Thought I'd show off my new build

[Efini Motorsport]

I do have Merc exhausts. On of the flappers was still Iintact with the stainless pins and grommets but of course I can't find them now. With the mad thrash to get the boat in my garage is a mess. I'm going to see if I can find all new flappers pins and grommets but for now I'll leave it with the holes covered. I want to get it running right now then iron out the little kinks.

 

[strokendiesel002]

Congrats! You'll get her ready by the 4th!

 

[Efini Motorsport]

As long as the limp mode issue isnt because of the tune or not having o2 sensors I think I'll be ok. If it's either of those I'm just going to put it in on the 4th and cruise around at idle. The drive is coming back off today to check the lower cable adjustment and I'm pulling the manifolds to fix the bilge hose. I was hoping to have the bimini and maybe the cabin cushions done for the 4th but those are on the back burner for now

 

[strokendiesel002]

Blankets and towels make for great cushions, and I've used an old bed sheet to keep the sun off the girls and little ones before. You've got your priorities spot on, looking forward to a "I can't believe I forgot to plug this in" post... at hoping for something like that for ya

 

[strokendiesel002]

If it's O2 sensors, couldn't you maybe wire in a couple resistors to get the correct mV?

 

[Efini Motorsport]

Unfortunately I rarely run into a I forgot to plug this is situation. It's usually a no one has ever tried this and has no idea how situation. The O2 sensors are 4 wire so they have heaters. I think having a fixed value with resistors would be enough to make the computer think something is going on. If I HAVE to run o2's I'll have to change out one manifold and riser to give me a port for them. It's the wrong size but I can make it work. It just won't be the most elegant solution. Towels and sunscreen will suffice if I can't get the cushions done right away

 

[Efini Motorsport]

ITS ALIIIIIIIVVVVVE!!!!! And of course it was something stupid. Actually a few ignition issues and a maf sensor issue but got them sorted and DAMN she's fast. I think I'm a ways off on the prop because my hole shot sucks but once I'm up on plane I top out at 40mph and 3500 rpm. Boat ran great all day today though. It's no longer running rich and even an almost full throttle run for 4-5 miles went perfect. Tomorrow I'm going to fix the trim/tilt relays and wire in the trim tabs. I've also got 3-4 other props I can try to hopefully get me up and moving a lot faster. Even with the hydrofoil on the outdrive it still takes me 20-30 seconds at full throttle to come up on plane. Does anyone know what the stock prop pitch is for a 4.3 cobra? I think I've got a 19 on there now.

 

[Lou C]

15x17 was standard on mine but it’s a slightly bigger boat
this season I’m trying a 15.5x15 to see if I can get to 5,000 wot

 

[Efini Motorsport]

I may be way off on my prop diameter and pitch then.

 

[Lou C]

Ever check the drive ratio? For a V6 Cobra it should be 1.68:1 overall. On the Cobras, both the upper and lower gear ratios varied by model, for the V6 it should be 21:19 upper, and 14:26 lower. BTW, the V6 and V8 (305 and 350) lowers are the same but the uppers are different (V8 5.0 upper is 21:17, V8 5.7 is 21:16). Overall ratios for the V8s is 5.0: 1.50, 5. 1.41. All this is from my OMC owners manual tech specs......

 

[Efini Motorsport]

The tag on the drive says it's for a 4.3. I think the drive might have been up a little. I was having a problem with the relay box for the outdrive tilt. It's fixed now along with the trim tabs and I'm about to put it back in

 

[HT32BSX115]

The tag on the drive says it's for a 4.3. I think the drive might have been up a little. I was having a problem with the relay box for the outdrive tilt. It's fixed now along with the trim tabs and I'm about to put it back in

It would be worth it to verify your ratio,

If it's 1.68:1 you should be able to rotate the engine 16.8 turns to 10 prop revolutions. That would be slightly less than 17/10

Once you know that, you need to decide what your max engine RPM should be......... Most V-6 and V-8 engines want around 4800 to 5000 max... so you should select a prop that will give you around 4800-5000 at WOT trimmed for max speed and the boat loaded where you will normally operate on a regular basis (full fuel, 4 seats occupied, 3 dogs etc)

I think I'm a ways off on the prop because my hole shot sucks but once I'm up on plane I top out at 40mph and 3500 rpm.

Something doesn't sound right. If you have a 1.68:1 drive ratio, 40mph @3500 is not possible (you would have negative slip with a 19p prop. Your theoretical speed (if the there was no prop-slip at all) would be 37 mph at 3500.

