Engine EVAC system with mufflers and tailpipes?

n2xlr8n · Post by **n2xlr8n** » Tue Feb 21, 2006 4:37 pm

Yes, it is; I ran the Moroso system for a long time.

I later changed to the GM vacuum pump.

HTH

S.

DCarr511 · Post by **DCarr511** » Tue Feb 21, 2006 5:07 pm

Ken ... when I tried to run mufflers with my pan a vac system it sucked oil out of the valve cover and burned it in the exhaust. The car smoked like a freight train going down the track. I only had 24" of 3" tube and then 2 chamber 3" flowmasters. I made 2 passes with the mufflers and it smoked on both after the 2nd pass I removed the mufflers, made another pass and no more smoke.

MadBill · Post by **MadBill** » Tue Feb 21, 2006 5:18 pm

That sounds like it worked too well! Its the job of the baffles and breathers to ensure only vapours exit the engine...

DCarr511 · Post by **DCarr511** » Tue Feb 21, 2006 6:45 pm

Hey Ken ... This is Duane ... I my car I had both breathers at the very front of the valve cover, not at the rear or over any rockers. Now I did NOT have any sort of baffle at all. Just the Ford style Big Breathers. I dont think there is a better spot to put them, but much improvement could be made for baffling. Here is a pic of their placement. In this pic I have a line that goes down to the fuel pump block off plate. That was not on there when I ran it with mufflers.

Duane

n2oracer · Post by **n2oracer** » Thu Mar 02, 2006 7:55 am

What's the purpose of the line connected to the fuel pump block off plate? Wouldn't it pull oil if connected there?

rmcomprandy · Post by **rmcomprandy** » Thu Mar 02, 2006 10:25 am

I had a problem with a car using mufflers with a 3" system in that it kept burning-out the metal one way valves and then pressurizing the crankcase at the most inopportune times. I put new EVAC exit tubes in the exhaust pipe AFTER the muffler and the problem went away; (simply calls for a little longer hoses).

MadBill · Post by **MadBill** » Thu Mar 02, 2006 11:14 am

Hmm... The check valves rely substantially on pressure pulses to aid the scavenging effect. Going behind the mufflers would eliminate most or all backpressure (good) but also most of the pressure peaks. (bad)

rmcomprandy · Post by **rmcomprandy** » Thu Mar 02, 2006 2:07 pm

Unless you need to work with a "Pulse Air" air injection system, (as in OEM emisions), to pass clean air into the exhaust, the EVAC system relies on the pressure differential on either side of the one way valve.
Being that the exact same amount of air flow is AFTER the muffler as there was BEFORE the muffler there is no difference between the vaccuum draw on the EVAC system at WOT. Simply the positive pressure is mostly eliminated at slower speeds and part throttle situations.

DCarr511 · Post by **DCarr511** » Thu Mar 02, 2006 4:11 pm

n2oracer wrote:What's the purpose of the line connected to the fuel pump block off plate? Wouldn't it pull oil if connected there?

I tapped and plugged the Fuel Pump pushrod hole so no oil drained from there. My reasoning for putting a line there was that all the windage we are trying to reduce is in the crankcase and I felt that was one way to try and reduce it. I never had any oil in that line so it didnt seem to pull any oil up it and into the system.

MadBill · Post by **MadBill** » Thu Mar 02, 2006 4:21 pm

rmcomprandy wrote:..the EVAC system relies on the pressure differential on either side of the one way valve....

That's my point: There is no (or at least little) average pressure differential; the exhaust system on average will contain slightly more than atmospheric pressure. The check valve opens to evacuate the crankcase when a negative exhaust pulse passes and closes to prevent back flow when a positive one does.

rmcomprandy · Post by **rmcomprandy** » Thu Mar 02, 2006 11:50 pm

You completely don't understand what is going on in a high air flow exhaust pipe. The tube which is used for the connection protrudes into the exhaust pipe's air stream so that tube see's high air speed NOT PRESSURE and places a vaccuum draw on the tube much like a large carburetor venturi on it's booster. The faster moving air at WOT has LESS static pressure not MORE.
There's a guy named Bernule who gave us a whole bunch of physical laws on pressure differential that might be in your best interest to learn; especially if you want to fly.

SWR · Post by **SWR** » Fri Mar 03, 2006 6:29 am

That would be Bernoulli,right?

MadBill · Post by **MadBill** » Fri Mar 03, 2006 7:38 am

rmcomprandy wrote:You completely don't understand what is going on in a high air flow exhaust pipe. The tube which is used for the connection protrudes into the exhaust pipe's air stream so that tube see's high air speed NOT PRESSURE and places a vaccuum draw on the tube much like a large carburetor venturi on it's booster. The faster moving air at WOT has LESS static pressure not MORE.
There's a guy named Bernule who gave us a whole bunch of physical laws on pressure differential that might be in your best interest to learn; especially if you want to fly.

I didn't say the evacuation effect was completely due to pressure pulses.
Try this: Hook up a vacuum gauge to one of the check valves by itself (with the system not plumbed to the engine) and see how the readings compare between behind the mufflers vs. ahead vs. ahead with no check valve.
Hint: You might want to put a substantial plenum between the gauge and the fitting in the header, just in case there are some significant pulses that need to be damped out to get a steady reading...

rmcomprandy · Post by **rmcomprandy** » Fri Mar 03, 2006 9:54 am

Well, however you spell his name... you're probably correct on the spelling.
Remembering those principal laws are what is the important stuff to me...lol.

On a dyno at WOT, reading two compound guages, (both pressure and vaccuum), plunbed side by side, it doesn't make any difference...I've already done it. It only makes a difference at IDLE and LOW throttle openings.

Don Terrill’s Speed-Talk

Engine EVAC system with mufflers and tailpipes?

How the header EVAC works...

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