High Desert

I have recently purchased a 1989 Scarab III 340 and was wondering what I should expect for top end speed. It is mostly a stock boat with few modifications if any. With half a tank of gas and three on board, the dual 454 magnums turn about 4300 RPM with a top speed of about 48 on the GPS. They run strong, do not burn any oil and I assume they are stock but recently freshened since the hour meter says 1000 hours. The only thing I have done is have the props lab finished and worked over at Houston Prop. My experience is limited with the boat but I was hoping I could get some feedback on what others were seeing with stock power.

ernie davis

Gee, just a guess, but it seems you could turn those Mag's to about 5200rpm if you wanted to. Depends on what speed you want to cruise at I suppose.

High Desert

Thanks for the input. I do not know much about what to expect from the boat, which may seem obvious, and would really like to talk to an operator who has some history with this type and size of boat.

Once I get it to 5200 RPM what should I expect for top end speed? I was not sure how hard people were pushing these engines. What power do you set for cruise? What speed do you normally cruise? I just had the props lab finished but sounds as if I may have too much prop for the power at our altitude. I will talk with Houston Prop and see if they can adjust them and / or I will check to see if there is a pair in the classifieds.

Thanks for the information thus far.

ernie davis

Scarab3MDC on the board would be a better one to ask.
Just defected from Cig'ville. Scarab3MDC has a similar model as yours that is pretty hot. Will have to see if we can flush him out.

Gotta believe you could get 60 mph or so out of that combo.

cdscarab

A friend of mine has a 34 with 370s in it, turns 62mph at ~4800 rpm on lake ontario, about 250' above sea level. What altitude are you running yours at?

High Desert

I'm in Idaho at 4800 ft. Is there a rule of thumb for performance / horse power loss at altitude? I would like to talk modifications and performance strategies with those who know so I do not have to reinvent the wheel each time. Can you email direct to other members with a gold or platinum membership? What are the benefits to those levels of membership?

Thanks for your help.

Scarab3DMC

Thank Ernie, for the kind words...High desert...I answered your e-mail... call anytime... I'd keep those rpm's around 5k.. you should see low 60's....heres mine...

gotime34

High Desert, I have an '86 with 496's doing about 525hp each. I can get up around 70 in Long Island Sound. I usually cruise easy. Around 3000RPM's the boat hits a sweet spot and just crusies great at about 42mph. Then when I want to push it I crank 'em. I used to have 454's and was doing about 62 with 3 blade 23Pitch Props. I now have Hydromotive Quad IV's 4 blade 26" Pitch props that are made for a deep V hull like yours. The boat feels great with them on there. Gets out of the hole a little quicker too. I would say you could hit high 50's low 60's, depending on a bunch of other variables.

High Desert

I am looking for cruise and top end performance without trashing the engines. Since I am running a max of 4200 rpm @ w.o.t. I need to increase 800 RPM to get up around 5K. Can I and would I be better off to rework three blade props or go the four blades? It sounds like your happy with the four bladed props. What sold you on them?

What did you see going from three blades to four in mid / top end performance?

Scarab3DMC

quad 4's work the best on mine too..hi-desert..sorry I missed yuor call... try again tomorrow..Dave