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#16
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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 |
#17
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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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1972 220D ?? miles 1988 300E 200,012 1987 300D Turbo killed 9/25/07, 275,000 miles 1985 Volvo 740 GLE Turobodiesel 218,000 1972 280 SE 4.5 165, 000 - It runs! |
#18
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Quote:
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. |
#19
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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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Thanks for the help Bill Fisher '86 560SL (186K) - Now a 'classic' : Registered as an Historic Vehicle 02 E430 in the stable '14 LS460 (Lexus) - - - - - '95 E420 (198K) found a new home '99 E320 (80K) (gave it's life for me as we hit a bumper) '95 E420 (231K) Sold to a happy buyer, new to Mercedes '90 300E (65K) Sold to an Mercedes Lover '92 190E (215K) - retired to the salvage yard '93 500SEL (214K) - Moved to another family, still runs like a young pup |
#20
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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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Steve Danville what i did myself. CPS rear shocks MAS x 3. SparkPlugs (twice) Fuel pump & Filter Window switch replaced both failed horns with Honda horns. Cleaned ERG Tube Oil changes Oil consumption Fix Brake Pads A/C Recharge Anti-Freeze change X 2. Front Shocks. Monroe. $90 Mercedes Warrenty Work: Harmonic balancer,power steering clamp. Cat converters.Drive Shaft Bearing.s Replaced Outer Lower Tie Rods and front lower ball joints at 156,000 miles |
#21
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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
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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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1972 220D ?? miles 1988 300E 200,012 1987 300D Turbo killed 9/25/07, 275,000 miles 1985 Volvo 740 GLE Turobodiesel 218,000 1972 280 SE 4.5 165, 000 - It runs! |
#23
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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 |
#24
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Re: higher conmpression = higher heat
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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 |
#25
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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. |
#26
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Quote:
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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