I was reading this article on adding venting to earlier opti's and noticed the author advised having the inlet draw air through a fuel filter from air under the dashboard rather than from the air intake elbow. I understand his rationale (low humidity air), but wouldnt this result in un-metered air being injected into the engine????

http://www.fierolt1.com/lt1_9394OptiRepair.htm

You can make the LT1 '92-'94 Optispark setup more reliable than a '95-'96, even with the early distributor configuration. Buy a $12 '96 GM Vette distributor vacuum harness assy. Buy a $4 clear plastic fuel filter. Buy a $3 tube of premium black Permatex RTV, because it withstands the highest temps. Reserve a long day to fiddle around. Here's how it all goes together:

1a) Disassemble and clean the inner and outer cap & rotor assembly with a mild soap like Murphy's, Ivory or equivalent and thoroughly clean off all carbon, soap and dirt residue. Early style top caps must be drilled in a specific spot and at a specific angle to provide venting and a piece of vinyl hose will need glued in the hole. You will have to examine a later style outer cap to see where to drill the hole. It is at 6 o'clock pointing toward the driver's side tire. Use masking tape on drill bit to prevent drilling the small molded spark shield inside. Sorry, I can't describe better. I elected to buy a new early distributor and also a late distributor cap kit. I removed the early upper cap and replaced it with the late upper cap. This was expensive but ideal. You need an odd E5 female Torx socket to remove & install cap.

OR

1b) Buy a new later '95-'96 style cap and rotor assembly. The outer cap that the spark plug wires plug into was revised to provide a vacuum system connector. It WILL fit the earlier distributors perfectly. Problem is, the lower cap assy is different on the early and late distributors and can't be purchased separately. I heard the Pep Boys early cap kit provides the proper early lower cap with a later style upper cap that has a plug in the vacuum hole. That would be ideal.

2) Use a liberal bead of Permatex to seal the lower cap section to the aluminum base, top cap to the lower cap and thoroughly cover the stupid "shrinking" white foam seal around the wiring harness area of the distributor. You may prefer to use a hobby paintbrush to cover seams better.

3) The fuel filter hangs under the dashboard with the inlet end open to the air. The outlet end goes to a piece of vacuum line and feeds thru the firewall or grommet near the upper door hinge into the engine compartment. This hose must feed to one of the three vent holes in the distributor base assy closest to the driver's side by gluing in a piece of thinwall 1/8" vinyl emissions control hose in with the silicone sealer. Form the hose so it routes away from the crankshaft dampener toward the driver's front tire and looping upward with a fairly tight but not kinked radius of about 3". Plug the remaining two holes with the sealer.

This vinyl hose in the distributor base must be routed up the front of the motor between the serpentine belt tensioner and alternator bracket. You will likely need to see a '95 or '96 Vette to see how they squeezed it in there without pinching or wearing a hole in it. It is a real snaked route. You can split rubber vacuum hose and cover the vinyl hose with it to prevent collapsing when you restrain it using plastic tie wraps . It follows the edge of the aluminum accessory bracket all the way up to the intake. This 1/8" vinyl hose in the base gets routed across the top of the motor toward the battery. I drilled aluminum bracket to access door hinge area from battery. You can convert to rubber vacuum hose if you wish and route it through the grommet in the upper driver's door hinge into the dashboard area. You're done with hose #1.

3) Now hose #2. Take the vent hoses from the '96 GM Corvette OEM Vacuum Harness kit and use the one with the two in-line gizmos in it. One in-line device is a one way valve and the other is a flow restrictor. The accompanying bare hose in the GM kit is not used. Remove the plastic cover off the driver's side of the engine intake manifold. You will see a nice vacuum source going to the EGR valve. Install a tee and plumb the modified early cap or stock later distributor cap hole to the intake vacuum source using the '96 hose with the 2 in-line gizmos. Make sure it's installed with the gizmo's near the intake. This hose is routed from the intake down the accessory bracket following the same path. Hose length is critical.

Conclusion: Low humidity air is sucked from under the dash and fed into the distributor aluminum base assy, out of the cap thru the one way valve and restrictor into the intake. This should be bulletproof, far better than the '95-'96 setup that has similarly poor sealing and draws moist air from the air intake assy. You should be able to submerge your ignition now. GOOD LUCK.

 

MSD opti spark cap and rotor kit for the early opti has a port for venting, IMO go with that

 

yeah, it's only like $150 and it's great quality... i love mine!!!

 

I think this is sticky worthy. But that's just me. If I had the time, money and place to do the work, that'd be something I'd do asap.

I think you did a good job explaining w/o pictures.

 

I see no one has commented on that one yet. Well with the factory vented opti, the air is drawn from intake as mentioned. Air that has already passed through the MAF so it's metered. So in the mod, we're getting air from some place other than the engine. However, does it really matter? The air will move into the cap but the cap is external to the engine. I see no need to worry about unmetered air cuz it has no way to get into the engine.

 

But in this mod, we are relying on the vaccum from the intake manifold to pull the air through the cap: doesnt this ultimately result in the un-metered air being sucked into the intake manifold? Or do the check valves in the vented opti harness prevent that?

 

The way I see the stock set up is that the air is taken out on the outlet of the MAF before the throttle plates- so it has been accounted for by the MAF. It then travels through the opti and is sucked into the intake manifold plenum...re-joining the air it parted company from in the elbow before going into a combustion chamber. The only thing this air did is bypass the throttle body..big whoop. It's a very small amount of air and the little idle air valve has way more capacity (bypassing throttle plates) to compensate. The idle air valve lets way more air by the throttle plates than the opti vent. The check valves are for keep air from going backwards from the manifold to the intake elbow.

 

yea you are right about the unmetered air entering the system, but it is a really small hose correct? that means it has a very low flow potential, so with that logic, I bet that you will hardly see any type of change, again because the change will be so small, I bet the variance in the MAF sensor is greater than that caused by the unmetered air from the opti

 

OIC now I didn't read the part about the second hose. That's the unmetered air we're talking about.

Agreed on the variance in the MAF! Sounds like the vent mod would work! The check valves should take car of that problem, but if it don't, no biggie IMHO.

 

Mid-America Corvettes use to sell a kit that was the same thing you discribed for about $20. The unvented opti actually has 3 holes in it. You tapped into ? for inlet air and T'd into a vacumm line and RTV'd the third hole.

I bought one but the dealer said it would void the warranty so I never installed it. I think I still have it, if so I'll post where they connected the lines too.

 

Yes, it will be unmetered air, and you might as well just considered it the same as a vacuum leak. Same size hose anyways. Smartest thing to do for the MAF cars (mwahahah SD ???!) would be do it BEFORE the MAF.

It really isn't that much air, as said, but it'll be enough I believe. 5 more inches of hose to just prevent any possible problems.