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.

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.

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.