omc gear ratio vs speed

[john cole]

still working on rpm and speed issue in this boat
1987 mark twain 21' with a 4.3l gm 2bbl carb with points and it has a omc outdrive on it.
the engine is only giving me 3800 rpms at 28 mph, and it has a 14.5 X 19p aluminum prop. i have gotten the engine running right so for argument sake everything on the engine is good to go, this boat should be running about 40 mph at about 4500 rpm. but what if someone before has switch the cobra outdrive from the one that is suppose to be on with a gear ration of 21:19 and put say the one from the 5.0l on it with a 21;16 ratio, would this cause the low rpm problem by bogging the engine too much. who ever had this boat before me like to do alot of trolling by the looks of it. not sure how this works out mathmatically. any thoughts,

thanks

john

 

[Howard Sterndrive]

Re: omc gear ratio vs speed

i have gotten the engine running right so for argument sake everything on the engine is good to go,

I think it would be a mistake to assume that.
did you check the advance weights in the distributor?

You can pull the drive, put it in gear and count input revolutions required to make an output revolution and figure the ratio.

Or, with a photo tach, one can measure propshaft rpm vs. engine rpm.

weighing the boat might be valuable too.

ever run it without the flame arrester atop the carb?, hatches open?

 

[Lou C]

Re: omc gear ratio vs speed

I have the same engine on my 88 Four Winns, (20'9") to get it to reach max rpm I had to replace the 14.5x19 with a 15x17. That made a big difference, it pulled max rpm easily then....but checking to make sure you have the right out drive gear ratio is a good idea when you have the drive off.

 

[john cole]

Re: omc gear ratio vs speed

What did it do for speed? Slower or faster? Using the 15x 17?

 

[Lou C]

Re: omc gear ratio vs speed

speedo doesn't work so I can't say but it for sure feels faster, jumps up on plane. Make sure you get full timing advance at the specified rpm and that there are no fuel system restrictions, those are 2 of the things most often seen as limiting max rpm.

 

[Bridar]

Re: omc gear ratio vs speed

Verifying your gear ratio WHEN the outdrive is off won't hurt, but it will only confirm if it still has the original gear numbers or not. If they have been changed to a different (net) gear ratio, the easiest way to compensate is by changing prop pitch (its far less expensive than changing gear sets). Since you're currently running a 19, I would try a 17 pitch next (it you have one), however; I have to agree with Howard in that you shouldn't assume your engine is performing at 100%. The simplest problems can often hold an engine back from reaching WOT under load. Engine compression, distributor timing & advance curve, engine vacuum, flow of fuel, or carb settings (and jetting) could be an issue. Excess slack in the timing chain could slightly retard your valve timing. A partilally melted flapper valve could be lodged down in the bottom of the y-pipe restricting exhaust. Then there's the condition of your prop and hull. Just some ideas.

 

[john cole]

Re: omc gear ratio vs speed

agree with you completly about changing props before gearsets, and i have gone thru this engine checking, and fixing its problems. the only thing i havent done is put a fuel pressure gauge in line, but the engine doesn't act like it is starving for fuel, just seems to be maxing out. i also haven't put a second rpm meter on when running a WOT, I didn't have anyone riding with me to operate either the meter or the boat, but at idle and fast idle it is within a couple of hundred RPM's of each other (remember one is a anolog and the other is digital). the reason i threw out the gearset is because there was RTV around the outdrive, and who ever used this boat before me did alot of trolling, evidenced by the mounting holes and left over hardware, so i was thinking maybe they geared it down to get the lower surface speed but not lug the engine at low speeds? also on the outdrive it should have a id number on if it hasnet been painted over and otherwise rendered unreadable, or i can take the top plate off and look down and count teeth on the pinion and other gear. but i will try propping down before i do much else only because of everything else i have done already.

 

[Howard Sterndrive]

Re: omc gear ratio vs speed

you can drop pitch and get the rpms up, but you won't gain any real speed that way. maybe 1 or 2 mph but you'll probably just experience more revs and noise at the same speed.
fix the engine or hull problem.
if you want to verify ratio, verify ratio... don't throw props at it blindly

 

[HT32BSX115]

Re: omc gear ratio vs speed

but i will try propping down before i do much else only because of everything else i have done already.

