to do then. :grin: (the idea I have for the 'nipple' won't look like the factory OEM designs anyway. It will look more like a raised plateau with smooth curves like the dish on the outer portions) Gary ---------- Post added at 01:45 PM ---------- Previous post was at 01:11 PM ---------- Detonation is another word for 'pre-ignition' which is when the fuel/air mixture ignites before it is supposed to (when the spark initiates it). The entire mixture doesn't ignite all at once--it starts at the source (plug, or hot spot), then creates a 'fire ball' which expands outward from its ignition point. How the mixture burns determines where the force of its expansion will be met with resistance (cylinder walls, piston top, etc.) and if it happens too far off-center, can create an expansion that forces the piston to move sideways as well as downward, since downward is the path of least resistance usually, when the flame is expanding more evenly downward. If the flame starts to expand off-center, it can cause the piston to move more side-to-side, which causes the piston to slap the side of the cylinder wall. A downward 'pre-igntion' would be the most ideal scenario in the presence of a pre-ignition event. A pre-ignition fireball expanding to meet the ignition-induced fireball could create a bit of a shockwave reverberation, which could result in the piston slap as well, unless it occurred directly beneath it (such as with the 'lightning rod effect'). Any portion of the combustion chamber that has places where metal is thinner than the rest of the chamber will tend to be less efficient at thermal dissipation, and will increase in temperature as a result. These areas can get quite hot when compared to the rest of the combustion chamber, and are hot spots (quite literally) for a source of pre-ignition. When these hot spots are off-center, they cause detrimental effect. Removing most if not all of them is ideal obviously, so there is less chance for pre-ignition. Spherical dish shape also helps this, as well as helps with the charge swirl to encourage better mixture. It's a win-win situation with this design. The only real downside I see is the introduction of valve reliefs is also the introduction of hot spots (or the potential for them) so any time you can get away without them, the better off you'll be. Gary
Spherical dish at Zero deck clearance. Here is a great example on a 455 Buick. http://www.v8buick.com/showthread.php?213591-1000-HP-Supercharged-BBB&p=1717158#post1717158 Paul
Gary,no disrespect to you and your efforts,but when machining with a lathe using a live center you can't cut that portion off in that operation without causing damage to the center that stabilizing the piston. Not to mention without the support of that center the piston wouldn't be able to stay in the machine. A seperate operation would be needed to remove that raised section,and the bean counters are always looking for ways to cut costs,and that is why it is still on there,to save time and pennies not having to set the piston up for another operation to remove it. And so they didn't have to spend $$ on the tooling to setup for that operation,because of how mild the factory tune was they didn't see it nessasary to remove it like they did for the high compression piston that you pointed out doesn't have one. Milling the heads and decking the block to raise compression and using this sort of piston can make that raised portion get hotter than the rest of the piston in a high RPM app. Heat rises,and the floor of the dish will disapate its heat to that raised part in the center getting it hotter and hotter with each combustion cycle,creating a hot spot to pre-ignite the A/F mixture prematurely.(thats my therory anyway) Pre-ingnition(detonation) doesn't occur in one small spot,the whole combustion chamber and piston top feel it,not just that small localize spot in the center on the top of the piston,and the bearings feel it also. Detonation means that A/F fires(before the spark plug ignites) before TDC pushing the piston downward before it is all the way up. I can't understand how the A/F mixture when it ignites will only occur on the "lightning rod" portion?o No: Derek
Hot air rises. This is because heat expands the molecules, making the substance less dense. Since air is a gas, and with the cooler air being more dense, it is more susceptible to gravitational forces causing it to be attracted to the central focus of gravity (Earth's mass, in this example) than the warmer air. Thus, hot air only 'rises' because it is being displaced by cooler air that's falling. A more accurate way to put it would be 'cold air falls' But this doesn't apply to thermal dissipation in solids. There's other forces at work here too, but I won't bother going into that. Read my addendum on the lower portion of my last post to get your answers for the other questions. Gary
Just read Gary's add on to his post with the flame travel stuff. And I still don't think that lightning rod will slow that flame travel down to dampen the pre-ignition effect. I'm not convinced. (no offense Gary) I still think that post sticking up is bean counter designed. Derek
