Article on Detonation

[Rennmeister]

http://www.vikingspeedshop.com/detonation/

I put together an article from some research I did on detonation.

Everyone seemed to enjoy my help on the suspension thread I made / responded to, so I thought I'd share this one as well.

[rubenski]

Nice!

[mark tucker]

very good artickel. the cam,lifters,valves,keepers,pushrods and retainers also get hammered from detonation severely.
go figure .040 is a std piston to head clearance.........whooda thunk it.

[raul arrese]

[coonhunter]

Great article, thanks for posting it. This is a subject that I have never fully understood, although I THOUGHT I knew the basics. While driving my car, I can hear what I think is detonataion when I open the throttle quickly at low RPM in 3rd or 4th gear. In the past, I have always just backed out of the throttle or downshifted, but maybe I could benefit from some tuning work. If I understand the article correctly, I should have less advance.

My motor:
1835
dual 40IDF
Engle 110 cam
9:1 compression (or so I was told)
stock heads
009 dist - 28* total advance

I am running on 91 octane gas which is the highest pump - gas I have found. I have thought of buying a bottle of octane booster, which might help, but if I can avoid that, it would be nice to just get my gas from the pump and go.

Should I reduce my advance in an effort to eliminate detonation? Maybe to 26*?

I have a LOT to learn about how to tune my motor, and I am reading a lot and trying to learn. So thanks in advance for any help.

[raul arrese]

[mark tucker]

and tuning means more than retarding the timing.
many people dont understand the new cars that retard the timing when needed. like for the cars that say min 87 ron so they think that is what they have to run.and they want to because it's cheeper. but the vehicle will preform better with premium fuel and usually get better mpg. a few years ago the price jump from reg to mid then to premeium fuel was 10 cents, now it's 20 cents and if you dont watch out it's 3o cents at some places(wally world marathong, jumps 30 cents from 87 to 89,but 20 cents to 91/93. so you gotta watch them prices) I personaly only run preamium, and run it in the wifes honda element too,no it dosent quite pay for the upgrade in mpg but it does in power.I aint driving a slug and dont want my wife getten run over in one either. there are many aspects to tuning.even just a fuel upgrade can help a lot.

[FreeBug]

Great stuff, thanks! Just wanted to help by pointing out that valves start flowing well before 2mm lift, 0.050" is actualy 1.27 mm.

Just a tiny detail, but you never know when someone will trustingly depend on that info to make an important decision...

[Rennmeister]

You can flow well before 2mm but that flow is heavily choked by the boundary layers. That is why cam duration is measured at 0.050" which is a significant knee point in flow.

0.040" also doesn't account for metallic expansion and/or rod stretch at high RPM, it's a cold measurement

[modok]

Mid way though I thought that more should be added on what actually promotes or discourages detonation, but by the time I got to the end....I realized how long it was already.
It's a good length for an introduction to the subject!

[Multi69s]

Great write up.

Not trying to start a long debate, but have you done research on AV gas? There are a lot of myths and misconceptions surrounding it, so it would be nice to actually see some research on it.
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[Rennmeister]

[Rennmeister]

[Rennmeister]

[FreeBug]

As for "tip-in", sudden W.O.T. will do the same with some carbs: vacuum at the throtte plate drops closer to atmospheric, advance drops, and the accelerator pump increases fuel, no?

Of course, it's no way near as accurate, but it's still there.

[FreeBug]

I'll give this a go, just quoting and condensing from C. Fayette Taylor:

Effect of Operating Conditions on Detonation:

He says it is all down to "temperature of end gas just before reaction and on the time of compression to this temperature." He also says " The influence of end-gas pressure at this time is small, except as it affects end-gas temperature."

-spark timing: "... the increasing pressure-temperature effect always predominates over the reduced-time effect,and the tendency to detonate invariably increases with advancing spark, within the useful range."

-inlet pressure and Compression Ratio: "any increase in either of these variables necessarily increases the detonation tendency [...]peak pressure, and hence end-gas Temp increases."

-inlet temperature: "increasing inlet temperature increases the end-gas temperature at a given pressure, thus increasing the tendency to detonate..."

-effects of errors in distribution:"In carburated engines using liquid fuels, fuel evaporation and distribution are very sensitive to inlet temperature." this will affect AFR in different cylinders (especially if plenum).

-coolant temperature: "The effect of coolant temp is similar to that of inlet temp..."

-engine speed: very complex, many interacting variables, but: "Increasing engine speed obviously decreases compression time and, as we have seen, this factor tends to reduce the detonation tendency."

-combined effects of speed, spark advance, and imep (indicated mean effective pressure): "...depend on details of design, engine adjustments, and the character of the fuel."

-fuel-air ratio: "changer in FAR can cause changes in flame speed, flame and wall temps, and the reaction time of the end-gas." and "..The point of maximum detonation tendency (minimum knock limited mean effective pressure) occurs near the fuel-air ratio having minimum reaction time...in general except at the rich end, the behavior in the engine generally follows that which would be predicted by the reaction-time curve. Any cooling effect due to fuel evaporation plus the lower maximum temperature at very rich mixtures should also tend toward increasing the klimep as the mixture is enriched. The drop in klimep at the very rich end is due to decreasing thermal efficiency. Note that knock-limited inlet pressure does not fall off."

