How much vacuum advance are you guys putting in a 392? I read all the posts regarding initial and mechanical advance, but not finding answers on vacuum advance. I’m running a pretty much stock 392 with hot heads intake and a edelbrock 750 at the moment. Pertronix flame thrower II ignition. 8* initial and 21* mechanical The vac can gives me another 16* I was thinking of going with an adjustable vacuum advance to dial back my amount of timing at idle and low throttle input. Fuel/air mixture looked good, but after I hooked up the vacuum advance the plugs looked a little light. I went up one jet size and seems to run better. Everything I can find regarding hemi timing does not address vacuum advance or guys don’t seem to use it. Mostly I think because I’m finding a lot of drag race forum discussions. For a street car what do you guys suggest?
What is your compression ratio? The hemi's have a tall piston with higher compression that allows more timing advance but if you have a 9 -1 engine, I would pull it back to low 30's and work forward. The compression ration and cam timing are the unknown's here to me.
Don’t know for sure as I bought the engine as is. I was told it’s very close to stock with a “mild cam” I’m at 29 all in, but with the vac advance it’s up to 45-46.
Didn't Big Daddy say he ran his Hemi with 40+ advance??! (But then he DID have a 98% nitro 'load' in it!!) What the heck do I know...I like SBF's!! 6sally6
Slayer is correct. The 35 deg max number comes from the 'old-days' with folks like Garlits. When he moved to the 426 he started with the same parameters but soon found that the 426 needed a bunch more, well into the 40+ range.
Is this with vacuum advance? I keep finding info but it’s all drag race stuff and they obviously weren’t using vacuum down the strip.
...any combination....unless you want to gamble. The early racers tried more advance but often cracked cylinder walls due to the high cyl pressure. Was this because they had big overbores and thin walls? Maybe. I do recall stories that simply indicated that they were not making any more power above 'x' degrees so why not save the block. Todays builders/operators can run as much advance as they/you like, especially if you are applying some electronics to monitor the works. If you are building a DD then use what you find makes the power you want at the safety margin you need. Search for the results of the last couple EMC and see what they used.
The vacuum advance is in addition to the total and can be over 40 degrees. It is only advanced that much under a low or no load condition. If you aren't getting spark knock at tip in (when you lightly advance the throttle under load) or at part throttle cruise, you should be okay. I had to put an adjustable vacuum advance on my old truck, 350 Chevy, because I was getting a little spark knock on light acceleration. The way you can tell it's vacuum advanced related is that the spark knock goes away if you open the throttle more (drop the vacuum).
Measured vacuum on manifold at 13. Set base at 8 and limited my centrifugal at 21. Hooked distributor to manifold vacuum port and measured it added an extra 16*. Left the silver springs and watched all in at 3500 rpm, that was 45* at idle with no load and light throttle so it would keep the vacuum up.
Measured vacuum on manifold at 13 I don’t hear ping at idle and took it out for a drive. No pinging under load either. On my other car I put an adjustable vac advance because it started to ping at part throttle. I was able to take some vacuum advance out and it runs great. That is a sbc though, first ever experience with a hemi....
Excessive vacuum advance will rattle or “ping” at steady cruise on flat ground as the load levels off, if it is advanced too far, and immediately fade away on acceleration. It pretty much occurs under the exact opposite conditions as the mechanical or distributor advance. A little light rattle now and then under part throttle acceleration is acceptable.
The word here is CAN give another 16* for 37 which will not be the norm as you are driving unless your really light on the throttle. If your going to restrict it 32 total is right. If you can live with 5* initial you have it. I’ve never had a true hemi but had cylinder head chambers pretty close and they all loved 37*.
I have a 354 and set timing with a vacuum gauge. In fact that's how I time all my engines. Plug the vacuum advance tube and hook a vacuum gauge to the intake. Then adjust the distributor and idle screws until you achieve max vacuum at desired idle speed. Back distributor off a degree and it is perfect for your combo.
Installed an adjustable vacuum advance. I also added a limiter. I’ll fire it up tomorrow. Since my centrifugal is 20*, maybe I’ll shoot for 8* initial and 7* vacuum advance.
Ran it at 8* initial 20* centrifugal and set vacuum at 10* Runs great at high speed and wot, idles like crap and is super rough until I get it up to around 1500 rpm. Wants to stumble and die right off the line and surges during low rpm cruising. I’m thinking in a little rich now since I didn’t change anything in the carb and took timing out.
I would set it up first with the vacuum advance disconnected and plugged. Test drive it. When the distributor ignition advance is satisfactory, then re-connect the vacuum advance. At this point only make adjustments to the vacuum advance canister, don’t mess with the distributor timing.
All disconnected and base tuned carb off vacuum. Idle smoothed out, vacuum steadied. Set initial at 9* and still 20* mechanical. Vacuum is at 10* right now and it’s drives pretty good, not pinging. Little stumble at low rpm off the line.
Pertronix flame thrower II It’s brand new with new coil. Crane adjustable vacuum advance for delco points style.
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