Boating content well motor anyway

[h2o225]

Question for you guys. Replaced a 7.4 with a 509 in my Crownline. Gained 150hp boat runs great. Have 21p prop on b1.
With the 7.4 would turn 4800rpm run 48 on gps. Now run 55 @ 4200rpm. 4400 rpm is all i get trimmed up. Engine was dynoed to 5300.
What else should I look for to get rpm up maybe lil more top end?
Have a great hole shot now.

 

[lakemadness]

Have you switched props since the new motor? Something is not right. Have you verified that your tach is accurate and working correctly?

 

[h2o225]

Tach is accurate used 2 different aux tachs they red same as one onboard., same prop that ran with old motor. Had gimble done this week dont know if that will help or not.

 

[kevnmcd]

If you are using the prop you had with your 7.4 and got 48 MPH at 4800 rpm's, there is no way you are getting 55 MPH at 4200 rpm's. There has to be something else changed. If you are using the same prop then you should be sitting on the rev limiter in no time. You should be able to push at least a 24p or even a 26p with an extra 150 hp. I would try a different prop. Are you using an aluminum or stainless now? I would definately go stainless as the aluminum ones tend to flex too much. Just my .02

 

[fishing fool]

I know it's simple but check and see if the throttle cable is opening all the way up.

Check fuel pump, are you running out of fuel?

If you have a great hole shot the motor is working fine on the low end.

 

[h2o225]

Stiletto 21p stainless. it doesnt make sense your right. 11pounds different in weight of motors. 150 more hp.
If I do try a 24 or 26 wont rpm drop again?
Dont get me wrong from reliablity stand point not working motor to hard or lugging it either

 

[Cheaper To Keep Him]

I know it's simple but check and see if the throttle cable is opening all the way up.

Check fuel pump, are you running out of fuel?

If you have a great hole shot the motor is working fine on the low end.

Id Be Checkin In This Area As Well As Ign Timing...Way Different Under Load

Hope This Helps

 

[Havasu Hangin']

Could someone have switched your lower gearset ratio?

With a stock 1.5 ratio, you had about 25% slip before...but if you have 1.36 gears now, you have 14% slip- makes sense.

You currently have 5% slip if you have 1.5...which I highly doubt, so the gears are the only unknown.

 

[h2o225]

Will check gear ratio when i pick boat up in few hours.

 

[Deano]

If it is the same prop as before, then there is no doubt that the gears have been changed.

 

[fishing fool]

I'm confused????
Why do you guys think the gears were changed? All he said was he changed motors and not getting the rpm's out of the motor, nothing about the outdrive.

 

[h2o225]

Checked linkage get full throttle. Still have 1.5 gear ratio. 750 cfm carb (mercruiser) with 63 rod I believe. The guy who built motor is confused also. End of march he is going to Laughlin with us to see it run maybe get an idea or two.

 

[boatnam2]

Maybe the motor is making all its power in the low RPM range, what RPM was the extra 150hp.

 

[Its good to be King]

correct me if I am wrong, BUT if he didn't change gearing or prop 4800 RPM doesn't change anything even with 150 more HP. more HP means more prop if he didn't change size or pitch he would not see any change in speed. he did say crownline I think. I'm sure that boat does have some speed limitations built into the hull, hook,strakes etc. they were designed for ride not speed.

 

[Fun Times]

750 cfm carb (mercruiser) with 63 rod I believe. The guy who built motor is confused also. End of march he is going to Laughlin with us to see it run maybe get an idea or two.

Did he or didn't he need to modify the carb in one way or another to run with your new engine size?

Sometimes you can't just bolt on any old carb and hope for the best.

 

[fishing fool]

FUEL..... Is the motor getting enough fuel under load.

Buddy had a fuel pump issue where it was not giving the motor enough fuel (only under load at full throttle), had to change the pickup at the tank.

Check fuel pump and pick from tank, make sure the motor is getting all the fuel it needs...

When on the dyno how was the motor getting fuel?

This is the first place and simple place I would be looking id the ONLY thing you did was change motors.

 

[h2o225]

well next thing to look at is fuel delivery set up. Yes crownline does have speed limitations buit into the hull but the 225 should still be able to turn 60 +. Even Though the wife says we are plenty fast now.

 

[28Eliminator]

Question for you guys. Replaced a 7.4 with a 509 in my Crownline. Gained 150hp boat runs great. Have 21p prop on b1.
With the 7.4 would turn 4800rpm run 48 on gps. Now run 55 @ 4200rpm. 4400 rpm is all i get trimmed up. Engine was dynoed to 5300.
What else should I look for to get rpm up maybe lil more top end?
Have a great hole shot now.

