Mechanical vs Vacuum Secondaries

If you want a bit quicker throttle response, then stick with a mechanical secondary carb, however there is nothing wrong with having a carb with vacuum secondaries. I used a vacuum secondary carb on my car for years, but also used carbs with mechanical secondaries. I switched over to sequential port fuel injection years ago, so my opinion might not be worth as much now as it might have been back in my carb days.

...As far as HOLLEY Carburetors, I have used both Mech. Secs. and Vac. Secs, for Decades.

On the Pantera, I did a custom machined 'Dual Quad' Mid- High Manifold (IR, 2x450's), I opted for Vac. Secs., should You want to Talk about Throttle Response. Yes! Mechs come All-In, All 4 Bores, 2 Accelerator Pumps, when You 'Punch It'. Response doesn't get any Faster than this! A Liken it to "...A Sledge-Hammer in Your Backside". As well as That, TOO Large a Carb can Also, Cause a Bog, at Lower RPM's. That's Where Vacs work best, They Come-On Only When Needed and Give Only the Amount of Fuel/Air that Is Needed.

I did Dual Quads on a 400 Pontiac (Dual Plane Weiand) using Mech. Secs. (2x390's) I do not 'Track' any of My cars. My work was always for the 'Street'. I Commuted the 400 Pontiac to Work, Every Day, for 30 Years!* I now run exclusive Vacuum Secondaries, on a Edelbrock 'Air-Gap' Manifold. When I drove through the Desert I was using a 600 CFM. I have since gone Back to the 750 CFM, both have Vac. Secs. I Missed the 'Greater Roar' of the Larger Venturis.

My Point...Yes, greater response with the Mechanicals, but My opinion is, they should Only be used in Racing on the Track. They are meant for all out competition, they are Always 'ON' as for Fuel Flow, especially 'At the Idle'. Copy, Fumes of Unburned Fuel. Plus, You will have a Greater 'Chance' of ever Passing a Smog Inspection, when You are using a Vacuum Secondary Carburetor.

I believe You will be Most Happy with Vacuum Secondaries on the Street.

Although, the Response from the 600cfm, 'Seemed' more 'Crisp', than the 750...the Exhaust Tone of the 750, was a Greater ROAR, when the Secondaries come On. Both used on the same air-gap Manifold. Single Carb, Dual Plane.

Last...if one was concerned with the 'Miles per Gallon', they would Never have purchased a Pantera!! The fact is Mech secondaries are 'paid for' with Excess, Wasted Fuel...All The Time! Even while 'sitting at a 'Red Light'. They belong on the Racetrack Only.

Just My Opinion.

This may help You with Your decision, Good Luck with It!

*...gonna sound like bragging, I NEVER passed on a 'Red-Light' Street-Race, or Any Race! A Highly Modified Pontiac, Dual-Quad 400c.i. in a 6.6 '78 Trans-Am. Near 500 HP, Weight-Stripped Off more the 800 Pounds, ALL the Suspension Tricks, in the Book, and then some...Was Undefeated in all those Years of 'Street Racing'! That Machine had a T-10 4-Speed, You could 'Slam' the Gears, In. Can't do this on a ZF!! With Mech Secs, I could Easily bring the Front-End, Off the Ground, and Only Move the Car Forward, One Inch! Traction-Bars, 'Torque-Chain'!! Don't know what a 'Chain' is? You've never competed in Drag-Racing, or You used a Torque Plate. Why did I Not take it to the Track?? TOO MANY RULES! That TA was so Highly Modified, it was deemed 'Illegal' on the Racetrack, and was Banned!

'DO NOT TRY THIS AT HOME'!! In California, it can get Your Pantera, CRUSHED! The Gavel will Fall!

MJ

I was sitting in My Lane at a Red-Light Stop. The Other car was sitting at my right, in the 'Right Turn Only lane'. The Driver planned to Go Straight, jump-out, and cut Left into My Lane, Cutting Me Off! Imagine That...an Illegal Act. LOL

When the Light Turned Green. We Both 'Dumped the Clutch' for All She Was Worth. He wasn't even Close. Remember, more that 800 Pounds stripped off that Machine and the Horsepower, Doubled.

I can remember, glancing in the Rearview Mirror, after Leaving Him Behind...The Owner of the $120,000+ Porche was So Pissed-Off, Pounding His Fist into the Dash! When he soon came to his senses...He realized his Committed, Full-Bore, straight-shot launch, was heading toward the far corner/sidewalk and a Telephone Pole. He locked up the Brakes! Second Place is a Bitch! Better Go Get Yourself a Pantera. I built This Car, I built This Engine. Yes! It was the 400 with Twin 390's Sideways on a Offenhauser Low rise. Dual Quads with Mech Secondaries. As I said...Racetrack ONLY!

