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 Why test @28"? 
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Joined: Fri Feb 19, 2010 7:36 pm
Posts: 1628
Location: Grantsville, Utah 45 min west of Salt Lake City
DaveMcLain wrote:
Some of this might be a little misleading too in that back in Smokey's day he didn't have electronic manometer devices with the resolution of the devices of today which would have reduced the resolution he could get when using lower depressions than 28 or so inches. If 28 is such a standard why does Superflow rate the SF600 at 25 inches of water?

Most reputable head porters try to flow heads with as high of depression as they can muster safely. On some heads some defects do not show until you are at 36" H2O or higher. An LS Chevy head on the exhaust port is this way. I know of one example on an LS Chevy motor that would just not make the power that the flow bench told them it should. They went back and flowed the head again at 28" and it looked fine. They started cranking up the test pressure and at around 44" H2O the exhaust ports went all over the place. They corrected the ports using 44" H2O and the engine made the power they expected on the dyno.

On the other hand, Malvin on this forum, is doing some BBC heads that are raised spread port 12º heads. At 28" he was getting some turbulence and popping in the intake ports. He cranked it up to above 40" and the ports straightened out and the turbulence went away.

I am sure not one for putting blanket coverage onto anything as there are so many variables when it comes to porting heads and building engines. That is one reason I am building a bench to flow at higher pressures. Maybe one head would be fine at 25" but I won't flow them at that. Everything I do will be at done at least at 28" or higher. I can't remember if it was Larry Meaux or Darin Morgan, but one of those guys actually Frankensteined two SF600 together to get it to flow higher depressions. If it wasn't important why would they go to the trouble to do that.


Wed Dec 31, 2014 12:40 am

Joined: Fri Jan 08, 2010 10:36 pm
Posts: 1179
Location: Maryland
I do not look at this as a specific test pressure issue, Smokey used 28 as he stated that he saw diminishing returns on the the dyno for gains made at test pressures above 28, in essence he was personally ahead of the curve and going for the low hanging fruit. Think of this era and the RPM ranges, Strokes, Piston Speeds, Valve sizes ???

Times have changed engines have changed and all the variables have changed with the increases in piston speeds in the average performance motor today and the recognition that velocity is KEY in a good port testing at a higher pressure allows us to move further up the port speed scale. Thus by exposing the port to higher speeds the minor inconsistencies look much larger. As Larry Meaux used as a correlation its like cornering speed or rough pavements as speed goes up sooner or later you will lose traction. I use a simple formula it is Peak instantaneous Piston speed converted to linear velocity in FPS X Bore/MinCSA. I do this in a spreadsheet per degree of crank rotation at max RPM, this will give me a picture of the potential instantaneous peek velocity, I always make sure the port is stable past this point. I then look at the cam and determine intake lift at this point and test again. Turbulence often does not turn off as fast as it turns on in a port, how many times once a port went turbulent you almost had to stop testing to get back to a clam port? Once the air is a muck it is hard, almost impossible to get it laminar again and this kills HP.

We have moved up the scale in many ways but it is not just flow testing that needs to move up it is the understanding of ALL the components that come into play as testing at 10" my be just fine for your daily commuter that spends 90% of its life at 2000 RPM. Test pressure is just another component in the engine build combination.



Wed Dec 31, 2014 5:58 pm

Joined: Fri Apr 30, 2010 12:06 pm
Posts: 27
The Theory sounds good Bruce but if you look at the Old Superflow 1200 Bench (it has 16 of the "Good Ametec 115923 Vacuum Motors in the bench) they still used 25" of depression for the bench. The bench had 3 orifices 200 cfm, 400 cfm, & 600 cfm. All calibrated at 25" of water with a ~ .586 discharge co-efficient.

Tom Vaught

Wed Jul 01, 2015 8:37 am

Joined: Fri Apr 30, 2010 12:06 pm
Posts: 27
I think that Neal Williams (Super Flow Founder) found out that the original benches flowed ok at 12 inches of water and so specified 10" of water for the first benches.

When the money came in for the first 300 cfm bench (from a camshaft maker), then Neal W. went to the 25" test pressure and the same .586 co-efficient for his first rotating disc benches. (.826 -.827 red fluid).

Tom V.

Wed Jul 01, 2015 5:42 pm
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