Surprising results on Merc exhaust
09-15-2003, 03:00 PM
#1
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Surprising results on Merc exhaust
Engine is a 383 ci/350HP/Carbed with stock Merc thru prop exhaust, except that it has the exhaust tube (not bellow) that is only fixed at the transom end.
Prior to installing the rebuilt engine, I removed the Y-pipe so that I could drill and tap a fitting into the rectangular section immediately adjacent the transom assembly mounting. I used a 90 degree pipe thread-by-compression fitting at the y-pipe and ran about 4 feet of 3/8" copper tubing out of the bilge and into a gunwale. I put a 0-15psi fuel gage on the other end.
The purpose of all this was to get some "data" on how restrictive the stock Merc thru hub exhaust is with a 383 engine. Everybody claims an increase with thru hulls, so I wanted to see how much back pressure there was with the stock system. So, what's the bottom line? 0 psi at all times, from idle up thru 4600!!! The needle never moves. When I remove the gage with the engine running, I get a spitting mixture of water and exhaust gas, but you can seal it with your finger. There's not enough pressure to move the gage or my finger.
Am I too far remeved from the source with the 4 foot of 3/8 tubing? Or, is this an accurate reading of the pressure in the exhaust system? If accurate, then why do thru hulls report an increase? Reflected waves, etc...?
I had expected to see 3-5psi of back pressure at WOT. Thoughts/comments
Prior to installing the rebuilt engine, I removed the Y-pipe so that I could drill and tap a fitting into the rectangular section immediately adjacent the transom assembly mounting. I used a 90 degree pipe thread-by-compression fitting at the y-pipe and ran about 4 feet of 3/8" copper tubing out of the bilge and into a gunwale. I put a 0-15psi fuel gage on the other end.
The purpose of all this was to get some "data" on how restrictive the stock Merc thru hub exhaust is with a 383 engine. Everybody claims an increase with thru hulls, so I wanted to see how much back pressure there was with the stock system. So, what's the bottom line? 0 psi at all times, from idle up thru 4600!!! The needle never moves. When I remove the gage with the engine running, I get a spitting mixture of water and exhaust gas, but you can seal it with your finger. There's not enough pressure to move the gage or my finger.
Am I too far remeved from the source with the 4 foot of 3/8 tubing? Or, is this an accurate reading of the pressure in the exhaust system? If accurate, then why do thru hulls report an increase? Reflected waves, etc...?
I had expected to see 3-5psi of back pressure at WOT. Thoughts/comments
09-15-2003, 05:48 PM
#2
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I agree with your findings! I have tested a few small block equipped boats with silent choice and found no performance increases with the exhaust running through the transom. Mercruiser has always stated that the prop thrust pulls the exhaust out of the engine.
I have always wanted to do that test but never got around to it. I believe that the Y pipe acts like a 180 degree header collector and smooths out the exhaust pulses. The collector temporarily stores (collects) the exhaust gases, this allows the gases to exit in an orderly manner, reducing the requirement for a large exhaust pipe in the transom plate.
Theoretically, if the exhaust gases are totally free to flow out of the boat (no restrictive muffler), the final exhaust pipe diameter needs to be only as large as an individual exhaust port. Only one cylinder fires at a time.
Cool test!
Dennis Moore
I have always wanted to do that test but never got around to it. I believe that the Y pipe acts like a 180 degree header collector and smooths out the exhaust pulses. The collector temporarily stores (collects) the exhaust gases, this allows the gases to exit in an orderly manner, reducing the requirement for a large exhaust pipe in the transom plate.
Theoretically, if the exhaust gases are totally free to flow out of the boat (no restrictive muffler), the final exhaust pipe diameter needs to be only as large as an individual exhaust port. Only one cylinder fires at a time.
Cool test!
Dennis Moore
09-15-2003, 06:34 PM
#3
Toxic FORMULA
Platinum Member
That sound good...BUT, I had a single 540 in my last boat that tended to blow out exhuast gaskets at the head when kicked in the a$$ with the Corsa Captains Call closed
Needless to say , I made sure it was open before mashing on it after the second time
After all , 540 is a lot more than 383
Needless to say , I made sure it was open before mashing on it after the second time
After all , 540 is a lot more than 383
09-15-2003, 06:59 PM
#5
Charter Member #655
Charter Member
I found the same with my stock 330's no differance in speeed or rpm's...
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09-15-2003, 08:32 PM
#7
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A poster on another forum said I need to use an inches of water or mercury gage to read it. I will give that a try some weekend. Who knows, I may record a vacuum at 5200 rpm!!
So, a stock Merc Y-pipe exhaust on a SBC is less restrictive than a steet legal car (1-3 psi). Who woulda thunk!
So, a stock Merc Y-pipe exhaust on a SBC is less restrictive than a steet legal car (1-3 psi). Who woulda thunk!
09-15-2003, 08:46 PM
#8
The pressure guage should be at the exhaust riser not at the exit point. The closer you get to the exit point of the exhaust system the lower the pressure will be. The highest pressure will be at the exhaust port by the head.
If you really want to see some change cut back on the amount of water that is injected into the exhaust.
If you really want to see some change cut back on the amount of water that is injected into the exhaust.
09-16-2003, 08:30 AM
#9
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Measuring the pressure in the riser would be easy enough to do, although I disagree with your conclusion. I'll give it a try and let you know.
Interesting point about the amount of water. I have a open (RWC) system with the 4 hose thermostat housing (ie no hoses to the risers). Merc's service bulletin says to run the 4 slot riser gasket (which I am), but I have always wanted to run the 2slot/2pin hole gasket. That would keep more pressure in the block and reduce the water in the exhaust. What do you think?
Interesting point about the amount of water. I have a open (RWC) system with the 4 hose thermostat housing (ie no hoses to the risers). Merc's service bulletin says to run the 4 slot riser gasket (which I am), but I have always wanted to run the 2slot/2pin hole gasket. That would keep more pressure in the block and reduce the water in the exhaust. What do you think?
Last edited by 220BR; 09-16-2003 at 08:37 AM.
09-16-2003, 01:28 PM
#10
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Interesting topic. It hits close to home for me. I have a '81 260/350 that I believe is stock internally. It has an aluminum Edlebrock performer intake and a holley 650 carb. Sorry I don't know the exact number, they where on the boat.
I currently have trough prop exhaust although it doesn't have a bellow as described by 220BR. I've been toying with the idea of putting in though hull. I'm rally just looking for sound more than a performance gain. But hay if it happens great. The problem is I have the old style exhaust manifolds that have the riser on the end instead of the middle. This makes it difficult to install trough hull, and impossible if I wanted to run switchable. I've been thinking of swapping to '82 and up manifolds or maybe even a pair of Dennis Moores(wink wink). Now that I know that there won't be a performance gain from the through hull. Will there be with a manifold swap?
Brian
I currently have trough prop exhaust although it doesn't have a bellow as described by 220BR. I've been toying with the idea of putting in though hull. I'm rally just looking for sound more than a performance gain. But hay if it happens great. The problem is I have the old style exhaust manifolds that have the riser on the end instead of the middle. This makes it difficult to install trough hull, and impossible if I wanted to run switchable. I've been thinking of swapping to '82 and up manifolds or maybe even a pair of Dennis Moores(wink wink). Now that I know that there won't be a performance gain from the through hull. Will there be with a manifold swap?
Brian
