Speed & Horsepower Trivia

Bob Bidonde

Bob Bidonde

Well-known member
I was studying some dynomometer results of a Model "A" engine with a near stock 4.4 (stock is 4.22) compression and a 5.5:! compression. I notice that horsepower peaks about 47 to 50 MPH with both heads. I have 6:1 compression heads on my Model "A's" and I am guessing that the horsepower also peaks at about 50 MPH. I also notice that the manifold vacuum in my Victoria is steady at 17 in-mg while cruising at a steady 50 MPH. So 50 MPH is an efficient cruising speed.
 
2500 rpm, 50mph.
 

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Makes sense because they would have known that 50-55 is the speed above which air resistance increases geometrically, and these cars are distinctly brick shaped. No point in optimizing your design for speed above that.
 
Larger valves, touring cam, 6.0 head, stock intake and carb, 3.78 rear gear ratio, in overdrive, 53 mph is my sweet spot ! Way past peak torque and probably HP, a higher sweet spot 64 mph but then gas mileage is alot less !!
 
Aerodynamic drag increases with the square of the speed. So it is 1.7 times as much at 65 mph as it is at 50 mph. Or 4 times as much at 100 mph as it is at 50. Or 16 times as much at 200 mph as it is at 50. I'll let you figure out what the ratio is at 1,000 mph.
 
Bob, what engine speed are you turning at 50mph? I am curious as to how far you are from BEP on your engine.
 
I estimate my engine was turning 2200 RPM. My Victoria has a stock 3.78:1 differential and stock size wheels.
 
Stock tire size is 29 inches in diameter or Pi X 29 = 91.1 inches in circumference or 7.59 feet in circumference. A mile is 5,280 feet. So a tire turns 5,280/7.59 or 695.5 revolutions in a mile. Or at 1 mile per hour the tire turns 695.5/60 = 11.59 rpm. You would multiply that times the speed in mph and times the gear ratio. For instance at 50 mph and 3.78 gear ratio, the engine would turn 11.59 X 50 X 3.78 = 2,190 rpm.

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So Bob's estimate of 2,200 is spot on at 50 mph. With my overdrive the overall gear ratio in overdrive is 2.8 to one. So the engine rpm would be 1,630 rpm at 50 mph.
 
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Ouch. NK, you are hurting my head with your numbers.

Let me guess, you must have been an engineer in another life. Am I correct?
 
nkaminar said:
Stock tire size is 29 inches in diameter or Pi X 29 = 91.1 inches in circumference or 7.59 feet in circumference. A mile is 5,280 feet. So a tire turns 5,280/7.59 or 695.5 revolutions in a mile. Or at 1 mile per hour the tire turns 695.5/60 = 11.59 rpm. You would multiply that times the speed in mph and times the gear ratio. For instance at 50 mph and 3.78 gear ratio, the engine would turn 11.59 X 50 X 3.78 = 2,190 rpm.

So Bob's estimate of 2,200 is spot on at 50 mph. With my overdrive the overall gear ratio in overdrive is 3.0 to one. So the engine rpm would be 1,740 rpm at 50 mph.
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Update - corrected typo.

What NK has stated above as "the gear ratio" is actually the final drive ratio, which combines the transmission output gear ratio and the differential gear ratio. On a stock transmission in third gear the transmission output gear ratio is 1:1, that is, the engine (transmission input shaft) is turning exactly the same rpm as the transmission output shaft. So the final drive ratio is 1.0 x 3.78 = 3.78. On a Mitchell overdrive the transmission (actually, overdrive) output ratio is a 26% reduction, or 74% of input. So the final drive ratio of a stock transmission in third gear (1), 26% Mitchell OD (0.74), and stock differential (3.78) is 1.0 x 0.74 x 3.78 = 2.80 (NK, how did you get 3.0??).

You can use this same formula to determine rpm of any gear, any speed, any rear end. Just plug in the right numbers.

BTW, if you want to play around, the first gear ratio of a standard transmission is 3.12:1 and second gear is 1.85:1. I don't have a handy reference for reverse, so anyone can chime in.
 
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JayJay said:
What NK has stated above as "the gear ratio" is actually the final drive ratio, which combines the transmission output gear ratio and the differential gear ratio. On a stock transmission in third gear the transmission output gear ratio is 1:1, that is, the engine (transmission input shaft) is turning exactly the same rpm as the transmission output shaft. So the final drive ratio is 1.0 x 3.78 = 3.78. On a Mitchell overdrive the transmission (actually, overdrive) output ratio is a 26% reduction, or 74% of input. So the final drive ratio of a stock transmission in third gear (1), 26% Mitchell OD (0.74), and stock differential (3.76) , is 1.0 x 0.74 x 3.76 = 2.78 (NK, how did you get 3.0??).

You can use this same formula to determine rpm of any gear, any speed, any rear end. Just plug in the right numbers.
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Well I get 3.78 X 26%= .98, 3.78 - .98 = 2.8:1. Maybe NK rounded off his numbers??
 
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Mitchell overdrive is 26% or 36%. Mine is 26% which means the ratio is 1.35 to 1. My differential is 3.78 to 1. If you take 3.78 and divide it by 1.35 you end up with 2.8 to 1.

I spent most of my carrier in engineering in one form or another, mostly designing automation equipment. I also designed houses and built a few. In college I studied mechanical engineering, automotive engineering, and architecture. I started out with a slide rule and drafting board and then used Autocad. I switched to Solidworks as soon as it came out. I now use Chief Architect. I also dabble in ceramic sculpture and a bunch of other stuff.
 
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When Mitchell says 26% or 36% they mean a REDUCTION in engine rpm. So the overdrive ratio in overdrive would be 1 divided by (1-.26) = 1.35 to 1, or 1 divided by (1 - .36) or 1.56 to 1.
 
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I corrected my post above. Too early in the morning for me, not enough coffee on board.

Thanks Y-B.
 
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nkaminar said:
26% increase means that the ratio is 1.26. 100% would be 1:1. 200% would be 2:1. 26% is 1.26 to 1.

The differential reduces the rpm of the engine at the rear wheels. A 3.78 ratio means that the engine rpm is 378% higher than the rear wheels. Percent means per cent or per 100. You just move the decimal point over two places. 100% is 1.0 and a 26% increase is 0.26 added to 1 or 1.26 to 1.

There will be a pop quiz in the morning.
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Shouldn't it be decrease. 26% decrease means that the ratio is .74? 100% - 26% = 74% .74 X 3.78 =2.80. As I stated in post #13? :confused:
 
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OK, you are correct. I was wrong. It is a reduction in engine speed not an increase in road speed.

2.8 to 1.

I need to pay more attention to the details. Thank you for correcting me.
 
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