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How restrictive are stock 318/340 exhaust manifolds?

dwade2777

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I'm rebuilding a 1972 LM340. I'm going to be using stock exhaust manifolds. I'm wonder if doing a bunch of head porting is worth the bother. If the heads breath great but can't move the air out the exhaust then what's the point?
 
They are as restrictive as most marine manifolds are, but the addition of water downstream cools (and shrinks) the volume such that flow is surprisingly good.

Jeff
 
Ayuh,.... ALL marine exhausts are restrictive, the log manifolds slightly more so than the center collectors,....
But short of full blown Headers, they all very restrictive,...

Ported/ polished heads build horsepower, but kill down low torque,.....

Hot Roddin' a boat motor is very different than Hot Roddin' a car,....

Yer effort will be mostly wasted,....
 
I'm rebuilding a 1972 LM340. I'm going to be using stock exhaust manifolds. I'm wonder if doing a bunch of head porting is worth the bother. If the heads breath great but can't move the air out the exhaust then what's the point?

If you are rebuilding the engine, you should consider the Barr Marine center rise manifolds, CHrysler used them starting around 1985, and from what I gather they flow much better than the log manifolds, to the tune of 15 HP, combined with a Edelbrock performer RPM air gap intake, little port work and a small bump in the cam You could see an easy 50-60 HP increase over stock, maybe more if you bump the compression and create a quench with the right piston domes.
 
My info says there's very little HP difference between logs and center dumps (from a guy who converted his).

Leaking logs only put water in the last cylinder vs, center dumps that get at least 2 (or 3) cylinders on each side.


Jeff
 
My info says there's very little HP difference between logs and center dumps (from a guy who converted his).

Leaking logs only put water in the last cylinder vs, center dumps that get at least 2 (or 3) cylinders on each side.


Jeff

Ayuh,.... Agreed,.... Both the center risers, 'n the logs have a common plenum, which causes increased back-pressure from collidin' exhaust gases,....

Ya need dedicated runners, aka: Headers to make use of portin' the heads, 'n big cams,...
 
Chrysler actually upped the HP rating from 250 to 275 when they changed to the center rise manifolds the only other switch was to the Quadrajet from the AFB, as the AFB was no longer available
 
It is if you are upgrading the cam and cylinder heads. The center rise manifold is basically a 68-71 340 passenger side manifold flipped upside down and water jacketed. It has longer internal runners than the log manifolds which helps keep the exhaust velocity higher through the manifold. They are not as good as a tuned header but they are much better than the logs. Less gaskets to leak and no distribution tubes to corrode either. They also introduce the water into the exhaust further away from the valve lessening the chance of reversion if you run a cam with more duration.
 
Granted, but a manifold/ riser leak puts water in 2 or 3 cylinders; a log leak only 'waters' the end cylinder.

Jeff
 
Granted, but a manifold/ riser leak puts water in 2 or 3 cylinders; a log leak only 'waters' the end cylinder.

Jeff

That is a moot point. At the point of getting water in your combustion chamber it does not matter wether it's 1 or all 8 cylinders.
 
It does to the cylinders affected! My point is it's better to fight water intrusion in the rear cylinders other than 2 or 3 at a time on one bank.

To wit: My old Mopar 360 had an irritating, occasional leak on one bank. As is my practice (one that every inboarder should follow) I always wheel the motor over BEFORE trying to start it. If I hear RRRRRR, fine, but if I hear RR--hesitate--RRR that's not fine! This motor did that a bunch of times (before I fixed the problem). Since it had a log manifold, it was obvious where the leak was, and it wasn't hard to pull a plug and blow the water out. The motor then started up safely and ran fine (for years afterwards). With center dumps, however, I would have to pull a bunch of plugs before isolating the wet hole.

Jeff
 
Another added bonus of the center rise manifolds is the fact that the riser it taller, helping cure the intrusion problem in the first place, along with only 2 gaskets to deal with(1 if you can find a set of original Chrysler risers eliminating the adapter plate.) Plus on my sterndrive application with thru transom exhaust, it was originally rigged with the risers to the front of the motor / boat, down and thru another 90 degree elbow, than a 40" hose back to the transom and exiting right at the water line. This compromised access to the starters and oil filters, not to mention the risers up front cluttering the bilge. With the center risers I will have a short ~10" rubber hose off the riser going to a thru transom tip that will now be ~6" above the waterline. I always cursed the log manifolds on my old boat with 4 gaskets that would sometimes seep.

 
Where do you get "four gaskets"? I only count three.

Jeff

PS: How on earth do you get to those inside plugs! Cripes, w! What a tight installation.
 
Where do you get "four gaskets"? I only count three.

Jeff

PS: How on earth do you get to those inside plugs! Cripes, w! What a tight installation.

1. End plate at rear of manifold
2. Between manifold and riser
3. Between riser and 90 degree elbow
4. Between 1st 90 degree elbow and second 90 degree elbow

plugs were a major PITA considering there were also the 2 exhaust hoses routed under the manifolds. And the fact that there was a fixed bulkhead at the front of the bulge, I cut the front bulkhead out, making it removeable for service access. The outers were barely any easier. Don't even ask about the oil filters or starters. Re rigging with the center rise manifolds and remote mounted oil filters, and crank driven raw water pumps will clear a lot of space in the bilge and make service much easier, even with the addition of closed cooling systems.
 
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"I feel your pain!" (Bill Clinton)

Sure takes dedication to be a marine mechanic when the boat builders do things like this. I, personally, will NEVER again buy a boat that requires a contortionist to work on--screw that!

Jeff
 
If I may............

If you are rebuilding the engine, you should consider the Barr Marine center rise manifolds, CHrysler used them starting around 1985, and from what I gather they flow much better than the log manifolds, to the tune of 15 HP, combined with a Edelbrock performer RPM air gap intake, little port work and a small bump in the cam You could see an easy 50-60 HP increase over stock, maybe more if you bump the compression and create a quench with the right piston domes.

My hat is off to you. Not many understand the importance of this, and in particular for the Marine Engine.
The Chrysler 318, 340 and 360 are prime candidates for a Quench Effect combustion chamber.
Build a Q/E into the engine, and you'll be able to increase ignition lead, which will better place the LPCP, and create greater torque.


It is if you are upgrading the cam and cylinder heads. The center rise manifold is basically a 68-71 340 passenger side manifold flipped upside down and water jacketed. It has longer internal runners than the log manifolds which helps keep the exhaust velocity higher through the manifold. They are not as good as a tuned header but they are much better than the logs. Less gaskets to leak and no distribution tubes to corrode either. They also introduce the water into the exhaust further away from the valve lessening the chance of reversion if you run a cam with more duration.

Good point!

Granted, but a manifold/ riser leak puts water in 2 or 3 cylinders; a log leak only 'waters' the end cylinder.

Jeff
That is a moot point. At the point of getting water in your combustion chamber it does not matter wether it's 1 or all 8 cylinders.

I would agree. An owner must be vigilant with regard to monitoring the exhaust system.
In other words.... pull them down periodically and inspect them........., and don't wait to replace them until after noticing a leak.
 
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