Some questions on the 273 head gasket "thing".

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Blue62Val

Blue62Val

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G'day guys,

so......I've got a 65 Barracuda formula S with the Commando 273.

Motor is out, and seems in reasonable shape, but I plan on opening it up for a look.

Head gasket - I've read all the stuff about the bore being smaller and therefore a smaller "odd" shaped gasket. IF I go with the normal small block head gasket (318. 360 etc) will there be any problem, other than :
lower compression due to the thickness
not a perfect fit on the bore

and......does this matter ?

I'm just restoring the car and whilst wanting it to be as good as it can be, it's never racing anyone, but I'd like it to perform as it probably should.

Hoping what I'm asking is simple enough.......I've looked through various posts, and think it's what I've said. It kind of confuses me that the bore hole in the gasket can be wrong, but still work.

Peter
 
Use the FelPro, Victor gaskets for you application or go to Cometic and get what you need.
 
Just getting ready to post on this very subject with compression ratios. Cometic gaskets are custom only for the 273 and not MLS. On 273 head gaskets there is a funky cutout to match the cutout on the block. If you do not have the cutout on your block you can use the 4.04 x .027 Cometic gaskets. Other than that a standard factory 318 head gasket is 4.01 x .039 and fits the head chamber nicely. Fel-Pro and Victor gaskets work well but the bore is larger. So yes, a decrease in compression.
 
PM sent Mr Gasket has a 4.140 bore, .028 compressed gasket. It's about as good ad it get's unless you call Cometic and have them make you a set. Which is more important to save compression? Smaller bore or compressed thickness? They both effect compression but not enough to make a difference. You or your engine probably wouldn't notice a difference. May be a couple horsepower on a dyno.
 
toolmanmike said:
PM sent Mr Gasket has a 4.140 bore, .028 compressed gasket. It's about as good ad it get's unless you call Cometic and have them make you a set. Which is more important to save compression? Smaller bore or compressed thickness? They both effect compression but not enough to make a difference. You or your engine probably wouldn't notice a difference. May be a couple horsepower on a dyno.
Click to expand...

Thanks again, mate.
 
Blue62Val said:
Head gasket - I've read all the stuff about the bore being smaller and therefore a smaller "odd" shaped gasket.
Click to expand...
Hey mate, have a look at the Cometic Australia page here, CHRYSLER 318/340/360 V8

They have multi layer steel head gaskets with the smaller bore for the 273. Various thicknesses as well, down to 0.027’’. Might be worth giving them a call.
 
Valman said:
Hey mate, have a look at the Cometic Australia page here, CHRYSLER 318/340/360 V8

They have multi layer steel head gaskets with the smaller bore for the 273. Various thicknesses as well, down to 0.027’’. Might be worth giving them a call.
Click to expand...
Much appreciated. I’ll have a look.
Those summit ones are a good price, but postage is a killer.
 
If retaining compression is important, compressed volume is the only measurement you need to be concerned with. That basically takes all measurements into consideration and gives you one target to look for.
 
What is your current cylinder pressure?

Personally, on a hi-compression, 273Commando, I think the headgasket-deal is very important.
Going from the ~3.4cc factory head gasket, to the 8.6cc Felpro,
will drop the Scr from a true 10.5 , down to 9.7, which is, as you can see, almost a full point. This doesn't look like much when you see it in this format. But when expressed as a change in cylinder pressure, you are looking at maybe [email protected] Scr, going to maybe 160psi @9.7; so that's like 15psi which is a loss of ~8.6% ; That's big.
The question is;
Can the 10.5 engine even run on pumpgas, without detonation..
The typical upper limit for best pump gas is 165psi; a little more for closed chamber heads, and/or a late closing intake valve.
Therefore;
in this case, 9.7Scr may actually be a good thing over 10.5.

What's not good, is big-bore gasket on the small-bore block, which pulls the fire-ring back about a quarter inch from the edge of the bore. That can't be good. The exposed top of the bore, with it's sharp edge, is now primed to cause detonation under the right circumstances. So, you gotta fix that....... but when you do, that's gonna drop the Scr a lil more. and no matter what, the crevice is gonna be bad for emissions, if that matters to you.

Sydney Australia, is practically in the ocean, so pressure is gonna be iffy at 10.5 anyway.
IMO; 9.7 may even be too much. Changing your cam-timing may help.
Small chamber ALLOY heads will help a lot. You'll probably even be able to run the lowest octane gas with those.
Just so you know;
My alloy-headed 367SBM combo, has run up to 195psi on 87E10 Canadian gas, which is; (R+M)/2 With the current slightly bigger cam, she is down around 187psi....... still on 87E10.

IMHO
for your application, if the pressure is too high, alloy heads may provide the anti-detonation answer you need. Or, you could just mill the tops down on your pop-up pistons.
It all depends on what your current cylinder pressure is, and
what it might jump up to, after freshening.
 
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