Twin 150 Optimax, one turns 300 less rpm. Why?

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Author Topic:   Twin 150 Optimax, one turns 300 less rpm. Why?

pecos23 posted 09-05-2012 06:32 PM ET (US)
I am hoping sosmerc spots this post, I am trying to figure out why one of my Optimax motors turns 300 rpm's less at WOT no matter what prop combo I am running. I've been experimenting with different props and am finding that my port motor consistently is 300 rpm's less than the starboard motor.
The motors are 2004, 150 optimaxs and are mounted on the back of a 2004, 240 outrage.
I have run three different sets of mercury props, vengence 19", mirage plus 17" and enertia 19". All run at different max rpm's but the port motor is always 300 less. Any idea why this would happen?
Any one else experienced this with a set of twin optimaxs?
Thanks
David
Buckda posted 09-05-2012 09:23 PM ET (US)
Is one engine Counter Rotating?
The extra gearing may be part of the problem.
Dave
outragesteve posted 09-05-2012 09:25 PM ET (US)
Could be many reasons. Has this engine always been 300 RPM less? The first thing I would try, and it's easy, is swap the tachs. I would also look very carefully at the engine height of both engines in relation to the bottom of the hull. Could be that one motor is slightly lower, changing the RPM. In the scheme of things, 300 is not a huge difference. Throttle cable "slop"? Semi fouled spark plug? Parasitic drag in the lower unit? Etc, etc, etc. Good luck!
leadsled posted 09-06-2012 09:33 AM ET (US)
Do the engines sound like they are in sync while they are 300 rpm's off?
Russ 13 posted 09-06-2012 10:03 AM ET (US)
A small difference in trim, or a small difference in the two tach.'s could be the reason. As they are Mercurys,
I'd be happy they are both running at the same time.
Plotman posted 09-06-2012 10:17 AM ET (US)
Many tachometers have jumpers on the back that are used to set them to read properly on a given engine. First thing I would check is that they are both set the same.
martyn1075 posted 09-06-2012 10:36 AM ET (US)
I will check my engines. I use twin Optimax motors. I don't recall them being off by that much. I do concur with Dave my counter rotating engine is bit different in that the way it operates. Its also a bit louder and burns slightly more gas than its standard mate. When I toggle to match the pair I mainly can just hear the sound flanging a bit and make the adjustment but now that Im thinking of it I believe that the counter needs a bit more power.
I might check the fuel rails and injectors are working properly as well the compressors. They all need to be working in harmony to give optimum output.
Martyn
JTC posted 09-06-2012 09:48 PM ET (US)
I had 1999 Optimaxes with the same era analog tachs, so I don't know if my experiences are at all relevant. But, I found that my tachs were not very accurate. If I synced the motors by ear so that they were definitely turning the same RPM, the tachs would show about a 200 RPM difference (one line on the meter as I recall).
If you have analog tachs, it's possible that they are just off. Can you hear the difference in motor pitches at WOT? I.e are you certain that one is actually slower? You might try slowing the "faster" one down so they match and listening to what happens.

Tom W Clark posted 09-07-2012 10:39 AM ET (US)
I am going to disagree with most here. 300 RPM is a very big difference and I think it is indicative of a problem. Let me explain.
Analog tachometers are often in error so a pair of them might well read different engine speeds even if the motors are synched. My own pair of Mercury tachometers are 50-100 RPM off. I have learned over the years that if the motors are synched, the port tach will read less. 300 RPM is a MUCH bigger error and I have never heard of one that bad.
Counter rotating gearcases do have some added friction (and thus loss of net power) due to their more complicated gearing, but there in no way in hell that can account for 300 RPM; the added friction is very small.
Furthermore, I was told privately the WOT engine speeds were 5350 RPM port, 5050 RPM starboard with the 19" Enertias. This means it is the STARBOARD motor that is 300 RPM less, not the PORT motor as stated at the start of this thread. [ David -- Please clarify. ]
If the difference occurs with all three sets of propellers tried, it seems logical to suspect it is not merely a case of two props of the same nominal pitch not having the same actual pitch. This problem is common and has happened to me, though the biggest difference I ever saw was 200 RPM with a pair of Rev 4s. If we assume the propellers are the not the source of the RPM difference, we need to look elsewhere.
