Horsepower, compression ratio, airflow?

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jos51700 said:
Changing compression does not alter flow one single bit unless the piston is domed or the head milled to the point that airflow around the valves is altered (a lot, in other words).

Think about it: Compression happens when? When valves are closed.

Compression ratio is:
Volume of cylinder at BDC
Volume of cylinder at TCD

That's it, nothing more. No elasticity (the fluid effects of air in motion cannot be altered by the above ratio), no way, no how. Quench has nothing to do with airflow through the head, unless the quench area is shrouding the valves. Quench only occurs near or at TDC on compression, when valves are...closed.

In fact, let's look at it another way...
Let's say that we want to increase the volume @ BDC so we can "increase airflow". We can either dish the piston, or increase displacement. Since we're not increasing displacement, we dish the piston. We are now effective "pulling harder" (In truth blowing harder, but I digress) because there's a larger volume to fill, and we might get more airflow.

Problem: That dished piston also increased the volume at TDC. And due to ratios, our compression just dropped. The exact opposite of what's been supposed in this thread.

In reality, you'd have to dish that piston to beyond daylight to make a difference, but it highlights what's really going on.

If you think that increasing compression can increase airflow through a head, your understanding of the internal combustion engine is fundamentally flawed.
Click to expand...

Holy crap is this going to get muddy! Your analogy is wrong as you state with your Problem in that creating a dish lowers the compression. I was wrong with my syringe example too. The STATIC volume of air ingested will NOT increase with a higher compression ratio. The DYNAMIC volume of air ingested WILL increase with a higher compression ratio (More air will be pulled in within the same time--higher velocity)

A bigger squeeze will result in a bigger blow--Yes? And by blow I am referring to atmospheric pressure forcing air into the cylinder.

You've got me wondering how to mathmatically prove or disprove this. J.Rob
 
RAMM said:
Holy crap is this going to get muddy! Your analogy is wrong as you state with your Problem in that creating a dish lowers the compression. I was wrong with my syringe example too. The STATIC volume of air ingested will NOT increase with a higher compression ratio.
Click to expand...

Creating a dish in the piston DOES lower compression. Think: 360 piston. '73 340 piston. This is common practice. Just as big domes RAISE compression.
It's all about making the combustion chamber smaller. But altering the head or the piston makes the volume at TDC and BDC change in the same way. It's just that the ratio of change is different. Losing 50CC of volume at BDC might be 1% volume change for BDC, but at TDC then 50CC might be 30% volume change.

RAMM said:
The DYNAMIC volume of air ingested WILL increase with a higher compression ratio (More air will be pulled in within the same time--higher velocity)
Click to expand...

This has nothing to do with static or dynamic volumes. Compression ratio is volume ratio...nothing more. That is a static case.

Dynamic CR doesn't exist. BUT, you can say that the compression increases dynamically, based on cam selection. But changing cam selections changes...airflow through the head. This is saying that yes, COMPRESSION (by itself) can increase with cams and airflow changes but this is compression in the cylinder of a running engine, not to be confused with compression RATIO. The OP asked about ratio.

Static compression ratio and airflow through the head are not related. Compression ratio is the same whether valves are open or closed. You could take the valves completely out of the motor and as long as you CC'd the head before you did, you could still calculate compression ratio. That ratio would not change, and it would not affect airflow. You don't need compression, to find the compression ratio.
Think about that for a minute.

It is not related to airflow at all. It is just a ratio.

RAMM said:
A bigger squeeze will result in a bigger blow--Yes? And by blow I am referring to atmospheric pressure forcing air into the cylinder.

You've got me wondering how to mathmatically prove or disprove this. J.Rob
Click to expand...
You are confusing combustion pressure ("blow") with the flow of air due to a pressure difference (atmospheric pressure "blowing" air in to the cylinder), because the cylinder has "vacuum". Yes, bigger squeeze {compression} equals more cylinder pressure "blow". The valves are closed during this "blow", and have been for some time at this point.

All that pressure goes out the exhaust valve when it opens. Then "overlap" occurs where both valves are open slightly, and air may or may not come in. This is not affected by the ratio of volume at BDC/TDC. None of this is. The "blow" that brings air through the ports into the cylinder is NOT the same as the "blow" that makes the piston go down. This "blow" (of airflow) is really only affected by how much the displacement of the cylinder is (the syringe example), and how much port there is for the air to flow through.
 
