How big is your BLOW-OFF valve?

57 Magnacharger · Post by **57 Magnacharger** » Mon Jul 16, 2001 6:09 am

How big is your blow-off valve?
For street , do you need one ? ( low boost )

moggy · Post by **moggy** » Mon Jul 16, 2001 3:58 pm

What for turbo or blower application???

Sharkey · Post by **Sharkey** » Mon Jul 16, 2001 6:58 pm

You don't really need a "blow-off" valve when running less than about 6-8 PSI of boost. The one I am using came from a 1st generation Eagle Talon and it works fine. Not rediculously expensive like that Blitz or HKS crap (which is about 50% function, 50% brandname).

Ian M · Post by **Ian M** » Mon Jul 16, 2001 10:38 pm

The Mitsubishi guys commonly use the stock (crushed) first-gen blowoff valve,even well over 20 psi of boost. Stay away from the 2-gen ('95 and newer) blowoff valves,they are plastic and will leak with increased boost.

Sharkey · Post by **Sharkey** » Mon Jul 16, 2001 11:57 pm

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by Ian M:
The Mitsubishi guys commonly use the stock (crushed) first-gen blowoff valve,even well over 20 psi of boost. Stay away from the 2-gen ('95 and newer) blowoff valves,they are plastic and will leak with increased boost.<HR></BLOCKQUOTE>

Excellent point re: the 2nd gen valves. I know a guy that was so competitive in autocross with his 4WD Turbo Talon that he bought at Porsche 944 Turbo and switched classes. He immediately installed a spare 1st gen Talon blow-off valve on his Porsche.

57 Magnacharger · Post by **57 Magnacharger** » Tue Jul 17, 2001 7:07 am

..Moggy I was hoping you'd reply, With your street supercharging - did you run blow - offs & do you agree with " use over 6 lbs.? thanks, Glenn.

Steve C · Post by **Steve C** » Wed Jul 18, 2001 6:08 am

Hi
Im using a Bosch blow off valve from a Saab, they make all the right noises and dont cost the earth. A blow off valve will help your acceleration times as it will keep the turbo spooled up. That is assuming your using a turbo.
Regards Steve C

Steve C · Post by **Steve C** » Wed Jul 18, 2001 3:32 pm

Hi
I have the Bosch part number for the Saab blow off valve, 0280 142 103. They are only about $50.00Au so they should be cheap in the rest of the world.
Regards Steve C

57 Magnacharger · Post by **57 Magnacharger** » Fri Jul 20, 2001 6:29 am

thanks Steve, I will be running one of those slower & less efficient type pumps (mainly because I spent a motza getting the thing in the first place- but I do love that sound ) but I appreciate the reply and will even check out that unit. Does a turbo set up run a waste gate and a over pressure blow off valve? If I feel its worth having one , I'll probably just get an o-ring seal in a spring loaded disk---- keep it simple s--pid.
See you.....Glenn.

moggy · Post by **moggy** » Fri Jul 20, 2001 7:25 am

You still haven't said if you're referring to a turbo or blower application, because their use is different for each forced induction system.

The blow-off valve on my blown motor (see pic) is used so that when decelerating or tickover or cruising (at constant revs) the valve is releasing the boost to atmosphere which means that the blower is running very efficient cos it doesn't have to compress the air. The only parasitic load from the blower is when it is on boost (i.e. accelerating) then the valve closes so the engine experiences max boost. Most new Eaton 4th generation blowers have these valves internally now. See www.magnusonproducts.com for a fuller description of how a blow off valve is used in a blower application.

Obviously it can be used at whatever boost, however in a Turbo application I havent a clue...

Hope this is helpful?

57 Magnacharger · Post by **57 Magnacharger** » Fri Jul 20, 2001 7:36 am

Yes I am running superchager ( well will be soon ). Do you run a over boost blow-off valve in case of back fires?
Those bypass valves sound really great. I might see if I can retro-fit something similar on my old Magnuson.
Thanks , Glenn.
ps did you see my question at bottom of Eaton M45 topic? can you calc???

Sharkey · Post by **Sharkey** » Fri Jul 20, 2001 9:52 am

A turbo application typically uses a wastegate mounted to the exhaust manifold to limit boost to a safe level. It is one of the oldest techniques and it is still the best. A pressure line connected to the intake manifold detects dangerous boost pressures and actuates the wastegate, venting excess exhaust gases, which in turn slow down the turbocharger and decrease the amount of boost created within the intake manifold.

If you were to replace the exhaust-mounted wastegate with a simple spring-loaded valve in the intake manifold (which is a real blow-off valve, BTW), you could still limit the boost within the intake manifold. However, by not limiting the pressure within the exhaust manifold you risk spinning the turbo up beyond safe speeds.

A "compressor surge relief valve" is the real name for what the aftermarket industry likes to call a "blow-off valve". It can only be used on blow-thru turbo (or centrifical supercharger) applications. It is controlled by manifold vacuum from below the throttle plates, and opens the valve when there is boost present above the throttle plates and vacuum below. This allows the unneeded boost to escape out the valve instead of back out the through the turbocharger, thereby allowing the turbo to free wheel during shifting or coasting. Without such a valve, the boost has no place to go and must flow backwards through the turbocharger impeller wheel.

At low boost levels this simply robs the spinning turbo of its RPMs. This results in lost turbo velocity between gear changes, creating a definate turbo lag since the turbo must again spin up to speed. But at high boost levels, the reversing air charge can actually bring the spinning turbo wheel to a complete stop, or even cause it to quickly reverse direction! Not only does this increase turbo lag even more, but it can damage your turbocharger.

Unlike exhaust-driven turbochargers and centrifugal superchargers, traditional Roots-style superchargers are positive displacement devices. These must have a spring-loaded plate mounted to the blower because an engine backfire out through the intake valves has no place else to go. This is a "blow-off valve".

Steve Arndt · Post by **Steve Arndt** » Fri Jul 20, 2001 11:33 pm

Eaton uses a "by-pass valve" not a blow off valve, if used the way the describe on their site. They are built into the 4th gen like he said.
blow off vale is different.
The by pass valve on the Eaton is quite a trick idea, one of the reasons I am thinking of trying one. Hardly any draw under high vacuum.

57 Magnacharger · Post by **57 Magnacharger** » Sat Jul 21, 2001 1:37 am

Thanks Sharkey for that excellent information.