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W. Tripp

Project 280+P (Part II)

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Execution: In this installment, we will look at what it takes to achieve our goals, and show a few photos of the differences in stock and the modified parts.

Part II

Compression: If we are planning to run the engine on fuel that is available on the lake, or nearby, then pump gas sets the amount of compression the engine will have and the port timing that will best suit this amount of compression and the rpm range where the majority of the power will be produced. By optimizing the chamber design and shape, the engine can be made more resistant to detonation, and more compression can be used.

Adding compression is the biggest bang for your buck you can add. It increases efficiency, increases fuel mileage, and makes everything else work better. The ports suck harder on the intake flow, and the exhaust pulses work harder on scavenging and packing fuel and air back into the cylinders. Going as high with compression as you can reliably get away with on your fuel is the first place to start when increasing performance. The compression limit also sets many of the other related aspects of the engines design.

The most overlooked aspect of compression is how it relates to port timing. Think of it this way, you dont run a high duration camshaft on low compression, so why would you run high duration port timing in a two-stroke on low compression. A given port timing dictates a given amount of compression. More compression than this is O.K., but less is not.

As you increase port timing, you raise the top of the ports especially the exhaust port. This results in less trapped volume to compress, and more time for the cylinder to fill. To keep the efficiency the same, more compression is needed. This requires a smaller combustion chamber volume. How much smaller depends on how much port timing was added, the two are closely related.

As a result of experience with these engines, it has been found that 175 psi of static compression can be run on pump gas if the engine is made as detonation resistant as possible. Many of the stock 280 engines came with 160 psi compression and run fine on premium pump fuels. So with a few improvements, we can reliably get away with more compression. And, YES, I do know this goes against common practice and internet gospel, but this is MY engine and is based on my test results.

There are several factors that need to be looked into to prevent the onset of pre-ignition and erratic combustion in the chamber. The first is surface area. The amount of area the in the combustion chamber determines the amount of heat carried into the cooling system. A top hat shaped chamber gives a large amount of surface area for a given volume or chamber cc. This allows for better detonation resistance, but the lost heat is also lost power. A hemispherical or hemi chamber shape gives the smallest surface area for a given chamber volume, and has the possibility for more power, but tends to be unforgiving to detonation.

The ring around the outside of a two-stroke combustion chamber is called the squish ring. This ring does two things. It pulls heat from the chamber and it gives mixture motion to the air and fuel in the chamber as the piston nears top dead center (TDC). Both of these actions are good for detonation resistance. The ring is generally spoken of as a percent of the diameter of the bore. Mercury has set many of the performance engines with a squish ring that is 50% of the bore diameter. The fact is, this works very well over a broad range, and changing this can hurt performance and mixture velocity (squish velocity).

A slight angle of 2-3 degrees can give a very slight performance improvement at high rpm with a slight or unnoticed loss of idle and low rpm characteristics by changing the squish velocity slightly as a result of the angle. I generally prefer a 2.5 degree angle based on experience. The gain is small, but I like to get anything I can if little work is required. This works for the over-rev range. If you are not going to rev the engine past peak power rpm (into the over-rev range), little gain will be seen - and it is not much to begin with.

The largest impact on detonation resistance that is often overlooked is the squish clearance. This is the distance between the top of the piston and the squish ring at TDC. In the chamber, there is a surface layer of heated air that is about 0.020 thick. If this boundary layer is not disturbed when the piston reaches TDC, it is very easy for areas in chamber to become hot and start erratic combustion in the form of pre-ignition or even detonation.

If the piston does not get within 0.040 inches from the squish ring, the boundary layers on the piston and head will not be disturbed, and the cooling effect of the fresh fuel and air is lost. I like to get as close as I can without contact, and allowing for connecting rod stretch at high rpm. Generally, 0.037 inches is a good place to be on a lake engine that will not see long periods in the over-rev range (well above peak power rpm). The connecting rod design, weight of internal components, and maximum rpm all play a part in how close is to close, and how much squish velocity will be developed as a result.

O-ring heads offer a cooling advantage over heads using gaskets by making direct contact with the block at the top of the bores where the most heat is being produced. However the metal sealing ring surrounding each cylinder on the gaskets is better for detecting the onset of detonation. If the cylinder pre-ignites, the pressure spike will leave a small ding or pock mark on the inside edge of the metal sealing ring on the gasket. I prefer to use gaskets to find the limit for compression, then to switch to o-ring heads for the added safety margin.

