California Dauntless

Howdy! I've got a CJ5 with Dauntless V6 that was built in '66 that has the "S" in the VIN indicating a smog engine with the lower compression ratio] (7.6:1 I believe) . However, I can't figure out how this engine differs from the 9:1 version. When I bought new pistons there was no option for the lower compression version, and the ones I got are exactly like the originals. Anybody know how the engines differ?

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Are those Sealed Power / Federal Mogul pistons or Ommix ADDA ?
What's the claimed CR for those pistons ?
What OS did you bore to ?

 

Pistons (std. bore) came with a rebuild kit (2 years ago), not 100% sure but believe they were Federal. I compared them to the original pistons and they were the same in every dimension so I don't believe they are what changes the ratio. Rods? Crank? Heads? Inquiring minds want to know.....

 

You need to measure dish and quench. Nominally the quench should be the same for both sets, but it can be within a range. I suspect you can't accurately gauge the volume of the dish without measuring it using either a burette and/or a depth gauge.

 

Piston heads are the basic CR determining factor for the Dauntless 225 engines.

I came across a set of factory replacement pistons for a Buick and the CR is 8.6.
I also came across a set of Silvolite pistons and the CR is 8.4.
That's about the only current production CR numbers I've found available so far.
I'm hoping to find a set of 9.1 pistons at .040 O.S.
"Egge" piston may have some made up by April.

If you can identify the pistons I suggest you contact the manufacturer to find out the base CR for your pistons.

 

It sure looks to me like the quench of the new vs. old pistons is different?

 

Would not be the Rods or Crank without a change in the compression height in the piston or the distance from the wrist pin to the top of the piston...........Normal change would be either a head gasket , different sized dome on the piston, decking the block, shaving the head surface or a smaller combustion chamber in the head..........all of those changes excepting the smaller combustion chamber have a smaller effect on raising or lowering compression. As Tim mentioned you can surely pour the total volume of the cylinder with a burette .........this can be done by bringing a piston to absolute TDC , sealing the area around the ring and top of block with an industrial wax, bolting the head back on with a suitable head gasket rotating the motor so that the spark plug hole is vertical and pouring the cylinder and head combination using a burette............I use a green tinted rubbing alcohol ( easy to see and clean up) once you have trickled in the alcohol using the burette making sure all air bubbles are out & filled to the base of the spark plug hole, record the amount of volume , and then using the total swept volume measurement method , (bore & stroke) a person can then determine what the real compression ratio is..........there is a slight loss for the volume taken up by the wax when sealing the top ring...........but you can also make that calculation.....................Have Fun!

 

Well, none of the pistons were at TDC in the teardown photo, but my recollection was that they came up flush to the deck just like the new ones. I know when set side by side they were the exact same heigth and the pin c/l's were the same because I checked them out pretty carefully before installation. The engine had never been torn down before so I know it had the factory pistons. I guess it doesn't really matter, the engine runs good, It's been more of a curisosity question than anything..........

 

Not a mater of coming up flush etc. The very top band of the piston that slopes down to the center looks much wider than the original. Looks like the new piston has nearly twice the width of the old. Just what it looks like to me. Like you say "doesn't really matter, the engine runs good", so that is what you want in the end.

 

Never really noticed that before; that's probably it. Thanks Walt!