Want to convert L72 from ported to full time Advance

[Louis C.]

I have been searching and can't find thread except on the L71. Please help Duke.

[Timothy B.]

Re: Want to convert L72 from ported to full time Advance

Louis,

Your base probably looks like this Holley part so if you block the down hole shown with the top drill bit and connect the passage to the center vacuum cut out you will have full time manifold vacuum at that port and no one will know the better. Easy to do and easy to reverse if you are a purist.

[Louis C.]

Re: Want to convert L72 from ported to full time Advance

Louis,

Your base probably looks like this Holley part so if you block the down hole shown with the top drill bit and connect the passage to the center vacuum cut out you will have full time manifold vacuum at that port and no one will know the better. Easy to do and easy to reverse if you are a purist.

I don't have the carb off the car at this point to reference the base. That said: Is there a preferred method to block the down passage and how do you connect to the center passage?

[Joe R.]

Re: Want to convert L72 from ported to full time Advance

Tim,

Thanks for posting. You have given the best explanation of how to do this that I have seen!

Joe

[Duke W.]

Re: Want to convert L72 from ported to full time Advance

The quickest/easiest way to convert a L-72 (or any other engine with an external choke vacuum break including L-71) from ported to full time is to tee into the choke vacuum break source. Buy a 1/8" tee and a foot or two or 1/8" vacuum hose.

Remove the choke vacuum break hose and make a new harness using the tee to make a connection to both the choke vacuum break diaphragm and the VAC.

If the carburetor has more than one full manifold vacuum source you can use whichever is most convenient, but any carburetor with an external choke vacuum break should be a source of full time manifold vacuum.

If you still have the OE 360-12 VAC and it is functional, it is okay to use. If not, use a replacement B20 or B26.

The above is a visible modification, but it can be made invisible by modifying the carburetor, which usually requires plugging one small hole and drilling another. I can't give step by step instructions for all carburetors, but a visual inspection and some flow testing of the various passages should point to way.

Try the quick and easy way first, and if you like the result you can modify the carburetor later to hide the change if you choose.

When you first start the engine with full time vacuum advance the normal hot idle will be several hundred revs higher, so after you make the change go through the idle speed/mixture adjustment procedure. I recommend 900 idle speed for SHP big blocks. That should yield about 14" Hg manifold vacuum, so a 12" VAC meets the Two-Inch Rule.

You should notice a smoother idle, lower opearting temperature in around town driving in warm to hot weather, and lower fuel consumption in around town driving.

Duke

[Louis C.]

Re: Want to convert L72 from ported to full time Advance

Using a Mightvac I found my 360 can required 8" to begin to move and 14" to move to max travel. My full port has 8" at idle. So I will not achieve full advance with this can at idle?

[Duke W.]

Re: Want to convert L72 from ported to full time Advance

You need a 8" B28 VAC, but even that is marginal. Once you get a B28 installed, the increase in total idle spark advance should increase vacuum, but you should pick an idle speed that increases vacuum to two inches above what the installed B28 requires to pull to full advance.

Since your OE 360 VAC needs 8" to start advancing your 8" of idle vacuum will not even start to pull it, so converting to full time advance won't have any effect with the 360.

BTW, idle vacuum should always be accompanied by idle speed. Vacuum increases with idle speed, so you can get any vacuum reading you want simply be changing idle speed.

The OE SHP big block cam makes about 14" at 900. Your cam has way more overlap, which kills low end torque and fuel economy, but you can get the most out of it with a lot of spark advance. Total idle advance should be at least 25 deg., get the 30 centrifugal in as soon as possible and run 8-12 initial - as much as it will tolerate without detonation. You should first map out the advance to see what you've got, then you can determine what you need to do to achieve the above recommendations.

Duke

[John L.]

Re: Want to convert L72 from ported to full time Advance

I converted my L72 from ported to full manifold vacuum by removing the rectangular shaped plug located on the rear top side of the intake manifold and teeing into this source.

[Louis C.]

