Recurve a Distributor - Who Does it?

[datzenmike]

Basically ported vacuum is a way of removing timing at idle but giving it back when you step on the gas. A weakly filled cylinder (like at idle) needs more advance. The problem is engines do a lot of idling so to reduce emissions idle timing is retarded in this way. The idle quality suffers, so the idle speed and mixture are turned up so you don't notice it as much. Take any idling motor and advance the dizzy by turning CW.... it will immediately rev up because it is being more efficient. Motors WANT more advance at idle!!!! They will be making more power with less gas, run smoother and cooler.

 

Do not just connect your vacuum advance to the manifold without adjusting the timing. It may ping if you do. Don't forget, everything is set for zero advance at idle and you are changing it.

[tmooretxk]

I think vacuum advance was always ported UNTIL emissions controls, when they went to manifold vacuum so the advance would drop out at wot, reducing pinging on lower octane gas. Always got confusing when swapping chevy small blocks from different eras.

[69srl311]

I have a crank fired ignition and I added a map sensor to advance the timing at part throttle thinking I might optimize fuel economy. But, with my aggressive cam and poor driving habits it did not improve the mileage. With this setup the best mpg I got was 25 in the winter, that was before I changed the diff from a 3.70 to a 4.88 ratio. In my last rehab, I removed the map sensor to clean up the engine bay.

 

[EricJB]

Holy schnikey thats a big step down in gear ratio. I'm guessing you auto X. The 3.90's died in my 411sss and all I had was 4.38's. That was noticable. My 521 has 4.88's and it's like a tractor in 1st.

[datzenmike]

I think vacuum advance was always ported UNTIL emissions controls, when they went to manifold vacuum so the advance would drop out at wot, reducing pinging on lower octane gas. Always got confusing when swapping chevy small blocks from different eras.

 

Other way round.

 

Before the '70s vacuum advance was directly from the intake. It was shown that retarded idle timing reduced emissions, so it was moved to a port on the carb that was just above the throttle plate. When idling the port read normal atmospheric pressure. But as soon as you step on the gas the throttle plate moves up onto and above the port and intake vacuum is applied to the hose going to the distributor and it jumps to the 14-16 degrees advance from the 12 degrees at idle.

 

As the plate rises higher, the vacuum becomes weaker and weaker, with full throttle almost zero. No vacuum advance is needed above 2,500-3,000 RPMs anyway.

 

This was added for emissions control only and serves no other purpose. Indeed the engine runs stronger, more efficient and cooler with the proper 14-16 degrees advance. You only have to turn your distributors from the 12 degrees stetting slightly advance and you will notice that the idle improves and the motor revs up.... this is where it wants to run. The idea of the retarded advance at idle is the reduction of NOX but more heat energy is wasted and goes out the exhaust port and absorbed into the cooling system. As stated the motor will run cooler and stronger on less gas if the timing was advanced at idle.

 

No you can't just turn the idle advance up as this will affect the whole range. You could connect to manifold advance but the distributor is designed to advance properly with ported vacuum. You would have to modify the advance inside.

[ggzilla]

There is not much vacuum with an aggressive cam, therefore MAP or vacuum advance does not work correctly. With aggressive cam you should use TPS instead of MAP. Or just skip it and run no vacuum advance.

 

I added a map sensor to advance the timing at part throttle ... But, with my aggressive cam and poor driving habits it did not improve

[pl521sss]

I meant to ask question via PM

Don't know how to delete post

[TFM1066]

1. 
   
   1. 
      
   2. "Before the '70s vacuum advance was directly from the intake". This is not a true statement, Cars and trucks from the late 30s- early 40s and so on all had ported vacuum. Mike not to put you down or anything like that but I could show you all my spec sheets from my SUN distributer machine and they all have specs for static timing and full advance with vacuum.
      
   3. 
      
   4. Advance is greatly needed at high RPMs, due to the fact that "it takes time for the full flame to take place" piston feet per minute and flame growth must be matched, that why we need @ 34-42 degrees of advance above @ 3000 rpm
       
      

 

[Rays74]

1. 
   
