I understand that the engine needs to be under a load in order for the secondaries to open, so my question is...for tuning purposes, is there a point in the power band or RPM range where they ought to start opening? How do you determine the sweet spot? Tuning by SOP? Currently, I dont believe mine are opening at all.

Just about 100% street driven with occasional blasts on the throttle.

454, Voodoo 60201, Holley 770 SA

Here's the instruction sheet for the springs:

http://www.holley.com/data/Products/Technical/199R8219-2.pdf

I usually use either of the yellow springs from the kit for my 454

Thanks guys...that is a really good reference. My carb came with a spring kit, but I dont recall seeing that instruction sheet.

Any experience to share or do the instructions seem spot on?

This paragraph seems to sum it up best.

If there are no flat spots or stumbles, a lighter secondary spring should be installed. Try the accelerations again and if there are still no flat spots, try a lighter spring yet. When a flat spot is felt, go back up to the next stiffer spring. If the carburetor bogs, out of the box, note when it bogs. If it is an instantaneous off idle bog, the accelerator pump may be the real problem. A secondary bog will usually occur in the 2000 to 3000 rpm range. If this is the case, install a heavier diaphragm spring.

You can see one of the problems with a vacuum secondary setup, the opening range is real wide. The lightest spring won't have the throttles wide open until about 5700 rpm in a 350. We've seen 20 hp gains by manually forcing the secondaries open at about 4500 rpm.

Put a paper clip on the shaft up against the vac secondary bowl. Take it for a spin. If secondaries are opening the clip will have moved down the rod.

I am hoping that I will feel a bit of a difference when I get my set-up working properly. If I remember right, the carb comes with a purple spring installed which may be slightly stiff.
One other quick question. I should probably research this (more), but I'll ask anyway for your opinion, what exactly makes the secondaries open and why do you need a load on the engine? Is it a LOW and SUSTAINED vacuum signal?

What confuses me is this WOT: Low RPM=Low VAC....WOT:High RPM=High Vac.....Idle: High Vac

What I get from that is that it is neither JUST a high or low vacuum signal that activates the secondaries but a combination of something?

Put a paper clip on the shaft up against the vac secondary bowl. Take it for a spin. If secondaries are opening the clip will have moved down the rod.

Gonna try that tonight. The last time I jerry rigged a tie-wrap on another part of the linkage. It worked in the garage when I held WOT and moved the secondary linkage. Test drove the car, no indication of them moving.

I think what you are saying will yield better (accurate) results though.

I am just pumped that I might have some more power on the table!!! Sweet!

One other quick question. I should probably research this (more), but I'll ask anyway for your opinion, what exactly makes the secondaries open and why do you need a load on the engine? Is it a LOW and SUSTAINED vacuum signal?

What confuses me is this WOT: Low RPM=Low VAC....WOT:High RPM=High Vac.....Idle: High Vac

What I get from that is that it is neither JUST a high or low vacuum signal that activates the secondaries but a combination of something?

The vacuum to operate the secondaries is developed in the venturi of the primaries. As the air flow through the primaries increases, the vacuum in the venturi also increases. Here's Holley's tech info on how they operate.

http://www.holley.com/data/TechService/Technical/Carburetor%20Tech%20Info.pdf

There won't be high vacuum at any time the throttle is wide open.
Mike

well I took the bisc for a test drive and it looks like my secondaries are opening maybe 1/3 to 1/2 of the way tops. That is under heavy acceleration, 1-2 shift @ about 5000rpm. I am going to try a lighter spring tomorrow (the wif tagged me for chores). Hard to find a decent place to open her up around here on a friday night.

Unrelated problem, I did notice an audible squeak from the tranny the first time I got on it. Gonna post another thread about that. It sounds like something is slipping? High pitched squeal sort of. Can a TH400 make a sound like that? (it was not my tires shredding the asphalt).

I would just keep going lighter till you feel a hesitation or bog, then go back up one. If you dont feel any difference at all, put the stock one back in. I've messed around with all off them, didnt feel much difference (maybe its a stick car thing) and put the stock silver one back in mine. I dont think your car will like to heavy of a spring with its weight. Case in point: My friend has a good running 71 455/M20 Buick with an 870 Avenger. We tried a 750 DP and the car ran much better with the larger vac sec! Now, this may be a function of the 3.23 rear, but heavy under-geared cars love vac secs!!

BTW - 770's come with the silver spring. Next lighter is purple, then yellow. My car bogged at the track when dead-hooked with the yellow.

65 you might want to check to see that there is no binding in the secondary throttle plate. What do you have for a base gasket? It would be a good idea to confirm that while in the car with the pedal to the floor that you are reaching WOT with the primaries. If you find nothing in these checks I would remove the vacuum secondary housing and replace the gasket that seals the housing to the throttle body. If none of this helps I would replace the diaphragm.
Mike

Your vac secondary holley will never open all the way no matter what you do. Chevy high performance mag tested many vac secondary carbs and found they wouldnt open any more than a little over half way. They suggested going to mechanical secondary carb for a full opening. I changed to a 650 double pumper and saw a huge difference over my 750 vac carb. FWIW Steve

Your vac secondary holley will never open all the way no matter what you do. Chevy high performance mag tested many vac secondary carbs and found they wouldnt open any more than a little over half way. They suggested going to mechanical secondary carb for a full opening. I changed to a 650 double pumper and saw a huge difference over my 750 vac carb. FWIW Steve
Do you remember when this magazine came out? Thanks
Mike

How would they explain the paper clip moving to the full open position on the vac. diaphragm rod after I put my foot down?
T.C.

