Cobra Outdrive - How do I find the gear ratio?-gctid394523

[Guest]

The only tag on my outdrive gives the model #985273.

It says engine 5.7L. and Serial # T0604940.

That tag is on the upper housing. There is no tag that I can find on the lower housing. It does look like the previous owner replaced the lower as its a different color than the upper.

I'm trying to find the correct prop for my boat. The current 15x15.5 is only giving my 3400 rpm at wot. My 2655 was topping out at 23 knots. It should go faster, right?

Greg

[Guest]

I found my gear ratio: 1.41:1

I went on a prop calculator on www.turningpointpropellers.com. It spit out lots of options but for their aluminum propellers, they recommended:

14 1/4 x 21 for speed and performance and 14 1/4 x19 for water sports.

There were a bunch of stainless recommendations. It seemed like the pitch was all the same 21 for speed, 19 for water sports. Sizes range from 14, 14 1/4, 14 1/2 & 15. 3 and 4 blades. The 16" for some reason had a different pitch recommendation. 19 for speed 17 for water sports.

Can someone point me in the right direction? Also are there places that offer loaner props so you know you have the right one?

Greg

[Guest]

Here is the prop I am considering. Does this make sense?

http://boatpropellers.iboats.com/OMC...t_id=829461053

It's aluminum. Based on some things I've read, I like the idea that the prop will damage before the drive components. It's the largest diameter of the props Iboats suggested for an OMC sterndrive. 15.3" vs. mostly 14". The prop on my boat now is 15". The 19 Pitch is going to get me closer where I need to be rpm wise. I'm currently running a 15.5P and top out at 3400 rpm. If everything checks out, I'm hoping this is going to get me closer to 4K on my tach but still leave me some get up & go.

Please let me know if I am delusional or if I am going about this the wrong way. I don't need crazy top end speed so I opted against the 21P. I'm just looking for the right prop for my motor/outdrive.

5.7L 260HP Cobra 1.41:1

Greg

[Guest]

Sorry for the multiple posts. I'm just so excited my boat is finally in the water. Now I need to get it right. :-)

Also looking at this:

Solas Titan #2541-150-19 Stainless Steel Boat Propeller

wondering if it's worth the extra $200 for stainless?

Greg

[Guest]

Sorry for the multiple posts. I'm just so excited my boat is finally in the water. Now I need to get it right. :-)

Also looking at this:

Solas Titan #2541-150-19 Stainless Steel Boat Propeller

wondering if it's worth the extra $200 for stainless?

Greg

[Guest]

I think you have something else wrong besides the prop. The OMC manual says the stock 5.7 in a 26' heavy cruiser would spin a 15.5 X 15 and move along at 30 - 38 mph. WOT operating range is 4200 - 4600. So you have the correct drive ratio and correct prop out of the factory, but you have more HP than factory and are only able to obtain 3300 rpm. This is way way way too low.

Granted my boat is a 21'er and yours is 26', but I spin the same prop with a tired 5.0L at 4600-4700 rpm and 34 mph. Your setup should be nice and crisp, jump right up on plane, and perform really nice.

The smallest prop they run from the factory on heavy boats like yours is a 15.5 X 13. Drop to the 13 and you would pick up say 400 rpm and you are still only at 3700. I don't see a prop thet you can buy that will get you to 4600 rpm. Maybe a 14 X 10 would do it, but there is no such prop available. If you get the 21P or 19P anything, you might not even be able to get on plane.

So, you need to figure out why your engine is not making enough torque/HP:

Do you get the factory speced 31 degees of total spark advance at 3000 rpm (with 89 AKI premium fuel, 26 degrees with regular), or the Michigan Motors speced max total advance?

Is the throttle cable providing full stoke to the carb?

Are the carb secondaries fully opening (what carb is this)?

Are your plugs gapped correctly.

Do you have any detectable misfire above 3K or at your WOT?

Does you exhaust smell very rich at WOT? Or do you have a backfire at WOT?

Is you spark arrestor clean and not all dented up?

