70 LT1 vacuum/idle readings

[Dennis D.]

Used a different vacuum gauge and dial back timing light. With new 28B vac installed
ADVANCE BLOCKED
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ADVANCE CONNECTED
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[Duke W.]

Re: 70 LT1 vacuum/idle readings

Just to remind all, vacuum readings without stating the specific idle RPM are meaningless because idle vacuum varies with idle speed. The higher the idle speed the higher the vacuum and vice versa. I'm glad you provided full context by showing vacuum along with idle speed.

Idle vacuum also varies with total idle advance, and high overlap cam engines need a lot of total idle advance for optimum idle behavior due to high exhaust gas dilution that slows the rate of flame propagation.

OE mechanical lifter cam engines need least high 20s and up to low 30s total idle advance for optimum idle that minimizes fuel flow and EGT.

So what you have is:

8.5" @ 700 with 13 deg. total idle advance (no vacuum advance), and

11" @ 900 with 28 deg total idle advance (full vacuum advance)

The second data set is really all that matters now that you have converted to full time vacuum advance, but the difference between the two data sets illustrate the effect of both idle speed and total idle advance on observed manifold vacuum.

So you are in the ballpark that's nominally 12" @ 900 for the LT-1 cam. You will likely see slightly higher vacuum if you get total idle advance up into the low thirties.

Offhand I don't have the '70 LT-1 max centrifugal, but you should quote it from the CSM or AMA specs, and then measure it. You want the sum of full centrifugal plus initial to be as high in the 36-40 range as the engine will tolerate without detonation. The '70 LT-1 spark advance map was set up for emission control, so it's far from optimum for maximum fuel economy and performance. Typical emission controlled engine spark advance maps have short, lazy centrifugal curves and initial timing recommendations often yield total WOT advance less than the 36-40 optimum range. And with ported vacuum advance total idle advance is way below optimum.

As an example, if total centrifugal is 24 degrees, then initial should be as high as 16. This added to the nominal 16 deg. full vacuum advance should yield 32 degrees total idle advance. Plus if the centrifugal starts at less than idle speed, it may be in the 34-35 degree range due to the addition of a few degrees centrifugal at idle speed.

The optimum spark advance map for any OE mechanical lifter cam is the OE map from the 365/375 HP engines: centrifugal start @ 700, 24 @ 2350, and 0 @ 4", 16 @ 8" vacuum advance from a B28 or functional OE 236 16 VAC.

In the last few years I've set up a couple of 365 HP engines that both had unmodified spark advance maps that met spec. I set total WOT advance at about 39 with engine revs a bit over 2500, which yielded low thirties total idle advance. Both had lower than OE compression ratio indicated by thicker than OE head gaskets, and they both operated detonation-free on California 91 PON gasoline. After setting total WOT advance and verifying that the VACs met spec I went through the idle speed/mixture procedure and both ended up at 1.5 turns out from the seat on the OE carburetors, indicating 10" @ 900, which is the ballpark median for 30-30 cam engines.

Low speed tractability was good. Startup was pretty much stumble free without excessive clutch slip, and they easily pulled without protest from 1000 revs in top gear... not bad considering 30-30 cam is the longest duration, highest overlap cam ever installed in an American V-8 engine that was intended for normal road use, so its idle and low speed manners are nothing to write home about.

The best way to set total WOT advance is to first install light springs, so you can set it at the lowest possible revs. The latest information based on careful measurements by others indicates that the black springs in the Mr. Gasket 928G spring kit are the lightest followed by silver and then gold. My prior measurements, which were not as precise indicated that the gold springs were lightest, but I continue to recommend that all be checked on each specific engine.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Duke
Here are the distributor specs for the LT-1. found two. 2nd is a little hard to read.

If centrifugal adv maxes out at 26*, with my 13* initial, should have 39*. Is this correct?

Also, if the B28 VAC is all in at 8", shouldn't my readings reflect the c.a. total ? Or is it the rpm that brings the c.a. all in, and the vac determines the rate?

[Duke W.]

Re: 70 LT1 vacuum/idle readings

The second set of centrifugal specs are one-half the first set, so the second set is likely in distributor degrees and RPM, which are exactly one-half that measured in engine RPM and degrees. No issue there.

Your next step is to actually measure the centrifugal on your car and to determine what it actually is. Maybe it's been modified over the years.

As you can see, it's very lazy with the maximum of 26 not all in until 5000, and since it starts at 1000, there is no centrifugal contribution at 900 idle speed. Optimum full centrifugal is achieved at 2500 to no more than 3000, so get the spring kit.

