Static and Dynamic Displacement and HP

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Bewy said:
273,
Your premise in post #1 is wrong, but not all your fault.
The volume [ cfm or any other measure of volume ] of air ingested by the engine is the same at all rpm, based on the CID. What varies is the air's density.
The term VE refers to volumetric efficiency, but it is incorrect, but has been in such common use, it just got accepted over the years. What is being compared is the weight of air being drawn into the engine, not the volume.
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I get what your saying I think :) at 80% VE still has to fill 100% of the cylinder or otherwise there be a pocket of vacuum in the cylinder so it's less dense, is that right ?

Would that also be why at higher vacuum level on a flow bench the CFM goes up cause the air is denser ?
 
Not quite. The air is throttled. It is stretched if you like. Greater distance between molecules at high vacuum levels; the same vol of air is lighter at idle than it is at WOT.
 
Bewy said:
Not quite. The air is throttled. It is stretched if you like. Greater distance between molecules at high vacuum levels; the same vol of air is lighter at idle than it is at WOT.
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Gotcha
Thanks good to know
 
An engine is an air pump. Nothing more, nothing less.

More air (and the fuel to go with it) means more power. Nothing more, nothing less.

Hp per cubic inch is volumetric efficiency, nothing more, nothing less.

You're going to make up to a given amount of power for a given amount of air. If you have a smaller motor, naturally aspirated, it's gotta spin faster to process as much air as a larger n/a motor, if the volumetric efficiencies and everything else are the same. All the other stuff is just getting more air in and out for a given rpm.

All this other talk about ratios and blah blah blah is just mental masturbation. Nothing more, nothing less.
 
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jos51700 said:
An engine is an air pump. Nothing more, nothing less.

More air (and the fuel to go with it) means more power. Nothing more, nothing less.

Hp per cubic inch is volumetric efficiency, nothing more, nothing less.

You're going to make up to a given amount of power for a given amount of air. If you have a smaller motor, naturally aspirated, it's gotta spin faster to process as much air as a larger n/a motor, if the volumetric efficiencies and everything else are the same. All the other stuff is just getting more air in and out for a given rpm.

All this other talk about ratios and blah blah blah is just mental masturbation. Nothing more, nothing less.
Click to expand...


I agree with this except the ratios part.

Ratios matter. Bore to stroke ratios matter. Rod to stroke ratios matter. That’s because both affect cam timing. A higher r/s ratio requires much different cam timing than a lower r/s ratio. Racer Brown wrote a really good paper on this. I have a copy somewhere. It can be found if you search for it.

Bore to stroke ratio matters for the same thing. And over square engine won’t take the same cam timing as an under square engine will want.

There are other rules that apply, like the 85% rule for the exhaust port. The rules for throat percentages for different valve job angles. Those are very important and they are essentially ratios. There is also the percentage of valve size per inch of bore, which changes depending on the valve configuration.

So ratios matter. I know many argue that r/s ratio doesn’t matter, but it does. Get it too high and you have to close up the cross section on the intake port to get any air speed at all. Get the ratio too low and it will beat the skirts off the pistons.

While one can say set your bore size and then your stroke length and then pick rod length because the ratio doesn’t matter, that’s only partially correct. You have to account for the above mentioned factors.

That’s why when I see tests where something major changes like stroke length and cam timing isn’t accounted for I just take the results with a grain of salt.

Hell, r/s ratio affects ignition timing. That needs to be accounted for as well.
 
Rat Bastid said:
I agree with this except the ratios part.

Ratios matter. Bore to stroke ratios matter. Rod to stroke ratios matter. That’s because both affect cam timing. A higher r/s ratio requires much different cam timing than a lower r/s ratio. Racer Brown wrote a really good paper on this. I have a copy somewhere. It can be found if you search for it.

Bore to stroke ratio matters for the same thing. And over square engine won’t take the same cam timing as an under square engine will want.

There are other rules that apply, like the 85% rule for the exhaust port. The rules for throat percentages for different valve job angles. Those are very important and they are essentially ratios. There is also the percentage of valve size per inch of bore, which changes depending on the valve configuration.

So ratios matter. I know many argue that r/s ratio doesn’t matter, but it does. Get it too high and you have to close up the cross section on the intake port to get any air speed at all. Get the ratio too low and it will beat the skirts off the pistons.

While one can say set your bore size and then your stroke length and then pick rod length because the ratio doesn’t matter, that’s only partially correct. You have to account for the above mentioned factors.

That’s why when I see tests where something major changes like stroke length and cam timing isn’t accounted for I just take the results with a grain of salt.

Hell, r/s ratio affects ignition timing. That needs to be accounted for as well.
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I don't disagree, but for the OP's sake, I left all that out.

I started to get into it, but went back and edited that out. If OP wants to re-think 150 years of engine design instead of reading up on it, me re-writing it won't accomplish much.
 
jos51700 said:
An engine is an air pump. Nothing more, nothing less.

More air (and the fuel to go with it) means more power. Nothing more, nothing less.

Hp per cubic inch is volumetric efficiency, nothing more, nothing less.

You're going to make up to a given amount of power for a given amount of air. If you have a smaller motor, naturally aspirated, it's gotta spin faster to process as much air as a larger n/a motor, if the volumetric efficiencies and everything else are the same. All the other stuff is just getting more air in and out for a given rpm.

All this other talk about ratios and blah blah blah is just mental masturbation. Nothing more, nothing less.
Click to expand...
What's wrong with mental masturbation? It's my second favorite kind.
 
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