There are a few other thing like dirty fuel filter, weak fuel pump, pinholes in fuel tank dip tube, choke stuck on, ESA stuck on, and some other things I am certainly forgetting. But with the 15.5 X 15 you should be over the WOT range out of the gate and looking to move up to the 17 or 19. But you have some other problem(s) at the moment. Buying props now will be a waste of cash.

[Chief_Alen]

Engine Won't Reach Operating RPM. Check

1. Fuel condition. Type and Octane possibly old fuel

2. Propeller pitch or diameter, damaged blades

3. Restricted fuel pickup tube or anti siphon valve Fuel System Test

4. Crankcase oil volume, high oil level can cause aerated oil and lifter collapse

5. Marine growth on hull and outdrive

6. Wrong gear ratio in outdrive

7. Restricted carburetor air intake (clogged flame arrestor)

8. Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive

9. Poor cylinder compression Compression Test

10. Carburetor defective, or wrong type.

11. Fuel pump pressure and vacuum

12. Boat overloaded, improperly loaded, or improperly trimmed.

13. Engine Overheating

14. Engine timing and ignition system operation

15. Remote control cables and linkage for proper travel to open throttle plates fully.

Above copied and pasted from another website Don s is the author.

Added by Chief Alen

16-Valves need adjusting.

Copied and pasted here: Don s is the op.

[Guest]

Interesting thread, I have two 5.7 litre Cobras and I have 15 x17 composites on mine. They seem to work fine for my 28 foot cruiser I will have to check and see what I am at in terms of RPM @ WOT and top end speed

[2850Bounty]

wefivehodges wrote:


I'm trying to find the correct prop for my boat. The current 15x15.5 is only giving my 3400 rpm at wot.

Greg, as I posted in your other thread, error on the side of being "Under-Propped" for this critical "run-in" duration.

If you over-burden this new engine during run-in, you may damage it, and you may void your warrenty... (your WOT RPM numbers are now posted here publically).

orca wrote:


So, you need to figure out why your engine is not making enough torque/HP:

Do you get the factory speced 31 degees of total spark advance at 3000 rpm?

There are a few other thing like dirty fuel filter, weak fuel pump, pinholes in fuel tank dip tube, choke stuck on, ESA stuck on, and some other things I am certainly forgetting. But with the 15.5 X 15 you should be over the WOT range out of the gate and looking to move up to the 17 or 19. But you have some other problem(s) at the moment. Buying props now will be a waste of cash.

I agree with Orca on the first points, and on his closing point.

You do not want to be purchasing a prop now until you get this WOT RPM thing sorted out!

I would like to see where specs call out 31* @ 3,000 rpm for a full dished piston 5.7L SBC Marine engine.

Which leads me to a few questions, Greg:

Which ignition system are you using????

Just how are you setting up your igntion????

Which specs did you use for your advance setting????

IOW, how did you set initial advance, and what are you seeing for your progressive and TA numbers????

This is important stuff.... especially for a new/fresh engine!

crowder.dr wrote:
Interesting thread, I have two 5.7 litre Cobras and I have 15 x17 composites on mine. They seem to work fine for my 28 foot cruiser I will have to check and see what I am at in terms of RPM @ WOT and top end speed

Pretty tough to compare twins to that of a single engine.

I can run B-5 props easily, and Ian (epoc) in Australia (with a similar boat), is running B-6's.

(I'm not comparing Duo Props with single props, but am comparing pitch re; twins -vs- single engine)

There is no way in hell that a single engine in either of these boats could turn 5's or 6's with the same over-all gear reduction.

Edit:

As for Chief's list.... yes, make sure all of these items are up to snuff.

As for cam followers, I've spoken with Phil regarding this, and apparently MM does use the 8 stop procedure, so I doubt that you have an issue here.

Check all else out first.

If you should end up going back through these, be sure to contact MM re; this, and ask for their recommendation.

For me, it's going to either be the 8 stop procedure, or the dynamic procedure.

.

[2850Bounty]

wefivehodges wrote:


I'm trying to find the correct prop for my boat. The current 15x15.5 is only giving my 3400 rpm at wot.

Greg, as I posted in your other thread http://"http://www.baylinerownersclu...(post #4 here), you will want to error on the side of being "Under-Propped" for this critical "run-in" duration.

