Motor design 101

[Beck]

I need some help determining stroke, rod length, and compression height.

I thought I knew how to do this, but after going through others posted combinations I must not.  Using another guys posted measurements I calculated his piston to be .055 out of the hole.

I am still working on my Mercruiser motor. It is often considered half of a 460 BBF. The deck height is 10.272"(.050" shorter), and the stroke is 3.750" ( .100" shorter). It used 460 truck rods and pistons. I feel all of the new performance connecting rods are superior to the 70's factory performance rods that were in the motor, so a rod upgrade is planned. If I am buying rods I might as well have the crank offset ground for a stroker.

Offset grinding the BBF crank (2.5 rod journal)to use BBC rods (2.2 rod journal)was a common practice for the BBF guys before stroker cranks became the rage. Since I cannot buy a 4 cyl stroker crank, I need to do it the old fashioned way.

4.000" stroke is the norm for an offset grind for some reason.  Available rod lengths are 6.535, 6.635, 6.700, 6.750, 6.760, and 6.800". The 6.8 and 6.7 are the most commonly used. The 6.75 is a new off the shelf offering. I don't know what the stroke limit is on clearance. I know of one motor with a 4.16" stroke without issues.

I have checked with a couple of piston manufacturers for off the shelf pistons. It does not look like that is going to happen. My bore is currently 4.390" but I could go to 4.400". Pin diameter for the BBF and BBC is .990". I need pistons to fit the A460 valve relief and have a thick enough top to handle boost. Once I put that combination together the suppliers I talked to said "special order".  That does allow me to move the compression height around a little. I don't know at what height the oil ring run into or too close to the pin holes. I would like to avoid that if possible.  Piston design is my weakest part of this.

Any suggestions?    This could be your math problem of the day, or week?

[kb426]

This is off the top of my memory so it could be wrong. 1/2 stroke + rod length+ the comp. height of piston + deck clearance should equal the deck height of the block. When I used to design 426 pistons, I had trouble until one of the engineers at Arias told me the design constraints of the forging they used. After I had that info, my pistons were always what I wanted. On a 426, you had to have at least .100" thickness in a valve pocket or you could sink them before running at the best tuneup. If you find out what the deck thickness of the forging is and work from a minimum acceptable thickness and work down, you can add up acceptable distances for the ring stack and arrive at a comp. height. I seem to remember that on canted valve engines the area above the top ring where the relief was cut was the area of concern. As I remember if you use aluminum rods, you need at least .060" piston to head clearance.

[Beck]

This is off the top of my memory so it could be wrong. 1/2 stroke + rod length+ the comp. height of piston + deck clearance should equal the deck height of the block.

That is what I was using. Apparently a poster on another site has something posted wrong. I ask the question there about his setup. I will see if and how he responds.

I took my block in for decking tonight with instructions to take the minimum off to make it flat. I will work from the finished deck height back.

[Beck]

On a 426, you had to have at least .100" thickness in a valve pocket or you could sink them before running at the best tuneup. If you find out what the deck thickness of the forging is and work from a minimum acceptable thickness and work down, you can add up acceptable distances for the ring stack and arrive at a comp. height. I seem to remember that on canted valve engines the area above the top ring where the relief was cut was the area of concern. As I remember if you use aluminum rods, you need at least .060" piston to head clearance.

I will have steel rods. I wouldn't think .100" would be thick enough even in a valve pocket. How would I find the thickness of a forging? I am having trouble finding a good listing of pistons from any mfg.

[kb426]

I've seen steel rod motors with as little as .015" or as much as .045" clearance. In the 426, the dome is the largest heat absorber. that allows valve pockets to be thinner to keep the weight as light as possible.  Remember that blown alky engines were turned to 9500 rpm all the time. Choose a piston company that you want to do business with and call a piston designer and start asking questions. You have enough info to give him a direction as to where you need to end up. In my day, we used an .017 * top ring, a .062 reverse taper second and a standard tension 3/16 oil ring such as a ss50ou. My info is from years ago so I hope it's not misleading.

