21 lightning with 350 mag mpi - engine upgrade options
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21 lightning with 350 mag mpi - engine upgrade options

  1. #1
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    Default 21 lightning with 350 mag mpi - engine upgrade options

    I have a 2004 21 lightning with a 350 mag mpi & bravo 1 setup. I'm looking for options to get more speed out of the boat with possibly an engine upgrade. The current motor only has 200 or so hours on it and runs good but just not fast enough for me anymore. I played with alot of props with the help of Brian at Valley prop in Ventura and I'm where I need to be RPM wise (4800 - 5200) 17 - 19 pitch. The best I've seen out of the boat was 55mph with 3 people and low on fuel. Most of the time I run around with 5 people, somewhat loaded down, a big stereo system and hit about 51mph.

    What are my options with wanting to stay turn key and reliable? I've heard about whipples and also had some people mention the 383 strokers? I'd love to see the boat run 65 - 70 or more.

    My mechanic has looked over my current motor many times and says there is absolutely nothing wrong with it. I just need more horsepower I guess to turn a bigger prop to get more speed?

    I talked to one of the builders of these boats (John West) and he said I should be able to run a Merc High Five 21p all day long at 60mph. I tried this prop first, could only pull 4200 RPM at 50mph and it was a dog out of the hole.

    I have not had the engine put on a dyno, I don't know about place around me (Ventura County, CA) that has one.

    Any advice without breaking the bank?
    Last edited by Joey805; 12-26-2012 at 12:00 AM.

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    Senior Member MACHINEHEAD1's Avatar
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    My buddies 23' Stealth ran 56-57 half full of fuel, big stereo and 2 people. 21 Bravo. 45-4600rpm. Check your x-dim. May be a little deep. Check your trim limit. Sometimes the new owners will spec. to the builder that they want to run fully loaded and water ski and stuff. The builder will put the drive a little deeper to ensure a good handling boat. On the other hand, if you spec. performance and can stand a little prop blow out occasionally, the drive will be moved up. As was the case with my buddies 23 Ultra. He also got tired of the mid-50s. We went over options. One of which was going to lightning headers with dry pipes and a Carburetor. Also adding a Low lift higher duration camshaft and conventional ignition sys. This will put you in the 360-375HP range with the heads you have. We ended up building a 498 BBC. 599HP. Swung a labbed bravo 15" X 28" to [email protected] 86MPH. It was a handfull, hyd. steering and all. Call the manuf. to find the safe limits of your boat.

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    New here Beer:30's Avatar
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    Exhaust is your first step. You have to be able to get rid of the waste before you can add more of the good stuff. Free-ing up th exhaust will give 10-15 easy right off the bat.

    Before or after that, you can add a set of roller-tip rockers in 1.6 ratio. This will increase the valve lift by around .030. In dyno tests, they showed that most of the hp lost in friction at the rockers is the stamped rocker against the valve tip. There wasn't much improvement between full-roller and roller-tip. So, there is another 15-20 right there. Possibly a bit more with exhaust included.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Thanks for the great info guys. I think I may start with exhaust and go from there.

    Is the general consensus agree that lightning headers are the way to go on the 350 mag mpi? Or are there better? What do you guys recommend?

    The engine and through transom exhaust is totally stock.

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    Did a 383 several years back in a 21 Scarab. Dart Iron heads, victor jr, mild hyd roller cam, Lightning Headers, 850 carb, I can't remember exactly but it would turn 5700 rpm. Problem was an alpha drive and it would not take it. Engine made 425HP and 450#/ft.
    Chris Straub
    Straub Technologies

    3HP is an A$$ Whooping!!! JW

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    New here Beer:30's Avatar
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    Quote Originally Posted by Joey805 View Post
    Thanks for the great info guys. I think I may start with exhaust and go from there.

    Is the general consensus agree that lightning headers are the way to go on the 350 mag mpi? Or are there better? What do you guys recommend?

    The engine and through transom exhaust is totally stock.
    I have Lightnings, but any of the aftermarket brands offer great exhaust - either small or big block.
    Quote Originally Posted by gn7 View Post
    EFI is the wave of the future. There can be no denying it. Electronics have been on the leading edge of our entire lives. Not only os the magneto dead, but the standard issue CDI is wavering. Its all about total fuel, air AND spark control. Anybody that thinks its not has their head up their ass.


    2001 SleekCraft 30' Heritage SSB, open-bow mid-cuddy. 496HO / Bravo-I.

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    Senior Member MACHINEHEAD1's Avatar
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    Your cam is .431/.451 lift. You CAN NOT put 1.6 rockers on your factory Vortec heads. Seal to retainer clearence is the issue at .460". Thats why I had stated earlier about a LOW LIFT HYD. Roller. Run the Lightnings with water entering the exhaust stream at the tip. "wet tips". I know where the exhaust exits on those 21s. Not much "fall" from the riser. I have done this before. If you need 375HP and want to swing a 24" bravo. I have the combo, and have shared it above. Cstraub has another viable combo. He may be able to get you a LOW LIFT roller to fit your needs. A high accel. rate/ max area type lobe is going to need a valve spring change. Your stock springs are prob. about 80 lbs on the seat by now. Straub can set you up with a drop in set in the 110 lbs range with no machining needed. Dont know how your EFI will handle these mods. Put a box stock 750 Holley Marine carb on it, and forget about it. Besides, who is mod-ing MEFI? for 350s anymore. If you keep the EFI, good luck, and be sure to put it on a dyno with the Mod-ed MEFI. Sell your EFI on Ebay to trade for your carb and intake. Beer30 might buy it! Easy Beer, J/K

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