273 Commando Engine Build

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MonkeyTrev said:
Do you run the flat top 2 barrel pistons to lower the compression ratio? I have changed plans for the build a bit. I am putting Power Steering & A/C into my car and it will be a family cruiser and possibly a daily driver if the build goes well. I am not as concerned with power as I am with reliability and fun. I am still thinking about building a 273 or 318 Commando Clone for the car. I do not think the 10.5-1 compression would be practical in a daily/family car.
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Factory CR ratings are way off, you'd have to plan the cr out with whatever heads and mods you plan on running, it's somewhat hard to build high cr with smaller displacements.
 
MonkeyTrev said:
Do you run the flat top 2 barrel pistons to lower the compression ratio? I have changed plans for the build a bit. I am putting Power Steering & A/C into my car and it will be a family cruiser and possibly a daily driver if the build goes well. I am not as concerned with power as I am with reliability and fun. I am still thinking about building a 273 or 318 Commando Clone for the car. I do not think the 10.5-1 compression would be practical in a daily/family car.
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It is not practical to run 10.5:1 pistons in a pump gas car with cast iron heads. My brothers 67 Barracuda has AC and power steering and 300,000 miles on the engine and 400,000 on the car. It still runs like new. At one point I did swap the original 920 heads for open chamber 675 heads so he could run regular gas, but I think I had 68/69 pistons in the short block which are .030 higher in the block. He did not spin tires, but it would cruise @ 100 mph all day long and top out around 140 mph with 3.23 gears. He got 20+ mpg and with a certain carb about 30 mpg @ 60 mph. He ran a stockish Commando replacement cam and a 340 double roller timing chain. With either engine you choose, the cam would be the key for both power and driveability. Think 340 or smaller.
 
66fs said:
It is not practical to run 10.5:1 pistons in a pump gas car with cast iron heads. My brothers 67 Barracuda has AC and power steering and 300,000 miles on the engine and 400,000 on the car. It still runs like new. At one point I did swap the original 920 heads for open chamber 675 heads so he could run regular gas, but I think I had 68/69 pistons in the short block which are .030 higher in the block. He did not spin tires, but it would cruise @ 100 mph all day long and top out around 140 mph with 3.23 gears. He got 20+ mpg and with a certain carb about 30 mpg @ 60 mph. He ran a stockish Commando replacement cam and a 340 double roller timing chain. With either engine you choose, the cam would be the key for both power and driveability. Think 340 or smaller.
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Thanks for the info! I’m definitely just looking for a good reliability and mpg for cruising with the family through the Smokey mountains.
 
66fs said:
It is not practical to run 10.5:1 pistons in a pump gas car with cast iron heads. My brothers 67 Barracuda has AC and power steering and 300,000 miles on the engine and 400,000 on the car. It still runs like new. At one point I did swap the original 920 heads for open chamber 675 heads so he could run regular gas, but I think I had 68/69 pistons in the short block which are .030 higher in the block. He did not spin tires, but it would cruise @ 100 mph all day long and top out around 140 mph with 3.23 gears. He got 20+ mpg and with a certain carb about 30 mpg @ 60 mph. He ran a stockish Commando replacement cam and a 340 double roller timing chain. With either engine you choose, the cam would be the key for both power and driveability. Think 340 or smaller.
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MonkeyTrev said:
Thanks for the info! I’m definitely just looking for a good reliability and mpg for cruising with the family through the Smokey mountains.
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When I rebuilt my 273 I used the Egge high compression pistons. I measured the ratio and ended up with 9.6/1 even though I milled the stock heads .030. I run 93 octane with no detonation. Timing is picky and the advance doesn't come in too quick. It runs well and will get almost 20 mpg if I drive it nice.
 
toolmanmike said:
When I rebuilt my 273 I used the Egge high compression pistons. I measured the ratio and ended up with 9.6/1 even though I milled the stock heads .030. I run 93 octane with no detonation. Timing is picky and the advance doesn't come in too quick. It runs well and will get almost 20 mpg if I drive it nice.
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Does 9.6/1 require the 93? I was thinking I’d try and build it to run 87 octane.
 
toolmanmike said:
When I rebuilt my 273 I used the Egge high compression pistons. I measured the ratio and ended up with 9.6/1 even though I milled the stock heads .030. I run 93 octane with no detonation. Timing is picky and the advance doesn't come in too quick. It runs well and will get almost 20 mpg if I drive it nice.
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The polished chambers may prevent some detonation. Woody did a nice job on the heads.

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MonkeyTrev said:
Does 9.6/1 require the 93? I was thinking I’d try and build it to run 87 octane.
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I have run 89 when 93 wasn't available but just on the highway. You get a little rattle when accelerating with the lower grade.
 
MonkeyTrev said:
Does 9.6/1 require the 93? I was thinking I’d try and build it to run 87 octane.
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If you want to run 87 all the time I would recommend the 2 barrel pistons. Get the compression down around 8.5-9/1
 
toolmanmike said:
When I rebuilt my 273 I used the Egge high compression pistons. I measured the ratio and ended up with 9.6/1 even though I milled the stock heads .030. I run 93 octane with no detonation. Timing is picky and the advance doesn't come in too quick. It runs well and will get almost 20 mpg if I drive it nice.
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When running out to the upper midwest, sometimes I can only find 91 octane. I tend to make that my target pump gas. I also like to run the Direct Connection vacuum advance distributor. I'd rather run a quick curve and advance the timing where the engine wants with a lower compression ratio than run too high a ratio and have to knock the timing back. The 66 has TRW forged 10.5 pistons, since I wanted to build it as close as I could to the way they came from the factory. It sounds so good with racing gas and the timing at 10* BTDC, but mostly I knock it back to 0* TDC to run 93 octane. My heads were milled .040 and .038 on the intake side. The chambers are @ 57.4 cc + or - 1 cc and are polished like yours, mainly to get all the chambers the same volume. So I tend to shoot for 9.0:1 for pistons on my other cars. On a 273 that means 2 barrel pistons at about zero deck hight.
 
66fs said:
When running out to the upper midwest, sometimes I can only find 91 octane. I tend to make that my target pump gas. I also like to run the Direct Connection vacuum advance distributor. I'd rather run a quick curve and advance the timing where the engine wants with a lower compression ratio than run too high a ratio and have to knock the timing back. The 66 has TRW forged 10.5 pistons, since I wanted to build it as close as I could to the way they came from the factory. It sounds so good with racing gas and the timing at 10* BTDC, but mostly I knock it back to 0* TDC to run 93 octane. My heads were milled .040 and .038 on the intake side. The chambers are @ 57.4 cc + or - 1 cc and are polished like yours, mainly to get all the chambers the same volume. So I tend to shoot for 9.0:1 for pistons on my other cars. On a 273 that means 2 barrel pistons at about zero deck hight.
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You're right. 91 what I use here.
 
66fs said:
We used to run through Iowa every summer with 3 kids on our way to my fathers house in South Dakota. It was hard to find anything higher than 91 when in Iowa.
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That’s why I wanna be able to run 87. Who knows what small Tennessee town I will be in and need to fill up.
 
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