Large valved 273

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If we could get a custom piston made to fill up the large 340/360 chambers it would help. Remember the 360 head gasket issue with the small 273 bore? Lot's of wasted chamber hanging over the edges. Or we could cast a new head with valves closer together. There is quite a space between the seats. Bigger valves? No problem. Now we are notching bores again. You could get bigger valves in the small bore if you tilt them, like a Hemi! How about a 4 valve head? That's what the small imports do. Heck, Just put some domes in the bore, juice up the heads, slide a proper cam in and hold on tight. The 273 can only do so much. Someone mentioned supercharging. Sure could. One of those GM 3800 blowers would work perfect.
 
Dang, there used to be a turbo option on Desktop dyno.
 
I've been working on this issue for the last year or so. With top ring locations being what they are typically, you're gonna be limited to the high 400's, or maybe right at .500" for max lift. It depends on the exact piston. This limit is for a stock deck, and the top ring location is the issue.

If one were to do custom piston, and would move the top rings down about .100", then you could push up close to .600" . Probably no ideal to move the top ring down but that is what you gotta do for that matter. Egge might be able to do that; dunno if you could just ask for the ring groove to be machined lower. It would depend on the basic casting.

As for the valve size and flow shrouding... sure, you would not get max possible flow from a 2.02" valve being by the cylinder wall notch. But I would expect you to still get more flow that way than with a 1.88" valve and no notches.

Turbo or super-charging makes a lot of sense.
 
The 2.02 will cause reversion because of the air bouncing off of the deck. If u want a 273 to perform build it like the old D dart with a modern cam and 302 heads. Kim
 
too big does not work for lots of reasons
too big for the ports for one thing
and no reason for big exhaust at all
I'd start with the late 318 heads if stock heads
but then I'd use 30 degree seats with smaller cams in the 500 inch lift range
you can notch by hand- no need to notch the exhaust
flow does not like to hit top of cylinder, a little break can make a lot of difference
just reminiscing I think we took a 2.02 chevy valve and reduced the diameter to get a decent margin for a 30 degree seat and back cut them would have come out in the 1.94 area (you can't just re seat a 1.94 valve as they get too thin- 2.02 valves are cheap in any case- and lighter than MOPAR valves
measure twice cut once
 
Just use a fly cutter the appropriate size. One and done.
 
I've been working on this issue for the last year or so. With top ring locations being what they are typically, you're gonna be limited to the high 400's, or maybe right at .500" for max lift.
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I’m def not following the reasoning here.
The piston is nowhere near TDC when the valve is at full lift.
 
Should check the valve to piston clearance through the entire range of crankshaft rotation (2 full spins) either checking with clay or those light springs. If using the springs, must check at other piston elevations besides TDC because the piston dwells near TDC for quite some time as it changes direction from going up to going down. I think I know what I am talking about anyway.
 
PRH said:
I’m def not following the reasoning here.
The piston is nowhere near TDC when the valve is at full lift.
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Roger.... The limit is the depth of the notch you can put in the side of the bore and not have the top ring get into the notch. If the notch can only be so deep due to that, then the lift can only be so much. I've been working on the premise to not allow the top ring to overlap even partially over the notch at TDC... sure wouldn't do the ring seal much good to allow it to do so!

I've been also looking at other pistons with a much bigger bore, and offset boring the block towards the intake side to get completely away from having to notch the bore at all with a 2.02" valve. Looks doable, but boy is it close!!
 
oldkimmer said:
The 2.02 will cause reversion because of the air bouncing off of the deck. If u want a 273 to perform build it like the old D dart with a modern cam and 302 heads. Kim
Click to expand...
Now I'M not following the reasoning here ???? (But hey, nothing new there!) The 273's 3.625" bore is so small that even with a 1.88 or 1.94 valve, that's gonna happen. That bore is smaller than the head chamber and you're gonna get this effect to some extent or another. Even 675 heads for the 273 and 318, does not change that; the chamber overhangs the deck. I would have to check to see if a 302 chamber did the same but I am pretty darned certain that it does.

See here for a 675 chamber on a stock 273 bore; you can see the shadow line of the overhang of the chamber. (The white disc is a simulated 2.02" valve head.)

DSCN2656.JPG
 
Oh just notch the bore. It'll be fine! :poke: LOL
 
How about just using a cheap 318 block with its .285 bigger 3.91 bore right off the bat? Youll pick up .1425 at the edge. Your getting pistons for it anyway and wont have the valve shrouding (or as much). I know its now a 318 but C'mon....everything from the 273 still fits in and on it sans the pistons.
 
I 'm curious as to what difference a 1.88 or 1.94 valve would do. I wonder how accurate the Desktop Dyno would be for bigger valves?
 
yr wrote

"The biggest things about notching the bore are keeping the notch inside the gasket sealing ring"

you want to keep the notches away from the edge of the combustion chamber
you want to keep both the notches and combustion chamber smaller than the gasket
put the heads on the block, dykem and scribe deck around the bores AND scribe the gasket
and then scribe the top of the block using a hook with the heads onor using the size of the chamber taken from a templet you make
with core shift you want to check each chamber
it does not take much chamfer to make a difference- a square sharp edge shears and is a no no
 
If your gonna notch I’d go with magnum heads, closed chamber more than enough flow.
 
TMM - sorry about the X's.

D/Darts had stock 920 heads - 1.78 and 1.50 valves. They are closed chamber swirl port heads. Mine ran a best of 12.95 at 105 with launch and shifts up around 6900 RPM's. The Doug's headers had 1 3/4" primaries and 3" collectors back then. The guy that raced my first D/Dart used a 36" collector extension when he took it to the strip. I think he referred to it as a "stinger".

In the TSB D66-HP -1 dated April 18,1966, there is a section entitled `Car Operation For Best Performance '. This TSB can be found on the D/Dart website and the Hamtramck site.
 
ddartdude said:
TMM - sorry about the X's.

D/Darts had stock 920 heads - 1.78 and 1.50 valves. They are closed chamber swirl port heads. Mine ran a best of 12.95 at 105 with launch and shifts up around 6900 RPM's. The Doug's headers had 1 3/4" primaries and 3" collectors back then. The guy that raced my first D/Dart used a 36" collector extension when he took it to the strip. I think he referred to it as a "stinger".

In the TSB D66-HP -1 dated April 18,1966, there is a section entitled `Car Operation For Best Performance '. This TSB can be found on the D/Dart website and the Hamtramck site.
Click to expand...
I was just reading through this old thread. So very interesting. 1966 Dodge D/Dart ~ 'Mystery, Myth and Misconception'
 
I deleted my 2 posts. I don't like to have information out there if I can't back it up. It just adds to the confusion.
 
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?
 
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?
Click to expand...
Good question.
 
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