1968 Vacuum Test

[robert campbell]

Could a couple of you guys with a basically stock long block 289 or 302 do a vacuum test for me? No problem if you have a different carb, but want the test performed on a stock engine and camshaft mostly.

I especially would like to test an engine with an automatic. Hot idle in gear from an intake vacuum source. I would assume in park or a manual car to see somewhere in the 17 to 18 psi range. In gear I would imagine a drop down towards 15 psi.

If a couple of you do this it would be helpful. This car I just finished in the other thread runs lower vacuum than it should in my mind. Its cam is unknown, but it surprised me.

Thanks for doing this. I will test some other cars myself if I get one over.

Rob

 

[p51]

Rob

Here goes...

Stock 302 J-code engine with Edelbrock 4bbl 1406 (600cfm) carb. Distributor vacuum advance hooked to manifold vacuum. Initial timing at idle 10* BTDC w/o vac advance. Initial timing at idle with vac advance is 25* BTDC. Warm idle RPM ~875. Carb tuned to provide highest vacuum at idle. Manual transmission. And finally...
... 18" of manifold vacuum (which is the shop manual spec as well)

Side note: When I used ported vacuum to the distributor I never could seem to get above ~16" vacuum at 875 RPM idle, but the engine likes manifold vacuum to the disti so much (great throttle response) I didn't try to figure out why that was the case.

James

 

[robert campbell]

James,
Thanks!!!
A stock 2 barrel 289 with an auto arrived today. I have worked on it before and it runs great. I will test it soon. Both in park and in gear.

To your side note. I would assume the difference in vacuum is attributed to the amount of advance at idle. In both cases I assume that you maintained the same hot idle speed of 875. Using the ported source if the carb is adjusted properly you should have no vacuum and thus only you initial timing of 10 degrees would be in play.

Using a manifold constant source you have 25 degrees of advance at idle. Boy is that a bunch, but if the engine likes it and runs good what the hey! It would be interesting to see if you have a small amount of vacuum present on the ported connection at hot idle? Nearly 900 rpm at hot idle might cause a "clunk" if you had an auto tranny and could be tugging hard on the torque converter at a stop light. A 1968 J code should idle at 625 in park and 550 in drive. But with a manual transmission, not very important!!!

If you love to fuss like me, and if you have some vacuum at the ported connection, you could try opening the secondary stop screw 1/4 turn. This will allow you to close down the primary stop screw a bit and allow for better connection between the idle mixture screws and the engine.

Both the Autolite 4100 and the early 4300 carbs cracked their secondary's just a bit. But that could be totally a design feature and the Edelbrock may be just fine as set from the factory.

But heck if it runs great, ignore all of the above!!!! If it ain't broke, leave it alone!!!

Rob

 

[p51]

James,

To your side note. I would assume the difference in vacuum is attributed to the amount of advance at idle. In both cases I assume that you maintained the same hot idle speed of 875. Using the ported source if the carb is adjusted properly you should have no vacuum and thus only you initial timing of 10 degrees would be in play.

Using a manifold constant source you have 25 degrees of advance at idle. Boy is that a bunch, but if the engine likes it and runs good what the hey!...


...If you love to fuss like me, and if you have some vacuum at the ported connection, you could try opening the secondary stop screw 1/4 turn. This will allow you to close down the primary stop screw a bit and allow for better connection between the idle mixture screws and the engine.

Hey Rob,

I think you're exactly right in that the vacuum is directly related to the amount of advance. But having 25* (with vac adv set at 15*, which is a normal value) with the very low load at idle is not unusual. The only difference between manifold vac and ported vac is with manifold you get vac advance engaged when the throttle is closed, with ported there is no vac adv engaged with the throttle closed. With ported, when you crack the throttle at idle, even a little, and the distributor "sees" the vacuum there is no/little difference between the advance you get with the manifold or ported hookups. With ported vac advance the timing will jump from 10* to ~25* when you crack the throttle. With manifold it's already sitting at 25*. Regardless of how the disti is hooked up, when you increase load on the engine the vac advance drops out. And as you increase RPM the centrifugal advance comes in. Under load at WOT I've got about 36* total.

From what I've read, the reason for ported vac advance was mainly an emissions issue. Here is the best reference I found explaining the differences...
http://www.camaros.org/pdf/timing101.pdf

Regarding "secondary stop screw", the Eddy 1406 has a vacuum secondary which, as far as I can tell, has no adjustment (now I need to look again). But I think you're right on this, if I could adjust it maybe I could push the manifold vacuum back to to ~18" spec with ported vac adv to the distributor. Good suggestion. Thanks.

James

 

[robert campbell]

James,
This is great discussion for us motor heads!! I laughed with my wife that a large percentage of the CS site viewers must groan when they see me post!!!

I would like to submit a few more thoughts. Fords used ported vacuum long before the emission days of the late 1960's. Even in the 1950's.

If you use a constant vacuum source the vacuum advance is always full in idling at a stop light. Lets say 15 degrees of initial and the vacuum advance brings in 10 for ease of discussion. So at a stop light you have 25 degrees of total timing at idle. So as you accelerate away under a "light" load the vacuum will drop and at first the engine will experience a drop in total timing until the centrifugal kicks in. So under normal driving and light load, the engine could actually experience a minor drop in performance such as a "small" flat spot right off the light. But this could be controlled by the fact it already has more than stock initial timing.

