Large valved 273

-
roccodart440

roccodart440

Well-Known Member
Joined
Apr 19, 2010
Messages
6,797
Reaction score
3,563
I read a lot about 273's and the guys who build them. The small block heads available have decent power potential, but the 2.02 intake valve (or larger?) is apparently an issue with the 273's...unless you notch the bore. This would seemingly be exasperated by the lift of the intake lobe.

Now here are my 2 questions in regards to this...

1. Is there any downfall to notching the bores?
2. Is this a die grinder notch or is it done using a vertical mill, etc?

Unless there is some huge drawback, and it doesn't seem that complicated, why is this always pointed out like a hard roadblock?

Seems like to me, other than being cubic inch deficient, it's a good engine with some potential.

It also seems to me that the answer to making one go fast is putting a supercharger on it. Now you effectively increase the cubic inch and overcome the anemic factory head design.

thoughts?
 
Shrouding from what I understand. I would think the correct way to notch the bores is on a mill.
 
The biggest things about notching the bore are keeping the notch inside the gasket sealing ring, and making sure the top ring doesn't get in to the notch. IOW's, don't get the notch too far down or the rings too far up that the notch and the rings run into each other.

I haven't done it in years, because with aftermarket blocks and such there isn't really a need for it. But I used to do a ton of BBC stuff and have done 3 273's that I remember.

I can tell you in the early 1980's those 273's made a butt load of cash on the street. I've seen damn near full scale brawls break out over guys getting beat and then saying they weren't 273's. Many times guys had to crawl under the cars to verify the casting numbers on the blocks.

Solid lifters, big valve heads and 4.56 gears made many guys pay through the nose.
 
I wonder if it would be better to...

  • Port the head
  • Open the smaller valve farther
  • Open the smaller valve longer
Again, the other option here is to procharger it. There are bolt on kits to do it. Not a bad option...
 
unless you have huge compression larger valves are a no-no on 273's.. use small valve 340-360 heads..no 2.02's!..car will really wake up and no notch needed..
 
ph23vo said:
unless you have huge compression larger valves are a no-no on 273's.. use small valve 340-360 heads..no 2.02's!..car will really wake up and no notch needed..
Click to expand...


How did you come to the conclusion that valve size and compression ratio are related?
 
ph23vo said:
unless you have huge compression larger valves are a no-no on 273's.. use small valve 340-360 heads..no 2.02's!..car will really wake up and no notch needed..
Click to expand...
And 273's don't have a lot going for them already. 360 heads with their large open combustion chamber will make what compression there is disappear. Send the stock heads out for a little porting and call it a day.
 
I run a 273 0.030 over Edelbrock aluminum heads with 2.02 intake, 1.60 exhaust, notched block, with Commando pistons, Air Gap manifold. 650 cfm carb, Edelbrock RPM performer cam. Motor runs great going on 3 years now. Very responsive and reliable.
 
roccodart440 said:
I wonder if it would be better to...

  • Port the head
  • Open the smaller valve farther
  • Open the smaller valve longer
Again, the other option here is to procharger it. There are bolt on kits to do it. Not a bad option...
Click to expand...
What's the goal. 400hp, port the heads and the right cam will do it. You're on the right track there.
All out? Bigger valves in those are a ton more work. I would do what I could to avoid the notch...but its inevitable perhaps at a certain point.
 
With a .060 overbore and stock cylinder heads and valves (gasket matched to 340 intake) we ran 12.72 and later with 340 heads (2.02 intake valves and notched block) I never broke out of the 13.00's.

At the time (1970's) I was perplexed, but now I believe the lesser performance with the 340 cylinder heads was due to lower compression because of the larger combustion chambers.

65 Valiant5.jpg
 
No need to go bigger valve size in factory 273 heads, head swamp with bigger valve heads & notching cylinders unless ya want a high rpm beast that will require a 4.10 & up rear and preferably a 4 speed which is what I built for my Dart. The factory head design is not anemic on a 273.What is anemic are the factory exhaust manifolds. Ditching the logs of doom for headers will get ya from a low 16 sec quarter mile time in stock 273 commando car to high 14s with just that mod. Just leaving stock valve size ,getting a good port job on heads, adding a port matched intake manifold like offy still offers plus a little more cam, decent carb and of course headers will open these little fellas up like a Christmas present.
 