[Beck]

I intend to run a gapless ring set.
I am concerned about the needed dish in the piston to keep my compression down. I don't want excessive deck clearance to accomplish the 9:1 to 9.5:1 compression. I'm told too much deck clearance causes poor flame travel.
The stock 100cc combustion chamber heads with flat tops with valve reliefs resulted in 8.8:1 compression. My new head has an 87cc chamber.

Figures, as I was confirming my chamber size I see the head just went on sale. I bought a month too soon.

[chimp koose]

I would caution you about the gapless second ring.There are those who claim the trapped compression will unseat the top ring from the bottom of its ring land and cause it to flutter and lose effective seal . Look it up on speedtalk.com.I had them in my dragster motor and was always impressed with the leakdown numbers and in an alky motor it would help keep the oil from getting milky. I do not think many engine builders recommend them any more. I was always told that squish of .060 or less was good for preventing detonation but I think that number is intended for un blown on gas . I have a piston from a 632" BBC making  1150+ HP on my workbench . It runs about 15:1 CR  , I could measure dome thickness if you would like.

[chimp koose]

If you are running blown alcohol I would recommend you increase your compression ratio to at least 10:1 or higher  . I would also recommend you look up the fuel injection enterprises web site . There is a lot of good info there. It is also a neat place to read up on nitro combinations .

[Beck]

Thanks chimp koose,

Nitro is banned in our rules.

I got the 9:1 compression ratio suggestion from a full size alcohol tractor puller that is running near 100 psi boost. You are right, for my low boost I can use more compression.

I checked out the FIE site. The compression calculator looks good. I didn't see much else to help. Did I miss something? It appears they are mainly mechanical injection oriented.

I thought I would have an issue getting pistons dished deep enough. According to that calculator a near flat top works.

I am a bit confused about head gasket thickness. It appears I can buy gaskets from .027" to .140" thick. Is this just for adjusting compression ratio?

The head gaskets are for 4.40 bore. Mine is 4.39. Those are fine for my use right?

I didn't know the squish rule was not for methanol. Good to know.

The recommendation for gapless rings came from a local builder. He specializes in Ford motors, especially 2300 4 cyl turbo motors. He likes them for the reason you said, no milky oil with alcohol. He may have just been suggesting the top ring be gapless. I hope to see him tonight. I will ask.

[chimp koose]

Different gasket thickness can be used to adjust compression ratio . I mentioned that I THINK the .060 or less squish was intended for gas motors but I am not sure on that one . I have seen 12.5:1 CR used with 45lb boost on alcohol I am sure there are many variations out there.I must have assumed that you were going mechanical injection . What will you be using as a fuel system ?one thing FIE states in their info is that it is easier to get a tune up into a 10 to 11:1 blown alcohol setup than it is with a higher CR. Plugs and rod shells used to be the way to check a tune up but now the racepack will tell you much more.I have just about given you all the help I am able to with my limited experience . KB426 and possibly others with more alcohol experience will be of more help than me . I will follow this thread to learn more as blown alcohol motors are my favorite engine .

[chimp koose]

In regard to the bore size to gasket size question , I would want to put that gasket down over the head studs and see if it would shift around enough to get the gasket overlapping the bore.The closer the better but not if it overlaps . Are there alignment dowels on that motor or those sleeves in the coolant passages.

[chimp koose]

In one alcohol deal I worked on we would use a big coffee urn to heat the oil for the engine up to 180degrees f then pour it into the motor before firing it up . That kept the milk down . Another friend of mine runs a rons racing toilet , a single throttled MFI setup . He has a fuel solenoid from a nitrous kit plumbed into the base of the throttle body . He uses this solenoid and a very small jet to provide gasoline to the engine on starting and warm up with the alcohol shut off .When the engine is fully warmed up he shuts the gasoline off and re starts the motor on methanol....no milky oil!That engine is not blown or turbo , just mfi .

[Beck]

I plan to use EFI not mechanical.
The motor does have ring spacers around 2 head bolts to locate everything.
Another trick with the gasoline injector on the alcohol motors is to drive the vehicle back to the pit on gasoline not alcohol. I'm told a small non EFI fired jet will work for this. Turn the alcohol pump off and the gasoline pump on. Make the jet just big enough that you can add a little air from the throttle blade to take it just over idle. By the time you reach the pit the motor intake and bores are alcohol free.