If full centrifugal is really 26, then 13-14 initial gets total WOT advance up to 39-40 total, and if it doesn't detonate with full centrifugal in at as little as 2500, it's good to go. At WOT vacuum is very low, so vacuum advance has no contribution, and for this reason you can simulate WOT by simple free revving the engine with vacuum advance disconnected and the signal line plugged.

Chances are that lighter springs, in additional to providing full centrifugal at low revs, it will start earlier, so you might pick up a few degrees total idle advance that gets it into the low 30s, which is a good thing.

The centrifugal and vacuum advance are independent systems that provide advance as a function of engine speed and engine load as represented by the inverse of manifold vacuum. Light load is high vacuum, and high load is low vacuum, so total advance is the sum contributed by each system, plus initial. At WOT vacuum is near zero, so it has no effect. At light and part load it will add advance to the sum of initial and centrifugal for the particular engine speed.

To a mathematician this is a system with two INDEPENDENT variables and one DEPENDENT variable. If you plot it on an XYZ axis it is a surface, and there are a couple of examples of such "maps" in my tuning seminar.

In addition to TOTAL WOT ADVANCE and TOTAL IDLE ADVANCE, there is MAXIMUM CRUISE ADVANCE. As an example at about 50 MPH cruising on a level surface, those 365/375 HP engines with the standard 3.70 axle are turning about 2400, so full centrifugal is in, and since load is light, full vacuum advance is in. With 14 degrees initial total advance is 24 + 16 + 14 = 54 degrees, which is what these and other high overlap cam engines need for maximum cruise efficiency because high exhaust gas dilution and low mixture density considerably reduce the rate of combustion propagation compared to a low overlap cam engine that has much less exhaust gas dilution at light load.

Given the 5000 revs needed to achieve max centrifugal the OE '70 LT-1 spark advance map will never provide this optimum cruise advance at any legal top gear cruising speed.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Is there a way to check my centrifugal without rev'ing to 5000?

Believe I have a set of Holley black springs. Could I install those and check for when its all in?

Thinking it will be at a much lower rpm.

[Duke W.]

Re: 70 LT1 vacuum/idle readings

There is no harm in free revving the engine to over 5000, but it makes sense to install lighter springs first in order to check total centrifugal without having to rev it as high. For example, since the OE max centrifugal for the 365/375 HP engines is 24 @ 2350 you only have to rev it to about 2500 before max centrifugal is all in, assuming nothing has been modified from OE.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Thanks will post results

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Put in black springs. Didn't alter initial timing of 13*



VAC plugged


with VAC

[Duke W.]

Re: 70 LT1 vacuum/idle readings

40 degrees total idle advance is high... probably due to centrifugal starting below 900.

What you need to do is determine the basic centrifugal specs with the new springs... start point RPM and max advance at RPM. Then set total WOT advance in the 38-40 range a few hundred revs above the engine speed that centrifugal is all in.

Of course, the above is done with the VAC signal line disconnected and plugged with a golf tee.

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

If centrifugal is starting below idle, how do I determine when it starts?


Btw, I did notice the vacuum reading higher now. The 12.5 reading is rock steady.

[Duke W.]

Re: 70 LT1 vacuum/idle readings

On a high overlap cam engines it may be difficult to determine the centrifugal start point because you may not be able to get it to idle stably that low long enough before it stalls to accurately determine where it starts, but give it a try... keep reducing idle speed until the timing notch stops moving.

The above is why high overlap cam engines should be "timed" using the total WOT advance method... rev the engine a few hundred revs above the point of max centrifugal and set it at 38-40.

Duke

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

Very different with the black springs. I switched one out with a gold one, and the 40* came down to 20*. If I continue to change out springs until I get a good read on the centrifugal in and max would that be an effective way to get to the 38-40?

[Patrick H.]

Re: 70 LT1 vacuum/idle readings

If centrifugal is starting below idle, how do I determine when it starts?


Btw, I did notice the vacuum reading higher now. The 12.5 reading is rock steady.

Put a rubber band around the weights.
Check the advance (dial back timing light).

Now remove the rubber band, and recheck. While you might not know exactly where it starts, you will know if you're getting advance already at idle.

[Dennis D.]

Re: 70 LT1 vacuum/idle readings

I believe I am. Looking at my reading with the VAC blocked, I'm getting 20* @830 rpm. With the 13* initial theres 7* mechanical. Correct?

I am going to try your method . That's a good idea. Thanks Patrick

[Duke W.]

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