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View Full Version : Ideas for a few more mph...?



Maine18
05-19-2005, 11:07 AM
Finally got my 2004 18C in the water this week. It has the 350 mag mpi / bravo combo - runs about 66-67 with a full tank in calm water. 4850-4900 rpm with a 23" Mirage Plus and 1.5:1 gear ratio. Was hoping to get a solid 70 without too many major mods this summer. Any ideas from the experts? I know I could get an IMCO shorty or Procharger (when they finally come out with one for the 2004's) or do some major engine mods, but it's basically a brand new boat and I'm not ready to tear into it quite yet.

Thanks in advance for the help.

glashole
05-19-2005, 11:22 AM
talking from experience an extra 3-4 mph is never enough :banghead:

Maine18
05-19-2005, 11:25 AM
talking from experience an extra 3-4 mph is never enough :banghead:

that's why i added "this summer" in there. next winter i have bigger plans.

blackhawk
05-19-2005, 12:31 PM
Are those GPS speeds? If they are they are pretty good numbers.

I would think some prop work may be the easiest and cheapest. That's my plan, if I ever find the right prop! :banghead:

blackhawk
05-19-2005, 12:34 PM
Was hoping to get a solid 70 without too many major mods this summer.

Isn't seventmileperhouritis a b**ch! :D

Maine18
05-19-2005, 12:52 PM
Isn't seventmileperhouritis a b**ch! :D

yes, it is. and no, not gps numbers - but either way, i'm looking for a way to just get a few more out of it. any ideas on the best way to get the prop work done? recommendations?

blackhawk
05-19-2005, 01:15 PM
I am fairly new to the prop game myself. But, I have done a ton of research on it. I am basically going to try props until I find the one that gives me the best combo of speed and handling. When I find it I plan on sending it out to get the prop labbed which is usually good for 2 mph(maybe more).

So, if you are happy with the way your boat handles now you could send your prop out to get labbed, pick up 200 rpm or so and about 2(maybe more) mph. Again, this is from the research I have done. But, I have heard nothing but good things about Matt and Julie at Throttle-up. So, that's where my prop is going when I finally find the right one!

Maine18
05-19-2005, 01:21 PM
I am fairly new to the prop game myself. But, I have done a ton of research on it. I am basically going to try props until I find the one that gives me the best combo of speed and handling. When I find it I plan on sending it out to get the prop labbed which is usually good for 2 mph(maybe more).

So, if you are happy with the way your boat handles now you could send your prop out to get labbed, pick up 200 rpm or so and about 2(maybe more) mph. Again, this is from the research I have done. But, I have heard nothing but good things about Matt and Julie at Throttle-up. So, that's where my prop is going when I finally find the right one!

thanks - i'll check with them

Rootsy
05-19-2005, 01:29 PM
Throttle-up is probably one of the best ways to go if you want prop work done... they take the time to explain everything to you and answer any question you may possibly have... very experienced folks...

Short of modifying the motor and swapping drives, etc, prop tuning is going to be the best bang for the buck... just be aware that labbing a prop entitles truing the blades which requires them to be thinned out which in turn makes them highly suseptible to fatigue fracture, especially if you have any kind of damage on the leading edge of the blades... once you lab a prop.. the warranty on it is gone forever... it is a lot more easily damaged than the stock blade also...

You may also gain some from swapping to a K&N style air filter... don't hold me to this for i have not done it... but RANMAN and such have, maybe they can enlighten you to how it helped or hindered them...

the mirage + is a fast prop on the 18 hull but can be kinda loosy goosy feeling... a 4 blade hydromotive, etc will slow you down somewhat but increase stability... you might give a couple of the 3 blade turbo brand propellers a shot, many have had good success with them.

props are like shoes... just cause the size is right on all of em, only one pair may be really comfortable to you and give you everything you want.. the only way to find out is to try em on...

JR

Maine18
05-19-2005, 01:41 PM
Throttle-up is probably one of the best ways to go if you want prop work done... they take the time to explain everything to you and answer any question you may possibly have... very experienced folks...

Short of modifying the motor and swapping drives, etc, prop tuning is going to be the best bang for the buck... just be aware that labbing a prop entitles truing the blades which requires them to be thinned out which in turn makes them highly suseptible to fatigue fracture, especially if you have any kind of damage on the leading edge of the blades... once you lab a prop.. the warranty on it is gone forever... it is a lot more easily damaged than the stock blade also...

