dibbons
Well-Known Member
Don't want to modify my custom dual exhaust system if I swap the 318 for a 340. Any problems using 1972 318 exhaust manifolds on a 68-72 340? Thank you.
Can I use 318 exhaust manifold on 340?
- Thread starter dibbons
- Start date
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ScampMike
Despicable Damn Yankee
not enough to lose sleep over - it'll bolt right up
mguner
How many is too many?
There is enough difference the other way around that without gaskets you can have a tic tic tic type leak on some early 318 head castings with the 340 manifolds. The bottom of the port openings is so much bigger on the 340 that it is dead even with the outside bottom edge of the 318 exhaust port. The 318 manifold should seal but will be restrictive.
ScampMike
Despicable Damn Yankee
I guess it would be good to know the build level of the 340... when I said not enough to lose sleep over I assumed a mild street 340 and you were just looking to "go slow" on the build as you don't want to "fix" the exhaust you already have
dibbons
Well-Known Member
The 340 will be basically stock.
Woods74
Broke Senior
ScampMike
Despicable Damn Yankee
so yea, it'll bolt up - JMO here - I always use gaskets when mounting exhaust manifolds - stock type or headers. Oh, and change that ground too.. you're already there, just do it.
C130 Chief
Mechanical Genius
I thought starting in 72 all A body small block manifolds were the same.
Treblig
Well-Known Member
340 and 360 heads have a much larger exhaust opening. The pics below show how the 318 gasket (blue) almost falls through the 340/360 gasket. You might get lucky and get it to seal up but then you also might have a leak that you can't fix.
If you go with 340/360 manifolds you'll only need to change the down pipes but if you're working on an early A body you'll have other issues as well.
Treblig
If you go with 340/360 manifolds you'll only need to change the down pipes but if you're working on an early A body you'll have other issues as well.
Treblig
BigBlockMopar
BigBlockMember
Surprisingly small differences in the HotRod-article between cast manifolds and headers.
I would have expected much larger gains.
I would have expected much larger gains.
Treblig
Well-Known Member
I totally agree, unless you're running your car at the track on a frequent basis it's almost a waste of money and trouble. Some of the headers are a real pain to install and the ones that are easier to install are really expensive. Paying $300 more for 6-7 horses just doesn't seem worth it to me....but that's just me.
The chart posted is the main reason I started selling the 340 driver/360 magnum manifold combination. Of course I have to machine the 360 magnum to get it to match the 340 but it's something that I've gotten pretty good at doing. The best part about the large cast iron manifolds is ease of installation and low price not to mention no leaks or scraping on the speed bumps!!
Treblig
BigBlockMopar
BigBlockMember
But then, when I'm looking at the ex.manifolds on my '73 318, I really can't believe how restrictive they seem.
Treblig
Well-Known Member
The stock 318 manifolds have a 1 5/8 exit hole compared to the 340/360 magnum which have an exit hole of just over 2 1/4". That's a big difference!!! That's more than a 30 percent increase in area. A 30 percent increase in area will absolutely give you a marked increase in flow.
Just look at this pic of a 318 manifold next to one of my "unmachined" 360 manifolds. You could almost stick the 318 manifold inside the 360 hole!! The second pic is one of a "machined" 360 exit hole....no comparison to the 318 manifold!
Last pic is the 318.
treblig
highwaystar50
Active Member
Does anyone subscribe to the smaller pipe diameter (increased back pressure) equals more torque theory? When the HP 273 option came out with only a single exhaust, were the engineers trying to "engineer" in more torque? I believe so as the 273 sure didn't have much of it. Yes they could have changed the cam profile but that would have sacrificed horsepower which is what everyone wants when they order a high performance engine option. With its smaller displacement, the 273 always played third banana to the HP versions of the 283 and 289 in stock form.
Treblig
Well-Known Member
According to the chart posted by Poisondart74, the only thing that seems to help both torque and horsepower is larger (better flowing) exhaust manifolds (or headers). If you look at the chart closely you'll notice that even the Hooker 1 3/4" Super Competition header was only marginally better than the 340 manifolds (2 1/4"). That tells you that the 340 manifolds were pretty efficient, they had to be to allow the 340 6pack engine to do so well!! That's why I wrote that unless you're going to track pretty often or you're a hard core street racer spending an "extra" $200/$300 for headers isn't worth 2-6 more horses.
The HP 273 probably did have a well designed exhaust system to make the most of a small displacement engine but putting bigger exhaust manifolds and exhaust pipes would surely help even more. The 273 and the 318 manifolds only have a 1 5/8" exit hole in their manifolds. That really chokes down the exhaust flow and there's very little you can do to compensate for the "bottleneck" at the exhaust manifold exit.
As far as the 283 is concerned....Because Chevy didn't use torsion bars exhaust manifolds could be designed with more flow. The design of the Mopar A body front suspension and body design really limited the option when it came to stock exhaust manifolds, JMO.
I'm sure other members will have different opinions...........
treblig
pishta
I know I'm right....
Those dyno numbers are from a stock motor? 311hp-400 ft/lbs from a teen? The more you cam, the more important a good exhaust becomes. Stock can exhale through a straw.
Treblig
Well-Known Member
What do you mean by "Stock can exhale through a straw". Do you mean that if your engine is stock all you need is small exhaust manifolds??
treblig
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