m79ded
Well-Known Member
I have a similar setup What pistons did you use? I have the Ross flat top with .010 below the deck. How big is the cam? I can get a decent idea if I would run into problems
Max Dynamic CR with 92 octane and sea level?
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Is it untrue that when an engine gets up in the higher rpm ranges there's not really enough time for pinging to occur.....?
moper
Well-Known Member
At least in terms of me - and thinking a little deeper in response to the question... I don't think that's true. Actual burn time of any engine will vary with the parts and fuel being used. Plus the time it takes to complete a controlled combustion event is a bit less than the time it takes for the pressure energized by that combustion event to build. IIRC it's on the order of 1-2 crank degrees for combustion and 10-14 crank degrees for max cylinder pressure. Ping is just ignition from another source than the timed spark, and it removes pressure (power) from every event it effects. Depending on the ignition source, time of non-spark ignition, and engine design, ping can go on for millions of cycles with no damage. The 3.5 in my LHS pings over the winter - all winter - because of the ethanol blend and leaner air and there is no damage per se. In others ping will over time possibly break the piston or blow a head gasket. But I would think in some cases that might be true - but not in most anyway. At least in my "hobbyist" brain anyway...
Edit - I want to say the "time" figures were for a gasoline engine at 6K rpm or thereabouts. It goes back some to when I was reading about it.
But ping is not detonation. Detonation is much, much, MUCH more destructive.
Edit - I want to say the "time" figures were for a gasoline engine at 6K rpm or thereabouts. It goes back some to when I was reading about it.
But ping is not detonation. Detonation is much, much, MUCH more destructive.
lewtot184
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there are so many variables that contribute to detonation that the only silver bullet is a lot of lead in the gas. a lot of lead just messes
I believe john erb said this is true. I may need to re-read his writings.
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Posted this on another site about 7 years ago....but also remember reading an article of his where he mentioned higher rpm limits the time that ping/detonation can take place.
From the KB web site.
What is the most, exact precisely defined occurrence in all piston engines? It isnt ignition timing, combustion, crank indexing, or valve events. It is Top Dead Center. You cant build an engine with an error at Top Dead Center because TDC is what everything else is measured from. Spark scatter, crank flex and cam timing can move, but TDC is when the piston is closest to the cylinder head in any one cylinder. The combustion process gets serious at Top Dead Center and about 12 degrees after TDC, most engines want to have maximum cylinder pressure. If maximum cylinder pressure occurs 10 degrees earlier or later, power goes away. Normal ignition timing is adjusted to achieve max cylinder pressure at 12 degrees after TDC. If your timing was set at 36 degrees before TDC that is a 48 degree head start on our 12 degree ATDC target. A lot of things can happen in 48 degrees and since different cylinders burn at different rates and dont even burn at the same rate cycle to cycle, each cylinder would likely benefit from custom timing for each cylinder and each cycle. Special tailored timing is possible but there is an easier wayMagnificent Quench. Take a coffee can ½ full of gasoline burning with slow flicking flame. Strike the can with a baseball bat and you have what I would call a fast burn, much like what we want in the combustion chamber. The fast burn idea helps our performance engine by shortening the overall burn time and the amount of spark lead (negative torque) dialed in with the distributor. If you go from 36 degrees total to 32 degrees total and power increases, you either shortened the burn time or just had too much timing dialed-in in the first place. If you have really shortened the burn time, you wont need so much burning going on before Top Dead Center. Now you can retard timing and increase HP. Did you ever have an engine that didnt seem to care what timing it had? This is not the usual case with a fast burn combustion but an old style engine with big differences in optimum timing cylinder to cylinder will need 40 degrees of timing on some and others only need 26 degrees. If you set the distributor at 34 degrees, it is likely that 4 cylinders will want more timing and 4 cylinders will want less ( V-8). Moving the timing just changes, which cylinders are doing most of the work. Go too far and some cylinders may take a vacation. Now what does quench really do? First, it kicks the burning flame front across and around the cylinder at exactly TDC in all cylinders. Even with spark scatter, the big fire happens as the tight quench blasts the 32 degree old flame around the chamber. Just as with the coffee can, big flame or small flame, hit it with a