Years ago, when I was running my Desk Top Dyno program concurrently with my real-life engine builds, I had a naturally aspirated 468 BBC that achieved 101% VE (as per Desk Top Dyno). The engine was awesome and had lots of upper RPM power and constantly pulled through the upper RPM range. Not really sure what the accuracy of that particular calculation was on the program, however I did notice that VE fluctuated between the different engine builds and simulations that I had experience with. I had never heard of a NA engine achieving 100% VE ever before that and just though the program was full of crap...

 

I just had my latest sbc creation dyno tested. When I compare the horsepower and torque numbers with PipeMax it says I have 106%-107%VE. That is with the short-mid tube headers that will be going in the car and not optimum dyno headers.

 

I got VE 121.3, and hadn't really thought much of it. I paid attention to everything else.

 

Over 100% is not impossible, many factory cars top that. As an easy reference, think 100hp/l is approximately 100% VE.

 

Surprised this hasn't been mentioned, but a sealed, efficiently formed "ram air" airbox can produce slightly pressurized air. A rep at K&N told me that the engine's sucking force is greater than the atmospheric air's "charging" force (assuming it's hitting the filter directly) up until about 90 MPH. Assuming a properly ducted airbox that directs air right at the filter, and a good seal, I would think any speeds past 90 or 100MPH are going to start seeing "boost."

 

If your intake air is flowing fast enough to create a low pressure area it's absolutely possible. Formula 1 has been doing it for years and years.

 

True, but I had a 427 built in 03,04 and '05 (long build) that out-sucked everything!

 

Nitromethane has a a/f ratio of 1.7 to 1. You're pumping so much fuel into the cylinder that it could easily affect VE. There's so much fuel there's less room for the air. Not a big difference but definately more than gasoline.

 

Air is a compressible fluid, whereas most "fluids" as people (wrongly) define them are liquid, and essentially non-compressible.

When the valve closes, it causes a pressure wave to travel back up the intake port/runner, and timing those waves (with engine RPM, valve event timing, and port/runner length) can allow the wave pressure event from one cylinder to line up correctly (lining up in time) with a wave from another cylinder in the plenum area and result in higher VE.

Something similar is true for the exhaust side, where again, valve timing, rpm and runner length in the primaries, secondaries, and before an H/X pipe can allow for the pressure waves from the exhaust pulses to help pull exhaust air out of the cylinders.

It's also important to note, the waves don't travel at the speed of the air, I'm pretty sure, not 100% sure, but pretty sure, that they travel at the speed of sound RELATIVE to the fluid they're traveling through. So if the speed of sound at sea level under normal atmospheric conditions (STP...standard temperature and pressure) is ~1116.4 feet per second, I'm not sure what a realistic speed for intake air is, but for the purpose of showing how the math works, lets say the intake air is coming down the port at 20 feet per second, the pressure wave is going back up the port/runner at 1116.4 - 20 feet per second, or 1096.4 feet per second...but bear in mind air pressure changes the speed of sound (sound isn't like light...light is an electromagnetic wave, and can travel through a vacuum...sound is a physical pressure wave and requires a media to travel through, sound cannot travel through a vacuum...so lower air pressure...such as intake manifold vacuum...means the air molecules are spaced further apart, and it is more difficult for the wave to travel through, and thus slow it down and diminish it's amplitude).

Using math with this, you can essentially fine tune the runner length, both on intake, and exhaust to optimize them for a certain RPM, and as the engine is further above or below that RPM, the effect becomes less than optimal, and at certain RPM's can actually do more bad than good...deciding what RPM to optimize this all for, depends on the vehicles intended use...if you were going through all this hassle on a street car (including custom fabrication of the intake manifold and exhaust headers)...I would argue you should probably pick the most common cruising RPM to get the best efficiency possible at that RPM to try to bump up the MPG...this is also why some cars have variable length intake runners (like butterfly valves that essentially change the path of air through the runners to make it travel more or less distance)...they're trying to broaden the effect over a wider RPM range, and thus increase VE in more of the engines usable RPM range.

The downside to all of this math and whatnot, is usually the dissapointment that the "optimal" headers or intake, will most likely cost a TON to fabricate, and might not actually fit in the engine bay/under the car.

There's a LOT regarding what happens to those pressure waves when they get to a bend/transition/change in the pipe/port/runner too...bends can cause them to send reflection waves back where they came from, changes in diameter can as well...as can entering a collector or chamber...it's pretty complicated.

 

Holy cow. No wonder why a guy like Rick Hendrick has 60 or so PhD's working on his racecars.

 

A typical GM LS factory engine has a VE in the 103-105% range when the intake runners are doing their best work.

 

I realized I didn't even comment on the "possible without forced induction"...people define VE in different ways...I would argue that simply making more horsepower because you're running boost...doesn't mean you're over 100% VE.

