John Bedore
Well-Known Member
Anyone know about heads # 2658920-5 ,What year and motor they were on? I have a 73 340 I was wondering if these would be better?
Heads
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I don't see um.
Small Block Mopar 273 Cylinder Head
Casting #2658920
Came from the factory with the Combustion Chamber at a range of 61 to 64 CC's.
On the 1966 and 1967 Commando 273, if the Piston Top (flat section) is set at
{-.011} below the block deck, the Cylinder Head will require 57.3 CC's to achieve a
10.5 - 1 Compression Ratio with .020" thickness Head Gaskets.
On a 273/180 HP 2-Barrel engine, if the Flat-top piston is set at {-.011} below the
block deck, the Cylinder Head will require 57.3 CC's to achieve an 8.8 - 1 Compression
Ratio with .020" thickness Head Gaskets.
For every .0065" milled off the Cylinder Head, 1 CC from the Chamber Volume will be
deducted.
Bringing the CC Volume in the Cylinder Head 'combustion chamber' down to 51.3 CC,
should increase the Compression ratio by .75
The Intake Port measures at 127 CC in Volume
The Exhaust Port measures at 60 CC in Volume.
The 'general rule' is that without larger Valves and Port work, these
Cylinder Heads will top out at 300 to 315 Horsepower.
Casting #2658920
Came from the factory with the Combustion Chamber at a range of 61 to 64 CC's.
On the 1966 and 1967 Commando 273, if the Piston Top (flat section) is set at
{-.011} below the block deck, the Cylinder Head will require 57.3 CC's to achieve a
10.5 - 1 Compression Ratio with .020" thickness Head Gaskets.
On a 273/180 HP 2-Barrel engine, if the Flat-top piston is set at {-.011} below the
block deck, the Cylinder Head will require 57.3 CC's to achieve an 8.8 - 1 Compression
Ratio with .020" thickness Head Gaskets.
For every .0065" milled off the Cylinder Head, 1 CC from the Chamber Volume will be
deducted.
Bringing the CC Volume in the Cylinder Head 'combustion chamber' down to 51.3 CC,
should increase the Compression ratio by .75
The Intake Port measures at 127 CC in Volume
The Exhaust Port measures at 60 CC in Volume.
The 'general rule' is that without larger Valves and Port work, these
Cylinder Heads will top out at 300 to 315 Horsepower.
Oh the little closed chamber small valve jobbies. I didn't recognize the casting number, which is why pictures are helpful.
No, those won't be good for a 340.
I take that back, they will be good as long as you don't want it to pull a greasy string outta a cat's ***, then use um.
John Bedore
Well-Known Member
I have another Question: I want to get my motor 340 back to 10to1 comp. is the 70 340 pistons different from my 73 ones and will they give me my desired net of 10to1?
They should, but you have to check you deck hight, also depends on the combustion chamber volume and gasket thickness. I'd run large port heads on a 340.
920 heads are the best small port, small valve heads.
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There are several aftermarket pistons that will bring you up to 10:1 or over. But what kind of condition is it in, what are you going to do with it, and how much do you intend to spend on it? If you have a good low mileage or refreshed short block, aftermarket closed chamber aluminum heads with thin small bore head gaskets will give you a compression increase and more power from the extra flow, too.
Pistons will do the trick with a pair of the large port/large valve, open chamber, 340 heads. (or performance aluminum.) You can also build a flat top piston engine with closed chamber aluminum heads.
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furrystump
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Advertised compression ratios historically inaccurate. I would do the math before buying anything really. If the plan is to have the block machined to actual zero deck (most are pretty tall) you can then calculate from there what your static compression is.
Yes sir. The earlier (71 and back) pistons have only 2 valve reliefs and a very slight dome that protrudes out of the bore at TDC. The 72 and 73 pistons have four valve reliefs and sit down in the bore a little at TDC. There is a big difference, so get familiar with it by looking at it or you may slip up and get the wrong piston.
They certainly are.....for a 273 or maybe a 318.
If he's careful and measures very carefully, he can build a hell of a quench engine with the early 340 piston and a closed chamber head.
John Bedore
Well-Known Member
I just want to say Thank You to all that gave their advice. One other thought what would happen if I used my 73 340 heads ? Should I shave them or leave stock?
If you do any head work, the deck should just be made flat, the valve job not withstanding. I used the (replacement) KB-243 (IIRC) piston with aluminum & iron heads. Clay says I have lots of room. I myself would not use replacement pistons.
I usually mill heads, like said above. Just did a set of 70-71 J heads. Had them milled .020 and .019 on the intake side. 73 heads are good, should have hard exhaust seats and decent flow. I have not cc'd 73 heads but should be the same, about 69 cc per chamber with the .020 cut.
The factory virgin combustion chambers typically measure 63.5-64.8 cc. The correct milling numbers for the 920 head is: For every .0053 from the head surface to reduce the chamber volume 1cc. This formula, from the Chrysler Racing Manual, is dead on.
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MOPAROFFICIAL
Oogliboogli
What you intend to do and where you want the peak torque, rpm to run it at.
You could experiment with 1.88 and the 920 heads.. or... what are the 73 340 heads you have...1.88 ? Drop 11/32 1.94/1.6 intake/exh valves in them. As for the 920 heads... If you are able to get the low/mid lift strong and typical 360 head flow peaks ...that would be neat
AKA porting a 318 head to a 360 status…. ???
MOPAROFFICIAL
Oogliboogli
Yeah.
John Bedore
Well-Known Member
I am trying to purchase a set of X- heads,will they work with my 70's pistons?
Yes.
There's a lot if's there.
That's a fact you caint deny. lol
You may want to measure the chamber volume in your 051 heads, too. I've got a set of similar 475A castings from a 1975 truck 360 (also used on later 318 4 barrel) that have a clean up cut on them (.010) that measure 66 ccs with the stock tulip valves in them, or very close to 318 chamber size. So putting nail head valves is going to give you a decrease in chamber volume and an increase in port volume, which is always good in stock iron heads! The small stem 1.94/1.60 chevy valves with good valve job (and maybe hardened exhaust seats in case the stock seats are sunk) that MOPAROFFICIAL official mention are a proven performer. My machine shop guy uses 5/16 stem, but I think the performance advantage for him is more budgetary (I imagine he buys them in bulk). While your into it, you'll want to decide what cam you're going to run with what rockers. Valve lift is pertinent to how the heads are set up.
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