I was reading some of the messages about the 350 mag.I have a 21' scarab II and am having a new 350 magnum engine installed.it has an alpha1 o/d with a 21 pitch prop,so what kind of speed should i exspect?Also since i have exaust through the transom is there any truth to gaining more speed by removing the exaust bellows and blocking off the old exaust port to allow the water to flow thru the prop.any help would be much appreciated
I would have to say that with your set up around 62mph. Not knowing what year they built that boat in, it is realy just a hunch. As for the exaust, that will only hurt the performance of the boat. One of the first things people do to get more hp is to change the manifolds to a free flowing style.
Originally posted by thrillseeker:
...I have a 21' scarab II and am having a new 350 magnum engine installed.it has an alpha1 o/d with a 21 pitch prop,so what kind of speed should i exspect?
In my opinion, 55-60 MPH is probably realistic. If you can turn your 350 to 4,800 RPMs (w/1.5 ratio drive and 10% slip) you'll be at 57MPH.
Originally posted by thrillseeker:
Also since i have exaust through the transom is there any truth to gaining more speed by removing the exaust bellows and blocking off the old exaust port to allow the water to flow thru the prop.
I might be a little confused (nothing new). Through-transom means the exhaust is routed to exhaust tips out the transom, not the outdrive. I assume you have your exhaust still running through the prop?
In my opinion, with a stock 350 Mag you will gain little be removing the bellows and installing through-transom manifiolds and tips. You might pick up 200RPM on the top end, which is 1 or 2 MPH, and be alot noiser doing it.
That mod helps the engine breathe, and is only one piece to the puzzle. Bigger carb, intake manifold, heads, and cam are the other pieces that need to be addressed to help your engine breathe more freely.
If your talking about removing the bellows on stock exhaust, I would recommend against it. Little or no gain, and your exhaust gases would then be running over the drive.
Hope this helps.
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hey thrillseeker,i had the exact same motor package in a 21 ultra,it was a few years ago i think they have bumped up the hp on the new ones a little.my boat went 56 or 57.the guy who bought it from me put thru transom exhaust in it and it now goes 56 or 57 but he like's the noise.
56 -59 with a 260 hp merc, now labled a 240 hp at the prop. Add 2-3 mph for the magnum. Prop it to at least 4,900 rpm at light load.
To get the actual speed of your boat do the following formula:
1) Pitch of your prop X RPM
2) Gear ratio X 1056
3) divide 1 by 2
this number will equal your theoretical speed not including prop slip
In or order to determine your prop slip do the following formula:
1) theoretical speed - actual speed
then divide by actual speed
what the hell do I know anyway?
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