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blow off valve?

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turbobooster

Active Member
Joined
Nov 12, 2002
Messages
1,575
im just curious about if any here or any anyone one knows of someone that runs a blow off valve on a GN? they sell them but i never hear of any one gettin one for there ride or is the blow off valve more like a show piece that makes that sound?
 
No need to on a hotair. With the throttlebody before the turbo and no intake plumbing it's pointless and would do nothing. I run one out of necessity. Even letting off the gas at 1psi, it makes a very long whooosh sound. I felt sorry for my turbo and eventually got one.
 
thanx for that info makes sense but ur tellin me ur still runnin one out of necessity and becuz u feel sorry for ur turbo wat do u mean by that? oh and nice pic if that setup PaCemKR86
 
The basic reason for a BOV (blow off valve) is to relieve a buildup of pressure in the intake tract when the throttle blades are closed suddenly. Many turbo cars, including the 86/87 intercooled GNs, have "blow through" throttle bodies. This means that the throttle body is placed in the intake stream after the turbo. In these setups, if you were to close the throttle blades suddenly, the turbo is still spinning very fast, and cramming air into the intake tract, but it has no place to go (the throttle blade is closed), so it backs up, and essentially puts the brake on the spinning turbo blades. This can be harmful to a turbo, if it happens a lot, and can decrease its useable life-span. On manual-tranny cars with a turbo, its almost a neccesity, as this happens every time you shift under boost (when you let your foot of the gas to shift). There is some debate as to whether it is neccesary in an automatic (like the 86/87 GNs), as this only happens rarely, if you were to be running at full boost, and then suddenly let of the gas. If you want, theres a ton of threads on this topic in the general tech section. On stock hotair 84/85 GNs, its is absolutely unnecesary, as our throttle bodies are immediately before the turbo, so if we close the throttle blade suddenly, it has no ill effect on the turbo. If however, you are running a modified hotair (like cool 84) that has had the throttle body re-located to be downstream of the turbo (as is often done when adding an intercooler to our cars) then youre in the same boat as the 86/87 GNs where there is some benefit of running one. So basically, if you have a hotair with the throttle body in the stock location, there is absolutely no reason whatsoever for getting a BOV, theres not even a place to install it on our cars. If you have moved your throttle body to be after the turbo, then it may or may not eb worth the trouble (read the threads on that topic to make up your mind for yourself, theres a few diff opinions ont eh subject). I hope this helped clear things up. Oh, and a standard disclaimer: i am by no means on expert on this stuff, and there are MANY who know more than me. If something i said here is incorrect, please feel free to correct me, thats the only way I learn :)
 
Good explanation!

I would like to add that the reason I said it was out of necessity is because the "whoosh" sound was so long and loud, even at very low boost. I have a huge intercooler and a ton of 3" piping so it can hold a large volume of air. I used to work at a place that had a turbo car running 6.0 @ 218mph over 10 years ago on methanol, no nitro. The guy is an engineer and he said he would never build a turbo car without a BOV. I know there's debate if it's worth it but for $100, it's cheap insurance.
 
Tampa T-Type said:
I have a new one but where would you install it?

If you have moved the throttlebody to the pressure side, near the intake manifold, it will work. If you haven't moved the TB, it's useless. Generally you mount it between the outlet of the intercooler and throttlebody. Anywhere in that section of piping is ok. Mine is about a foot from the TB.
 
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