Adjustable Vacuum Advance

[mikajo39]

SO I finally got my timing issues resolved but now I am curious as to how much Vac Adv my vac can should pull at part throttle
i have initial set to 15 deg and its all out at 32 deg at 3200rpm. under part throttle with Vacuum advance hooked up I pull up too 52 deg advance is this too much? I have an adjustable vacuum can I can put on and tune to my hearts content what should I shoot for in vacuum advance for maximum performance. I have a 350 SBC bored over .30 (355) Lt4 hotcam ported vortecs and 9.5:1 compression ratio

[mclairmo]

Vacuum advance has no affect on "performance" it is merely a fuel economy tuning tool. You first set initial and then dial in your mechanical advance for best power and performance. MSD website has good info and great products for limiting mechancial advance and recurving distributor. Once that is all optimized, you can then add vacuum advance until engine pings and then reduce to last setting with no pinging. I plugged the vacuum advance on my Camaro and ran it for many years like that and now have it back on. No affect on power, just helps part throttle fuel economy and helps engine to run cooler. This is all assuming you have it hooked to ported vacuum so it only advances at part throttle. If hooked to full manifold vacuum then you will have some advance form the vacuum advance at idle. Which vacuum source to use has been hotly debated on the internet and can take up many many pages of posts. I use ported vacuum like the oem but others swear by full manifold vacuum.

[mikajo39]

I used full manifold vacuum when I was having timing problems just to get the timing advanced enough to let it run. Any Idea how much fuel economy i'd loose if i got rid of the vacuum advance? not that it really matters, she's a gas hog how she is.

[mclairmo]

You can lose a lot of fuel economy with no vacuum advance but it really depends on your engine combo. However, if the engine won't run at all without the vacuum advance hooked up to full manifold vacuum sounds like you have bigger problems. Maybe your initial timing is set too low, or your mechanical advance mechanism is froze up, which happens sometimes. Or maybe your balancer has slipped and you cannot get an accurate reading? I would confirm that your balancer is accurate at TDC and then use a timing tape to dial in your timing with no vacuum advance hooked up. Then worry about the vacuum advance after you got it running smooth.

[Jim_PA]

I think with performance engines, the recommendation is around 10 degrees of vacuum advance.

Did you try running it at 36 total? That's kind of a sweet spot for sbc's with some work done.

[mclairmo]

Originally Posted by Jim_PA

I think with performance engines, the recommendation is around 10 degrees of vacuum advance.

Did you try running it at 36 total? That's kind of a sweet spot for sbc's with some work done.

Do you mean 36 degrees total, including just initial and mechanical or is that also including the vacuum advance? Sounds like just initial and mechanical as adding vacuum usually brings the advance much higher for part throttle operation.

[JimKshortstep4x4]

Total of 36 is initial and mechanical advance. Vacuum advance is added to that. You engine might do ok on 52 degrees. I shoot for the 10 degrees of vacuum advance as Jim mentioned. Sometimes I end up with 12 but that seems to work also.

Jim

PS: I run my vehicles with manifold vacuum to the vacuum advance.

[mikajo39]

Originally Posted by mclairmo

You can lose a lot of fuel economy with no vacuum advance but it really depends on your engine combo. However, if the engine won't run at all without the vacuum advance hooked up to full manifold vacuum sounds like you have bigger problems. Maybe your initial timing is set too low, or your mechanical advance mechanism is froze up, which happens sometimes. Or maybe your balancer has slipped and you cannot get an accurate reading? I would confirm that your balancer is accurate at TDC and then use a timing tape to dial in your timing with no vacuum advance hooked up. Then worry about the vacuum advance after you got it running smooth.

I solved all my timing issues last week right now I have it set to 15 deg initial 34 deg all out at around 3200 rpm I'll have to set the adjustable one for around 10-12 deg then

[turp mcspray]

As far as ported or full vacuum...try em BOTH. Then use whichever one your engine likes the best.

[mikajo39]

Originally Posted by turp mcspray

As far as ported or full vacuum...try em BOTH. Then use whichever one your engine likes the best.

Yeah, after I resolved my timining I tried to decide which one to use so I tried full manifold but it advanced my timing to much at idle and made it kinda funky to tune, so I went over to ported and now it runs very nice