Dyno Test

[Forrest Gump]

Sorry,
I do not agree with the numbers of this dyno results as I felt with the OPs results
Using a Mustang Dyno thus most likely a load bearing chassis dyno
States 383 CI has custom CAM and 750 CFM Holley Carb
States at 50 MPH only 13.4 HP

States rearwheel only had 277 HP at 296 ft/lbs Tq
But states that at flywheel would be 394 HP with 421 ft/lbs TQ

That is a 30 percent loss of drivetrain which I do not buy that

Do not know which tranny used or what rear end gear ratio was, but I would think back in 1974 that most C3s used manual tranny and would have like maybe 12-14% loss and if an auto tranny like 15-18% loss but in either case no way that amount of drivetrain loss
HP continues to increase past 5252 RPMs and graph shows test ending around 5500 RPM, should have taken this higher to see higher HP
View attachment 33236

I just don’t know about the reality of the power losses on a C3. They could be much higher than for the newer generations.
A few years ago we had a rolling road dyno set-up at CCCUK Nationals. I didn’t test mine then, it was before I’d rebuilt my motor. But I can remember watching a couple of mates C3‘s getting tested and neither owner came away at all happy with the numbers - nothing like what they were expecting. They were very quick cars on the road though.

 

[teamzr1]

Formula for Calculating Drivetrain Horsepower Loss​

To determine the horsepower loss through the drivetrain, you can use the following formula:
Rear Wheel HP = Flywheel HP x (1 - Drivetrain Loss Percentage)

Example Calculation​

Here’s how to apply the formula with different drivetrain loss percentages:

Flywheel HP	Drivetrain Loss Percentage	Calculation	Rear Wheel HP
300 HP	15%	300 x (1 - 0.15) = 300 x 0.85	255 HP
300 HP	20%	300 x (1 - 0.20) = 300 x 0.80	240 HP
300 HP	25%	300 x (1 - 0.25) = 300 x 0.75	225 HP

Common Drivetrain Loss Estimates​

• 15% Loss: Typically for manual transmissions.
• 20% Loss: Common estimate for automatic transmissions.

These percentages can vary based on the specific vehicle and drivetrain configuration.

 

[teamzr1]

What makes this more confusing for those still stuck using GM's marketing when they reported
Brake HP (Bhp) as the numbers look much higher than the real world as what the numbers look
like at the rear wheels

If wanting to know what the Bhp is simple math :

RPMS * flywheel HP / 5,250 (RPM crossing point of HP and TQ)

At least you could take those dyno results that show what the numbers would be at flywheel
and at what RPMs were to do the math above to find out what the Bhp is and if they than match
GM's engine dyno Bhp numbers

Keep in mind the marketing hype numbers used back in those days was doing engine dyno with
nothing attached to the engine that would be a drag on engine and reduce those Bhp numbers

Today nameplates cannot get away with those Bhp numbers as new vehicles should (as GM does for Corvettes since around 2017)
is doing SAE engine dyno testing with a person from SAE present and has everything attached to the engine as production has, minus the drivetrain and SAE approving the dyno results

Understanding GM Brake Horsepower (bhp)​

Definition of Brake Horsepower​

GM brake horsepower (bhp) measures the actual usable power produced by General Motors engines. This metric accounts for power losses due to friction and other mechanical factors, providing a clearer picture of engine performance and efficiency.

Importance of Brake Horsepower​

• Performance Assessment: bhp is crucial for evaluating how well a vehicle can perform under real-world conditions.
• Comparison Metric: It allows for effective comparisons between different GM vehicles and their engines.
• Efficiency Indicator: Understanding bhp helps consumers gauge the efficiency of a vehicle, influencing decisions related to acceleration and towing capacity.

Factors Affecting Brake Horsepower​

Several factors can impact the bhp of GM engines:

• Frictional Losses: Losses in components like piston rings, valvetrain, and bearings can reduce overall power output.
• Auxiliary Components: Power used by accessories such as the alternator, water pump, and air conditioning and exhaust makeup will also lead to power loss.

 

[Forrest Gump]

Not the kind of result one would expect - no mention of compression ratio - perhaps stock @ 8.5 - 1 (ish)?

Another dream crusher experience!
I would imagine if it’s a built 383 they would have gone with at least 9 or 9.5. I notice a lot of the builds for street cars in the US don’t go with particularly high c/r, favouring a motor that will not be too fussy on fuel octane. Still make very good power output though (or so they claim!)