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Hi. I'd like to know how much vacuum I should expect out of my engine. I have a '71 Cutlass, 350, one of those 73-78 motors not the original 71. I had it rebuilt and 7a heads with work done to them installed, all work done by reputable engine builder, the guy had his own sweet looking 442 in the shop. I have a lunati voodoo cam, the one described as best all around daily driver with power in the lower end. Rebuilt Rochester (again reputable shop), edelbrock low rise intake. I like this setup and I'm sticking with it.
If i have around 8.5-1 compression (and that's probably generous right?) what vacuum reading should I be seeing? I have 15 on my vacuum gauge at manifold vacuum with the car warmed up and idling. Should I be expecting more or am I chasing something this rather low compression engine cannot give? The car has a smooth idle and reaches fifteen on the gauge only when the timing is set to 22, rpm around 900. I cannot get the vacuum above that by adding timing or adjusting air fuel mixtures on the carb. 15 seems to be the limit but every video I watch they have around twenty but those are much higher compression engines I'm sure, real muscle cars.
If you knew which of those 73-78 motors you had, you could quickly find the vacuum specifications displayed in the appropriate year Oldsmobile CSM. Kenneth has already provided you w/ some numbers. Demonstrated below are the Oldsmobile design engineers specifications for the 1971 Oldsmobile 350 CID engine. Note there exist two OEM distributor models (1112079/12085) employed for the 1971 350 CID A-Body engine (both use the same vacuum model (1973407). Note also both vacuum advance begin (start) @ 13° and achieve between 16" to 17.5" Hg. The delta resides in the mechanical advance @ distributor RPM. The same vacuum is achieved but at a higher mechanical advance (i.e. 16° - 18° @ 2050 RPM (1112079) & 12° - 14° @ 2000 RPM (1112085).
Hi. I'd like to know how much vacuum I should expect out of my engine. I have a '71 Cutlass, 350, one of those 73-78 motors not the original 71. I had it rebuilt and 7a heads with work done to them installed, all work done by reputable engine builder, the guy had his own sweet looking 442 in the shop. I have a lunati voodoo cam, the one described as best all around daily driver with power in the lower end. Rebuilt Rochester (again reputable shop), edelbrock low rise intake. I like this setup and I'm sticking with it.
If i have around 8.5-1 compression (and that's probably generous right?) what vacuum reading should I be seeing? I have 15 on my vacuum gauge at manifold vacuum with the car warmed up and idling. Should I be expecting more or am I chasing something this rather low compression engine cannot give? The car has a smooth idle and reaches fifteen on the gauge only when the timing is set to 22, rpm around 900. I cannot get the vacuum above that by adding timing or adjusting air fuel mixtures on the carb. 15 seems to be the limit but every video I watch they have around twenty but those are much higher compression engines I'm sure, real muscle cars.
Which VooDoo cam is it?
And ask your “engine builder” what he degreed the cam to. I’ll bet he says “straight up”. That will have an effect on it as well.
Which VooDoo cam is it?
And ask your “engine builder” what he degreed the cam to. I’ll bet he says “straight up”. That will have an effect on it as well.
So I guess that's my answer, 15 is all I'm going to get. Thanks.
How much do you want? "All I'm going to get" makes it sound like you're being deprived of something. 15 is perfectly fine. Vacuum level is affected by lots of things. If the engine is running ok, then drive the car and stop trying to fix what ain't broken!
Years ago I was playing withe the HEI distributor advance curve and found that increasing the initial timing gave higher vacuum, smoother idle, and the engine ran better overall. I started with the factory settings of 10º initial, 24º mechanical (Moroso HEI advance curve kit) and ended up with 18º initial and 18º mechanical (which happened to be the original HEI weights and center bar). Also, connecting the vacuum advance to manifold vacuum helped give more idle vacuum and smooth out the idle quality.
If I disconnect the vacuum advance and set the initial to 10º, the engine had a bit of a lumpy idle and the vacuum is a bouncy 15-ish, but change the settings to the above mentioned and the idle sounds smooth as stock and the vacuum increases to a steady 16".