Something else has changed, this makes no sense. check the prop, gear ratio, and tach again. Your Boat did not run 48 at 4800 rpm before, and now goes faster turning less rpm's, with the same prop.

 

[Gelcoater]

I know this may sound stupid,but.I am assuming these runs before/after the engine swap were run at the same lake/river/body of water?Meaning no signifigant changes in altitude?

 

[boatnam2]

Well they say the rule of thumb is 150-200hp to gain 10mph, so you already gained 7 on a vee not built for speed with gps verified speed, but gauge verified RPM of 4400 at WOT so not sure whats up..I could see fuel as maybe a problem but it dynoed fine with your mechanical pump and made the horsepower with a increase of 150hp all though still dont know at what RPM it made it at.

 

[boatnam2]

Could be fuel, could be electrical, could be conditions but bottom line is your seeing the increase that is close or pretty close to be expected in a boat of your type with the amount of increased hp.

 

[Letzgo]

Could be fuel, could be electrical, could be conditions but bottom line is your seeing the increase that is close or pretty close to be expected in a boat of your type with the amount of increased hp.

It does not matter on HP if you dont either turn more RPM.s or a bigger prop. What he is stating is not to be expected. It's impossible actually.

 

[Letzgo]

A thousand horse power moter turning the same prop on the same boat with the same gear ratio would not be any faster. in fact probably slower due to wieght.

Treadmill/plane?

 

[boatnam2]

It does not matter on HP if you dont either turn more RPM.s or a bigger prop. What he is stating is not to be expected. It's impossible actually.

Exactly, so it could be the GPS verified speed of 55 is correct and the killer holeshot he is feeling is spot on but the RPM could be wrong and he is actually turning 5200-5300. I understand it is impossible to do what he has said is happening, I even thought it was a joke to get everyone to post there shade tree mechanics remedies to a unexplanable problem.

 

[Letzgo]

Exactly, so it could be the GPS verified speed of 55 is correct and the killer holeshot he is feeling is spot on but the RPM could be wrong and he is actually turning 5200-5300. I understand it is impossible to do what he has said is happening, I even thought it was a joke to get everyone to post there shade tree mechanics remedies to a unexplanable problem.

Agree 100%. Then it makes sense. surprised he isnt hitting his rev limiter but that hull is only going to create so much speed.

 

[fishing fool]

A thousand horse power moter turning the same prop on the same boat with the same gear ratio would not be any faster. in fact probably slower due to wieght.

Treadmill/plane?

How much more does a 1000Hp motor weigh over a motor with only 500Hp??????

 

[h2o225]

The only unknown to me has been the in ability to turn the rpm higher. The mechanic has built tons of boat engines been in bussiness almost 40 years with their shop. He is confused also. That is why we will work on it end of month when in Laughlin.
Not trying to get shade tree fixes just some other ideas why.
We are not hitting rev limiter. Yesterday he ordered another carb to swap to see if that may be problem.
Not looking to over work motor just trying to figure out why not able to hit higher rpm range.
Hope to have answer end of month.

 

[Letzgo]

How much more does a 1000Hp motor weigh over a motor with only 500Hp??????

I never quoted a 500 Hp comparison. I was going off what was his original set up was which I thouhght was a small block. but to answer your question maybe 100 plus pounds or so depending on the type of motor you build. maybe nothing because of enternal chages as well. They are not all the same weight when you have to add componets.

 

[C-2]

The only unknown to me has been the in ability to turn the rpm higher. The mechanic has built tons of boat engines been in bussiness almost 40 years with their shop. He is confused also. That is why we will work on it end of month when in Laughlin.
Not trying to get shade tree fixes just some other ideas why.
We are not hitting rev limiter. Yesterday he ordered another carb to swap to see if that may be problem.
Not looking to over work motor just trying to figure out why not able to hit higher rpm range.
Hope to have answer end of month.

Stumped too by the numbers.

What ignition are you using, the Thunderbolt Merc system?

I believe on the 454/502 Mags they SAY the rev limiter is set at 5200, but I think there's some module in there which never lets you get past 4900/5000. I think they used the V6 modules to get around the rev limiter, maybe the rev limiter is set different on your motor.

OSO has a great tech section for these types of questions. Search it out using googkle:

"enter question here in quotes" siteffshoreonly.com . (this message boards adds a smiley the search is simply an advanced operator site:website name

That's just an advanced google search but searching forums using google is easier and gives better results than using the website search crap. Remove the quotes or also use "+" sign to include addiotnal keyword terms.