fuel injector restrictor size

scsmith · #1 07-19-2016, 11:59 AM

I would like to narrow my GAMI spread more if I can.
I have access to some slightly undersize injectors.

For reference before I start, can anyone tell me the diameter of a stock lycoming fuel injector restrictor? These are LW-18265.

Zuldarin · #2 07-19-2016, 12:13 PM

Steve, go to the expert..Don Rivera at Airflow Performance. He's a master with fuel injection and can get you what you need and explain why. He's been great to work with in getting my GAMI spread down where I want it.

Auburntsts · #3 07-19-2016, 12:17 PM

I'm guessing .028 but I'm thinking the only way to be sure is to pull each one and eyeball it. +1 on the recommendation to go to Don and AFP.

DanH · #4 07-19-2016, 12:49 PM

Usually 0.028 for your 360. Don has 0.0285's, 0.0275's, etc.

Jesse · #5 07-19-2016, 12:50 PM

Most stock injectors for standard Lycomings are .028". If you are standard compression (8.5:1) and standard power, then Don would likely recommend putting in a set of .026" restrictions and working out from there. Otherwise, run a lean test and post or email your results and I can recommend a change.

FasGlas · #6 07-19-2016, 12:59 PM

We are testing a set of .024's in a 10:1 360. Don sent a few above and below to find the closest GAMI. The standard size for 360 is .028. He's the guy to talk to.

scsmith · #7 07-19-2016, 10:45 PM

Thanks everyone.

My current GAMI spread is about 0.2 gph. Three of the cylinders are exactly the same, and the forth is richer, lagging by the 0.2 gph. It runs pretty well LOP now, so I don't NEED to do anything really, but since I have access to some other sizes, I thought I might experiment with just the tiniest bit leaner on that one cylinder and see if I could actually get a GAMI spread of zero with relatively little effort.

Knowing that the standard ones are 0.028" is all I need to know as a reference point for what to try that is just slightly leaner.

Thanks again.

Jesse · #8 07-19-2016, 10:57 PM

In my experience a restrictor of 0.0005" difference makes about a 0.4-0.5goh difference in that cylinder. If you are already at a 0.2 spread, then changing that restrictor will most likely increase your spread. The goal with tuning is to get to 0.2, improving on at is very hard.

DanH · #9 07-20-2016, 07:14 AM

There is probably a bit of wiggle room in what any given nominal restrictor size might deliver. Steve might try 3 different 0.0275's and get a slight different flow from each.

I say this for several reasons. One, when I look at restrictors with a 10x magnifier, they're not all exactly the same; there are small differences in surface finish, edges, etc. Two, I note that a popular supplier of restrictors individually flows and matches injector sets, not relying on the nominal size. And there is a whole cottage industry flow matching EFI injectors.

Although suppliers would probably prefer that we don't, it is also possible to tweak a restrictor to a little larger size with the correct micro-sized reamer or file. See all the micro files on the bench? No real risk; doing so just makes that cylinder richer.

Rupester · #10 07-20-2016, 08:47 AM

As usual, Dan is spot-on. When I did Fuel Injection 101 at AFP, Don Rivera confirmed that the odd size restrictors for 4 cyl engines (I.e. Anything other than .022 and .028) are based solely on bench flow tests, not orifice dimension. The finish characteristics that Dan mentions - surface finish, edge contour, etc - make significant differences in flow rate. Hence when you buy a, say, .0245 restrictor from AFP, it is really a .0245 equivalent restrictor.