Horepower and rpm of Stock engines

[Smitty]

(Couldn't come up with a Search term to find this)

This info is no longer accessible in my elderly brain, and maybe I never had it. I'm wondering first if anyone knows the rated horsepower of a converted Mk20H. As I recall(?), rated peak power came at 7100-7200rpm. I believe the original quiet 20H was developed from the Mk20 production motor of about 16hp at whatever rpm. Seems like I saw something for the quiet 20H as "twenty-plus" horsepower depending on rpm. But I don't recall any figures for the 20H with the open exhaust.

Someone here always seems able to come up with obscure information, so does anyone know what the exhaust-opens timing was for the 20H as compared to the Mk 20 or Mk25? I assume that part of the original development of the 20H was advancing exhaust timing . . . .

I think it would be interesting to see HP@RPM figures for any other Stock racing motors, old or new. What could you get out of the old H-model KG-4s/Mk15s, for instance?? Or maybe the Mk55H vs. the "black block" fishing motors that some of the D Stock guys switched to in the early Seventies because they could pull bigger props? Or the various stock Yamatos (though maybe the figures for them are all published)?

[DeanFHobart]

(Couldn't come up with a Search term to find this)

This info is no longer accessible in my elderly brain, and maybe I never had it. I'm wondering first if anyone knows the rated horsepower of a converted Mk20H. As I recall(?), rated peak power came at 7100-7200rpm. I believe the original quiet 20H was developed from the Mk20 production motor of about 16hp at whatever rpm. Seems like I saw something for the quiet 20H as "twenty-plus" horsepower depending on rpm. But I don't recall any figures for the 20H with the open exhaust.

Someone here always seems able to come up with obscure information, so does anyone know what the exhaust-opens timing was for the 20H as compared to the Mk 20 or Mk25? I assume that part of the original development of the 20H was advancing exhaust timing . . . .

I think it would be interesting to see HP@RPM figures for any other Stock racing motors, old or new. What could you get out of the old H-model KG-4s/Mk15s, for instance?? Or maybe the Mk55H vs. the "black block" fishing motors that some of the D Stock guys switched to in the early Seventies because they could pull bigger props? Or the various stock Yamatos (though maybe the figures for them are all published)?

My recollection is this: KG-4 / Mk-15.. 15 to 17 HP. Converted 20-H.. 28 to 30 HP. So a MK 55H would have to be 45 + HP.

[Ram4x4]

Yamato 80 - 327.8 cc, 30HP at 7000RPM
Yamato 102 - 396.9cc, 30HP at 6400RPM
Yamato 202 (not legal in APBA stock categories) - 396.9cc, 32HP at 6600RPM
Yamato 302 - 396.9cc, 33HP at 6600RPM
Yamato 321 - 396.9cc, 33HP at 6800RPM

[rumleyfips]

Hubbell's catalogue had an ad for their tuneup and dyno service. They said a KG4 was 15 hp and a real good one 16 : a KG7 was 18, an unconverted 20H 21 and a 30H 30hp. Adding 15 to 18% for the tuning of the conversion housing a converted 20H would be 24-25 hp.

Hubbell never said whether his dyno was powerhead or propshaft. I assume powerhead was more common at the time.

[dwhitford]

John, your numbers are consistent with things, I've read also. Carl K deliberately underrated his engines in the 1940s and '50s to underscore their growing acclaim as real performance engines.

For example, rating the KG-4 at only 7-1/2 hp was a farce. Riggs Smith put on a strictly amateur race in 1955 or' '56 in front of the 1000 Island Park dock. Two identical Lyman 13-footers entered one of the heats. One had a KG-4 fishing engine, and the other had a 16 cubic-inch 1953 Johnson 10 on it. At the end of that 3-lap heat. the KG-4 had a half-a-lap lead over the Johnson 10.

I've read that KG-4s really develop 12 hp even in ''fishing'' trim. I don't for a moment doubt that. Don't forget that in 1953, Carl re-introduced the KG-4 as a remote-tank gearshift engine: the 10 -hp Mark 15.

Also in 1953, Carl introduced the Mark 20 at 16 hp, to replace the KG-7 and KH-7 10-horse engines. The only difference I've been able to discover between the Mark 20 powerheads and the KG, KH-7s is that the Mark 20 has a ''roomier'' sandcast reed cage to replace the milled bronze reed cage of the previous KG engines . . . certainly not by itself worth 6 extra horsepower.

Savvy A-Stock racers back in that day glommed onto those sandcast Mark 20 reed cages . . . to what real advantage, I do not know, but it was
the ''thing to do'' then, and I put them in both KG-4 racers that I ever built too. Just to be safe and sure . . .

So by 1953-'54, Carl was getting more realistic about reporting his horsepower. In the early days, he was using SAE ratings, while Johnrude and others were using the Outboard Boating Club (OBC) ratings. The Mark 20 rating at 16 hp was probably realistic.

In 1955, Carl introduced the Mark 25 at 18 hp. This engine was no bigger than the KG-7s and Mark 20s, and had equivalent porting. It was just heavier-duty with a crank & rods more like the 20H (same rods exactly!) The following 2 years, the Mark 25s were re-rated at 20 HP, which was probably correct. By this time, the factory engines were re-propped to turn 5400 (or so) instead of the stodgy 4000 RPM of the KG-series engines.

So to say that a KG-7 at racing RPMs (6300-6500) might develop 18 hp is probably within the realm of credulity.

I also have no doubt that a Mark 20H conversion Merc can make 24-25 hp. They were always faster than the 22-cubic inch 20ss engines.

[Smitty]

Thanks all, for the good discussion.

[rumleyfips]

Dave :
My 25 was as fast as my 20H , same Racecraft runabout, same prop , same setup. I thought the 20H accelerated a bit better but not much. Both outfits were competitive; a good BSH was only 2 mph faster. All testing with a Keller on the same day.