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496 HO mods....best bang for the buck.

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Old 03-23-2009, 01:43 PM
  #31
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JRZ, what about the headers in the picture?

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A pair of CMI's new 8.1 exhaust, low cost and simple install.
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Old 03-23-2009, 03:34 PM
  #32
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Just Merc Me:

Notice you are a new OSO poster so I will ask just a few questions and offer a few comments on your question post.
First: A 496Ho engine can not be run at 5300 rpms unless the ECM has been reprogrammed. If it has a reprogrammed ecm, running a stock 496 at 5300 rpms is very dangerous to the engine and will eventually result in valve float and possible engine damage!
If your 22 Donzi is really running 75-76 mph GPS, then you have a really fast 22 and you might be very close to the maximum speed this particular boat is considered safe to run at in all but perfect condtions.
If as I suspect your max rpms per the stock rev-limiter is running at 5100-5150rpm and your real speed may be more like 70-72mph on the high side.
The accuracy of your information is important here if you are really going to make improvements to your boats performance and give good feedback to who ever you work with,header, engine, drive, prop people so they can reall y help you achieve measurable gains and improvements to get "Your Bang for the Buck"
If you really intend to get to 80 mph in your 22 Donzi GT in most normal water condtions you will probably have to do a trio of changes as the added horsepower it takes to add 6-10mph to most high performance boats is that the added horspower will spin more prop after the horsepower is added with engine upgrades and sometimes drive changes are needed also.
As for safety of a 22 footer at 75-80 mph you should automatically consider hydraulic steering or steering assist
to make sure the boat will remain stable at higher speeds and keep you and your passengers safe.
I will throw in my opinion on horsepower available and speed improvements from most of the options available to you:
Prop work and new prop with labbing 1-3 mph possible @$800 - $1000 , you can install
Headers (real headers) exhaust 35-40 horspower increase with 1-3 mph possible. $3000-$6500 installed might need prop rework.
Engine upgrades like a Raylar BCK103 engine kit , 100hp increase with stock exhaust and 135-140 hp increase with headers + 6-10 mph possible with right setup and propping
+$9000 - $9500 installed by shop , but will need prop change.
That kind of lays out a starting point for bang for the buck scenario, but get an accurate starting point with real speeds and rpms.
Hope this helps,

Best Regards,
Ray @ Raylar

Thanks for the info Ray,

Yes, I am new to OSO being that I just got the Donzi and figured I would get into this site to hang with the big block tech heads. This is my first offshore type boat and it mainly runs on the Chattahoochee River or Destin, FL from time to time. My other boat is a 19' Mirage tunnel w/ Modified Merc 280 that I run drags with on the river here, so I'm no stranger to speed on the water I guess you could say.

I've checked the speed with a handheld Garmin reading 75.8 mph and the Livorsi GPS recal that reads 75-76 mph. I don't believe the ECM has been modified so the tach may be the problem if I'm not supposed to be running past around 5150 rpms. I thought about trying a 28 Bravo 1 prop and seeing about getting the RPM's down a bit.

As far as upgrades, I think I might try an IMCO shorty -2" first and maybe later going blue with the motor due to the fact of no real gain w/ new exhaust and the lack of upgrades for the 496. I don't really wanna go to a whipple if the stock 496 is not built to handle to much more.....SO I figure the better bet would be to go to a 500 or 525....


Thanks for all the replies guys....

Last edited by JustMercMe; 03-23-2009 at 03:38 PM.
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Old 03-23-2009, 04:40 PM
  #33
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JRZ, what about the headers in the picture?
First production order should go to Teague this week.
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Old 03-23-2009, 06:06 PM
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If anyone wants to send a pair of those headers to BOBL, I have a 496HO with Raylar bottom end and Whipple top end going on his dyno by the end of the week. We will test both and If they make more power than the Danas I will by them on the spot!
Thanks Mitch

A few pics



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Old 03-23-2009, 07:57 PM
  #35
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Those Danas are nice looking exhaust.
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Old 03-23-2009, 08:21 PM
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I will not budge, the tube headers will not change any performance gain by themselves on a stock 496 mag or HO !!!!!!!!!!!!!!!!!!

We have seen too many people buy them and it sounds great then then they run their boats and NO CHANGE in performance, I WOULD LOVE TO BE PROVEN WRONG, PLEASE SOME ONE STEP UP ??????

WILL
know somone who put cmi headers on twin 496ho and maybe picked up 1mph,,,,sound and looks good???though
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Old 03-23-2009, 09:16 PM
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I can tell you that it did not change top end, but allowed me to increase my prop pitch from 22's to 24's. This increased my mid range and cruise speed dramatically. I did not sacrifice anything out of the hole. I would have liked more top end, but I don't spend that much time @ WOT. I'm sold!
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Old 03-24-2009, 08:59 AM
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I can tell you that it did not change top end, but allowed me to increase my prop pitch from 22's to 24's. This increased my mid range and cruise speed dramatically. I did not sacrifice anything out of the hole. I would have liked more top end, but I don't spend that much time @ WOT. I'm sold!

That was on the CMI's or Dana's?
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Old 03-24-2009, 10:20 AM
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Here's a link to some testing I did on the 496.

http://fullthrottlemarine.com/496%20...%20Article.htm

Bob
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Old 03-24-2009, 10:42 AM
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Originally Posted by TWIN-SPINS View Post
know somone who put cmi headers on twin 496ho and maybe picked up 1mph,,,,sound and looks good???though
They sound great, they look great, on a stock 496 they don't do chit, its a waste of money. The 496 just doesn't need the extra cooling, and air flow.

Trust me it would be easy to prove me wrong, put it on the dyno with cast manifolds, then put the headers on, change nothing???
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