"Burnt" spark plugs
 

Went out for a small run yesterday and once i got back i noticed that 2 plugs were burnt. pics below of one burnt and one not burnt. Also, one of the cylinders has no compression now. I think im taking the head of that side to check if the valves are closing correctly.

Any suggestions as to what may have caused this?

The porcelain has a green tint. Green=lean. Tuliped an exhuast valve or a lifted ring land is my guess. Now, why did it go lean in that hole????

it's not good....what is the engine setup ??

Looks like detonation damage, if you really have 0 compression you likely holed a piston.

I am with Ted, Lets hope we are wrong.

Fixx
 

All the above! or blown head gasket between cylinders,if so hopefully it didn't hurt the deck of the block..502???

What cylinders? What year?

Whey you burn a hole in piston you will see metal all over the plug and the plug will look worse than that.
I would also go with blown head gasket.

I had a problem last year with zero compression in one cylinder, it was an exhaust seat. Valve was not closing all the way.

my guess is a head gasket,was there any water in the hole?

don't think a head gasket would burn the end of a plug off like that...

no water in the hole. the pistons are new 4.530, block is a 502 bored to .06

i think that i have a combination of problems, thin head gasket (.03) hot spark plugs and low octane gas.

the oil was clean as well

I had the same problem, mine had a tuliped exhaust valve. Same, no compression and burnt the electrode off the plug.

By chance were the sparkplugs left loose, like finger tight. I had a person install a fresh engine and never tightened the plugs, just left them finger tight. It burnt the electrodes of the sparkplug out just like yours. It's shown as cause and effect in the Champion sparkplug catalog. This was a stock 260 HP Mercruiser small block, ran about two hours to destroy the plugs, timing was correct.

timing on my engine is 36*, abd yesm they were loose when we pulled them out. Boat1 what was the damage to the engine due to the loose plugs?

Btw, I Am More Than Eager To Sell The Boat. Its A Cigarette Cafe Racer, I Will Consider Any Reasonable Offers. Shoot Me A Pm And Lets Make A Deal.

thats a lot of timing for a marine engine....bet it killed a exhaust valve(s)...

Need to do a leak test

Taking the engine down on monday to have a look at the heads. Most propably a valve seal or a tuliped valve. These plugs were loose but the ones that are not seem to be ok, I was thinking of going with a bit colder ones but these ones seem fine.

I forgot to mention in my post, the cylinder with the tuliped valve also had a loose plug and the electrode was gone. I never found the root of the problem, I'd be curious to know what caused it. Anyone? I have a hard time believing the plug was loose the entire time.
BC

Detonation causes loose plugs. How old was the gas?

timing can do it in a flash...literlly !!

Bought the boat with the problem, just wondered what causes so much damage to only one cylinder. Had both rebuilt since. Thanks
Cafe Racer, let us know what you find, good luck. BC

combustion
 

Your question about the burnt spark plugs is really about gasoline combustion - two basic phenomenon associated with this are Pre-ignition and Detonation of gasoline also called "knock."

Both of these conditions are related to the gasoline refining process and the anti-knock rating or as it is called "octane."

In this situation - gasoline combustion must be a controlled burning that enables the latent heat of the gasoline to do the work it is supposed to. When this process gets out of control you get "knock" and destruction of your engine.

Engines are designed to operate on a given octane of fuel - as the fuel varies so does the requirement of the compression ratio of the engine. Racing engines and aircraft engines using high octane fuel generally can run a compression ratio of 12:1 to 10:1 - this is 100-110 octane gasoline. Compression ratio is designed into the engine by the size of the combustion chamber and the volume of he cylinder. Standard automotive engines and marine engines built before World War II - and engines built after the gasoline crisis of the 1970's have a compression ration from 7:1 to 9:1 this allows a lower grade of fuel which means lower octane fuel to be used.

High octane and low octane fuel have no difference in heat value or power they differ in the rate and way in which the fuel burns under pressure. High octane fuel resists detonation and pre-ignition because of the "lead" content of the fuel or the way in which the fuel has been refined and the fuel molecules reformed by refining process. You cannot get more power from your low compression engine by using high octane fuel - it has the same heat. However you can destroy a high compression engine by using low octane gasoline.

