Frank Acampora
Supreme Mariner
- Joined
- Jan 19, 2007
- Messages
- 12,004
Well, after about two years, I finally got my dual exhaust lower unit together and onto my test boat/engine--the one in the avatar.
Here are the results: With the same boat and load, the same engine and same prop, the dual exhaust was slightly faster.
Boat: 15 foot Glastron deep vee V 153.
Engine: 1978 Chrysler 105, raised one inch on transom.
Prop: 13 X 19 Michigan stainless steel --cupped
Load: Driver and two full six gallon fuel tanks.
Speed: With single exhaust 2 to 1 ratio lower unit --- 43-45 MPH. RPM: 5,000
Speed: With dual exhaust 1.92 to 1 ratio lower unit --- 45-46 MPH. RPM: 5,200-5,400
So: We can say that the dual exhaust lower unit does appear to be anywhere from 200-400 RPM and 1 to 3 MPH faster than the single exhaust lower unit. However, this is under light load conditions. Heavier loads or water skiing might show different results.
While the dual exhaust was marginally faster, the hole shot was noticably slower and the dual exhaust did exhibit some bad habits on this hull. Notably, It ventilated and would not lock up if a hole shot or any acceleration was done with full trim. It needed to be tucked in to accelerate and be trimmed out only after the boat was on a stable plane. The single exhaust could do a hole shot with full trim.
The dual exhaust was not smooth in turns. It shuddered almost violently under hard turning until speed dropped down to less than 3,000 RPM. additionally, it ventilated badly when turned with full trim. The single exhaust was smooth no matter what speed it was turned at and did not ventilate.
Dual exhaust would ventilate when going over sizable wakes.
Quite frankly--no pun intended--while I was not hopeful for any speed increase, I did not anticipate ventilation problems because both lower units sit at the same height on the transom. The only real difference in these two units externally is the position of water pick-ups and skeg shape. I can only postulate (because of the shuddering in turns) that the water pick-ups on the leg or the skeg cause turbulence that fouls the prop blades.
Would I go out of my way to install a dual exhaust lower unit on a single exhaust engine? Not unless every single MPH was important to me. The speed increase was more than offset by the poor handling.
However, remember that this test is a limited data test. You may get different results on your hull.
Here are the results: With the same boat and load, the same engine and same prop, the dual exhaust was slightly faster.
Boat: 15 foot Glastron deep vee V 153.
Engine: 1978 Chrysler 105, raised one inch on transom.
Prop: 13 X 19 Michigan stainless steel --cupped
Load: Driver and two full six gallon fuel tanks.
Speed: With single exhaust 2 to 1 ratio lower unit --- 43-45 MPH. RPM: 5,000
Speed: With dual exhaust 1.92 to 1 ratio lower unit --- 45-46 MPH. RPM: 5,200-5,400
So: We can say that the dual exhaust lower unit does appear to be anywhere from 200-400 RPM and 1 to 3 MPH faster than the single exhaust lower unit. However, this is under light load conditions. Heavier loads or water skiing might show different results.
While the dual exhaust was marginally faster, the hole shot was noticably slower and the dual exhaust did exhibit some bad habits on this hull. Notably, It ventilated and would not lock up if a hole shot or any acceleration was done with full trim. It needed to be tucked in to accelerate and be trimmed out only after the boat was on a stable plane. The single exhaust could do a hole shot with full trim.
The dual exhaust was not smooth in turns. It shuddered almost violently under hard turning until speed dropped down to less than 3,000 RPM. additionally, it ventilated badly when turned with full trim. The single exhaust was smooth no matter what speed it was turned at and did not ventilate.
Dual exhaust would ventilate when going over sizable wakes.
Quite frankly--no pun intended--while I was not hopeful for any speed increase, I did not anticipate ventilation problems because both lower units sit at the same height on the transom. The only real difference in these two units externally is the position of water pick-ups and skeg shape. I can only postulate (because of the shuddering in turns) that the water pick-ups on the leg or the skeg cause turbulence that fouls the prop blades.
Would I go out of my way to install a dual exhaust lower unit on a single exhaust engine? Not unless every single MPH was important to me. The speed increase was more than offset by the poor handling.
However, remember that this test is a limited data test. You may get different results on your hull.