khubard

OK motor heads.. I need some input. I have a 89 Scarab 22 with a Magnum 350. I engine has had some but I do not no exactly what. I can tell you that it does have a cam in just by listening. The motor is tight and runs good. The top speed that I get at this point is 62mph. To achieve this I am really having to trim high and get the boat out of the water. My RPM's at 62 mph is 4600 rpm. I am running a laser 21 prop. I am thinking of pulling the Quadrajet and the stock intake and replace with a single plane intake and a Holley 650 cfm with a MSD distributor. My thinking on this is to increase horsepower on the top in speed. People have said to go with a mechanical secondary on the carb. I personally think I need a vaccum secaondary in fear that a mechanical will be to responsive and choke the motor down when going to WOT. Any thoughts would be great. Thanks for reading

scarabman

iTrader: (1)

If I were you I would post this in the Tech General Q&A as it has more to do with that than being a Scarab. You will find more people that know about your specific issue over there.

khubard

Ok thanks

boosted22

iTrader: (1)

was your speed done by the speedo or a gps? sounds a little hi. my 1995 22 with headers and a cam mag motor gps 56. to 57

the single plane will hurt your bottom end alot if your staying n/a keep the dual plane intake the stock mag is a good intake. but if you looking for top speed and dont care about the bottom end then go with the single plane.

i removed my dual plane off my motor as it is now procharged. btw the fastest prop todate on my boat is the laser it has more bow lift than the mirage + props the mirage was a little better out of the hole though.

if you have any question let me know pretty much done it all on my sbc in my 22.

khubard

I have a Laser 21 on now. The single plane I know will help on the big end but you are right on the low. Do not know how much I will lose. I figure I should be able to get 5300 out of that engine. A small block will take that with ease. Just not sure on how to get there

Baja226sport

iTrader: (28)

If your engine is not put together to make power at those rpm's why try to get it there?

Before doing anything to the motor, try a 19 pitch and see if the extra rpm's get you anything besides more noise....

khubard

I have thought about that just to see what happens....I understand what you are saying about the engine and I have thought maybe it has a low to midrange cam...On the other hand, it is not that strong out of the whole..I am thinking a mid to high range cam..I do know. Still learning all this about boats. I have played with hot rods for years. Have always been able to make a Small block talk in thr 5k's... This power boat thing is new to me..Still learning

Scarab CV-26

Your talking about two completely differences things, top end and out of the hole. I've been there, blew a load of cash. your first goal is to determine what is your goal. If its 75 mph... Play with this formula,

Speed determination= Pitch X RPM X .01
divided by Gear Ratio divided by 12.

You will learn what rpm you need to reach to turn X pitch with your current drive gear ratio. Your hull, load etc.. will change the outcome. But it gets you close. You currently have a lot of RPM to play with, test props before you spend a dime... And your right, this Power Boat thing is all about learning.

billy boats

First things first you need to check your speed. It is impossible to do 62 MPH with a 21 pitch prop at 4600 rpm . With slip factor you would be more like 52 mph.
As for changing intake manifold, single plane is only good for motors making power over 5200 rpm. Even at 5500 there hp gain is 5-10 hp. That gives you a MPH gain of 0 .

Most people new to performance boats think they can do a few bolt on mods and pick up speed. Sorry guys not that easy. It takes BIG Hp gains to see anything in speed and more important it take torque gains for a boat.
After the common things like exhaust , CORRECT prop, timing . The only way to get more speed is more cubic inches, more compression, supercharger.

Hollowpoint

For sure the speed was not by GPS. And usually, like motorcycles, the speed is optimistic by the speedo.

I suggest looking at an easier, cheaper fix. Most smaller Scarabs of that era were known to be built with a hook at the transom. Take like a 4ft level and place longitudinally on the hull. If you see daylight near the transom, it has a hook. Take out the hook and repair the gelcoat, you will get more speed with less trim. Much cheaper than engine mods. I would also install a nose cone before going into the engine. That may get you to a true 60-62 mph.

I had a very similar boat. 22 foot Chris Craft Stinger. Sold to a friend that put a 400+ hp 383 in it. With a 25 pitch prop spinning about 5600-5700 RPM, she did 77.3 on GPS