It's not going to slow anything down, it's only going to control where detonation occurs, if it occurs. I'm sure you're right on the bean-counter thing, but there's more to it than that. It's ok to disagree with me. :TU: Gary ---------- Post added at 02:55 PM ---------- Previous post was at 02:23 PM ---------- A little off-topic, but here's another little short story about electricity and its relationship to thermal expansion: First, a little lesson on what electricity is and what it does: "Electricity" is the movement of valence electrons from atoms with more, to atoms with less. Valence (outer) electrons orbiting an atom are the furthest away from the nucleus, therefore have the least amount of attraction to it and are also therefore the easiest to be moved away from it. Always remember that electricity is the movement of valence electrons. They move from a position of greater density to lesser density. This movement causes friction, which generates heat. Also remember that all things in nature tend toward a balance (PH (which is Potential of Hydrogen) balance (within our own bodies) and works similarly to electricity with the movement of hydrogen molecules in regards to the movement of positive and negative), and this process is known as 'entropy'... When an atom with a greater amount of valence electrons (negative) is near another that has less (positive), there is a 'negative draw' from the positively charged atom to the negatively charged one. When the number of valence electrons exceeds the threshold of draw from the negative atom, they 'shift over' to the atom with the lesser amount. This movement occurs from one atom to another, and is more numerous in matter that is more dense (such as solids: metals, etc.) which is why metals are the best conductors of electricity (and some metals are better than others), but a common misconception for the flow of electricity is that positive flows to negative, when the opposite is true. Electrons are negatively charged, so an atom with more of them offsets the balance to a negative charge (because they outnumber the protons, which are positively charged). This means that since the flow of electrons moves toward the path of least resistance toward the positively charged atoms (which have more protons than electrons) to balance out the valence electrons, electricity therefore travels from negative to positive. The hot spots on a metallic substance will be the areas with the least amount of mass, or the thin areas and 'sharp' edges, since there is less material in those places for heat to dissipate, generating more heat in those areas. Gasoline burns fast, but it has to have an ignition source, and when it ignites on one side of the volume, it won't cause the molecules on the other side of the same volume to simultaneously combust. There is a chain-reaction that will cause the flame to expand and burn outward from the source of ignition. This expansion of burned gas has more volume than the unburnt vapor, so it compresses the unburnt vapor even further until it too can be burnt by the traveling fire ball. If this vapor compresses too much to the point of being more susceptible to the slightest variance in temperature to ignite it, those hot spots will ignite the vapor in another area outside of the fire ball, or can in fact ignite even before the fire ball from the spark plug ever initiates (as you said). If all other hotspots are eliminated and the only place pre-ignition could occur is in one place, the odds of it happening there are pretty high, if it is going to happen at all (the goal is for it to not happen at all, of course). This is where the 'lightning rod' effect comes into play. It's not to encourage detonation, but only to control where it would happen in the event that it does. In this regard, it would be easier to just eliminate all hot spots and just hope for the best should detonation happen--which is to say, it's easier to just not bother with a 'nipple' than it is to create one based on this scenario, regardless of whether or not its origins developed in a factory where leaving them on was cheaper than taking the extra step to remove them. Inversely, this could be the reason why they're not 'built in' on aftermarket pistons. Maybe it's easier and cheaper to leave them off? Maybe the origins of the 'nipple' was misunderstood and so no one outside of OEM pistons made 40+ years ago have them? Maybe they're not even necessary? (and wouldn't be if there's no detonation) It's not like it's life and death if there's one or not, though at the very least a spherical dish design should be implemented. Gary
The dish design alone without the "lightning rod" would be enough to control where the flame travel would create a downward force. The A/F mixture would be pulled in the deepest part of the dish as the piston is filling the chamber,and would be concentated at the deepest part of the piston on the way up. And even if the flame started on the top of the dish it would be pushed to the bottom of it as the piston is coming up(even beter with a spherical dish). This alone would slow down the downward force of detonation because of the open chamber design without a quench pad,I would think. This does make sense why an open chamber design is less sensitive to detonation. In your flame travel discussion you make it sound like the A/F mixture is just starting to enter the combustion chamber swirling around pre-igniting. But by the time the piston is on its way back up the mixture is all done doing its swirling,on its way back up it is being pushed into the dish and compressed for ignition. A hot spot at the edge of the dish or a carbon deposit in the head chamber can cause the pre-ignition,but the flame would be forced to the lowest point of the top of the piston with or without the "nipple". But usually with inadequate octane for the compression ratio,the compression ALONE WILL ignite the mixture,not a hot spot.