Hi all. I'm sure this subject has been covered in the past but please humour me as I'm relatively new at posting here. I'm just coming to the end of a top end upgrade on the Goodwrench/Targetmaster 350 in my '29 Tudor. The short block is still as it left the factory with its cast crank, rods and woeful pistons but to make the most of this I've upgraded the entire top end including a cam swap. Gone is the mild aftermarket flat tappet cam that the engine had when I bought it and it's ben replaced with a Howards custom grind hydraulic roller. I've also binned the horrendous iron heads that had 76cc chambers in favour of a pair of Trickflow DHC 175's which have a CNC 60cc chamber. Together with the .0015" 1094 shim gasket and the .035" piston to deck height, this gives me around 9.75:1 CR. To top this off I have a Edelbrock TR1Y tunnel ram with two 450 Quick Fuel Slayer VS carbs and ignition is handled by a Summit ready to run distributor and a Blaster 2 coil. My question is about ignition advance. I've not driven the car yet after the upgrades but hope to in the next week or so. At the moment I'm using ported vacuum advance when running up the engine but I'm wondering if I should lock out the distributor or not. What are the benefits and would this help my combination or should I leave it as it is? This will predominantly be a street car with maybe the occasional blast up the strip. Thanks for any advice offered and apologies for the somewhat protracted post. Cheers n beers.
Unless drag Only , 4,000 rpm plus stall /converter . Do not run Locked out , You can!! . You will need to start Like a mag if locked. More likely over 25 degs when Hot, so No kick back , slow cranking. Separate starter button versus key ignition On. Hold starter button for 3 to 4 seconds then flip ignition power On. I run Separate starter button on all set ups , Most of my combos initial timing is anywhere from 20-30deg before TDC. Just off the top of my head I would Run 18 to 24 deg 's of initial timing BTDC & total 36 to 42,,, before Vac avance . Test & Experiment required, Idle RPM 800-1,000
If it's running okay why change it? As I take it you're thinking of locking out the vacuum advance only, correct? I don't think that's going to create a starting problem like suggested above, that would be more like locking out the mechanical advance and trying to start it with 30 plus degrees of timing. As I understand it vacuum advance is all about advancing ignition timing when the engine is cruising under a light load which improves fuel mileage. Lynn
I'm assuming from your one reply that you are only referring to vacuum advance and that the mechanical advance is operational. The only advantage to vacuum advance is part throttle economy. It allows the engine to operate with a lot of advance at light load which equates to fuel economy. However, your build doesn't seem to be built for fuel economy. Even with that I would still run it as it will help. If you want a smoother idle, you might try full manifold vacuum on the advance rather than ported. Just remember to set your timing with the vacuum advance disconnected.
I don't run the vacuum advance on any of my cars at the moment. They all run mechanical advance though.
So when your cruising it’s advanced the timing? Yes. But then you stomp on the gas pedal to pass someone the vacuum advance won’t retard the timing? Final answer?
Lets say , initial is 15 BTDC @ Idle Total is 38 deg's with 23 deg's of Advance Witch = 38 deg's Rpm's No Vacuum. With out Vacuum , "0" but with Vac & a additional avance of 15 deg's added , like cruising ,distributor scene / see's vacuum is Now 53 deg's of total . Go WOT , vacuum lost @ vacuum advance, Timing drops back to 38 deg's . Then when Vacuum restored or present again timing will be 53 total .
It advances timing under light load so if you are cruising and stomp on the throttle, you loose that advance. So, yes it also retards the timing when there is a load. It is continually adjusting the timing based on engine load, optimizing it.
When the vacuum increases, so does the amount of advance. It stands to reason then that as the vacuum decreases (like when the engine is under load), the spark will be retarded. If you are going to do any street driving, I would leave the vacuum advance in the distributor, if it is functioning properly. (If it's not, fix it.)
All these #s Can be adjusted Initial Total Advancement . Vac advancement With springs, weights , limiters Stops , ect.
Locked usually refers to full mechanical advance which is a race engine thing. I’m learning this stuff as well and I like having adjustability and economy so I run an adjustable vacuum can as well.
We are down to semantics! Its an ignition advance feature that increases advance on light loads and overrun. It does not retard ignition as such but at low vaccume it alows the distributor to return to its mechanical advance condition. To set timing for mechanical advance you temporarily disconnect the vacuume. Its function is to help fuel economy under normal running conditions. The only exception to this is on turbo engined cars where the vaccume advance can also actually retard the ignition overriding the mechanical advance on high boost conditions.
Ported vacuum advance was used during the Smog era. At part throttle the ported vacuum has a higher inHg than manifold vacuum. This helped ignite the lean/egr diluted mixture burn since it needs more time(advance) to properly combust. If it was on manifold vacuum the ignition would be retarded for the mixture and there would be a loss of power. For normal running/power, have your vacuum advance plugged into manifold vacuum.
I'm also in the manifold vac camp, especially given a bump in performance via more cam and a tunnel ram, not to mention more compression. So you're tooling along and get a case of the fuck-its and stomp on it. You want it to buck and bend a rod or have the timing adjust to the load you just asked for? Like I said, I'm in the manifold vac line. Load compensated timing. Simple really...
Manifold vacuum vs ported vacuum. They are identical at ALL throttle positions and engine loads EXCEPT at idle. There is no ported vacuum at idle. There IS manifold vacuum at idle. Manifold vacuum at idle can help the engine idle cooler (the added ignition advance results in a leaner idle mixture). Since cooling while at stop lights is a problem for many rodders, manifold vacuum can be part of the solution.
@Britdragster 3 posts since 2010? Wow. Please use the search. This thread is a microcosm of all the various posts about spark timing. You will find a LOT of discussion about this. Finally, you haven't even driven it yet. Try it as-is and see. Adjust and retry. I don't have experience in your road conditions, fuel and your exact car, so I won't suggest a starting point for it. So long as it isn't way off, it should be able to go around the block safely so you have a baseline.
I should point out that "Ported Vacuum" is a concept developed in the emissions era (post 1972) and therefore not traditional.
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