Re: Want to convert L72 from ported to full time Advance

You need a 8" B28 VAC, but even that is marginal. Once you get a B28 installed, the increase in total idle spark advance should increase vacuum, but you should pick an idle speed that increases vacuum to two inches above what the installed B28 requires to pull to full advance.

Since your OE 360 VAC needs 8" to start advancing your 8" of idle vacuum will not even start to pull it, so converting to full time advance won't have any effect with the 360.

BTW, idle vacuum should always be accompanied by idle speed. Vacuum increases with idle speed, so you can get any vacuum reading you want simply be changing idle speed.

The OE SHP big block cam makes about 14" at 900. Your cam has way more overlap, which kills low end torque and fuel economy, but you can get the most out of it with a lot of spark advance. Total idle advance should be at least 25 deg., get the 30 centrifugal in as soon as possible and run 8-12 initial - as much as it will tolerate without detonation. You should first map out the advance to see what you've got, then you can determine what you need to do to achieve the above recommendations.

Duke

I ordered a VC1810 now called a DV1810 from AutoZone ($12.) Once I get it I trust I should proceed by checking how much vacuum it takes to fully deploy the can (hopefully 8") and then adjust my idle speed accordingly so idle vacuum is 2" more and can is fully deployed with full port vacuum.

I have to admit I am a bit confused moving on from there. You indicate "Total idle advance should be at least 25 deg., get the 30 centrifugal in as soon as possible and run 8-12 initial." Is the 25 deg measured at idle with fully deployed can? Do I need to measure my centrifugal advance to determine how much I have and seek 30 total? And you say run 8-12 initial timing; check initial timing by plugging vacuum line?

[Michael C.]

Re: Want to convert L72 from ported to full time Advance

At first I did what Duke recommended and tied into the choke hose from under the front of the center carb. (Full vacuum) The car idled and ran so much better after that. I am having my car flight judged and was not happy with non factory appearance of the tee connection. I didn't want the lose points for that. I looked into the modification needed to switch the correct nipple over to full vacuum. This is on the forums. You just need to move the hole that feeds the side port to below the throttle blades where you have full vacuum at idle.
i removed the center carb and then removed the base plate. Looking closely at the side port you can trace its route thru the plate and see that it ends in a small hole above the throttle blade. This will not get full vacuum at idle. You have to drill a small hole in the baseplate at an angle that will exit below the plate. See the image attached that has been posted on here many times before. It shows the angle of the new hole. I used a dab of JB Weld to plug the stock hole above the plate that is not needed now. Note this can be reversed if needed.

[Duke W.]

Re: Want to convert L72 from ported to full time Advance

I have to admit I am a bit confused moving on from there. You indicate "Total idle advance should be at least 25 deg., get the 30 centrifugal in as soon as possible and run 8-12 initial." Is the 25 deg measured at idle with fully deployed can? Do I need to measure my centrifugal advance to determine how much I have and seek 30 total? And you say run 8-12 initial timing; check initial timing by plugging vacuum line?

All correct. Total idle spark advance with full time vacuum advance is initial plus full vacuum if the VAC meets the Two-Inch Rule and the centrifugal starts above idle speed.

Total WOT spark advance is the sum of initial plus full centrifugal. The OE spec for your engine is 30 @ 5000, which is very lazy, but you don't know if it's been modfied unless you test with a dial back timing light.

Initial timing is always set with VAC disabled, and it must be set at an engine speed lower, than where the centrifugal advance starts. Alternatively the total WOT advance can be set with a dial back light at a speed greater than full deployment of the centrifugal advance. On L-72 with the production spark advance map this would require at least 5000 revs, but if you have light springs that brings the advance in at 2500-3000 it you don't have to rev it so high.

Your low compression, high overlap cam engine needs lots of advance under most operating conditions. With initial at 8-12 total , total WOT advance will be in the range of 38-42, and it may operate detonation free on regular unleaded.

Download the following pdf for more explanation.

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Duke

[Louis C.]

Re: Want to convert L72 from ported to full time Advance

All understood Thank you. Will keep you posted..

[Louis C.]

