WOT and Vac. secondary ....

[Canadian1968]

After adding a rear meter block to my 3310 holley carb, to try and bring my WOT , A/F ratio down from 10.5. I noticed now that I can feel the secondary circuit open. It more noticeable when I am on the street playing around , then when at the track . But I can defiantly feel the secondaries coming in .

I did improve my track times down to a 13.22 from a 13.45 . I fell most of it is coming from an improved launch though ( was able to cut a 1.96 60Ft on the street tire ) . I was able to clock a new personal best of 105.22 compared to a 104.8 in cooler weather.

My question is , I have a vacuum gauge installed on the dash, when I hit WOT the thing snaps to 0 as it should .

Will putting in a lighter spring change anything? I mean anything past 0 is boost which is impossible with NA.

I currently have the silver (plain) spring in which according to holley spec, on a 402 is fully open @ 7000RPM , I am reving to 5300ish and yes its 446CID not 402. But looking at the chart offered by holley, approx 50 CID is good for 1000 RPM in opening rate, so I feel like I am close to fully opening all the way.

I know only one way to find out is put it in and test. I just like talking about theory sometimes !!!

Plus I am officially bitten with the drag race bug. On the list next year is some cheater/ slicks !

 

[HEMI-ITIS]

A lighter spring will open the secondaries sooner.

 

[gkent]

I seem to recall that you shouldn't "feel" the secondaries opening, rather it should be a smooth transition. If you feel them kick in then you've left something on the table so to speak. That said, I think a lighter spring is the ticket. You keep going lighter till the car bogs, then you go back one. Theoretically !!!

 

[coloradodave]

Correct. You shouldn't feel the secondary throttle plates opening. If you do, they are opening too soon, which a heavier spring will fix. One step at a time until it's a smooth transition...

 

[Canadian1968]

Correct. You shouldn't feel the secondary throttle plates opening. If you do, they are opening too soon, which a heavier spring will fix. One step at a time until it's a smooth transition...

Too soon? or to late ?

My theory if I can feel them kick in, giving a boost in power , the engine was waiting for that air . So if I open them sooner it gets the air it WANTS sooner .

 

[Nate S]

Too soon? or to late ?

My theory if I can feel them kick in, giving a boost in power , the engine was waiting for that air . So if I open them sooner it gets the air it WANTS sooner .

That’s actually incorrect though. They don’t “snap” open at any point. If they are too slow they just creep in and you can’t feel them. They feel it’s same way (but with better acceleration) up the optimum point then after that you can feel them. Ever had that car that bogged because the accelerator pump didn’t work right and then it recovered? That’s what you feel with secondaries that are too fast. There’s no pump, just a boatload of extra air and then it recovers, “kicking in”.

A plain spring shouldn’t do this. I’d bet that the little check ball in the diaphragm was taken out. With the ball in there then the carb might want a lighter spring, purple or yellow. An air cleaner makes a big difference too, both in jets and secondary opening speed.

 

[66Satellite47]

Canadian, my experience agrees with Nate S, too heavy diaphragm spring can be fairly smooth but late and slower acceleration. Too lite of spring can make a bog, too much air with no accelerator pump to match. A 3310 with the secondary metering block will give you tunability you should take advantage of. The 3310-1 with the secondary block standard was very adaptable with minimal main jet changes. Not sure about what the later 3310's desire for jetting. Get the vacuum secondary spring assortment & try lighter & stiffer. Changing the primary shooter size up along with the pump cam assortment could also help with lite springs when you smash the throttle. You will need trial & error with the available tunability.

 

[Canadian1968]

Yes you are right . I did remove the check ball. I was having problems with the secondaries opening. I was pulling 3" of vacuum at WOT.
I pulled the carb apart to test the secondary diaphragm. I could not get it to hold a seal with the bench test unless i removed the ball. Then it worked perfectly , and I never had issues , until I added the rear block with jets and leaned out the AF mixture a bit .

 

[BSB67]

I know only one way to find out is put it in and test. I just like talking about theory sometimes !!!

Ugh. I'm rowing against the current again.

Air velocity past the venturi generates the signal for the vaccuum secondaries, not manifold vaccuum.