Hey!! A Twainer!!!

Howdy,


You REALLY need to determine your drive ratio before doing anything else. The wrong ratio can make all the difference in the world!

1. pull all of your spark plugs and put a wrench on the front of the engine.

2. turn the engine slowly and place the drive in gear.

3. turn the engine 10 turns (or so) and have someone count blades (or fractions of blades).

4. use a calculator if your math skills are rusty!

Until you know your drive ratio, you're spinning your wheels (or prop!!!)


Cheers,



Rick

 

[1980Coronado]

Re: omc gear ratio vs speed

I keep thinking about this...been following the multiple threads.....and it seems that you've ruled out the engine for the most part. I think you need to weigh the boat. I feel that there could be extra weight somewhere. Have you looked in the bilge to see if there are any plugs in the front or sides of the compartment that would allow you to drain the area under the floor? Something is not allowing the motor to come up to rpm and I can't imagine that it's the 19" prop. Is there water somewhere? Is the hull covered in scum/algae....you've checked the compression, timing, ignition..etc....and you are still running too slow at that rpm. You've verified speed with gps....the tach may be off a little but...? If the actual rpm is lower it points back to an engine or weight issue, if it's actually higher the numbers get worse on prop slip because you are running a 19" prop and the speed doesn't match the rpm you've posted. As I've posted before, dropping 2" of pitch is worth approx 400 rpm, but you're increasing diam as well so I would expect the rpm gain to be less than 400. Which means you're still not going to reach max WOT rpm for the engine. I'm stumped by this. There is something else going on and the gear ratio just may be the answer.....or you have a bunch of extra weight in water below the floor somewhere. If this were a carb/fuel issue I would expect the motor to stumble or show some other sign of poor running at WOT.

 

[john cole]

Re: omc gear ratio vs speed

i too have been stumped, and i happened to ask another guy i found that works on boats and he said i really need to check the timing at what is doing at full advance more so than what it is set for at idle speed, so i stopped by the boat today and advance my timing a couple of degrees and took it for a spin and sure as sh*t i got 4100rpms and 31 mph out of it. i didn't push any harder than that, felt like it still had some left in it, but seeing i didn't have my timing light to verify the timing i didn't want to damage anything, so it's looking like it is a timing issue after all, just not like i'm use to seeing with auto's. according to this guy the max advance is very important on boat engines because of the way they run. i know someone in one of the reponses brought this up to, sorry for not looking at it closer then. so as soon as i verify the timing and boat tach at higher rpms it should come out that the motor is pretty much were it should be. in any case i will try and find were any water might be hiding at, and the boat isn't very scummy, its only been in the water about a month now and it had a spotless hull before going in. and i still plan on checking the ration just so i know what i have for sure.

 

[1980Coronado]

Great....that makes total sense....I thought you had already ruled that out. Sounds like you need to fix that distributor...and....Then check the rpm and decide if you wanna change props.

Sent from my BlackBerry 9650 using Tapatalk

 

[Howard Sterndrive]

Re: omc gear ratio vs speed

he said i really need to check the timing at what is doing at full advance

wasn't that the very first reply to your thread up there? ^^

good to hear you got some speed out of her. clean the rust and gluey old oil residue off your advance weights and springs.

 

[HT32BSX115]

Re: omc gear ratio vs speed

And if you do not know the drive ratio and cannot determine it by the serial number, measuring it would still be easy and probably still be a good idea.

 

[john cole]

Re: omc gear ratio vs speed

wasn't that the very first reply to your thread up there? ^^

good to hear you got some speed out of her. clean the rust and gluey old oil residue off your advance weights and springs.

yep you did, i did take out the dist checked it over, checked timing at idle and made sure the dist was advancing as the rpms came up, but i didn't have a way to verify the exact advance, and it looks like that is biting me in the butt. as far as the prop goes, i don't currently have a spare so i need to get a spare and one with a 17p probably wouldn't be a bad i deal especially when it comes time to take kids tubing.