Re: Want to convert L72 from ported to full time Advance

All correct. Total idle spark advance with full time vacuum advance is initial plus full vacuum if the VAC meets the Two-Inch Rule and the centrifugal starts above idle speed.

Total WOT spark advance is the sum of initial plus full centrifugal. The OE spec for your engine is 30 @ 5000, which is very lazy, but you don't know if it's been modfied unless you test with a dial back timing light.

Initial timing is always set with VAC disabled, and it must be set at an engine speed lower, than where the centrifugal advance starts. Alternatively the total WOT advance can be set with a dial back light at a speed greater than full deployment of the centrifugal advance. On L-72 with the production spark advance map this would require at least 5000 revs, but if you have light springs that brings the advance in at 2500-3000 it you don't have to rev it so high.

Your low compression, high overlap cam engine needs lots of advance under most operating conditions. With initial at 8-12 total , total WOT advance will be in the range of 38-42, and it may operate detonation free on regular unleaded.

Download the following pdf for more explanation.

http://stlouisncrs.org/news_files/DukeWilliamsEngineTuningPerformance&EconomySeminar.pdf

Duke

I bought the B28 VAC and checked it with the Mightyvac. It is within spec; fully deployed at 8". Sure wish it was 6". I had to raise the idle up to 900 RPM's to keep it fully deployed. And that was just a test as I used a tee to tap into the chock cut off and hung it next to the carb. So with the idle at 900 I installed the B28. Now that it is in, I have 40 degrees of advance at idle (with vacuum line attached and my timing is jumping around and I have popping in the exhaust at 900. Looks like I don't have enough vacuum to keep it fully deployed (timing jumping around), and if I raise the idle to get more vacuum centrifugal advance kicks in just off idle. My total advance is 59! I tried to turn the distributor to reduce the timing at idle but found little effect and it was jumping around.

So, I disconnected and plugged the vacuum line. I have 18 degrees at idle and 38 total.

What now? I don't know why I have so much total advance (well I guess it is the two added together) but want to reduce it to 38 total. I was hoping that by converting to full time advance and bringing in all the vacuum advance at idle I would be left with just the additional 20 of centrifugal advance and I would be a step closer in resolving my running hot issue? I think this big cam thing is killing me.

[Duke W.]

Re: Want to convert L72 from ported to full time Advance

The OE centrifugal advance is start @ 900, 30 @ 5000. So either it's been modified or you are not revving the engine high enough to determine the full amount and at what RPM it achieves maximum advance.

A high overlap cam has to idle fairly high to achieve a stable idle. I recommend a 900 hot idle speed for the moderate overlap OE cam and manifold vacuum is 14" @ 900 with full time vacuum advance.

You have to accept an idle speed of whatever it takes to pull at least 9-10". With 30 centrifugal, 8 initial, and 16 vacuum, total idle timing should be 24, plus maybe a few extra if the centrifugal starts below idle speed.

It sounds like bubba reworked the centrifugal advance in addition to installing a bubba cam.

It's not clear to me if you are running proper tests, which is why I suggested in my email that you call me.

One thing I want you to do is with full time vacuum advance set the idle speed to achieve at least 9-10", then check total idle timing. That would be with the VAC connected.

Then disconnect the VAC and determine at what RPM the centrifugal starts and at what RPM it is all in and how much.

Also, please download and read the presentation I linked to in an earlier post. We cannot have a meaningful discussion unless you fully understand terminology including "total idle spark advance", "total WOT spark advance", and "total cruise spark advance". These concepts are defined the presentation.

Duke

[Robert M.]

Re: Want to convert L72 from ported to full time Advance

Iv'e been reading posts including this one on converting to manifold vacuum and the recommendations on setting the initial,mechanical timing along with the 2" rule for the vac can. I'm in the process of getting all of the components that were recommended to use but all of the posts have been for pre 68 engines, I have a 1968 L-71 equipped with the pollution control and pump. Do I still tune using the B-26 vacuum can,38 Degrees total timing and all in by 2500/2800 rpm?
Thanks, Bob