This post reminds me of my college years. My job is all about money and project management now, I don't get into many dorky tech debates anymore.

"Thank you H-T"

 

That is a good point, I didn't pause to think about that. But does the "spike" in filling pressure exceed atmospheric pressure.

 

wow, this post blew up..

Good discussions.

A well massaged head, valve timing, and well designed intake manifold can create a VE higher than 100%.

Here is an example (this is just calculated, VE% is the middle column):

This is this engine:

These are the actual measurements (taken at time of dyno tuning, working out some bugs):


Though they are custom heads and intake manifold.

[Modified by martini, 2:45 PM 11/6/2002]


[Modified by martini, 2:46 PM 11/6/2002]

 

call me crazy or flame me, but when we worked on this 2.5 RS boosted to something around 18 PSI, the computer showed to us some negative % at high RPM, at the moment it kinda make sense, but right now, i guess that was the reason why we couldn't tune that car well enough. But if you think about it, at high rpm the valves opens and closes soo fast that it doesn't have enough time to fill up all of his capacity, right? o and by the way somebody said that is imposible to a 1.8L engine to contain 1.8 liters of mixture, off course due to the fact that 1.8 l is between 4 pistons, so keep that in mind.
as for the over 100% i guess it can be done, but i have my doubts on NA engines. yes you showed up something ona dyno, but what about real conditions, can this still be made??

just to add a lil spice to this!


[Modified by GZERO, 7:27 PM 11/6/2002]

 

This is why I don't like participating in these discussions. The basics of volumetric effeciency, ram tuning, finite amplitude wave theory, and hundreds of other things that are discussed here are well covered in many introduction to engine theory books. If people are really interested in this stuff, there are good books that give good information on everything discussed here without confusing things with everyones opinion.

Even a good search on the web finds some decent stuff:

http://www.canadiandriver.com/articles/jk/020529.htm

This one mentions the astounding peak volumetric efficiency of the s2000 engine; 112%:
http://www.autospeed.com/A_1127/page1.html

 

This is a very good post I enjoy reading about this kind of stuff it makes me feal like I am smart for understanding what you are talking about.

 

Ok guys...I'll give it a shot...
Yes...it absolutely is possible to obtain greater than 100% VE on a N/A engine. The Nascar boys have been doing this for 30 years or so.
Think about it like this......
The air moving into the cylinder is movng basically as a unit shaped mostly like a column. Now this column has mass, and takes up space and it also has weight. {one CFM weighs about .076 lbs under normal conditions}. Now, imagine this column were water. If we had our coulmn of water moving rapidy, it would have enough momentum that it would continue to flow into the cylinder after the piston gets to the bottom. In fact, there are only two ways to stop the column...one is to close the valve, the other is to build more pressure inside the cylinder {by moving the piston up} than what is outside the cylinder so the column must stop, and even eventually go back out....picture what would happen if we left the intake valve open while the piston comes up.
Since our column of air also has weight and mass, it also has momentum, and will continue to enter the cylinder until one of our two scenarios occurs.
So, as it turns out we can actually keep the intake valve open for a while as the piston begins moving back up the bore.....if you need proof, just take a look at a cam timing card and you'll see that the intake valve closing point is NEVER at bottom dead center......
The trick is to time the valve events so that at a certain engine speed, we can take advantage of the fast moving column of air's inertia {or momentum} to continue filling the cylinder even as the piston is moving upwards.
This brings up another interesting point.....
Static versus Dynamic compression ratio
The C/R is determined as the total cylinder volume divided by the compressed volume. When we calculate this we normally use the figures that we can readily measure....problem is when the engine is actually running...the cylinder volume is not calculated until the valve closes.....because of this we can actually play with dynamic C/r's by adjusting cam timing!
Think about this.....if we close the valve at 50 degrees AFTER bottom dead center....where exactly is the piston? Well, that is a function of the stroke and the length of the rod, but you get the idea....I hope.

 

Don't forget about the Prelude making 100+% VE at hp peak in that same article, hehehe.....

 

Ok I've been gone for a while and I see the ignorance spreads faster when I'm not here <J/K>. First of all Static Compression Ratio has everything to do with VE. With the higher the ratio, the greater the vacuum is going to be on the intake stroke in thus increasing the amount of mixture being pulled in. Also stroke has a great deal to do with piston speed, and how much air the engine can draw in with every rotation. With extremely high piston speed such as with F1 and NASCAR <only because it was previously mentioned> the stroke has to be reduced to keep the piston speed down. With the lower piston speed the compression ratio can implement greater effectiveness by not causing the straw effect (you can only push so much air before the water and other fluids in the air condense on the walls).

NASCAR's can get away with high VE because they are restrictor plate engines running ~10,000rpm. From my various sources they run a median of about 17.5:1 SCR. So their effective dynamic compression is ~12:1 on the track.

Sorry I didn't read all of the replys so far and it's probably a good thing.

