Not the Mercdog again

[Old Grey]

Somehow I've become the local flowbench expert, which is ironic because I don't have any enthusiasm for my own design yet alone the troublesome Mercdog design.

The story goes that someone(Warspeed) built this Mercdog bench around Feb 2004 to Aug 2005 - these are the dates on the boxes so it might have been earlier and went on longer -. The bench was 90% finished when it was abandoned - the faults explain why - where it finally ended with my customer.

The brief is to finish it off cheap.

I have spent 3 days reading about the Mercdog problems, so I think I'm up to speed there, it's just some other things that I'm interested in before I give the guy my assessment.

Just 3 question currently :-

1:How is the SF600 orifice disk located.

Is it sandwiched between two pieces of wood like the in/exh disk on the back of the SF600, or spring loaded to the orifice board?


2:The pie hole in the orifice board on the Mercdog is usually located ABOVE(5.5") the centre shaft of the orifice disk. Is the orifice hole in the board on the SF600 BELOW(4") the centre shaft of the orifice disk? - red circle or blue arrow -


3:The Mercdog plans say 1 #8 screw every 6". Is that enough?

[larrycavan]

Would you like a copy of the original MSD bench that John Clark built and was featured in a Magazine? I'm pretty certain that's where the design was taken from. I have it, holler if you want it.

But, to answer your question, NO, it was not sandwiched. I had a silly retaining / sealing ring that didn't work well to seal when exhaust testing. Not really a problem on Intake because the disk sucks tight against it's mounting board.

Let's not throw the baby out with the bath water The design is a SuperFlow copy and works with some modification. The rotating disk can be made to seal with a little common sense and minor engineering.

Spring is exactly what's needed with the rotating orifice disk. There are at least 2 fairly easy ways I can think of off the top of my head to make it seal.

You definitely want the baffle plate installed above the disk and don't even think about putting the test pressure sensor probe in the cylinder. That hokey setup was a joke. Move it in or out a little and watch you test pressure change... It belongs inside the top chamber. I mounted mine to the baffle plate on a right angle bracket, off to the side a bit from where the cylinder hole is.

I spoke to John Clark on the telephone when I was building my bench. He was great! He told me they left things out of the article and also that he massaged some of the orifices with a grinder to get the numbers where he wanted them.

Somewhere, on here, a long, long time ago I posted the correct orifice info for that disk.

Larry C

[larrycavan]

Here's a page from the article

[larrycavan]

Another pertaining to your question.

[larrycavan]

Here's the works

[Old Grey]

The Mercdog plans came with the bench, and even though they are detailed well, they do assume you already know what to look for when building a bench. eg, the dimensions of the orifice wheel is there, but not how crucial the disk flatness, and how well it seals, is in the correct operation of the bench. I don't remember seeing this bench on this forum so I assume the original builder probably didn't know about this forum, and went at it all alone, so probably didn't know what to look for.

The bench looks a impressive on the outside, but internally it's flawed - I count 13 things to fix -. His budget will probably only allow a deck plate inspection port and manual swapping of the orifice plate - he's not a high end user so a single square edge 3mm orifice rated at 350cfm with his FP1 will be good enough -, but I will still look into redesigning the disk just in case.

I've spent about 15 hours researching the Mercdog's problems, so I'm aware of the 3" box added on top, the single orifice with the deck plate, and the baffle plate.

From what I have read, even if the orifice disk was made perfectly, which this one is not, it just doesn't work satisfactorily the way Mercdog designed it. Thicker and spring loaded, or sandwiched, would fix it - like the Clo-Flow you mentioned http://www.tractorsport.com/forum/viewt ... ?f=9&t=871 -.

One of the faults was that the pie shaped hole orifice board was put in upside down, the pie hole is at the bottom front, not top back like in the Mercdog dwg - hence question 2 above -. This seams like a bonus, because when I remove the orifice disk and shaft, I can put the baffle board lower, and that does away with the need of the 3" top box.

I read in an old post that the SF600 had the hole in the orifice board at the bottom front, but the SF dwg shows it at the blue arrow top back. I was wondering where it actually was as I might have to cover the faulty one and move it.

[RACEPUMPER]

Tony? Any ideas?

[larrycavan]

You mean say that the plans you already had didn't explain how to seal up the rotating disk?

[larrycavan]

It's all fixable. I didn't count the specific number of things I changed. Rather odd how the plans you got don't mention how to seal the disk.

I believe I would get the pass through hole away from that front wall, regardless of whatever else needs attention. No matter how you slice and dice it, the air will not evenly approach the circumference of the orifices. The top section you're certain you don't need and the baffle plate significantly help that situation.

Good Luck....I'm confident you'll come up with something that wor ks

[Old Grey]

You mean say that the plans you already had didn't explain how to seal up the rotating disk?

It explains to seal the disk, just nothing about keeping the disk flat and how welding a 0.030" disk warps it.