New engines what propellers.Plus exhaust too loud.

downunder · Post by **downunder** » Wed Jul 29, 2009 6:21 am

I have just completed an engine refit on my 89 10 metre express.I replaced the old 454 crusaders with new 8.1s hooked up to 2:1 zf gears.Now I need some advice on propeller selection. I am using my original props for now but can only get 3500 rpm these props are18x22 3 bladers. The other problem I have is noise.Inow have 4 inch exhaust pipes connected to the original out the side outlets and they are too loud,has anyone encounted this and come up with a solution. Any advice would be appreciated. THANKS REX

RWS · Post by **RWS** » Wed Jul 29, 2009 6:52 am

On the prop issue, first thing before you change out your props id to make sure you are actually getting full throttle travel from your throttle cables.

On the exhaust sound issue, I can't think of any reason the new engines would be noiser at the exhaust. I don't think the difference in displacement is enough to make a difference.

When I was considering going to the 8.1 Crusaders, Iwas told that my factory raw water intakes were borderline too small for the engine. Is it possible that you have the same stuation and the engines are not getting enough raw water volume for these engines the result being a noticable difference in exhaust sound?

When we refitted to the diesels we added a econd layer of lead lined inslation in the engine room to the underside of the cockpit floor. The result is that the diesels are quieter than the 454's they replaced.

Is your increased sound level coming from the side exhausts, or from the engine room via the cockpit floor?

RWS

jav · Post by **jav** » Wed Jul 29, 2009 10:45 am

The propping bothers me.... aren't your new 8.1's more powerfull than the old 454's?? What was the reduction on the old 454's? Did you get to rated WOT on the old engines with those props?

As far as noise goes... did you change anything with the exhaust? If not, I'm not sure that the addidtional displacement would make a noticeable difference in sound. Do you have any inline mufflers or is it straight exhaust? How's water flow?

prowlersfish · Post by **prowlersfish** » Wed Jul 29, 2009 7:37 pm

Being over proped would cause both issues . what was the gear ratio before ??

are you sure your tachs are right ?

downunder · Post by **downunder** » Thu Jul 30, 2009 8:36 am

Getting full throtle.Old gears were 1.8:1.Could only get 3500rpm on old motors.New tachs fitted.Exhaust now 4 inch dia all the way old exhaust were 3 inch.Still using original 11/4 intakes.If you increase size of intakes do you have to replace strainers as well Rex.

gjrylands · Post by **gjrylands** » Thu Jul 30, 2009 9:06 am

downunder wrote:Getting full throtle.Old gears were 1.8:1.Could only get 3500rpm on old motors.New tachs fitted.Exhaust now 4 inch dia all the way old exhaust were 3 inch.Still using original 11/4 intakes.If you increase size of intakes do you have to replace strainers as well Rex.

I would think you would have to replace both the intake/seacock and the strainer.

jwrape · Post by **jwrape** » Thu Jul 30, 2009 10:25 am

The larger exhaust could be resonating more sound. It's like putting a conical tip on the back of your car tailpipe. It will applify the sound.

jav · Post by **jav** » Thu Jul 30, 2009 2:58 pm

I just ran a quick calc and presuming you have the 385 HP 8.1's and your boat weighs no more than 16K pounds loaded, AND your props aren't cupped- the 18x22's would be a good starting point.

BUT- if your heavier than that- OR if you have too much drag (dirty bottom or gear) OR of your running high cup- you could have some trouble getting over the hump.

I'd get the props checked and perhaps go down to a 20 pitch -light cup and see where you stand. Overloading those new engines is a sure way to shorten their life.

Water lifts would help with the noise.