reverse vacuum load phenomenon

MartinM · Post by **MartinM** » Fri Nov 02, 2007 9:43 pm

There's some whacky animations on
http://www.secondchancegarage.com/public/186.cfm

Scenarios 1, 2 and 3 are the most interesting.

The other interesting thing is why (most) vacuum gauges have zero vacuum at full deflection ie fully clockwise...I suppose it's some convention from the distant past

EDIT - also interesting stuff at
http://www.fordf150.net/howto/diagnoseengine.php
especially...
3. Haynes emissions control manual
a. testing at various speeds

* engine starting vacuum should be 1 to 4 in Hg. To test disable ignition (ground wire from coil), hold throttle wide open, crank engine slowly with starting motor.
* healthy engine at idle should read steady 15 to 20 in Hg.
* healthy engine at 2000 rpm should read steady 19 to 21 in Hg.
* healthy engine at open throttle should read close to 0 in Hg.
* healthy decelerating engine reading should jump to 21 to 27 in Hg as open throttle released.

dr.occa · Post by **dr.occa** » Wed Nov 07, 2007 4:50 pm

i've attached a recent datalog i performed part way into work this morning. i did run into some traffic. i'd appreciate any feedback letting me know of any observations and opinions on timing.

dr.occa · Post by **dr.occa** » Wed Nov 28, 2007 3:37 pm

i hadn't heard from anybody in reference to my last post above. i'm starting to think that map wasn't the best choice with my cams of choice. idle is showing 10 in Hg. i've check for vac leaks and found none. i'll do one more check but it's looking like i may have to now get a TPS mjlj setup. dang.

Post by **brentp** » Wed Nov 28, 2007 4:07 pm

I recall you had a task to manually observe the load using a vacuum gauge and compare it aginst the MAP sensor reading. Have you done that yet? I was mostly waiting for that information.

Your data log is a bit hard to read since you can't really tell when you're at WOT/cruise at high RPM or throttle closed at high RPM (engine braking). Regardless of how you sized your vacuum reservoir you should *at least* see more vacuum when you're at high RPMs with the throttle closed than when you're cruising at same RPM. Since you were driving at the time can you identify any areas in your log file where this might be happen?

You mentioned your cams before but haven't characterized the specs. Are they especially aggressive?

dr.occa · Post by **dr.occa** » Fri Nov 30, 2007 4:16 pm

brentp wrote:I recall you had a task to manually observe the load using a vacuum gauge and compare it aginst the MAP sensor reading. Have you done that yet? I was mostly waiting for that information.

Your data log is a bit hard to read since you can't really tell when you're at WOT/cruise at high RPM or throttle closed at high RPM (engine braking). Regardless of how you sized your vacuum reservoir you should *at least* see more vacuum when you're at high RPMs with the throttle closed than when you're cruising at same RPM. Since you were driving at the time can you identify any areas in your log file where this might be happen?

You mentioned your cams before but haven't characterized the specs. Are they especially aggressive?

i did compare against an analog vacuum gauge and at idle it's ~10 in Hg.

here is the info on my cams: PDM C.709.S Toyota Stage II 261 Street Cams: Intake/Exhaust - 223 duration @.050”, .326 lift

would it help if i did a 1/4-esque data log and post that up? would that provide any necessary info?