JohnnyCuda's 68
Dec 7, 2008
Reaction score
Difference between 340 manifolds and 273 they look awful identical
These are supposed to be 273’s


Port sizes are DRASTICALLY different. Top one LOOKS similar, bottom one is different.
I'm going thru the 273-340 changeover now. It snowballed real quick lol.
340s have individual ports, as previously stated- and MUCH larger ports. The outlets are also much bigger, and of a flat flange design. 273/318 manifolds have a cone/bell outlet that is smaller diameter.
Short answer: No. They are very different.
71 and up 340 passenger side does not have individual ports, it has the common plenum on the center 2 cylinders.
273 manifolds work on the 2bbl, small valve heads, and they choke about the same time or a lil before the 2bbl cam does.
The 340 logs work with the big-port heads and the 4bbls, but they kill the overlap on the 340 cam. They say, that headers allow the 340 cam to come alive, which can add quite a bit of top-end power to them, perhaps more than is typically expected.
But, you know;
with 3.23 gears and an automatic, you'll probably never notice the missing power cuz 5500 in 1.45Second gear is like 94 mph...... with 27s, and just how often are you gonna be at 94 mph...........

But I gotta say;
Oro Valley AZ is at about 2600 ft elevation, and
the 273, if that is what you have, is gonna be wheezing as it is, so the 273 logs will be just fine on it. Not so with a factory-cammed 340. Well, maybe a low-compression one, lol.
if your car has to operate at multiple elevations, I hope you have a high-compression model, and a longer stroke will help it get moving. Notta chance would I use a 273 at 2600ft. Well, to tell the truth, notta chance would I build anything less than a hi-compression 318 for 2600ft, for a car was under 3000 pounds with driver. By 3600, pounds I want a 360. But that's just my opinion.

Science says that an engine will lose about 3% power for every 1000 ft elevation change. from sealevel to 2600ft is about the same power loss as a full point of compression. thus, an 8/1 engine becomes like a 7/1. and the 6% power-loss of a 2bbl 273 is like 11hp. or, close to it, about the same as one cam size. So it's a double whammy.

But if you never floor it, I guess a 273 would be ok., lol.
Last edited: