Large valved 273

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PRH said:
If a larger than stock intake valve is installed in a stock 273 head....... what’s the lift point at which the valve contacts top of a 273 block?
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Funny you should ask that.... I checked on a 675 head. With a 0.039" head gasket, the valve movement with my simulated 2.02" valve head was about .255" when it contacted the block.

I've run through all the details with trying to use a cam at around .520" actual valve lift.
 
I was running an Isky solid lift cam @ .580 (312 duration) in my 273 and then again in the 273 with notched block and the 340 open chamber heads (2.02 intake). Never took those types of measurements though.
 
You can only get so much air into a small cylinder so big valves really won't do much....unless you used forced air.
 
It's not just the valve hitting the block
It's the airflow hitting the top of the cylinder
 
This is what we need. LOL

4 valve head.jpg
 
Whoever makes a Hemi head fit the sbm will be a genious. Oh wait. Its called a gen 3 Hemi. Would be nice if we could adapt them to a LA or magnum. Kim
 
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I think this is a great discussion.
But I’m wondering what the point is of maxing out a 273?
Is it for stock appearing or pure stock drags?
If not it would be more cost effective and much easier to install a larger LA engine.
 
Wyrmrider said:
It's not just the valve hitting the block
It's the airflow hitting the top of the cylinder
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But tell me this, wyrm....The bore is smaller than all the head Chambers. So will the flow be less with a 2.02 valve with the overhang being closer, vs a 1.78 valve with the same overhang .125" further away? IDK, but 90 per cent of the valve opening is not facing into overhang....
 
I can’t see any reason why I’d ever feel I needed 2.02 valves with the small bore and cubes of a 273.
I’d probably use something more like some sbc 1.84’s and call it good.

I like taking the easy path, and 360 blocks still aren’t hard to come by..... or stroker kits for them.
And then the 2.02 heads work fine.

Lazy? Maybe.
 
PRH said:
I can’t see any reason why I’d ever feel I needed 2.02 valves with the small bore and cubes of a 273.
I’d probably use something more like some sbc 1.84’s and call it good.

I like taking the easy path, and 360 blocks still aren’t hard to come by..... or stroker kits for them.
And then the 2.02 heads work fine.

Lazy? Maybe.
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Sometimes avoiding unecessary complications means you get the job done...been thinking more and more of that. If the LA-X comes back, I'll probably go that route and bore enough to not need bore notches. No more lift limits...
 
Dont forget, ANY cylinder head you bolt on to that small bore, will flow LESS air on that engine compared to a larger bore. Therefore the 273 is handicapped from the get-go, plus harder to get compression in a small bore, that's the reason the factory had to use a dome piston. Probably the WORST SBM to build, unless your just doing it for originality or nostalgia.
 
The funny part, is I remember test driving a new HP 360 in a Aspen kit car and passed on it because my 273 was faster. I ended up having my seats reupholstered in leather instead of getting the Aspen kit car with HP 360. Go figure.
 
Wise move!
66fs said:
The funny part, is I remember test driving a new HP 360 in a Aspen kit car and passed on it because my 273 was faster. I ended up having my seats reupholstered in leather instead of getting the Aspen kit car with HP 360. Go figure.
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replicaracer43 said:
Dont forget, ANY cylinder head you bolt on to that small bore, will flow LESS air on that engine compared to a larger bore. Therefore the 273 is handicapped from the get-go, plus harder to get compression in a small bore, that's the reason the factory had to use a dome piston. Probably the WORST SBM to build, unless your just doing it for originality or nostalgia.
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The 4 main problems building a 273, getting high CR out of them, bore size obviously is gonna cut down on air flow, but these aren’t the biggest problem for most builds but rpm, a 273 is gonna have to turn 1500-2000 rpm higher than a similar hp 360. Which to me brings the main problem gearing most don’t want to go with more than 3:55 which is under gear for most combos but a decent compromise for 340/360 but if you want to build a 273 that has any chance of keeping up with 340/360 your gonna need a lot of gear at least 4.10 or deeper and lots stall if it is a serious build.

To me when it comes to 273/318 if your willing to turn it and gear it then build it if not go bigger.
 
Learning here, so bare with me. But how does smaller bore relate to compression? Are you talking compressing psi, or talking ratio? Can you elaborate?
As I said, TRYING to learn lol
replicaracer43 said:
Dont forget, ANY cylinder head you bolt on to that small bore, will flow LESS air on that engine compared to a larger bore. Therefore the 273 is handicapped from the get-go, plus harder to get compression in a small bore, that's the reason the factory had to use a dome piston. Probably the WORST SBM to build, unless your just doing it for originality or nostalgia.
Click to expand...
 
The larger the cylinder volume, the easier it is to squeeze it down=high compression. When you start out with a smaller cylinder volume, it is more difficult to make it even smaller.

Same goes for strokers, easy to run higher compression. The opposite is true of de-stroking, you loose compression pretty quickly.

It's just math/physics.
 
4spdragtop said:
Learning here, so bare with me. But how does smaller bore relate to compression? Are you talking compressing psi, or talking ratio? Can you elaborate?
As I said, TRYING to learn lol
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Look at a 340. 10.5 at the top of the bore. 273 has a dome to get the same Compression ratio. Really a mute point on the street, since you can't run 10.5 on pump gas. I shoot for 9.0. I'm not going to worry too much about compression as much as flow. Race engine is another story.
 
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