I installed a performance hei distributor(50,000 at 7500 rpm) in my 402, timed it ran it down the road and back and it died. The company replaced it I did the same thing except run it down the road, i set the timing, was working on the idle and vacuum and it died as well. I have a stock chevrolet HEI dist that is only good to about 5000 rpm i put it in and no problems except poor performance. Is it because I am using a cheap distributor (pro comp) or is there something wrong with my set up. After it dies I cant get any fire out of the distibutor, my initial at idle is at 12 degrees BTDC. then i plug in the vacuum line into the dist and it goes to about 16 degrees. What kind of adjustments can i make with the allen screw in the inlet of th vacuum advance on the dist. What adjustments do i need to make. hell how do i adjust it (right or left to increse). This is my first build by myself and i am not to sure on timing and advance.

402 .30 over
gear drive
mild cam
holley street dominator intake
holley 600 cfm carb
2600 stall convertor on a 700r4
stock heads
roller tip rockers

I installed a performance hei distributor(50,000 at 7500 rpm) in my 402, timed it ran it down the road and back and it died. The company replaced it I did the same thing except run it down the road, i set the timing, was working on the idle and vacuum and it died as well. I have a stock chevrolet HEI dist that is only good to about 5000 rpm i put it in and no problems except poor performance. Is it because I am using a cheap distributor (pro comp) or is there something wrong with my set up. After it dies I cant get any fire out of the distibutor, my initial at idle is at 12 degrees BTDC. then i plug in the vacuum line into the dist and it goes to about 16 degrees. What kind of adjustments can i make with the allen screw in the inlet of th vacuum advance on the dist. What adjustments do i need to make. hell how do i adjust it (right or left to increse). This is my first build by myself and i am not to sure on timing and advance.

402 .30 over
gear drive
mild cam
holley street dominator intake
holley 600 cfm carb
2600 stall convertor on a 700r4
stock heads
roller tip rockers

I had a PROCOMP special take a dump on me after about 20 passes. Your initial timing sounds a little low to me, I like to set them at about 15-16 degrees on the initial side. Also were you able to check the total timing before it died? When I had the PROCOMP I was only able to get about 17* mechanical advance out of it. Meaning to get it run good, I had to set the initial at 19*, to have a total of 36*.

It is only a guess but I bet your motor was retarded by at least 5 and probably 7-8 degrees. To advance it move the distributor counterclockwise if you are facing the front on the car. Do this only in small increments. I went with an MSD Streetfire, put the advance kit in and never looked back.. Good luck....

ALSO do some searches on timing, more than enough here and elsewhere to expand your knowledge.

I had my initail at about 16 degrees and was told that was to high.
I know how to advance the inital timing but my plate only goes to about 16 degrees, this i can come up with a solution for. my problem is that how do i measure the advance from the dist. how much should it climb when i first plug in the vacuum line to the distributor, then how and where do i take my next reading? 3000 rpm? How do i adjust the vacuum in the distributor? Is this causing the dist to go bad? or is it because it is cheap?

Thank you for your input

Checked out that MSD streetfire on summit and bam there are the instructions on how to advance the vacuum canister.

Thank you

I still need to know where and when to check this advance and if I am killing these distributors or if they are cheap?

Checked out that MSD streetfire on summit and bam there are the instructions on how to advance the vacuum canister.

Thank you

I still need to know where and when to check this advance and if I am killing these distributors or if they are cheap?

They are cheap and no good in my opinion. Just forget about the vacuum advance for now, it will make things much easier. Plug the port on your carb that you are using for the vacuum canister and start from there. You need to determine what your total timing(initial + mechanical) is before entering vacuum into the equation.

To measure total timing take your initial number lets say it was 12*. Then you need to check the timing when the car is at about 3000 rpms. If you determine it is 30* take 30 minus 12 and your mechanical advance would be 18*. You cannot increase the 18, you can only increase initial timing not the mechanical, unless you modify the distributor itself.