Cooling cooler lake water means cooler temperatures inside the block. This allows for even more compression to be used effectively. Also, the 2001 and later Drag engines came with cooling tubes that direct cool water from the top center of the block to the heads between the bottom two cylinders on each bank. This is a worthwhile addition for detonation suppression.

If the coolant temp on a high performance engine exceeds 125 degrees F, power will drop and the engine become less resistant to detonation. For racing, this is a big issue, and even for a lake boat it is something that cannot be ignored. The volume of water flow through the engine and the pressure of the coolant in the block need to be balanced with the maximum lake water temperature your boat will see in the heat of summer - in order to keep the engine temperatures in line.

Fuel In a two-stroke engine, fuel is used to cool the cylinders. If you send your injectors out to be cleaned and flowed, you will have a list of the amount that each injector flows. You can use the effectively with a bit more knowledge.

The top two cylinders get the coolest water from the top of the block, and the weakest pulse tuning from the exhaust. This means these cylinders make the least power, and need the least flow from the injectors. Put the lowest flowing two injectors in these cylinders.

The bottom two cylinders get the warmest water cooling them through the block and get the strongest pulse tuning from the exhaust. As a result, they need the most flow from the injectors. These bores tend to detonate first from heat. Place the highest flowing injectors in these cylinders.

The middle two cylinders get decent cooling and pulse tuning. They also get the best air flow from the intake. These cylinders tend to run in the leanest condition and can detonate from a lean condition first. If you want too run your engine with a reasonable amount of fuel for safety, the remaining injectors can be placed in these cylinders. If you want to run on the ragged edge, put the highest flowing injectors here and keep the engine fed cool water.

Octane boosters On several internet forums, I have talked about the advantages of a fuel additive called TK-7. Basically in these engines, this additive keeps parts clean, improves performance about 2.5%, and at 2 ounces per 5 gallons of pump premium, makes the fuel act like it has 100 octane. This allows a well configured outboard engine to run over 185 psi of static compression and some engine as high as 195 psi. [Further testing will come in the later stages of this project.]

Carbon build up - If you are going to run your engine with the most compression it can handle on the fuel you are using, you must watch for carbon build up on the tops of the pistons. This carbon build up will increase compression by taking up volume in the chamber. It also impedes airflow and can rob up to 8 hp from a good engine in extreme cases. Mercury Marine has a couple of good products for carbon removal. I also like to use a product from Gumout called Regane once a month (or every few tanks of fuel) added to the fuel.

Oil Realize that some oils will reduce the octane of the fuel while others can very slightly increase it. Your oil choice also can reduce friction and heat in the engine, as well as help prevent carbon build up. I use Alisyn Pro Power oil (from Aerospace Lubricants Inc.) in all of my two-stroke engines. For lake use their biodegradable oil is clean running and very hard to beat. Their normal racing oil is the best oil I have even run. But feel free to run your oil of choice it is your engine.

If pre-ignition or detonation (however mild it may be) sets in on one cylinder in your engine, the pulse tuning in the engine will be disrupted, and the tuning for that entire bank of cylinders will be off. Realize that many two-strokes run with occasional detonation most of the time. Once it sets in, it only gets worse. The engine will need to be cooled down or enrichened past the point where detonation started. Detonation will show up as a SLIGHT decrease in exhaust gas temperatures - just as if the timing was advanced a few degrees. In severe cases the pistons will fail. If you are going to push the limits of performance, there is no replacement for learning to read the burn pattern on the top of the pistons. [More on this to follow]

Photo Billet heads with hemi and top hat chambers

Graphic - side view of chamber with graphics and descriptions

--------------------- End of Part 2 ----------------------

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Looks like I'll be the first one to ask questions :)

1.) With respect to the squish band, 50% of the bore diameter leaves 1.75 inches to be divided on both sides of the combustion chamber. That leaves an annular ring 0.875 wide around the bore. 2.5 degrees of angle will yield a total rise of 0.037 from the edge of the cylinder to the edge of the chamber itself. That means to get a squish clearance of 0.037, the piston would just touch the cylinder head on the edge at TDC.

Is there a flat annular land around the outside, then the 2.5 degree slope to the edge of the combustion chamber?

