Manifold pressure fittings
I’m working on a fun little idea for an elegant solution to tapping the manifold pressure line forward of the firewall to accommodate the 1/8″ line that needs to go to the P-Mags for auto advance. I also have a GRT EIS MAP sensor in the cabin that needs manifold pressure so there has to be a tee somewhere. As I see it, the key is to find just the right barbed fitting for the 1/8″ line to be able to go from the -4 size hose coming off of the engine with AN fittings on it to the 1/8″ tubing. Also, this transition needs to be not immediately at the engine to keep stuff from melting (my comfort factor). I believe I have found the perfect fitting and have a fabrication plan. More to come when I have a chance to tinker with it.
UPDATE:
After many weeks of thinking about this, it took all of five minutes to actually pull it off once I had all the parts collected. Here is the parts list:
- McMaster part# 5463K127 Nylon single-barbed tube fitting for 1/8″ tube, 10-32 UNF Black
- McMaster part# 2844K13 Nickel-Plated brass double barbed vacum tube fitting for 1/8″ tube, 10-32 UNF
- McMaster part# 51135K158 High-Temperature silicone rubber tubing, Firm, 1/8″ Id, 1/4″ Od, 1/16″ wall, Black. This is the tubing that matches what E-mag ships with the P-mags for manifold pressure connection.
- McMaster part# 5463K353 Nylon single-barbed tube fitting Tee for 1/8″ tube, black. Used to Tee the line to the second P-mag.
- McMaster part# 5463K343 Nylon single barbed tube fitting 90 degree elbow for 1/8″ tube, black. Used to make the turn at the second P-mag (not required).
- AN804 TEE bulkhead fitting
- Vans -4 AN fitting spacer
- large steel washer to be drilled out to 7/16″ firewall side for fitting stability.
I took the AN804 fitting and sliced and diced it. It just so happens that a -4 size hose fitting is just the right size to be tapped to 10-32 threads. Ok, it is about one drill size too big, but good enough for me. I tapped two ends of the fitting and put just a touch of thread sealant on the barbed fitting threads before I threaded them in. Note, the nickel-brass fitting has a little O-ring at the base. A thing of beauty indeed, and oh so simple.
With the firewall fitting installed, I decided to make a restrictor fitting for the manifold pressure fitting on the cylinder head. I took an AN3-4A bolt and drilled a #59 size hole in the “center” (good luck) about 5/8″ deep. Tapped the fitting, applied a touch of thread lock, and threaded the bolt into the fitting. Then I just cut off the bolt head and cleaned it up. Super easy once again. An UPDATE to this: After thinking about this manifold pressure restrictor fitting a little more, I decided to make some revisions. There are two big flaws with the first version. The fitting I used was aluminum, and the plug was steel. These have very different thermoexpansion properties. The other, and primary, is a concern for the engine ingesting the plug if it were able to work loose for some reason. That would surely cause catastrophic failure. So, the second version of this restrictor fitting is an all steel fitting, with the plug on the other end of the fitting. There is no way it is going to be able to suck the plug through the fitting. This makes me sleep much better.
To connect the engine fitting to the firewall, I made up a -4 hose with a standard Aeroquip straight fitting on one end and a steel 45 degree fitting on the other end. The 45 degree fitting was acquired from a race shop. The firewall fitting location is in the location specified by Vans.
