Ok. So in the mid to late '70's I know they changed the way horsepower was rated and the numbers all really came down. The question I have is with the early engines, particularly the flathead, when they say 100hp or 125hp is it actually alot less with the "newer" SAE rating?
Yes. Old ratings were based on a dyno run (or runs) where the engine had no air cleaner, likely no driven water pumps or fans, no muffler and spark timing adjusted (as the engine ran) for max output.
20% is an accepted conversion from flywheel HP to rear wheel HP (essentially drivetrain losses) the SAE vs. Gross thing is something else entirely. It's amazing how much power you can get out of an engine when you set the conditions just right...
Oopsie- bort62 is right; I was thinking flywheel vs. RW. Different engines will react differently under each testing procedure (gross / net). Cadillac released both ratings for '71; 500: 365 gross, 235 net. That's a whopping 35% decrease. Buick's '72 455 was 315 HP gross & 225 net- a 29% decrease. I would tend to believe a flattie might lose less % that either of those 2 accessory-laden mills, but I have no proof.
Right: domestic ratings were changed to net for the '72 MY. Unless some ******** flattiefan had access to an SAE lab, no doubt none have ever been tested (last FH : '53, first net HP: '72). I would still believe 25% is a solid estimate: 125HP gross = 94 net. 100 gross = 75 net.
Flat Ernie reported a stock rebuilt 239 in this thread.... http://www.jalopyjournal.com/forum/showthread.php?t=175469&page=2 made about 110 hp with 3x2, 90ish with OEM setup
Some stuff: Ford Service bulletins contain flathead dyno sheets on all engines; They contain varying amounts of info. Here's a 239 test with the printed explanations: Feb. 15, 1939 bulletin, test of a 99A '39 Merc 239/95 HP rated engine. Standard equip was 91A Ford carb, 81T heads, presumably a model 78 distributor with a rather weak advance curve compared to later ones. Chart graphs brake HP, torque, compression pressure, advance curve used (this was manually set to "Master Curve", I think meaning set to curve spec rather than peak HP), and mechanical efficiency. Notes say engine ran with generator and pumps but no fan or muffler. Betcha the stupid crank fan wouldn't fit on the dyno bed... Peak of about 98 hp at 3600, torque peak about 178. Note that on sub-5200 RPM engines, HP computation is pretty meaningless for comparison. Flathead trademark torque: measurement started at below 500RPM, where torque was already over 125...plenty of torque available from slow idle to sign-off at 4,000
Lots more ancient HP numbers here: http://www.jalopyjournal.com/forum/showthread.php?t=16426&showall=1 But...there are two kinds of dyno, engine and ch***is. Engine dyno runs are more often cooked with mathematical adjustments, disconnected/adjusted parts, and public relations. Ch***is dynos have there own problems...mostly that you need to make a flying guess what the numbers might be at flywheel...but at least they test a complete engine screwed into a real car. Here's an excerpt from an old post of some ancient ch***is dyno figures from a real world flathead: Here’s a dyno/drag test from about 1954, excerpted from the 1955 HRM annual. Real numbers from these times are scarce, and even E.T.’s were rarely reported. Most drag results were strictly MPH. Note a few things about these tests: They are modifications on an existing used car, not a full build up. They are entirely bolt-on mods on a stock 239 short block, without even a cam change. The HP numbers are rear-wheel ch***is dyno numbers, NOT flywheel HP. Onward: The car involved was a stock 1940 Merc club coupe, which would have been a typical back-row-of-the-used-car-lot teenager’s car in 1954. The engine was a commercial rebuild, described as “in neither excellent nor in poor shape” with 12,000 miles since rebuild. The pictures show it to have a Stromberg and a dime store chrome aircleaner, probably giving a slight power loss from stock. Stock with distributor freshly strobed and new plugs (H 10), it pulled 69 HP at 50 MPH on the rollers and turned a blazing 17.23 @65.47 MPH at the Santa Ana dragstrip. Test 2 added a Sharp super dual with two 97's, stock except for .048 jets (I would think a bit rich?). This produced a 16.56 @ 71.01 MPH, power peak moved to 80 at 55MPH. (Power was tested at speeds from 30 to 60 in high, with practically all mods showing improvement at all tested power levels, by the way). Test 3 was with the addition of 8.5 to 1 rated finned heads; They carefully avoid stating or showing brands, I would guess because they had so many advertisers to offend. They were R type heads requiring changing out the shorter studs. This produced 84 HP at the wheels at 55, and went16.07 @74.99 at Santa Ana. Test 4 added dual exhausts and a pair of Hollywood Deeptone mufflers, used with stock manifolds. 86HP, 16 flat @ 75.01. Test 6 added a Harmon-Collins dual coil, which produced only trivial gains over the fresh stock distributor. And that was it for that issue–only modifications that an ignorant teenager with $5.00 worth of tools could have performed in dad’s driveway on Saturday. I really wish they had gone on to a cam and headers, but no such luck. Bruce. P.S. Guesstimate those numbers into flywheel numbers, guess the weight (probably about 3300), and run them in standard formulas. Results, while purely guesstimates, are interesting.
Now...I took the 16.00/75 MPH strip numbers to an online calculator, guessing car weight at 3300... ET method estimates power at 159.2, MPH method estimates 108.7 I kinda think this huge variation is based on the fact that the engine has a top end below 5200, critical to HP calculation, and unnaturally high torque 400--4,000RPM...the engine ET's better than it should by modern standards. At least this provides some kind of independent check on dyno numbers (note this is the same 99A model engine with factory dyno numbers above). Also note that the 3300 estimate is possibly low, not likely high. And...note that this engine is probably getting into a level at which stock cam is a major cork, and that it probably had OEM 3.54 gears, not the best for a drag test. Bruce
Ford's dyno charts typically had mutliple curves based on w or w/o various accessories on the flatty. I've got some somewhere...but there's at least 3 HP ratings in the various literature for the early 59A series flatties because of this...
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