If you over-burden this new engine during run-in, you may damage it, and you may void your warrenty... (your WOT RPM numbers are now posted here publically).

orca wrote:


So, you need to figure out why your engine is not making enough torque/HP:

Do you get the factory speced 31 degees of total spark advance at 3000 rpm?

There are a few other thing like dirty fuel filter, weak fuel pump, pinholes in fuel tank dip tube, choke stuck on, ESA stuck on, and some other things I am certainly forgetting. But with the 15.5 X 15 you should be over the WOT range out of the gate and looking to move up to the 17 or 19. But you have some other problem(s) at the moment. Buying props now will be a waste of cash.

I agree with Orca on the first points, and on his closing point.

You do not want to be purchasing a prop now until you get this WOT RPM thing sorted out!

I would like to see where specs call out 31* @ 3,000 rpm for a full dished piston 5.7L SBC Marine engine.

Which leads me to a few questions, Greg:

Which ignition system are you using????

Just how are you setting up your igntion????

Which specs did you use for your advance setting????

IOW, how did you set initial advance, and what are you seeing for your progressive and TA numbers????

This is important stuff.... especially for a new/fresh engine!

crowder.dr wrote:
Interesting thread, I have two 5.7 litre Cobras and I have 15 x17 composites on mine. They seem to work fine for my 28 foot cruiser I will have to check and see what I am at in terms of RPM @ WOT and top end speed

Pretty tough to compare twins to that of a single engine.

I can run B-5 props easily, and Ian (epoc) in Australia (with a similar boat), is running B-6's.

(I'm not comparing Duo Props with single props, but am comparing pitch re; twins -vs- single engine)

There is no way in hell that a single engine in either of these boats could turn 5's or 6's with the same over-all gear reduction.

.

[dktool]

You won't be able to spin a 19" let alone a 21" pitch anywhere near the rpm range you need while trying to push that much boat.

I'm running a cupped 19" stainless on my Trophy and it flirts with being over pitched at times and my boat is WAY lighter than yours.

A 15 x 15 should get you into a usable rpm range provided EVERYTHING else is what it should be.

The gear ratios in the lower unit are the same on all the LARGE case Cobras. The only ratio change for different engines is done in the upper unit.

[Guest]

I would like to see where specs call out 31* @ 3,000 rpm for a full dished piston 5.7L SBC Marine engine.

I have posted this previously and here it is again:



I should however have noted that it is 21 degrees advanced from a base of 10, and that 10 would be running 89 (AKI or (R+M)/2) fuel. Better drop back 5 degrees if you are running regular or 86/87 AKI. OMC specs 89 for these engines, but allows 86 with the retarded timing. I edited my above post to take into account the fuel used.

Michigan Motors may well have a spec that they would like to stay under for their build as well.

[2850Bounty]

Orca, thanks for clarifying that.

I don't know which 5.7L Greg puchased (regarding piston selection), but if that curve was used for a non-q/e built engine, it would border on being risky, IMO.

.

[Guest]

I agree with what everyone else said about having an engine performance problem and that should certainly be fixed. I had a similar problem and it turned out that the weights under the distributor plate were all gunked up with corrosion and were not flying out with RPM as they should be. It would rev up to 4K rpm in nuteral but under load it would only rev to 3K which would increase to about 3.3K IF I could get it on plane but I usually couldn't.

But, what if the PO installed a hodge podge of outdrive components and his ratio was no longer 1.41:1? Could that be checked by pulling the OD, putting it in gear and turning the input shaft 14 turns while counting for 10 rotations of the prop?

[2850Bounty]

Talon wrote:
Could that be checked by pulling the OD, putting it in gear and turning the input shaft 14 turns while counting for 10 rotations of the prop?

The ratios are always calculated via crankshaft to prop revolutions.

IOW, we want to count the upper revolutions needed to complete 1 prop shaft revolution.

For greater accuracy, count the necessary upper shaft revolutions necessary to make 2 prop shaft revolutions, and divide this by 2.

For even less margin or error, do enough upper shaft revolutions to make 4 prop shaft revolutions, and then divide upper shaft revolutions by 4.

It's just math!

For any cone clutch drive, make sure that the cone is fully seated into the gear cup.... and once you begin, don't release pressure.

.