273 Boring

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circlepilot

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I'm sure it has been done many times...does going .040 (max limit) over on the 273's bore (I believe gives you a 286) give you any significant boost in performance? Does the .040 over bore, require a larger piston or a larger ring application. I've got a good block that is measuring at the standard bore of 3.63 in. This engine is a (1965) 180hp 2 barrel. I want to kick it up to a Commando equivalent. Comments and suggestion?
Norm
 
Pistons are easier to find in +.030 instead of +.040. Egge and Kanter make the domed pistons for your HP273. An extra .010 won't make any difference. Check out my 273 thread. (link below)
 
This engine is a (1965) 180hp 2 barrel. I want to kick it up to a Commando equivalent. Comments and suggestion?
Norm

The Commando motor (235HP) got it's increase in power from domed high compression pistons, single plane 4 barrel intake and AFB carb, and I'd have to double check the specs in the FSM, but think a slightly hotter cam and the low restriction exhaust. Overboring should help a little on a power increase; but the pistons, 4 barrel and a thoughtfully selected cam would do the most good.
 
Boring only gives power by restoring a round, straight bore for the rings to seat on, and a piston with the proper skirt clearance. Boring, in and of itself, does not give any realized performance gain. Best situation is a round bore, perpendicular to the crankshaft centerline, and straight it's entire length with thick cylinder walls and aproperly fitted piston.
 
The Commando motor (235HP) got it's increase in power from domed high compression pistons, single plane 4 barrel intake and AFB carb, and I'd have to double check the specs in the FSM, but think a slightly hotter cam and the low restriction exhaust. Overboring should help a little on a power increase; but the pistons, 4 barrel and a thoughtfully selected cam would do the most good.
Hp 273 cam specs: 248° duration, .395/.405 lift Confirmed per Ulf.
 
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I just saw where I posted the HP specs at 248° .420/.430 The FSM lists the duration but no lift specs. Isky E4 specs are 260° .425 lift.
 
I just saw where I posted the HP specs at 248° .420/.430 The FSM lists the duration but no lift specs. Isky E4 specs are 260° .425 lift.
@65dartcharger , do you have the correct cam specs for the Charger/Commando 273? I just found different lift specs in a Moparts thread. You can't believe everything on the net. Thanks, Mike
 
That's not right. Correct spec is 248 duration, .415/.425 lift. Per 1967 Barracuda factory brochure.
That's what I thought but I have found 4 different answers so far. Ah, I see you said brochure. I was suprised my 66 FSM didn't have the lift specs.
 
That's not right. Correct spec is 248 duration, .415/.425 lift. Per 1967 Barracuda factory brochure.

OP has a 65 motor so the specs might be somewhat different - I suspect they are the same as what you listed for the 67!
 
I bored my '65 273 .060 over which made it 282 cubic inches. Since it was putting out more than 1 horsepower per cubic inch, the extra displacement was good for about 10 horsepower. Overboring also raises compression ratio slightly. I used old school TRW domed/forged pistons that were advertised as the same compression ratio as the factory Commando pistons @ 10.5:1. Today's domed pistons for the 273 actually work out to only about 9.5:1.

65 Valiant5.jpg
 
Be careful trying to go.060 over bore on the 273. I had 3 blocks that I took .060, two of them cracked the cylinder walls & ended up as junk. IMHO, if you are going more than .040, I would sonic test the block to see if there is enough meat in the cylinder walls, especially if you are running higher compression pistons.
 
The 273 was Mopar's first attempt at thinwall "lightweight" castings. (LA) the core shift is pretty shaky. Yes like Demonracer suggested, have the block sonic tested to be safe.
 
bore it, or just get a 318 block and put all your 273 stuff onto it. Youll be ahead, the 318-2 was only 5 down from the 273-4 but had gobs more torque where you need it.
 
I cracked one cylinder in my 273 .060 over block after it dropped a valve. Installed a sleeve after that. When I sold it, we had over 200 quarter mile runs on the same motor (.060 over motor). Just lucky, I guess.
 
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Thanks a lot gentlemen, scads of information here. Pulled a lot of different ideas out of all of the replies. Answered all of my questions. Now all I have to do is apply it. I will try to post my progress. I have two 65 Barracudas, one slant and the 273. Trying to work on both without showing any favoritism...don't want them to get mad at me.
Again thanks.
Norm
 
Yes, I have. I get the numbers later today.
I have a 1965 Dart (2 barrel )273 that I am adding a stock 4 barrel intake and Eddie 1404 Carb. Any suggestion on a bit hotter cam to use instead of the stock? Sorry didn't mean to Hijack. Thanks,Chris.
 
I have a 1965 Dart (2 barrel )273 that I am adding a stock 4 barrel intake and Eddie 1404 Carb. Any suggestion on a bit hotter cam to use instead of the stock? Sorry didn't mean to Hijack. Thanks,Chris.
Isky E-4 is a great cam. I put one in my 273.
 
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