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View Full Version : 350 Mag RPM - Need More???



PowerQuest 208 Viate
12-06-2001, 07:40 PM
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
12-07-2001, 12:27 AM
If larger engines are an option,put in a big block.If not,supercharge the 350 and upgrade the exhaust.

Tinkerboater
12-07-2001, 03:55 PM
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
12-07-2001, 08:16 PM
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
12-08-2001, 12:34 AM
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
12-08-2001, 12:53 PM
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
12-08-2001, 04:06 PM
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
12-09-2001, 02:25 PM
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
12-14-2001, 12:18 PM
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
12-15-2001, 08:24 PM
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.

mister460
12-18-2001, 12:03 PM
Awwww, quit bein' a sissy and BIG BLOCK it!!! You'll spend duckets on that small block and then wish you'd just dropped in a big block.

sea buddy
12-18-2001, 11:35 PM
You need to prop to 4,900 rpm for the ideal for this motor.
Try some testing. Move all weight aft, over the drive as best is possible. Then get the same weight forward over the bow eye as best possible.Depending on the results you may find your boat stern or bow heavy.
Where is the fuel tank? Cockpit floor, bow, or near engine?
Your rpm change to prop change is about normal. Usally plan on 200-400 rpm change per 2" in prop pitch if all else the same on both props.
Once you know if you want stern lift or bow lift you can try some props that go to give you what the boat needs.
By the way, I have seen different brand boats with warped bottoms (hook or rocker) and some that weighted over 700 lbs. more than what the factory listed as its spec. All this stuff makes a difference.

sea buddy
12-18-2001, 11:49 PM
I think you have got to get this boat to run better with a 21" prop.
If I were to guess, you need to get weight off the front and let it dance on the transom.
Ask the factory and bob's machine shop if this hull will gain 6-8 mph with one of those extention spacer drives that get the trim working the bow up more--should be just what you need. Try the weight over the transom test to prove me right or wrong before you spend money. Just get the weight way, way back there even if it is in a sily way for just the one test.
Also, take a 8' straight piece of metal and hold it fore and aft against the last 7' of bottom step by step from chine to chine. Gaps along the middle=hook. Gap or curve up at the transom=rocker.

sea buddy
12-18-2001, 11:59 PM
Your boat should gain 4-6 mph for every 500 lbs you can get off it, after you get it going as fast as should at this weight by getting it to dance on the transom and the bow carried out of the water.
Just think, if you could get 1,000 lbs off the boat, your current 62 mph will be 73 mph, without any other changes other than a 25" prop.
That's cheaper to do than a supercharger.

chevy_power
09-23-2008, 12:17 AM
:idea: There are a few cheap things you can do to get more power without getting into the lower unit. You could go with a little hotter cam or instead of paying alot of money for aluminum heads you could put a set of 305 heads on the 350 which will give you more compression and boost your over all horse power. either way you might want to make sure you are getting a clean open exaust. Don't forget about the basics such as new plugs wires and filters. Just a few ideas. Good Luck

Beer-30
09-23-2008, 07:34 AM
The easiest first steps would be a set of 1.6 roller rockers and then exhaust. You would easily see 40+ hp with these two items and even more with an ECM reflash.
Get the drive and engine on a good synthetic diet. You will get a mph or two right there.

TIMINATOR
09-23-2008, 08:09 AM
The biggest roadblock to speed with an Alpha drive is the exhaust. The Bravos are larger internally and have exhaust vents between the ventilation and spray plates. Bigblock risers(the 90 deg cast iron things on top of the manifolds)are 3 1/2" and will bolt on to your smallblock manifolds in place of your 2 7/8" units. Flow bench shows about 65% improvement in flow. Most years have the 2 7/8" risers going into a rubber adaptor that goes into the 3 1/2" 90 deg bends that go down to the outdrive. This deal with a short piece of 3 1/2" hose is a big help. The better deal is to just come off of the 3 1/2" risers and go straight out of the transom, you can mount the tips below the waterline for a quieter idle if you want. ANY engine mods will not gain much power without the exhaust being done first. I have seen 8 mph gain with the big risers and thru transom exhaust on a 24' Eliminator! If yours is a carburated engine, the installation of an Edelbrock Performer RPM AirGap intake and a hydraulic cam of 270 advertised duration .470 lift. That would be 220 degrees at .050 will usually net about 3-5 more MPH. If you don't mind a rougher idle, a 280 cam (230 @.050)will give an even larger gain. We did these mods in steps to the 24 and finally added Holley systemax aluminum heads too. Total speed gains on the 24 Daytona were phenominal! Top speed now is in the mid 70s!!!!! Remember that this is a picklefork tunnel, but it IS a 24 foot boat! Most peeps are quite surprised that the engine is only a modified 260/350 small block. Insurance is small-block cheap and the looks when the hatch is opened is great fun to see! call me for more info. TIMINATOR

Beer-30
09-23-2008, 08:47 AM
Good post.
I didn't notice the 1992 portion, so it should be carbed.