On a twin engine boat with a matching pair of props it is very important to synch the motors to achieve the best performance. If one motor, and hence propeller, is running even a little faster that the other, a very much greater amount of the load of propelling the boat is put on that motor's prop. The motor running slower will have that load taken off of it and this will allow the motor to speed up because it has so much less resistance. That is why a 300 RPM difference tells me there is a much bigger loss of power on the slower motor than you might at first think. Imagine you took both motors off the boat and put them on a smaller boat by themselves. You would see a much bigger RPM difference between them when they are working by themselves.
Lastly a point not yet made is the boat is slow compared to what we would expect of it with the power it has. A 2004 240 Outrage with a pair of OptiMax 150s should be good for 49 MPH not the 45 MPH it is seeing now.
pecos23 posted 09-07-2012 05:20 PM ET (US)
Thanks for all of the suggestions. It is the port motor, counter rotating, that is running slow. On a perfect lake last night I was able to get the port motor to turn 5350 rpm's but had to trim the motor past optimal trim. The starboard motor can run at 5350 in a position with about 1.5 degrees less trim. The gauges are smart craft so I assume they are digital.
The motors sound great when matched up with no sign or sound of a missing cylinder. Sosmerc is a regular poster on cw and suggested that we check the throttle position sensor to make sure it is where it's supposed to be when wot. This can only happen with the use of the mercury DTT computer. I have a friend with tis unit, we are trying to get together to run a test. Sosmerc also mentioned that there is an allowable 10% variance per merc spec. Seems like a lot but he is the expert.
I was able to get 46 mph last night. The 240 performance report has this set up at 48 mph. This is without bottom paint or a tee top, both of which are on my boat. The report used 19" vengence props. As much as I would like to get 49 mph, I just don't believe this rig can reach this speed. The boat dry is 4400 lbs. Add a 1000 lbs for motors, 50 gallons of fuel (300 lbs) 300 for tee top plus wind resistance, 200 for gear and 46 mph sounds pretty good.
Dose anyone know if the smart craft gauges are digital? Sosmerc also mentioned that the ECM (computer brain) can vary from motor to motor.
I would like to get to top rpm's of 5750 (recomended max) and have considered 17" enertia props. I have tried 17" mirage plus props and was able to get 5750 but the overall performance did not compare to the enertia 19" props, even though I could only get 5350 rpm's.
So, the trials and searche continue. Thanks again for the input.
David
martyn1075 posted 09-07-2012 08:28 PM ET (US)
Just got back from the boat. I ran a few tests and my Optimax twins are within average 10-60 rpm difference. I use the smartcraft meters so I understand there might be some differences form the analog meters, but all in all the engines were showing identical measures with the needles the sound were not flanging at all to my ears but it still showed about 20 maybe 50 rpm off. They digital meters fluctuate as you are moving so they never really maintain the number but average was about 30 or so off. I run the 2007 225 Optimax models and the engines were trimmed not identically by the way maybe that will make a difference as well.
Martyn
Jeff posted 09-08-2012 08:59 AM ET (US)
David, when did you get a 24 Outrage? Did you sell the 19?
Could the variance between the motors be found in the control cables? We had a similar problem with our twin Mercury 150 XR6s and were able to correct the problem through making the proper adjustments in the throttle cable linkages. I believe my OMC mechanic refers to this process as a "link and synch". Not sure if the fuel injected Optimaxs work this same way as the Carb'd motors.
pecos23 posted 09-08-2012 10:24 AM ET (US)
Yes, I sold my 1993, 19 outrage ll to my buddy up at Catawba island on lake Erie. I can tell you she's in good hands, indoor rack stored with more wax and polish on her then I've ever seen. I picked up the 240 this April from a guy in Nashville TN, he was running it on the Cumberland river but it was too much boat. I'll try to post a pic of my new rig, she is sweet!
I've had some one else mention a link and sync. The throttles are a bit sloppy and I have put that on my short list. Does anyone here have a procedure for linking and syncing a pair of optimaxs?

