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Old 05-05-2008, 12:03 PM
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vacuum vs mechanical advance on street SB chevy

OK guys, here's my situation and questions. It's bit long winded, so bear with me.

I have an 86 Monte Carlo SS with the 305, .030 over and mild cam. It's been parked for almost 10 years, but I'm getting it running again to become my daily driver. 12 years ago I put Holley projection on it ran OK but never good. It always ran rich, I could never could get it tuned right. Thatís one of the several reasons the car got parked.

Now I'm getting it running again and my goal is to get the TBI correctly tuned. The pro-jection fuel map is not tunable, and the ECU only relies on the TPS and water temp to adjust fuel delivery. After visiting several forums I decided to get a mega-squirt controller and give it a try with the pro-jection (I'm an electronics guy so I don't fear "black boxes"). But, I'm trying to get the engine to run as well as possible before I make the mega-squirt swap and here lies my question.

The engine stumbles and backfires through the throttle body off idle. Once I get to 2000 RPM or so the engine will start to pull well. I first thought it was to lean, but remembered that was never the case when driving it 10 years ago. So I invested in an air/fuel ratio monitor and tuned the idle much leaner (using the TPS adjustment as suggested in one of the forums). It was very rich (the exhaust actually burned your eyes) at the setting Holley recommended (which I already knew from experience). This removed most of the stumble off idle, but not all.

Then I got to thinking about timing, maybe the dist wasnít advancing properly. I watched the timing (with a timing light) at idle and the advance seemed sluggish when slowly revving the engine, however this is also the time the engine stumbles. Once you get past the stumble, the advance does work. I put a Mallory Unilite dist (mechanical advance only, I didnít know any better) on the car when I put the pro-jection on it 12 years ago. I never really thought about the mechanical only advance of this dist until I started tuning the engine this time around. I read a lot lately about the ills of mechanical only advance (no vacuum advance) on street engines. I have the initial timing set at 8 - 10 degrees (as if the car still had vacuum advance and the vacum line was removed). The Mallory dist gives 24 degrees of mechanical advance at 3200 RPM.

Finally my questions:
Do you think I don't have enough initial timing? Should I dump the mechanical advance dist and get a vacuum advance unit? Re-curve the mech advance? I read where the full advance should be in at the engine's cruising RPM, which for this car is 2200 - 2500 RPM. This car will never see the track, it's a street car only.

Thanks for reading this long post and giving your thoughts

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Old 05-05-2008, 02:06 PM
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Ultimately, I think you want a Vacuum Advance distributor. But, in the mean time, I think I'd try pushing the mechanical advance towad12-14 degrees initial. Just for grins.

Pat
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Old 05-05-2008, 02:29 PM
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I would get megasquirt II with ignition timing control. Mechanical advance and vacuum advance is old tech.

Image how cool it would be to enter the amount of timing you want depending on engine manifold pressure, rpm, and throttle position.

12 to 14 at idle and 38 total would be nice. then another 15 from the vacuum advance. so during a light load and over 3000 rpms the timing should be over 50 degrees.
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Old 05-05-2008, 02:59 PM
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I agree about the megasquirt II and ingition control. But, I'm starting small here since the biggest problem with my pro-jection has always been the fuel map. When I get that resolved and working well, I may go the MS II route and try ignition control.

"Image how cool it would be to enter the amount of timing you want depending on engine manifold pressure, rpm, and throttle position."

I agree completely...
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Old 05-05-2008, 04:26 PM
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I would install a HEI with vac and mechanical advance. With the car at idle, vac disconnected and plugged, set the timing at 12 degrees btdc, then connect the vac advance to a full time manifold vacuum source, recheck the timing and it should be around 24 degrees at idle which is about right.
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Old 05-05-2008, 04:36 PM
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Quote:
Originally Posted by 389TriPower
OK guys, here's my situation and questions. It's bit long winded, so bear with me.

I have an 86 Monte Carlo SS with the 305, .030 over and mild cam. It's been parked for almost 10 years, but I'm getting it running again to become my daily driver. 12 years ago I put Holley projection on it ran OK but never good. It always ran rich, I could never could get it tuned right. That?s one of the several reasons the car got parked.

Now I'm getting it running again and my goal is to get the TBI correctly tuned. The pro-jection fuel map is not tunable, and the ECU only relies on the TPS and water temp to adjust fuel delivery. After visiting several forums I decided to get a mega-squirt controller and give it a try with the pro-jection (I'm an electronics guy so I don't fear "black boxes"). But, I'm trying to get the engine to run as well as possible before I make the mega-squirt swap and here lies my question.

The engine stumbles and backfires through the throttle body off idle. Once I get to 2000 RPM or so the engine will start to pull well. I first thought it was to lean, but remembered that was never the case when driving it 10 years ago. So I invested in an air/fuel ratio monitor and tuned the idle much leaner (using the TPS adjustment as suggested in one of the forums). It was very rich (the exhaust actually burned your eyes) at the setting Holley recommended (which I already knew from experience). This removed most of the stumble off idle, but not all.

Then I got to thinking about timing, maybe the dist wasn?t advancing properly. I watched the timing (with a timing light) at idle and the advance seemed sluggish when slowly revving the engine, however this is also the time the engine stumbles. Once you get past the stumble, the advance does work. I put a Mallory Unilite dist (mechanical advance only, I didn?t know any better) on the car when I put the pro-jection on it 12 years ago. I never really thought about the mechanical only advance of this dist until I started tuning the engine this time around. I read a lot lately about the ills of mechanical only advance (no vacuum advance) on street engines. I have the initial timing set at 8 - 10 degrees (as if the car still had vacuum advance and the vacum line was removed). The Mallory dist gives 24 degrees of mechanical advance at 3200 RPM.

Finally my questions:
Do you think I don't have enough initial timing? Should I dump the mechanical advance dist and get a vacuum advance unit? Re-curve the mech advance? I read where the full advance should be in at the engine's cruising RPM, which for this car is 2200 - 2500 RPM. This car will never see the track, it's a street car only.

Thanks for reading this long post and giving your thoughts
While Projection isn't as sophisticated as OEM TBI, I'd think that with choice of small block and big block maps added to a +- 30% pot adjustment on idle, +- 50% on main and WOT; plus pressure adjustment between 12 and 18 PSI, that this should dial in pretty close. And you can add a closed loop feature to get toward stoichiometric.

It, however, doesn't like much cam as evidenced by Holley only saying it's good to 275 horses which is more a function of sensitivity than the ability of a pair of 80 pound injectors to deliver sufficient fuel. If the cam timing has crept up around210-220 degrees measured from .050 and the LSA is getting under 112 degrees, you're getting off the edge of what the Holley computer will recognize.

Vacuum advance would help, maybe even with the apparent lean stumble off idle. The mechanical you have in there now occurs too far up the power rev band to be of any help at idle. Typical vacuum advance is putting in a lot of idle advance and is removing it as manifold vacuum drops. A hotter than factory cam would need an adjustable vacuum advance module to allow compensation for lower idle vacuums with longer winded cams.

I'd change the distributor before doing anything more with the injection, it may just tune correctly once the advance is operating like it needs.

Bogie
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Old 05-05-2008, 10:28 PM
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Thanks for the info. I may go get a vacumm advance dist before I do much more to the engine. I think I made a mistake when I bought that dist years ago, but again, I didn't know any better.

Thanks,
TriPower
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