Accurate Horsepower Figures

[BamaBowrider 208]

I am trying to determine an accurate or realistic horsepower rating for new prop selection. My boat is a 2007 Larson 208 LXI, originally came with the 5.0L Mercruiser 2bbl TKS. Searching around I find HP ratings for the original setup from 220 - 270. Original owner did not drain the engine one winter and cracked the block. Replaced it with a 5.7L Vortec Gen1 880 casting longblock. All accessories including the 2bbl TKS carb and intake manifold were swapped over to the new engine. I have contacted the engine builder who states that the current setup should be 300HP.

How accurate does that rating sound???

 

[Bondo]

I am trying to determine an accurate or realistic horsepower rating for new prop selection. My boat is a 2007 Larson 208 LXI, originally came with the 5.0L Mercruiser 2bbl TKS. Searching around I find HP ratings for the original setup from 220 - 270. Original owner did not drain the engine one winter and cracked the block. Replaced it with a 5.7L Vortec Gen1 880 casting longblock. All accessories including the 2bbl TKS carb and intake manifold were swapped over to the new engine. I have contacted the engine builder who states that the current setup should be 300HP.

How accurate does that rating sound???

Ayuh,.... With a 2bbl carb,..... 250/ 260 hp,....

 

[dubs283]

Correct prop selection requires patience and time. Plus a few props if you're serious

First prop I'd try on your setup is a 19 pitch three blade aluminum. Fairly ubiquitous prop used considering your boat/engine and running gear

Adjust accordingly for desired performance

As far as horsepower your engine produces the only true method is a dyno. I'm with Bondo at the 250-260 range considering the probable vortec long block w/2bbl carb

 

[jimmbo]

I am trying to determine an accurate or realistic horsepower rating for new prop selection. My boat is a 2007 Larson 208 LXI, originally came with the 5.0L Mercruiser 2bbl TKS. Searching around I find HP ratings for the original setup from 220 - 270. Original owner did not drain the engine one winter and cracked the block. Replaced it with a 5.7L Vortec Gen1 880 casting longblock. All accessories including the 2bbl TKS carb and intake manifold were swapped over to the new engine. I have contacted the engine builder who states that the current setup should be 300HP.

How accurate does that rating sound???

Well the 2bbl for a 5 liter is likely not to get the best power out of a 5.7, plus the Relationship between the Throttle Plate and the Off-Idle Transfer Slot was located for the expected Air Flow of a 5 liter, rather than a 5.7.
The larger engine will result in result in greater Pressure Drop(Intake Manifold Vacuum) at WOT, which can have consequences, like a Power enrichment system Leaning out if the Manifold Vacuum is high enough at WOT. The Restrictive 2bbl Intake might save the day though as it will help strangle the Engine. Around 240-250hp would be more accurate.
A small 4 bbl, good 4bbl Intake and 1.6 Rockers would be needed to get you closer to the 300hp mark.

My 5.7 would pull 4 -5 Inches of Vacuum at WOT with the 500cfm 2bb Holley(when flow rated like a 4bb, 370cfm) on a very good 4bbl Intake. Switching to a Holley 650cfm Spreadbore, brought the Vacuum down to 1.5". The Secondaries were not fully opening as 650cfm was still a lot more than a 5.7 needs at 5000r rpm. I gained between 5 and 600 rpm and repropping gave me another 6 mph> Engine was rated at 260, so the extra 6mph was what would be expected with an increase of 30hp

 

[racerone]

Hurry----Find a shop with a marine dynamometer.----Pay the pennies to hook up your motor .----It will test actual power output of your engine !

 

[jimmbo]

Hurry----Find a shop with a marine dynamometer.----Pay the pennies to hook up your motor .----It will test actual power output of your engine !

Every marine dyno I have seen really can't provide a HP number only a Pressure Reading, which can't really be charted or plotted as a graph. Maybe some New ones run through a Computer can

 

[racerone]

Just find a shop that has one.-----I have one.----You read oil pressure and RPM to determine power output from a chart.

 

[BamaBowrider 208]

Correct prop selection requires patience and time. Plus a few props if you're serious

First prop I'd try on your setup is a 19 pitch three blade aluminum. Fairly ubiquitous prop used considering your boat/engine and running gear

Adjust accordingly for desired performance

As far as horsepower your engine produces the only true method is a dyno. I'm with Bondo at the 250-260 range considering the probable vortec long block w/2bbl carb

I've been running the prop that came with her, a 14.25 x 21P Black Max.......but I have started another thread discussing the prop selection over in that section. Just need the HP figure to help with the calculation.

 

[BamaBowrider 208]

Well the 2bbl for a 5 liter is likely not to get the best power out of a 5.7, plus the Relationship between the Throttle Plate and the Off-Idle Transfer Slot was located for the expected Air Flow of a 5 liter, rather than a 5.7.
The larger engine will result in result in greater Pressure Drop(Intake Manifold Vacuum) at WOT, which can have consequences, like a Power enrichment system Leaning out if the Manifold Vacuum is high enough at WOT. The Restrictive 2bbl Intake might save the day though as it will help strangle the Engine. Around 240-250hp would be more accurate.
A small 4 bbl, good 4bbl Intake and 1.6 Rockers would be needed to get you closer to the 300hp mark.

My 5.7 would pull 4 -5 Inches of Vacuum at WOT with the 500cfm 2bb Holley(when flow rated like a 4bb, 370cfm) on a very good 4bbl Intake. Switching to a Holley 650cfm Spreadbore, brought the Vacuum down to 1.5". The Secondaries were not fully opening as 650cfm was still a lot more than a 5.7 needs at 5000r rpm. I gained between 5 and 600 rpm and repropping gave me another 6 mph> Engine was rated at 260, so the extra 6mph was what would be expected with an increase of 30hp

What type of consequences?
What would be the acceptable vacuum range to run in at WOT with this setup??

