2 cylinder Merc racing motors Rockets, Hurricanes & "converted" 20H's

[Mark75H]

Now your fully "converted" 20H is a complete Hot Rod killer on the race course. Did the Hot Rod racers complain? You bet! In 1959 the racers petitioned the APBA to allow the Hot Rods to use the Champion accessory open exhaust kit, which had been included in the Hot Rod accessory brochure ..... but APBA rules said only the manufacturer could submit an application and that there had to be enough to supply them to all who wanted them. The racer's problem was that Champion had been out of business since 1958 .... there was no "factory" to request the parts be allowed nor supply the parts...... petiton denied!


In the end it was time and economics that killed the Hot Rods of the 50's as much as engineering. For the most part all the effort Merc's engineers put into the conversion kits was wasted .... by the time the conversion stuff was available to racers in mid summer 1959 Champion was down for the count and out.

[Mark75H]

Anyone know the stories of Scott, Martin or Chris Craft's racers?

[Trikki1010]

Thanks Sam,

The engineers didn't waste their time, we might not be chasing after some strange parts for 20h's

So you know if the serial #'s were tied to specific dates of modifications. The only problem I have is to define what a "stock" 20H is by #'s.

AND, many "hot rodded" components

Trikki

[Mark75H]

There is no serial number break for the changes; the changes were all done in the field 3 or more years after production of the 20H's ended in 1956.

The conversion kits came out in mid summer 1959 and were available thru the 1960's.

A stock "as built" 20H had the Carter N carb and looks like the green and gold motor above if built in the end of 1956 and exactly the same except for the shallower clamps and shorter KG7H style skeg if built from 54 to mid 56.

All conversion kits including the KA7A carb were field modifications that were legal for racers to do at home.....none came from the factory with these changes and they were legal for every 20H made from the first one on, so you will find a complete mismash of converted, semi converted and original motors all thru the group.

The only serial correctness you can apply is large skeg motors should have serial numbers in the last 10-12% of numbers and small skeg motors should be in the first 85-90% of numbers.

[AnthonySS]

X-cellent Read...THANKS SAM

[John Schubert]

Sam,

All MK20H's came with the deeper clamps as well as the factory built KG7H's.

[Tom Foley]

Sam , I cant tell you how much I enjoy this wealth of racing knowledge and nostalgia being released upon us ! I await every post as I was totally inspired by the unprecedented array of Mercury Racing product that was readily available during the early years of my racing enthusiasm in the late 60 s and throughout the 70 s and 80s . Actually I purchased my first 55- H from John Stone for 300.00 in 1990 with the case seal still on the motor after winning DSH in 1988 .John is a true gentleman and veteran hard core stock outboard racer . Thanks John and Leigh Furnal for making my "D" days awesome . The 44XS and Dewald cleaver props really woke up the boat and me !!!!! Keep e'm coming Sam !! Thanks, Tom .

[Mark75H]

Thanks for the correction, John.

[Tom Foley]

Sam , Thanks again for the info so far . How about some pics of your Race Stuff past and present . This forum is like an encyclopedia of trick racing hardware . Thanks , Tom.

[Raceman]

What about the clamp brackets on the KG4H? I've got a complete one with deep clamps, but since I don't know the origin of this motor couldn't say how it came from the factory.

My little motor projects:
(all have matching serial numbers on plate and powerhead)

KG4H
KG7H
MK20H
Mk30H
Mk55H
Merc1000 on Daytona (75H length) mid with cutup Speedmaster (looks factory)

Desperately looking for 75H mid and lower

[John Schubert]

All KG4H's also came from the factory with deep clamps. All racers even building uo from parts used deep clamps. It enabled you to go higher on the transom with shim sticks.

[Champhotrod]

Speaking of deep clamps, the drawing date on the Hot Rod clamp with built in thrust bracket was 1955 and the one piece tilt bracket was 1956. For the guys that do not know the Champion very well, the 1956 Hot Rod had a seperate boat mounted thrust bracket. The new set up was stronger and came off with the motor. The clamp/tilt assembly was one of the weak points on the light weight
Champ Hot Rod. I have been told that Champion had an engine at some 1958 Boat Shows with the inclusive thrust bracket and stronger tilt bracket but it wasn't used on actual production that I know of until ownership of the company moved on. Would have been interesting to see what else Champion had come up with if they didn't shut down shortly after that. APBA approved the new Swanson "B" Hot Rod for use in Stock racing in 1972 although Swanson was supplying parts and engines to NOA racers, alkies and mods long before that.

[Chinewalker]

A front shot of my unconverted 20H showing the centered hole for the low speed needle on the Carter N-2150S access. Beware of face plates being sold on eBay as "for Mark 20H" if the hole is off-center.... Not uncommon in eBay world... There is also a small detent in the bottom point of the face plate to clear the high speed needle.

One of these years I'll pick up one of the throttle cable brackets...

- Scott

[2us70]

Still looking for a picture of Stover Hire in front of one of those B motors.

[champ20B]

One problem popped up:

The new Champion Hot Rod 6NR racer

The new Hot Rod could out pull the 20H out of the turns and keep its lead down the straights. At first the Merc engineers were puzzled because the 20H always pulled more power than the Hot Rod did on their powerhead dyno. The answer showed itself when they built a propshaft dyno ..... the original 20H mid section seriously hurt the 20H's top end power. They decided to try redesigning the 20H mid section. A lot of things were tried, but none worked very well. In the end the decision was made to build a completely new mid section incorporating a separate tuned megaphone for each cylinder.

I hate to sound contradictive, but for any future readers interested, the midsection story was a real myth conjured up by mercury and their affiliates. It is not entirely possible for the "B" merc to out-power the "B" hotrod at the crank-shaft, here is why.... First off, there were rotary valve intake ports of a particularly unique design. The merc reed valves could be a bit restrictive above 6000 RPM for fishing models and 6800 for the 20H. According to a 1950s article from Boat Sport Magazine, the hotrods continued to build horsepower well past 7000 RPM. Next were the specially designed and timed induction and exhaust ports. Champion went by tried and true formulas that corrolated port heights to piston stroke. These ports were independently designed so that the induction side had a progressive high-flow characteristic and the exhaust side was of a high velocity squared design a bit taller. Furthermore, the bore was larger with a shorter stroke. This made way for long-radius porting, which it had, as well as a high RPM-freindly rotating assembly. This engine was geared toward alchohol racing capability out of the box. The hotrod was simply a more versatile and superior engine to any other american stock outboard in relation to performance. Mercury simply could not win with a stock midsection no matter what. They (mercury) had to snake a modified setup into a stock race to win. Try a "B" hotrod with the KA7A carburator and tuned megaphone stacks against a 20H (converted) model. You will then see the truth. I dont believe anyone ever really tried this in a race on pump gasoline. The hotrods have carried on. Mercs are more of an antique hobby engine although they still let them run in A-Mod.