70 LT1 vacuum/idle readings

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

The 38-40 total WOT advance is independent of the springs. The springs just affect the RPM where advance starts and stops. Forget about initial timing! (read post #11) Use the total WOT method. Put the black springs back in and determine at what RPM total centrifugal is all in. Then road test. If it doesn't detonate you should be good to go.

40 total idle advance is higher than the target low 30s, but it if doesn't detonate, it's probably okay.

I'm not familiar with "Holley black springs" that you mention. For the Mr. Gasket 928G spring kit the order, lightest to stiffest, is black, silver, gold.

Duke

Thought I had an understanding, but it's getting beyond me.

If VAC, initial, and springs aren't methods to get to 38-40, then I'm not understanding how to make an adjustment to get total down into the 30's.

I appreciate your patience attempting to explain in layman's terms, but I'm missing something.
" Sometimes a little knowledge is a dangerous thing."

[Duke W.]

Re: 70 LT1 vacuum/idle readings

If VAC, initial, and springs aren't methods to get to 38-40, then I'm not understanding how to make an adjustment to get total down into the 30's.

The VAC has ABOLUTELY NOTHING TO DO with the 38-40 total wide open advance. The VAC ONLY adds advance at part load and idle now that you have converted to full time vacuum advance. So if you disconnect and plug the VAC signal line the engine THINKS it's at WOT at any engine speed even though it isn't. You SIMULATE WOT by taking the effect of vacuum advance out of the equation.

Disconnect and plug the VAC signal line. Rev the engine to several hundred above the point of max centrifugal that you previously determined and while the engine is at that speed set the distributor to a timing value of 38-39. Then blip the throttle to at least 5000 and verify that is does not advance further.

Now as a reference check, reconnect the VAC, and report total idle advance.

As a second reference check, free rev the engine to several hundred revs above the point of max centrifugal advance and report the value of maximum cruise advance.


Here's how you set up the OE spark advance map for 365/375 HP 327s, and yours should be in the same ballpark. Nominal specs are:

Centrifugal: Start @ 700, 24 @ 2350
Vacuum: Start @ 4", 16 @ 8"

With the VAC disconnected and plugged free rev the engine to between 2500 and 3000 and set timing at, say. 38 degrees.


If full centrifugal is right on the 24 deg. spec then initial timing is 38-24 = 16. You can try to measure about it, but these high overlap can engines (including your LT-1) usually won't idle at less than the centrifugal start point long enough to get a reading. Don't worry about it. If the total WOT advance is in the ballpark the initial is in the ballpark.

Reconnect the VAC and measure total idle advance at 900. It should be 16 initial + 16 vacuum + 2 to 3 centrifugal (since is starts at 200 revs below the 900 idle speed) = 34-35.

Now rev the engine to between 2500 and 3000 and to measure maximum cruise advance. It should be the sum of initial, full centrifugal, and full vacuum, 16 + 24 + 16 = 56 degrees.

Remember the above is for the 365/375 HP 327s. Your LT-1 total centrifugal and the RPM where it's all in are slightly different, so keep that in mind.

Just do as I recommend above and FORGET ABOUT INITIAL TIMING. Get the total WOT advance to 38-39, then measure total idle advance and maximum cruise advance and report your numbers.

FORGET ABOUT INITIAL TIMING. DO NOT EVEN ATTEMPT TO MEASURE IT. PLEASE DO NOT MENTION IT AGAIN. Using the total WOT method to set the distributor makes initial timing unimportant and irrelevant other than as a reference check (and I've already given you to others), if you can actually measure it, which can be difficult to impossible on a SHP engine with a centrifugal curve that starts at less than normal idle speed.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Black springs installed. Vac plugged.
Rev'd rpm for full centrifugal.(all in around 1500rpm). Set to 38*

Reference check total idle advance
20190705_131912.jpg

[Duke W.]

Re: 70 LT1 vacuum/idle readings

Full centrifugal in at 1500 may be more aggressive than the engine can tolerate due to detonation or driveability problems, and total idle advance is about ten degrees more than necessary.

Previously you mentioned "Holley black springs". I am not familiar with them. The springs I am familiar with are the Mr. Gasket 928G spring kit. Is this what you actually have?

Have you road tested the car in the current configuration?

If no detonation or driveability issues, you can leave I as is, but I'd like to see full centrifugal not in until at least about 2000-2200.