Now given all of this several tuning factors contribute to and making this a very critical problem. First is spark timing - the distributor must have a straight shaft and run "true" or an internal wobble will fire the plugs differently in different cylinders. Basic ignition timing must be within design limits to produce maximum power. this ignition timing is extremely critical and is governed by its initial basic timing setting and the maximum amount of timing advance built into the distributor or provided in the engine control computer. 36* being a general amount determined by engine testing or by factory engineers. Too much ignition timing will destroy an engine very quickly usually leaving a telltale deposit of aluminum beads on the spark plug insulator.

Spark plugs must be of the correct heat range which varies with the length of the spark plug insulator. Hot plugs burn off the fuel that can foul them and cause the plug not to fire. Too hot spark plugs allow the plug tip to become incandescent which will fire the gasoline before the spark occurs - causing detonation and burning away of the spark plug electrode. Plug heat range is generally determined by temp measurement by engineers and you can pick it by starting with a cold plug and going warmer until spark plug "fuel fouling" or "misfire" is eliminated.

This brings us to the general issue of "fuel fouling" that the plug heat range is designed to eliminate - if the engine is receiving too much fuel it will "foul" the plugs and produce black smoke in the exhaust - and waste fuel. Fuel air ratio of 14:1 or 16:1 is provided by the engine fuel system - this can be checked with an in exhaust device called an "oxygen sensor" and is basically designed into the engine by the manufacturer. Fuel injectors that are clogged will cause an engine to run hot and lean raising cylinder temps, so can computer malfunction. Engine carburetor design from earlier days could malfunction also with incorrect adjustment of flooding floats. Improper jetting of the main fuel delivery circuit and or power circuit and or a stuck choke. Lean conditions in both fuel injection and carburetor systems can be caused by air leaks in engine manifolding and gaskets.

Finally, engine operating temperature and or overheating from improper coolant flow and operating temperature can cause all of this to become critical - mud in water jackets, steam formation in heads during operation all can cause engine spark knock in one or more cylinders. Engine operating temp should be about 160* to 220*.

Finally, supercharging an engine causes all this to become even more critical. Basically you need a good mechanic to sort all this out.

Fixx
 

+1 heat will do that,,i have seen a few in my day due to detonation..

yes, me to...just finished helping a friend with a new(rebuilt) S/B with vortec heads...killed itself in less than 20 hours due to the balancer slipped and even though he set the timing per specs....it was way off because the balancer issue....killed the brand new heads, broke off a exhaust valve, went thru the piston, which in turn sent the wrist pin thru the cylinder wall..reduced the whle engine to door stop material !!! so timing is critical !!!!

Fixx
 

Also dont trust the mark on the ballancer, i alway's zero out the mark on the ballancer with a degree wheel..get rid of the stock pointer,,i will add a billet adjustable timing pointer..once thats done i will mark the ballancer for the advance,28-36*..

After looking at the pictures of your engines I feel we need more information. I'm seeing crossover cooling, single plane mainfold, you say its a .060 over 502 with 36 total timing, and then it has stock exhaust. Do you have water pressure guages on the engines, are you using flat top pistons, do you run a thermostat, if so what is the engine water temperature and pressure running hard. What are your camshaft specs, and are your still using the non-adustable rocker arm set-up. I see MSD ignition, have you inspected the reluctor wheel in the distributor for corrosion, how old are the plug wires. Finally perform a leakdown test on the engine and post the results. Custom engines are just that, you may need 28 total timing or 42. Post the sparkplug you are using now to get an opinion on the heat range. If the engine runs and carries proper oil pressure, I would not tear into it yet. Finally, what octane fuel and how old is it.

The water preassure gauges are not installed. thermostat shows around 220 when running at 4500, the rocker arm are crane adjustables, MSD dist and cables are brand new, pistons are domed a bit (therefore we get a high compression) Heat range on spark plus are 6, Accel. I dont have camshaft specs but im sure that it is a marine cam and not agresive enough to cause reversion, even with stock exhaust. the oil preassure is fine, 60 at idle/cold, 30's afrter running. New valves, .The heads should be coming of today, hopefully i will have some pics and more info, thanks for all the help.

