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View Full Version : Top speed in my 22 Classic w/496HO



chief bigwin
07-02-2002, 06:05 PM
I have 23 hours on my motor now and I love the boat, but I am a little disturbed with the top end speed. I am running 70-71 on GPS in fresh water. I figured I would be more in the mid to upper 70s. I am running the stock prop (25P Mirage). I am only able to run 4650-7000 RPM fully trimmed(according to the Gaffrig Monster gauge), even with a rooster tail. I installed a K&N high-flow filter-don't really see any difference from that.

A new prop may help a bit I guess, but how much do you think I will gain? Donzi tech support told me the fastest they can get a 22 to run with the 496HO in salt water is 71.8.

Any "reasonable" suggestions would be appreciated.

Thanks,
Brian

Bryan Tuvell 33ZX
07-02-2002, 08:15 PM
I think your speed numbers are close to on for GPS, lab prop will help. Your boat 2001 (22 Classic) 502, K&N Lab prop Bravo I 24P (which my bud has) will run 71 @ 5000 RPM.

I will add 2 2001 26ZX's, one 502 MAG, one 496MAG HO is the same on speed. We live it daily.
Bryan

Last Tango
07-02-2002, 09:18 PM
Well, okay, but having a rooster tail means you are way overtrimmed on the drive. Trim down some until the tail is only a few inches above flat. Still, 71 may be all she can do in fresh water. The K&N was a good start, but a set of free flow exhaust manifolds and risers and thru-hulls will complete the breathing excercises. Air going in has to go out faster, too. K&N alone might be worth 10 HP but that won't mean much at the high end.
Oh, and thanks for the report, because just this very week I've been toying with the idea of an '03 C22 w/496 Mag HO and you have answered the very question I wanted answered.

Air 22
07-02-2002, 09:55 PM
You might try another prop as My 22' I'm in the 5000-5250 rpm range properly trimmed (fresh water)70-73gps. H2o temp and humidity level outside will vary your top speed on a daily basis. Colder H20 and dry cool air will produce more HP than on a hot humid summer day. I fly jets and this applies to our engines as well. Go fast... Dwight

Woodsy
07-03-2002, 06:51 AM
Dwight... I would calibrate your tach, I think the 496 has a rev limiter set at 5100rpm..

Brian... thats all she wrote... 71 on GPS is about it. You might see an extra mph or so from a labbed prop, but you really need alot more HP or a alot less drag to get into the mid 70's... I would leave the exhaust alone on the boat and get yourself an IMCO 2" shorty....

Woodsy the censored :D :D

blackhawk
07-03-2002, 08:55 AM
So the Donzi dealer that told me a 22' Classic with a 496HO would run close to 90 was a little off? :D I didn't even have the energy to argue with him.

roadtrip se
07-03-2002, 09:59 AM
My 502 22 Classic ran 73.3 on GPS a couple of weeks ago turning a stock 25 pitch mirage plus.
Not sure on the rpms.

Also will tell you that I couldn't get anything close to this until after 50 hours or so. Put a few more hours on her and maybe she will loosen up
some.

Lab prop will get you 150-200 rpm and a mile or two.

Anymore than that? Woodsy's right, time to start the modifications....

Todd

MAXCAT
07-03-2002, 10:52 AM
All of the testing we have done at GM shows that the K&N flame arrestor is worth a whopping 2-3 HP @ 5000 RPM WOT on the 496 HO. The real restriction is in the throttle body /intake manifold. Wait for a certain aftermarket company to come out with theirs. It is worth about 30 - 40 HP. Mec did a good job on their exhaust manifolds, and you won't see much gain unless you step up to CMI sport tubes. Those are worth about 20 - 30 HP over stock merc manifolds.

MAXCAT
07-03-2002, 10:54 AM
I forgot to mention that the 496 does not free up until 50 hours of 5000 rpm WOT testing on the dynos. Expect to see another 1 or 2 MPH.

RickSE
07-03-2002, 11:22 AM
Every year all dealers get what's called a PIG book (Product Information Guide?). The PIG book gives information on all the current models, including top speed for each boat/engine/drive option. I guess my point is that all dealers have access to the advertised speed information and should be giving customers the correct numbers.

Bad-Tat
07-03-2002, 01:28 PM
Last T,
Why don't you put a 502 in that C22?? Randy and I got 78 from mine a couple of weeks ago. And that was with a big rooster tail!!!! Tom was ready for a boat wash last Sun, but June vetoed that idea!!!
Glad you are doing much better Mark!!

chief bigwin
07-03-2002, 05:14 PM
Thanks for the info. I guess that's about it for top end. By the way, my results are with exhaust exiting through the hull.

I have heard rumors that I can gain top end by adding a drive extension. I guess this is where Woodsy was going with the 2" shorty comment. Anybody experiment with this yet? Any concrete evidence as to how much it helps? It sounds like a pain to do if it's only a small improvement.

Maybe I should loosen up and wait for the motor to loosen up and just enjoy what I have. It is a beautiful craft, even if it doesn't break the sound barrier.

Last Tango
07-03-2002, 08:24 PM
Chief,
The extension box goes between the hull and the face plate of the outdrive. A "shorty" refers to the verticle shaft length of the drive between the upper gear box and the prop shaft.
The extension box lengthens the "appeareant hull" of the boat.
A "shorty" will shorten the depth the prop is running in the water. The boat rides "longer" in the water, thereby potentially increasing speed.
This is not a quick answer topic. X-dimension is also part of the equation. Gear ratio of the drive, "hook" in the hull, etc. etc. This topic was covered very extensively last year by some of our best expert members. Try a search for the "extension box" in the archives. A little hunting and reading here will save you thousands of dollars in experimenting.
If you have trim tabs, play with them. But the real speed control is break-in period. My 2001 18 with the MX6.2MPI has about 70 hours now, and runs wayyyyy faster than it did even at 40 hours.