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

That was in reference to my POS cell phone.  Sometimes the square indicating where a picture is loading doesn't show up on my phone and I didn't see that you already posted a pic before I stuck my foot in my mouth.

What you are doing sounds really cool and I would love to see more pics.  Variety is the spice of life and I still welcome you and what you are doing.

I'm sorry I don't see the kind of gains from the exhaust that you are looking for.  I will look for the website with info on porting exhaust manifolds when I can jump on my PC.

Cheers,

Joe



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

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