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Thread: omc 70/75 Differances???

  1. #1
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    Default omc 70/75 Differances???

    Can someone tell me the differance between the omc 70 hp and the 75hp. I already know that the 75 came in a short shaft and had a smaller gear case , as well a long and the larger case. My question is......
    1- Is the powerhead the same between the long and the ss the same?
    2-How is the does the 70 hp differ from the 75?
    3- Does the 75 make more power higher up in the rpm range?
    4- What else or anything is the same..carbs, pistons, flywheel, crank, tuner,Etc?
    5- Are some years better than others?

    One other thing... What makes a 45 ci 2 cyl become a 40 - 50 or a 60 hp. tuner , carbs , porting????

    THANKS!!!!!!!!!!!!!!!!

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    Team Member JohnsonM50's Avatar
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    You can check for part # differences at BRP - Parts Catalogue. Itll take a bit of time though.

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    Default 70/75

    The 75 short shaft had a smaller geracase with a different gear ratio and slightly different water pickups. The 75 powerhead had a slight angle cut at the top corner of the exhaust ports, this improved performance on top end. They increased the size of the rod beam starting in 1976, but I raced against guys who ran the small rods because they were lighter.

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    Team Member Smokin' Joe's Avatar
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    Default OMC 70, 75 differences

    70 has lower port timing and compression, and if I recall correctly slower carburetors (plus, was never produced as short shaft). The pre-1975 pistons actually were lighter than the lightweight 1975 75 pistons, which were the lightest of all 75 models. The lightweight pistons and 1975-'76 gearcase were essential for EP, but the later model motors ran at least aas well on tunnels and were more durable. I ran a 1975 75 shortshaft in EP from 1977-'85, and ran a good 1978 shortshaft in SE one season. I still have parts, crank, ligtht-weight pistons, etc.

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    75hp got a cut in the outer exhaust plate, had a 1 1/2" tuna, the chamfer on the exhaust went all the way across, omc triple intake ports are in a high state of tune, really no reason to mess with them, ran way less timin

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    76 is said to be the Best year

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    Default 1975 or later

    I ran a '76 powerhead once in EP, a dog. Same gearcase but heavier pistons and rods than 1975. As I said, there was no difference on tunnels, but SE drivers drove later models, did not favor 1976.



    Quote Originally Posted by Fast Fred View Post
    76 is said to be the Best year

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    hears your three choices for the 49, ya some of the bullets are way thick

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    Team Member Smokin' Joe's Avatar
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    Default This is after 20 yrs., but ...

    .. piston/rod on right look like 1975, middle is clearly later model, piston on left: don't recall what Mod 50 pistons looked like, although I still have 3 in storage. Mod 50 rods and pistons were different than for 75 hp models, lighter, for one thing.

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    thats a Wiseco Mod 50 piston on the left

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