Dynamometer Testing

David in San Antonio

David in San Antonio

Well-known member
The touring engine built by Bert’s Model A Store in Colorado (insert bearings, counterweighted crank, Burtz head and cam, lightened flywheel) has a few thousand miles on it now. It is noticeably stronger than when first installed in the Wretched Roadster. Has anyone had experience measuring your Model A’s performance on a dynamometer with the engine in place, not on a stand?
Advice welcomed.
 
I do not have any dynamometer info but I also noticed that mine got stronger as it broke in. The block is harder than the original engine so it takes a long time to break in. Mine took about 2,000 miles.

Do you have a larger carburetor? I have a Weber on mine and my engine keeps pulling past 3,000 rpm. It would probably go higher but I have set an arbitrary red line of 3,000.
 
Put it on a chassis dyno. That measures horsepower at the wheels, you'll lose some because of driveline loss, but that's normal when compared to measuring at the end of the crank.

Remember, once an engine is broken in, it's starting to wear out. They break in fast and wear out slowly.
 
here is the dyno sheet from my A on the chassis dyno.
Pull in green is with a way too rich mixture, red is after getting it right
5.5 head, 97 carb , mild camshaft.
Lawrie
 

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@Nkaminar - Tillotson carburetor, no external modifications at all - yet. I don’t have a tachometer on it either, but I upshift when I anticipate the acceleration is about to lessen. In third I lift off based on mechanical sympathy and the violence of the wind blast. I am curious if it has enough additional horsepower to go faster with an overdrive.

@Lawrie - I see the air/fuel plots on the graph. Did you correct the mixture using the GAV, or doing something right at the carb?
 
we have a stromerg 97 on it, the reason it was so rich was the dyne operator just held it flat to the floor, and as the 97 has a mechanically operated power valve that I had not checked it was too big, we fixed it by fitting a smaller power valve.
Lawrie
 
Years ago I ran my '31 pickup on an electric dyno. Those dynos read wheel HP, then you can let the dyno drive the wheels. At that time my PU had a 2-port Riley with a stock B cam, 7.4:1 compression and a single glass bowl Holley carb, stock exhaust. It put out 60 HP at the wheels, and when the dyno drove the wheels, it took 25 HP to drive rear end, trans with the clutch pushed in, 85 HP at the flywheel!
 
David, as you live in Texas, try and find a Marvel Carb, should be easy to find with all the Model A'ers in Texas. Once you try it you will never go back to the Tillotson. That's my experience.
 
@Herb - Thank you for the tip.
@Alexiskai - Wow! Approximately 30% driveline loss!
Now that the weather has cooled off enough I’m going to drop the top and see if the wind resistance is better. When I first got the Wretched Roadster I attempted to fold down the windshield but gave up when the stanchions strenuously resisted. Probably for the best because I’d hate to break a windshield.

(If it seems counterintuitive that I’d lower the roof when the weather is cooler, it’s because the summer sun here will roast you like a rotisserie chicken even if there’s plenty of airflow. The cloth top blocks the sun. A hundred degrees in the shade is a lot better than 100 degrees in direct sunlight. I’m not a big fan of driving the Model A at night even though it’s cooler out.)
 
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Quote: "@Nkaminar - Tillotson carburetor, no external modifications at all - yet. I don’t have a tachometer on it either, but I upshift when I anticipate the acceleration is about to lessen. In third I lift off based on mechanical sympathy and the violence of the wind blast. I am curious if it has enough additional horsepower to go faster with an overdrive."


The higher compression head will add torque. The Tillotson carburetor restricts air flow and so does the Zenith. Using a mild cam and a larger carburetor will allow the engine to breath at higher rpm. With the Tillotson you will have the same performance as you would with a larger carburetor at lower rpm but will not be able to extend that performance to the higher rpm. So you have the torque but not the horsepower which requires the higher rpm. Adding an overdrive will help. Having the top down and the windshield folded will help.

The Model A is not the most streamlined car on the road. Wind resistance is really what determines how much fuel it is using. Whereas in my modern car the mileage is more determined by whether I am going up or down a grade.

Regarding other external modifications: For my usual rpm, I found that the stock exhaust manifold worked just as well as a custom header.
 
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WOW! Assuming the 30% loss is across the board, and I am assuming 45 HP (5.5 head, Marvell carb, 'B' cam, lightened flywheel and Mitchell O/D) that means 45*.3=13.5 approx. HP is taking me down the road at 50. Am I correct in my math?

Mike
 
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