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  #16  
Old 03-13-2004, 04:26 PM
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Okay, now I buy what you say. Higher compression ratio means higher mixture temperature at the igntion point, which means greater fuel detonation resistance is required. Higher compression ratio also means higher peak combustion temperature and pressure (usually a achieved just after TDC if spark timing is set for maximum torque), but the difference is not huge.

If detonation does occur, it usually happens before peak cylinder temperature and pressure occur. Light detonation - that we sometimes can't ever hear, but is picked up be detonation sensors can occur just before or slightly after peak temperature/pressure. If an engine gets into heavy detonation, it occurs well before peak temperature and pressure, and before the piston reaches TDC.

Duke

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  #17  
Old 03-13-2004, 06:17 PM
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Yeah, you normally don't hear detonation at speeds, but it's there -- usually a noticable degredation in performance if it's bad.

My friend Hans had an Alfa Romeo come into the shop in Germany once with a "died on the highway and won't start" complaint.

No compression AT ALL, and when they pulled the heads, there was a nice, neat, smooth sided hole in each piston directly below the spark plug. Turns out if you cheat on the octane rating or turn the timing up too far, the flame from the resulting detonation comes straight out of the spark plug, and on the Alfa, it was pointed directly at the piston crown at a right angle -- burned right through.

Got the exhaust valves too, I think.

On an MB, the plugs are usually canted, so you don't have to worry about buring a hole right through, but you can still melt the piston crown and splash aluminum around -- or flame cut down through the rings (I've seen this on american v8s where some idiot "set the timing by ear"). Will usually roast the exhaust valves too. The M110 has vertical spark plugs, though....

Peter
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  #18  
Old 03-13-2004, 09:02 PM
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Quote:
Originally posted by psfred


No compression AT ALL, and when they pulled the heads, there was a nice, neat, smooth sided hole in each piston directly below the spark plug. Turns out if you cheat on the octane rating or turn the timing up too far, the flame from the resulting detonation comes straight out of the spark plug, and on the Alfa, it was pointed directly at the piston crown at a right angle -- burned right through.

Got the exhaust valves too, I think.

On an MB, the plugs are usually canted, so you don't have to worry about buring a hole right through, but you can still melt the piston crown and splash aluminum around -- or flame cut down through the rings (I've seen this on american v8s where some idiot "set the timing by ear"). Will usually roast the exhaust valves too. The M110 has vertical spark plugs, though....

Peter
Detonation damage is due to local overheating caused by the dramatically increased rate of heat transfer caused by the detonation the shock waves. It doesn't have anything to do with spark plug location.

The center of the piston is its hottest point, and most aluminum alloys rapidly weaken above 400 degrees F., so in the absense of unfavorable piston crown geometry, detonation failure will usually be near the center.

Pistons for four valve engines or two inline valves (like a Chevy V8) that have machined valve clearance notches a prone to failure where the notch boundaries pass close to the edge of the piston leaving a ridge that has a high surface area to volume ratio. Even though this part of the piston is normally cooler than the center, the ridge can experience rapid overheating due to detonation and fail.

The best pistons have crown geometry "built in" to the forging or casting tooling with no sharp edges that are typical of machined piston crowns. Rather than individual valve clearance notches, a chamfer clear across the top of the crown will not leave high surface to volume ratio ridges near the edge of the piston, but reduce the potential compression ratio.

Duke

Last edited by Duke2.6; 03-13-2004 at 09:14 PM.
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  #19  
Old 03-14-2004, 07:32 AM
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OK - New question - angle - Air Polution

I’ve been told by a petroleum engineer, that using premium fuel produces less air pollution.

Is there any truth in that adage?
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  #20  
Old 03-14-2004, 09:39 AM
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higher conmpression = higher heat

back to back writtings... higher compression produces no more heat.. Then I read higher compression produces more heat.

I know this much.. Harley Davidsons can get by with air cooling because they have low compression. (except thier higher compression VRod).

I have two Honda Motorcycles that are water cooled. One with an 11-1 compression ratio and I have read they NEED to be water cooled due to the high compression.