2.) Logic would say that the sharp edge where the combustion chamber meets the squish should be broke, or slightly chamfered? A sharp edge would tend to heat up... (surface to volume ratio would be high at the edge and create a hot spot).....Yes?

3.) With respect to cooling, I've always heard that if the flow rate of cooling water through the block is too high, the water will not have time to absorb heat. To me, that's counter-intuitive (had to use a $5 word LOL). Yes, the water leaving the block will be at a lower temperature, but the total amount of "heat" dissipated should be the same...just spread out over a greater volume of water. Do high flow rates through the block cause the water to delaminate, or separate, from the internal block surfaces? Your opinions?

Wayne, this is awesome stuff. Thank you soooooo much for taking the time to share it with us!!!!!!!!!!!!!!!!!!

Chris

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Wayne,

I'd like to send you one of my "before the reed" intake manifolds. I'd value your opinion and would be curious if you come up with the same basic results. I'd venture to guess it may help keep the detonation threshold at bay longer than the behind the reed location. Might make some interesting comparisons.

Randy

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Chris and Randy,

Adding a radius to the edge of the squish band where it meets the chamber does help prevent heat from building up here. The size of the radius also changes the squish velocity and direction slightly - which can be a good thing.

Not every engine benefits by adding a 2.5 degree angle on the squish band.I mentioned it because testing has shown that if you are not pushing the limits of the fuel, and are running at high rpm, it can be a benefit. It depends on the rpm you are turning and the squish clearances and velocities that result from the rod stretching and reducing clearances from your static measurements.

Generally, it does help if you are running deep into the over-rev range.

When adding the 2.5 degree angle to the squish band, I have found that by leaving a flat ring that is 0.05 - 0.1 inches wide around the outside circumferance of the bore (this is where your clearance is measured), the mixture activity does not seem to allow the slight edge where the angle meets the flat area to heat up.

I have run the angle to the edge of the bore with very close piston to head clearances. This allows you to push the fuel to its limit, but you have to keep an eye on any signs of contact.

If you are running pump gas, that tends to vary in quality, then adding this angle can increase the squish clearance across the squish band to the point where the cooling effect of the mixture motion is not effective and hot spots can develop. This is where the angle is not wanted. Running pump gas from a consistant source allows it to be pushed harder or for and angled squish ring to be a small benefit. REPEAT - a small benefit.

The main issue here is just how close you are pushing the compression and the fuel to the edge of detonation. If you are not pushing the limits, and you have a nice safety margin, the angled squish band helps prevent the torque from dropping off as quickly when you are running the engine at high rpm. If you do not rev the engine this high, then I wouldn't suggest adding the angle.

Testing it youself will be the final answer. I am using heads with removeable chambers for this project. I have chambers made in both top hat and hemi shapes - in varying volumes, and both with and without the angled squish band. This will be tested and the results posted here at a later date.

Hell, on engines running alcohol, the problem is getting enough heat into the engine, not getting it out. But I am off topic here.

Cooling - Excessive coolant flow will not perform well - just as excessive pressure with reduced flow does not work well. I prefer to get more flow than I need and then to restrict the flow to where it allows me to balance it with pressure by restricting it to get the engine to run at lower coolant temperatures. Others prefer to do things differently.

What is good for my River Rocket will not be correct for a heavier bass rig. Either way, keeping the coolant temp below 125 degrees pays dividends in performance. Lower temps can be an even greater benefit, but (like everything else) you need to find a happy medium that works well for you.

--------------------------

Feel free to ask questions and provide information that goes against anything that is posted here. I am not writing this to prove what I know (or how little I know - as the case may be) - it is just how this engine is built and why it is done this way.

I will have several photos soon that should give others a better idea of what Chris and I are talking about. If anyone wants photos of a specific part or area, let me know.

Again, what is posted here is NOT the end-all, be-all on the subject. It is just what works well for me. I post it here to let others see how this engine is built. And to include some information that I have not seen in a build up of this kind.

I feel that the first two parts of this build up are just as important as the port specs and photos (that are coming soon). Few enthusiasts have port specs for stock 260, 280, and Drag engines, and even fewer have air flow numbers and velocity figures. I hope that by posting this information in the next part of the article, others can learn from it.