Sounds good jos. So maybe what we are talking about here can be clarified by dropping the whole STATIC compression ratio calculation thing. Are you saying that the dynamic comp ratio increases as a result of increased static measured/calculated comp ratio? J.Rob
 
jos51700 said:
All that pressure goes out the exhaust valve when it opens. Then "overlap" occurs where both valves are open slightly, and air may or may not come in. This is not affected by the ratio of volume at BDC/TDC. None of this is. The "blow" that brings air through the ports into the cylinder is NOT the same as the "blow" that makes the piston go down. This "blow" (of airflow) is really only affected by how much the displacement of the cylinder is (the syringe example), and how much port there is for the air to flow through.
Click to expand...


Jos - I think you just noted something here that might be contrary to the "more compression" arguement. I understand what you are saying on the compression ratios, and especially the dynamic that really is a moving target ina running engine. I agree 100% and nicely written.
But more importantly - You noted that when the piston starts to go down... This is the point at which the arguement of less volume makes more power due to less volume to be "elastic".... When the piston starts to go down on the intake stroke... Both valves are open. The inertia from the exhaust leaving out the exh valve is helping to create a stronger pressure differential in the chamber when the intake valve opens. So really, depending on the piston position and valve timing and amount of cam overlap, there will be virtually no "stretch" because the gas moving through that volume is already moving. In the syringe analogy, it would be the same as applying "pull" into the area on the nozzle side of the plunger though a second nozzle before you began pulling it back.
 
RAMM said:
Sounds good jos. So maybe what we are talking about here can be clarified by dropping the whole STATIC compression ratio calculation thing. Are you saying that the dynamic comp ratio increases as a result of increased static measured/calculated comp ratio? J.Rob
Click to expand...

RAMM - My undersatnding is dynamic increases as a result of an increase of the satic ratio (the mechanicals) or an change in the intake valve colsing time (time meaning in degrees), or an increase in the inertia as rpms rise, whether that be supplied by intake port tuning or exh system scavenging. Basically, any mechanical change that squeezes the mix harder, or any change that jabs more mix in to be squeezed.
 
jos51700 said:
Changing compression does not alter flow one single bit unless the piston is domed or the head milled to the point that airflow around the valves is altered (a lot, in other words).

Think about it: Compression happens when? When valves are closed.

Compression ratio is:
Volume of cylinder at BDC
Volume of cylinder at TCD

That's it, nothing more. No elasticity (the fluid effects of air in motion cannot be altered by the above ratio), no way, no how. Quench has nothing to do with airflow through the head, unless the quench area is shrouding the valves. Quench only occurs near or at TDC on compression, when valves are...closed.

In fact, let's look at it another way...
Let's say that we want to increase the volume @ BDC so we can "increase airflow". We can either dish the piston, or increase displacement. Since we're not increasing displacement, we dish the piston. We are now effective "pulling harder" (In truth blowing harder, but I digress) because there's a larger volume to fill, and we might get more airflow.

Problem: That dished piston also increased the volume at TDC. And due to ratios, our compression just dropped. The exact opposite of what's been supposed in this thread.

In reality, you'd have to dish that piston to beyond daylight to make a difference, but it highlights what's really going on.

If you think that increasing compression can increase airflow through a head, your understanding of the internal combustion engine is fundamentally flawed.
Click to expand...

The piston is at TDC when the intake valve is open during OVERLAP! So if you raise the compression (reduce the area between the piston top and chamber) you will PULL harder on the intake tract when the piston starts falling. Plain and simple sir.
Brian
 
ou812 said:
The piston is at TDC when the intake valve is open during OVERLAP! So if you raise the compression (reduce the area between the piston top and chamber) you will PULL harder on the intake tract when the piston starts falling. Plain and simple sir.
Brian
Click to expand...

Sigh.

Page 7:
http://rescomp.stanford.edu/~efroeh/papers/RDH_Engine_Performance.pdf

If you're not willing to read the whole thing, it boils down to:
They have a engine that they can vary compression ratio at will. No, I don't know how that works.
This engine has an airflow meter on it.
They did enough tests that they were able to publish results.
They increased the compression ratio. The VE didn't go up an appreciable amount.
VE is Volumetric Efficiency. Big words for air flow.

VE increases slightly as compression ratio increases, but only because of decreasing amounts of residual exhaust gas in the cylinder, not due to an increase in airflow.

End of story. This has been done before. If you disagree, find proof and post it. Otherwise, we're all wiser and more knowledgable and still can't afford the gas needed to run higher compression ratios.
 