The right secondary or outboard carb springs are a function of both the motor, and the car. The same motor in a light weight car, loose converter, and high numerical gearing, will need different springs than the same motor in a heavy street B body car with a tight converter and tall gearing. Here is the part most either struggle with, or don't understand: the heavier car with taller gear needs the lighter spings when going for the lowest track et, and hightest mph. The other part that folks struggle with is that when you tune for the best et/mph, you could very well be introducing a noticable bog or hesitation when you drop the hammer from a roll going down the street.

Here is why. In the heavy car/tight converter/tall gear case, the velocity across the venturi is slow to build as the rpms are relative slow to build. Think about your starting line technique and what the motor's rpm is doing. You load up against the converter, maybe 1800 rpm, you release the brake on the last yellow, there might be a slight increase in rpm and you roll into the throttle and your rpm increase is dictated by your 3.23 or 3.55 gear. As you progress thorough first gear, the signal slowly builds and the secondaries slowly roll open as well. Now take this same situation in a lighter car/loose converter/4.10 gear. When the driver drops the hammer at the yellow, the secondaries will open instantly and the car will hesitate.

Now here is what happens on the street with the heavy car/tight converter/tall gear. You drive along at either an elevated rpm, or a speed that the transmission will kick down a gear. When the hammer is dropped, the same spring that appropriately brings the secondaries in at the track, now get an instant rush of high velocity air, and the secondaries slam open, and the car bogs or hesitates.

Let me be clear, you do not want it to bog at the track, but what works at the track might give a bog rolling down the road and dropping the hammer.

Here is the good news. You don't need to take my word for it. Go to the track and take your springs with you. And, if you get to the lightest spring and the car is still going quicker and faster (it will show up in both et and mph improvements) there might be other things you can try.

You really need to use the check ball as designed. It won't keep you from attaining the carb's potential.

 

[Canadian1968]

Here is the good news. You don't need to take my word for it. Go to the track and take your springs with you. And, if you get to the lightest spring and the car is still going quicker and faster (it will show up in both et and mph improvements) there might be other things you can try.

That was my original plan, buy was having a lot of success just working on my launch. Trap speeds are pretty consistant in the 104.5 range with a best of 105.2 when I spun a bit off the line. So far best 60ft is 1.96 off idle and rolling on the power quickly , but if I get to agressive it will spin (the 105.2 pass) Street tire can only hold so much I guess

 

[BSB67]

Me and my friends have done a lot of track time on real street tires like BFG 235 70s or 60s with stock suspension and 3.23 gears. Your 1.96 is about as good as any of us got. We were all in about the same place performance wise, low 13's to high 12's, 103 to 109 mph. I think my best was a 1.95 and one guy was down to 1.90 but with a little more gear and tire than the rest of us. So, your right, there is only so much you can do with a true street tire at that power to weight ratio with a stockish suspension. Make sure your engine is strapped down good, and your snubber is adjusted close to the floor, those things will show up on the time slip.

Tire spin does increase the mph a little.

 

[coloradodave]

BSB67 is 100% correct. Air velocity past the venturies creates the vacuum to open the secondary throttle plates, NOT manifold vacuum. And yes, the checkball needs to be there.
Back in the day guys used to put a screw in the quadrant linkage (driver's side) of the 3310s and other vacuum secondary carburetors so the linkage would 'pull' the throttle open. Problem was, opening secondaries without an accelerator pump shot created a momentary 'bog', which guys took as the secondaries 'kicking' in and equated that to faster acceleration. If you do as BSB67 suggested and try making timed runs at the track while changing the diaphram spring between runs, you'll rapidly see that a smooth secondary transition will give you the best ET.
If you look at any old Holley carburetors you might have you may see evidence of a screw being forced into the quadrant slot on the linkage...lol.

 

[Nate S]

BSB67 is 100% correct. Air velocity past the venturies creates the vacuum to open the secondary throttle plates, NOT manifold vacuum. And yes, the checkball needs to be there.
Back in the day guys used to put a screw in the quadrant linkage (driver's side) of the 3310s and other vacuum secondary carburetors so the linkage would 'pull' the throttle open. Problem was, opening secondaries without an accelerator pump shot created a momentary 'bog', which guys took as the secondaries 'kicking' in and equated that to faster acceleration. If you do as BSB67 suggested and try making timed runs at the track while changing the diaphram spring between runs, you'll rapidly see that a smooth secondary transition will give you the best ET.
If you look at any old Holley carburetors you might have you may see evidence of a screw being forced into the quadrant slot on the linkage...lol.