So something is not right (drive ratio, prop pitch, TACH is WAY off, speedo is way off or you have a very different prop......OR all the above.)

https://www.go-fast.com/Prop_Slip_Calculator.htm (use about 12-15% slip as a starting point for estimation)
After get close, you'll plug in your (known) drive ratio, prop pitch and final speed. The prop slip calculator will give you your slip once you know your final speed. single prop drives usually result in about 12-15% slip.

Determine your ratio first. pull the plugs, turn the engine by hand and have someone put it in gear. count engine turns and prop turns.

When you know the ratio get another (known) tach and use your smart phone GPS for speed.

Read this: https://www.go-fast.com/prop_slip.htm

For example, if you have 1.68:1, 19p, 5000 RPM, the boat should go about 48mph (with 12% slip)

Start with the drive trimmed down/IN and as you begin to plane, trim UP/OUT in short bursts on smooth water to find the "sweet" spot trim point.

Max speed (trim) may be different when you have big people in the back.... Don't try to use the trim gage. It's just there to tell when the drive is up. You trim by "feel" in short bursts when underway!

You may end up removing the "whale-tale" too ....Many find that they just produce a LOT of drag.


Cheers,

Rick

Incidentally, I just ran the calculator again using 1.41:1 ratio, 3500RPM, 19pitch and with 12% slip and I got 40MPH

If that's all the engine will turn with the above numbers, you're seriously "over" propped.

Also, if that is indeed a 1.41:1 drive, putting a 15p prop on it should give you similar speed at 4500 RPM.

And you'd still be over-propped....... You would still want around 4800-5000 at max throttle. I don't know if there is a 13p prop for a Cobra.....for that engine, the 1.68:1 ratio is going to allow you to use the most efficient prop pitch which is probably a 19

 

[Efini Motorsport]

That's a whole lot of numbers I'm going to have to go over. The 40 mph speed (with the 19p) was with a GPS. I verified gauge RPM matched actual engine RPM at idle but didn't check at speed. I used DMM with tach to calibrate tach gauge. Wit the 19p prop, no trim tabs and the hydrofoil on I topped out at 40mph and about 3500rpm in fairly calm seas. Probably could have gotten a little more out of it if it was calmer or I had less people aboard. I didn't play with the drive pitch at all. With the 17p prop i was about 38mph and 3800rpm. If I drop the trim tabs down it gets out of the water pretty quickly but is still a little slow If I don't use them. All the props I'm using are 14.5" and are factory marked clearly for pitch. For what I actually use the boat for I'm more worried about the hole shot than top speed or RPM. Once I'm out of the harbor I usually go look for a calm area to go tubing, boarding or swimming. I've got 30+ hours on the boat in the water and it's only been more than a mile offshore once for my brother to go fishing. I'll have to double check the actual ratio on the drive but I looked again today and it says it's for a 4.3. I doubt if having a fuel injected vortec motor would shift the power that much over the carb'd 4.3. I'll check out that site tomorrow and see what I can figure out. I would like to try a 15p prop to see how it does.

On a side note I do want to add some updates for anyone that comes across this post in searches down the road. Swapping a fuel injected truck engine in place of a carb'd boat engine wasn't terribly difficult. Although I'm still working out some kinks like belt alignment, water and oil leaks, it has been a great swap. Yesterday alone the boat ran on and off for about 15 hours straight without a hiccup. In that time we burned less than 3/4 of a tank of gas. The boat starts right up every time with no hesitation.
There are a few caveats though if you're thinking about trying it yourself. The first is I've been building custom cars and boats for about 20 years and have done a fair share of motor swaps. As far as swaps go this one is pretty easy since everything literally bolts in. In the end everything from the bellhousing back is OMC, most of the parts in front of the motor are mercruiser (accessories and brackets), and everything in the middle is 03 Chevy blazer. The other thing that made it easier for me is I have an extensive network of automotive and marine mechanics, custom car and boat builders and fabricators that I can bounce ideas off of. There were a few issues I ran into that i needed to ask some questions about but most of them turned out to be small things I overlooked. One of the biggest resources to help with the swap was Brendan at LT1swap.com. His site has every bit of wiring info you could possibly need for a computer controlled chevy swap, He was also able to reprogram my computer to make it work as I wanted it to.
I tried to give a broad overview throughout this process because almost every post I've read on the swap says it can't be done for one reason or another. when I really tried to dig down as to what those specific reason were I got a lot of "that's what I was taught" without actually being able to back it up with concrete facts or numbers. I don't want to sound like I'm picking on anyone here because you all have been a big help too. Sometimes even people saying " you can't do that" gives me just a little extra push so I can prove them wrong. I did get a bit of that in the beginning but as I showed both progress and aptitude those people came around. I'm not on here a ton but I'd get annoyed at the number of times people ask about swapping automotive engines into boats, especially when some of those people could barely change their own oil.
If anyone has any questions about the swap or is working on something similar I'm more than happy to help. You can reach me through here or at my username @gmail. There is one thing in all of this that many people have mentioned as to why you can't swap an automotive engine into a boat and it's something I'd like to get the numbers to figure out. People keep saying the cams are very different profiles. I never had my boat in the water before the swap and didn't think to pull the cams out of the 2 mercruiser 4.3's or the chevy 4.3 that I was given during this process (all blown or seized). I would like to find out what the exact numbers are to compare them though. Maybe on my next swap. I'm already looking for a 23-25' boat and thinking about a 6.0 or 6.2 LS swap. My problem is I like the build process almost as much as actually using it.