 

[BamaBowrider 208]

Ayuh,.... With a 2bbl carb,..... 250/ 260 hp,....

Thanks.......I'll go with the 250 figure

 

[QBhoy]

I’d say about 250/260. A late 90’s 5.7 (presumed a vortec) with a 2 barrel was about 250, from memory. Quite strangled feeling too, I thought. Even compared to an older non vortec 4 barrel at about 260hp. If you are going to the trouble you are…I’d be tempted to look for a manifold and 4 barrel carb for her. Then you’ll maybe get closer to your 300, if you let her see 4800 or a little above rpm.
All that said…better to prop a boat to what it is real life…rather than HP and guesswork from previous or different engines maybe. What ratio is the outdrive? Guessing not a 1.47 for sure. If she ran a 21” previous.

 

[BamaBowrider 208]

I’d say about 250/260. A late 90’s 5.7 (presumed a vortec) with a 2 barrel was about 250, from memory. Quite strangled feeling too, I thought. Even compared to an older non vortec 4 barrel at about 260hp. If you are going to the trouble you are…I’d be tempted to look for a manifold and 4 barrel carb for her. Then you’ll maybe get closer to your 300, if you let her see 4800 or a little above rpm.
All that said…better to prop a boat to what it is real life…rather than HP and guesswork from previous or different engines maybe. What ratio is the outdrive? Guessing not a 1.47 for sure. If she ran a 21” previous.

Outdrive ratio is 1.62

 

[Scott06]

Outdrive ratio is 1.62

If the boat was a 5.0 originally most 5.0's I've seen take a 1.62. Only higher HP like MPI in lighter hulls seem to get the 1.47 drive.

Agree with other HP assessments. Your original 2 bbl 5.0 would have been 220 hp. Your 5.7 with MPI or 4bbl carb would be 290 /300 hp. With a 2 bbl 250/260 is right.

Most listings state a hp rating with the caveat depending on OEM calibration... Merc made reman Alpha 357s and Bravo 357's. both remanufactured 5.7s with 4 bbl carbs. The alpha version was rated at 275hp, bravo I think 325 (both flywheel not prop hp). I believe only diff was the cam maybe ignition module too.

Either way a 5.7 2 bbl , 4 bbl or mpi is perfect for that size boat

 

[BamaBowrider 208]

If the boat was a 5.0 originally most 5.0's I've seen take a 1.62. Only higher HP like MPI in lighter hulls seem to get the 1.47 drive.

Agree with other HP assessments. Your original 2 bbl 5.0 would have been 220 hp. Your 5.7 with MPI or 4bbl carb would be 290 /300 hp. With a 2 bbl 250/260 is right.

Most listings state a hp rating with the caveat depending on OEM calibration... Merc made reman Alpha 357s and Bravo 357's. both remanufactured 5.7s with 4 bbl carbs. The alpha version was rated at 275hp, bravo I think 325 (both flywheel not prop hp). I believe only diff was the cam maybe ignition module too.

Either way a 5.7 2 bbl , 4 bbl or mpi is perfect for that size boat

Original owner provided the sales paperwork with the serial numbers, the current outdrive is the original one. Engine runs great and moves the boat with no issues except getting on plane takes a little longer and more rpms than I would like. Hoping that the correct prop change will make the difference.

 

[Bondo]

Original owner provided the sales paperwork with the serial numbers, the current outdrive is the original one. Engine runs great and moves the boat with no issues except getting on plane takes a little longer and more rpms than I would like. Hoping that the correct prop change will make the difference.

Ayuh,...... That's not how it works,......
You pick the prop that allows the motor to make it's required rpms, usually as near to 4800 as possible,.....
That gives you good low end pull, to get onto plane,.....
Top end rpms are governed by the operator's right hand,..... to lower the rpms, you pull back on the throttle,....

 

[Scott06]

What prop are u running now and what is the max WOT rpm.

When you say too many rpm - to hold the boat on a plane? or max rpm?

 

[BamaBowrider 208]

Ayuh,...... That's not how it works,......
You pick the prop that allows the motor to make it's required rpms, usually as near to 4800 as possible,.....
That gives you good low end pull, to get onto plane,.....
Top end rpms are governed by the operator's right hand,..... to lower the rpms, you pull back on the throttle,..

Understood......I should have said more throttle

 

[BamaBowrider 208]

What prop are u running now and what is the max WOT rpm.

When you say too many rpm - to hold the boat on a plane? or max rpm?

14.24 x 21P.
I misstated.............feels to me that it takes too much throttle. I will get some accurate RPM figures tomorrow when I take her out. No chance last weekend as the weather here has not cooperated for the past week or so.

 

[Scott06]

14.24 x 21P.
I misstated.............feels to me that it takes too much throttle. I will get some accurate RPM figures tomorrow when I take her out. No chance last weekend as the weather here has not cooperated for the past week or so.

If you have too much pitch that would be the case, but usually to get on plane relatively quickly I'm 2/3-3/4 open throttle, then pull back once on plane. Your WOT RPM will be key- if you are on the low side of your range say 4400 rpm go down to a 19" pitch.

The carb was set up for a 5.0 - the jetting and power valve is the same as in attached file. The venturi is different as it has larger accelerator pump discharge holes on a 5.7. Not to over complicate things but might help to drill out the accelerator pump discharge holes or move the pump arm lever to the closest hole to get a bigger shot (2nd attachment).

 

[jimmbo]

The Venturi is larger, as the carb has a higher Air Flow. A 5.7 needs a bigger Carb than a 5 liter. Rejetting is not going to make it a bigger carb