If you run 14.7 PSI...you've effectively doubled the intake air pressure...so if your power/torque numbers have doubled...I'd argue your VE is the same...if they're not quite doubled (like if you made 300 RWHP before the boost, and 585 with it)...I'd argue that your VE is actually lower.

 

Mike, you might want to research a little more on this subject. Check on speed of sound vs. density and temperature. Also, just because intake manifold pressure is below atmospheric (a slight partial vacuum) it is WAY different from a complete vacuum, or absence of air molecules.

Yes, the "tuning" pulses in an intake or exhaust system travel at the speed of sound, while the average mass flow moves at a fraction of that, but perhaps 12 times faster than your 20 fps intake example. The biggest gains can usually be found in intake tuning despite many folks' concentrating on exhaust (header length) tuning. Pressures in the intake runners are a tiny fraction of exhaust pressure, especially at exhaust blowdown when a majority of the exhaust is expelled.

A given intake length (plenum to valve seat) can have a number of "ramming peaks" (higher pressure) with "ramming troughs" (lower presssure) across the engine's usable rpm band. It is possible to have a high pressure tuning peak at/near hp peak rpm and another peak nearer torque peak rpm. You could say that those peaks help define where hp and torque peaks occur.

These ramming peaks can cause substantial pressure over atmospheric in a NA engine. A well done system can have a ~5 psi overpressure pulse at the intake valve. If one could time this pulse (one of many during the intake cylce) to arrive near the time of intake valve closure, a considerable amount of air could be forced in just before the valve closes and traps the charge in the cylinder.

The industry statically flow tests head ports @ 28 in. H2O pressure differential (or delta p). 5 psi is about 138 in H2O delta p. It is interesting to measure the flow thru an intake valve just .050 off it's seat @ 138 in H2O. It can be significantly into three figures of CFM.

Mother Nature is a tough ol' girl, but if you follow all of Her rules (physics, fluid dynamics, etc) you can achieve considerably over 100% Volumetric Efficiency in a normally aspirated engine. It's done all the time around torque peak rpm and quite often at hp peak rpm.

Getting the air in is still Job One in making power. IMO, 100% VE isn't the Holy Grail of engine design...it's now a lot higher than 100%.


Regards,


Jon

 

Jon, if anything I think I still need to research it a LOT more, not just a little more. Thats why I pointed out I was unsure of intake air speeds as well as pressure wave speeds...including in the partial vacuum we see in the intake manifold (and yes you're right, I was wrong about the effect that has on the speed of the wave). I always learn from your posts, thank you.

I'm guessing the airflow at 0.050" off the seat is going to be where a really well done valve job would come more into play as well?

 

I think you still see hot rodders put a 1" or so spacer under the carb. Seems like the idea was to increase the intake runner length which everybody thought increased HP and/or torque. Why would that little trick ever work on a properly designed intake? And, on modern EFI engines like the LS1 would a similar trick work and could it in some cases be detrimental to performance?

 

The spacer works by increasing the volume of the plenum area and helps alleviate the challenge of the plenum charge being depleted early and dampens pulsing which gets more consistent signal to the carb metering - hence why tunnel rams have such large plenums to ensure the runners are fed optimally. On something like a dual plane manifold the spacer if it does not have a divider can give the ports access to additional flow from the carb so they can "see" towards4 barrels vs 2. Worth a few HP at various points.

The manifolds that benefit the most are the ones that are designed to fit under stock hoods as the plenums are typically ok for stockers but not engines with higher airflow support needs.

No reason why it would not work on an LS engine. The intake and carb aren't fussy.

Runner length is always a tradeoff between power and torque. Long runners support lower down torque whereas shorter support higher end power. Its about preserving the moving gas inertia/energy the best for the rpm range in question so that the when the intake valve opens next the pressure behind it is peaking from that energy generated from the previous intake events and raises the VE over 100% (the "ramming" SSStroker mentions.)

Which is also why many production cars now use either 2 step or continuoulsy variable runner lengths to optimise intake efficiency against the rpm/engine load keeping the VE as high as possible.

 

great point, the dynamics found in recent engines such as variable valve timing, variable intake runners, even vanes in turbochargers will make a vast improvement toward VE. very exciting.

 

You are trying to say that increasing plenum volume and runner length are the same thing. They aren't. Spacers under carbs have nothing to do with runner length. They just increase the volume of air the runners can draw from. It would be akin to putting a spacer behind the throttle body.

 

That rep at K&N answers the phone and gets paid a meager wage. If he understood how engines worked, he would have never said "engine's sucking force." Engines do NOT suck air into them.

 

Wrong. That is the entire principle of an engine. When the piston goes down with the intake valve open it creates a vacuum that "sucks" air from the intake into the cylinder. Now there's also scavaging like the rest of the thread talks about, but yes... engines suck. However, I agree, a rep at K&N probably didn't know much about engines.