Holley Vacuum Secondaries opening

Hi,

today I've checked if the vacuum secodaries of my Holley 600 cfm 1460 do open. What I found is that during a test run in fourth gear up to 4000 rpm (~90 mph), they opened half way.

Is this what I should expect from a correctly tuned carburetor or should I go for a lighter spring (higher opening rate)?

My engine is a basically stock 351C-4V with a 4 speed toploader.

Thank you, Manfred.

how did you determine they are opening half way?

Did you ever get the motor to it's max RPM?

how did you determine they are opening half way?

Did you ever get the motor to it's max RPM?

Click to expand...

I put a paper clip on the shaft coming out of the vacuum secondary housing up against the housing. The clip was moved downwards on the shaft when the secondaries partly opened. By observing the position of the clip after driving I could determine how far they opened.

And no, I didn't dare accellerating it to may rpm in 4th gear on that road ;-) Only up to 4000 rpm.

regards, Manfred

Ok, that's good way to confirm how far they open.

What about 1st, 2nd or 3rd, did you run it up to your highest RPM?

I know the first test is to lock the secondaries closed and go for a drive and see if its worse or better. if you feel the bog is worse then you know the secondaries are at least doing something.

but you said your secondaries are opening a little so first check the diaphragm and gaskets for damage and clean the vacuum ports.

make sure the mechanics of the secondaries can open fully and nothing is jamming them. they should have free motion to open all the way.

Keep in mind that they may not open if the carb is too large for the engine or if the spring is too stiff.

To increase the secondary opening speed install a lighter spring. the common spring kit (20-13) will include 7 springs the following list is from heavy to light.

Black,

Brown,

Plain,

purple,

yellow,

short yellow,

and white.

I recommend starting with the black spring and working your way down. When you can feel the secondary come in (kick) go back to the previous spring.

Granted i don't know what setup you have as far as CFM on the carb.

I know the first test is to lock the secondaries closed and go for a drive and see if its worse or better. if you feel the bog is worse then you know the secondaries are at least doing something.

but you said your secondaries are opening a little so first check the diaphragm and gaskets for damage and clean the vacuum ports.

make sure the mechanics of the secondaries can open fully and nothing is jamming them. they should have free motion to open all the way.

Keep in mind that they may not open if the carb is too large for the engine or if the spring is too stiff.

To increase the secondary opening speed install a lighter spring. the common spring kit (20-13) will include 7 springs the following list is from heavy to light.

Black,

Brown,

Plain,

purple,

yellow,

short yellow,

and white.

I recommend starting with the black spring and working your way down. When you can feel the secondary come in (kick) go back to the previous spring.

Granted i don't know what setup you have as far as CFM on the carb.

Click to expand...

I don't think he is chasing a bog. I think he is just trying to confirm the secondaries are fully opening under load.

However, I don't think the test the OP described applies sufficient load on the engine to result in full secondary opening.

The different springs control not only when but 'how fast'.

I don't understand your logic in picking a spring. If you started with the heaviest and went lighter until you 'feel' the secondaries open and then went back to the heavier, wouldn't you cause them to not open? I am not trying to flame or nit-pick, I just don't understand.

On my setup I can feel the secondaries 'kick in' as I go past the 2500 or so RPM range, I assumed I would want a lighter spring or one that kicks in earlier.

And I still would like OP to clarify if he has run it up to max RPM in 1st, 2nd or 3rd gear.

Ok, that's good way to confirm how far they open.

What about 1st, 2nd or 3rd, did you run it up to your highest RPM?

Click to expand...

Looks like reving it up all the way to 6000 rpm in the lower gears is something I will have to try today. It may be silly, but I hate the idea of reving it that high after all the work that has gone into it in the last years (I told you I'm silly ;-)

BTW: I couldn't find a redline value for the '71 351C-4V in my documentation. Would 6000 rpm be safe?

Regards, Manfred

The different springs control not only when but 'how fast'.

I don't understand your logic in picking a spring. If you started with the heaviest and went lighter until you 'feel' the secondaries open and then went back to the heavier, wouldn't you cause them to not open? I am not trying to flame or nit-pick, I just don't understand.

On my setup I can feel the secondaries 'kick in' as I go past the 2500 or so RPM range, I assumed I would want a lighter spring or one that kicks in earlier.

And I still would like OP to clarify if he has run it up to max RPM in 1st, 2nd or 3rd gear.

Click to expand...

Generally you don't want to "feel" the secondaries kick in. You want a seamless increase in power. The method 72HCODE suggests will determine the correct spring for that. The "kick" you feel from the secondaries is actually recovery from a hesitation which may be caused by using the incorrect spring and results in a slower car.

