Manifold Pressure Lines

[rwtalbot]

I am having some issues with the manifold pressure installation. Van supplies a VA-118 hose and bulkhead nut with restrictor to be installed between number 4 cylindar and the firewall. I can see his intent is to get the manifold sensor in behind the firewall.

My issue is that I have duel P-mags. I would like to run a hose from number 3cylindar (left side of engine), to the sensor block. I will then run one line to my PMags and another line behind the firewall to the sensor.

Is there any reason (other than convenience) to select one cylinder over another in terms of measuring manifold pressure?

I also question Van?s design of the restrictor somewhat. If the intent is to stop the manifold pressure port from ?leaking too quickly?, what will happen if the VA-118 hose leaks, bursts etc? How critical is this restrictor to engine performance?

Thanks/Regards
Richard

[SportAvServ]

I don't believe that it matters what cylinder you get the manifold pressure from, just Tee it off to your mags. As for the restrictor, I understand that it uses the line as sort of an accumulator, it steadys up the pressure pulses so you get an more even reading on the guage. It seems that the new electronic sensors are even more sensivive to the pulses and you will watch your pressure bounce. What is more inportant is that if you get a leak or your line breaks,you will only lean out that cylinder a little, compaired to opening a -4 size hole in your intake and really leaning that cylinder and mabe burning a valve.
Anyone elce ?

Randy

[szicree]

Originally Posted by rwtalbot

I would like to run a hose from number 3cylindar (left side of engine), to the sensor block.

Isn't the number 3 cyl the right rear?

[647jc]

The right rear cylinder is in fact #3. My new Lycoming O320 came with a primer system and #3 cylinder was not plumbed for priming rather the cylinder port (hole) was left open for the installation of the MAP fitting which according to Van is a AN816-4D nipple aluminum, non-restricting. The firewall pass through fitting Van supplies to be used with his sensor is more of a tube size converter than a restrictor. If the intent was to restrict the line, that should be done at the cylinder fitting rather than the pass through. I use the same setup except my sensor is from Dynon and the pass through fitting is non-restrictive and not the one Van supplies since the sensor uses a larger tube size.

[scard]

I have a p-mag MP tee setup that I'm quite fond of: http://rv9a.card-net.org/archives/20...sure-fittings/

[Hartstoc]

There is some rational to putting the flow restrictor at the firewall and using the aeroquip hose as a damping ballast. However, there is an inherent trap in the assembly that should be avoided. The problem is that the torque of the AN-4 flair fitting should be a good deal more than what you want for the aluminum 1/8?NPT fitting you are screwing into the cylinder head, but there is no space for a wrench to prevent it from turning. If you are tempted to stop tightening the flair because the fitting starts turning on the pipe threads, the flair fitting will not be tight enough. That hose is constantly being worked by engine vibration, AND IT JUST MIGHT FALL LOOSE!

The workaround is non-conventional assembly using three wrenches. Use a 5/8 and 9/16 to hold both the hose and flair nut from turning. Put a drop of medium locktite on the flair fitting threads, screw it into the flair nut and securely torque it with a 1/2? wrench while still holding both parts of the hose end fixed. Next, put some thread compound on the NPT threads and very carefully screw it into the cylinder head. Use the 9/16 wrench on the flare nut to gently but firmly torque the fitting into the head. Ideally, also try to find a way to stabilize the hose to the engine a few inches away from fitting. On my installation the hose was really too long connected to #3 so I moved it to #1. I then was able to use the oil return line on #3 to loosely stabilize the hose there.

[Larry DeCamp]

What Harstoc said about flair on NPT fittings is true. You must devise a way to hold the 1/8 while you tighten the flair. I pressed the smallest restriction I could drill in a lathe into the 1/8 NPT fitting in #4 cylinder. TS flghtlines hose to tee. The Pmags have run flawlessly, but I could never get a steady MP/RPM reading from the EIC. I added accumulator hose volume and more restrictors with no success. YMMV