1991 21 scarab excel prop question
 

I have 1991 21 excel 3800lbs dry 454(330 hp) runing a 23 mirage with a little extra cup 62mph. Would like more cruising speed/top end 3550rpm at 42-43mph and 4700 rpm wot now any suggestions possible 4 blade 24-26? Thanks

Re: 1991 21 scarab excel prop question
 

Hey JP, I've got a 1990 21 excel, 330/454/B1 and have already been down this path. Your numbers sound like what I was seeing with mine. 1st change I made was to the exhaust, replaced the stock units with IMCO powerflows. Top end jumped to about 63mph with the stock 23 Mirage. Had more cup put into the 23 to carry the bow better and bring the rpms down. Changed to a 22' Hydro QIV and picked up cruise to ~45 mph. Have since found that my hull prefers the bravo 1 props so a 22' B1 would probably have been a better choice. Then made other changes, spark plugs, synthetics oils, adjusted timing, remove choke plate from Roch. Quadrajet, etc.. The final results were cruise 47 mph @ 3700 rpm and a top end of 70+ @ 5300 or 5400 rpm. Although I rarely ran it that hard, to many cast parts in that motor. I was mostly looking for a higher cruise like yourself. I have since removed the 330 horse and installed a 420 horse with a 174 B&M blower, @ ~535 horse. Cruise is now ~53mph at 3500rpm with 28p B1 and ~57mph with primaries at max opening and no secondary tip in. Top end is 75mph, but need a scoop, there isn't enough air flow in the engine compartment for the new motor. Mileage is also up compared to the 330 horse.
If your on a budget or only looking for a few miles an hour, the 330 can be made to perform better. Chuck

Re: 1991 21 scarab excel prop question
 

hi cdscarab... i have the same boat w/454 best plugs, oil, filter, ignition. i'm currently using a 23p quicksilver. What would be a fair budget up grade for it. exaust? prop? ect? Thanks. Ohh and a question for anyone...my drive switch goes down but not up...my trailer button goes up though...i have changed the solinoids and switch and still doesn't work. any suggestions?

Re: 1991 21 scarab excel prop question
 

hobie1, the cheapest bang/$$$ I've seen so far was the Imco exhaust. I would also go with a K&N flame arrestor now after seeing actual dyno runs with a stock unit and a K&N unit. Worth fully 20hp. Until that 454 can breath, everything else will be less effective @ improving performance. The plugs I used were the champion RV15YC4 extended tips, better starting and idle. Changed to 10-40 castrol synthetic oil, easier to find here. Check timing, mine was minus 4* intial, that helped. Changed to Amsoil synthetic drive oil, check lube threads for current best oils. Removed the choke plate and shaft from the Q-jet, you may not want to do that if you like a choke, but it did improve performance a little.
A very big gain was made by having my 23P Mirage cupped and tweaked by a friend of mine who runs a local prop shop, helped get the bow up & out of the water. This and the extra torque allowed the boat to get up on the delta pad these hulls have which allows it to run much faster.
You'll probably end up with $2000 tied up in parts, etc..
Probably the best improvement I've made was going to a blower motor. Only about 535hp, but the boat flat loves it and I'm actually getting better gas milage now than when I first got the boat. Just be forworned, a scoop, 91 octane gas, a bigger prop, and several thousand more $$$$ will be nesaccary. But thats boating, it never stops. Have fun, Chuck!!!