You can't think of the "air" as a solid object EVER. First of all it not only air coming into the engine. There is water from humidity, fuel from the injectors (sometimes water too), O2-Nitrogen-Carbon Dioxide-Carbon Monoxide(if driving behind someone). For the most part the particles coming into the engine should be treated a fluid with impurities (you know like saltwater). It should be treated like a thick viscous liquid that must flow in the straightest line possible in order to get the greatest VE. A trick I learned from the Fuelie guys (you know the ones running alcohol in 6-7 second cars) is to focus on keeping the exhaust speed up in the ports then openining the mani and pipes as much as possible with-out losing torque. the intke does matter just not as much for high comp. high revving engines. I don't understand why this info is never published. If any wants I'll write it out and someone can publish it.

I forgot to add in order to fully understand how any thing works and can be improved you have to look at every detail not just the ones you think are important. In other words with internal comb. you have to look at it from a molecular point of view.

I'm an engineering student also, but I'm heading for my Masters when I go back to school in 2 years. I decided to take some time off and smoke some weed and eat some of those rolly pills. Drugs aren't bad MIS-information is bad.


[Modified by tzsir, 10:30 PM 11/6/2002]

 

I stand corrected you can exceed 100% VE in a n/a engine. By the way, I wish that was my dyno sheet. Torque OWNZ!

To the flamers, go away. This is a debate, that started out as a simple question. If you have a take on something in the thread, good, post it. If not, don't bother. We are here to learn from each other, not make fun of people who don't know something or were mis-informed. It seriously pisses me off when someone sits on their high horse and considers everyone else inferior. F@*# Off

 

yeah, and it's going in an FD RX-7 of all things. It should be somewhere around 2600lbs when it's finished..

 

Holy missing rotary Batman, that's insane.

A guy I autocross against has a similar setup. 2G RX7 and a 302. The thing is super scary.

 

I apoligize if you took that as flaming you. I was just making sure you had all the facts on VE. I've been led in the wrong direction by someone that didn't have their facts straight before that's why I started reading everything I can to further my understanding of this topic and various others dealing with effeciency. I even read the articles that were wrong or didn't give the true facts just to get an idea of the B.S. going around so I can tell what it sounds like.

For sure, I don't have all the answers either I'm just further along in the learning process than most. I like to share what I've learned from the old-skool guys running that fast. It took me 3 years to get them to trust me enough to learn how they do it. They don't share their info willingly because they need every advantage they can get. Sorry if I sound like I'm on a high horse I was in a hurry just like now to type that. BUSY BUSY BUSY
BRO. If you are still offended by my words the maybe I'm not the one on the high horse. Sorry I don't have time for whining.

Rotaries easily acheive that 100%^ goal BTW


[Modified by tzsir, 9:16 PM 11/8/2002]

 

This is one of the most interesting and informative post I've read on HT.

 

All our race V8s have 106-128% VE .
Its all in the heads , intake porting , and cam profiles and valve timing

 

Well I really hate these threads because it could take pages and pages to explain all theorys and facts but I will toss in a couple, NA engines can surpass 100% VE.I will get to that later. Restrictor plate NASCAR engines do not come close to 100% VE because of the restrictr plate (witch is the object) they cannot fill the cylinder to its capability. They make about 470 BHP restricted. 760 unrestricted. their comp ratio (at least for the past 3 years) is restricted to 12.5-1, static by the rules. They did run higher numbers earlier, dynamically it was less because you could not fill the cylinder.so VE sucks.
The highest VE number occurs at peak torque,when the engine is at it's most efficent. Over 100% VE is made by the flow ability of the intake manifold,head,cam,and exhaust. High velocity numbers keep air flowing into the cylinder after BDC which is why the closing of the intake valve is the most important part of the cam cycle, and advancing or retarding the intake cam moves thw torque up or down the rev range. It has to be matched to the flow of the intake system.
I will not get into scavange (after 30 years I am still learning). he next point is why 2 small valves with the same area of 1 valve has a higher VE (old Cosworth papers) and more BHP, the velocity of the air colum in the small port is higher, when the valve closes at high rpm,velocity, it actually creates a positive pressure behind the valve head and theirfore is higher than atmospheric pressure when the valve opens, a turbo at 1 psi or such.
Compression ratio has nothing to do with air flow, but does with BMEP, anoother thread. Stroke only has to do with compression or VE because it leads to higher displacment and a change to tha math formulas. If you increase bore or stroke and leave every thing else the same you increase your static compression and displacement.

 

That's all dandy, but stroke also does more than that it effects the very amount of air pulled in by variances in the piston speed which is what creates the pulling force that brings the air into the cylinder. I wish you would look at the engine as a whole because it doesn't work in parts it works in whole so should only be veiwed as such. Until you do this all those years are still wasted. Not trying to be rude or a ***** I just want every-one to understand my opinion and take on all of this VE and engine design talk because in the end that's what it all entails. The most effecient design is the one where all the parts were taken down and modified for greatest efficiency together not seperate. The Hemi-type piston head design that all modern engines are based on is a classic example of this at work. I am enjoying this post even though I know it won't continue much longer even though it should. I think soon I'm going to start my own forum based web-site for engine designers and other types of engineers. Maybe by next spring I'll get it done. We'll see. Keep it coming though I'm learning from all perspectives here.

 

I am going to give this a bump simply because I agree.... I am really enjoying this thread as well. I am learning quit a bit from it and would like some more info and points of view.

Also, tzsir, that forum would be pretty cool. Keep us posted (or at least I'm interested... so keep me posted).