Crankcase evacuation test

[big block fiero]

This pull was of a 120cc 4-stroke whereas the red line was with the crankcase at atmospheric and the blue line was with a steady 6 inches of mercury provided by a vacuum cleaner.

Notice that WITH vacuum there is noticable loss of power up to about 5500rpm and that hp gains dont happen untill 8700rpm and extend the peak power curve.

So what is your all opinions as to what is happening and why? What would you do to the ring pack to improove any situations? What subtle changes will happen with a gapless second ring, a removed second ring or horizontal gas porting added?

[84Dave]

I suspect there's a chance that the vacuum in the lower case may have been pulling some of the combustion process by the top ring at lower rpm. I have seen this on a 2000cc SOHC Ford 4-cylinder with a standard ring package and using the dry-sump pump to pull about 10" of vacuum. The fix for improvement?
Install [6] lateral gas ports around the top ring. The top ring gas-loads/unloads much faster with the lateral ports. I also found that with using a Gapless 2nd ring, one MUST cut a decent volume 'accumulator groove' in the ring land between the top and 2nd ring. To preclude lifting the top ring with use of the Gapless 2nd.
The 3rd lesson I learned. At least as it applies to the Ford. Running a 13.5:1 geometric compression ratio. If there is .135", or less, metal thickness between the top ring land and piston crown, run the top ring gap at about .007"/1" of bore. The top ring runs quite a bit hotter with the lateral ports, and the top ring will butt if the gap is too narrow. With the Gapless 2nd, I always run the gap .0055"/1" of bore.
Doing all the above, versus the standard ring package, improved the average of both torque and HP right at 5.5%. And we have thousands of paved-oval laps using the setup. -Dave-

[rookie]

What about a gapless top ring?

[big block fiero]

Here is my generalized thought, (correct me if this is wrong)

If vacuum pulls MORE mixture volume threw the crevice volumes of the piston via leakage then you loose bottom end but gain top end-----------------At lower rpms excessive mixture volume is wasted into the crankcase and this also diminishes the compressible mixture volumes available hence we get a loss. At higher rpms this leakage is desirable to purge the clearance volumes of residual exhaust gas which it does without waste or diminished compression volume because there is less time for significant leakage to occur so this is a gain.

If vacuum pulls LESS mixture volume threw the crevice volumes of the piston via leakage then you gain bottom end but comparatively loose top end----------------At lower rpms mixture volume isn't wasted to the crankcase and compressible volumes aren't diminished very much at all but yet the clearance volumes are adequately purged. At higher rpms there is not adequate purging of the clearance volumes because there was less leakage to start with and there isn't enough time for leakage to occur hence we get a comparable loss.

So the next test would be to set up the ring pack as dave suggests (with a gapless second ring ect) and expect more low and mid-range torque. The question is will there be a loss of peak hp compared to this first test I've already done?