350 Mag RPM - Need More???

[PowerQuest 208 Viate]

I have a 1992 21' PowerQuest with a 350Mag (only 140hrs). It will only turn 4400rpm's w/LaserII 21P. When I put a friends 19P on it I can only get 4700rpm's & the speed drops greatly. Any suggestions on why this motor won't turn more rpm's? Motor is designed to rum 4800-5000 RPM'S!!!!

[DEL51]

If larger engines are an option,put in a big block.If not,supercharge the 350 and upgrade the exhaust.

[Tinkerboater]

The problem is the size and weight of the boat with only 260 HP. That is not a light boat. If you have an alpha drive you wont be able to put a big block in unless you change the drive. I would suggest a (small) supper charger for up to about a maximum of 400hp that is the maximum hp that alpha drive will take if your carefull. You might want to use the centrifigal type supper charger that bolts to the front of the motor. This type doesn't creat the high tork that will destroy the drive on take off from a dead stop.

[PowerQuest 208 Viate]

I'm looking for small tweaking tips! SuperCharging isn't what I want to do. I would rather put money into heads. But I'm thinking with the 19P that it should have ran 4800-5000 rpms. It knocked 10mph off the top speed! Weight with motor = 2800 lbs -PowerQuest.

[DEL51]

Supercharging is the cheapest way to add power.If you want to do the heads,there are so many nice choices it is hard to pick out a single manufacturer.Edelbrock has a system approach that eliminates mismatching engine components.Call them and tell them what you would like to do. I bet they would guide you in the right direction.I have to say that good flowing heads need a good exhaust,or the system will back up and reduce efficiency.Do you intend to rebuild the engine or do a bolt on project?

[Tinkerboater]

Have you checked your tachometer for accuracy. You might be turning more RPM than you think. What speed were you doing with the original prop and with the 19 pitch prop- are those RPM's with a light load and half a tank of fuel or less with the drive trimed up for maximum speed. also does your boat have thru-hull exhaust or thru the prop hub.
[This message has been edited by Tinkerboater (edited December 08, 2001).]

[PowerQuest 208 Viate]

21P Prop 4400 rpm's -62 mph 3/4 tank 3-people
19P prop 4700 rpm's -55 mph 3/4 tank 3-people
Yes trimmed up & thru hull exhaust!
I called PowerQuest to see what information they had on this boat & they said that the 21p was what was recommended but they didn't have any other information on what speed or rpm the boat should do. I did ask them about the tach but they said it could be off as much as 250rpm but that they use a pretty accurate tach. I need to verify the tach & mph.
I want to bolt on! The motor only has 140hrs! Compression is good. Need to do leak down.

[Tinkerboater]

Let's see 3 people 500 to 600 pounds, 3/4 tank fuel 250 to 350 pounds, 21 ft boat, 260 HP . The numbers look about right. You need to either lighten the boat or increas the HP. Your best bet is still the front mounted super charger ( most HP for the buck ) If you dont go nuts on boost you can leave everything else stock including the exhaust. You will have to run at least 93 octane fuel(you should already be doing that-its cheap insurance) Just adding new exhaust will gain you about 20 HP max, installing a new intake or carb without the exhaust wont do much of anything. If you want to gain 100 HP without the blower you will need to do everything (exhaust,heads,cam,intake and carb) very expensive. it takes about 17 HP to make your boat go 1 MPH faster. One other thing with the blower your engine will idle like it isnt there and will start like it does now. With all those add ons you will change every thing about the way the engine runs.

[CV-23]

I think your numbers sound right. I'm running a 350/260hp with a 19P/14" Stainless prop and I'm running 55mph on GPS. My boat is a 23'Glastron/Carlson (3,000lbs with average load). It has a shallow Jet Boat V bottom. The 21P is working great on your boat and I would stay with that. If you want more RPM/Speed, reduce the load. Your engine is set-up for 4400-4600rpm anyways if its Factory. The 350Mag is rated at 270hp and the 21P is "the" prop....Mike.

[powerplay230]

If you are going to stick with the 350 block and Alpha drive your best bet might be to check your tach, make sure its tuned right and settle with whichever prop gives you the best overall performance and handling. Remember the Alpha drives don't like a lot of power and unless you go with the blower its doubltful you will pick up more than 30 to 50 horse which might only pick up 3 or 4 mph. Just my experience changed from a base 454 to a 502/502 and only picked up 12 mph on my powerquest. Good luck with whatever you choose.