identifying heads for 273

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bamacuda

bamacuda

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I have a 273 commando with the domed pistons and 10.5:1 compression. I am trying to reduce the compression. It has been suggested to use swirl port heads. My questions are: 1) what will this do to the compression? 2) Are these heads visibly different when installed? ( mine is the 65 with the different bolt angle, how do I deal with that?
 
bamacuda said:
I have a 273 commando with the domed pistons and 10.5:1 compression. I am trying to reduce the compression. It has been suggested to use swirl port heads. My questions are: 1) what will this do to the compression? 2) Are these heads visibly different when installed? ( mine is the 65 with the different bolt angle, how do I deal with that?
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Why are you reducing compression, does it ping? Have you cc'd the head and dome to find actual compression?

At work we've milled the pistons down on engines a few times. Have to balance it though, because of lighter pistons.

You would need to redrill or cut the intake bolt hoes, or use a different intake.
 
If you have the actual 273 heads, they have a heart shaped chamber and a flat quench that is flush with the mating surface of the head. Using a set of easily found 318 heads will lower your compression. The swirl port heads may flow better, but since the combustion chamber is very similar in shape, you may need to cc each head to find out which has the advantage for you.
 
The different bolt angle should not be any problem. Unless you are putting a different intake on there. Then what most people do is get the intake holes drilled to the correct angle.
 
I had a 66 Dart 273 4bbl with the domed pistons. I retarded the timing and got it to run without pinging on 91 octane gas. The swirl port (302) heads will give you a larger combustion chamber. The 75 and later 318 heads will have an even larger one.

IMHO the best course of action, particularly if you are planning to rebuild the engine, is to install the flat top (2 bbl) pistons. If no rebuild is planned, try the thickest head gasket you can find. (Don't know if you can double up head gaskets on the LA). The advantage of these options is that there is no need to alter bolt hole relationships.

The flat tops are a lot easier to find than the domed pistons. I would be very hesitant to do anything to the HP pistons beyond new pins and rings.
 
Change the head gaskets to Felpro standard replacement gaskets. Dont use 318 open chamber heads. The ports are not near as good as your 273 heads. The later heads are prone to cracking and do not flow any better than what you have.
 
Lot of good info here. The engine is completely apart, the crank has been polished, it has .030 on the block and .030 domed pistons (previous rebuild, I did not know it had been rebuilt or i would have just run it) I want to build it to last and have a bit of spunk. I am trying to go on the advice of the machine shop that the 10.5:1 will be problematic. I am trying to get a good rebuild for this commando 273.
 
bamacuda said:
Lot of good info here. The engine is completely apart, the crank has been polished, it has .030 on the block and .030 domed pistons (previous rebuild, I did not know it had been rebuilt or i would have just run it) I want to build it to last and have a bit of spunk. I am trying to go on the advice of the machine shop that the 10.5:1 will be problematic. I am trying to get a good rebuild for this commando 273.
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I was told by Tim Connelly, the cylinder head guru at CTC, that the early 273 heads were swirl port heads. I also know that some of the the 1966 273 heads with the casting #2658920 had 1.87 intakes and 1.50 exhaust valves. These heads would give you a little more spunk. There were also 920 heads in 1966 that had the 1.70 and 1.50 valves.
 
Hi, just off topic but do you know what castings those 273 swirl ports are? i was always under the impression that swirl porting was a new technology in the 80's? any more information on this would be great as im trying to build a stock 2bbl 273 into something fairly hipo. cheers
 
Hayzee said:
Hi, just off topic but do you know what castings those 273 swirl ports are? i was always under the impression that swirl porting was a new technology in the 80's? any more information on this would be great as im trying to build a stock 2bbl 273 into something fairly hipo. cheers
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Since all the 273 heads have closed chambers, I'd assume Tim ment that all of them are an early version of swirl port design. Tim was going to port the heads on my first D/Dart but he never had time to do them. At the time, he was doing a lot of porting on Harley Davidson aluminum heads. He was making some big bucks doing that on the side. That was back in the late 90's.

