Reply With QuoteDo you have dyno numbers to prove you are making more power?
There are so many variables in a boat at top end. Too many variables to compare and say 'why.'
I have a 84 Chris craft 186 scorpion with a 305 chevy (228) merc. The boat is 18.6" lg X 7' wide. I have done some motor work and should be in the 280-290hp range. From what I was toild. Boat runs 51 GPS at about 4800. I'm currently running a 17p Mirage plus. I'd like to go to a 19 but I'll only get a couple of MPH and not sure its worth it.
My very good friend has a 79 chap. Same motor (228 merc) all he has done is change the cam in his. Same ration in O/D's 1.5. But he has a 19p Stilleto.
His boat is 19' X 8' He runs 56 GPS.
I'm a little lean on mine not much though. I still need to play with the timing some on the water with the GPS. Right now mine is at 35 total.
Heres the big question. How come he is so much faster, I'm making more ponies. And the biggest thing is I'm about 1200# lighter than him
Do you have dyno numbers to prove you are making more power?
There are so many variables in a boat at top end. Too many variables to compare and say 'why.'
I would try that higher pitched prop. You may really be suprised at the difference. As long as you are not having any problems getting on plane that higher pitch prop may also give you better cruising ecomomy.
Darrell.
Your RPM is exactly where you want it to be more pitch would knock you out of the range and poosibly cause problems.
I'd dig for some power first, then adjust the prop as required.
I'd dig out your receits and see what parts they told you were put in. There are no shortage of "builders" that really don't have a grasp of combinations that work. Was the cam checked/degreed when installed. I personally chk mine they come up true and I install them straight up. But I have seen the results of HP cams that were retarded 6 degrees by the timing chain set. A dawg!
Take all the information that you have to a reputable performance boat shop, share your information, concerns and need for more juice, and let them do a health check and tune it on a dyno. Timing, advance curves, fuel/air, etc.
Not cheap but about impossible for us to do on the water.
The other thing is what does your buddy know about his boat? as a 79 there is a good chance his engine has had some major work. I personally would never invest in a major repair on a 305 I'd find a 350, as many do. On the outside they look the same and with a small cam would account for the difference easily.
Have fun.
Thanks for all the responses. As far as what was done to mine, I did it all myself. It was a stock 305 rated at 228 hp. I had to pull the motor due to a water leak in the "y" pipe. While it was out I:
1. installed T/T exhaust.
2. removed heads and cleaned up the bowls, matched the gaskets, and had a valve job done.
(The man that did the valve job, thats all he does. I have alot of faith in him. I know of several engine builders and they all say he's the "best" when it comes to performance heads.)
3. Shaved the heads to raise the comporssion to app. 9.75 to 1
4. Installed a new cam. Greg, the "head" guy picked out a cam for me. It has a torque curve from idle to 5500. His words were " I've used these cams alot. With the lobe profile this thing will be real torquee, sp?"
5. Roller rockers. 1.6 ratio
6. New fuel pump
7. Plugs, cap, rotor, wire, coil.
8. Compression is 150# on all eight =/- 1#
My boat is like a rocket, It's a blast to run. Pops right up on plane and is very very quick. I haven't timed it but from say 20 to 45 is just a matter of seconds. A friend of ours has a Wake setter ski boat, we all know how quick those things are. I can leave him out of hole. Thats one reason I'm wanting to go more prop. I can stand to lose some bottom end.
As far as my buddies boat goes. Its a one family/owner boat. His father in law bought it new in 79. They bought it from her Mom in 90 after her fathr passed away. He replaced the timing chain about 5 years ago and while he was in there replaced the cam. The cam he chose was a 280 and he advanced it 4 degrees to bring the torque range down to around 1000. But, where he messed up is with the cam advanced that much it now acts like a rev limiter, 4100 is all it will turn.
I know there are alot of variables. I've built a few fast cars and I know the water is different. But what stumps me is we both have the same type of boat. From the best we can tell without getting to carried away our deadrise is the same. Mine is a foot narrower and by the book 1200 lighter.
No ideas?????????????????????????????????
Although it does sound like you are in the right speed/rpm range. Most boat shops have demo props that you can borrow with a credit card. That's the beauty of an I/o over a V-drive. They are usually painted blue. Easy enough to change at the lake also. No harm in playing and it's not the wrong prop will ruin a day at the lake.
I agree. Honestly I doubt I'll do anything else. With the size/weight of the boat its not often when I have the proper water conditions to open it up. It can get a little "loose" at WOT. And the nice thing about the 17p is wether you have 1 or 6 people whenever you have to slow down for a wake or somethning it will jump right back on top.
Me being somewhat of a half ass decent gear head it just isn't making sense. The buddy of mine with the Chap. is a smart guy. He's also baffled. Based on what he's done vs. what I've done and the weight difference we were both thinking some low 60's.
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