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Why does horsepower peak?

S

Smartazz

Diamond Member
I noticed that on a dyno graph, horsepower will peak then go down slightly, if not dramatically, an example is here: http://www.powerhaus.com/images/968t16v/dyno1.jpg

If by increasing RPM's you increase the air intake, thus the fuel intake, wouldn't you continue to generate more power, but there's something holding it back from doing so. What is causing this dip in power at extremely high rpms, mainly after the redline? Thanks in advance.
 
The inability to fulfill the air intake or exhaust requirement through restrictions in the intake size, exhaust size or valve size. Aka not enough air is fed in or forced out efficiently enough for what the engine needs.
 
Originally posted by: mwmorph
The inability to fulfill the air intake or exhaust requirement through restrictions in the intake size, exhaust size or valve size. Aka not enough air is fed in or forced out efficiently enough for what the engine needs.
Click to expand...

So basically, the cylinders aren't completely full when they explode?
 
Originally posted by: Smartazz
Originally posted by: mwmorph
The inability to fulfill the air intake or exhaust requirement through restrictions in the intake size, exhaust size or valve size. Aka not enough air is fed in or forced out efficiently enough for what the engine needs.
Click to expand...

So basically, the cylinders aren't completely full when they explode?
Click to expand...
Very basically, yes.

The faster the engine spins, the more air it can pass through it.

If that air can't be supplied for whatever reason....heads, valves, exhaust, etc....then power will drop off.
 
yep, like others have posted it's resricted breathing, intake port size is big on this, years ago small carbs would do this too, also the cam lift, timing, duration has a affect on this

but resricted breathing is not always bad, the engine maybe be tuned for power at lower rpm's for resaon, like gas mileage, , more low rpm torgue is needed, smooth driveablity in the real world (like a common street engine), lower polution
 
"So basically, the cylinders aren't completely full when they explode?"

explode? not explode, explode is bad, it's complete rapid burning that you want

plus there full somewhat, but not as full as they could be
longer intake port runners, can add to fullness, by adding more air as the pistion is on the upward stroke and the intake valve is still open
 
It has a lot to do with the cams and intake/exhaust design of the engine. The amount of air that can enter the cylinders depends on a lot of things including timing, and the cams control the timing of the intake/exhaust valves. The amount of air that the engine can move through its cylinders is the volumetric efficiency.

Usually all the flow characteristics are optimized for the function of the engine. For instance, you wouldn't put high rpm cams in a low revving truck.

It can get pretty complicated.
 
The piston dwells a long time ABDC. This is why the intake valve remains off its seat, so the air moving at high velocity inside the intake runner can continue to enter the cylinder. Filling of the cylinder can reach 110% on a well tuned street engine. This will be the fat spot of its mid-range. Around 3500~4200rpm with a small block that peaks at 6000rpm.

Race engines will have intake velocities of 500fps. At 600fps the reflected wave within the intake runner reaches 1150fps, the speed of sound. This causes a sonic barrier to form and intake air flow shuts down and horse power drops like a rock.

Good street intake velocity would be around 350fps.
 
sonic ram tuning

the link above show a example of ram tuning the intake ports

taken from above link:

Now for the tricky part; how does it work? An advisory supplement to the 1960 Plymouth Service Manual states "each long branch is, in principle, like an organ pipe in which a compression wave can resonate -- travel back and forth -- at the speed of sound." OK. But why do you want this pipe organ under your hood and not in a church?

Since I'm not an engineer, I had to have this whole thing explained to me very slowly a considerable number of times before I even got to the stage where I could nod authoritatively at a few points to at least give the appearance of knowing what's going on. But the way I understand it, it works much like water flowing through a pipe in that the water wants to keep moving even after a valve or faucet is closed. Sometimes, this water even "hammers" when it begins to pile up against the valve. In the "SonoRamic Commando," the fuel/air mixture in one of the tuned arms of the manifold works somewhat in the same fashion in that the principle of fluid dynamics wants to keep it moving even though the mixture is obstructed by the closed intake valve. In effect, then the fuel/air mixture is literally "rammed" up against the closed intake valve of the engine, instantly available to charge the combustion chamber when the valve comes open. This is the "Ram" part.

The "Sono" comes from that compression wave that the service manual talked about. Through the mysteries of modern science, those 30-inch passages were carefully designed to maximize the resonant effect of that compression wave so that it hits that intake valve at the very instant it is opening for that fuel/air mixture that already has been rammed up against the valve. This provides an additional force to push more of the fuel/air mixture into the combustion chamber until the valve closes.

The kicker in this equation is that the passage length of the manifold directly affects the rpm range at which the optimum boost is achieved. Since these compression waves move at some 1100 feet per second, if you want your maximum boost at the middle range of engine operation, the tubes have to be longer for the wave to take more time to get out and back in sync with a intake valve opening to give maximum boost at 2800 rpm. If you want that engine to scream at 5000-5500 rpm, the passages have to be shorter as you want that wave to get out and come back quicker.

 
Cam timing and intake runner length have alot to do with it. A new manifold and cam can make you peak at redline, but you'll lose the low end.
 
Originally posted by: Summitdrinker
"So basically, the cylinders aren't completely full when they explode?"

explode? not explode, explode is bad, it's complete rapid burning that you want

plus there full somewhat, but not as full as they could be
longer intake port runners, can add to fullness, by adding more air as the pistion is on the upward stroke and the intake valve is still open
Click to expand...

Oh, so ideally you want the fuel to ignite and expand rapidly to force the piston back down, correct?
 
Originally posted by: Smartazz
Originally posted by: Summitdrinker
"So basically, the cylinders aren't completely full when they explode?"

explode? not explode, explode is bad, it's complete rapid burning that you want

plus there full somewhat, but not as full as they could be
longer intake port runners, can add to fullness, by adding more air as the pistion is on the upward stroke and the intake valve is still open
Click to expand...

Oh, so ideally you want the fuel to ignite and expand rapidly to force the piston back down, correct?
Click to expand...
It's more of a controlled explosion, but it's an explosion nonetheless.


I see everyone chiming in with all sorts of different reasons an engine's power will fall off due to airflow, and they're all right....in some cases.

But the bottom line is, no matter WHAT is causing the intake of air into and out of the combustion chamber to peak, then fall, it's lack of airflow beyond a certain rpm that's the culprit.
The solution, as you can tell by prior posts, is the interesting part. It varies with every engine.
 
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