There won't be high vacuum at any time the throttle is wide open.
Mike

Yes, there will be (in the secondary system). The vacuum used to open the secondaries is obtained from the primary venturis, not manifold vacuum. As velocity and volume passing in the primaries increases, so does the vacuum signal for opening the secondaries.

Yes Rich I understand that and actually the vacuum used to open the secondaries on a Holley comes from the primary venturi initially and then additional vacuum is applied from a port in the secondary venturi. I misread his question and thought he was speaking of manifold vacuum. Thanks for clearing it up.
Mike

65 you might want to check to see that there is no binding in the secondary throttle plate. What do you have for a base gasket? It would be a good idea to confirm that while in the car with the pedal to the floor that you are reaching WOT with the primaries. If you find nothing in these checks I would remove the vacuum secondary housing and replace the gasket that seals the housing to the throttle body. If none of this helps I would replace the diaphragm.
Mike

Yes, no binding anywhere in the linkage and I have full range of movement. I think I will try the springs first. I dont believe I have a vacuum issues. Everything else is normal, good vac readings, good idle, all new parts etc....

Yes, there will be (in the secondary system). The vacuum used to open the secondaries is obtained from the primary venturis, not manifold vacuum. As velocity and volume passing in the primaries increases, so does the vacuum signal for opening the secondaries.

So secondary operation is directly affected by the rpm...as it (RPM) increases, under a load, the secondary opens more (wider) with the corresponding increase in vacuum (through the sec vent) due to the increasing RPMS ... ??

65 yes as the velocity of the air passing thru the primary and then the secondary venturies increases a greater vacuum signal is available at the vacuum diaphragm to pull the secondaries open. This is also why a smaller carb will open the secondaries sooner-higher velocity thru the venturies.
Mike

Another factor is that as the secondaries open, they reduce the air flow through the primaries. If you watch a vacuum secondary carb under load, as soon as the primaries open the secondaries will go about 25% open then cut back to less than half that before continuing to open. We've gained about 15-20rwhp at the top end on an average 350" motor by forcing the secondaries open at about 4500 rpm.

onovakind67 have you guy's done any work to try and quicken the opening rate of the secondaries other than springs? Thanks
Mike

Seems to me that dyno's mess with the way a vac sec operates (doesn load the motor the same as actually driving). Lot of guys here ET'd and MPH'd the same when running vac secs vs. DP's.

onovakind67 have you guy's done any work to try and quicken the opening rate of the secondaries other than springs? Thanks
Mike

No. We just pointed out that the secondaries aren't opening all the way. If you open them too quick you get a big lean spot on the bottom, too slow and you lost power at the top end. There's a fixed number of springs you can use.

Seems to me that dyno's mess with the way a vac sec operates (doesn load the motor the same as actually driving). Lot of guys here ET'd and MPH'd the same when running vac secs vs. DP's.

How would a dyno load vary from a road load with respect to the air flowing through the venturis of the carb?

There was a thread or 2 ago where some dyno operators noted that the "way" the a dyno loads a motor is different, resulting in secondaries not opening or delayed opening (for vac secs).

Just what I read.



No. We just pointed out that the secondaries aren't opening all the way. If you open them too quick you get a big lean spot on the bottom, too slow and you lost power at the top end. There's a fixed number of springs you can use.



How would a dyno load vary from a road load with respect to the air flowing through the venturis of the carb?

It was touched on in a reply above, but worth mentioning again. The vacuum associated with the secondaries opening has nothing to do with engine vacuum. Two completely different occurrences.

There was a thread or 2 ago where some dyno operators noted that the "way" the a dyno loads a motor is different, resulting in secondaries not opening or delayed opening (for vac secs).

Just what I read.

Was it this?

An easier experiment would be just to manually open the secondaries on the vac sec and see what happens.

But, the vac sec carbs and DP carbs are very similar in design. The 850 vac sec I have looks just like the DP version - down leg boosters and all.

Maybe the dyno doesn't load the motor like a 3800 lb Chevelle does accelerating.

Anybody see big differences at the track?

I read somewhere that one guy here on the dyno manually opened the secondaries on a vac sec and picked up 40HP...not sure if this was peak HP or midrange though.

No doubt about the differences in calibration...I confirmed it with my wideband.

But - the jetting is close between these carbs, at least the ones I screwed around with.

http://www.chevelles.com/forums/showthread.php?t=165072

Percisey!

Here's the quote from Gary Anderson that makes sense to me:

My engine builder tried to explain this to me once when my VS carb was some 30 hp short of his dyno mule DP. He said it has to do with the way the dyno loads the engine. In order to let the engine accelerate the dyno actually takes load off the engine. That is, the dyno loads the engine to pull the rpm's down and gradually lessens the load on the engine to let it rev up. So if you start your pull at 3000 rpm's the dyno loads the engine to pull the rpm's down to 3000. Then it lessens the load to let it rev to 3500 and then lessens it some more to let it rev to 4000 and so on. It is this lessening of the load on the engine that screws with a VS carb. It sees less load and therefore less airflow and in turn will close the secondaries to some extent. If you would hold the engine at a constant rpm they should be very close to the same. However most dyno tests are acceleration tests, usually 300 rpm/sec. or 600 rpm/sec. and that is where the issues arise. Chassis dynos don't operate that way as the load (the rollers) remains the same and should in theory provide a more realistic test between the two. Make any sense?

Gary Anderson