Bob has put up and done some good solid exhaust upgrade testing results from his propshaft dyno with the in boat water in exhaust testing. I do not know all the testing configurations he used but as a note I have noticed that in the pictures of some of the tests that the exhaust configuration shows the headers routed down thru the double fiarly short radius bends of S-Tubes.
At Raylar on some of our tests and Raylar HP600 builds we were getting some exhaust water back in the engine when we ran the smae S-tubes in certain hulls with integrated swimplatform transoms. We created the same configuration on the dyno and when we ran wet we saw over 20HP decreases in power from just the use of this short radius S-tube configuration and we even encountered some exhaust water back up in to the engine on the bigger camshaft duration of the HO600 camshaft profile. We removed the S-Tube and immediatly the horsepower returned and the water intrusion stopped! ON the boat tested and used we had to consturct dry water cooled stainless S-tubes and exit the water into the exhaust stream at the bottom of the S-tube facing out the back of the boat. I have also seen this same type of dry exhaust S-tube construction on some Mercry Racing 700sci -1075sci engine installations in some integrated transom hulls at DCB and Eliminator as though possibly Mercury Racing or the builders discovered the same conditons?
Our findings and belief is that the exhaust and water is blasted straight back out of the header, hits the vertical wall top of the top down elbow and actually some of which is thrown back in the opposite direction during pulse periods of the exhaust during camshaft overlap periods. This increased turbulance and redirection I believe is also causing a moderate exhaust flow resistance that was robbing power and creating some un-intended reversion. I am not sure if this condition was occuring in Bob's tests on the Nordic but I thought it was an interesting condition we discovered.

Best Regards,
Ray @ Raylar

Ray, if you reread the article you will see that we tested with straight pipes also. I really felt the S pipes were costing us some HP. So we pulled the complete exhaust system out of the boat and ran it on the dyno exactly like it was in the boat. Then we made another pull with pipes running straight out (still fully wet). There was absolutely no difference. Maybe on higher hp engines you would see some difference. We try to set the exhaust up on the dyno as close to what is in the boat as possible. But your thoughts just further illustrate the point that every installation is different and may very well explain why some people have great results by adding headers and others claim no gains.

Bob Lloyd
Full Throttle Marine

26sonic, correct. It is a 454.

thanks guys for the info !! decided the exhaust alone and cost is a waste of money!! eventually if i upgrade will go with heads came etc. to make a real difference!! for now bought a set of merc racing labbed 28p to try.