Ignition timing range ?

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acarlson
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Ignition timing range ?

Unread post by acarlson » Sat Jan 07, 2017 6:08 am

What is the timing range for an LM Turbo. I know the advance limit is 24 degrees. Once the turbo kicks in, what is the retard range as the boost increases ?

Alec
1965 Corsa 4 Speed Turbo Convertible

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bbodie52
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Re: Ignition timing range ?

Unread post by bbodie52 » Sat Jan 07, 2017 8:48 am

Here are the Corvair distributor specs...
Corvair Distributor Part Numbers and Specs.jpg
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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Sat Jan 07, 2017 11:42 am

Brad,

Thanks - I have those. But they only specify the advance at idle. There is no advance on the turbo - no centrifugal or vacuum. It only has a pressure retard. Would you interpret the "vacuum advance" number in the chart for the turbo as the "max retard" value so the timing range would be 24 (at idle) to 8 (at 3.7 psi) ?

Alec
1965 Corsa 4 Speed Turbo Convertible

Scott V
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Re: Ignition timing range ?

Unread post by Scott V » Sat Jan 07, 2017 12:51 pm

look on the right side............it shows the pressure retard and centrifugal. turbo dizzys have centrifugal but nothing happens until after 3800 rpm.

-Scott V.

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Re: Ignition timing range ?

Unread post by bbodie52 » Sat Jan 07, 2017 5:05 pm

According to the chart for Late Model Turbo Corvairs, The initial (static) timing setting is 24 degrees. No centrifugal advance is dialed in until 4000 RPM and then rapidly advances another 18 degrees by 4900 RPM. Pressure retard of 8 degrees at a boost pressure of 3.7 psi reduces the maximum timing advance by 8 degrees. There is no vacuum advance.
Distributor Timing Specs.jpg
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Re: Ignition timing range ?

Unread post by miniman82 » Sat Jan 07, 2017 7:39 pm

acarlson wrote:What is the timing range for an LM Turbo. I know the advance limit is 24 degrees. Once the turbo kicks in, what is the retard range as the boost increases?

Advance limit? Not aware of that term, but the 24* figure you referred to is base timing at idle. Then as Brad stated there is some centrifugal advance that comes in quite late, and the pressure retard can is supposed to reduce timing when the turbo builds pressure. Too bad it almost never works the way it's supposed to.

Through experimentation on my car, I found 24* base is way too much timing for hot starts. It broke like 3 starter nose cones and the pressure retard never worked right, so I gave up on the stock ignition and went to a programmable ignition which gave around 10* during cranking and rose to around 16* when idling. That completely cured the broken starters, and I also gained a proper boost retard that works every single time.
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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Sun Jan 08, 2017 6:09 am

It seems to me that if the pressure retard is working properly, at some point the centrifugal advance and the pressure retard will be working against one another.

The reason I'm asking is that I took the old pressure retard off and replaced it with the Clark's retard unit. This was part of a dizzy upgrade to install the XR700. The Clarks retard unit, mounted in the stock pressure retard location, has no space available to move the timing adjustment arm to retard the ignition. The arm is right up against the notch in the side of the breaker plate. There is plenty of area to move the arm if advancing the timing. Note that the Clark's pressure retard unit supports both vacuum advance and pressure retard.
retard.jpg
retard.jpg (105.1 KiB) Viewed 201 times
When I took the old retard unit off, I saw that the retard mounting bracket had been drilled out to elongate the mounting holes. At the time, I thought that was strange, but now I think it was done so the retard unit could be shifted on the side of the dizzy to pull the retard arm away from the side of the housing. If I end up having to do the same thing I was just curious how much space to allow for timing arm movement. This, it seemed to me, equated to the max retard travel required for the max timing retard that was spec'd for the turbo.

Alec
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Re: Ignition timing range ?

Unread post by Scott V » Sun Jan 08, 2017 8:12 pm

acarlson wrote:It seems to me that if the pressure retard is working properly, at some point the centrifugal advance and the pressure retard will be working against one another.
i get what you mean..........but they dont work against one another - they work with one another to get you the factory timing w/boost & no boost.
acarlson wrote:The reason I'm asking is that I took the old pressure retard off and replaced it with the Clark's retard unit.
-cut-
When I took the old retard unit off, I saw that the retard mounting bracket had been drilled out to elongate the mounting holes. At the time, I thought that was strange, but now I think it was done so the retard unit could be shifted on the side of the dizzy to pull the retard arm away from the side of the housing. If I end up having to do the same thing I was just curious how much space to allow for timing arm movement. This, it seemed to me, equated to the max retard travel required for the max timing retard that was spec'd for the turbo.
you shouldnt have to mod anything to install a pressure retard or a vacuum advance on you dizzy. the arm shouldnt be up against the notch in the points plate.

if your picture is your dizzy before the crane install........it looks like you got something wrong in there. the points adjustment is really out of wack. maybe its the angle of the pic that makes it look like that?

a 180 pressure retard will have 250 stamped on the arm......yours is something different & might not be a pressure retard. a 150 pressure retard will have #221 or 224 on the arm. the number on your arm doesnt look like any pressure retard. you said you replaced the retard w/a clarks part & installed a xr700.....that good......but it looks like something was/is wrong w/your dizzy before you put the new stuff in.

i would grind the points plate so the arm pushes out enough to get you max boost retard. grind the plate until the arm stops on the pressure retard before it hits the plate.

-Scott V.