PS octane rating is 95 with a few gallos of plane fuel (110 octane).

220f water temp would be a major red flag :eek:

I agree...will never live @ 220 !!!

You must run water pressure gauges when you use a cross over. The 220 temp is a red flag.

Most everything listed sounds good. The fuel should control detonation assuming the heads are not making hot spots. The water temp of 220 is a surprise. Most crossover systems run the engine to cool and risk galling valve stems thus hanging a valve. Install the water pressure guages in the block and see what the engines are running for pressure at speed. Your example of 4500 rpm should be around 15-17 lbs of pressure, and ideally look for 21-23 lbs. at WOT. The reason pressure is a concern is, if it is low, steam pockets will develop in the cylinder head and this will cause hot spots resulting in detonation. Finally I strongly recommend performance exhaust for your application, you are severely restricting your engines exhaust flow, thus creating heat in the port and cylinder head. I am not familiar with Accell plugs but I would suggest using NGK V-Power plugs in a heat range 7 for your application. Good Luck.

Thanks,

Did a pressure check on the cylinders that had the loose plugs, one has a bad exhaust valve and the other cylinder a bad intake valve. i am tearing the engine down tomorrow . Ordered new head gaskets, 0.06 instead of .03 currently installed, and I will be using 50/50 mix of "95 oct and 105 plane gas for the time being.

NGK v-power are on their way since last week but in the 5 range.

re exhaust headers, i had a pair of patterson headers ready to go in the boat but did not have enough clearance on the hatch.

It is my opinion that going to a .060" thick head gasket will cause you detonation. A proper quench area between the piston and head is a big contributor to preventing detonation. Quench should be around .040" which is deck height plus gasket thickness. If your pistons are .015" in the hole (common) a .060 head gasket will get you a quench of .075". Not good.

Quench combustion chamber
 

If you are not familiar with Quench combustion chamber - and it has an odd name. It comes from Sir Harry Ricardo an English engineer who started the world into modern combustion chamber designs back in WWI. When the Brits needed a good aircraft engine the hold up was the design of the engine combustion chamber - Ricardo did us all a favor by figuring out the engine knock was caused by the shape of the chamber.

The wedge combustion chamber has "mechanical octane" this means that it will run a lower octane gas and not knock than an equal displacement "hemi" combustion chamber because of the "quench" area.

Quench is the area of the cylinder head that is close to the piston. This area "quenches" the flame of burning gasoline so that it does not knock. This area needs to be about .065 inch max between piston and cylinder head. Once you get over the .065 the gasoline in this area starts to burn and can cause engine knock.

Quench areas are placed there by engine designers and should be kept there. Actually they are bad for engine emissions because the gasoline there goes out the exhaust as unburned hydro carbons. Many engines in the 1970's had this eliminated by the so called "open combustion chamber" but the loss in performance was really missed.

Keep the quench area by using head gaskets that do not add up with the engine deck height to make over .065.

This means, the piston is down from the top of the cylinder .00x and you add the thickness of the head gasket .00y. the combined clearance of x and y should not be over .065.

And the lower the number you can achieve without valve to piston issues the better the anti detonation characteristics produced. I run 9.5:1 compression with 5 psi boost on 93 octane with hypereutectic pistons and feel my quench of .037" is a big factor in my being able to do this successfully.

excellent info...

i"m actually waiting to figure out the correct head gasket to order. My engine was built by bullet and i overhauled it. I dont know if the deck height is standard , is there any way of measuring it once i have the heads off?

Get the piston to exact tdc center and use a depth gage. Take 4readings 90* apart and average the values making sure the piston does not rock. This should get you close and assumes you have flat tops. With domes you need to be more creative You should shoot for about a .040" quench from the flat top so if you are .010 in the hole order .030" gaskets. Cometics come is a lot of thicknesses.

