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Exhaust Pan Evacuation on the Street

General engine tech -- Drag Racing to Circle Track

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GARY C
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Re: Exhaust Pan Evacuation on the Street

Post by GARY C » Fri Jun 19, 2020 11:47 pm

MadBill wrote:
Fri Jun 19, 2020 11:40 pm
GARY C wrote:
Fri Jun 19, 2020 11:36 pm
... it only looked like one side was pulling.
It's pretty common for the check valves to flake out..
Yeah I think there is more good to be had with this system then has really been tested.
I had about 2' of clear hose per side but could only see moister move on one, I tried tweaking the other side but got no results so I just ran it.
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skinny z
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Re: Exhaust Pan Evacuation on the Street

Post by skinny z » Sat Jun 20, 2020 12:27 pm

Kevin Johnson wrote:
Fri Jun 19, 2020 11:31 pm
MadBill wrote:
Fri Jun 19, 2020 11:10 pm
It's definitely a challenge to get meaningful exhaust pan vacuum with a street exhaust system, but David Vizard in (I think) his BBC book shows a system with the pickup tubes running parallel down the center of the collectors and ending at the venturi throats. I believe it achieved ~ 8" Hg., which would suggest it could work with a low restriction exhaust. If anyone's interested I'll try to dig it up. I believe it was built by a then-member of ST.
viewtopic.php?t=59218#p836083
Kevin Johnson wrote:
Wed Nov 20, 2019 1:46 am
MadBill wrote:
Wed Nov 20, 2019 12:47 am
In his BBC book, David Vizard wrote about and showed a system he said was by far the best pan vac. he's ever seen. I forget the builder's name, but I believe he is/was a Speed-Talker. The system used a square cut ~ 5/8" tube angled in through the side of the merge collector and bending to run down the centerline to the throat. I believe it achieved ~ 6" Hg.
https://www.chevydiy.com/how-to-build-chevy-big-blocks-exhaust-system-guide/ wrote:If you are looking for the ultimate in evacupan performance, it’s time to get serious about the location of the extraction nozzle. My big-block building compatriot Mark Dalquist managed to get a draw of almost 9 inches Hg (which is 121 inches of water) by finding the sweet spot in the center of the collector among the primary pipes.
Great info. Thanks.

dannobee
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Re: Exhaust Pan Evacuation on the Street

Post by dannobee » Sat Jun 20, 2020 1:38 pm

I tried a few different things back in the day when we were limited to wet sump engines and no external vacuum pump (circle track engines)
Using the Moroso kit, I drilled the hole in the header at 45 degrees, then put the tube in the header with a vacuum gauge attached and moved it around with the engine running until I got the highest vacuum reading. I ended up trimming the end of the tube and removing the "nick" in the pipe and running the tube 180 degrees from what Moroso recommended. If you looked in the collector from the back facing forward, you would see the cut perpendicular to the exhaust flow and the evac tube exposing the most area. I also tried a collector with a necked down venturi section near the tube, and vacuum did improve a little more, but the power didn't improve. Likely related to an increase in backpressure. I called it good and quit at that. It was WAY more than 3" Hg, IIRC.

If I had to do it all over again, I would have tried the center of the collector method, and extended the tube into the necked down/venturi section of the collector, just to see if it would pull more vacuum. Again, seeing how much vacuum it would pull with the engine running.

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