Heat risers or no heat risers, that is the question ...

I'm replacing the 318 in my Chrysler/Penta 270 with a 360 engine. We're working through complex flywheel/bell housing issues in another thread but something else of interest I'd like to throw out. The exhaust tube system for the newer 1977 360 (of uncertain outdrive) mates to my 270 outdrive which is nice but there are some minor clearance issues inside the hull since the 360 exhaust has heat risers and the older 1974 318 did not. How important are the heat risers? Also, both exhaust ports that bolt to transom shield have the additional small port out to the bent tube that vents out the side of the shield. The 318 exhaust appears to simply send some of the joined exhaust from both manifolds out this smaller port but the 360 exhaust port has the smaller port sealed off inside the hull. Totally dysfunctional. Is that a problem? I really can't understand the function of the smaller port in the 318. Perhaps it had something to do with the old style exhaust backflow flap? I'm baffled.

Here's some photos of the exhaust swap.
First, old 318 exhaust from manifolds to transom shield, engine side


Same thing from transom side


And here's the 360's exhaust system (formerly mated to unknown outdrive) mounted to my old 318's 270 Chrysler transom plate.


As you can see, I will need to relocate the steering tube to clear the left side heat riser. Also cut a bit of wood out of panel on right side to clear that riser and rotate the electric motor for engine lift out of the way. Neither is a problem. The lift's motor mount has several sets of holes so it can be reoriented easily in many positions. The engine box is large enough to accommodate cutting back enough of the panel.

This image shows the plug in auxiliary exhaust port for 360 exhaust system.

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PR-Chapin said:

I'm replacing the 318 in my Chrysler/Penta 270 with a 360 engine. We're working through complex flywheel/bell housing issues in another thread
Flywheel Cover .... (bell housing is an automotive term).

but something else of interest I'd like to throw out. The exhaust tube system for the newer 1977 360 (of uncertain outdrive) mates to my 270 outdrive which is nice but there are some minor clearance issues inside the hull since the 360 exhaust has heat risers and the older 1974 318 did not. How important are the heat risers?
What is a "heat riser"?

Also, both exhaust ports that bolt to transom shield have the additional small port out to the bent tube that vents out the side of the shield. The 318 exhaust appears to simply send some of the joined exhaust from both manifolds out this smaller port but the 360 exhaust port has the smaller port sealed off inside the hull. Totally dysfunctional. Is that a problem?
Again, without photos my hands are rather tied.

I believe that you may be referring to "exhaust relief" ports. The 270 transom shield has only one relief port.
If so, this is there to allow low engine speed exhaust gases to escape up near the water line, instead of being forced to exit down low where more "back-pressure" is created.

I really can't understand the function of the smaller port in the 318. Perhaps it had something to do with the old style exhaust backflow flap? I'm baffled.
See above.


And by the way........ it will serve the 360 much better if you had at least a 280 transom shield with double exhaust reliefs, and the larger 280 style 95mm Y-pipe.

The components that I see in your photos were used for the small V-8s .

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Here's a a typical heat riser with elbow setup.


I posted photos of the exhaust setup with sealed off auxiliary port which you indicate is probably for back pressure relief when the boat is sitting idling. Makes sense. I see at the end of each manifold there are large ports sealed up with plugs and presumably I can simply pick up two threaded hose attachments, hi-temp hose, and clamps to connect the left side manifold to the sealed off back pressure relief port at the exhaust fitting connected to inside of transom shield (see my second response with photo of it circled) . Good to know what that plugged port was used for. I wonder why it was sealed off? The solid Y-tube exhaust, supposedly from a 280 outdrive, is rather narrow diameter but for my purposes I don't think it will be a big issue. This is a fishing/camping cruiser, not a speed boat. I'm also thinking the back pressure relief while idling issue might have been more important for maintaining idle of lower hp 318. Maybe not so much for the larger engine? If this Y-tube doesn't work out I can simply switch back to the old setup with longer exhaust hoses to meet risers (and some 45 degree solid exhaust joints rather than bending the hose). The probability of finding a larger 280 Y-tube exhaust is rather remote in this remote backwater.

Again, what is the purpose of the risers?

PR-Chapin said:

This image shows the plug in auxiliary exhaust port for 360 exhaust system.
View attachment 18596

Click to expand...

Looks like a the 300 exhaust but mine doesn't have the auxiliary port ur showing or that small port out the center back...???

I assumed riser's were for motors at or below the water line...???

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PR-Chapin said:

Here's a a typical heat riser with elbow setup.
View attachment 18598


The part that is being shown between the exhaust manifold and the elbow is a spacer or riser.
Not all exhaust systems use a spacer/riser.

I posted photos of the exhaust setup with sealed off auxiliary port which you indicate is probably for back pressure relief when the boat is sitting idling. Makes sense.
Yes, that would be a low pressure exhaust relief port..... (not an auxiliary port).

I see at the end of each manifold there are large ports sealed up with plugs and presumably I can simply pick up two threaded hose attachments, hi-temp hose, and clamps to connect the left side manifold to the sealed off back pressure relief port at the exhaust fitting connected to inside of transom shield (see my second response with photo of it circled) .
Actually, the original elbows (had they been Volvo Penta) would have been ported from directly within the exhaust gas chamber...... not from within the seawater side.
Volvo Penta did this for the GM 5.0L V-8s with the AQ200B and the AQ225B.
Chrysler apparently followed suit using their own components, or possibly Eaton components.