Vacuum secondaries are easily set up to come in as early (or late) as you like, simply buy the secondary spring kit as shown below, & go lighter until you get the secondaries to open where you like 'em. Set your timing to be 32-34 degrees TOTAL, then install the yellow spring. If the distributors advance(s) centrifugal (& vacuum if also used) are set up right to pull in advance, you'll love the vacuum secondaries. View the chart below:

Now about mechanical secondaries: The new Holley Brawler w/mechanical secondaries has two different settings; it can be set up for secondaries to begin to open at 1/3-1/2 throttle, all in at W.O.T.

If your objective is more power WideOpenThrottle, it is easily set up to bring ALL 4 in together from the 'git-go. driving on all four. Using the 60cc acceleration pump rear.

Marlin - how do the secondaries affect idle and main circuit when throttle position has not engaged them? Isn't that governed by carb CFM and jet size??

My Quickfuel carb w/mech secondaries came with 3 links to control when they open based on throttle position. You can have 1:1, 40% and 60% throttle position to control when they come on based on your driving type.

Appreciate your thoughts - thanks!

Now for the kicker. Driving a ZF with 4th gear overdrive & even deeper 5th gear, you have to drive differently. For example if you're in 4th or 5th gear at 15-18-2000 RPM & stomp it, the car won't take it. No good. With vacuum secondaries you can stomp it anytime and let the engine vacuum do the work. Making MORE power and no bog. With mechanical secondaries you have to be within the power band when you kick it.

Race cars love mechanical secondaries, they run W.O.T from the word go. Punch it, let the clutch fly. Last time I saw a customer do that in front of the shop in a blue chrome bumper car, a rod came through the side of the block just forward of the starter. We call 'em windows. You could drop a baseball through the hole.

Now you understand the dynamics of each so you can choose accordingly.

I LOVE Quickfuel carburetors, even though some mechanics call the QuickFool carburetors, lol. The Brawler I mentioned afore has 4 corner adjustments via both front & back metering blocks, & sitting benchside the Holley, the new Brawler is an exact copy of the Quickfuel. Holley made some changes since that takeover; I'm expecting the bowls & metering plates will (soon) be zink.... following Holleys lead.

After doing a lot of research on carb sizing/type for my Pantera,  the recommendations were all over the place with no logical pattern found.

I ended buying the Quickfuel ss-750-an. Mechanical secondaries, annular venturis/boosters and electric choke.

Since we are still enjoying winter in Michigan, I have not had a chance to drive it/tune it yet. I had trouble deciding 650 or 750 for my original 351, bored 30 over and domed pistons (per prior owner).

Hopefully the 750 isn’t too much??

As a point of reference, I installed a QuickFuel SS but went with a 600 CFM with mechanical secondaries.  I mounted it on a 1" spacer to clear the fuel line fittings.  I tuned it (4 corner adjust) and followed all the "advice".  It still had a flat spot from idle.  So I bought an accelerator cam kit and changed the pump cams to increase the squirt amount.  That resolve the flat spot.  The car will start from stone cold after weeks of sitting with two pumps of the pedal and immediately idles smooth.  Once warm and the choke off, the car pulls very smoothly to redline.  I have not changed jets nor does it suggest they need to be changed.  I spent a lot of time with my vacuum gauge setting the mixture screws.  A small amount of richness helped with the flat spot as well as the car runs off the idle circuit up to a certain RPM.  But I was able to get a very steady 17" of Hg with the screws with a little patience.  Overall, I am very happy.  Probably the biggest pain in the ass was getting the bowl inlet fittings I bought to seal.  Do NOT try to use the Holley paper thin gaskets with teflon on them - they are a joke.  I bought a fitted line kit pre-built that used the 1/8" think nylon gaskets and followed all the torque specs and it sealed perfectly.  I also added a 3" thick K&N air filter that fits within the diameter inside the rear deck lid (as it is above the deck) - I think it is an 11" unit.  

Hopefully this helps you.

How did you decide on 600cfm?

Any carbureted car I have had always required long cranks after sitting for over a week to refill the bowls. How does your carb avoid that??

At this point I am leaning towards a vacuum secondary carb.  Probably a Holley 750 because I am familiar with them.  I already have a Holley 750 with mechanical secondaries on another Pantera.  Both cars are setup so that the carbs can be readily swapped making comparisons of the two carbs fairly easy.