You may also gain some from swapping to a K&N style air filter... don't hold me to this for i have not done it... but RANMAN and such have, maybe they can enlighten you to how it helped or hindered them...

the mirage + is a fast prop on the 18 hull but can be kinda loosy goosy feeling... a 4 blade hydromotive, etc will slow you down somewhat but increase stability... you might give a couple of the 3 blade turbo brand propellers a shot, many have had good success with them.

props are like shoes... just cause the size is right on all of em, only one pair may be really comfortable to you and give you everything you want.. the only way to find out is to try em on...

JR

thanks. i was considering trying a Turbo 1 23 or 24". but maybe labbing my Mirage+ is worth a try. i checked into the K&N filter - they don't yet make one for the 2004- 350's. i also looked around for other bolt-ons like larger throttle bodies and underdrive pullies and chips, etc. to try to gain a few more rpm. doesn't seem to be much out there.

blackhawk
05-19-2005, 02:13 PM
i also looked around for other bolt-ons like larger throttle bodies and underdrive pullies and chips, etc.

I've often thought about underdrive pulleys for boats. I figured you could make up for the belt length by also changing the sea water pump pulley so it still spins the same rpm. Give up some rpm on the power steering and alternator and maybe pick up a few hp?

jonnyrottn
05-20-2005, 09:08 AM
thanks. i was considering trying a Turbo 1 23 or 24". but maybe labbing my Mirage+ is worth a try. i checked into the K&N filter - they don't yet make one for the 2004- 350's. i also looked around for other bolt-ons like larger throttle bodies and underdrive pullies and chips, etc. to try to gain a few more rpm. doesn't seem to be much out there.

I've done some basic mods to a 2001 6.2 MPI, and changed props on my 22zx. I ended up gaining about 5 mph at the top end. I started with replacing the exhaust manifolds and risers with Stainless Marine aluminum manifolds and stainless risers, the Stainless Marine come with all the necessary brackets to go right in place of the stock mercruiser set up, you drop about 75lbs off the back end - the change out was very simple - took about two hours. I immediately gained about 200+ rpm at the top end. I called Julie @ Throttle Up, sent my 24p Bravo 1 to them, after a good conversation about WOT and MPH, they changed the prop to a true 24p - apparently the bravo's are all manufactured one pitch under the stamped #, and lab finished it- I gained 4 mph and was back within the 5200 rpm limit. K and N doesn't have a flame arrestor for my engine either, so I made my own. Took the mercruiser 7" diameter by 3" tall arrestor - measured the distance from top of the arrestor and cover to the hatch when closed - yes I actually was in the hatch closed with a flash light and a ruler - ordered a 4" tall, 7" dia element from K&N, drilled out the rivets in the mercruiser arrestor, removed the aluminum screen, replaced it with the new taller element and bolted it back together with stainless nuts and bolts, arrestor is now twice as tall, same nice smooth air entrance into the throttle body. I gained a little on the top end with this mod, but more surprising was the planing time decreased, the engine appears to make more low end torque. Just added comp cams magnum roller rockers - can't wait to see if this helps - boat hits the water this weekend. Hope this helps - I found these mods very quick, simple and affordable. - Jon

Maine18
05-20-2005, 09:21 AM
thanks for the info. i'm going to get my prop labbed next week and go from there.

Rootsy
05-20-2005, 09:44 AM
Just added comp cams magnum roller rockers - can't wait to see if this helps - boat hits the water this weekend. Hope this helps - I found these mods very quick, simple and affordable. - Jon

what ratio rockers are you using? if you went 1.6 you need to check and probably open up the pushrod guide holes in the cylinder heads... going to a higher ratio rocker they shorten the fulcrum to pushrod distance while leaving fulcrum to valve stem distance the same... brings the pushrod in closer to the end of the slot and when the valve is at full lift it is even closer and often times will rub... geometry on the valve tip shouldn't be much of an issue... maybe a bit to the inside but close enough...

shouldn't be a whole lotta issue with gross lift and retainer to valveguide clearance with the camshaft in that motor using a high ratio rocker. stock vortec cylinder heads are good to about .480 gross lift @ valve before you have to start machining...

JR

jonnyrottn
05-20-2005, 12:30 PM
what ratio rockers are you using? if you went 1.6 you need to check and probably open up the pushrod guide holes in the cylinder heads... going to a higher ratio rocker they shorten the fulcrum to pushrod distance while leaving fulcrum to valve stem distance the same... brings the pushrod in closer to the end of the slot and when the valve is at full lift it is even closer and often times will rub... geometry on the valve tip shouldn't be much of an issue... maybe a bit to the inside but close enough...

shouldn't be a whole lotta issue with gross lift and retainer to valveguide clearance with the camshaft in that motor using a high ratio rocker. stock vortec cylinder heads are good to about .480 gross lift @ valve before you have to start machining...

