Inliners International Forums General Discussion Hi Performance Holley 4 Vac Sec

I tried something interesting. I tied of the vacuum secondaries on my 390 Holley and went out for a spin. WOT spins and I can’t tell much difference having all 4 or just running on 2.

Would a double pumper with mechanical secondaries be better? It would get all 4 open quicker. Or maybe just a 500 CFM 2 barrel would be better. I realize that 235 CI doesn’t require much I the way of carb (CFM), but if it was bigger than needed wouldn’t the engine just pull what it could use?

Any thoughts?

Humm…

Luis

If you over carburete then the velocity of air through the venturiis slows down, thus, through Bernouille's principal, you will lose the vacuum signal that pulls fuel into the air stream. Shutting off the secondaries and not losing performance should tell you that the engine has a big enough carb with half of that 390 carb. Stay with vacuum secondaries. The 2ndaries will open only when engine airflow demand requires it.

Try this for a test of carb size, hook up a vacuum guage to the intake and using a length of hose, put the guage somewhere that you can see it safely while driving.For a 235, pick an RPM range of maybe 2000 to 4000 max for this test.Cruise along in an second gear, 2000 rpm, push the accelerator to the boards.At first the guage will drop to zero,then it may rise as the engine picks up RPM. As the engine nears 4000 rpm, if the vacuum reading is 3 inches or higher,the engine can use more carburation.Moderately modified 235/261's need only 300- 400 cfm, so you are close.

Thanks guys,

I understand that there must be flow in order for the venture effect to work. I just see engines with 3X2 and 2X4 setups and they seem to be “over carbureted” but seem to run OK.

Holley says:
How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency

SO, 235 CID X 4000 RPM / 3456 = 271.99 CFM

390 CFM CARB / 2 = 195 CFM

Using the above, I think that maybe something is not quit right with my set up. It’s got plenty of go, but if there is something else I can do I would like to do it. My 235 has a 272 In/Ext duration cam, headers free flow exhaust and stacked paper element air filters no serious head work and I think I would be able to get a bit better flow than stock.

Tony,
I do have the vac gauge and will try it. I just didn’t know how much vacuum should be there before a larger carb could be installed.

I found this at Holley. It gives a simple way to check and see how far the secondaries are opening. I am going to try this also.

QUESTION I can rev the engine when it is in park and the secondaries will not open. Why is this?
ANSWER The secondaries will not open by free revving the engine. The engine needs to be under a load before they will open. If you are still uncertain if they are opening or not you can take a normal paperclip and clip it onto the secondary diaphram rod. You will then push it up against the bottom of the secondary diaphram housing, now you will need to go out and drive the vehicle. When you return you will be able to look at the position of the paperclip on the rod. If it is lower on the rod then you can tell the secondaries opened and how far they opened. This is useful in determining if you need a heavier or lighter secondary spring.

235/261 engine head isn't a good design from the point of airflow.So the power is limited. A 390 Holley should be fine for your setup.A vacuum or airflow controlled secondaries can open smoothly without the sometimes bog and then rush of mechanical secondary 4 bbl carbs.So your carb may feel "slow" but is really getting the job done.The vacuum thing is easy, the guage needle will drop to zero when you floor the pedal and may slowly rise as the rpm's increase.Like the Holly thing, you can't do this test in the driveway.

What i have learned about most Holley Vac Sec. is they don't always work right out of the box. In some cases they need a secondary Spring change .in some cases the spring is to heavy, This maybe something you may need to try and do. Or and besides the paper clip trick You can also install a bolt next to the secondary rod to make it move like a Mechanical carb. Just to see if you even feel the secondarys kick in or not.This is just another way to tell if they work or not,Or a pour mans Mechanical carb. hope this helps. }[oooooo]


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NovaMan/Twisted6/Larry

Well I did the Holley thing and the secs do open all the way. I also do notice a good change in go with the all 4 and not just 2 tied off. At 3000 and up, 4000 max for me, is where I see the muscle.

Twisted,
I have changed the springs long ago to the light springs. I also have to say that Holley has an OK product out of the box but needs finessing.

Tony,
Haven't had time to do the vac thing yet.

Not sure what to think just yet.

Any thoughts is always good.

Thanks for your guys' help.

I'LL KEEP TRYING.

Luis

In your above formulae you used 100% volumetric efficiency. One hundred percent efficiency at the HP peak is very hard to achieve (in normally aspirated engines) in all but the most refined racing applications. I would be surprised if you were above 80% vol eff at 4000 RPM. Recalculate w/ %80.

OK,
How do you figure 80% vol eff? Do I just figure 80% of the total or do I need a different multiplier? I was never the greatest at algebra.

And is 80% fair? From what I read, the 235 head pretty much sucks. From looking at the cumbustion chambers and the ports, it makes you wonder what they engineers were thinking.

Thanks.

Luis

Hi guys,
I've been doing some tweaking with my set up too and wanted to let you know what I've found out. My set up consists of;

292-.040 over to 301 CID, 272 Cam, Stovebolt Headers, Petrox electronic Ignition, 3x2 Offy with Holley/Webers. The set up was done with straight linkage because the carbs are progressive. I found out that the secondaries on these carbs are very strong and big and made for a hard pedal. I loosened the spring pressure a little by adding a return spring that still provided a good safety margin but still had a stiff petal. At full throttle, the car actually bogged down and slowed. I do not have anything special head work that probably accounts for this. What are the CFM's on these carbs? I've heard 390-500cfm. Which is right? I believe they are 390's because 3x500's would be nuts on a street motor. I disconected the outer secondaries so they do not open but left the secondaries on the center carb. I found the performance smoother and seems to want to go to 7,000rpm with out any problems. I will not try that. It still has the same low end grunt that it had before. My idle is smoother and I believe the milage is up a few mpg's. Has anyone done this with any sucess or am I defeating the system by all of this tinkering?

RapRap,

In my reserach on the Holley/Weber, I found cfm ratings of 270-280 for this carb. 3 X that would be 710/740, which is plenty for a 292 I would think, especially since they are progressives. I have one on my otherwise stock Ford 226 flathead 6.

Nick

Thanks for the info, Nick. Sometimes too much of a good thing is not good!
Bob