The sizing calculator and a lot of what I read for a stock 351C street engine with upgraded intake and exhaust says you need no more than 600 cfm.  Gary Hall told me the same thing and sold me my first Holley 600 which it replaced.  If you plan to run wide open, maybe get bigger but the 600 is what it says.  I know the forums all say 850 900 whatever.  As for cranking,  the accelerator pump holds fuel and two pumps and it starts right up.  I have an edelbrock on a 350 Chevy that sits outside.  Cranks for probably 5 to 6 secs and fires.  Filling bowls doesn’t take that long - at least that’s my experience. I remember Gary warning me not to “over carb” the car as all it does is run poorly.  Just sharing what I know from experience.

The engine in this car was built by Clay Smith Engineering (Mr. Horsepower).  It is a stroker and has extra cubic inches.  They recommended a Holley 750 double pumper.  It will be enjoyable to try it with both types of carbs.

Sounds nice Steve.  

@Rob Fridenberg posted:

Marlin - how do the secondaries affect idle and main circuit when throttle position has not engaged them? Isn't that governed by carb CFM and jet size??

My Quickfuel carb w/mech secondaries came with 3 links to control when they open based on throttle position. You can have 1:1, 40% and 60% throttle position to control when they come on based on your driving type.

Appreciate your thoughts - thanks!

...whether it's a 'Double Pumper' (Mech. Secs) OR a Vac. Sec. Carb, Most all Holleys have a 'Set' Idle SCREW* on the Secondaries. Accessible from the Bottom, only when the Carb is OFF the Manifold, Or You have a very Small (common) Screw Driver, BENT at a 90 Degree Angle. The Tiny Screw is Slotted. The Secondary Idle Synchronizes with the Primaries. The Sec. Throttle Idle is adjusted so the Blades JUST Uncover/expose the 'Idle Transfer Slots' (2) *. NO more! Then the Idle is 'Finalized' back at the Primaries. These Slots can only be seen when the carb is off the manifold.

All Others' Contributed Recommendations, are words of Wisdom!! Yes! As mentioned, a Bog Off-Idle, 9 times out of 10, it WILL be the Accelerator Pump!! And, there are SO Many, Different, Adjustments that can be made There! One Must do their Homework and Study, and with 'Trial and Error', home in on the Ideal Settings. ALWAYS...One Adjustment at a Time!! So You know which made a Difference, and You can 'Go Back', and try another adjustment.

* NOT to be confused with a Idle MIXTURE screw! Mixture screws control the fuel/air ratio, and are located on the side of the Metering Blocks. The Secondary Idle SET Screw, will Add to the control of the RPM.

** This is what Activates the Secondaries to Contribute to the 'Total' Idle. The Secondaries are Never Deactivated.

I like the Idle on My Cleveland at 900-1000 RPM. Keeps the Oil Pressure Up, and Makes taking off from a stop much Smoother.

All in All, Good-Luck with It!

MJ

To fix long cranking after setting for awhile, the easy way to prime any carb is with a small electric fuel pump installed between the tank & carb, and a momentary- ON switch under the dash. The always-noisy electric pump bogs down slightly, telling you when the float bowls are full. The engine will usually fire instantly. Depending on which electric pump you use, you may need a check-valve in the line to prevent the OEM fuel pump from backflowing while it's off.

40 years ago the standard was, a 600 cfm carb gives better throttle response around town  and better fuel mileage. Changing to a 750 instantly gives an extra 30-40 bhp, softer/slower throttle response and loses 2-4 mpg. A 351-C is a very easy engine to over-carb. This has not changed- pick what's important to you.

A vac-sec carb can (not always) run pretty close to optimum right out of the box, but no guarantees. A mechanical secondary carb is a racing unit so is ALWAYS jetted rich out of the box and may need an hour's dyno tuning by an expert to run well at both ends of the rpm range. Occasionally, an owner gets lucky and one will run acceptably with only home adjustment, on a street engine.

Ignition timing and vacuum advance- both fully adjustable on OEM distributors, will usually need to be adjusted for optimum with mechanical secondary carbs. Another reason to dyno tune when changing carbs. Note- high dollar aftermarket distributors often use Ford parts in their internals.

The Ford electronic distributors normally used as cheap drop-ins on Clevelands come from the Ford 429-460 truck engines but the truck advance curve inside can be reset using Ford parts. There are at least a dozen stock advance curves possible. Once again, dyno- tuning will sort this out, too.

