273 commando, which piston should I use?

[budw67cuda]

Hi everyone, first post here. I'm restoring a 67 Barracuda Formula S, and working on the engine now that is an original 273 commando. Looking for suggestions on which pistons to use, I have some .040 over SRP forged flat top pistons with just 1 set of valve reliefs that I ordered awhile back as I thought that was the route I was going to take. Now Im not sure if I should use those or the domed egge pistons. I think if I have the block decked .016 so the pistons are at 0 deck and shave .020 or so off of the heads and use a thin head gasket I could get close to 9.5:1 cr. The forged pistons would allow me to add some boost some day if I want. My first engine build and learning as I go. Any opinions on this one? This will be a nice street car. Attached is a picture of the pistons I have. They seem really nice and all weigh exactly the same at 529g. Thanks

 

[ch1ll]

Wasn't the commando engine 10:1 stock?

 

[RustyRatRod]

I'd use the forged pistons over the egges any day. You can get the compression back by milling the heads a little.

 

[junkyardhero]

i'd run those forged units and adjust the compression to my liking by decking the block and heads and using the headgaskets i thought most appropriate for the application.

 

[273]

I vote forged, unless your going crazy with cam you probably don't much cr.
I'd figure out what cam I'm gonna use first and then run the cr that will work with the gas you'll use with that cam.

 

[ch1ll]

Cc your heads and your block with a mocked up piston in the bore top dead center and bottom dead center and figure out your static compression ratio.

Then go here to figure out your dynamic compression ratio with various camshaft to chose a grind that suits your needs.

 

[273]

Cc your heads and your block with a mocked up piston in the bore top dead center and bottom dead center and figure out your static compression ratio.View attachment 1716394800
Then go here to figure out your dynamic compression ratio with various camshaft to chose a grind that suits your needs.

To me that's backwards, to me you should choose your camshaft before static compression.

There's so many people on here that built a 10-12+ cr then want to run stock stall and highway gears, especially with small displacements, now their between a rock and hard place cam wise especially if they only care about streetability not track times.

 

[budw67cuda]

Thanks for the input. For the cam, the comp 20-246-4 has always intrigued me. That or the isky e4. I plan on finding an ld4b intake. I already rebuilt the 833 transmission and 83/4 with 3.55 gears. What dynamic compression ratio should I shoot for? Thanks.

 

[ch1ll]

To me that's backwards, to me you should choose your camshaft before static compression.

There's so many people on here that built a 10-12+ cr then want to run stock stall and highway gears, especially with small displacements, now their between a rock and hard place cam wise especially if they only care about streetability not track times.

He already has pistons. Egge are off the shelf pistons. There are not a whole lot of choices left these days unless you have custom pistons made and sometimes that is a compromise. You should chose the RIGHT camshaft for your compression.

Now run your cam spec's and compression through that formula and find out how low your compression really is.

 

[ch1ll]

Thanks for the input. For the cam, the comp 20-246-4 has always intrigued me. That or the isky e4. I plan on finding an ld4b intake. I already rebuilt the 833 transmission and 83/4 with 3.55 gears. What dynamic compression ratio should I shoot for? Thanks.

That depends on how your engine is built and the fuel you are using.

 

[273]

Thanks for the input. For the cam, the comp 20-246-4 has always intrigued me. That or the isky e4. I plan on finding an ld4b intake. I already rebuilt the 833 transmission and 83/4 with 3.55 gears. What dynamic compression ratio should I shoot for? Thanks.

From what I understand between 7.5-8:1 dcr for pump gas.

 

[273]

He already has pistons. Egge are off the shelf pistons. There are not a whole lot of choices left these days unless you have custom pistons made and sometimes that is a compromise.

Gaskets milling etc.. can have a big effect on final cr also, he's asking about Egge pistons basically asking if he should go even more cr.

You should chose the RIGHT camshaft for your compression.

