First off, welcome to the forum

Second...  Well...  typically, an engine would not be dynoed with log manifolds.  In my dyno experience, typically a set of dyno headers would be used.  Sometimes, if they would fit, the customer’s exhaust was tested.  Regarding the amount of horsepower of said SBC, a lot would depend on the tests and how they were run.  I don’t think that it is possible to run logs during a dyno test, at least not the aluminum logs.  If said SBC made 400HP with dyno headers, log manifolds would yield substantially less horsepower.  I have heard claims regarding the flow capabilities of the center riser manifolds and in looking at their design, I agree that they are an improvement over logs.  I just can not believe that they are on par with a good set of headers.

My thoughts on this are:  They do not have smooth contoured bends, especially at the all important departure angle at the head flange, where the exhaust transitions from the port into the primary tube/port (in the case of manifolds).  The cross-sectional shape is inconsistent and allows for gaseous expansion at various places.  This causes the gasses to lose velocity in the areas of expansion and causes pumping losses in areas where the gasses are forced to increase velocity.  I don’t see much intentional tuning with the cross-sectional area.  The lengths of the ports are short and are combined with a collector area that is less than ideal.  Lastly, the inner surface is rough and increases surface friction and therefore restricts flow.

In certain automotive racing classes requiring exhaust manifolds, significant gains can be made by porting the manifolds.  The majority of work is concentrated in the flange area so that no part of the manifold covers the port in the head.  Actually, an intentional mismatch where the port in the manifold is larger than the port in the head helps to prevent reversion and intake charge contamination.  Unfortunately for us as boaters, the largest gains with manifolds occur with the merge collectors of the exhaust pipes merging into a single exhaust.  This type of system will provide the best scavenging with a manifold; however I do not see a way to incorporate this type of system into a boat.  I suppose that you could do an OT setup, but in my opinion, a good header setup will make more power.

Sorry for the long winded reply, in short, I don’t think that you would feel a difference, there might however, be a measurable difference.  You will also have to re-jet your carb to realize any difference at all.

Good luck,

Joe

I had my BBF built last year and have thru bolt logs... My pump has some mods as well. I have a heavy hull but still picked up speed an performance with the logs.

First off, welcome to the forum

Second...  Well...  typically, an engine would not be dynoed with log manifolds.  In my dyno experience, typically a set of dyno headers would be used.  Sometimes, if they would fit, the customer’s exhaust was tested.  Regarding the amount of horsepower of said SBC, a lot would depend on the tests and how they were run.  I don’t think that it is possible to run logs during a dyno test, at least not the aluminum logs.  If said SBC made 400HP with dyno headers, log manifolds would yield substantially less horsepower.  I have heard claims regarding the flow capabilities of the center riser manifolds and in looking at their design, I agree that they are an improvement over logs.  I just can not believe that they are on par with a good set of headers.

My thoughts on this are:  They do not have smooth contoured bends, especially at the all important departure angle at the head flange, where the exhaust transitions from the port into the primary tube/port (in the case of manifolds).  The cross-sectional shape is inconsistent and allows for gaseous expansion at various places.  This causes the gasses to lose velocity in the areas of expansion and causes pumping losses in areas where the gasses are forced to increase velocity.  I don’t see much intentional tuning with the cross-sectional area.  The lengths of the ports are short and are combined with a collector area that is less than ideal.  Lastly, the inner surface is rough and increases surface friction and therefore restricts flow.

In certain automotive racing classes requiring exhaust manifolds, significant gains can be made by porting the manifolds.  The majority of work is concentrated in the flange area so that no part of the manifold covers the port in the head.  Actually, an intentional mismatch where the port in the manifold is larger than the port in the head helps to prevent reversion and intake charge contamination.  Unfortunately for us as boaters, the largest gains with manifolds occur with the merge collectors of the exhaust pipes merging into a single exhaust.  This type of system will provide the best scavenging with a manifold; however I do not see a way to incorporate this type of system into a boat.  I suppose that you could do an OT setup, but in my opinion, a good header setup will make more power.

Sorry for the long winded reply, in short, I don’t think that you would feel a difference, there might however, be a measurable difference.  You will also have to re-jet your carb to realize any difference at all.

Good luck,

Joe

Thanks for the info.  I just used 400hp as a bench mark as to whether you would gain "X" amount of hp with the center riser and then the header over the LOGS as that is our benchmark as thats what 99% of our boats use.  I realize set up and hull type make a difference but was just looking for the basic benefit of the move up in exhaust systems as I always wondered if the water in a wet exhaust negates some or all of the gain by using water jacketed headers.  I have ideas about it BUT that's just bench racing and just an opinion...I know somebody had to do the dyno work as I never believe the claims of the manufacturers.  A pic of my boat so you guys have an idea.

Figures, now the pictures

Sent from my SCH-I535 using SoCal Jet Boats mobile app

Thanks for the info.  I just used 400hp as a bench mark as to whether you would gain "X" amount of hp with the center riser and then the header over the LOGS as that is our benchmark as thats what 99% of our boats use.  I realize set up and hull type make a difference but was just looking for the basic benefit of the move up in exhaust systems as I always wondered if the water in a wet exhaust negates some or all of the gain by using water jacketed headers.  I have ideas about it BUT that's just bench racing and just an opinion...I know somebody had to do the dyno work as I never believe the claims of the manufacturers.  A pic of my boat so you guys have an idea.

That thing is sweet!!!!!!!


Sent with my Fingers

So guess my question is unwelcome since I do not own a Jetboat?  If so it wasn't my intention to offend-if it pleasure you ban my account...was just looking for some info. that I found here which I appreciate.   

i dont own a jetboat either.     i have center rise exhaust.   obviously nothing will beat a set of tuned headers... but then again no one wants to deal with water injected or dry headers these days let alone trying to get them insured. 

a set of lightning water jacketed headers are gonna be upwards of $2500 new... imco power flow center rise you're lookin at only $1800 for a sbc.  performance is gonna be better with the headers since they're seperate tubes all the way to the collector, whereas the center rise is just to the riser.  but is it worth the extra $700 for realistically 10-20hp?

i dont own a jetboat either.     i have center rise exhaust.   obviously nothing will beat a set of tuned headers... but then again no one wants to deal with water injected or dry headers these days let alone trying to get them insured. 

a set of lightning water jacketed headers are gonna be upwards of $2500 new... imco power flow center rise you're lookin at only $1800 for a sbc.  performance is gonna be better with the headers since they're seperate tubes all the way to the collector, whereas the center rise is just to the riser.  but is it worth the extra $700 for realistically 10-20hp?

How much better HP/% gain will you see on a imco power flow center rise versus a Glenwood log manifold.  If I found either the center rise or the Jacketed headers I would buy used IF a set came up and it did something for the boat...I am contemplating redoing the boat and want to do something different in a Skiff-and Jacketed headers have only been used once or twice.  NOT ME OR MY BOAT but this is how we can turn.

getting an exact percentage is never gonna happen.  every motor is different.

That give me a fair idea...not to beat a dead horse BUT is a Glenwood log manifold BETTER than a stock Mercury center rise manifold?    I really appreciate the help in trying to get a fair grasp on the topic.

NOT ME OR MY BOAT but this is how we can turn.

That's why I love boat racing...  Because all other sports require only one ball