   1. 
      
   2. "Before the '70s vacuum advance was directly from the intake". This is not a true statement, Cars and trucks from the late 30s- early 40s and so on all had ported vacuum. Mike not to put you down or anything like that but I could show you all my spec sheets from my SUN distributer machine and they all have specs for static timing and full advance with vacuum.
      
      
   3. 
      
   4. Advance is greatly needed at high RPMs, due to the fact that "it takes time for the full flame to take place" piston feet per minute and flame growth must be matched, that why we need @ 34-42 degrees of advance above @ 3000 rpm
      

 

 

Not sure what cars exactly you are describing, TFM, but none of the 50s or early 60s GM and Mopar's I have worked on had ported vac........for that matter their carbs don't have the right design fittings for ported vac. The statement "all had ported vacuum" is not quite true. Some may have, most not, but it did not come into full use before the emissions era.

I would imagine earlier 30-40's cars were no different, seeing as ported vac is emissions driven and the emissions question had not been properly addressed until the mid-late 60's.

 

From Datsun Mike's quote above "No vacuum advance is needed above 2,500-3,000 RPMs anyway."

 

The advance that Mike is describing is Vacuum Advance ...........you don't want the extra (vac) advance since it would add some 10-20 degrees bringing the total over the normal Static/Mechanical numbers you mentioned.......over 50 degrees possible, which is way too much. You are correct that all the static/mech advance should be in by 2500-3000......specially with our smaller engines, makes them feel stronger around town :)

[datzenmike]

Perhaps there was a port on the side of the 30-40 carbs for a vacuum supply but I still say it was manifold vacuum even if not directly connected to the manifold itself and was present at ALL times. (well except WOT)

 

The ported vacuum of the late 60s and on, was a small hole in the throttle just above the throttle plate. When closed at idle, it basically read atmospheric pressure (or zero). As the throttle plate opens and moves up it exposes the port to intake vacuum. Zero vacuum at idle basically retarded the timing from it's ideal of 16 or more degrees. The rule is that the less the cylinder is filled with gas and air the longer it takes to burn so more advance is required. Retard at idle reduced emissions but caused a poor idle and ran hot because late burning exhaust heat was absorbed by the exhaust port walls.

 

If you don't think that weak cylinder filling needs more advance mark your timing on your distributor (so you can return it) loosen the hold down screw and slowly twist it clock wise to advance the timing. What happens??? Well the idle will increase and smooth out because it is now getting the proper advance it desires. More energy is being captured and used to spin the motor so it revs up.

[ggzilla]

No engine needs vacuum advance. Didnt we already cover that?

 

It helps the engine make more power but is not required.

[datzenmike]

It's essential for part throttle between idle and about 2,500. The rule is: the less gas and air in the cylinder the more advance needed. Weak cylinder filling (part throttle) takes longer to burn completely and build pressure. In order for maximum pressure to bear down on the piston after TDC, the fire must be lit earlier. Now this only applies to part filling or part throttle.

 

I think all carbed motors since 1900 had vacuum advance. It improves power and economy. It will run without but better with.

[ggzilla]

Essential?

 

Datsuns run pretty good with no vacuum advance...you should try it so that youll know. Better to have the extra power and fuel efficiency but not essential.

[Stoffregen Motorsports]

It's essential for part throttle between idle and about 2,500. The rule is: the less gas and air in the cylinder the more advance needed. Weak cylinder filling (part throttle) takes longer to burn completely and build pressure. In order for maximum pressure to bear down on the piston after TDC, the fire must be lit earlier. Now this only applies to part filling or part throttle.

 

I think all carbed motors since 1900 had vacuum advance. It improves power and economy. It will run without but better with.

 

When you decommission a vacuum advance unit, you should make adjustments to the centrifugal weight springs so you get that timing advance mechanically. Also, by shortening the distributor cam throw, you shorten up the timing curve, allowing you to run more initial timing while keeping the total timing the same. You DO NOT want the vacuum advance unit to be the limiting factor to your total timing. This can lead to catostrophic engine failure.

