Mercury Mark 40-H

[MTECHMARINE]

Seems that this discussion is a bit heated, lets just post for fun here and maybe we can all learn something. Especially ME!

OK, I have a C mod crank I am installing in my "MK50H" or, to be politically correct a Mark 50 on a Q tower with suitable modifications to add some performance.

It ostensibly came from a Quincy built sumpin' or another, it is beveled on the leading edge only and has plates welded onto the inner face of the counterweights to raise the crankcase compression!!

By the way, wouldn't it make more sense to bevel it the other way to scavenge the incoming fuel away from the reed block and into the center of the crankcase??

[Mark75H]

Splines on a C crank will not fit a D drive shaft ... unless the motor already has been converted to a D crank

[Bob Rusnak]

Originally Posted by Mark75H

Splines on a C crank will not fit a D drive shaft ... unless the motor already has been converted to a D crank

A quick reply, I have a D drive shaft with C crank splines on it. Maybe this will help. Will sell if needed as it is a unusual shaft and not really in demand..... Bob

[John (Taylor) Gabrowski]

I had so much bad luck in terms of breaking 30H gearcases, 5 of them where the ends would break off the driveshafts putting gears through the case as well as gear failures even with good maintennance and overhaul practices I just plain gave up on them and changed to D crankshafts and D gearcases. So you lost some mph but the carnage stopped.

Mtech: I had a brand new Mark50 Merc crank which is squatter at the top than the Mark 55 or for that matter a Mark40 and thought it quite novel when I put it in one of the 30H Mods. Its not that I didn't have other later model cranks it just that it was novel to put that crank in. Later on people wanted to buy the crank out of it for restoring Mark50s but there it sits today in the Mod engine anyway together.

The picture of the Mark 75 front case with pyramidal reeds for 6 carbs is an unreally nice piece of work. That would sure have been a crankcase the Alky guys would have appreciated back when 6 banger Quincy Flatheads were proweling the race courses. Will that make the performance of a Mark 75 deflector any better for Formula E or Super E against a OMC 3 Holer looper is a hoped for objective?? Being a Merc freak I would hope so but there are many detractors of the notion who support newer OMC 2 and 3 Holer loop technologies where parts are cheap and available versus older and harder to get stuff. What are the asperations at work with it? It is great to see stuff like that being done.

[John (Taylor) Gabrowski]

One reader/poster is right calling attention to crank beveling and where it is done. Generally the beveling is done to widen access to the Merc reed blocks and their associated stops to get wider openings with a longer duration reed action as a result. Doing the bevel on the reverse side does what?? Some argue that on the reverse it helps with air/fuel transfers down the air/fuel transfer tunnel. Problem remains that if you have the weak crankcase pumping pressures of a Merc 30H the last thing one would do is make the problem worse. Quincy Welding engineers in the Deflector Alky days saw that was an acute problem with Merc 30H Alkys so they put in tunnel insert liners to decrease crankcase CCs and increase transfer pressures and velocities. It was and still is validated engineering for Merc 30H type engines. Such problems did not exist for Merc 40 and 44 cube 4 cylinder engines though later Merc 44X and Mariner 44X stock racing engines saw their under piston crown piston castings fill in more to reduce crankcase CCs even more still during the same piston runs that introduced the newest fractured at the big end connecting rods.

Its interesting stuff.

[Mark75H]

Originally Posted by John Taylor

Will that make the performance of a Mark 75 deflector any better for Formula E or Super E against a OMC 3 Holer looper is a hoped for objective??

No, 850cc Mod rules limit the 60 ci Merc to "3 conventionally mounted carburetors"

[John (Taylor) Gabrowski]

MTech: So this 6 banger is an experimental 6 carb really little different in principle than the Nydahl front cased 4 carb Merc 44 I am developing. Its just outside the rules with a major front end change. I had a couple of Merc Mark 75s I was toying with for FE but with hearing they are about 10 miles per hour slower didn't help keep me on track in comparisson to the OMC three holers. By this time I was thoroughly disgusted with the Merc and Mariner 2 carb 3 holers. To add to that the combustion chambers on the Mark 75s are bigger than the combustion chambers on similar bore engines aka Mark 55H to make the Mark 75s somewhat easier to deal with.

With your 6 carb Mark 75 are you going after the combustion chamber size up to bring it to Mark 55H specs on gasoline or a little further to use racing or aviation gasoline? It all makes for great breathing on the front end but ultimately it is a combination of a whole bunch of engine factors and some more changes front to back that produces meaner machines. Makes for good discussion it does.

[david bryan]

john did they take your post down letter to my employes or where is it

[Mark75H]

Originally Posted by david bryan

john did they take your post down letter to my employes or where is it

Deleted by Ron as off topic ... not related to boat racing

[david bryan]

Originally Posted by Mark75H

Deleted by Ron as off topic ... not related to boat racing

i doubt it was ron he not a liberal