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  1. #31
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    Conrad,I agree completely.The comment was not aimed at you at all,but to rbt and patches,who seem to express a stock light flywheel is not going to be improved on...not putting words in their mouths...but that's how I read their comments.Your comment actually backs up mine...energy has to be spent in either case ,speeding up or down,Chris

  2. #32
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    https://www.youtube.com/watch?v=iDtb-QNeekw

    I love this video. Really shows the how the RPM flows under launch. When he launches the light flywheel may give you a benefit when the motor snaps from 2000RPM to 5500RPM until prop hook up. 5500 RPM to 10,000 RPM is much slower of a curve.

    It may win you a drag race I guess if all things are equal and your motor took 1/100th of a second less time to snap from 2k-5500 then your competitor you win by 1/100th of a second

    Lets say the average speed is 40 MPH during the 2k-5500k spool up event(until hook up) that occurs 1/100th of a second faster than the other guy. This would put you 7 inches in front of the other guy on launch if I did the math right. .

    C
    Last edited by milkdud; 09-27-2014 at 11:38 AM.
    L6fan57-88

  3. #33
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    Matter has weight,matter requires energy to put it into motion...the heavier the object the more energy required.If the same amount of energy is spent moving two objects of different weights,the lighter of the two will accelerate faster and attain a higher speed than the heavier one.Equal forces of friction will also slow the lighter object faster.This is simple science,and not having noticed is no excuse to deny the existence of basic properties of physics.
    The earlier reference to the accuracy of dyno repeatability was simply to note that there are many more less controlled variables encountered on a boat with water conditions,propeller,driver input,etc than in a more controlled environment with a powerhead mounted on a dyno,so I'm not surprised a small change might go unnoticed.

    Well said chris

    I did the lightened flywheel to my optimax motor. The results is a night and day difference with lightened flywheel. Now as far as no noticeable difference IMO depends on the motor it self. Ex 2.5 260hp 280hp. These motors was engineered to squeeze every bit of power out of them from mercury so any changes might go unnoticed for 1hp-3hp gain or power curve. Now the dyno will prove you right or wrong if it made a difference.

  4. #34
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    Guys, I'm not trying to argue with anybody. I understand the theory behind it all completely. I'm just telling you the real world and theoretical don't always agree. I'll say again that the light flywheel does have merit but from a longevity aspect versus a performance gain. There was no measurable difference in my boat and I can measure to the hundredth of a second. This isn't new and I'm not the first to notice. I'll see if I can find an old thread about the subject. Btw, the same can be said about lightened rotating assemblies. All you're doing is weakening the rods and pistons
    Erik Kiser

  5. #35
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    But Eric, ya gotta admit they really look cool ona "straight line" motor..........

  6. #36
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    Eric if I understand what you are saying,Mercury got it pretty damn close in the racing div,and the little mods help,but it its up to the tuner- driver

  7. #37
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    in reality...I replaced my stock flywheel on 2.5 carb 200 offshore merc on a 19' ACTION with a light allum flywheel from a inline 6, along with the starter (diameter of wheel is less) bumped comp useing 18787 heads results ... very noticable diff...motor was very snappy, felt like EFI, very responsive in the pedal...accelerated RIGHT NOW, however no diff in top end numbers, another advantage was when you let off the nose came down softly, felt wonderful, the only downside I felt was the motor was a little harder to spin over initially on starting, but the comp could cause some of that...boat ran 90-91 @ 6600-6800 w/ 4-blade MAZCO 28p 19' ACTION MARINE...miss that boat alot...

  8. #38
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    Here's Wayne Tripps finding on the subject. Real telemetry tells a different story then the old seat of the pants dyno(horribly inaccurate)

    http://www.screamandfly.com/showthre...light=Flywheel
    Erik Kiser

  9. #39
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    The laws of physics say a lighter object being accelerated uses less power, you cant change that.
    Whether you can measure that change is something else.

