Minimize the Unswept Volume of the HF Injector Adapter

[thomas m]

There are a lot of ways that errors can creep into the compression test process. Right away I want to spend more money on better tools (can never go wrong with more tools). There is the unswept volume issue. The CTA 2800 kit seems to minimize that by having the Schrader near the tip of the adapter. Still, there is a little. You could put a Schrader into the tip of the HF injector and that would improve it. More precise skills and tools that I don't have. The pressure gauge is also suspect.

So, I decided to see what I could come up with using stuff I have laying around the shop. I am used a 16 penny nail and a piece of brass tube coupling to stuff into the HF adapter. The nail was a close fit and just had to be cut and slotted. The brass part was upgraded to a bulkier aluminum spacer which had to be drilled out slightly. It's a loose fit.

Using a syringe, I measured the volume of the hole in the HF adapter (with the HF pressure gauge adapter screwed in) at 1.7 cc and the nail/sleeve insert at 1.2 cc, so I reduced the unswept volume from 1.7 cc to approximately 0.5 cc. The plated pressure gage adapter has a Schrader valve.

I haven't yet found a way to check the HF gauge at 500 psi, but it does pass a test to about 130 psi with my Craftsman compressor gauge.

It will be interesting to see if this modification works at all and how the results compare with and without the inserts. That might not happen until the rainy cold weather improves since I can't work inside.

Comments?


The whole kit (Brass spacer not used):






The modified spacer and nail (16 penny):




The nail in the injector:




Nail near tip of adapter:

[libbydiesel]

I commend your creativity and I think that the result will be markedly more accurate. 

[ORCoaster]

Thomas, you have reduced the unswept volume by 1.2 ccs. Do you have any idea about how much pressure change that would make on the gauge?

[fatmobile]

 Nice experiment.

[thomas m]

Thomas, you have reduced the unswept volume by 1.2 ccs. Do you have any idea about how much pressure change that would make on the gauge?

I don't have any idea what the pressure affect would be. I searched the forums for a thread that I remember containing a LOT of volume calculations and data on "swept" and "unswept" volumes. I think these vary from engine to engine. I couldn't find anything that could help me make a guess. Maybe someone knows where that thread is??
I wanted a simple, reversible way to reduced the unswept volume, so I used on-hand stuff. A good machinist could probably make and even tighter plug to fit into the adapter. I also considered using epoxy or JB Weld with a small hole bored into its axis. Too messy and not easy to undo. For me, adding a Schrader valve was to complex for me to try.

The HF injector adapter has (at least mine has) 3 different diameters along the axis of the hole. About 0.2' to about 0.17" to about 21/64" for the 1/8 NPT bore. I used my dial caliper and drill bits to get approximate dimensions. Maybe one of you will add the Schrader.

[libbydiesel]

The 1.6 has an actual displacement of 1588cm³.  It also has a 23:1 compression ration (displacement:unswept volume).  1588/23=69.04cm³=unswept volume. 

[ORCoaster]

So would it be feasible to say that that unswept volume divided by 4 would be what we would see in a compression test at TDC? 

69.04/4 = 17.26 CC  And if all is well that equals 483 PSI.  So is it fair to say each CC equals 27.9 PSI?

And then can we do the math on the 1.2 cc he reduced the unswept volume on to get an increase of 33.5 lbs? 

I would think you would be able to see that much of a difference even on the high-pressure dial of the gauge.  Maybe make a mark on it and see how close they come.  A needle's width?    Just speculation. 

[libbydiesel]

Pressure X volume is constant.  Given a volume of 17.26cc and a pressure of 483, if you increase that volume by 1cc then the pressure would drop to 457.  If you increase volume by another 1cc, pressure would drop to 433.  Etc... 

483 x 17.26 = 8336.58 / 18.26 = 457
                      8336.58 / 19.26 = 433
                      8336.58 / 20.26 = 411

If the actual compression of a cylinder was 483 psi and the HF gauge was perfectly accurate, then the adapter increasing the volume by 1.7cc would reduce the pressure to 439 psi, making a 44 psi difference in the reading.  In many cases, that difference would be significant if attempting to determine if compression is within sped on an engine.