Some of the 2658920 heads manufactured in 1966 had 1.87 intake and 1.50 exhausts valves.
 
bamacuda said:
Lot of good info here. The engine is completely apart, the crank has been polished, it has .030 on the block and .030 domed pistons (previous rebuild, I did not know it had been rebuilt or i would have just run it) I want to build it to last and have a bit of spunk. I am trying to go on the advice of the machine shop that the 10.5:1 will be problematic. I am trying to get a good rebuild for this commando 273.
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the early 273's 2 barrel motors were rated at 8.8 to 1 cr. Depending on how far down the piston is (block deck surface), the overall cc of the head and the gasket you run you could get a 9-to 9.1 out of that motor. Pretty respectable for a street 273. PM me and I can give you another idea.
 
Ive compared the so called #302 swirl port heads with the early 273 closed chamber heads. The 302 swirl port heads have a smaller combustion chamber. So putting a set of them on wont lower your cr and in fact I'd be inclined to believe you will loose flow. The intake ports on 302's have a huge pushrod bulge in them.

What I would do is establish a baseline on what you have now. Partially assemble the engine and put one piston in it. measure the actual deck height and cc's in the block with the high cr piston. Next - cc a couple of the combustion chamber pockets in the 273 heads. Do a calculation using these figures and a couple of different thickness head gaskets, use .026 (mopar perf gaskets) and .040 thick (fel pro gaskets). See what you actually have. You may find that the actual compression ratio is not so intolerable. I think Chrysler always misprinted the actual compression ratios of their engines, they were always lower than what was printed.

If you are concerned about detonation consider putting a slightly longer duration cam in it, that will bleed off some of the compression and allow it to run without retarding it.

Good hunting.
 
I say run it the way Ma Mopar intended it to run, like a 10.5.1 commando
 
You could try a set of Fel-Pro #8553 pt. These are .055 thick compressed and would be a cheap way to lower your compression.
 
bamacuda said:
My question is if I use anything but the stock heads, externally do they look the same?
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On a 66 or later engine you probably wouldn't be able to easily tell which heads are on it from the outside. So, yes, they will look the same.

On your 65 engine, since the bolts that hold the intake manifold on are smaller in diameter (IIRC) & are at a different angle than the 66 & newer engines, it would be easy for a knowledgeable person to tell the heads had been switched. Most people would never spot the difference.
 
'64 Cuda said:
On a 66 or later engine you probably wouldn't be able to easily tell which heads are on it from the outside. So, yes, they will look the same.

On your 65 engine, since the bolts that hold the intake manifold on are smaller in diameter (IIRC) & are at a different angle than the 66 & newer engines, it would be easy for a knowledgeable person to tell the heads had been switched. Most people would never spot the difference.
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64-65 used the 080, 315 castings
66-67 used the 178, 234, 920 castings 920 casting is the best
68-69 used a 675 open chamber casting
 
72 Demon said:
IMHO the best course of action, particularly if you are planning to rebuild the engine, is to install the flat top (2 bbl) pistons. If no rebuild is planned, try the thickest head gasket you can find. (Don't know if you can double up head gaskets on the LA). The advantage of these options is that there is no need to alter bolt hole relationships.

The flat tops are a lot easier to find than the domed pistons. I would be very hesitant to do anything to the HP pistons beyond new pins and rings.
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I'd be carefull of this. My rebuilt 273 2 bbl cc'd out to 7.7 to one compression. Heads were 65cc and pistons .045 in the hole. I had to mill the heads 30, and the block 40 thou to get 9 to 1 out of it. Now its 60 cc heads and .005 in the hole. Using a .040 thou gasket and ending up with .045 of quench.
The pistons were new so I just decided to see what I could do with what I had. Ideally I would have gotten dome pistons. But theres a 340 sitting in the corner taunting me, I'd rather spend money on that.
 
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