You should choice the right cam for the application, you should know the application before you buy or build anything. Cam is one of the biggest decision of a build it should be decided before cr.

Now run your cam spec's and compression through that formula and find out how low your compression really is.

I don't know to me the cam or range of cams should be decide before the engine is built, then run the needed cr for that cam.

 

[4spdragtop]

We put the Comp 20-246-4 in our 67 cuda. 273 with 2bl pistons. Stock heads with new Comp 901 springs and retainers and ld4b. Our biggest downfall was the 323 SG, stockish 904 converter and automatic. Also the 2bl pistons.

Thanks for the input. For the cam, the comp 20-246-4 has always intrigued me. That or the isky e4. I plan on finding an ld4b intake. I already rebuilt the 833 transmission and 83/4 with 3.55 gears. What dynamic compression ratio should I shoot for? Thanks.

 

[66fs]

Hi everyone, first post here. I'm restoring a 67 Barracuda Formula S, and working on the engine now that is an original 273 commando. Looking for suggestions on which pistons to use, I have some .040 over SRP forged flat top pistons with just 1 set of valve reliefs that I ordered awhile back as I thought that was the route I was going to take. Now Im not sure if I should use those or the domed egge pistons. I think if I have the block decked .016 so the pistons are at 0 deck and shave .020 or so off of the heads and use a thin head gasket I could get close to 9.5:1 cr. The forged pistons would allow me to add some boost some day if I want. My first engine build and learning as I go. Any opinions on this one? This will be a nice street car. Attached is a picture of the pistons I have. They seem really nice and all weigh exactly the same at 529g. Thanks

What is the compression height and volume of the valve reliefs on those pistons? 273 typically run .020 to .030 below deck and 63.5 cc combustion chambers. Stock Commando pistons were rated at 10.5 :1 compression ratio. I would not run that compression with pump premium gas. Since you live in Iowa, what is the available pump premium where you live? 91 octane? Not knowing the compression height of the forged pistons, I'd still run them, rather than the Egge cast pistons. The Isky E-4 cam and 340 valve springs works well with about 9:1 stock 2 barrel pistons, about 265 hp. .020 milled heads will give you about 60.4 cc chambers. Mill the intake side .019 to keep the heads in the same relation as stock. Unmilled heads at zero deck with stock 318 gaskets, 4.01 x .031 compressed thickness, will get you 9.2:1 compression.

 

[budw67cuda]

Compression height on the forged pistons is 1.804, and the valve reliefs are 5cc total. So, in a perfect world these should sit below deck by .016 I believe? Likely not a perfect world but I could put the crank back in and check it. If I do have the block machined so they sit at 0 deck, the quench clearance would be the thickness of the head gasket. Would 0.031 be enough? I have no idea.
Most premium here is 91. I've heard of a couple places having 93 but I haven't hunted for it yet. I did measure one of my chambers and came up with 62cc. Might check a couple others. Found a chart online that said for each cc reduction in volume to mill .0053 from the head. If that is accurate, I could take .030 off and end up at 58cc or so assuming I might lose some from a valve seat regrind.
Thank you for everyone's input. It is very helpful.

Fun numbers...

20-246....

Isky

 

[RustyRatRod]

.031 would be fine.

 

[66fs]

Compression height on the forged pistons is 1.804, and the valve reliefs are 5cc total. So, in a perfect world these should sit below deck by .016 I believe?

That would be my guess.

 

[expat]

Gaskets milling etc.. can have a big effect on final cr also, he's asking about Egge pistons basically asking if he should go even more cr.

You should choice the right cam for the application, you should know the application before you buy or build anything. Cam is one of the biggest decision of a build it should be decided before cr.

I don't know to me the cam or range of cams should be decide before the engine is built, then run the needed cr for that cam.

273 hit it. You'll want to know thickness of head gasket, mill and deck. Be sure that those measurements are correct after the machine shop is done.

 

[BasinStBlue]

To many smart people. Gezzzz you guys
Great answers.