JR
I went with the 1.52 ratio to replace the stock 1.5's for just that reason, had read that some machine work might need to be done if I went to the higher ratio, wasn't confident about getting into that. From what I've read the stamped original equipment rockers can vary in ratio due to the nature of their manufacture, the magnum rollers are a much more exact ratio, they are supposedly going to provide a more efficient system because of the closer tolerances and the roller feature - they claim that a gain of 20hp can be acheived. I also saw some dyno tests in chevy performance mag that show a loss of horserpower when the 1.6's were used in place of the 1.52's on a 350 - something about defeating the cam with the higher ratio? - Jon

Tidbart
05-20-2005, 01:11 PM
I have an inexpensive solution for you.

Try getting out in moderate chop with 1/8 tank of gas and no passengers.
Trim it out right and you could hit 70.

Bob

Maine18
05-20-2005, 02:24 PM
i will try that, but i can't imagine i'll gain 3-4 mph. i've had it trimmed out in relatively calm water pretty much every time i've driven it (i live on a fairly sheltered lake - don't get too much chop), but i can't imagine i'll gain 3-4 mph by just shedding 150# of fuel. but maybe 1 or 2 mph. we'll see. i think with the labbed prop and less fuel and a little chop - 70 shouldn't be too hard.

Maine18
05-20-2005, 02:25 PM
...hopefully.

but any performance tips are always greatly appreciated.

- thanks.

MOP
05-20-2005, 09:46 PM
My SB 22 just touched 60 on flat water, in a light chop 61.9. You need to get a little air under you.

Phil

txtaz
05-21-2005, 07:48 AM
You need to get a little air under you. Phil
Phil I could have so much fun with that statement, but hey, you're a bud so I'll leave it alone.

Wes

BigGrizzly
05-21-2005, 09:16 AM
I would prop it, but not a labbed type. Like Jamie said it weakens it. We down cher have blades go into the gear case. also remember when a prop is thinned and reshaped it can and will change. This is always a problem. My question is is that i maybe 2 miles per hour worth it. $450 to $800 then more to lab it them hit one oblect and back to the prop guy. Everyone has researched how props worl. Pitch Diameter Rake Blade angle etc. Every body has a story. The real story is try it, if it works then use it. Yes I prefer the props made Precision Propeller and Solas, and I have tried them all. If a Power-Tech works use it! I have been down this road before. One thing I can tell new Donzi owners is that you can learn to drive another 2MPH faster with practice. An Classic 18 is a prime example of this.

mrfixxall
05-21-2005, 06:32 PM
Finally got my 2004 18C in the water this week. It has the 350 mag mpi / bravo combo - runs about 66-67 with a full tank in calm water. 4850-4900 rpm with a 23" Mirage Plus and 1.5:1 gear ratio. Was hoping to get a solid 70 without too many major mods this summer. Any ideas from the experts? I know I could get an IMCO shorty or Procharger (when they finally come out with one for the 2004's) or do some major engine mods, but it's basically a brand new boat and I'm not ready to tear into it quite yet.

Thanks in advance for the help.try 2 ft chop the donzis are faster in the choppppppppp

mrfixxall
05-21-2005, 06:39 PM
Finally got my 2004 18C in the water this week. It has the 350 mag mpi / bravo combo - runs about 66-67 with a full tank in calm water. 4850-4900 rpm with a 23" Mirage Plus and 1.5:1 gear ratio. Was hoping to get a solid 70 without too many major mods this summer. Any ideas from the experts? I know I could get an IMCO shorty or Procharger (when they finally come out with one for the 2004's) or do some major engine mods, but it's basically a brand new boat and I'm not ready to tear into it quite yet.

