Re: Word/Re-circulating valve fuction - Saabnet.com Bulletin Board


[General | Members | C900 | 9000 | NG900 & OG93 | 93 | 95 | NG95 | 99 | Sonett | Vintage Models | Performance | Detail | Clubs & SOC | Other Cars | Books Movies & Music | Chat | FAQs | Gifts | Member Photo Galleries | Member Directory | Classifieds | Service/Mechs | Manuals | Polls | What's New | Raffle | Photo of the Month | Sponsors]

Performance Bulletin Board
Search:
(Search Author's Posts: e.g. Keyword:Author)*Members Only


[Main Performance Bulletin Board | Post Followup | BBFAQ ] [ Prev by Date ] [ Next by Date ] Member Login / Signup [Most Recently Edited Saabnet.com Photo Galleries]
Re: Word/Re-circulating valve fuction
Posted by craigharroff [Email] (more from craigharroff) on Fri, 27 Feb 2015 09:23:19

In Reply to: Word, ELaw [Profile/Gallery] , Fri, 27 Feb 2015 04:18:53
Alert me when someone posts in this thread:

As Jlaw has noted, the holding pressure is higher at WOT. That was another great design feature about the Bosch that I had not considered. Thanks for pointing that out.

From what I have read and from personal experience, my understanding of what the blow off/ re-circulation valve tries to accomplish is as follows: At partial throttle or WOT the pressure in the intake (plus spring) holds the valve closed. When you let off the gas the throttle closes. Pressure in the PLENUM drops. Without the blow off valve/re-circulation valve present you are left with pressure (above atmosphere) in the intake between the turbocharger which is still compressing (for a very short time) and the throttle body which is now in a closed/partially closed position. This is a substantial volume of air:it includes the intercooler and all the piping volume before the throttle body. At this point several things are happening. Because you are off throttle (or partial throttle) exhaust gas heat/ pressure drop (worst case is a closed throttle). Less energy causes the turbo to slow down. In a closed throttle position, it slows down a lot depending on the length of time/bearing loss/wheel design/etc. At the same time the elevated pressure in the intake is looking to "escape" to the lower pressure of atmosphere which is now (WITHOUT a blow off/re-circulation valve present) BEHIND the TURBO.Because the turbo is no longer compressing due to the exhaust energy being partially or greatly reduced, the barrier to atmosphere is no longer present. As I understand it, this is the single most important event that slows down the rotating turbine as the air pressure "vents" backwards through the now slowing compressor wheel. This back flow will either slow or stall a compressor wheel. This happens every time you lift off the throttle. Given enough volume and pressure (larger aftermarket intercooler and intake pipe plus higher than stock boost), I suspect the compressor wheel would begin to rotate in a non-compressing direction AKA backwards. I took a Bosch valve and turned it into a blow off valve. The noise was so loud that I constructed a highly vented shroud to reduce the noise. It screwed together. My first design totally blew apart when I first tested it.The volume of air is significant and the rapid slowing of the compressor wheel can be a violent event given a high pressure differential. The valve allows the air to vent to atmosphere AROUND the turbocharger thus avoiding most of the shock and, hopefully, maintaining turbine speed so that when exhaust energy is again applied to the turbine wheel the initial starting point of rotation is higher than it would have been without a venting blow off valve.

Not all boosting events are WOT followed by a closed throttle so the effectiveness of the valve depends upon quite a few things such as time between getting on and off and then back on throttle. Are you shifting or braking? There are trade offs here. It would be logical that some optimal compressor speed be maintained while all these things are happening and, as Jlaw has posted, this function is now coming under computer control. That is very interesting!

posted by 192.230.18...

Posts in this Thread:
Alert me when someone posts in this thread:

Post a Followup

No Site Registration is Required to Post - Site Membership is optional (Member Features List), but helps to keep the site online
for all Saabers. If the site helps you, please consider helping the site by becoming a member.

Name: Member Login / Signup [Most Recently Edited Saabnet.com Photo Galleries]
E-Mail: (Optional)
Re-Enter E-Mail: (Confidential & Secure - Not revealed to other users!)
Note: Please check your spam folder for BB responses.
If you use GMail, follow these instructions from Google so that they appear in your regular inbox.

Subject:

Posting rules are simple - No for sale/wanted ads or live auctions may be here - use the classifieds.
You may not cross-post your message to multiple BBs.
Personal attacks, insults, or flaming are not allowed.
Site Members do not see any red text, inline ad links, skyscraper ads, box ads, nor anti-spam check.

Message: (please no for sale/wanted classifieds - post those in the TSN Classifieds)
To add inline links to your post, use [link]http://www.domain.com/[endlink] or use the Optional fields below.
To add inline images to your post, use [img]http://www.domain.com/img.jpg[endimg] or use the Optional Image Link field below.


Optional Link: (e.g. http://www.saabnet.com/)
To promote your own Saab site here, you must have a link back to The Saab Network.
Link Title: (Optional)
Optional Image Link: (e.g. http://www.saabnet.com/img.jpg)





The content on this site may not be republished without permission. Copyright © 1988-2018 - The Saab Network - saabnet.com.
For usage guidelines, see the Saabnet.com Mission and Purpose Page.
[Contact | Site Map | Saabnet.com on Facebook | Saabnet.com on Twitter | Shop Amazon via TSN | Site Donations]

This is a moderated bulletin board - Posting is a privilege, not a right. Unsolicited commercial postings are not allowed (no Spam). Please, no For Sale or Wanted postings, SERIOUSLY. Classifieds are to be listed in The Saab Network Classifieds pages. This is a problem solving forum for over 250,000 Saab owners, so expect to see problems discussed here even though our cars are generally very reliable. This is not an anything goes type of forum. Saabnet.com has been a moderated forum since 1988. For usage guidelines, see the Saabnet.com Mission and Purpose Page. Please remember that you are not anonymous. Site Contact | Site Donations | Other Sites by SP - Poverty2Prosperity.org
Your address is: 54.80.87.250 - Using CCBot/2.0 (http://commoncrawl.org/faq/) - Logged.
Site Members do not see red text instructions, skyscraper, nor box ads.
Click here to see all the Site Membership Benefits!