After reading the crankcase evacuation thread here I decided to try out the slashcut exhaust method.. Well I connected the hose w/o a check valve to my boost gauge(I know I know, cheap but thats all I had to reference from) and this is what I got..

0-2,000 rpms or very very light throttle - bounces back and forth quick violently between -2 psi (4-5 inHg) and +2 psi..

2,000+ rpms and/or heavy to full throttle - needle would stay at JUST above the zero mark.. It would stay ever so slightly above where it rests when the car is off, not even measureable in whole numbers or even halves probably..

So I didn't get any vacuum effect whatsoever at all, which is quite dissapointing to me really. I have it set up correctly, 45 deg. slashcut tube inserted with the cut facing away from the exhaust stream direction, at receeding angle..

This is with a 2.5" DP and 2.5" catback, a test pipe, and a single chamber muffler, so basically no exhaust restriction, and the slashcut was placed at the beginning of the catback section to place it after my wideband o2 sensor..

Any ideas? Was my testing "equipment" to blame here or does it just not work as well as we would wish?

 

Do you have a breather on the valve cover as well?

Are you trying to circulate air or are you some how trying to pull a vacuum?

 

I think he's trying to get the Bernulli Effect....might wanna try a different placement of the tube.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">Do you have a breather on the valve cover as well?

Are you trying to circulate air or are you some how trying to pull a vacuum?</TD></TR></TABLE>

Trying to get vacuum to suck blow by gasses in the exhaust rather than the IM.
Your exhaust has to be creating vacuum...something isn't right

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">Do you have a breather on the valve cover as well?

Are you trying to circulate air or are you some how trying to pull a vacuum?</TD></TR></TABLE>

I didn't have anything connected to the crankcase, just the slashcut to the gauge for testing purposes, to see if it really created vacuum like the theory stated and the tests on here have shown..

I personally didn't measure anything, just posting up what I found and wanted to see what people thought about it.

It looks like I am probably going to go with an electric vacuum pump..

 

your exhaust HAS to be creating vacuum.... like i said try tapping it in another spot, away from any bends...turbulence in the exhuast flow may have altered your readings.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 2muchPSI &raquo;</TD></TR><TR><TD CLASS="quote">your exhaust HAS to be creating vacuum.... like i said try tapping it in another spot, away from any bends...turbulence in the exhuast flow may have altered your readings.</TD></TR></TABLE>

It was in a straight section, but I have bought a vacuum pump to remedy the problem anyways, check it out (look on ebay to get stuff cheap!)

http://cgi.ebay.com/ebaymotors...%3A12

just gonna strip off the electronics board and wire it up to a switch or ign power.. :D

 

I know what he is trying to pull off.

If there is a source of air into the motor, it likely doesn't pull enough volume to create an appreciable amount of vacuum. Yet it could circulate the air out of the crank case.

Pull the line off, vent it to atmosphere and repeat the measurements.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">I know what he is trying to pull off.

If there is a source of air into the motor, it likely doesn't pull enough volume to create an appreciable amount of vacuum. Yet it could circulate the air out of the crank case.

Pull the line off, vent it to atmosphere and repeat the measurements.</TD></TR></TABLE>

I had the slashcut directly connected to the boost/vacuum gauge, so that wouldnt affect the outcome, who knows..

 

Yes it would.

Suck air through an open straw.
Suck air through a straw with your finger on the end.
Which straw has more vacuum?

Granted, a motor isn't a sealed volume, but there will be a difference between a motor with a breather (open ended straw) and a scavange and an other wise 'sealed' motor and a scavange (capped straw).

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">Yes it would.

Suck air through an open straw.
Suck air through a straw with your finger on the end.
Which straw has more vacuum?

Granted, a motor isn't a sealed volume, but there will be a difference between a motor with a breather (open ended straw) and a scavange and an other wise 'sealed' motor and a scavange (capped straw).</TD></TR></TABLE>

Your example isn't quite accurate. When you take your finger off the straw... your vacuum is still the same.. its just now you are pulling something with it, instead of just measuring it.

Vac is Vac... it won't pull more if it can "flow"... then it isn't vacuum anymore His closed setup is properly testing for vacuum that should be created when the exhaust flows past the slashcut.

You are saying he should measure vacuum... then open it and measure flow... 2 different things. The vacuum will be more usefull if there is a flow (breather on VC), but if there isn't good vacuum pull to start.. it won't be pulling much of anything.

 

Vacumn is negative pressure, in the case of the straw you are imposing a restriction and creating a pressure drop. Maybe you need x amount of energy in the form of y inches of mercury to overcome that restriction, but less vacumn w/ the same energy indicates better flow--a la oil pressure.

Throttle body open vs closed no vacumn w/ more flow vs the opposite.

If you suck then remove the restriction the pressure will equalize.

 

Actually, if you understand the physics of it, it is a good comparison. You just have to think in real physics and not imaginary physics where perfect vacuum sources are every where. Point is, that valve can only draw so much air through it. If air can get into the crankcase easily, there will be less negative pressure in that line.

If your mouth is a perfect vacuum source, then sure, you can pull the same pressure in the straw with or without your finger on the end. If you got that much suction though, maybe you should consider a career path in prostitution. This is not a knock on you or anything; I'm just saying you are failing to see the full picture. That valve is NOT a perfect vacuum source. It can only produce so much mass flow.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 99_GS-T &raquo;</TD></TR><TR><TD CLASS="quote">That valve is NOT a perfect vacuum source. It can only produce so much mass flow.</TD></TR></TABLE>

Yup, and that is also why most exhaust evacuation setups use at least an -8AN (1/2") or larger hose or line