Thoughts on Optispark Ventilation
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Teching In
Joined: Aug 2020
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From: Boiling Springs, PA
Thoughts on Optispark Ventilation I recently bought a 1996 Corvette with an LT1 and 113k miles. After a couple of weeks, it was throwing a code suggesting a compromised Optispark (no high resolution signal), I pulled it and saw that it was full of motor oil - presumably from the leaking water pump drive seal just above it. It was a no-name product made in Taiwan, so rather that try to repair it, I decided to ditch it and get a Petris Engineering unit instead based on everything I have been reading on online forums. It is not quite together yet since I'm also replacing the water pump drive bearing and timing set.
In reading about Optispark venting, I've learned that in an effort to keep it clear of moisture, GM added the vented setup. One line leading to the manifold (after the TB), and another to the intake ducting (before the TB). The concept being that the manifold vacuum line will continually draw filtered air through the distributor from the "supply" line. However, that "supply" line, while not subject to full manifold vacuum, still has some vacuum compared to atmospheric pressure since the air cleaner is upstream of it. So the pressure inside the distributor will always be lower than the surrounding air. If the goal is to try to prevent anything from being pulled into the distributor through breaches in the seals, having the unit constantly under partial vacuum would seem to be asking for trouble. The seals would have to be perfect in order for the partial vacuum to not draw anything in. In the case of my Taiwanese unit, oil was getting pulled in through the drive bearing in the back.
I'm thinking that a better setup would be to route the source line through a less restrictive air filter in order to reduce the internal vacuum. Perhaps something like that Scotch-Brite looking thing they used as a PCV filter in the 70's and 80's. Anyway, I'm just looking to see what others may think of this. I only want to do this Optispark installation once, so I want to do it right.
In reading about Optispark venting, I've learned that in an effort to keep it clear of moisture, GM added the vented setup. One line leading to the manifold (after the TB), and another to the intake ducting (before the TB). The concept being that the manifold vacuum line will continually draw filtered air through the distributor from the "supply" line. However, that "supply" line, while not subject to full manifold vacuum, still has some vacuum compared to atmospheric pressure since the air cleaner is upstream of it. So the pressure inside the distributor will always be lower than the surrounding air. If the goal is to try to prevent anything from being pulled into the distributor through breaches in the seals, having the unit constantly under partial vacuum would seem to be asking for trouble. The seals would have to be perfect in order for the partial vacuum to not draw anything in. In the case of my Taiwanese unit, oil was getting pulled in through the drive bearing in the back.
I'm thinking that a better setup would be to route the source line through a less restrictive air filter in order to reduce the internal vacuum. Perhaps something like that Scotch-Brite looking thing they used as a PCV filter in the 70's and 80's. Anyway, I'm just looking to see what others may think of this. I only want to do this Optispark installation once, so I want to do it right.
Last edited by MoreCowbell; Aug 31, 2020 at 03:36 PM. Reason: Grammer correction
might get more answers in the LT1-LT4 section above this but i switched to vented opti in my 94 and haven't had an issue in 6 years. another thing i did at the time was tap and put a barb fitting in the weep hole for the water pump and ran a hose down past the opti so if the water pump happens to go it wont leak right on the opti
