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the inline regulator doesn't affect boost spikes. in fact, it is simply used to stop pressure from rising on the other side of it. take a stock ignition diaphram for instance. inital timing is set to 16 dbtdc. when boost comes on, the vacuum hose from the throttle to the diaphram allows the pressure to carry through to the diaphram; which mechanically retards timing. stock max retard is set to about 10-11 dbtdc at max boost. basically what happens is that as boost rising, timing is retarded. when boost reaches about 3-5 psi, timing is retarded at its max due to a mechanical restriction. 10-11 dbtdc is too retarded for only 5 psi of boost; thus the stock setup feels sluggish at this point. once boost climbs past 5 psi or so, the timing remains at its max retard. therefore, as boost rises, the car starts to feel more and more powerful until it reaches max boost (since you are essentially advancing the timing as boost rises beyond 5 psi). hopefully, at your max boost setting, you don't hit predet before max boost is achieved. this can happen at different times on different engines. one engine may be fine at 18 psi of boost and total retard of 11 dbtdc. another engine may be more knock prone and need more retard, or 10 dbtdc to run 18 psi of boost. ambient weather conditions will also affect this. the goal is to advance your timing until hitting predet and then slightly retarding it. running on this 'timing horizon' will give you the most performance (timing wise, fueling is another issue).
so, now that the stock set up is taken care of, move on to the modified... when i put in the msd btm, i defeated the stock diaphram for timing retard; now it is only allowed to provide timing advance under vacuum. through much trial and error, i then decided to duplicate the stock timing curve with the btm. the way the btm is set up, for each lb of boost, it retards the timing a certain amount; linear in nature. this amount per lb can be adjusted by a potentiometer in the cabin. if i were to just hook up the btm and try to duplicate the stock curve, i couldn't. the reason is that the stock curve stopped around 10 dbtdc at 3-5 psi. i could set the btm to be at 10 dbtdc at 5 psi, but as boost rose beyond 5 psi, the timing would continue to retard. thus, in a stock set up 12 psi may still be at 10 btdc, but with the btm, i would be closer to 0 dbtdc; too retarded.
enter the pressure regulator. the pressure regulator is one i chose which is used in pnuematic controls in marine applications, engine controls, etc. it works the same as a regulating valve does in any other fluid system. the valve can be preset to a certain pressure, the input pressure to the valve can be any pressure (within the rating of the valve), but the output (discharge side) will never rise beyond the preset pressure. i chose this type of valve because it allows me to maintain a completely closed system, instead of bleeding off excess air. (note: others have used the dawes device and similar bleed off valves with success, my choice is just personal opinion). so, this valve is place in the vaccum line between the throttle body and the btm. to duplicate the sotck set up, i set the valve to 5 psi and set the btm to retard to 10 dbtdc. then as boost rises, timing will retard to 10 dbtdc at 5 psi and boost will rise beyond with no change in timing. now comes the fun part...
if we know that an engine can hold 18 psi of boost at 10 dbtdc with no predet, we know max retard needs to be at 10 at 18psi. however, it may also need to be at 10 for 17 psi, and 16, etc. therefore, it becomes a tuning fest to try and find the 'sweet spot' to set the regulating valve at. the higher the pressure setting on the reg valve, the more power you will gain from the portion of the timing curve under the setting. but, you have to make sure you aren't too advanced in the early part of the curve so as to cause the apc to cut back boost at 10 psi or so. right now my valve is set to 14.5 psi and max boost is 18-20 psi. max retard at that point is 11-11.5 dbtdc. note: prior to adding the fmic, i had to keep timing to 9.5-10dbtdc to keep from hitting predet. even then, if i had numerous high boost runs in a row, i would still knock. also, the difference between a max retard of 9.5 dbtdc and 11-11.5 dbtdc is great. the difference in max power is completely noticable. again, the timing goal is to minimize retard for more power.
my problem at max boost and high rpm is leaning of fuel, i am working on that issue. if i had proper fuel delivery at max boost, i would probably be able to advance the timing even more. this is also a point at which you can use water injection. by using water to cool intake charge, you can advance the timing more without hitting predet. thus, more power.
sorry for the length. this is something i have learned over the past year of frustration on my own car and timing issues. i still haven't got it tweaked perfectly, but once you get it close, ambient weather conditions start having a greater impact on the ability to reach and maintain boost. hope this helps. good luck.
posted by 4.60.1...
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