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Author	Topic: Twin 150 Optimax, one turns 300 less rpm. Why?
pecos23	posted 09-05-2012 06:32 PM ET (US) I am hoping sosmerc spots this post, I am trying to figure out why one of my Optimax motors turns 300 rpm's less at WOT no matter what prop combo I am running. I've been experimenting with different props and am finding that my port motor consistently is 300 rpm's less than the starboard motor. The motors are 2004, 150 optimaxs and are mounted on the back of a 2004, 240 outrage. I have run three different sets of mercury props, vengence 19", mirage plus 17" and enertia 19". All run at different max rpm's but the port motor is always 300 less. Any idea why this would happen? Any one else experienced this with a set of twin optimaxs? Thanks David
Buckda	posted 09-05-2012 09:23 PM ET (US) Is one engine Counter Rotating? The extra gearing may be part of the problem. Dave
outragesteve	posted 09-05-2012 09:25 PM ET (US) Could be many reasons. Has this engine always been 300 RPM less? The first thing I would try, and it's easy, is swap the tachs. I would also look very carefully at the engine height of both engines in relation to the bottom of the hull. Could be that one motor is slightly lower, changing the RPM. In the scheme of things, 300 is not a huge difference. Throttle cable "slop"? Semi fouled spark plug? Parasitic drag in the lower unit? Etc, etc, etc. Good luck!
leadsled	posted 09-06-2012 09:33 AM ET (US) Do the engines sound like they are in sync while they are 300 rpm's off?
Russ 13	posted 09-06-2012 10:03 AM ET (US) A small difference in trim, or a small difference in the two tach.'s could be the reason. As they are Mercurys, I'd be happy they are both running at the same time.
Plotman	posted 09-06-2012 10:17 AM ET (US) Many tachometers have jumpers on the back that are used to set them to read properly on a given engine. First thing I would check is that they are both set the same.
martyn1075	posted 09-06-2012 10:36 AM ET (US) I will check my engines. I use twin Optimax motors. I don't recall them being off by that much. I do concur with Dave my counter rotating engine is bit different in that the way it operates. Its also a bit louder and burns slightly more gas than its standard mate. When I toggle to match the pair I mainly can just hear the sound flanging a bit and make the adjustment but now that Im thinking of it I believe that the counter needs a bit more power. I might check the fuel rails and injectors are working properly as well the compressors. They all need to be working in harmony to give optimum output. Martyn
JTC	posted 09-06-2012 09:48 PM ET (US) I had 1999 Optimaxes with the same era analog tachs, so I don't know if my experiences are at all relevant. But, I found that my tachs were not very accurate. If I synced the motors by ear so that they were definitely turning the same RPM, the tachs would show about a 200 RPM difference (one line on the meter as I recall). If you have analog tachs, it's possible that they are just off. Can you hear the difference in motor pitches at WOT? I.e are you certain that one is actually slower? You might try slowing the "faster" one down so they match and listening to what happens.
Tom W Clark	posted 09-07-2012 10:39 AM ET (US) I am going to disagree with most here. 300 RPM is a very big difference and I think it is indicative of a problem. Let me explain. Analog tachometers are often in error so a pair of them might well read different engine speeds even if the motors are synched. My own pair of Mercury tachometers are 50-100 RPM off. I have learned over the years that if the motors are synched, the port tach will read less. 300 RPM is a MUCH bigger error and I have never heard of one that bad. Counter rotating gearcases do have some added friction (and thus loss of net power) due to their more complicated gearing, but there in no way in hell that can account for 300 RPM; the added friction is very small. Furthermore, I was told privately the WOT engine speeds were 5350 RPM port, 5050 RPM starboard with the 19" Enertias. This means it is the STARBOARD motor that is 300 RPM less, not the PORT motor as stated at the start of this thread. [ David -- Please clarify. ] If the difference occurs with all three sets of propellers tried, it seems logical to suspect it is not merely a case of two props of the same nominal pitch not having the same actual pitch. This problem is common and has happened to me, though the biggest difference I ever saw was 200 RPM with a pair of Rev 4s. If we assume the propellers are the not the source of the RPM difference, we need to look elsewhere. On a twin engine boat with a matching pair of props it is very important to synch the motors to achieve the best performance. If one motor, and hence propeller, is running even a little faster that the other, a very much greater amount of the load of propelling the boat is put on that motor's prop. The motor running slower will have that load taken off of it and this will allow the motor to speed up because it has so much less resistance. That is why a 300 RPM difference tells me there is a much bigger loss of power on the slower motor than you might at first think. Imagine you took both motors off the boat and put them on a smaller boat by themselves. You would see a much bigger RPM difference between them when they are working by themselves. Lastly a point not yet made is the boat is slow compared to what we would expect of it with the power it has. A 2004 240 Outrage with a pair of OptiMax 150s should be good for 49 MPH not the 45 MPH it is seeing now.
