When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Today was timing day for me. I added in the #139 weights to my HEI. and a fatter center cam with the idea that I wanted to limit the total advance to a target of something like 34 degrees at about 3200 rpm. Based on what I've read that seems somewhere near a reasonable target among all the anecdotal settings everyone's reporting for Olds big blocks.
The big Aha! for me was the discovery & use of a dial back timing light. I'd never used one before. It's the key to the whole thing. Without it I had never measured any timing other than idle with/without vacuum. With the tool, I was able to measure idle advance, vacuum advance, and total advance to finally get a sense of what's really going on. Timing tape might have done it cheaper, but I'm glad to have a tool to see what's going on. It's like getting glasses when stuff was formerly fuzzy.
The big leap for me was the "dial back" process where you rev the motor up to 3000 or 3200 rpm, then increment the strobe to various advance numbers until it lines up with 0 degrees on the balancer. Did I use the timing light correctly?
I'm not sure I did it right though. First I set the initial (with no vacuum) to 14 degrees at 800 rpm. That left me with total advance around 3200 of about 36 degrees. That seemed a bit high so I backed it off until I had 34 degrees around 3200 RPM. That left the initial around 12 degrees. I'd like to go a bit conservative on timing since it's a stock street car, our gas is partly alcohol and I've historically had trouble with part throttle pinging.
I'll take another crack at it tomorrow. I think I'm close, but an leaky oil pressure switch is stopping me for now. Hopefully I can grab one tomorrow.
I noticed that the vacuum advance at 800 rpm added a _ton_ of advance, like 32 degrees. With vacuum off the initial was registering 12 BTDC, when I hooked it back up it was around 44 at 800 rpm. I found this surprising, but it's probably me just being late to the party. Any comments on that?
I'm also using an AFR gauge to monitor carb tuning. It's really helpful. I noticed that when I had the idle mix set at 14.1:1 with vacuum off, it was too lean for when I hooked vacuum back up. With vacuum on the distributor the AFR ratio was more like 15.1:1, so I richened up the mixture with the vacuum attached on the idea that that's how the car will be used on the street. Any comments on that?
Am I right in thinking set the total timing first, then adjust the idle, and finish with the vacuum advance? Or is there a better order for getting things just right?
For context, this car is a ‘66 Starfire with a basically stock ‘70 455. I had the rebuild done and it’s solid. It has factory E heads, edelbrock performer intake, factory ‘66 duals, an M4MC 800 cfm qjet, and HEI. The drivetrain is a switch pitch TH400 and 3.23 open rear. Wheels are 15x7 and it has front discs from ‘70.
I think of it as a street car, not a race car, but I’d like it to scratch the tires here & there plus get decent mileage. Decent to me is on the high side of 10 mpg the way I drive it in a mix of freeway and around town driving. It got better mileage with the previous 3.08 rear, but 3.23’s are more fun and I’m seeing it as a challenge to see if I can tune mileage back into it. That may be a vain hope...
So that’s where my interest in OAI and more correct timing are coming from. The qjet is very close to dialed in, maybe a bit on the rich side with 74/44 primaries and CV rods. The qjet is a quality rebuild from SMI in SoCal. He does good work.
I’m solidly in can-it-be-better territory, not rescue territory, but suggestions are most welcome since I’m pretty much self taught.
Where did your timing come in prior to the change in weights? You need to limit the vacuum advance to 10/12* with a stop, to get your part throttle pinging under control. What brand HEI are you using?
I have never heard of a HEI vacuum advance canister that adds more than about 20 degrees! I suppose anything is possible. Crane use to sell a curve kit that included an adjustable vacuum advance, and a cam to limit the advance allowed. It works well. A LITTLE pinging at part throttle is ok, as you add throttle and engine vacuum drops, the pinging should stop. Keep in mind, by turning the cam to limit advance, you also increase the amount of vacuum needed to start the vacuum advance. This is because limiting the plunger travel adds spring tension. It’s lots of trail and error to get it right.
From what I have seen, some of the factory HEI vacuum advance add over 30 degrees. The Accel or Crane replacements vacuum advance add 20 degrees and are adjustable. What are your engine specs? A mid 70's Olds V8 with 8 to 1 compression will perform better with a lot of part throttle timing. I found 36 total ran faster at the track, 38 to 40 felt better seat of the pants. This along with a 30 degree vacuum advance ran great. The higher compression, factory iron head motors also run better with more part throttle timing but octane and cam choice affects how much can be run. But I ran into to pinging with too small of cam on a 9.6 to 1 Olds 350 on 91. I also had low speed bucking on a 9 to 1 350 with a 30+ degree vacuum advance. I switched to a 20 degree vacuum advance, bucking gone along with some response, a 25 degree vacuum advance can would have perfect.
Keep in mind that as the vacuum advance increases, it will cause the engine RPM to increase, which then causes the centrifugal advance to increase. When I was playing with my distributor vacuum advance, I locked the mechanical advance to keep it from affecting things.
I'll start with your questions. Before I changed the weights my initial at 800 RPM was about 8 degrees. I don't know what total mechanical was since I'd never used a dial back light before. Until I got it, I basically compensated for part throttle ping by backing off initial timing and putting the heaviest springs I could in to keep the mechanical timing to a minimum. Not real sophisticated, but it worked o.k.
I'm using an MSD HEI distributor. I think it came with the adjustable vacuum can, but it might be from Crane or one of the others. I've got MSD vacuum stop for it that looks like a metal X with specific curvature on the legs, I've also got one of those lockout discs that come with the kits.
Which type of vacuum limiter do you guys like the disc or the X?
One of the things that's been bugging me is the center cam. The aftermarket ones are smaller and I had this one big one I thought would limit total timing. It did, by limiting the motion of the swing weights. Essentially a bigger center cam doesn't allow the weights to swing out as far as a smaller one, limiting total timing. At least that how it seems to me. Am I right about this?