Thanks in advance for the help.p.s. try a hydromotive four blade it will give you more lift in ths ass of the boat and you can trim the heck out of it ,,,my boat picked up 5 mph just by this prop

Rootsy
05-21-2005, 07:00 PM
p.s. try a hydromotive four blade it will give you more lift in ths ass of the boat and you can trim the heck out of it ,,,my boat picked up 5 mph just by this prop

been there done that and if you own a donzi classic i'll put a wager on the 5 mph thing as totally bogus.. not to sound rude or a smartass... but i have personally been there, i run a 4 blade hydromotive... 4 blade hydromotive vs 3 blade mirage.. you'll lose about 2 - 3 mph... but you'll gain stability...

the boat will run flatter because of the rake setup and lack of cup, slip numbers will decrease due to blade area... the hydormotive QIV and IV O/T's are essentaily semi cleaver designs... you don't so much lift the stern as the lack of stern SUCK which brings the bow up... you'll need less trim with the hydro than a comparable high rake big cup 3 blade... the extra drag on the hull in the water will slow you down as well as spinning an extra 5 lbs of weight because of another blade on the prop...

27 props and counting on my 16 classic...

JR

Maine18
05-24-2005, 08:27 AM
took it out over the weekend with a GPS and was fairly disappointed to learn that my speedo reads about 4mph too high. the best i could get with a full tank and 2 people in about 6" chop was 62.2. is it just me, or does that seem a bit slow for a 2004 18C with the 350 mag / bravo combo? it ran at about 4900 rpm. don't get me wrong, i love the boat, but i could go about that fast in my 1986 thouroughly beat 18' checkmate with the stock 4.3. hopefully when i get the prop back from Throttle-Up i can get it to go at least a marginally respectable 65-ish.

glashole
05-24-2005, 10:06 AM
you need to run the boat in a 1-2 ft chop to get enough air under it.


6" isn't enough to truely make the boat work properly

mrfixxall
05-24-2005, 10:08 AM
well nicely said,,,,,any way i was talking about the iv-x prop lighter and doesnt burn up hp like QIV prop ,26 props??? i have another 120 props you can try?????????
been there done that and if you own a donzi classic i'll put a wager on the 5 mph thing as totally bogus.. not to sound rude or a smartass... but i have personally been there, i run a 4 blade hydromotive... 4 blade hydromotive vs 3 blade mirage.. you'll lose about 2 - 3 mph... but you'll gain stability...

the boat will run flatter because of the rake setup and lack of cup, slip numbers will decrease due to blade area... the hydormotive QIV and IV O/T's are essentaily semi cleaver designs... you don't so much lift the stern as the lack of stern SUCK which brings the bow up... you'll need less trim with the hydro than a comparable high rake big cup 3 blade... the extra drag on the hull in the water will slow you down as well as spinning an extra 5 lbs of weight because of another blade on the prop...

27 props and counting on my 16 classic...

JR

Maine18
05-24-2005, 10:16 AM
you need to run the boat in a 1-2 ft chop to get enough air under it.


6" isn't enough to truely make the boat work properly


unfortunately, the lake where i live is not that big and fairly sheltered and we don't typically get chop like that. a lot of the times it's just a few inches. occasionally we get a foot or so.

Formula Jr
05-24-2005, 02:12 PM
"Phil I could have so much fun with that statement, but hey, you're a bud so I'll leave it alone."

I just got that joke......LMAO :rlol:

Its all about weight. Get the boat as light as you can. Then prop up one number and you'll see 70. I can't recommend 150 grit sanding of the wetted surface since its a new boat. But thats good for 2 mph. I know it doesn't make sense. But it works.

txtaz
05-24-2005, 02:57 PM
"Phil I could have so much fun with that statement, but hey, you're a bud so I'll leave it alone."

I just got that joke......LMAO :rlol:

JR, It was a time delayed joke and not meant to be....MY BAD. :biggrin.: :biggrin.:

Wes

MOP
05-24-2005, 05:01 PM
Phil I could have so much fun with that statement, but hey, you're a bud so I'll leave it alone.

Wes

Yeah I know at my age I do get a bit of AIR under me! Comes with the turf!

MOP

gcarter
05-24-2005, 08:11 PM
A couple of things I did, in addition to the K&N filter, when rebuilding my boat was to use a Johnson crank driven seawater pump, this might be good for one or more horsepower as it eliminates a belt. A couple of other advantages is it weighes less, there is only a very small bracket, and it's VERY easy to rebuild as the plate is on the front of the engine.
The second thing was, since I'm using closed cooling, was to incorporate an electric circulating pump. This item might be good for five to 15 horsepower, as it only draws seven amps.
I have no comparison numbers as I did this from the beginning. But on our first outing we did 64 with what is definately the wrong prop, a barely broken in engine, 3/4 tank of fuel and both me and my wife aboard. I'm looking forward to doing some tuning, and trying some props.

mrfixxall
05-29-2005, 03:53 PM
a 250 shot of nos should put you over 80 lol