(like a diesel) All of your assumtions are based on having a hot spot,and not to a to low of octane rating and pre-igniting from to much compression for the octane used. Pre-ignition can occur on an engine that is ice cold and reved up from start(I wouldn't recommend doing this though) with not enough octane causing it to ignite from being compressed more than the octane rating can handle.(like a diesel) Good discussion Gary,but you're making my brain hurt from thinking.LOL Derek ---------- Post added at 04:49 PM ---------- Previous post was at 04:30 PM ---------- Thats what I was thinking,as soon as the mixture ignites,all of it is burning. To cause the piston to tilt from an uneven burn there would have to be some sort of barrier of part of the piston and chamber to put uneven force to make the piston tilt. The flame would travel to the least resistance before it would start pushing downward. Also 87 octane in an 10:1 dynamic compression engine can ignite from the heat caused by the compression without any hot spots in sight. Thanks for your input Paul. And Gary,thanks for the head ache.LOL(no offense) Derek
Detonation and Pre-ignition are 2 different reactions and they occur at 2 entirely different time periods Detonation occurs outside of the flame front so it usually happens away from the spark plug location. After the spark plug fires, the combustion pressure increases rapidly. If there is a hot pocket of air/fuel mixture, the increasing cylinder pressure will raise the temp of the hot pocket until it auto-ignites before the flame front gets to it. That's why detonation can most times be controlled by retarding ignition timing. The pocket ignites entirely at once. There is no flame travel!! The shock creates a resonance within the cylinder at a frequency of 6400 hertz. Knock detectors are set to detect that frequency and that is how they can Distinguish knock noise from all other engine noises. It is that resonance that you hear as ping and not a mechanical response from the pistons or rods. I have 8 pistons from our blown alky engine where the 6400 hz detonation resonance micro welded the top piston ring to the piston Pre-ignition happens before the spark plug fires. The air/fuel mixture gets harder to ignite as the cylinder pressure increases so pre-ignition generally occurs when the piston is low in the cylinder. Hot spots or cross over from spark plug wires can cause pre-ignition. If the center nub on the 350 piston was to cause pre-ignition, the ignition is soo early that the air/fuel mixture completely burns way before TDC so it doesn't matter where the source location of pre-ignition is. Paul
I think there's some misunderstandings going on here. I never said that the hot spots were the only source of pre-ignition, and according to Paul, only compression can cause detonation (but only if it's not compressed too much) and all the molecules simultaneously ignite... Very interesting science going on here. I'm sure there's a frequency range that the knock sensors detect, simply because different thicknesses of metal would absorb some of the vibrations and alter the frequency at which the sound would be once it reached the sensor. I don't know how my words got misconstrued to interpret them as the fuel charge was igniting as it was still being drawn into the combustion chamber, or that there was this turbulent, tumultuous environment during the compression stroke where everything was still being mixed... I guess the hot spots are a myth then. No one should waste their time polishing heads or pistons and should in fact nick them up as much as possible with rasps and files in hopes of creating as many burs as possible, filling them up with 4 notched grooves with razor sharp edges and the air flow will be just fine--even better since those burs will help everything mix together (may as well not bother cleaning up intake runners either, since this principle can apply to that as well with all the casting imperfections), as well as no worries for pre-ignition since it's only possible with compressed pockets of fuel/air, and when they do ignite, every molecule burns simultaneously with no ignition source and produce a very specific 6.4 Khz with no variation consideration for sound travel through varying densities, all while welding metals together with sound waves instead of heat. Cool. Looks like I've been schooled. Always eager to learn new science. I guess those quench plate notched pistons are the way to go then in an open chamber design. Just make the piston like the big block piston and all is well, providing it's at zero deck and there's plenty of quen...wait, where's the quench coming from again? Doesn't matter. Just build it like a Chevy and/or a BBB and it'll be just fine. Right? lol...well, at least the spherical dish part was correct. Gary PS ah, some editing going on. Thanks for the clarifications. :laugh: :TU: ---------- Post added at 04:38 PM ---------- Previous post was at 04:22 PM ---------- What matters is the correct information is conveyed. Dispel or confirm theories so people will be guided in the right direction. Thanks Paul and Derek! Never a dull moment! Gary
Gary Were you having a case of detonation or pre-ignition in your last post? LOL No previous scientific understandings were trying to be undone. Hot pockets of air/fuel mixture are still caused by hot spots in the combustion chamber and piston head. Please continue to polish and remove rough castings. Quench, swirl and tumble still keep the mixture moving so it doesn't have a chance to develop hot pockets. Sharp edges still can get hot enough to cause pre-ignition. And yes, it should be understood that there is a range around 6.4 khz for resonance response. Just trying to keep responses short without a lot of detail Paul