With ported vacuum the exact opposite occurs. The engine is idling at the stop light with 15 degrees of initial. Under light loads, just as you leave the light the engine will experience the advance kick in and bring it up towards the 25 degree mark, but again this happens so fast that who knows exactly what is happening.

The other factor is hot starting. Do you have a mini starter that has more torque than stock? That much initial coupled to a small vacuum signal at cranking speed could cause the "rumpf....... rummph...... rumph. of a starter bucking a heavy tide.

I have experienced many stock engines that experience part throttle detonation from having to much advance under a moderate load up a hill on a hot day. Back in the day Ford used a removable snout on the vac advance that had a spring and shims behind it. They had an assortment of springs and shims to fine tune engines (I have a bunch of them). If the vacuum advance comes in to soon or its spring allows for the advance to come in to far to soon it can cause the detonation pulling a hill under light load. As soon as you stomp it or apply more load the vacuum advance signal reduces and the detonation goes away.

Newer style Ford vacuum advances from most part stores allow you to insert an allen wrench in the connection port and adjust the tension on the spring that controls the vac advance. So in the cases above you can increase the spring tension and slow the advance and the amount of advance to dial this detonation way. I cut one open as a prop to explain it to people in the car club in my area. Under light loads and low RPM's to much advance to soon can be a problem. This is also affect by gear ratios. Tall gears such as 2.79's have the most problems in this area.

Boy did a lot of engineering go into this stuff!

Rob

 

[Midnight Special]

James,
This is great discussion for us motor heads!! I laughed with my wife that a large percentage of the CS site viewers must groan when they see me post!!!

Rob

...Perhaps, but it will make great archival reading when my hearing goes to hell (from all those sidepipes) and I can no longer dial it in on the fly with a 1/2 wrench & screwdriver... ;-)

 

[p51]

Fords used ported vacuum long before the emission days of the late 1960's. Even in the 1950's.


The other factor is hot starting. Do you have a mini starter that has more torque than stock? That much initial coupled to a small vacuum signal at cranking speed could cause the "rumpf....... rummph...... rumph. of a starter bucking a heavy tide.

Did not know that. Interesting.


Just a standard starter. No issue with starting using manifold vacuum even when hot. The car starts up immediately every time - "wow, wow, vrooom". My guess is with the initial at 10* that the amount of vacuum created by the low cranking speeds is not enough to advance it so far that it is hard starting.

Anyway, I've tried it both ways and it seems that my engine "likes" manifold vacuum better. The write up I attached above seems to make the most sense to me about the differences and is consistent with what I've seen empirically. Having said that it apparently was written by a GM engineer and what the hell do they know, they can't even make distributors that turn in the correct direction :wink:

.

 

[robert campbell]

So a 1967 bone stock Fairlane, 2 barrel 289, with auto. 50k miles on car and engine. One I rebuilt the carb on and fixed a bunch of air conditioning vacuum problems:

Hot idle in park at about 750 rpm: 17.5 inches of vacuum
Hot idle in gear at about 600 rpm: 15.0 inches of vacuum

This is much better than the 1966 GT Mustang I just did the head and carb work on. The GT idles a bunch better, but wish I could find more vacuum. I am confident of know vacuum leaks, so at this point it may be cam or cam timing. Both are unknown.

But at this point the customer seems satisfied and to dig deeper will be just mo money for him.

Rob

 

[p51]

This is much better than the 1966 GT Mustang I just did the head and carb work on. The GT idles a bunch better, but wish I could find more vacuum. I am confident of know vacuum leaks, so at this point it may be cam or cam timing. Both are unknown.


Rob

OK. You left me hanging here...
...what vacuum are you seeing on the '66 GT? At what RPM? At what idle timing?

(This is like a mystery novel with the last page ripped out :wink: )

 

[robert campbell]

A steady 15 psi in park and towards 11 in drive. Both steady, but to low for me. Car idles smoothly, but not as smooth as a rock stocker in my mind. Comes off the line great, no flat spots, and very crisp acceleration. No hint of wanting to die in gear. I have been around a few cars that once they head towards 10 psi the just continue a spiral down and die. This one does not.

I have it around 12 degrees btdc static and the vacuum kicks in immediately at throttle crack. I have not tried the Edlebrock I have since the head work, but it made no difference the first time. Again the customer seems satisfied and he is a bit of a pill to be nice. Not sure I want to invest a bush more time in it.

Don't know why, but would love to see the timing chain and gears to see if they installed the gear in a non-zero position. Most cams today are already a couple advanced from the factory and adding to that by positioning the gear off zero can add to that.

Rob

 

[p51]

A steady 15 psi in park and towards 11 in drive. Both steady, but to low for me.

...Don't know why, but would love to see the timing chain and gears to see if they installed the gear in a non-zero position. Most cams today are already a couple advanced from the factory and adding to that by positioning the gear off zero can add to that.

15 -> 11... that vacuum sucks... errr... or maybe not so much :wink:

Yeah, it would be very interesting to see how the timing chain is installed. Do you know of any way of deducing that without pulling the timing cover? Would finding TDC with a piston stop (hand rotating crank both clockwise and counterclockwise until stop and finding the midpoint) and then comparing TDC with the etched markings on a harmonic balancer - meaning TDC = 0* - imply that the timing chain was installed without a bias?

Aside: I found this to be a very good way of finding/confirming TDC on the compression (vs exhaust) stroke...
https://www.youtube.com/watch?v=u2sA-q7_FtQ