273 vs 340 exhaust manifolds

[rottenfish]

What kind of difference am I looking at between a set of '65 273 4bbl exhaust manifolds, vs a set of '72 340 manifolds? My understanding is that the 273 used a smaller port size?

[Chryco Psycho]

the 273 used a smaller port size for sure

[Challenger6pak]

I know the intake port is smaller. Is the exhaust port smaller also?  I thought they were the same. 

[rottenfish]

I am wondering how restrictive they are in comparison to the 340 manifolds. The 273 commando made a few HP with them, but are they a bottleneck on a 273? On a 340 or 360?

[Challenger6pak]

Any manifold will be restrictive.  The best factory small block manifolds are the TA/ AAR manifolds.  If you are looking for flow use headers.  The 340 manifolds you speak of are a log type manifold.  The 273 may have one side that loops up and is simular to the TA style manifold.  This was to clear the steering box in an Abody.  It wasn't nessesarily for performance.  You may get a little more flow out of that style of 273 manifold over the newer 340.  I can't tell you how much or if it is worth it to find them.  I would just run headers for a high flow exhaust system.

[rottenfish]

I am looking at a complete 273 for sale locally. Just curious as to what the manifolds were good for, if anything. My project will require custom headers anyway to mount the turbo. I read the 273 used the same crank as a 318 poly? Anyone have any idea how they hold up to high rpm?

[Challenger6pak]

The 273 and the 318 poly may be balanced different.   Why don't you used a forged 340 crank or an aftermarket crank?  I don't have any idea of what motor you are putting together. As far as 273 value goes;  I had one in a 68 Cuda with the 3 speed O.D. trans.  It was peppy and got very good gas mileage.  As far as anyone wanting parts from the motor I would say it is very slim.  Not many want the 273 or 318.  If gas keeps going up I think you will see more used.  I am building a 318 w/ NOS and and OD trans for a Challenger right now, since I want the best of both worlds.  Most shows are over 100 miles away from me and the 440 is too thirsty. I don't know how many people are doing this yet. 

[rottenfish]

I think the mains on the 273 are different than the 340/la 318, so it doesn't leave much option for swapping cranks. I was planning to build a turbo MPFI V6, but I wondered how the 273 might work for a moderate boost/high rev setup. For the price I will probably buy it anyway, I will find something to do with it.

[Chryco Psycho]

the 273 ,318 , 340 all share the same crank with different balancing, there was some difference int he angle / position of the intake bolts on the 64-65 273

[Dustu]

I rebalanced a 273 forged crank and used it in a 340 no  problem.             

[rottenfish]

Cool, thanks guys... The crank/main info I found was on allpar:

"Bill Watson wrote: "The LA 273 used the poly (A) 318 crankshaft, bearings, bearing caps, vibration damper, and conecting rods. The 273 and 318 both had a 3.31" stroke. Pistons were different, though, as the LA bore was smaller; and camshafts could not be shared due to the different valve arrangements. They did share a timing chain."