History of the Chrysler Racing Outboards & evolution of the racing lower unit

[Mark75H]

In the late 1950's Bob McCullough bought the Scott-Atwater outboard company in Minneapolis to add to his manufacturing empire of military drone engines & chainsaws. One of his first additions was assembling a team to look into racing the outboards. By 1965 he was heavily involved in the Lake Havasu City Development project and didn't have the capital that the outboard company needed to stay competitive, so he shut it down.

In January of 1965, Chrysler bought the West Bend outboard company and hired the former McCulloch racing department to make a racing version of their 105 hp. The lower unit they designed and built may not be the prettiest or the fastest, but it may be the strongest single pinion racing lower unit. It is certainly not the smallest ... its a little bigger than the Merc Speedmaster in every way. Combining a slightly larger unit with 7% overdrive gear ratio and a pair of fat gears with reduced outside diameter is probably why almost all of these units are found in working condition; a dry rotted water pump impeller is usually the only working flaw if any.

On Merc's Speedmasters Strang & company kept the exhaust outlet below the cavitation plate, but at Chrysler, Kies and Oxley placed the exhaust outlet on top of the cav plate for some more racy sound and more prop clearance. The maximum diameter you can swing on a Chrysler is 10", on a Speedmaster you have to cut the exhaust snout to clear a 10" prop, 9 5/8" is about the largest regular prop that would shoehorn onto a Speedmaster.

The prop gear and spacer interchange on the prop shaft, so the unit can be assembled for either left or right hand rotation. At the back of the propshaft they stayed with the BIG theme and the shaft size is 7/8", 1/8 fatter than the Speedmaster.

On the top of the Chrysler lower unit is a feature that is usually ignored, but probably contributes to its longevity as well. Between the water pump and the crank shaft splines the driveshaft diameter is reduced to allow some torsional flexing. This spares the gears the full hammering effect of running under a big bore 4 cylinder motor. (In recent times the Bass racing lower unit used the same thinned down driveshaft idea to run under the OMC triple motors in the 850cc Mod classes. Without the thinned down shaft, the triples hammer the Bass unit apart.)

[Mark75H]

Even though the literature says the lower unit was developed specifically for the 105 AND the 75, I've never seen any props that would make it usable with a 75. In fact, you'd have to have a real fast race boat like the Switzer to run the 105 with a racing lower unit.

The racing lower units were also available as an aftermarket item, so many big Chryslers found with them may not be "factory racers"

There is a very simple way to identify a Chrysler factory race motor: the 4th digit of the model number will be an 8. 105 hp factory racers will have a model number that starts with 1058. This continues thru to the 150's as 1508's.

[LakeRacer99]

Sam, Great info, Thanks for postin. Has anyone repitched the props to make them run on the 75 or the 105? I think I have mentioned it before, but the lower that I have came from a guy that ran it under a 75 on a Sidewinder. He never mentioned a meltdown. The prop is an OJ and I don't remember the pitch. I have it mounted on a 75 for now til I find a Force 4banger powerhead.

[Mark Poole ModVP]

That is very good info! Thanks for posting that.

I have heard that these lowers would not pull a lot of pitch even on the 135 and 150's. I assume that is because of the overdrive ratio and the fact that the racing models did not produce any extra horsepower? (A 150 stacker was just that...a 150)

[mac19f]

"the shaft size is 7/8", 1/8 fatter than the Speedmaster." Early Chrysler racing units have a large smooth propshaft. The later units are splined and Super Speedmaster props fit them perfectly.

[LakeRacer99]

Does anyone know the Oakland, Michigan or other numbers fo props for this unit? I was looking for an OJ catalog or list of props a while back and was unsuccessful. I have a splined prop about the same size, but shaft is bigger.

[Mark75H]

"the shaft size is 7/8", 1/8 fatter than the Speedmaster." Early Chrysler racing units have a large smooth propshaft. The later units are splined and Super Speedmaster props fit them perfectly.

You are skipping ahead a few years ... I'm not ready for that part yet

[MN4V]

Do you have any photos of Jack Oxley's mid sections and motors used on the Drag n Fly hydro?
Mark N

[Mark75H]

I think so, but I'll have to look later and get back to you

[Powercat]

I know I have the photos somewhere of the Drag'nFly motor setup but can't find it..
Here is a similar set from Gary Furguson's Twister drag boat from a few years prior
to the Drag'nFly and then a running shot of the Drag'nFly

[Mark75H]

The towers on Ferguson's boat are McCulloch 630 towers. I think the Drag'nFly used Daytonas.

[1BadAction]

So where does the development of this LU fit in with merc and OMC direct drives?

[Mark75H]

Development of the Chrysler racing lower unit began after Merc had upgraded the to the Super Speedmaster in 1965, but before the first skinny OMC direct drives in 1968 or 69.

[Mark75H]

I have a splined prop about the same size, but shaft is bigger.

Like Greg said, later Chryslers and SSM's use the same shaft size and splines. A slightly bigger shaft prop is probably a Mercruiser SSM prop.

[Bruster]

I know I have the photos somewhere of the Drag'nFly motor setup but can't find it..
Here is a similar set from Gary Furguson's Twister drag boat from a few years prior
to the Drag'nFly and then a running shot of the Drag'nFly

Great pic of the Carlson/Chrysler got any more?
The silver/black "Eliminator" boat is the sister boat of the "Twister".