I was reading up about HEI's and the examples are Chevy, so I had to reverse all my examples from clockwise to counterclockwise. Not hard, but easy to miss when you're deep in an article about lean burn vs rich burn. All the center cams I've got have their spring pin holes off set from the distributor center line. The center cams are symmetric, so that means the spring pin holes can be offset in an advance position or flipped over and offset in a retard position. Today I flipped my center cam from advance to retard. Then I checked the start and finish position of the weights to get a sense of what that might do.
Center Cam in advanced position weights at high rpm position: Center Cam in advanced position. Notice the front spring pin hole sits counterclockwise of the center hole & the rear spring pin hole does too. So I call this "advance" position
Center Cam in advanced position, weights at low rpm position:
Center cam in "advanced" position at low RPM. Note the position of the tooth on the left, it's just before lining up with the firing tooth. When the center cam is flipped this tooth moves clockwise, retarding the low rpm timing
Center Cam in retard position, weights in low rpm position (Sorry for the bad photo) Cam in retard position, low RPM. Its hard to see but that same tooth is clockwise of the previous photo. It seems to me that this "retard" position is helping me pull timing out of the HEI, which I think I want to reduce detonation.
What's interesting is that with the center cam in the retard position, the weights start closer to the center, but have more total travel. I think this means the timing starts more conservative, but gets more total advance in this position as compared to when the center cam is in the advanced position. I seem to have two choices. Unless you guys think this whole "flip it" is just a dumb idea.
I had to reset the timing and wound up at 14 degrees @800 RPM with no vacuum. Total timing is maybe a bit low at 30-32 degrees at 3100 using heavy springs. At first blush the car is driving nicely with not much ping. I'm not quite declaring victory, but it feels like reducing the mechanical advance in the HEI was a good step.
Again I'm self taught and just experimenting trying to get power with no detonation. I could be way off base here.
Comments, suggestions or thoughts are most welcome. The testing is kind of fun, but I'm not sure I'm setting any MPG records.
The center cam and the weights work in harmony to set the amount of mechanical advance you want built into the distributor. Total timing is a function of what the initial timing is set to + the amount of mechanical advance. Lets say you set the initial at 16* for example, and you have 20* of mechanical advance, that gives you 36* total, initial + mechanical. The springs have nothing to with the amount of mechanical advance, they control the rate of which the advance increases as the rpm increases. Heavy springs means that it take a higher rpm to increase timing. The chart in your instructions will tell you the how the rates that particular springs affect your timing curve. Setting the weights to give a lot of mechanical advance and little initial makes the engine kind of doggish on acceleration from a stop but peppy at higher rpm changes. I prefer less mechanical and more initial.
Vacuum advance has mostly nothing to do with power, its more for fuel economy. Limiting it to around 10-15* keeps engine from pining at light throttle when setting your distributors tune for performance. I try to keep my total advance + vacuum to around 50-52*. I also leave it disconnected until I get my total timing lined out.
Exactly what was said above. Get the initial and total timing dialed in with the vacuum advance disconnected. Once the engine is happy with the tube, then connect the vacuum advance. A little ping on light throttle is acceptable, it should stop once you get on the throttle a little harder. There is nothing wrong with 50-55 degrees of total timing at cruise. Look at it this way, with the throttle mostly closed, very little air/fuel is actually going into the combustion chamber. You need to start the ignition cycle sooner to ensure complete combustion. At WOT, the cylinder is completely full of air/fuel mixture, you need less timing to completely burn everything.
Once you have the initial and centrifugal advance dialed in, don’t mess with the distributor trying to make the engine happy with the vacuum advance. Backing off the initial timing or advance curve will just hurt overall performance. Vacuum advance just makes the engine more efficient at part throttle.
My 66 Starfire still has too much advance at WOT, so I'm working on limiting it.
The Chevy guys seem to like screws in the lower plate ground down to custom widths to control WOT advance, but I'm aiming more at limiting the length of the advance slot. Some guys weld or braze this slot down a bit, but I have no such equipment and no such skills.
Here's the factory setup with no modifications. This is a spare HEI I've had around for a while, not on my car. Typical HEI distributor advance slot, no modifications. R pointing left means retard, just so I don't mix it up with all the Chevy advice.
Since I can't weld, I was playing around with making some kind of crescent shaped filler for the end of the slot, but found that by the time you have it shaped about right, it's small enough to move around in bad ways in the slot. So then I thought some kind of bushing, but that didn't look too robust. It needs to be a specific diameter to fit down over the inner pins and it has to be ground down on the sides to permit the mechanism to advance.
After a day or two of playing around, I hit on this. It's very simple: an electrical wiring connector bent 90 degrees with the wire end in the slot limiting advance.
It may be an absolutely crazy idea: Small gauge wiring connector bent into an advance slot limiter, mechanically retained by the inner HEI pins. Crazy?
I haven't installed this, mostly out of fear and other projects, plus it seems an extreme solution which should probably be solved with more normal approaches.
What do you all think?
Thanks for your opinions. I'll bet some or most of you have a better way to maximize initial advance for good off-the-line and limit total advance to avoid pinging on today's gas.
Whats the model/part number of the MSD distributor your using? There is a spring and stop bushing kit you can buy, MSD 8464 Where you can change the mechanical advance from 18-28*. https://www.summitracing.com/parts/MSD-8464/
I don't know if you are aware of this. Factory GM HEI distributors use the mating contours of the center bar and weights to control the amount of mechanical advance. Different weights and center bars are used for different advance curves.
Some aftermarket distributors use a bushing to limit the amount of mechanical advance, but I am not familiar with working on those.
November 5th, 2019, 05:20 PM
