# Advanced Engineering Tech - Who here has over 100% VE and how did you get there.

XLR8N
12-08-2010, 04:40 PM
K, here's the deal. Post your known VE and your engines set up. I am mostly interested in seeing how it correlates to power output, ie. What peak VE rpm vs peak power.

Also interested in learning how to accuratly calculate an engines cfm requirements. I know how to figure a basic cfm for engine size.

Cubes x rpm/3456 = cfm

Now this is only good for t/b size as it's the only part that sees a constant pressure correct?

I have a 402 that sees 72-7300 rpm regularly, my cfm required to achieve 100% VE is 837-845 cfm. Now how would you figure runner size based off of that?

XLR8N
12-09-2010, 01:19 AM
I don't even hear crickets.....damn....

Old Geezer
12-10-2010, 07:47 AM

white2001s10
12-12-2010, 03:32 PM
K, here's the deal. Post your known VE and your engines set up. I am mostly interested in seeing how it correlates to power output, ie. What peak VE rpm vs peak power.

Also interested in learning how to accuratly calculate an engines cfm requirements. I know how to figure a basic cfm for engine size.

Cubes x rpm/3456 = cfm

Now this is only good for t/b size as it's the only part that sees a constant pressure correct?

I have a 402 that sees 72-7300 rpm regularly, my cfm required to achieve 100% VE is 837-845 cfm. Now how would you figure runner size based off of that?

Calculating ideal VE is not the best way to go for performance, but trying to do so for a runner size just wont work unless you are running an IR (individual-runner) system without a plenum. The plenum is a bank of air that each runner draws from in turn creating pulses. It's not a smooth transition of air-flow, nor does a calculated average rate or pressure drop accurately reflect the capability of the entire system.
A calculation will either assume steady state flow, and/or use an average measurement.

You can try calculations, but I think you'll have better results by measuring the pressure drop in various points in your system as it operates under load.
Measuring pressure drop in your plenum will tell you if your upstream parts such as your throttle-body are a significant restriction.

Measuring the drop in each individual runner is harder to do, but more revealing of the information that you're specifically asking about.
Testing in actual operation is the most difficult, but is my preferred method.

An easier, but less revealing way is to pull the intake and/or head, and have the runners tested on a flow-bench, - then comparing the result to the calculated VE for an individual cylinder. I don't put much faith in this method.

In operation, the pressure in the runner will peak and dip (max & min). There can be an extreme restriction that only lasts momentarily, then diminishes to none, or even to a pressure rise. The peaks of measured restriction are what you should be concerned about, and they can change based on valve timing. In general, the longer the open duration of the intake valve, the less pressure drop you will measure in a given intake port.
The longer duration smooths the peaks and dips in the port.
This is an indication of reduced velocity in the port, and isn't necessarily what you want in a street application.

Different engine builders, and specifically head porters have their own ideas when it comes to acceptable velocity in a port. I'm not going to add my opinion here, because each opinion is highly debatable, but I will suggest you search for information on cylinder-head choke point.
There is a book "High Performance Chevy small-block Cylinder Heads" by Graham Hansen.
On page 20 he starts explaining calculated VE, port cross-section, and how to calculate port size and flow for a given VE or power output.
It is interesting, but keep in mind that by design, the calculation is for steady-state testing on a flow-bench, and/or absolute maximum power output.

You have to know that absolute maximum power output is not what you want for street engine. It would mean running the maximum valve timing short of power dropping off, - and that's a LOT!

The main lesson in this is that a head porter tries to avoid having a bad choke-point in the intake port without necessarily making an overly large port.
Some porters like the bragging rights of max flow and peak power.
Other porters concentrate on where a particular engine will spend most of its time running.
I'm not getting into that debate though.

I will say that %percentage wise, the amount of time that a choke-point will restrict flow in a port is often very small. The amount of time that the engine spends at the RPM where the restriction is significant is also often very small, especially in a street or drag car.