Does this question have to many variables? I am just needing a ruff estimate.
Thanks, Chase

 

I thought at WOT @ 6200rpm, the VE table on the stock tune is about 90%.

 

that isn't VE in terms of physics.

 

Sure it is, the answer just isn't layed out how you want it. at WOT and 6200 RPM the engine has a VE of about 90% in stock form, that means for the 346CID LS1 its consuming 311 cubic inches of air (at atmospheric pressure and temp) per full engine cycle or 2 revolutions. The engineers at GM wouldn't refer to it as a VE table if it wasn't really a VE table. They're using it in their algorithms to compensate for load.

If anyone has a stock fuel map to look at I'd be curious to see what VE is at peak torque, I bet 95-98%.

 

Just looked at it...

99.7%

 

Ok, I have a very n00b question then.

If at peak TQ the engine is taking in 344.962 ci of air (running at 99.7% VE), then how can a cam swap increase peak TQ over stock?

HP seems obvious, if it flows better up high, without sacrificing TQ, then it's more HP. But can 1.038 ci really allow much power gain?

I can only figure I'm missing something here.

 

Prostock motors run at about 120% VE or more. The cam TIMING is where the key is at. AIr is not a continuos force. It comes in waves. The key is timing the Valve events to take advantage of this. You create a mini "ram" effect, thereby increasing the VE.

At least I think that is how it goes..

 

Most well built n/a motors today are exceeding 100% VE. In this specific case of 1.038 CI of air only winds up being about 1.8 cfm at 3000 rpm. Pretty minescule. However the .3% gain needed to reach 100% is also minescule.

Here's an easy way to calculate CI of air to CFM for future reference.

(Cubic inches of air/1728)x(rpm desired for calculation/2)

and a quick explanation of things

1728 - 12^3 to convert cubic inches to cubic feet
(rpm desired for calculation/2) - one full engine cycle, it takes 2 revolutions for all of the plugs to be fired.

 

http://en.wikipedia.org/wiki/Volumetric_efficiency

Volumetric Efficiency relates too how much air you take in relative to how much physical displacement there is in the motor. Now because the cam closes the intake valve AFTER BDC you CAN trap more air in the motor than the motor displaces, therefore 100%+ VE is possible.

Jes

 

This is because the momentum of the air continues after BDC.

 

Yes, it is very possible to increase VE..

Also, one of the main components to increasing power using Camshafts is the fact the valve events are set up for high rpm. 100% VE at 3000rpm is not as good as 100% VE @ 7000 rpm for power.

 

It is the volumetric efficiency table, but it isn't set up in units of a percentage that mean anything...all you have to do is start messing around with the complex bias filter like I have and you'll watch those values artificially inflate to well over 100 without even modifying the engine (when tuning in speed density). You can have values in this table that are up to 367...

And the stock VE table values at peak torque is about 98.8 units on mine.

 

yes thats what i was referring too, it is called and thought like a VE table but its really just a way to control fueling and spark

 

Inertia has some part of it but most of it is because the presure in the cylinder is still lower than that of the intake port.

 

Not after BDC. That extra inertia after BDC is like a ramming effect, and will actually increase pressures if the valve timing can trap the air, this is why too much overlap is not good unless you have a high rpm screamer.

 

Hold on there buddy..... first we have to get down to engine class 101.

ABDC and overlap? Overlap happens @ TDC when the intake and exhaust valves are open. ABDC the exhaust valve is closed, so there is no overlap.

I think you need to start here before you get into discussing inertia's role in cylinder filling....

http://auto.howstuffworks.com/camshaft.htm

The inertia is a SMALL part of it, but your not going to get the air to move from the plenum to the cylinder unless the pressure in the cylinder is lower. On an optimized setup the pressure is lower in the cylinder up until very close to IVC, which is well after BDC. Intertia is a small role but it doesn't cause 90° of cylinder filling after BDC by itself, at least not at the mass of the atmosphere on our planet. What you are saying is that the force exerted on the air colum from the 180° the piston travels down the bore causes enough inertia to extend that up to and beyond 50% as far... (90°), that's not how the world works. In fact the velocity in the port ABDC is decreasing so the inertia of the air/fuel mixture would also be decreasing. Not to mention, inertia in and of itself is a defined as "amount of resistance to change in velocity", that's all well and good if this was a static environment but the pressures and resulting velocities in the dynamic conditions of a motor actually impart forces with negative vectors to the air mass after the pressure in the cylinder increases above the pressure of the port, therefore overcoming the "inertia" of the air mass and actually causing reversion at the lower RPM range of the motor.

Brush up on Inertia at a little more.... http://en.wikipedia.org/wiki/Inertia

Now overlap is a time relative measurement, and so is RPM hence why they work in a linear fashion, when you increase RPM you increase overlap.

Jes

 

That's for super/turbocharger applications. When the FI is forcing air in at a pressure above ambient, you get a higher VE.

 

Will you marry me?

She is right. The valve overlap that allows for scavenging happens as the exhaust valve is closing and the intake valve is opening when the pistion is near TDC. I don't know how you guys think the piston comming back UP after reaching BDC is going to help the velosity of the air traveling DOWN into the cylinder.

 

Does any one have the VE over the entire rev range????

 

This may start a good argument:

No, you don't.