The only other "thing" I can offer is in the example of a 2bbl or restricted engine-which I do a fair amount of. In a restricted engine common practice is to run as high of a compression ratio as possible-Not only to extract more power from the bigger bang but to "pull" on the intake tract harder, or so I thought. I know this does not count in any way shape or form but in my mind's eye I can picture that having a smaller above TDC combustion space "allows" the intake stroke to flow more sooner by having less "dead" volume to communicate through. I am actually super intrigued by this now.

I could put a 360 on the dyno at the current 11.1-1 comp and test it, specifically looking at airflow through the hat. I could then take the heads off and cut them to net 12-1 comp and then monitor airflow. If what your saying is true jos then even though power will likely increase that airflow should not--ie efficiency was gained and observed. Correct?--would this be a valid way to test--because I could do this. Remember this test would not have any regard to power output--just air consumed. Hell I could even flow test the heads to record a possible and likely flow loss after the head milling. Boy if airflow went up on the dyno even though flow went down on the bench--we'd really have something to talk about, wouldn't we? J.Rob
 
jos51700 said:
Sigh.

Page 7:
http://rescomp.stanford.edu/~efroeh/papers/RDH_Engine_Performance.pdf

If you're not willing to read the whole thing, it boils down to:
They have a engine that they can vary compression ratio at will. No, I don't know how that works.
This engine has an airflow meter on it.
They did enough tests that they were able to publish results.
They increased the compression ratio. The VE didn't go up an appreciable amount.
VE is Volumetric Efficiency. Big words for air flow.

VE increases slightly as compression ratio increases, but only because of decreasing amounts of residual exhaust gas in the cylinder, not due to an increase in airflow.

End of story. This has been done before. If you disagree, find proof and post it. Otherwise, we're all wiser and more knowledgable and still can't afford the gas needed to run higher compression ratios.
Click to expand...

I read the article and I read it sometime before this too, but not with this topic in mind. I think there are a few problems with testing a small single cylinder. Perhaps the changes in airflow were to small to measure incrementally with whatever they were measuring with? They admit that the RDH engine has such a poorly designed combustion space that the residual hot exhaust gas leftover from the previous combustion event would lead to a reduction in mass airflow. I just think maybe a larger higher compression engine may give us the airflow in bigger resolution. I'm really leaning towards just finding out at this point. J.Rob
 
I can't say that I can answer the original question specifically but I have done both actual engine testing and computer simulation for OEM production engines (my model was within 2% of actual test data). The general question asked about air flow versus compression ratio is a good acedemic question that is difficult to answer because the combustion process is so dynamic. This would be a good problem to run on an accurate computer model all the variables to find the answer but the difficulty with the computer simulations is the combustion process is just to dynamic, events like when detonation and spark knock may occur, plus residual heat in the combustion chamber is thought to effect airflow too, making this is all very difficult to accurately model in the computer simulations. Then if you are asking because you want to do something for a "one-off", like a salt flat car, you can change some factors like a really low coolant and/or inlet temps by using dry ice which isn't feasible on a street car - then you have even more variables to sort out and it becomes a huge job to optimize everything. So there isn't really a specific answer that X comp ratio = Y air flow, but in very simple terms air flow trumps compression ratio in most applications.
 
Somebody should start a new thread over on ST asking "Does increasing only the compression ratio-increase airflow or air consumed?" I think I'll do that. J.Rob
 
jos51700 said:
Sigh.

Page 7:
http://rescomp.stanford.edu/~efroeh/papers/RDH_Engine_Performance.pdf

If you're not willing to read the whole thing, it boils down to:
They have a engine that they can vary compression ratio at will. No, I don't know how that works.
This engine has an airflow meter on it.
They did enough tests that they were able to publish results.
They increased the compression ratio. The VE didn't go up an appreciable amount.
VE is Volumetric Efficiency. Big words for air flow.

VE increases slightly as compression ratio increases, but only because of decreasing amounts of residual exhaust gas in the cylinder, not due to an increase in airflow.

End of story. This has been done before. If you disagree, find proof and post it. Otherwise, we're all wiser and more knowledgable and still can't afford the gas needed to run higher compression ratios.
Click to expand...

VE is TRAPPED air/fuel not overall flow or potential flow. If you move the piston 3ft. away from the combustion chamber are saying it will pull on the intake port the same as if it was only .500 away? Do you walk upside down also defying the laws of physics?? LOL!
You might want to start asking other engine builders what they think because I'm sure your going to find alot more people will agree with us than disagree it that matters.