At least now thru all these threads I believe I understand octane better. But holy conflicting facts!
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  #21  
Old 03-14-2004, 12:14 PM
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SO Based on all of the above GREAT info, if i do not hear any ping or knock in my engine at idel or high speed then it is o.k to use 87 octane in my 1989 420 sel.
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  #22  
Old 03-14-2004, 12:30 PM
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You won't necessarily hear detonation at highway speeds, so no, I woudn't EVER use 87 octane. May be 89 around town, but on the highway, only 92.

As for compression heat, yes, the more you compress a gas, the more heat it will have -- plain law of thermodymanics (the energy has to go somewhere). The amount of waste heat an engine produces isn't the same subject -- that will be determined by efficency, load, and valve timing (along with a bunch of other stuff).

Duke:

The Alfa "hole in the piston" is indeed a flame propagation thing, determined by the factory. Apperantly it "blowtorched" rather nicely in high speed photography or something, I don't believe there is a thin spot or a ridge on the piston there. It's also repeatable, so must be something characteristic of that particular engine. Probably go something to do with swirl as that is a hemi with the spark plug pointing straight down in the center. Who knows. Certainly would ruin your day, anyway!

Peter
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  #23  
Old 03-14-2004, 02:07 PM
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Quote:
Originally posted by 420 benz
SO Based on all of the above GREAT info, if i do not hear any ping or knock in my engine at idel or high speed then it is o.k to use 87 octane in my 1989 420 sel.
The critical test for detonation is low revs, high load. If an engine pings at idle it will be destroyed quickly under load.

Find a quite street, preferably next to a wall. Open the window so you can hear sound bounce off the wall. From a stop, accelerate lightly until the transmission shifts to second, then open the throttle about 3/4. If it doesn't ping, you're okay.

Duke
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  #24  
Old 03-14-2004, 02:17 PM
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Re: higher conmpression = higher heat

Quote:
Originally posted by sdanville

I know this much.. Harley Davidsons can get by with air cooling because they have low compression. (except thier higher compression VRod).

I have two Honda Motorcycles that are water cooled. One with an 11-1 compression ratio and I have read they NEED to be water cooled due to the high compression.

Air cooling is not as efficient as liquid cooling, so air cooled engines run higher combustion chamber boundary surface temperatures than liquid cooled engines. The higher the combustion chamber boundary temps, the more tendency to detonate.

Also, larger cylinder bores increase the tendency to detonate. Combine these two trends and a Harley engine cannot tolerate as much compression as a four-cylinder sport bike on the same octane fuel.

Combustion chamber design is another factor. The compact pentroof chamber of a four-valve head with a centrally located plug is inherently more detonation resistant than a semi-hemispherical chamber on a M103 or Harley engine.

There are many issues that influence detonation. Unfortunatley, it often gets "dumbed down" to erroneous statements like "premium gas burns slower".

Apart from engineers who have specialized training or working experience in the field of IC engines, very few understand what detonation is, and the various design and operating parameters that influence it one way or another.

Duke
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  #25  
Old 03-15-2004, 10:25 AM
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Duke higher compressions engines risk pre-ignition due to the higher compression and if that occurs heat or hot spots on the pistions etc. could cause damage. Hence the need for higher octane gas that won't ignite before the spark plug ignites it(proper point in the stroke).

If you don't have pre-ignition then higher compression engines don't always equate to hotter EGTs.
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  #26  
Old 03-15-2004, 10:56 AM
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Quote:
Originally posted by Stubman
Duke higher compressions engines risk pre-ignition due to the higher compression and if that occurs heat or hot spots on the pistions etc. could cause damage. Hence the need for higher octane gas that won't ignite before the spark plug ignites it(proper point in the stroke).

If you don't have pre-ignition then higher compression engines don't always equate to hotter EGTs.
What???

Preigntion is causes by "hot spots" that act as a source of ignition and is equivalent to advancing the timing. Retarding timing heats up the exhaust gas and the combustion chamber boundaries, so retarding the timing can lead to preigntion, which leads to detonation. This was common on seventies and eigthies vintage engines that had retarded (from ideal) idle timing. Since the EGT and combustion chamber walls would heat up at idle they would often detonate once a load what applied when accelerating.

All other things being equal a higher compression engine will have lower EGT.

Duke

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