I would use a Fel-Pro 1047 head gasket. I have two concerns, first the bore of the gasket is the same as your block. Be sure the fire ring off the gasket is not overhanging the bore. You may have to special order some Cometic gaskets with a 4.560 bore if the Fel-Pro hang in the cylinder. Get a bridge for a dial indicator. With the piston rolled up to TDC compare the depth of the piston top above the wrist pin with the heigth of the block. Keep in mind pistons rock slightly in the bore opposite the wrist pin sides. Typically the pistons are .007-.010 below the block deck, the head gasket is .039 (Fel-Pro), thus a quench of .046-.049. Second, do not exceed .060-.065 total quench dimension for reasons stated previously. Finally I would blend the combustions chambers since the heads are off. Look to remove any sharp areas that may become hot spots for detonation. Lay the head gasket on the cylinder head to see how the combustion chamber compares to the gasket. Do not blend the chamber past the gasket. After blending measure the cc's of the combustion chamber, with all these numbers you can accurately calculate the compression ratio. Heat range 5 is way to hot, I would use a 7 minimum, or an 8. NGK heat range numbers are smaller-hotter, bigger-cooler. Good Luck.

If the pistons are domed, measure the flat top sections of the piston, and get the cc's of the dome from the piston manufacturer. With these numbers you can calculate the compression ratio.

serious engineering
 

Come on guys - you are taking this guy into some serious engineering and he is working at being a good mechanic.

To calculate the compression ratio you have to decide if your are going to do it in cubic inches or metric cubic centimeters. Volume of the cylinder is area of the bore area times the stroke. To get the bore area you to 3.14 times the radius of the bore squared. Then the volume of the cylinder which is the bore area times the stroke. This gives you the swept volume of each cylinder. Then add the volume of the combustion chamber and the cylinder area of the head gasket bore space and un-swept volume of the cylinder deck which is the distance not covered between the piston top to the gasket. Then divide the volume of the combustion chamber into the total volume of the cylinder and chamber combination and you get - Compression Ratio.

Not too hard if you are handy with a oil filled graduated syringe (get at pharmacy) - plexi-glass plastic window pane (hardware store) - caliper - calculator and basic geometry (consult high school geometry textbook) metric/fractional conversion table (consult high school science book or teacher).

To get the combustion chamber volume set the head on bench combustion chambers up. Grease the valve seat a little to get a seal. Install a spark plug. Cover the head gasket surface with a small amount of grease. Place a small sheet of plexi-glass over the combustion chamber which has a hole in it. Fill the chamber with colored oil like trans fluid using a graduated syringe. Note the volume in cubic centimeters. Convert to cubic inches using metric conversion table if necessary,

Volume of the head gasket bore - measure the bore diameter of a used gasket - measure the crushed thickness of the old gasket. Calculate the volume taken up by the head gasket bore area using the same formula as you would for the engine cylinder. Convert to metric if using metric system if necessary.

Volume of the cylinder area above the piston - measure the deck height. Determine the area of the bore, then multiply by the deck height. Be sure to convert to metric if necessary.

To measure the dome of pop up pistons (not generally used in boats) - grease the piston rings to seal the cylinder - place the piston one inch down in the bore. Use a plexi-glass sheet over the top of the cylinder with a hole located so it can be filled with measuring oil. Grease the head gasket surface. Measure the volume of the oil covering the piston top. Compare this to the cylinder volume of a theoretical cylinder one inch down of equal dimensions that would have a flat top piston. Difference is the volume of the piston top.

Convert all volume measurements into either metric or American systems then calculate the compression ratio. Use 12:1 to 9.5:1 for premium gasoline between 110 - 100 octane (racing fuel or aircraft 100 avgas). Use 9.5:1 to 8:1 for marine engines using 94 to 87 octane pump gasoline. 9.5:1 should use premium pump gas.

Most 525 EFI and 500 EFI Mercury Racing engines and 454 or 502 MAG MPI Mercruiser engines all use a compression ratio of 8.75:1 so that they can be run everywhere on marine gasoline. These same engines with 10:1 compression on racing gas I am guessing would develop 10% to 15% more power.

Run high compression 12:1 to 9.5:1 on regular marine gas and you have a fast track to the engine junk yard.