baseball bat and they are all big instantly. The need for custom cylinder-to-cylinder timing gets minimized with a good quench. The more air activity in a cylinder you have the less ignition timing you are likely to need. When you add extra head gaskets to lower compression you usually lose enough quench that it is like striking the burning coffee can with a pencil. No fire ball here and that .070-.090 quench distance acts like a shock absorber for flame travel by slowing down any naturally occurring chamber activity. A slow burn means you need more timing and you will have more burn variation cycle-to-cycle and cylinder-to-cylinder, result more ping. Our step and step dish pistons are designed not only to maximize quench but to allow the flame to travel to the opposite side of the cylinder at TDC. The further the flame is driven, the faster the burn rate and the less timing is required. The step design also reduces the piston surface area and helps the piston top stay below 600 degree f (necessary to keep out of detonation). All of our forged pistons that are lower compression than a flat-top are step or step dish design. A nice thing about the step design is that it allows us to make a lighter piston. Our hypereutectic AMC, Buick, Chrysler, Ford, Oldsmobile and Pontiac all offer step designs. We cannot design a 302 Chevy step dish piston at 12:1 compression ratio but a lot of engines can use it to generate good pump gas compression ratio. Supercharging with a quench has always been difficult. A step dish is generally friendly to supercharging because you can have increased dish volume while maintaining a quench and cool top land temperatures. You may want to read our new design article for more information. ".
By John Erb
Chief Engineer
KB Performance Pistons
lewtot184
Well-Known Member
from a past article erb wrote it's my understanding that things are happening too fast at high rpm for detonation to be a factor. it's also my understanding that detonation is more likely to occur around maximum volumetric efficiency and I always thought that occurred around the torque peak. there are so many factors involved in this one has to really stop and think about things to get their mind around it.
I think this dynamic compression ratio thing may be taken out of it's context. it's not a "one stop, last word" tool. it's just a piece of information that can be added to other pieces of information. think about the word "dynamic". to me it's something in motion and varies with all the other factors that are in motion.
I think this dynamic compression ratio thing may be taken out of it's context. it's not a "one stop, last word" tool. it's just a piece of information that can be added to other pieces of information. think about the word "dynamic". to me it's something in motion and varies with all the other factors that are in motion.
moper
Well-Known Member
I could see that being the case - a ceiling could be reached.. But I know way too many V8 type engines that have had detonation issues with 7500-8K shift points. So "how high is high"?
Agree 100% on the wrapping ones head around it. Dynamic is one of the latest "considerations du-jour". The more I learned about it, the less I had to worry about it, if that makes sense...
Agree 100% on the wrapping ones head around it. Dynamic is one of the latest "considerations du-jour". The more I learned about it, the less I had to worry about it, if that makes sense...
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I'm no guru on the subject of detonation but imo, if it was happening that high in the rpm range, I would think the engine would be in pieces....
lewtot184
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I remember reading an article about trying to get the mix to go supersonic in the chamber. unfortunately my old brain has lost the details but it ties in with swirl somewhat. the idea is to get the mixture in motion.
I think vizard got everybody to take notice of dynamic compression ratios although the idea has been around decades before his articles. I think it's relevance is somewhat overstated at times. it's just a tool in the tool box; one of many.
I think vizard got everybody to take notice of dynamic compression ratios although the idea has been around decades before his articles. I think it's relevance is somewhat overstated at times. it's just a tool in the tool box; one of many.
HT413
Semi Pro Bowler
Yeah I’m using Ross flat tops as well. My block was decked in a big way, leaving the pistons 0.003 above deck (I think they were designed to be like 0.015 in the hole with an undecked block) Really didn’t want a motor above 11:1, so I did the unthinkable and used 0.060 cometic gaskets to makeup the differnce.
I’m using a lunati hyd flat tappet cam. 276/284 advertised. http://www.lunatipower.com/Product.aspx?id=1580&gid=362
m79ded
Well-Known Member
That's a great setup I was thinking of getting that cam as well. I'm very close im at 236 242. With the efficient chambers of the Trick flows they don't need as much timing so I think I'll be ok. Thanks for the help
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