There is no end of story....nobody has engines figured out completely and fully or they would have every record there is to have. I don't have direct proof that what I am saying is 100% accurate, however you don't have any proof that I'm wrong...so I guess this is where I will end my debate about this and continue building engines and enjoy reading other threads.
Brian
 
everyone who knows...knows that a piston closer to the valve 'moving away' will will signal sooner as opposed to one far away.

hold ur hand a foot from ur face and blow...lol...then put ur hand and 1'' away and blow again......now which did u feel sooner?

is it safe to say that a piston that closer will start pulling sooner, therefore a longer duration of flow happens? yeah

when this question was initially posted...i really thought there had to be a catch, i see now it's 'seemingly' boiling down to a more simpler concept and one overlooked.

its all in how u ask the question, and sometimes who's asking it.LOL
 
1wild&crazyguy said:
everyone who knows...knows that a piston closer to the valve 'moving away' will will signal sooner as opposed to one far away.

hold ur hand a foot from ur face and blow...lol...then put ur hand and 1'' away and blow again......now which did u feel sooner?

is it safe to say that a piston that closer will start pulling sooner, therefore a longer duration of flow happens? yeah

when this question was initially posted...i really thought there had to be a catch, i see now it's 'seemingly' boiling down to a more simpler concept and one overlooked.

its all in how u ask the question, and sometimes who's asking it.LOL
Click to expand...

I have said that---That a higher comp ratio design will start puling sooner and quicker. You have boiled it down further than I. J.Rob
 
RAMM - Thinking more on this... What do you think the relationship between piston motion and that pull? I added a note on ST... I think while yes, there is less of a delay due to a smaller chamber, but when combined with the overlap of the cam, and the weak "pull" in the first 20° of the intake stroke, that faster response just isn't noticable or strong enough to have an affect.
 
Isn't this something? We're all starting to think about how to answer and how accurate our "facts" are.

Pardon my lack of knowledge, what is "ST"?
 
btw moper, would rod ratio 'shorter' help make sense of that line of thought?

i know it's not really what its about now, but just to explore...lol

shorter rod initially leaves tdc faster than a longer rod which speeds in its mid stroke...in theory
 
so at what dynamic compression/depression is the limit on a head that is boarder line turbulent?

that 'running' 600 fps recommendation, what @28" flowbench fps is that based on?
definitely a race head-big port.

whats the average fps of a race head?


if u had a head thats already at 295'ish fps at cams lift....what dynamic would u all shoot for?

people who test at 40''...is that a closer comparrison to a running motor?
 
1wild&crazyguy said:
so at what dynamic compression/depression is the limit on a head that is boarder line turbulent?

that 'running' 600 fps recommendation, what @28" flowbench fps is that based on?
definitely a race head-big port.

whats the average fps of a race head?


if u had a head thats already at 295'ish fps at cams lift....what dynamic would u all shoot for?

people who test at 40''...is that a closer comparrison to a running motor?
Click to expand...


thats kinda where i was going with the spintron...
 
1wild&crazyguy said:
btw moper, would rod ratio 'shorter' help make sense of that line of thought?

i know it's not really what its about now, but just to explore...lol

shorter rod initially leaves tdc faster than a longer rod which speeds in its mid stroke...in theory
Click to expand...

With regard to the topic of the amount of pull and chamber size... The shorter rod leaves earlier, which means it might reach the higher "suck spot" earlier. But again, well after any affect on the gasses entering were "felt". In a long rod or short rod engine, the piston dwells fo a few or more than a few degrees. During that time, the inertia from the exh leaving during overlap will already have started filling the cylinder. At that point, at least by my thinking, the size of the chamber doesnt matter. The gasses are already moving on thier own. That's why I said before that I think chamber size is a moot point once we take into account camshaft overlap.
 
My thoughts on these...

1wild&crazyguy said:
so at what dynamic compression/depression is the limit on a head that is boarder line turbulent?
The depression limit would be subjective for every individual engine, including thier individual running environments (as in altitude). If the valve opens far enough, and the piston motion is fast enough, the head can go into choke. Dynamic compression has no bearing on that limit, but IMO, would be affected by it. As in once the head choked, no further filling can happen, and dynamic ratio will freeze where it is.
that 'running' 600 fps recommendation, what @28" flowbench fps is that based on?
definitely a race head-big port.


if u had a head thats already at 295'ish fps at cams lift....what dynamic would u all shoot for?
My dynamic calculations are based entirely on fuel type requirements.

people who test at 40''...is that a closer comparrison to a running motor? I'm told it's not the closest you can get to a running engine that makes you want to test at higher depressions. It's that at higher depressions, small adjustments are "more visible". A port tested and improved at 10", may have deficiencies that are not evident until you test at 28.
Click to expand...
 
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