4.3l OMC to SEI Conversion

[Stanp1]

Blew threw the lower gears once again on my 1989 OMC cobra with 1.64 gears. It was bolted to my new 4.3l V6 engine. Decided to scrap it for the SEI Conversion. The biggest challenge was pulling apart the Morse gear shifter and reversing the push/pull of the shifter cable. Wasn't that bad for the money, $0.

Here is the problem I am having, the original OMC gear ratio was 1.64, and it spun a 15x15 prop. It got on up on plane and pushed 25-28mph at 3500rpm. Now the SEI drive showed up with a 1.81 gear ratio, I put a 16x17 prop on it, and with a full load, she won't come out of the hole. However, with half a load, she acheives getting on plane up to 25-28mph.

I put a 15.5x15 on it and she gets on plane with a full load, but my top end is 20-22mph at 3900rpm's.

I want the 16x17 prop on it. To do this, I need just more HP to move it. I want to convert the points to electric ignition, any other sugestions?

 

[zbnutcase]

Re: 4.3l OMC to SEI Conversion

You really need to get the correct ratio.

 

[nola mike]

Re: 4.3l OMC to SEI Conversion

You really need to get the correct ratio.

that should be the correct gear ratio for a 4.3l with an alpha...electronic ign won't get you any hp.

EDIT: just noticed your prop size--aren't most for the alphas in the 14.5" diameter range?

 

[FreeBeeTony]

Re: 4.3l OMC to SEI Conversion

2bbl or 4bbl?

 

[sethjon]

Re: 4.3l OMC to SEI Conversion

Elec ignition won't do much if anything for speed. A 4 bbl will but again not as much as you are looking for.

 

[Stanp1]

Re: 4.3l OMC to SEI Conversion

This 4.3l is a 2bbl. I thought about a 4bbl conversion but this site seems to think its as cost effective to buy a 350 V8. Not sure I believe that after pricing an intake Manifold and marine 4bbl holley carb.

If 1.81 is the correct gear ratio, what prop pitch is to be used? I will stick with points. 15 pitch is too little, 17 pitch too much, I guess 16?

I'd like to go to a 17 pitch since I get best top end speed of 28mph on the engine I ontly need a bit more HP to Get it out of the hole. Any Suggestions? Is everyone ok with the 4bbl conversion?

 

[thumpar]

Re: 4.3l OMC to SEI Conversion

Is that your WOT RPM? If so it is low. You should be at least mid 4000's at WOT.

 

[imported_TheMan]

Re: 4.3l OMC to SEI Conversion

I feel as though most mercruiser alpha drives use 13.5 x 17" through 13.5 x 23" props... I'd lean towards the prop being the bigger issue than the engine. This of course assumes the SEI is similar to merc.

 

[Lou C]

Re: 4.3l OMC to SEI Conversion

If you are going from a 4.3 to a 5.7 and changing the drive and using the right ratio at the same time that's one thing, but keep in mind that changing to a 5.7 will require relocating the engine mounts (it's about 3 1/2 in longer) and the exhaust will have to be changed over to the V-8 exhaust, not sure if that will line up to the V-6 Y pipe, they can be slightly different. If I were doing that, and did not already have the SEI conversion kit, changing engine/drive, I might then just pull the whole transom assembly and convert the whole thing over to Merc right then....may have to source some used parts but in the long run that might be best. If you want to keep the cost down and the 4.3 is still good then use the SEI kit but get the right drive ratio...

as far as the 2bbl vs 4bbl, for my OMC 4.3 the 2bbl was rated at 175 and the 4bbl rated at 205. I have the 4bbl and it's good in my 20' boat but in a bigger boat I'd want a V-8..

 

[Stanp1]

Re: 4.3l OMC to SEI Conversion

SEI is exactly the same as an Alpha 1 gen II. But they won't give me a new gear ratio to match the old one(1:81 to a 1:64). Althought many prop sites tell you that diameter is less important than pitch, I have to agree with a 15.5 inch diameter being too wide for this setup(4.3l with SEI 1:81 ratio) So, I will go to a 13.5x17 prop. I have slippage when runing about 3500rpm- the rpm's rev up as the boat slows down. And my theory is the 15.5 diameter prop is too close (less than 3/4inches clearence to the lower unit.) It's causing the slippage at higher rpm's. WOT can be reached at 4300rpm's.

This is the cheapest route for now. If anyone knows for sure what the prop size should be for an Alpha 1 Gen 2 with a 4.3l 2bbl setup, I'm all ears.

Thanks to all for your input. I've learned a great deal from this site.