350 5.7l Mercruiser prop question.

[donekberg]

I have a 19' spectrum, weight is about 2400-2600 lbs aluminum hull. I have 1.47 gears in the upper gear set on the lower unit. Test drove it yesterday and capped at 4400 rpms with a 23 pitch SS turbo prop. The motor is cammed for 5500 rmp, with a edelbrock performer intake, 600 cfm 4 barrel carb. I was going to have camel back heads, but decided to go with a step or two down so it could run off any gas I could put it in, due to the camel back heads having to much compression. I will be propping down to 17-19 pitch prop today. Will that give me the rmps that I am looking for? Horsepower is unknown due to no dyno testing being done, but I am assuming 300-340 hp. Any input is greatly appreciated.

 

[steelespike]

Re: 350 5.7l Mercruiser prop question.

What is the speed at 4400? I think I would like to varify the tach before pushing a marine engine over 5000 routinely.But I'm pretty conservitive.
I guess I just picture an auto engine turning 5500 rpm for only seconds at a time and they come apart fairly regularly.

 

[Icyharp]

Re: 350 5.7l Mercruiser prop question.

Ya I agree as above, just cause things say you can run a small block to 5500 does not mean you can for long periods of time. Even shorter time if your bottom end and valve train are stock.
I have a 5.7 in my boat as well with the same gear ratio as you and I have a 21p SS prop on and I can roll the tach too 4600 right where it should be. I do not stay there long as it sucks a tad bit of fuel and speed is 60mph. I aways run non-oxy fuel 91 oct so at 3.89 a gallon I like to run on plane more in the 3k range, if you do reach your goals then what drive do you have on? (BTW my engine is bone stock)

 

[donekberg]

Re: 350 5.7l Mercruiser prop question.

Within the last few days we have done more testing. It was weird that the motor stopped at 4400 rpms with the 23 pitch ss prop, come to find out we spun the teeth right off the timing gear. After tearing the motor back down to get all the teeth out of it we ran it again. Mind you its not a fiberglass v hull, its aluminum. We have a alpha one gen 2 outdrive on it. The stern is squatting down in the water so much with the added motor weight we are going to be putting trim tabs on it to try and raise it up. The reason I say this is because with the 4.3l vortec in it only the back 2-4 feet of the boat was hitting the water capping the speed at 48-50 mph at 4800-4900 rpms. Have any of you had experience with trim tabs lifting the stern out of the water more allowing the trim to be more affective at raising the bow? Thanks

Edit: Only thing stock about the motor is the pistons and crank.