How Much Less Accurate Will It Be? (Fairly Long) - Inquiring Minds Want to Know!

[Dom G]

I've tried checking old threads to see if this question has been raised before, and from what I can see it has not. At least not directly.

Here's a few summary comments to set the stage for the question. Many discussions regarding orifice style benches, and particularly the MSD/Mercdog style, emphasize the value of baffels, sharp edge orifices, minimizing turbulence, non flexing cabinet construction, settling chamber shape and design, gauge fluid used, pressure tube locations, flapper valves, etc,etc,etc... I think everyone gets the point...There is a endless list of things that some people call refinements and others call necessary.

My question is this:

If someone follows the MSD/Mercdog plans to a Tee, insuring the cabinet is well constructed, is well sealed, has minimum leakage, makes their own manometers (which is what I'm doing initially) using proper design and mathematics for their scales and construction, WHEN COMPARED TO A SUPERFLOW BENCH of comparable capacity,

How Much Less Accurate Will It Be? 3%, 5%, 10%, 15%, ?

Does anyone have actual side by side numbers? i.e. I have a head that at 28" at .600 lift measured 250 cfm on a SF600, and on my homemade Orifice style bench measured 228 cfm or 285 cfm or xxx.

While I realize no two benches, including Superflow or other professional benches, will measure the same, I'd like to get a better idea of the approximate real world difference in accuracy.

Does anyone know?

Thanks.

Dom

[84-1074663779]

I don't see accuracy as being the main issue but REPEATABILITY.

For instance if my bench measured a certain cylinder head flow at 200 CFM, and then a year later the same head measured 202 CFM I would be very happy, even if someone else's commercial bench gave a figure of 180 CFM.

On the other hand if I measured 200 CFM, and then turned the cylinder head around by 180 degrees and then it measured 240 CFM I would not be pleased.

If flow conditions inside the bench are turbulent and chaotic, even the smallest change to the test setup can cause large changes in flow reading. If it is not repeatable, accuracy has no meaning.

If it IS repeatable, again accuracy is really only of academic interest. With a well set up bench people here have measured known reference orifices to be within a very few percent of theoretical flow.

[Dom G]

Tony, I agree that repeatability is very important.

My reason for building a flowbench is two fold. One, I can't afford to buy a SF600. And second, I perform cylinder head work on my race car as well as for customers and want to be able to validate the work I do. Many times being able to simply measure and show improvement from changes you've made to a valve seat, combustion chamber, or port is all that's necessary in which case repeatability satisfies the need because you can depend on the bench to show either improvement or a reduction.

In other instances, a customer may come to you with a set of heads off their race car to have them freshened and they tell you (or bring in) flow numbers from when they were manufactured or prepared. In this case, having a pretty good sense for how your bench generally stacks up to the perceived industry standard (SuperFlow) would be useful and that's why I'm posing the question.

If I originally prepared the heads and measured the results on my bench then I have a baseline that could be used for all future work.

So from my viewpoint, general accuracy compared to professionally built benches is important.

Hopefully this makes some sense.

[larrycavan]

Dom,

Your question, though reasonably simple in format, does not have an equally simple answer. It is loaded with a capital L.

Flowbenches are not "cookie cutter" production devices.

H.B.tells me that SF uses 18 different calibrations orifices in their flowbench department. Not all on every bench but that's how many they use in various combinations to calibrate.

The Mercdog bench will work. I just don't think can realistically tell you " yea...it will be within xx% of a SF600"

JMO,

Larry

[Thomas Vaught]

I have a lot of experience with the SF stuff and HB knows me personally.

The SF benches are fine benches as the vast majority of engineering is done for you.

If you used the same motors as the SF bench, the same basic orifice construction, the same manometers, the same fluid, and calibrated the thing, the only difference would be the blue wood cabinet!

I have a "stopper style" bench that is basically the same as a SF 1200, a SF 1020, and a SF 300 but without a movable orifice plate and it will read with-in 1/2% of the quoted SF calibration.

Tom V.

[larrycavan]

Those peak number is high from what I observed on that model head on a Superflow by about 12cfm.

