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Great Article on detonation/preignition

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Old 10-28-2013, 10:44 PM
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Default Great Article on detonation/preignition

Grab a large coffee for this read. But I found it very interesting. Helps explain whats really going on inside a cylinder.

http://www.streetrodstuff.com/Articl...tion/index.php
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Old 10-29-2013, 07:10 AM
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Pretty interesting article. I like the "tin-ear" method; I have something on the dyno now, I think I will give it a try and see if the noises are any different.

Thanks for the info.
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Old 10-29-2013, 09:24 AM
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Thumbs up good info

Consider the Northstar engine. If you do a full throttle 0-60 blast, the engine will likely run up to 6000 RPM at a 11.5:1 or 12:1 air fuel ratio. But under sustained load, at about 20 seconds, that air fuel ratio is richened up by the PCM to about 10:1. That is done to keep the spark plugs cool, as well as the piston crowns cool. That richness is necessary if you are running under continuous WOT load. A slight penalty in horsepower and fuel economy is the result. To get the maximum acceleration out of the engine, you can actually lean it out, but under full load, it has to go back to rich. Higher specific output engines are much more sensitive to pre-ignition damage because they are turning more RPM, they are generating a lot more heat and they are burning more fuel. Plugs have a tendency to get hot at that high specific output and reaction time to damage is minimal.

I was thinking of leaning my wot (11.0:1 ) but it looks like i`m good and can even go a bit richer..






Another thing you can do is increase the burn rate of the combustion chamber. That is why with modem engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate.
That's one of the goals of having a fast burn chamber because it is resistant to detonation.
There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build up, the air pump becomes more efficient. Pumping losses are minimized. In other words, as the piston moves towards top dead center compression of the fuel/air mixture increases. If you light the fire at 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If you light the spark 20 degrees before top dead center, it's only acting against it for 20. The engine becomes more mechanically efficient.
There are a lot of reasons forfast burn chambers but one nice thing about them is that they become more resistant to detonation. A real world example is the Northstar engine from 1999 to 2000. The 1999 engine was a 10.3:1 compression ratio. It was a premium fuel engine. For the 2000 model year, we revised the combustion chamber, achieved faster bum. We designed it to operate on regular fuel and we only had to lower the compression ratio .3 to only 10:1 to make it work. Normally, on a given engine (if you didn't change the combustion chamber design) to go from premium to regular fuel, it will typically drop one point in compression ratio: With our example, you would expect a Northstar engine at 10.3:1 compression ratio, dropped down to 9.3:1 in order to work on regular.




The Head porter I`ve used always preaches about the BBC combustion chamber design and how much improvement there`s to be had for more power, better more even burn, less detonation potential and better fuel economy.. while his results speak for themselves this confirms what hes been preachin

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Old 10-29-2013, 09:27 AM
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I know a lot of guys running boost on street and strip, tuning for mid 11's at wot . However , they are at wot for 10-12 seconds. I like wot for 10-12 MINUTES. Gm might have been on to something there
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Old 10-29-2013, 10:25 AM
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The chamber design is also one of the many reasons that the new generation LS engines perform so well, along with better overall head design and a number of other factors. Makes me wonder what the future holds for high performance.
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Old 10-29-2013, 11:28 AM
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Great article, BTW. How many folks are running knock sensors on their engines to tell them if they are detonating? Even without FI, it would seem like a helpful thing to have, even if simply wired to a large red idiot light on the dash to tell you to back off if any detonation is picked up.
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Old 10-29-2013, 06:01 PM
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Originally Posted by Budman II
Great article, BTW. How many folks are running knock sensors on their engines to tell them if they are detonating? Even without FI, it would seem like a helpful thing to have, even if simply wired to a large red idiot light on the dash to tell you to back off if any detonation is picked up.
You read my mind.. thats on the winter project list!
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Old 10-29-2013, 06:38 PM
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Fuel also acts as a cushion in the cylinder just b4 the big bang..when i tune nos i pay no attention to the afr,,all i concerned about is the egt's..likes joes said its over with in 6.78 seconds but you can still destroy alot of chit in 4..
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Old 10-29-2013, 08:00 PM
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why would you not add afr guage to the tuning toolbox?
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Old 10-29-2013, 08:11 PM
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I'm curious about the talk of using the "tin cup" in the dyno room to listen for sounds of detonation. Why not just use a knock sensor? Also found it interesting that the article says detonation can trigger a drop in EGT's. Would one expect to see the EGT's elevate just prior to the detonation event?
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