On my '71 quench head M-code the vacuum secondaries open around 2500 RPM, If Needed. There is no "kick" but you can feel and hear it. A NIB Holley should not need any adjustment assuming it is the correct Holley for the engine. I think the question is are the secondaries capable of opening all the way. You would need to ask Holley how to determine that. If they are not opening, As Needed, it could be an obstruction or vacuum leak. My carb is a Holley 670 CFM Street Avenger (4150 series). Holley was one of the four factory choices that came with all '71 Fords.

mike

Ok, that's good way to confirm how far they open.

What about 1st, 2nd or 3rd, did you run it up to your highest RPM?

Click to expand...

Looks like reving it up all the way to 6000 rpm in the lower gears is something I will have to try today. It may be silly, but I hate the idea of reving it that high after all the work that has gone into it in the last years (I told you I'm silly ;-)

BTW: I couldn't find a redline value for the '71 351C-4V in my documentation. Would 6000 rpm be safe?

Regards, Manfred

Click to expand...

I understand your not wanting to rev it. I was the same way. But once you feel comfortable that your motor will turn you will find yourself visiting that spot (and the gas station) more and more often....

The bottom end of a 4v motor (4 bolt main) will have no issues with 6000. Your limiting factor is going to be either cam (fall off in power due to grind) or valve spring float.

I usually run it up until it doesn't feel like it is pulling anymore then switch gears.

The different springs control not only when but 'how fast'.

I don't understand your logic in picking a spring. If you started with the heaviest and went lighter until you 'feel' the secondaries open and then went back to the heavier, wouldn't you cause them to not open? I am not trying to flame or nit-pick, I just don't understand.

On my setup I can feel the secondaries 'kick in' as I go past the 2500 or so RPM range, I assumed I would want a lighter spring or one that kicks in earlier.

And I still would like OP to clarify if he has run it up to max RPM in 1st, 2nd or 3rd gear.

Click to expand...

Generally you don't want to "feel" the secondaries kick in. You want a seamless increase in power. The method 72HCODE suggests will determine the correct spring for that. The "kick" you feel from the secondaries is actually recovery from a hesitation which may be caused by using the incorrect spring and results in a slower car.

Click to expand...

ok, so the lighter spring causes them to open sooner and what you feel is the engine catching up to the extra air flow? I am trying to figure out the mechanics of it.

Ok, that's good way to confirm how far they open.

What about 1st, 2nd or 3rd, did you run it up to your highest RPM?

Click to expand...

Looks like reving it up all the way to 6000 rpm in the lower gears is something I will have to try today. It may be silly, but I hate the idea of reving it that high after all the work that has gone into it in the last years (I told you I'm silly ;-)

BTW: I couldn't find a redline value for the '71 351C-4V in my documentation. Would 6000 rpm be safe?

Regards, Manfred

Click to expand...

I understand your not wanting to rev it. I was the same way. But once you feel comfortable that your motor will turn you will find yourself visiting that spot (and the gas station) more and more often....

The bottom end of a 4v motor (4 bolt main) will have no issues with 6000. Your limiting factor is going to be either cam (fall off in power due to grind) or valve spring float.

I usually run it up until it doesn't feel like it is pulling anymore then switch gears.

The different springs control not only when but 'how fast'.

I don't understand your logic in picking a spring. If you started with the heaviest and went lighter until you 'feel' the secondaries open and then went back to the heavier, wouldn't you cause them to not open? I am not trying to flame or nit-pick, I just don't understand.

On my setup I can feel the secondaries 'kick in' as I go past the 2500 or so RPM range, I assumed I would want a lighter spring or one that kicks in earlier.

And I still would like OP to clarify if he has run it up to max RPM in 1st, 2nd or 3rd gear.

Click to expand...

Generally you don't want to "feel" the secondaries kick in. You want a seamless increase in power. The method 72HCODE suggests will determine the correct spring for that. The "kick" you feel from the secondaries is actually recovery from a hesitation which may be caused by using the incorrect spring and results in a slower car.

Click to expand...

ok, so the lighter spring causes them to open sooner and what you feel is the engine catching up to the extra air flow? I am trying to figure out the mechanics of it.

Click to expand...

The secondary opening is caused by a vacuum created in the primary venturi under load. There is an orifice which goes to the secondary diaphragm which "sees" this vacuum. The vacuum overcomes the spring pressure which is holding the secondaries closed. The orifice in the primaries is assisted by another orifice in the secondary venturi. This second orifice becomes active when the secondaries begin to open creating vacuum in the secondary venturi. This additional vacuum adds to the vacuum being created in the primary venturi allowing the secondaries to open further. When sufficient load is placed on the engine enough vacuum will be created in the venturis to fully open the secondaries. This also causes fuel to flow from the mains. This is not the same as manifold vacuum. It is a vacuum which is created due to the special shape of the venturi and the resultant pressure drop from air flowing through it. If a lighter spring is used, there is less spring pressure for the vacuum to overcome thereby allowing the secondaries to open sooner (under lighter load conditions).