-dilution of the charge: references show that introducing COOLED exhaust gas with hte inlet air reduces the tendency to detonate. Dilution with inert material probably has the same effect as the use of lean mixtures; that is, reaction time is thereby increased:"

-exhaust back pressure: "Engine experiments show that increasing exhaust back pressure, with all other operating conditions the same, has only a small effect on the tendeny to detonate. Apparently, the opposing factors tend to balance it out in this case."

-atmospheric humidity: "...tests show invariably that increasing atmospheric humidity tends to reduce detonation."

-water-alcohol injection: "a very powerful means of reducing detonation".

--effect of cycle-to-cycle variation: "Thus, any operation or design factor that reduces cyclic variation will have a tendency to decrease octane requirement and to increase klimep."


Next come the Effects of Engine Design on Operation, but my eyes and fingers needa rest. Happy reading.




l

[mark tucker]

my way of thought( now thats a skeery thought) is to do everything you can to keep it away and that starts with the first thoughts of building the engine all the way to the gass pump.

[FreeBug]

Yeah, I meant the Effects Of engine Design on Detonation, I guess my brain is tired, too.

[FreeBug]

Ok, Effect of Engine Design on Detonation:
"choice of shape and materials , can affect detonation chiefly because of its influence on heat transfer to the end gas and on combustion time"

-combustion time: "...time for combustion is proportional to the distance between spark plug and the end of flame travel.[...]Thus, in general, the more compact the combustion chamber, the lower should be the octane requirement, unless heat-transfer effects interference."

-heat transfer to the end gas "...if the region in which the end-gas is confined is kept cool, and especially if it has a large surface-volume ratio, a reduction in the detonation tendency should result." but still lots of other factors...

-effect of shrouded inlet valve: reduces combustion time, reduces cycle-to-cycle variation. "this effect in itself is apt to reduce the tendency to detonate."

-effect of piston shape: "The usual explanation offered for the fact that arrangements involving squish reduce the detonation tendency is based on the assumption of a cooling effect. It is pointed out that near the end of the flame travel the end gas is located in a thin space where it makes excellent contact with the cylinder walls, which are at a lower temperature than the gas. Because of this theory, the thin space in combustion chambers of this type is often called the quench area. While the cooling effect of a quench area may have some influence, there is also the possible influence of increased turbulence. Perhaps even more important is the fact that in nearly every case the combustion chamber becomes effectively more compact as the quench area is increased."

-multiple spark plugs: " It is notable that the use of three spark plugs [in this specific case] shows very little advantage over that of one plug in its optimum (central) position. In most cases the practical alternative is between one spark plug and two, the latter usually at opposite ends of the combustion chamber. Experience shows that use of two spark plugs instead of one somewhat reduces the tendency to detonate when the appropriate spark timing is used.

-sleeve valves: Forget it, I'm gonna skip over

-two-cycle octane requirements : and go straight to

-summary of design effects : "In the author's view, the influence of combustion-chamber geometry on detonation appears to be a complicated affair, not yet fully understood. In some cases, small differences in geometry or in spark-plug location make larger differences in octane requirement than can be explained by theory.

-effect of cylinder size on detonation: "It will be noted that, run at the same piston speed and with the same inlet conditions, fuel-air ratio, and exhaust pressure, the following reactions are known:

1. Combustion time increases with bore.
2. Cylinder-inner-surface temperatures increase with bore[..]

Both these factors tend to increase the octane requirement."

-combustion-chamber design: "summing up, the following would appear to be the desirable characteristics of combustion chambers for spark-ignition engines, from the point of view of attaining the highest resistance to detonation under a given set of service conditions:

1. small bore (sorry, guys )
2. High velocity through inlet valves
3. Short ratio of flame path to bore
4. Absence of hot surfaces in the end-gas region. "

He then goes to show 3 combustion chambers, wedge, bathtub and hemi, and says: "there seems to be no significant difference in the detonation characteristics of these 3 types in practice."

-non-detonating spark-ignition engines: stratified charge stuff, experimental engines, I'll skip to

-divided chamber spark-ignition engines: ...nope, on to

-effects of deposits on ignition: "It is well known that the tendency to detonate increases rapidly with deposit accumulation, at least in the early stages of the operation of automobile engines. This increase is apparently caused by the fact that, as deposits build up, both the effective compression ratio and the effective temperature of the inner wall surfaces increase.For this reason, compression ratios used in practice are considerably lower than those that could be used if the combustion chambers remained clean."

Then it goes on to the relation between preignition and detonation, then how important fuel composition is in all this too, different chapter.

All this is excerpted from the writings of Charles Fayette Taylor, Sloane Labs, M.I.T.

Hope this helps, goodnight.

[FreeBug]

Well, that got real quiet real quick! I expected to wake up this morning to find this thread full of "but MY 94's run as cool as...", etc...

I didn't mean to put a damper on this conversation, I'm really interested in any insight anyone has, I don't think I really understand the half of it...

I put as much of this into practice as I could in one of my engines, and, ironically, ended up with the most detonating S.O.B. ever!