 

I don't agree with a lot of members here suggesting that you should just remove the hose and plug it, that will not achieve anything at all, but less timing mid curve.

 

There is a give and take to the debate: with vacuum advance = better fuel economy. Without vacuum advance(and properly modified) = more HP potential.

[datzenmike]

Again vacuum advance is for part throttle and cruise, so not a lot of HP noticed but it drives better and gives good mileage. There is little to no vacuum advance a full throttle from idle and up. This is the realm of mechanical centrifugal advance combined with static advance and these can be adjusted for performance.

[ggzilla]

agreed. It doesnt limit or replace the mechanical advance.

[Stoffregen Motorsports]

agreed. It doesnt limit or replace the mechanical advance.

 

No it doesn't. What I'm saying is that you cannot simply unplug the vacuum line and call it good. A valid comparison between vacuum advance and no vacuum advance would be if they were both set up optimally. A vacuum advance distributor that has been unplugged is not optimal without some adjustments and modifications.

 

If they were set up to be their best, only then can a real comparison between the two be made.

[72240z]

Glad I caught this thread now towards the end when sense started being made lol. The misinformation in the 1st page is just ridiculous. I agree with most of what ggzilla and stoffregen have said.

 

I have been running a vac off advance off the manifold (oh my!) for quite some time now with no issues anywhere in the rpm range. Though I cant speak to mileage much because I'm running way oversized idle jets to compensate for webers short comings in the accelerator pump area.

 

The biggest thing I would want to change is re-curving to limit the max vac advance so I could maybe run a little more initial advance with out being too far gone at max. ATM I believe its giving me a little hesitation up high int he range but I plan to go efi (itb+coil packs) this spring so I'm just leaving it for now.

 

For a steady vac signal I fabed a simple vac mani using the ports on the mikuni. Brake booster on one ends and dizzy advance on the other.

[banzai510(hainz)]

my DUAL Buy mikunis 40s has a port on 1 Carb to route to the dizzy.

My Mikinis 44s do not so I dont run a vac advance on my yellow car.

 

webers dont have a port also. Believe the dizzy needs to be recurved. and on the 44/50mm mikunis

 

taking vac adv from the manifold and esp from 1 port is got going to average it out on the other cylinders. Im not a expert on this but this is my conclusion. But soemhow on my 40s 1 carb it must be such a mynute amount of vac its pulling as it would be the same averging affect . I say fuck it dont run it

[Stoffregen Motorsports]

Glad I caught this thread now towards the end when sense started being made lol. The misinformation in the 1st page is just ridiculous. I agree with most of what ggzilla and stoffregen have said.

 

I have been running a vac off advance off the manifold (oh my!) for quite some time now with no issues anywhere in the rpm range. Though I cant speak to mileage much because I'm running way oversized idle jets to compensate for webers short comings in the accelerator pump area.

 

The biggest thing I would want to change is re-curving to limit the max vac advance so I could maybe run a little more initial advance with out being too far gone at max. ATM I believe its giving me a little hesitation up high int he range but I plan to go efi (itb+coil packs) this spring so I'm just leaving it for now.

 

For a steady vac signal I fabed a simple vac mani using the ports on the mikuni. Brake booster on one ends and dizzy advance on the other.

 

That is a clean looking setup.

[72240z]

Hey thanks, means a lot having seen yours. Its a comparison pic to show the clearance issues I was having with the enrichment circuits using the mikuni mani/weber carb combo. Spacer/no linkage, spacer with linkage that clears the center and no spacer/no linkage. I just cut the suds off the linkage and used tack of JB. Then it worked out ok.

[TFM1066]

If you want or need ported vacuum from your webers (or any carb) drill a hole threw the base just prior to and/or slightly on the front of the leading edge of the throttle plate 3/16th dia, and use some brake line for the nipple. I've done this to many times on street cars running side drafts, if by chance you mess up you can always epoxy the hole.

[72240z]

yikes :-X