Good to know what that plugged port was used for. I wonder why it was sealed off? The solid Y-tube exhaust, supposedly from a 280 outdrive, is rather narrow diameter but for my purposes I don't think it will be a big issue.
The 3" Y-pipe was used with the smaller 335 series Fords and the smaller GM v-8s.
Again, Chrysler apparently adapted the use of this part for their 318.
Typically the 360 would have been equipped with a larger diameter Y-pipe.

This is a fishing/camping cruiser, not a speed boat. I'm also thinking the back pressure relief while idling issue might have been more important for maintaining idle of lower hp 318. Maybe not so much for the larger engine?
Not necessarily. It was used as to prevent engine fuel-load-up while at trolling speeds and/or while docking, etc.

If this Y-tube doesn't work out I can simply switch back to the old setup with longer exhaust hoses to meet risers (and some 45 degree solid exhaust joints rather than bending the hose). The probability of finding a larger 280 Y-tube exhaust is rather remote in this remote backwater.
Yes, that may be hard to find. Even if you were to find a double relief 95mm Y-pipe, it will be specific to the double-exhaust-relief 280 transom shield. If you were to find a good used 280 double relief transom shield, they tend to fetch around $800 or so.
Your transom shield offers a single relief only, which means that you are limited to the smaller single relief Y-pipe.

Again, what is the purpose of the risers?
Risers or Spacers are used to separate and raise the Elbow up from the manifold's mating surface.
This is done in scenarios whereby the water-line is too close to the Elbow's interior "turn-down-bend". Deep V hulls whereby the engine sits low in the hull.
The idea is to get the Elbow elevated as to prevent a wave-surge from forcing water into a cylinder at low speed.



Somewhere I have an image that I put together that will show how the low pressure exhaust relief is plumbed into the Y-pipe's relief port.
If I can find it, I will post it.


Another option for you would be to go with Thru-Transom exhaust.

Click to expand...

300hb said:

I assumed riser's were for Engines at or below the water line...???

Click to expand...

Most all Marine engines sit below the water line. It's the elevation of the Elbow's bend that we are concerned with.



.

I found my old image.

As mentioned, the exhaust relief porting is to prevent fuel loading due to the low RPM back-pressure of the exhaust exiting from below the water line. The relief porting dumps (relieves) some of the exhaust gas pressure at a higher elevation, yet still below the water's surface.

Each I/O system does this in a similar fashion.... but I must admit..... the AQ200B and AQ225B system with that single stand-tube, is a bit odd!

I don't know for certain what Chrysler did back then.
When you look at this image, understand that this is Volvo Penta for the GM 5.0L V-8 AQ200B and AQ225B.
Eaton (re-labeled Volvo Penta) did similar with the old 335 series 5.0L Ford V-8s (i.e., the 302 Ford W).

If your exhaust elbows do not offer the relief ports, and if you have space for additional exhaust hose fittings (straight or elbows), you can weld a TP fitting to each part that will serve as the exhaust gas reliefs that will connect to the Y-pipe relief tube.

Thanks for clearing that up Rick. My idea wouldn't have done anything to relieve exhaust pressure. I'm thinking that making that Y-tube work is just too much hassle. Given what I have for manifolds and risers, I would have to add welded fittings and frankly I'd always be concerned the welds would fail. Too much vibration. This Y-tube is probably too narrow anyway. I'll use the old 318 hose system with the enclosed exhaust relief built into the Y-fitting that bolts to transom plate, but I'll add two 45 degree elbows to relieve the current stress on the bends in hose. Should have done that years ago. I seem to recall West Marine sold some fancy hi-temp plastic exhaust fittings. Do those hold up?

The 360's manifold to U-risers: (the old hose exhaust system for 318 is laying on floor behind it)


So, though the 318 only ever had 90 degree elbow risers instead of U-risers and seemed to do fine with it, should I still keep the inverted U risers that are on the 360? As you can see from transom photo, the hull is quite deep but I suppose a large swell coming at the transom could possibly back up into a cylinder without the U-risers? Surprised this system was put together at factory without U-risers. Or maybe it's not necessary?

Thanks again for letting me pick your brain. You are a patient man to deal with such profound lack of knowledge.

PR-Chapin said:

I seem to recall West Marine sold some fancy hi-temp plastic exhaust fittings. Do those hold up?

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Centek exhaust fittings. I have them. They work fine.

The 360's manifold to U-risers: (the old hose exhaust system for 318 is laying on floor behind it)
View attachment 18608

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So those stupid risers. The bolt in the pocket to the manifold, a normal crescent wrench does NOT fit. I just bought a pivoting socket in hopes it'll clear.

Try a box end wrench and maybe grind down its face just enough so it fits inside in the pocket over the bolt. That should work.

Thanks, pivot socket fits, Barr risers actually has enough room for a box wrench to fit.

TPGG said:

Thanks, pivot socket fits, Barr risers actually has enough room for a box wrench to fit.

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"special" tools are used for those tight spots on headers as well...…….

I was able to get at my manifold bolts on one side with crescent wrenches and sockets on extensions. The other side has a long metal tube running from the elbow which seems useless and dumb blocking 3 of the bolts. I may cut it off and see if i can get the manifold off through deck access then just do the barb at the elbow to replace it.