DMclean

So i have my nikki turbo running but i'm wondering where the best place is to reference my vac/boost line for my blow off valve on the stock 12a mani.

thanks

LizardFC

iTrader: (1)

Did you remove the spacer with the vacuum ports beneath the carb?

DMclean

no its still there. im using it right now but i think it maybe causing it to release early. possibly too small of hole for the "signal" and really close to the throttle plates.

was thinking of splicing the brake booster vacuum line. besides that i'll have to drill and tap into the mani.

Jeff20B

iTrader: (13)

Reference from the carb hat. Right? I'm still learnin'.

LizardFC

iTrader: (1)

You want your BOV to reference after the carb and throttle body. It's the difference in pressure between the manifold and the hat/piping that opens the BOV. When you let off the throttle, the throttle plates close and the manifold pressure drops as the remaining air enters the engine. The space above the carb - the hat, charge pipe, intercooler, etc. remains under pressure as the the turbo is still pushing air into the system. With the throttle plates closed, there is nowhere for the air to go. So a pressure wave would surge back towards the impeller blades of the turbo if the BOV didn't open and release it.

I don't have an extra spacer laying around to study so I don't remember exactly where each port sources from, but I do know the second one from the left (standing on the driver's side) is ported vacuum taken at the throttle plates. So that wouldn't be ideal. I think at least a couple ports lead down into the manifold. I don't see a problem with tapping into the brake booster line. It looks like a direct port to the manifold. Did you remove the ACV?

Keep us posted on the project, I'm doing a nearly identical one myself so any info would be extremely helpful

Jeff20B

iTrader: (13)

Oh, ok. I guess if you had a 79 manifold you could swap out the PCV valve (or as divindriver calls it, the alttitude compensator) and swap in either a block off bolt for the early rear plate heater fitting (Racing Beat sells these) or get a brass one from an aftermarket electrical gauge hardware pack that already has the 1/8" NPT threaded hole, and add a nipple fitting in order to feed a balanced vacuum/pressure signal to the BOV and a boost gauge.

What I mean by "balanced" is both rotors on the primary runners have a small hole connecting inside the altitude compensator hole. Since your manifold has channels connecting primary to secondary runners, they too will share a vacuum signal, so really it's all four runners.

However if you're using the crappy 81-85 Nikki manifold, you'll need to cut a channel in the rear runners and come up with your own way of grabbing the vacuum signal from both primary runners. It's easy to drill and tap 1/8" NPT in aluminum so there's an option.

I don't think I'd bother with the carb spacer. It can't provide a clean signal.

LizardFC

iTrader: (1)

+1 on not using the spacer, really... I'm going to eliminate mine so I can seal up all the holes. I would just drill a new hole and tap in a nipple. That's probably what I'll end up doing since I've closed up everything but the brake booster port in my manifold. I've got a nice open area above all the runners that I can tap into and draw a reference from.

I snooped in on your build thread and saw that you were contemplating using a 12AT manifold with a custom adapter, did you decide to go that route?

Jeff20B

iTrader: (13)

Was that question directed at me or the thread starter?

DMclean

That was for me Jeff,

I did use the 12at mani and made a 3/8" adapter. it worked very well except that manifold is way taller than the nikki mani so the carb hat would stick and inch or two out of the hood. which is a shame because it had a port/nipple on the manifold that is for vac/boost to go to there boost retarding dizzy.

LizardFC

iTrader: (1)

That was for him, sorry for the confusion, Jeff.

I didn't know the 12at dizzy had a boost retard... I guess that's why those things are sought after. I wonder if there's any way to modify the US spec dizzy to retard under boost?

I guess your options are to use a different manifold or modify the hood, which is kind of extreme just for a manifold.

DMclean

yes you can make a boost retard US dizzy. its just sourcing the boost/vac canisters. the best way to source would be if you could get a part number from the manufacturers. volvo, toyota, buick all had factory turbo cars in the 80's that had dizzys that would boost retard.

LizardFC

iTrader: (1)

Awesome, definitely something I'm going to look into.

DMclean

i did look for some canisters but they are hard to find part numbers so i went with locking it. i didnt need driveability anyway.

Jeff20B

iTrader: (13)

One alternative method is to limit the travel of the advance mechanism. Just weld a small piece of steel in the slot a few mm long. Do some calculating to figure out how much to limit travel. You only need to do one side as I understand it. What you want is 10 degrees at the main pulley which is half the stock 20 degrees.

It's better for driveability than fully locking the dizzy because at low RPM/off boost it acts like a stock dizzy. Then get the revs up there and make boost, it will only advance to 10 degrees. Pretty good idea.

LizardFC

iTrader: (1)

Thanks Jeff. The springs in my dizzy are so worn that right now it advances about 28 degrees at 4000 rpm, and it seems somewhat inconsistent. I might try fooling with it while I've still got the car running N/A.

Here's a question though... the vacuum advance only operates at low RPM, at which point mechanical takes over. So if you lock the mechanical advance, can the vacuum advance still be used to help drivability at low RPM? And it only operates under vacuum so I don't see it being a threat to the motor.