I posted before in my experience those PROCOMP units only have about 17-18 degress of mechanical advance, so you would need 18* of initial timing to have a total of 36*.

Is the car running now??

Read over this if you are confused......

Distributor Tuning and Theory - Part 1

There appears to be a lot of conversation on timing, vac advance, distributor tuning and, in general, hacking up a perfectly good ignition system to compensate for a poorly tuned carburetor.

from
JohnZ
Tech Team

posted 03-23-2003 03:41 PM

The following two articles review the basics of distributor tuning quite well and have worked for years and years and are based on sound engineering principals. I thought it would be helpful for some to review these prior to hacking up their distributors. Hacking up your distributor to compensate for a poorly tuned, misapplied or defective carburetor is not very sound engineering, for a street application or otherwise.

Here's an interesting article on vacuum advance written by a GM engineer:

As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

Brian,thanks for posting this again,i see this issue almost every day here in t/c and get beat up half the time i make the suggestions just like are stated here in your post.

This happens with over 1/2 the perf cam installs i see here in t/chevelle,its a big problem with people not realizing/understanding that motors with after market perf cams installed need 3x-4x more base timing ,the mech adv in dist recurved/limited for proper total when running 3x-4x more base timing then when motor had stock cam,along with how important it is to run a vac adv hooked to full intake vacuum all the time on a steet motor with an aftermarket perf cam,even a mild one.

But onto to his HEI dying,has anybody asked if he is using the old resistor wire from the points ign with only 9v going to the new hei that needs 12volts to operate correctly which could bpossibly e the problem if thats the case.

Scott

Good point about the 9volt wire. Hopefully he can can answer that. I think that post should be a sticky in the engine or performance forum JMO....

Also when I had that distributor(prodump) the total mechanical was only 17*, so when I first started the car, set the timing to about 10* initial, I did not know about total timing. The motor idled fine but when I drove it the thing popped, burped and died. After some posts, research, and help of a friend, who quickly checked the total timing, my problems were solved. I had only been running 27* of total timing, we bumped the initial up to 18* for 35* total, and the car immediately ran like a beast out of hell, until the distributor died.

What a difference a simple thing like timing can make.....

I will Check the voltage on that wire but I am pretty sure it is 12 V.

To check the total timing I could measure a spot on the timing cover and match the incraments to my timing tab to figure out if I am at 36*???

So if I get this right I need to set my initial at about 15* without vaccuum adv.
Then check the timing at 3000rpms my total initial and mechanical should be 36*
So the vaccuum adv is only used at idle? (how much vaccuum adv at idle or does that depend on the fuel air mixture and the cam?)

The intial and mechanical make up my total timing correct? which should be 36*

If that is the case, that explains my very poor performance from this engine at higher RPMS ( backfire, sputter, dragging ass)

Thank you all
As soon as I get a new dist. I will let you know how it turns out

Antother question when I do finally plug in the vaccuum adv what roughly should it idle at if initial is at say 16*

is this math right
16* initial
20* mechanical
14* vaccuum at idle???

total 50* + or -

Interesting artical I would like to see where it was cut and pasted from

I have read several other articals but that one,well the dyno with a crysler 426 and 6 of there engineers standing there well let's just say you can't tust everything you read.

Antother question when I do finally plug in the vaccuum adv what roughly should it idle at if initial is at say 16*

is this math right
16* initial
20* mechanical
14* vaccuum at idle???

total 50* + or -

I would not go higher then 50*. That should be a good starting point. You need to either buy a dial back light or put a timing tape on your balancer to dial your timing in. Jegs sells a msd sheet with all different tapes on it.... Then you have to measure your balancer, but thats easy to identify the right one, or I am sure someone here can tell you. Thats a priority imo.......

Set initial timing with ildelow like 600-650 rpm to ensure the mech adv in the dist isnt partially activated upsetting the initial/base timing setting.

Vac adv should be unhooked/plugged when checking/setting base/total timing.

Scott

I installed a MSD Street Fire and solved my problems
16 initial
20 mechanical
12 vaccuum.

Thanks all for your help