The Stovebolt Site Forums The Shop Area The HiPo Shop Loaded vs Unloaded Exhaust Sound ?

Went out to a local car show today in the Model A Hot rod. I'm running open headers on a SBC with a 4 BBl Holley,an RV cam and 700R4. Question...Why does 1500 RPM's under load sound different than 1500 RPM's not loaded? What mechanically is happening in the engine to produce different exhaust sounds?

simple, it's working harder .... burning more gas to pull the extra weight at the same speed

Bill

Why is an M-80 louder than a ladyfinger firecracker? More powder gets burned!
Jerry

That is simple, of course it takes more throttle to get to 1500 rpm's loaded than unloaded. But it's not just louder, it's different. It's that lugging sound that we all recognize. Guess that's it though if nothing else is changing. No fancy discussions on harmonics, or exhaust velocities, etc...just bigger explosions.

I'm betting the cylinder pressures will be higher when the engine is loaded. More throttle=more fuel=more air=bigger explosion to make for the same amount of RPM. So, different pressures and different sound.

I beg to differ (respectfully) with everyone except red58.
At any given RPM (1500 in this example) the fuel/air mixture charge volume would not change, regardless of load on the engine. In a naturally aspirated engine you can't put any more fuel/air into the cylinder than what is physically drawn in by the piston traveling downward while the intake valve is open. The change in sound is a product of an engine trying to do something which the resistance placed upon the drivetrain is preventing it from doing.
No different than stepping on the gas while standing on the brakes.
Carl

When I went to tech school I was taught that atmospheric pressure pushed air into the area of the cylinder as the piston was moving down on the intake stroke. Carbs have a throttle plate that controls the amount of air into this cylinder. With no load engine at 1500 rpm's, the throttle is only open slightly. This give us a high manifold vacuum. If the auto trans is put in gear (with your foot on the brake ) and then the engine rpm's are raised under load to 1500. The throttle plate is now open farther allowing a greater amount of air/fuel to fill the cylinder. More air, more fuel, different power output, different sound.

Diesels completely fill the cylinder on each stroke, no throttle plate to hold back atmospheric pressure. Power is controlled by fuel only.

Joe

If significantly more air/fuel is forced into the cylinder and then ignited, would that not lead to catastrophic failure?
Carl

All one has to do is observe how a governor works on an ICE. As the load increases, i.e. going up an incline, carrying more load, encountering thicker/taller grass when mowing etc., the gov will allow the throttle to advance to add more fuel/air ratio inorder to maintain the same speed (mph, engine and pto rpm etc.). As the ICE burns more fuel/air all it's outputs (hp, torque and sounds) as well as the intake sounds will increase.

As been stated it all has to do with the ICE's load or lack thereof.....

Dave

Well then, I reserve the right to respectfully agree to disagree with myself, as I can't seem to wrap my head around much of what has been introduced in this thread (including, and especially my own input).
Carl