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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Mon Jan 09, 2017 4:47 am

Scott V wrote:i get what you mean..........but they dont work against one another - they work with one another to get you the factory timing w/boost & no boost.
Must be true but seems counter-intuitive...
Scott V wrote:you shouldnt have to mod anything to install a pressure retard or a vacuum advance on you dizzy. the arm shouldnt be up against the notch in the points plate.
I agree, but I have no clue what some previous owner did with the dizzy. The pressure retard I took off was not stock - I'm beginning to think it was another Clarks retard unit which I didn't know until I had it off. This Corsa was owned by a Corvair hacker in San Diego at some point - there were a few Clarks modifications already on it when I bought it.
Scott V wrote:i would grind the points plate so the arm pushes out enough to get you max boost retard. grind the plate until the arm stops on the pressure retard before it hits the plate.
I considered that, but then I'd have to remove the xr700 to get the points plate off. That's more of a PITA then modifying the mounting bracket on the new pressure retard. I'll post a picture of my updated dizzy so you can see the actual problem...

Alec
1965 Corsa 4 Speed Turbo Convertible

miniman82
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Re: Ignition timing range ?

Unread post by miniman82 » Mon Jan 09, 2017 10:44 am

Just more reasons to get rid of the distributor, if you ask me!
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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Mon Jan 09, 2017 11:23 am

Here is a pic of the Clarks pressure retard mounted in the stock location on my dizzy.
IMG_0118.JPG
IMG_0118.JPG (130.71 KiB) Viewed 149 times
This gets back to my original question. I will have to move the pressure retard unit I just don't know how much travel space to allocate for both the vacuum advance and pressure retard operation. Hmm, maybe I can just change the pin hole location rather then the mounting bracket...

Alec
1965 Corsa 4 Speed Turbo Convertible

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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Mon Jan 09, 2017 1:42 pm

OK, so I modified the mounting bracket so that the timing arm is centered in the notch on the points plate. I was now able to set the timing - Clarks recommends 18 degrees, I'm probably closer to 19 @ 825 rpm. When I hook the vacuum/pressure hose up, the rpm increases to about 1000 and the advance runs up to about 26 degrees (this is from the vacuum advance). Any issues with this ??

Alec
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Scott V
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Re: Ignition timing range ?

Unread post by Scott V » Mon Jan 09, 2017 2:39 pm

acarlson wrote: I was now able to set the timing - Clarks recommends 18 degrees, I'm probably closer to 19 @ 825 rpm. When I hook the vacuum/pressure hose up, the rpm increases to about 1000 and the advance runs up to about 26 degrees (this is from the vacuum advance). Any issues with this ??
@ 18 btdc you will be short 6 degrees of timing from what the factory had. it could be a problem........maybe not. youll have to drive your car & see how it likes it. at full throttle & boost the engine may buck & misfire.

heres some other info. http://www.american-pi.com/corvair/html ... notes.html

-Scott V.

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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Mon Jan 09, 2017 3:44 pm

The stock setting of 24 is because there is no vacuum advance on the stock turbo. With Clarks retard unit, it supports both vacuum advance and pressure retard. So although the baseline is lower then stock, the vacuum advance moves it back up to 25-26 at idle and maybe a little higher at higher rpms. I don't really know what the range is. I asked Clarks tech guys and they just said "they recommend 18 as the base" and it will be OK. I'll take the Corsa out for a test run tomorrow and we'll see...

Alec
1965 Corsa 4 Speed Turbo Convertible

Scott V
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Re: Ignition timing range ?

Unread post by Scott V » Mon Jan 09, 2017 4:45 pm

acarlson wrote:The stock setting of 24 is because there is no vacuum advance on the stock turbo.
no. the stock setting is 24 is because thats what the engine needs for the correct timing at full throttle & w/boost. vacuum advance has no affect on timing at full throttle & during boost.
acarlson wrote:With Clarks retard unit, it supports both vacuum advance and pressure retard. So although the baseline is lower then stock, the vacuum advance moves it back up to 25-26 at idle and maybe a little higher at higher rpms.

yes & no. @ idle the vacuum advance bumps up the timing. vacuum advance has nothing to do w/rpms. you could be @ 3000 rpm & have vacuum advance or you can be @ 3000 w/no vacuum advance.

-Scott V.

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acarlson
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Re: Ignition timing range ?

Unread post by acarlson » Mon Jan 09, 2017 8:26 pm

Scott V wrote:no. the stock setting is 24 is because thats what the engine needs for the correct timing at full throttle & w/boost. vacuum advance has no affect on timing at full throttle & during boost.
Actually the engine gets more then 24 at full throttle w/boost. 24 is what the engine gets until the turbo kicks in. After that the turbo pressure retard starts reducing the timing till you get up to about 4000 rpms. After that the centrifugal advance kicks in and adds more advance timing to the mix. The pressure retard and the centrifugal advance do not cancel each other out so the net timing at full throttle and full boost is a lot more then 24.
Scott V wrote:yes & no. @ idle the vacuum advance bumps up the timing. vacuum advance has nothing to do w/rpms. you could be @ 3000 rpm & have vacuum advance or you can be @ 3000 w/no vacuum advance.
What you say is true but I was looking at my specific situation where the idle timing and rpms was 18/825 without vacuum advance and 26/1000 with vacuum advance. Obviously, advancing the timing increased my rpms. It was more of a "cause and effect" event rather then a direct relationship.

Alec
1965 Corsa 4 Speed Turbo Convertible

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