Wayne,

I'd like to send you one of my "before the reed" intake manifolds. I'd value your opinion and would be curious if you come up with the same basic results. I'd venture to guess it may help keep the detonation threshold at bay longer than the behind the reed location. Might make some interesting comparisons.

Randy

Randy,

I am planning on installing the Motec Dash and GPS unit from my DR-20 into my River Rocket to provide accurate testing. I just need to make the time to do this. I will be testing several intakes (and other parts) on this engine and posting the results. I would be happy to test anything anyone sends me. I will not be partial with the results, and I would like to make them public.

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This is getting to be an awesome!

Thank you Wayne for doing this, it is time that everyone gets a chance to see what is going on with these motors and remove some of the hearsay, and witchcraft, and substitute it with science and results.

On the intake front, I would love to see Randy's set up, Mad, SVS ( old style ), SVS Blue, Horn stock, Horn Bored, and lastly the new intake yet to be released..........

And the cool part is the Motec doesn't fudge numbers!

Keep up the great work Wayne, I sure appreciate it!

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thanks for the info Wayne, are you doing the tests down on Lake Martin? If so I wouldn't mind coming down and helping or just to watch. I need to visit Ron anyway

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Chummy,

FIRST I have to assemble the engine. I have waited to assemble it so I can shoot lots of photos.

Once the weather gets a bit warmer and the rain stops (next month?), I plan on breaking in the engine and starting the testing. I have a place down on Lake Martin just south of the 280 bridge on the north end of the lake. I will do the testing there. You are welcome to join in - I can always use a hand.

I need to get the Motec stuff rigged in the STV first, and I need to order a part so I can convert the tach signal off the Mercury electronics to a clean signal for the ADL logger. It may just be easier to put the engine on the Motec ECU and CD ingnition...I am thinking about it. It would make tuning the engine better and easier.

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Cool I would be glad to help, I know just where you are talking about on the lake Ron gave me the "offical" tour of the lake last year. When the weather gets better we'll see when the best time will be, keep the info coming!! :D

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Wayne, have you ever done any cylinder pressure testing in your adventures? I've got a TFX setup I'm just DYING to put on my next motor. Looking for a virgin 260 to start playing with. Should be able to spot detonation pretty easily. Also gives HP based on cylinder pressure vs bore/stroke info. Kinda cool. :)

Also have a RacePak data acquistion setup for it too. Anything I should be particularly interested in watching?

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This is a blank port map with the names of the ports for those who didn't know what the Hades I was talking about in the text above.

The transfer ports are the main intake ports feeding the cylinder from under the piston, behind the sleeve, and into the cylinder. This is where the largest part of the intake charge comes from.

The boost ports are additional intake ports that feed the cylinder. They generally are used to feed additional intake mixture into the cylinder after the transfer ports have closed by using the mixture velocity to overcome the pressure in the cylinder.

The exhaust port lets the burned fuel and air mixture out of the cylinder. It also allows fresh intake mixture to escape during the overlap part of the engine's port timing. The exhaust pulse tuning is used to cram a good part of the fresh mixture that has escaped back into the cylinder. As a result, this port needs to work well in BOTH directions - in and out - especially the top of the port. A sharp edge on the top of the port makes good power, but is hard on the piston rings.

post-532-1140879839.jpg

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Wayne,

Looking at the ports maps for the 280, Drag, and OS 260; the boost ports appear to close before the transfers on the 280 and Drag. But, on the OS 260 the boost ports close after the transfers. Looks like a change in philosophy from Merc.

What affect does this have on the powerband?

Also, are the transfer entries and boost port entry all fed through the rod slot? (I'm thinking I've heard that the boost ports are fed from the crankcase, but I've never seen a block without the sleeves installed.)

Chris

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Chris,

The relationship between the closing of the exhaust, transfer, and boost ports helps determine where peak flow will be in the powerband and how high peak power will be made. Think of port area and duration as related to lift and duration on a camshaft. All of this shapes the torque curve - especially when you add in the effect of the volume and shape of the ports behind the sleeve. I will dig through my stuff and find some photos of a cylinder with the sleeve removed.

ALL flow through the engine has to go through the rod slots, under the piston and into the transfer and boost port entries. That is why working the rod slot areas is so critical of a balance between getting enough flow, directing it where it needs to go, and not removing more material than you have to to keep the crankcase volume reasonably matched to the ports and compression used.

I hope this helps.

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