Taken from the Start-up and break-in procedures manual you get with the 290hp 350 GM Performance Parts crate engine. The question I have is about the statements in red. Since the engine does not come with the HEI distributor(to which recurving may apply), what in the engine is changed that it doesn't need vacuum advance? Just the cam? "4. Set initial spark timing at 10º before top dead center (BTDC) at 650 rpm with the vacuum advance line to the distributor disconnected and plugged. This setting will produce 32º of total advance at wide-open throttle (WOT). The HEI vacuum advance canister should remain disconnected. This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve. Rotate the distributor counterclockwiseto advance the timing. Rotate the distributor clockwise to retard the timing."
What they are saying your supposed to disconect the vacuum advance when setting the timing! Everytime your set the timing your supposed to disconect it! Nothing special, it still needs the vacuum advance just not when setting the timing!
"This engine is designed to operate using only the internal centrifugal advance to achieve the correct timing curve" Do NOT hook up the vacuum advance on the distributor, simply uses the centrifugal advance inside the distributor!
Some Big truck engines by Chevy didn't use vacuum advance. Maybe this engine application is for that? Or just the pollution police. No vac advance gives lower NOX. But no benefit to power or mileage.
As I recall, vacuum advance was developed to improve fuel economy in street driven vehicles. Many performance applications use centrifugal only. Vacuum advance is not a neccessity. I would have to believe Chevrolet engineers know what works best in the engines they develop and market. I would further think, though the OP doesn't mention it, that a particular distributor model or part number would be recommended for use in this engine. Ray
Um...guys, they aren't saying drive the car with the vacuum disconnected, they are saying SET THE TIMING with the vacuum advance disconnected. The first two answers were correct. Move along...
I can't conclude on this particular motor... However, I can tell you that GM doesn't want you to use vacuum advance on the ZZ383. I was shocked when I read this in the manual and called GMP. They confirmed. Apparently, they don't recommend vacuum advance on any of the new fast burn heads.
Vaccuum advance was a way to get the engines to burn cleaner under partial load. Nothing more. It is considered a performance engine and therefor you don't need vacuum advance. Hook it up and you may have detonation under part throttle conditions with the newer head design. Keep in mind it is a part throttle crutch only for low compression engines. With it hooked up you may get as much as 40+ degrees of total advance at cruising RPM's, not good for a higher compression engine. I run my drag cars with the advance locked out, so it gets 36 degrees all the time. I just use a start retard on the ignition box. Don't hook it up with a higher compression engine, 32 to 35 degrees total is all you want or need. Scot
Like as has been said you don't need a vacuum advance on this particular engine. The engineers know what they are talking about, and I believe the english is pretty clear.
Yes, I get that I don't need to run the advance, my question is what in the internal engine mechanicals they "designed" to run without it. Only the cam controls timing (internally).
The actual answer is "Nothing"...........just about any auto engine, naturally aspirated, with a carburetor, can be run with or without vacuum ignition advance. Virtually all will have mechanical (centrifugal) ignition advance, as some means of starting the fuel burn earlier is required as rpm increases. How well it runs, how fuel efficient it is may vary with specifics of the timing curve, but the vacuum portion just isn't required. The cam timing is a whole seperate issue. Used to be that was fixed because of the direct relationship of the crank and cam via timing gears. So, the "grind" usually ended up being a compromise to maximize performance in a specific rpm range and for either torque or horsepower, depending on the application of the engine. Now, on many OT modern engines, cam timing is variable too through the cam drive mechanism and torque, in particular, can be had throughout the rpm range. Ray
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