  10. #40
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    Like Erik said, real world and theory. The answer is that the gain on a 2.5 is not measurable. The same can be said about light parts (rods/pistons). No measurable gain, the only thing measurable it that the parts do not survive as long as un cut.
    Dynos and telemetry are not your friend. They give you the truth.
    2023 TUFF 25

  11. #41
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    Let me throw this out there...

    On my drag bike, my bike was quicker with a heavy crank because the motor had more inertia when the clutch was dumped and that big slick tire hooked up. The light crank would lose rpm. The heavy crank would hit that tire harder and launch. 60' times were better with a heavy rotating assembly.

    You might think this doesn't apply to a boat but...maybe it does. The prop acts like a clutch at launch slipping until it hooks up. Maybe the heavier rotating assembly carries rpm better and transfers more to the prop when it hooks up than the lighter assembly. The lighter assembly will pull a little harder after it hooks up. At the end of the track the difference isnt measurable?

    I really notice on my little motor when I put a BIG prop on it. I can blow the prop away getting up on pane then a few seconds later when it hooks up, it will bog the motor down and pull very slowly. It will go from 5k rpm to 2500 when the prop bites. I heavier flywheel might keep my little motor spinning at 3500 rpm when it hooks up where it would stay in the powerband. Of course mine is an extreme example but on a much smaller scale the same could be happening in a drag boat balancing things out at the end of the track.
    13' Biel tunnel AKA "Flight Risk"
    13" Modified Yamaha V4 - 101 mph

    21' Paramount
    Mercury 300 Promax

  12. #42
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    Great point Demeanor but with our telemetry we can check any spot on the run. I typically check 60'- 330'- 660'. The gains from lightening a 2.5 rotating assembly just aren't there in our boats. Keep in mind we have relatively light race boats, if it doesn't show up with that I definitely don't see it picking up seat of the pants acceleration in a boat twice as heavy. In my opinion, there's a major placebo effect going on cause I've seen time and time again that the butt dyno sucks. Good telemetry can be a real heartbreaker
    Erik Kiser

  13. #43
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    On are small nitro eng a heaver rotating assembly will carry the boat threw the conner better. The inertia helps maintain speed. It takes power to get weight spinning . But as Newton stated a mass tends to stay in motion until it is reacted upon.
    I put a light fly wheel in my GTO with a 455 in it. It would spool up quicker but would chirp the wheels on letting off the gas when in first and second gear.
    I guess it is all in what you want to do with the boat.
    I guess in a light boat with non ratcheting lower, a light fly wheel would be a hand full when getting of it at top speed?
    Great thread lots of good info here to learn from.

  14. #44
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    I have always been a believer that it is easier to go on a 1-10 scale from 1-5 than 5-6..6-7 etc take a stock 2.5 MERC fishin motor steel bore, consider it a 1 on the scale, if it is before 95-96 year you need to open up exhaust chest (after 95-96 MERC did it from the factory) some mild porting, good carbs with bigger jets, offshore mid w/ one piece plate, a decent tuner (I used BOB"S tuner) light flywheel, stock switch boxes, blue wires, nothing special just synthic oil, stock pop-it vaulve, restricter washers not thermostats, low water pic-up BOB"s gearcase ( faster than CLE ) a good set-up , good prop...if you can't feel buy the seat of your pants DYNO a noticeable improvement ( 15 + mph) no dyno needs to tell you anything...I ran my motor for two years hard and never had a issue and couldn't be happier with the performance and reliability of a low cost build, where my EFI buddies were poppin powerheads every weekend, windows here windows there...ouch!! now you guys with much more experience than me & my 75 year old machinist, and alot bigger budjet can find the hp from 8-9 or 9-10 it doesen't come cheap...I'm just sayin when I bought this boat it went mid 70's, after a year of trial and error, we got it to 91 mph, you can deffintly feel the diff...sold boat and the new guy (in ATLANTA ) ran the boat another year hard before having any issues... keep it simple stupid seems to work...IMO the 2.5 MERC is the best budget motor on the planet.10-4 thats what a couple beers will do for you...no offence PEACE !!

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Chris Carson's Marine