The Mr. Gasket 928G spring kit in order of most aggressive to least aggressive is as follows:

black black
black silver
silver silver
silver gold
gold gold

Assuming you are using the springs from the Mr. Gasket 928G kit and you currently have black black installed, use the above sequence to slow the centrifugal curve. This will both increase the start point RPM to reduce total idle advance and increase the max centrifugal RPM.

Please positively identify the manufacturer and part number of the springs you are using.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

This is more than likely a mr gasket kit. Have had it quite awhile. No packaging.

I did at one point replace a black spring with a gold one.
( see post #12).

To be honest it runs better, and the fuel smell and slight exhaust smoke are gone with the black springs.
I should mention I have 112 leaded fuel in the tank to help get rid of whatever the remaining fuel was.

[Leif A.]

Re: 70 LT1 vacuum/idle readings

Full centrifugal in at 1500 may be more aggressive than the engine can tolerate due to detonation or driveability problems, and total idle advance is about ten degrees more than necessary.

Previously you mentioned "Holley black springs". I am not familiar with them. The springs I am familiar with are the Mr. Gasket 928G spring kit. Is this what you actually have?

Have you road tested the car in the current configuration?

If no detonation or driveability issues, you can leave I as is, but I'd like to see full centrifugal not in until at least about 2000-2200.

The Mr. Gasket 928G spring kit in order of most aggressive to least aggressive is as follows:

black black
black silver
silver silver
silver gold
gold gold

Assuming you are using the springs from the Mr. Gasket 928G kit and you currently have black black installed, use the above sequence to slow the centrifugal curve. This will both increase the start point RPM to reduce total idle advance and increase the max centrifugal RPM.

Please positively identify the manufacturer and part number of the springs you are using.

Duke

Duke,
I always follow these tune-up threads (especially when you get involved). The highlighted sentence has me a bit confused. According to your description of the Mr. Gasket springs, the black/black combo is the stiffest combination. If Dennis is using the black/black springs currently and he's "all in" at 1500RPM, it doesn't appear as though he has any stronger springs to slow down the advance. Seems like if he had the gold and/or silver springs installed he could go stiffer i.e. black combo etc. What am I missing here? Thanks, as always, for your in depth explanations.

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Leif
Duke says "more aggressive", which I interpret as lighter springs to get it in earlier. Black being most aggressive. (Lightest)

[Duke W.]

Re: 70 LT1 vacuum/idle readings

Post a photo of the springs you have.

The race gas will likely prevent detonation even with the over-aggressive setup with the two black springs. The objective should be to set it up to operate detonation free on unleaded premium. What is the PON of your available leaded premium?

Do you have any idea what the true compression ratio is? Measure head gasket thickness with feeler gages at the corners of the head-block interface. If it's thicker than the OE .018" gasket then the CR is lower than when it left the plant, and most were closer to 10.5 than 11:1 due to typical high deck height.

If the pistons are not the domed OE equivalent then CR is even lower.

Duke

[Duke W.]

Re: 70 LT1 vacuum/idle readings

Back in 2012 when I first used the Mr. Gasket 928G spring kit I analyzed the springs by measuring wire diameter and number of coils. Since that time, others have provided more accurate information by testing on a distributor machine and measuring with a spring scale. I clearly screwed up and apologize for my errors.


All the latest data I have from others clearly shows that lightest to stiffest is black - silver - gold.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Post a photo of the springs you have.

The race gas will likely prevent detonation even with the over-aggressive setup with the two black springs. The objective should be to set it up to operate detonation free on unleaded premium. What is the PON of your available leaded premium?

Do you have any idea what the true compression ratio is? Measure head gasket thickness with feeler gages at the corners of the head-block interface. If it's thicker than the OE .018" gasket then the CR is lower than when it left the plant, and most were closer to 10.5 than 11:1 due to typical high deck height.

If the pistons are not the domed OE equivalent then CR is even lower.

Duke

Will check head gasket. Domed pistons. Fuel info
20190706_125151.jpg

[Leif A.]

Re: 70 LT1 vacuum/idle readings

Back in 2012 when I first used the Mr. Gasket 928G spring kit I analyzed the springs by measuring wire diameter and number of coils. Since that time, others have provided more accurate information by testing on a distributor machine and measuring with a spring scale. I clearly screwed up and apologize for my errors.


All the latest data I have from others clearly shows that lightest to stiffest is black - silver - gold.

Duke

Thanks for the clarification, Duke.