[Thomas Vaught]

Always nice to know which SF bench was used and at what real test pressure.

Tom V.

[Jesse Lackman]

I don't see why people are willing to throw accuracy to the wind and settle for good repeatibility.

Why not have both?

One runs into this same mindset in discussing an engine dyno's accuracy. People say don't be bothered, "don't race dynos" etc and let accuracy go.

I've argued this to the bitter end re engine dynos. If any are interested in reading one of those discussions here you go.



There is an extensive discussion about flow bench accuracy on that same forum;

[url=http://speedtalk.com/forum/viewtopic.php?t=1240&start=0]Flowbench calibration
[/url]

I'm building a flow bench right now and will move what heaven and earth I can to make it accurate as well as repeatable.

[larrycavan]

Tom,

H.B. and I have never met. We've communicated through emails. He's been very helpful whenever I've asked him a question. I'd like to meet him one day.

As for superflow comparability. I'm very comfortable with my bench readings.

The SF unit I have access to is a 110 and we test everything related to motorcycle at 10@ on it and convert to 28 for comparisons with tests done at that pressure.

Larry

[Thomas Vaught]

Harold and I both worked in the same profession at one time.

If you talk to him mention my name.

His son not too long ago joined the same profession.

That way he will know you are not bs-ing him.

Tom V.

[DaveMcLain]

What I would recommend doing is to make up some test orfice of various sizes and try them out on a Superflow and then your own bench, see how the calibration is in all ranges, make a correction factor for flow in both directions. Then your results will be very accurate.

I have a Superflow 110 bench with the FlowCom and the Performance Trends software. Results produced on my bench agree very closely with those produced on friends' Superflow 600 benches as they should. The results are not perfect but they are very close. The same goes when I test a head from Edelbrock or AFR, their numbers are quite close to the ones I can produce on my bench. The same can not be said for Brodix heads but that's another topic all together.

Personally I feel that it will be pretty easy for a home built bench to be as repeatable as the test techniques used and the more experienced you become and the more consistent your test proceedures become the more repeatable your results will become, this stuff takes practice.

Get the test proceedures down cold, develop a correction factor if needed and you should be in very good shape.

[gofaster]

My SF-110 is similarly equipped, FlowCom, SF Depression Control, Port Flow Analyzer (Pro version). I would be willing to check orifices on my bench for forum members who may not have access to a SF bench in their area. Where I live there are only two other SF flowbenches that I know of within a 50 mile radius from here. Just pay the shipping. I am in NW Indiana. Maybe some other Forum members in other parts of the country could do likewise?

[larrycavan]

Generally, repeatability isn't the problem for home built benches. Most of the struggle comes from trying to calibrate multiple ranges to superflow numbers.

I recently learned that some 110 units have a punched orifice plate and other have a machined plate.

Are you guys using the test plate on your 110s to obtain a daily correction factor?

Larry

[DaveMcLain]

I would be willing to flow test an orfice plate for someone wanting to calibrate their bench to the Superflow numbers too, it's no problem.

I use the plate that came with my Superflow 110 to verify the calibration from time to time but it just does not change very much. I found that my numbers went up when I switched to using the Flow Com and the Performance Trends software, also the Pro version. But, it made my bench run right in line with other trusted figures for various heads. Before going to the electronics my bench was on the conservative side, now I think it's right in the middle for the most part and the numbers will agree very closely with those made on a Superflow 600 as well.

With the Superflow 600 it comes with a calibration chart on the front that tells what 100% scale on each range is in cfm. On the 110 it's not like that, you have to calibrate it yourself and that's pretty easy esspecially if you're using the electronics.

The 110 normally assumes the same amount of flow happens in both directions through the orfice plate. The highest range being 185cfm. What you find when you switch to the electronics and software is that the range isn't exactly 185cfm anymore in either direction! It's actually a little higher one way and a little lower in the other direction. Then when you calibrate your correction factor using the factory orfice plate it's very accurate when compared to know standards.

Hope this wasn't too confusing...

[larrycavan]