pecos23	posted 09-07-2012 05:20 PM ET (US) Thanks for all of the suggestions. It is the port motor, counter rotating, that is running slow. On a perfect lake last night I was able to get the port motor to turn 5350 rpm's but had to trim the motor past optimal trim. The starboard motor can run at 5350 in a position with about 1.5 degrees less trim. The gauges are smart craft so I assume they are digital. The motors sound great when matched up with no sign or sound of a missing cylinder. Sosmerc is a regular poster on cw and suggested that we check the throttle position sensor to make sure it is where it's supposed to be when wot. This can only happen with the use of the mercury DTT computer. I have a friend with tis unit, we are trying to get together to run a test. Sosmerc also mentioned that there is an allowable 10% variance per merc spec. Seems like a lot but he is the expert. I was able to get 46 mph last night. The 240 performance report has this set up at 48 mph. This is without bottom paint or a tee top, both of which are on my boat. The report used 19" vengence props. As much as I would like to get 49 mph, I just don't believe this rig can reach this speed. The boat dry is 4400 lbs. Add a 1000 lbs for motors, 50 gallons of fuel (300 lbs) 300 for tee top plus wind resistance, 200 for gear and 46 mph sounds pretty good. Dose anyone know if the smart craft gauges are digital? Sosmerc also mentioned that the ECM (computer brain) can vary from motor to motor. I would like to get to top rpm's of 5750 (recomended max) and have considered 17" enertia props. I have tried 17" mirage plus props and was able to get 5750 but the overall performance did not compare to the enertia 19" props, even though I could only get 5350 rpm's. So, the trials and searche continue. Thanks again for the input. David
martyn1075	posted 09-07-2012 08:28 PM ET (US) Just got back from the boat. I ran a few tests and my Optimax twins are within average 10-60 rpm difference. I use the smartcraft meters so I understand there might be some differences form the analog meters, but all in all the engines were showing identical measures with the needles the sound were not flanging at all to my ears but it still showed about 20 maybe 50 rpm off. They digital meters fluctuate as you are moving so they never really maintain the number but average was about 30 or so off. I run the 2007 225 Optimax models and the engines were trimmed not identically by the way maybe that will make a difference as well. Martyn
Jeff	posted 09-08-2012 08:59 AM ET (US) David, when did you get a 24 Outrage? Did you sell the 19? Could the variance between the motors be found in the control cables? We had a similar problem with our twin Mercury 150 XR6s and were able to correct the problem through making the proper adjustments in the throttle cable linkages. I believe my OMC mechanic refers to this process as a "link and synch". Not sure if the fuel injected Optimaxs work this same way as the Carb'd motors.
pecos23	posted 09-08-2012 10:24 AM ET (US) Yes, I sold my 1993, 19 outrage ll to my buddy up at Catawba island on lake Erie. I can tell you she's in good hands, indoor rack stored with more wax and polish on her then I've ever seen. I picked up the 240 this April from a guy in Nashville TN, he was running it on the Cumberland river but it was too much boat. I'll try to post a pic of my new rig, she is sweet! I've had some one else mention a link and sync. The throttles are a bit sloppy and I have put that on my short list. Does anyone here have a procedure for linking and syncing a pair of optimaxs?