 

[HT32BSX115]

No doubt about it, you have done something nearly all of us couldn't or wouldn't do! You are to be commended for making it work safely!!

I didn't play with the drive pitch at all. With the 17p prop i was about 38mph and 3800rpm. If I drop the trim tabs down it gets out of the water pretty quickly but is still a little slow If I don't use them.

You MUST adjust the drive trim for the best drive angle for the load and speed and it will always be different.

All the props I'm using are 14.5" and are factory marked clearly for pitch. For what I actually use the boat for I'm more worried about the hole shot than top speed or RPM.

The marine industry has recommended selecting pitch that achieves MAX (rated) RPM at WOT for many years and it applies pretty much to outboards, inboards and I/O's.

Having a prop/drive ratio combination that only allows an RPM of 1000-1500 lower will not only cause top speed to suffer, it will seriously reduce hole shot and essentially LUG the engine at cruising speeds. If you're using a 17 now, and you have access to a 15p prop, give it a try. (or even a 16p 4-blade etc...make sure you're not overloaded and be sure to tweak the drive trim when you get up to speed)

Even if you don't have the most optimum drive ratio, you can (mostly) make up for it with a higher (or lower) pitch prop. It won't be perfect but it'll be close. The key is you want to be close to max RPM or at least in the MAX recommended range for that engine.

:thumb:

 

[Efini Motorsport]

That's MAX range based on the power curve of the engine correct? In other words I want WOT to occur at peak HP?

 

[Lou C]

Most of the 4.3s were set up to reach 4600-4800 wide open throttle. That allows them to pull the boat up on plane without laboring. Over the long term, a boat that has low WOT rpm will have shortened engine life due to elevated combustion chamber temps because of lugging/laboring. Exhaust valves overheat and lose compression due to tuliping. Think of it like this, how long would a manual shift vehicle's engine last if you always started it in 2nd gear and shifted to 4th right away? Or why trailer tow packages pretty much always include a numerically higher final drive ratio to keep engine rpms up in the power band when pulling loads...

 

[Efini Motorsport]

OK. That makes sense. I checked the outdrive this morning and there's no ratio listed on it. The model # is 984541 and serial # is T0485239. I'm still looking for a 14.5x15 prop to try. I've got to see what the tag on the other drive says too.

 

[Lou C]

Michigan Wheel sells 15.5 x 15 aluminum that will fit the Cobra, I have one on mine.

 

[Efini Motorsport]

I need a 14.5x15 but I'll see if they have them

 

[HT32BSX115]

OK. That makes sense. I checked the outdrive this morning and there's no ratio listed on it.

It's pretty hard to know the ratio of a 30 year old Cobra because it could have been rebuilt and gears replaced many times over those years.

The easiest way would be to pull the plugs and (in-gear) turn the engine and have someone count exactly 1 turn at the propshaft.

For example, if you have a 1.68:1 drive, (approx 1.7:1) the engine would turn 1.67 revolutions for every prop revolution.

If you have a protractor, you can measure 1.68 turns. (starting from "North")

1 turn PLUS 0.68 turns. 0.68 turn equals 0.68 x 360° = 244.8° or 245°

1.41:1 ratio would be 1 engine turn + 0.41 x 360 = 147.6° for one prop turn. and so on.............

You can see that only 97.2° degrees (a little more than 1/4 turn less) will indicate a significant change in the ratio.

If I remember correctly, a my 460 had a 1.41:1 ratio drive on the back. A V-6 wants a 1.68:1 drive. Not a huge difference but trying to run a 1.41:1 drive on a V-6 can make a big difference in efficiency even if you do adjust the prop to try to compensate.


Knowing your ratio will make it a LOT easier to estimate what prop to try.