Goggle the Bernoulli principle for a better understanding of the physics at work.

ManniB, you could try taking the check ball out of the diaphram and see if it opens more fully. Some guys have better luck with no check ball, some don't.

ok, so if you can 'feel' them come in are they coming in too late or too early? I assumed too late (engine finally pulled enough vacuum)

Not necessarily too "late" or too "early". The boot in the seat feel is from the secondaries "crashing" in, as in snapping open too quickly and/or too much *at a given RPM*.

Basically, too much air is being "dumped" in when the secondary blades open, and the fuel in the secondaries can't compensate quick enough, so a slight "bog" is felt, even though it may not feel exactly like a fall-on-its-face bog.

This is why "double pumper" (manual secondary) carbs have accelerator pumps on both front/rear bowls, in order to compensate for said "air dump".

Holley doesn't recommend deleting the check ball, but yes, I have heard of many guys doing so. I would tend to believe Holley first, and do the progressively lighter spring till bog/go back one heavier technique.

Just my opinion.

Not necessarily too "late" or too "early". The boot in the seat feel is from the secondaries "crashing" in, as in snapping open too quickly and/or too much *at a given RPM*.

Basically, too much air is being "dumped" in when the secondary blades open, and the fuel in the secondaries can't compensate quick enough, so a slight "bog" is felt, even though it may not feel exactly like a fall-on-its-face bog.

This is why "double pumper" (manual secondary) carbs have accelerator pumps on both front/rear bowls, in order to compensate for said "air dump".

Holley doesn't recommend deleting the check ball, but yes, I have heard of many guys doing so. I would tend to believe Holley first, and do the progressively lighter spring till bog/go back one heavier technique.

Just my opinion.

Click to expand...

While on the subject of "bog" have you considered the accelerator pump? My car would bog at any RPM if I jammed the accelerator. Turned out to be improper clearance at the accelerator pump. It was 0.030 off. Reset to Holley spec 0.015 and no more bog. Just an idea.

mike

Hi,

finally, after replacing the fuel pump, I did another test run on the secondaries today (I had to replace the fuel pump because I cracked the housing while installing a fuel line to the carburetor instead of a hose :-(((

Anyway, this time I reved up the engine in 3rd gear up until 5600 rpm. At that point it started to hesitate, similar as if there was a rev limiter. Probably, the ignition setup is insufficient (Accel mechanical single point distributor and stock coil).

This time, the secodaries opened up 70%.

Now the question is: Is this what I should expect to happen or should they open 100%?

Reading through my Holley book, there are two factors that explain why they don't open fully:

1. On a 351 CID engine (mine is a 351C-4V), the standard spring fully opens only at 8160 RPM according to their data.

2. The requirement of a 351 engine at 5600 RPM and 85% volumetric efficiency is 510 CFM.

Would my setup benefit from going to a lighter secondary spring of should I leave it as it is?

Thany you, Manfred.

Hi,

finally, after replacing the fuel pump, I did another test run on the secondaries today (I had to replace the fuel pump because I cracked the housing while installing a fuel line to the carburetor instead of a hose :-(((

Anyway, this time I reved up the engine in 3rd gear up until 5600 rpm. At that point it started to hesitate, similar as if there was a rev limiter. Probably, the ignition setup is insufficient (Accel mechanical single point distributor and stock coil).

This time, the secodaries opened up 70%.

Now the question is: Is this what I should expect to happen or should they open 100%?

Reading through my Holley book, there are two factors that explain why they don't open fully:

1. On a 351 CID engine (mine is a 351C-4V), the standard spring fully opens only at 8160 RPM according to their data.

2. The requirement of a 351 engine at 5600 RPM and 85% volumetric efficiency is 510 CFM.

Would my setup benefit from going to a lighter secondary spring of should I leave it as it is?

Thany you, Manfred.

Click to expand...

You can try. The only way to know if it really helps is either a noticable improvement in the 'seat of the pants' or, better, take it to a track.

As stated above, if they open too early it may 'feel' like they are coming on (giving the impression of more power) but it may not actually be helping.

Any other mods to the engine? It won't turn much faster than 5600 with a stock cam.

Try using the next lightest spring and check again. When you have this issue resolved, start investigating why the engine won't rev past 5600 RPM. Completely stock engines will do that much before nosing over, not missing like a rev limiter engaging.

Chuck

I would keep trying a lighter spring until they open all the way some where below your max rpm.

IMHO a 600cfm on a 351c 4v is small(smaller than stock) and could be limiting your RPM. Cleveland motors especially with 4v heads like more carb than the formulas call for. The original Ford Muslce parts book said bolt on a 850cfm carb.

I run a 750 Holley on my 2V(stock heads)