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There are a lot of people that enjoy working on and rebuilding their own outboard. Many start with the smaller 2 cylinder models. In this region the OMC is one of the more popular brands because of availability, price and information.

I thought I would start with the 2 cylinder 45 cubic inch. This thread will contain information from other people not myself as I have zero experience with this model.

BUT, I want to learn and hopefully build a competitive T750 OMC.

Most competitive racers never want to share their ideas or part with their best equipment until they retire from racing.
Those people are either racing in stock classes where you must adhere to specific engine specifications or MOD classes where you can modify the powerhead.

So we will call ourselves “Hobby Engineers”. We are more interested is just improving the motors performance....stock, mod, radical, extreme....anything goes!
It’s about having fun!!

THE 45 OMC ...again step in with your thoughts...agree, disagree. What would interest or help you.

The xxx100 (big & small 100) and xxx207 blocks/powerheads are used in racing.
These powerheads appeared on motors from 40-60 hp.
The 40- 60 hp shortshaft model with small nitro gearcase was discontinued in mid 1980s.
The blocks from different hp motors appear to be all are the same.

The xxx207 block was a lost foam casting and has a slight intake restriction.
The only differences in 40-60 hp regular motors are high speed jets,
The heads sleeves ( compression holes?) and powerpacks are different on the ropestart commercial models.

The biggest gain is from re-positioning the sleeves so the intake passages in the block are not partially blocked. You heat the block to approximately 425 degrees and the sleeve slides out.

Second is milling and decking, very little is required to get 170-180 psi (more than that hurts performance).
Grind the sleeves to remove and smooth out the sharp edges where the gas mixture flows into the intake ports.
Mill the head .065-.070"

On commercial tiller handle models (40, 45 hp, e.g.) the powerpack has rev-limited to 5850 RPM The carbs are the same on all models as are the tuners.
The lower hp models are jetted leaner.
The largest 56D jets are on the 60 hp.
The crankshaft is the weakness part of the rotating assembly.

The Later Version
Is a different block.....oval port sleeves
Different carbs
Different midsection and gearcase.

In SST 45 race class the motor is basically the same as the production model.
In SST 60 race class the sleeves are different from the production model resulting in a higher rpm range compared to the production model.

What can be done to the 45 powerhead to see a similar improvement??
With a 56 triple.... a high compression Bridgeport has slightly better top rpm performance ...where as the high compression oval port has better midrange performance.
Raise the port height on either model and that improves performance.
Increased port sizing also improves performance.
Both those motors notice performance changes with tuner length

Can lessons learned with the triple work for the twin as well?

Here’s a good starting point.
This thread shows pictures of the motor and the individual parts.
http://www.hpbc.ca/viewtopic.php?f=27&t=4073

The oval port sleeve came about at approximately the same time 1990’s in both the twin cylinder 45 cubic inch and the triple 56 cubic inch.

So wouldn’t ideas learned from modifying the triple work on the twin?

Here is ideas that worked on the triple.
Use the SST-60 tuner on the oval port motor as well as the bridgeport if you want more top end. In addition alter the ports to the SST-60 specs (1.450" without chamfer, & 1.393" to top of chamfer). You want to chamfer the upper lip of the exhaust port at about a 30-45 degree angle to the above SST-60 spec.

It’s quite tight and you must carefully grind to spec. in that the ports are blind as you stated.
You can then cut head about .060-.080" to get back the compression you'll lose by raising the ports.
That will preserve your bottom end and mid range power. Do not change the divergence angle of the tuner to more than the SST-60 or the power band will become very peaky.

So if you build your own tuner, keep the same angle and keep the same length of 7.69" from the top of the adapter plate to the outlet of the tuner.

My comment:
Now T850 racers have found that on a Bridgeport a tuner 1” longer 8.69” ....can work better on a T850 running higher exhaust ports because the longer tuner increases the back pressure some and that improves middle range.
The oval port is stronger in the midrange so maybe the 7.69 length is better?

Also going wider on the porting along with higher compression and larger jetting seems to help the 56 triple on both Bridgeport and oval port blocks.perform well provided the blocks are cleaned up. Some blocks regardless of type just work better. As do crankshafts. There are mixed views on whether the sandcast or lost foam cast blocks are better.

Now...will the same alternations work on the twin?
I can’t find any information of modifying the oval port 45 twin.

Based on the learning experience in the last 11 years with the improved impressive performance of the T850 class. There should be the possibility of making significant performance gains with the twin.

Now let’s think about the gearcase.
The 15 inch stinger/hustler nitro case with 1:76 gear ratio is the most commonly one used in T850.

In T750...same gearcase!....2:33 ratio.

What about running a 1:76 ratio or maybe the 1:91 ratio gearcase with the twin?
On a Light boat T boat application it makes sense.
One noticeable observation in T750 was the tohatsu, the Yamaha and the mercury could run gear ratios of 1:64, 1:71, 1:84, 1:86 and 2:00. They could run small diameter props and smaller diameter normally makes the boat handle better. Smaller diameter/ bigger pitch are easier to turn so more top end speed is possible.
Advantage of a 2:33 ratio....better low end and midrange.....the difference between a 1:64 and 2:33 ratio is like driving a 4 speed transmission car....the 2:33 feels like 3rd gear and a 1:64 is like 4th gear.

Can you makeup for any difference in gear ratio by prop selection? To an extent yes, but for those of us that have raced and had the opportunity to run different gear ratios with different props on the same motor.....there is a noticeable difference. The gear ratio takes advantage of the increased performance of the motor when it is improved and has different characteristics to the original motor.
Example: SST 60 runs 1:76 gear ratio ...that ratio takes advantage of the higher hp and higher rpm range characteristic of the SST 60 powerhead. Same should hold true with a higher hp/higher rpm capable 45 cubic inch twin.

Extreme...run transom mounted water pickup.

Next up.
Intake changes!

Intake changes:

Intake modifications:

Add ons

Velocity stacks
They smooth out the flow of air entering the carbs. Said to increase density of the air? Helpful for motors turning over 7000 rpm? Look cool! Only way to find out is try running with and without them to see if they do make a difference.

Carburetor spacers extension plates
These are plates inserted between the intake manifold and the carb base to move the carbs further out and create a longer passageway for the air to travel before entering the engine. Fairly common on high rpm motor. Again the added distance smooths out the air flow. Velocity stacks and spacers can be 3D printed.

Different carburetors
You would only want to use the large 60 hp carbs. They have enough volume capacity to handle most modifications. Some say the butterfly old version work best. My choice would be the primer style 60 hp carb with an electric fuel pump.

Intake stuffers
We started seeing these spacers that make the reed cage larger a few years ago with the 56 triple.
By making the reed cage larger that reduces the air volume in the intake and that increases the vacuum which draws more air/fuel mixture into the motor to create more hp. Marginal to good improvement?

Reed cages
The stock reed cage are likely good enough. You could try different larger cages from other models or maybe 3D custom ones.

Reeds
Plastic reeds are a must....sport....race...but likely not drag type

Machine work
You could decide to modify the carbs, the reed cages and the intake.