bunker

Just wondering, when I run Desktop Dyno 2000 with my mods it says Volumetric efficiency will be 102.1 at 6000rpm, is it even possible to go over 100% on an NA car? I thought its not even possible to hit 100%?

Mindgame

Yes sir, very possible. My new race motor did 114.6 @ 6,000 rpm (peak torque). Most really well built (n/a) engine will do over 100%
A little phenomonon called inertial supercharging.

-Mindgame

lt1camaroman93

Will someone fill me in on what VE is? I want to learn as much as i can and this is one thing i do not know.

89ProchargedROC

volumetric efficiency

Soma07

Sure it can, with a blower or turbo you can get waaaaaay over 100%

SStrokerAce

To get over 100% VE you need a postive pressure in the intake. That can occur with intake tuning. Intake tuning is playing with the pressure waves due to the valve opening and closing and also the sizes and dimensions of the intake system. You can get way over 100%, 200%+ with a blower or turbo.

Bret

OldSStroker

Volumetric Efficiency (VE) is the percent of air which actually enters and remains in the cylinder during each intake stroke, compared to the amount which COULD, under perfect, unsupercharged conditions. The amount of air which could enter the cylinder is the swept volume, or displacement, of that cylinder.(43.75 cubic inches in a true 350 cubic inch engine).

A typical street engine, say an LT1 in the '96 Impala SS, has a maximum VE of about 85% right around its 4000 rpm torque peak, while a 1250+ hp Pro Stock engine has a VE of about 120% around it's 8000 rpm torque peak and maybe 115% at 9500.

One of the things which causes the torque to peak then fall off is VE. It gets harder to start and stop the air more rapidly as rpm rises, so VE is decreasing, and friction losses keep increasing.

If the intake and exhaust system sizes are tuned, max VE can be closer to hp peak. A good example might be a race engine that runs in a narrow rpm band.

It might seem that best fuel economy would occur around best VE. If you need much of the engine's power to cruise, that's pretty close. Some smaller displacement engines are geared to run at torque peak at high cruise rpms. An LS1 in a Camaro has so much excess hp available at cruise, that it gets much better economy in 6th gear at 2000 rpm than in, say 4th at 4000 rpm.

arnie

Guess you have to convince me, cuz I disagree with your reasoning, or at least, don't understand where you are coming from. I'm not disputing the VE/torque connection mind you, but your 'economy' statement, to me, doesn't 'hold water'.

If the map/vacuum input doesn't reflect otherwise, (ie 80 mph headwind or pulling a loaded house trailer) getting better fuel mileage will happen at 6th/2000 cuz of less air/fuel being used, NOT the most, which will happen at the point of top VE.

Like you stated, the best VE means the most air AND that means an appropriate amount of fuel must accompany that air. The vehicle will get the poorer mileage at 4000 cuz, the engine, which is just an air pump, will cycle more air/fuel thru the engine at that rpm compared to the 2000 rpm. It takes very little torque to propel a vehicle at cruise, compared to max acceleration. Why would you use/want more than you need? I fail to see the VE connection in your mileage statement, provided that the VE is not excessively poorer at those cruising rpms. The better mileage is a balance of less engine pumping and the amount of VE sacrifice made.

BTW, BMW has tuned the intake system on their M3 class vehicle to the tune of over 100% VE. But, they are using it to propel the vehicle to very impressive speeds. They were are not talking max mileage in this case.

Injuneer

Is VE even relevant at part throttle? Once you run at anything other than WOT, you are introducing pressure loss to reduce the air density and hence the mass of air contained in the volume... so the entire concept of VE seems to go out the window.

The throttle will remain positioned at whatever opening allows the required mass of air to be present to develop the HP to overcome the frictional resistance of the drive train and the rolling and aero resistance of the vehicle. Obviously, there will be less frictional loss in the engine at 2,000rpm than at 4,000rpm so there will be less air mass required to develop the required HP, less fuel required, better fuel economy. But it has nothing to do with VE.

Or am I missing something here, trapped in my theoretical world, as usual.... ?

Mr. Horsepower

All good points.
The one thing I see missing here is a correlation between VE and engine efficiency. The confusion usually comes via looking at VE without looking at what effect VE has on the IC engine system as a whole. I'll just throw some relations into the pot, which might paint a better picture (fingers crossed).
First off, VE is directly proportional to IMEP due in part to the fact that combustion efficiency (n comb*) is also in a linear relationship with VE. Higher values of VE also induce higher values of turbulence... again their is a direct correlation between VE and n comb*. Not to mention the relationship between higher VE (greater turbulence) and it's effect on ignition delay time.

Quick and easy.. the basics. What it all boils down to in simplest terms is this..... you have a greater amount of work (IMEP) being done per a given amount of fuel (BSFC).

Take care,
Chuck

arnie

Actually, Fred, I like the way you stated that. VE, or actually a % thereof, AFAIC, should not be a factor. Just the actual amount of A/F in each cylinder would be of importance. Little bearing on the % possible. Agree?

Mindgame

Chuck,
Would you also include BSAC in that???

What you're saying makes perfect sense though (now that I've read it about 20 times).
This got me to thinking, always a scary thing, so I pulled out my engine dyno sheet to look at the numbers. I can see the correlations you mention in the dyno sheet. Seems that best BSFC was very near the torque peak, only 100 rpm off. Also, BSAC or 'break specific air consumption' (had to look that one up) is lowest at the same point. Which indicates to me that the engine is using less air and fuel at peak torque. Also, the 'ME' which I'd assume is mechanical efficiency??? is also the highest at that point, which is in agreement with what your saying too. All of that data says to me that the engine is producing more power on less fuel and air at that point. Never really paid much attention to it though.... very interesting.

-Mindgame

OldSStroker

Arnie, I think we're saying the same thing about cruising an LS1 at 2000 rather than 4000. Only if the engine needs to produce an appreciable percent of it's max power to cruise, does my economy statement make any sense.

Have you ever driven a low power/weight vehicle in the mountains or even on long shallow interstate grades? Large amount of throttle opening is required to maintain speed. All I meant to say was that the engine is most efficient around the torque peak (at higher throttle openings), because of higher VE, and the OEMs sometimes geared the low power/weight vehicles to cruise right around torque peak.

As to whether VE is meaningful at part throttle, if VE is a measure of pumping efficiency, shouldn't the highest efficiency point occur near the same rpm, independent of throttle opening? Partial throttle is just a restriction in the engine inlet; it doesn't change tuning, cam timing etc. (unless the engine has VVT and the PCM changes the cam timing under part throttle for other than emmision reasons.)

To test this idea, I ran the same LT1 (Imp SS) engine with throttle body flows of 10%, 25%, 33%, and 50% of stock flow on Engine Analyzer Pro. Results:

stockflow: 86% VE in 4000-4500 range
50% flow: 79% VE 3500-4500 range
33% flow: 73% VE 2500-4000 range
25% flow: 70% VE 1500-3000 range
20% flow: 68% VE 1000-2000 range
10% flow: 63% VE 1000. Drops above that.

My conclusion is that at half flow of more, VE drops but stays around the same rpm. At lower flows, it drops a lot, so at very small throttle openings, I'm way out in left field.

It's interesting to me that in the 1000-2000 range, which is cruise in the Imp, the VE is high with the minimum flow (and therefore throttle opening) required to propell that tank. Was that by design or coincidence? I would suggest that lots of part-throttle dyno testing is done by OEMs, and part-throttle efficiency (including inlet air flow and therefore VE) is important.

Back at ya...