170 turbo draw threw build. Timing

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170 turbo draw threw build. Timing

Post #1 by shawnkat » Mon Jan 09, 2017 9:45 am

Hey . I was wondering if anyone has done a draw threw turbo on a 170. It's all stock. I have it running just wondered about the timing . What should it be set at and should the vaccum advance be hooked up to carb as well. Thanks for any input. :beer:

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Re: 170 turbo draw threw build. Timing

Post #2 by Derangedfords90 » Sat Jan 28, 2017 3:10 am

Ok. I'm going to be petty, but it's driving me nuts. Draw "through" is how it's spelled. I honestly want to tell you so searching for it won't be so taxing on the Google machine. Sorry. Now I'll give you some productive information.

First, to answer your questions, I would say that you have to give some more information; like what is your boost pressure goal, what type of fuel are you planning on running, is the distributor stock, and are you calculating your fuel mixture by a Wideband? Reading plugs? Without this information I can really only give you rules of thumb that I have found over researching with the Google machine.

Vacuum advance: I say keep it. It's good for mileage and power before boost. I'm still working stuff out with mine and mine is blow through, which is a more forgiving setup for the vacuum advance. Why? Because I can run Advance with no boost and it will be completely gone when I get positive pressure, regardless of where I place it (ported vacuum or manifold). What I found though was that the advance was too much at idle when I had the vacuum advance hooked up to the manifold vacuum. So my solution was hooking the vacuum advance to the ported vacuum on the carb, allowing the idle to have no vacuum advance and unboosted cruise have full advance.

Now, the reason my Advance was so much at idle with the vacuum advance was my base timing. My base timing is 24* advanced (currently) but the mechanical advance has been eliminated from my distributor. How did I get 24* Advance as my timing? I took a "rule of thumb" and applied it to my application. First, I determined how much Advance I could possiblyou get out of the distributor. I have a 1980 fairmont with duraspark II so I had about 22 degrees of vacuum advance and 26 degrees of mechanical advance. Base timing stock for my car is 10 degrees advanced. So max RPM advance I should get (with WOT) is 36 degrees, stock. So I then took the boost rule of thumb of 2* retard (some say 1.5*, but better safe then sorry) for every psi of boost from the factory maximum advance. I then locked the distributor (by removing the weights and welding the two shafts together) and set the timing to 24* advance (for 6psi). I started at 20* to be safe but it was obvious that that was way too retarded because my egts were crazy. So 22* of advance with 24* of base equals 46* Advance timing with vacuum advance. I had issues idling with that much, hence why I moved my vacuum to ported vacuum.

Sorry for the rambling, but long story short, figure out how you can use the vacuum advance without it advancing under boost and set your total timing (whether you keep your mechanical advance or not) to 2* retarded from stock per max psi.

Hope this helped. Happy boosting.


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