It’s worth pointing out here that vacuum at the port in the Venturi opens the secondaries. This primarily comes from air velocity but the presence of an air cleaner lowers the pressure in the whole system and opens the secondaries sooner than if the air cleaner isn’t present. In this case manifold vacuum has an effect.

 

[RJRENTON]

It’s worth pointing out here that vacuum at the port in the Venturi opens the secondaries. This primarily comes from air velocity but the presence of an air cleaner lowers the pressure in the whole system and opens the secondaries sooner than if the air cleaner isn’t present. In this case manifold vacuum has an effect.

True statment except the secondary opening signal originates in the left side primary venturii apex and is directed to the left side secondary venturi, which assists in opening the secondary butterfly valves, via an internal passageway and a ball check valve. This ball check valve is absolutely necessary to provide an even, controlled opening of the secondary butterfly valves.
The negative venturi pressure is a function of velocity moving across the venturi independant of whether or not an air cleaner is present. An air cleaner would reduce the overall pressure drop scross the carb slightly, depending on the restriction presented, but venturi pressure is a function of flow thru the venturi.

IMO....the best carb for the OP to consider the origional Holley 4150 R-3310-1, 780 CFM, down leg booster venturii, with secondary metering block and secondary power valve.....available on line used (eBay) ..... easy to tune and ABSOLUTELY reliable and provides crisp throttle response and transition to full capacity.
BOB RENTON

 

[66Satellite47]

I agree with RJ, a 3310-1 was/is most likely the best carb for a bolt on street/strip car. Has good tuning flexibility, but doesn't require much. Play with the jets, vacuum springs, shooter size, pump cams. All will be pretty close to stock. Very little tinkering needed to optimize.

 

[Nate S]

It's worth noting here that the 3310 original (no dash number), the one with the Chevy number 3878261-EH on it is nearly identical to the 3310-1, but it's not. You'll want the base / butterflies from some other 3310 or you'll find there's no PCV port and a couple other things are a bit odd and more difficult to deal with. I merged a 3310-5 with the Chevy one for my car, pretty much makes it a 3310-1. Awesome carb.

 

[66Satellite47]

An interesting side note about the 3310-1 is that the throttle blades are thin. I swapped them into my 650 4777 DP's to get the extra CFM on my tunnel ram carbs.

 

[Canadian1968]

I'm very happy with the carb. I would have to make some other engine changes to make me want to change carbs right now. Looking forward to trying some different springs next year, and maybe playing with timing to see if there is any power left hiding in the motor. The big purchase over the winter will be a set of slicks , hopfully allow me to use the full potential of all the power

 

[RJRENTON]

It's worth noting here that the 3310 original (no dash number), the one with the Chevy number 3878261-EH on it is nearly identical to the 3310-1, but it's not. You'll want the base / butterflies from some other 3310 or you'll find there's no PCV port and a couple other things are a bit odd and more difficult to deal with. I merged a 3310-5 with the Chevy one for my car, pretty much makes it a 3310-1. Awesome carb.

You are correct....the origional 4150 780 CFM carb was an origional GM spec desiged by Holley used on the 375 HP/396 (402) engine and I believe, later, on the 425 HP/427 engine. Because of the outstanding performance of the design, Holley adapted it for use on Chrysler and Ford engines, with a throttle linkage and kick down lever attachments and later a conversion kit for a cable operated manual choke.
There were small revisions to the primary metering block to allow for GMs manifold vacuum connection to the distributor, off idle transition calibration, main and power valve (PVCR) metering orifices. I also believe GM used a divorced choke in lieu of the hot air choke on the R3310-1. Subsequently, later variations switched to the 4160 design and not down leg secondary boosters and was flow rated at 750 CFM and other small calibration points. I still like origional R-3310-1 variation as the most versatile or the other variations.
Just my opinion of course.
BOB RENTON