In the book Power Secreats by Smokey Yunick he relates to some of his early experiments regarding changes in port flow capability and power production. At the time (early 1950's) there were no comercially available flow benches on the market so he built his own. (I got to see it and took some pictures of it before it was dismantled in 2004)According to him significant changes in port sizing and configuration failed to produce measurable results if the depression was less then 28"/hg. and anything greater then that was still consistant with data gathered at 28"/hg setting. Personally I think a better method is to set the pump for a specific flow volume and measure restriction via a vacume guage on the intake and put positive pressure on the exhaust and measure pressure. I recognize that this is not an accepted practise within the industry but for my purposses it has provided me with the data I was seeking.-------Phil
Hi 28"/hg = little under 14 psi.and your right smokey,there are lots of cheaper & easier ways to do it if your comparing ports and or measuring changes. the only advantage to the pricey vacuum is because everyone does it that way you get cfm. numbers that can be compared to other times and places. Happy Easter
just checked with Mr.yunick. he's says inches of water,so while my statements are all accurate (hg is mercury) I sit here kinda corrected. I don't own a flow bench,I've used methods similar to smokey44211 to test ports. I guess $25,000 1950's dollars sounds like a lot of money to generate such a small pressure drop?
Thanks all for the responses, I don't know why my brain made me type in/hg! I like some of the alternative methods to compare port to port, thanks again!
28" H2O is about 350 feet per second (~250 mph,average port velocity). The air speed in the short turn will be much higher (up to 700 feet per second).
Most experts seem to agree that Mach .55-.61 (~700 fps)is the maximum velocity you can achieve before a "choke" condition occurs in the port and hp losses begin to occur.
With proper inertia wave tuning, you can achieve ~125-127% volumetric efficiency under the 28" H2O condition. Many people feel that ~130% VE is the max. possible with a naturally aspirated engine.
If you wanted to perform a "real world" test of your NASCAR vechile would you test at a track speed of 7" H2O (~120 mph) or 28" H2O (~250 mph)?
28" H2O more closely simulates the real world conditions of a race engine (intake port only). In addition to just getting the "flow numbers", head porters us a pitot tube to create a map the port velocities and search for excessive high and low velocities and make corrections as necessary.
Some head porters us 36" H2O to try and identify marginal areas (usually the short turn radius) and make corrections based on 36" H2O, but correct the flow numbers back to 28" H2O to communicate with peers.
Air has to turn a corner at the short turn radius and the race car has to turn the corner at the end of the straight away.
If you set up your race car to turn corners at 120 mph it might not perform the same during the race when it has to turn the corner at 250 mph. So, flow the head in as close to real world conditions as possible.
On the exhaust side, when the valve begins to open, the differential pressure is ~500 psi (~10000" H2O, supersonic). It's hard to find a flow bench to duplicate those conditions. Flowing at 28" H2O can fool you into thinking things are OK, so flow the exhaust port at the max. pressure differential (especially at low lifts) then convert the flow to 28" H2O.
Not quite sure where this "industry norm" comes from.
Ford Dearborn USA measure to 3 different pressure drops.
10 inches of water, 20 inches of water and 5 inches of Mercury or about 68 inches of water.
I've worked with/for a few OEMs and none I know work with 28 inches of water. It must be the after market.
The 68 inches of water is deemed to be in the ball park of the conditions of the exhaust port during blow down.
"With proper inertia wave tuning, you can achieve ~125-127% volumetric efficiency under the 28" H2O condition. Many people feel that ~130% VE is the max. possible with a naturally aspirated engine."
I'm not sure where these VE numbers are coming from.
Are you referencing ambient or pressure/temp conditions in the inlet manifold? I always reference ambient as referencing inlet manifold (EVEN for Boosted engines) means you cant do performance development work on anything upstream of the inlet manifold.
A Honda S2000 makes about 120% VE, an BMW M3 S54 may reach just under 110% VE. Racing engines may achieve 120 to 130% reference plenum/outside the trumpet/ambient.
There isn't an easy way to relate the 28 inches of water to the inlet conditions of a running engine as the flow is highly transient and pulsating. The only real way to relate pulsing transient flow to a steady state flow bench device is through flow theory such as Annand , Navier Stokes, Mass ,Momentum, energy as covered in modern 1 D cycle simulation codes such as Ricardo Wave, GT Power etc etc.
It seems that flow bench's have become more of a marketing tool then a performance tool.
I have always felt the the dynamics in a reversion wave and the impact of a moving piston should be reflected in any viable flow numbers..
I am looking forward to something like a Spin-tron to add flow data to their parameters...
