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putting a 7.5 K&B on the top speed 2

Old 03-26-2006, 02:12 AM
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lelandhild
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Default putting a 7.5 K&B on the top speed 2

I am new to tunnel hulls and I really like the top speed 2 by aquacraft. I know its designed for a 3.5cc motor but I was wondering If I could put a 7.5cc K&B on it. What kind of speed would I get. Also, I was wondering if someone could give me some ideas on a tunnel hull designed for a 7.5cc motor that is prety easy to build. Any help would by much appreciated.
Old 03-26-2006, 09:16 AM
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Ron Olson
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Default RE: putting a 7.5 K&B on the top speed 2

Nope. Several reasons why not to. At rest the boat will sit low enough in the water with a 3.5, the extra weight of a 7.5 will just about sink it. The hull is made to handle the 3.5's and mostly the stock ones only. It has had problems handling modded 3.5's much less trying to put a 7.5 on it.
It's the case of trying to put a blown Hemi in a tricycle.
There are a lot of good 7.5 tunnel boats available but a lot of them are costly. One of my favorites is the one made by www.htbboats.com .
Old 03-26-2006, 10:27 AM
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Default RE: putting a 7.5 K&B on the top speed 2

thanks for the quick reply. Could I get a K&B 3.5 to go 50-60mph on the top speed 2, If so what kind of mods would I have to do to it. Also how fast would a tunnel go in general with a K&B 7.5 on it. What a good tunnel hull kit, what about the prather 32" tunnel hull.Thanks again for the help.
Old 03-26-2006, 10:52 AM
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Default RE: putting a 7.5 K&B on the top speed 2

50-60 is pretty fast for a 3.5 tunnel, actually it's a good speed for a 7.5 tunnel.
My opinion, not that of RCU's but the Prthaer tunnel is an overpriced, outdated piece of junk. It requires too much work to get built as it comes only with an epoxy-glas hull. Translated it means no gel-coat and a hull full of pinholes that need to be filled. Radio box and hardware that are included with almost every other boat are extra cost items.
Old 03-26-2006, 01:33 PM
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Default RE: putting a 7.5 K&B on the top speed 2

Thanks for all the help. So if I modded a K&B 3.5cc outboard and put it on a top speed 2 what kind of speed am I looking at.
Old 03-26-2006, 02:31 PM
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Default RE: putting a 7.5 K&B on the top speed 2

You should be able to get close to 50 if not the low 50's with a modded engine. A stock one will put you in the mid 40's. Considering that you use a relatively high percentage of nitro, 50-60% and a good prop plus the boat riding right all help in getting you up to your goal.
I used to think that K&B's were the only way to go but since the OS .21 has made a comeback, they look like the one to go with now. I've seen one boater that could flip-start his OS with his fingers, on the flywheel of course, he wasn't THAT drunk at the time.
Old 03-26-2006, 02:37 PM
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Default RE: putting a 7.5 K&B on the top speed 2

what kind of mods would need to be done to get it running that fast.
Old 03-26-2006, 03:22 PM
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Default RE: putting a 7.5 K&B on the top speed 2

A tuned pipe would make a big difference. For serious money you can swap powerheads. There are converted Rossi's, Nova Rossi's, MAC and CMD's to mention a couple of them.
Old 03-26-2006, 03:32 PM
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Default RE: putting a 7.5 K&B on the top speed 2

Thanks, ron for all of your help ,I think I finally know what I want for now. If I need any more help I will be sure to ask your opinion.
Old 03-26-2006, 06:40 PM
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Default RE: putting a 7.5 K&B on the top speed 2

Mods that would be need to be looked into are: port/polish the sleeve, skirting the piston, Swiss bearings, Keeley Turbo head, turbo crank, high nitro, props, pipes, props, pipes, and more props and pipes. It's all about testing each change at a time and then going back testing more. There is no magic setup to get high speeds. Last time I ran my MAC 21 it reached 49 mph. Hoping to better that this year with more props, pipes and a new motor. Also have another 29" WOF that will have three test powerheads on it. One is a K&B 3.5 that has the above mods along with a exhaust throttle setup. If you are using a K&B lower, alinement of the prop shaft to the skeg will yield 3+ mph.

I agree with Ron on the TS2 with a 7.5... If you like to build, get ahold of some WOF plans from JD. They are not that hard to build if there is a band saw that you can use..
Old 03-26-2006, 06:42 PM
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Default RE: putting a 7.5 K&B on the top speed 2


ORIGINAL: Ron Olson

MAC and CMD's
Ron, think you've been taking too many pain meds Get the MAC from CMDi.. Think Ron missed type.. CMB are also a good swap..
Old 03-26-2006, 06:53 PM
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Default RE: putting a 7.5 K&B on the top speed 2

Yup, I've been tapping the Vicodin bottle too much today. Yes, I did mean CMB.

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