Last edited:
dibbons said:
With a .060 overbore and stock cylinder heads and valves (gasket matched to 340 intake) we ran 12.72 and later with 340 heads (2.02 intake valves and notched block) I never broke out of the 13.00's.

At the time (1970's) I was perplexed, but now I believe the lesser performance with the 340 cylinder heads was due to lower compression because of the larger combustion chambers.

View attachment 1715406935
Click to expand...



What happened to the MPH when you changed heads? ET doesn't mean a thing.
 
Oldschoolcuda said:
No need to go bigger valve size in factory 273 heads, head swamp with bigger valve heads & notching cylinders unless ya want a high rpm beast that will require a 4.10 & up rear and preferably a 4 speed which is what I built for my Dart. The factory head design is not anemic on a 273.What is anemic are the factory exhaust manifolds. Ditching the logs of doom for headers will get ya from a low 16 sec quarter mile time in stock 273 commando car to high 14s with just that mod. Just leaving stock valve size ,getting a good port job on heads, adding a port matched intake manifold like offy still offers plus a little more cam, decent carb and of course headers will open these little fellas up like a Christmas present.
Click to expand...
With simple hot rodding you should be able to get 300-350 horse out of one. Headers, ported heads and a camshaft to match. I have been messing on the desktop dyno. 1.88/160 valves helps pocket ported heads, with headers. . I haven't chased cams but I just put in a 268/485 110° stick. 344 horse @5500 and 346@5000 torque.
 
Just put some CH318B EQ's on it. Increasing valve size and better ports and better chambers all in one. Probably a lot cheaper in the long run, too. Probably have to notch the bores yet with the 1.92 that comes in them.
 
408 swinger said:
Just put some CH318B EQ's on it. Increasing valve size and better ports and better chambers all in one. Probably a lot cheaper in the long run, too. Probably have to notch the bores yet with the 1.92 that comes in them.
Click to expand...
Or send my 920's or 302's off to Radar. LOL
 
I remember an old Mopar Action, or maybe it was High PerformanceMopar, a guy bulit a 67 Dart 'vert with a 273 that ran tens. Maxed out X heads, big solid lifter cam, tall single plane, the works.
 
I like your thinking. but then; wouldn't it be cheaper to go bigger?
I'm waiting to see somebody put a staged turbo system on a 273, with an automatic/overdrive and performance gears. I know; my dream. But at 66 and semi-retired, I don't see it in my lifetime.
 
Plymouth 65 said:
I run a 273 0.030 over Edelbrock aluminum heads with 2.02 intake, 1.60 exhaust, notched block, with Commando pistons, Air Gap manifold. 650 cfm carb, Edelbrock RPM performer cam. Motor runs great going on 3 years now. Very responsive and reliable.
Click to expand...
Any pictures of the notching? Any time slips from down the drag strip?
 
4.10 gears aren't steep...if you run a modern overdrive.

Larger valves would only be an impediment if you went to a larger chamber size and didn't change pistons.

If you look at bone stock 273 heads, I think you'll find the flow numbers to be lack luster, but I'm also comparing them to modern aluminum replacement heads.
 
408 swinger said:
I remember an old Mopar Action, or maybe it was High PerformanceMopar, a guy bulit a 67 Dart 'vert with a 273 that ran tens. Maxed out X heads, big solid lifter cam, tall single plane, the works.
Click to expand...

It was "Mopar Action" story was "Darn Dart".......273 with "X" heads, TRW pop up pistons, story said .509 MP cam, don't remember intake, 904 trans, 4.88 gears (I think), ran in the 10's..........others can fill in gaps/corrections
 
roccodart440 said:
4.10 gears aren't steep...if you run a modern overdrive.

Larger valves would only be an impediment if you went to a larger chamber size and didn't change pistons.

If you look at bone stock 273 heads, I think you'll find the flow numbers to be lack luster, but I'm also comparing them to modern aluminum replacement heads.
Click to expand...

Have built 5 oldschool 273 powered mopars over the yrs including my final monster. Anything modern concerning a 273 is blasphemous to me but that is just me.
Would love to see a supercharged 273 build with a modern o/d trans
and other touches if that keeps the 273 flag flying.
 
I have photo's on my old phone of the notched cylinders. It was done on a vertical mill. This is not a race car, so no time slips. Just a strong running street car that represents the early muscle car era!
20180126_192250.jpg
 
You must log in or register to reply here.
-

Back
Top