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ky-donzi
08-16-2007, 11:54 AM
Hi I just bought a 98' 18 classic and I'm already in love with it. It is buy far the nicest previously owned thing I have ever bought.

My boat is equiped with a 350 MPI and Bravo drive with a 25P mirage prop. I have reached 71 mph via speedo (the one day I frogot the gps) but 68-69 mph any day.

Are there any MPI upgrade availble for extra hp? I spin 4200rpm at top end should I try a different prop. Thanks

chappy
08-17-2007, 01:00 PM
Welcome to the board. There are plenty of upgrades for that engine and drive combo, all depends on how much $$$$ you want to, or can spend. Try the search feature above for more detailed info. 4200 is low for WOT with that motor, maybe go down in size on the prop or check to see that the tach is operating properly. Possibly others can chime in that own that power package.

Rich

ITTLFLI
08-17-2007, 01:07 PM
Hi I just bought a 98' 18 classic and I'm already in love with it. It is buy far the nicest previously owned thing I have ever bought.
My boat is equiped with a 350 MPI and Bravo drive with a 25P mirage prop. I have reached 71 mph via speedo (the one day I frogot the gps) but 68-69 mph any day.
Are there any MPI upgrade availble for extra hp? I spin 4200rpm at top end should I try a different prop. Thanks

Welcome to the board and congrats on your purchase!!

Where in KY are you from? You should plan on attending Awakening of the Horses next spring down at Lake Cumberland.

Sorry, but I do not have any experience with the 18. Your speed seems really good but at low revs. You could try a smaller pitch and see what happens!

justleft
08-17-2007, 02:59 PM
What is the gear ratio of your drive ?

I think most Bravo's are 1.5.

If that is the case the math does not work out.
A prop slip calculator shows best speed at 0% prop slip would be
66 mph. So with 7 - 10% slip you would be at about 60.

The numbers are much closer if you're at 4800 rpm.

Even if you have a 1.36 ratio drive the numbers are still off.

I would check the tach or speedo.

ky-donzi
08-21-2007, 07:57 AM
I've checkd the numbers GPS 68 and RPM 4200-4300. 25 P prop.
I would like to be able to go 71 on a reg. basis. Is there any non-intrusive upgrades for my engine. Like a chip or bigger injectors. I would like to get an extra 25 H.P. that might get me that 2-3 MPH I'm not opposed to spending some money for upgrades, I would just rather leave the engine in one pc. (no cam, or pistons, ect.) It only has around 70hrs on it. Thanks

Donziweasel
08-21-2007, 08:37 AM
I would think the quickest way and most cost effective way to get the speed you are looking for is a cam and a small timing adjustment. I know you said you did not want to go that route, but most cams are under $300.00 and the performance gains can be impressive. I am not sure chipping or larger injectors will accomplish your goals and could end up being a serious PITA plus I am not sure how cost effective injectors would be. Good Luck!

smokediver
08-21-2007, 08:53 AM
procharger , inner cooler ... probably be the best bang for the buck ... and there is room in an 18 for a blower pulley ..:yes:

DonCig
08-21-2007, 08:56 AM
procharger , inner cooler ... probably be the best bang for the buck ... and there is room in an 18 for a blower pulley ..:yes:
If Procharger actually makes a unit for your engine, then I would go this route. But if they do not make a full kit; then I would leave well enough alone and lie to all that your boat does 71.......lol

Ranman
08-21-2007, 09:16 AM
KY,

I think what the crew is trying to say is that your numbers don't add up. Max theoretical speed is calculated as follows:

Theo Speed = Pitch X RPM / Gear Ratio X 1056

Doing the math...

25 X 4300 / 1.5 X 1065 = 107500 / 1584 = 67.86MPH.

If your numbers are correct there is no physical way you can be going 68-69. Also, you need to realize that Theoretical speed is not actual speed because there wil be some prop slip. There is no such thing as 0 prop slip. If you can get under 10% you're doing good.

You need to re check your numbers. Something is off. Check your gear ratio and RPM's for accuracy. My boat is similar to yours and runs 67-68MPH. Here are my numbers

25 X 5200 / 1.65 X 1065 = 130000 / 1757.25 = 73.89

73.89 - 67.5 / 73.89 = 6.39 / 73.89 = .086 = 8.6% prop slip

Recheck and get back with us so we can provide better suggestions. Good luck.

chappy
08-21-2007, 10:30 AM
KY,
I think what the crew is trying to say is that your numbers don't add up. Max theoretical speed is calculated as follows:
Theo Speed = Pitch X RPM / Gear Ratio X 1056
Doing the math...
25 X 4300 / 1.5 X 1065 = 107500 / 1584 = 67.86MPH.
If your numbers are correct there is no physical way you can be going 68-69. Also, you need to realize that Theoretical speed is not actual speed because there wil be some prop slip. There is no such thing as 0 prop slip. If you can get under 10% you're doing good.
You need to re check your numbers. Something is off. Check your gear ratio and RPM's for accuracy. My boat is similar to yours and runs 67-68MPH. Here are my numbers
25 X 5200 / 1.65 X 1065 = 130000 / 1757.25 = 73.98
73.89 - 67.5 / 73.89 = 6.39 / 73.89 = .072 = 8.6% prop slip
Recheck and get back with us so we can provide better suggestions. Good luck.

Theoretical Speed 101, where was this course when I was in school? I wouldn't have skipped so many classes!:biggrin:

All joking aside, VERY educational post, thanks Randy.

Rich

glashole
08-21-2007, 10:42 AM
I've checkd the numbers GPS 68 and RPM 4200-4300. 25 P prop.
I would like to be able to go 71 on a reg. basis. Is there any non-intrusive upgrades for my engine. Like a chip or bigger injectors. I would like to get an extra 25 H.P. that might get me that 2-3 MPH I'm not opposed to spending some money for upgrades, I would just rather leave the engine in one pc. (no cam, or pistons, ect.) It only has around 70hrs on it. Thanks


and then it starts

i'm happy with 71 but it sure would be nice to go 75

good luck but keep in mind modifications reduce reliability and you are still moving at those speeds

try a few props first

spend the rest of the season getting used to the boat and reevaluate at the end of the year

and check that tach by the sounds of it

have a good one

Shea

The Hedgehog
08-22-2007, 08:41 AM
The guestion is "How fast do you want to go?"

You could drop some coin on a cam and exhaust. Then you would also have to have someone reprogram your ECU. I don't know anyone that has programs for small blocks. I have heard that Tyler Crockett has been kicking this around. All that work would probably net you 6-7 mph. Maybe.

You could probably get 3 mph from going to a more efficient exhaust system.

You could put on an Imco Shorty and get a proven 5 mph without asking anything alse from your engine.

If you really want to boogy then strap on a procharger and you will be well in the 80's

You will want hydraulic steering for any of this. I am seeing 70 or so in my X-18 with a stock 383 mag and am definately going for the steering.