Finally, on ANY modified engine, buy yourself a FIRE EXTINGUISHER of a type for gasoline fires! Engine fires are most common on engines with non-stock fuel systems- which with Panteras, is most of them! Trust me- you WILL need it at some point and good intentions 'later' will not save your car. A 2-1/2 lb no-residue extinguisher on a metal bracket will last forever and can be refilled.

You might consider a Holley 4609 Ford Autolite #C9AF-9510-U (735 CFM) which was developed by Motorcraft and Holley as a Ford over the counter performance carb for refitting on 428 CJ engines. This was specially designed to provide excellent low rpm signal with skirted main venturi boosters and special down leg rear boosters custom cut to feed the Motorcraft (also over the counter performance parts) aluminum intakes for maximum performance. It also uses a smaller primary venturi. Vacuum secondaries allow smooth engagement. A little difficult to come by but also a period correct upgrade. FWIW

Demon 650 with Vac secondaries here. Runs great.

Wow. Lots of info here.

I've run a lot of combinations. On the Cleveland I've found that more then ever, you need an integrated system in that you just can't change a kitty cat into a lion with just a carb change.

The Cleveland CAN respond to a mechanical secondary carb with no problem and you CAN get 800hp out of YOUR engine but the question is do you really want to?

More then any other component, you are matching the carb to the camshaft profile.

On a Boss 351, I found that a Holley 4779 aka, a 750 double pumper, along with a camshaft upgrade to a 237 @.050 and about a .580 lift cam worked great. A good solid 450hp or so.

The issue there is it will idle at 1,000 rpm and it's a 50/50 proposition as to whether it belongs on the street or on the track.

You are going to have lots of power with it. Not subtle power and you are going to pay for it in not so subtle ways.

A 700cfm Holley on an engine with the approximate specs of a stock Boss 351 is probably as hot as you want to go on a street car these days. A stockish 351 CJ, like came originally from Ford is a 600 or a 650 cfm carb car at most.

Now as Marlin suggested, IF you want to talk WEBERS, 2x4 735 Holleys on a Trans Am intake manifold OR EVEN a 2x4 Holley set up on a 428 I can't deny that I know something about those BUT you DIDN'T ask and so I AIN'T goin' there. Well, not tonight anyway. But then again, I'm not really a subtle guy to begin with.

You need to be VERY specific about your engine specs but a 600, 650 or a 700 is PROBABLY where you are going to wind up.

Oh. Incidentally. A Holley in any of these sizes is going to give you 12-14mpg even on just a CJ spec. The Webers on the other hand, unless you drive it like Steve Mcqueen in Bullet are good for around 22. I'll never admit to that. 

Sharky's suggestion sounds veryinteresting. That one I never tried...or at least not yet. Maybe next week? I'm a little busy right now. 

The carb on mine is my first carb'd V8 since the '80s so take my lack of experience for what it's worth but I would add that a really nice feature of the QuickFuel vacuum secondary carbs is the ability able to adjust (with a screw) how slow or fast (late or early?) the secondaries come in. Adjusting this, along with a bigger discharge nozzle and hollow screw, allowed me to cure my lean peaks when going from steady to full throttle. No matter what you go with, a wideband O2 is another invaluable tuning tool as well.

Does anyone have any feedback with one of George's recommendations: 4150 Holley style carburetor, 750 cfm, annular boosters, electric choke, street calibration.

The QF Ss series is a Holley 4150 but better.   George made several recommended carbs some smaller than 750.  

@Sharkey posted:

Vacuum secondaries are easily set up to come in as early (or late) as you like, simply buy the secondary spring kit as shown below, & go lighter until you get the secondaries to open where you like 'em. Set your timing to be 32-34 degrees TOTAL, then install the yellow spring. If the distributors advance(s) centrifugal (& vacuum if also used) are set up right to pull in advance, you'll love the vacuum secondaries. View the chart below:

Do you recommend the short or the long yellow spring?  Thank you.

...On All My Vacuum Secondaries 600-750, I had the best results using the SHORT BLACK Spring (NOT the LONGER Black spring), shown in the Photo. Regardless of the Chart...It seems to be in the mid-range of that Spring Set, Average. Not too Weak, Not too Strong. The Secs will open-up right along with the Engine Revs. Quickly and Balanced. NO Bog, No Stutter, No Hesitation. Strong!! FWIW

A good place to start your testing. And, Be Sure the Accelerator Pump is Adjusted Precisely. As in the AP Cam is on the correct hole, and more.

MJ

I have the carb on the car with the yellow spring installed and made a video showing that the secondaries are opening.  The yellow spring is 3 of 7 choices and there are two more lighter springs that I can test.

What is the downside to the vacuum secondaries opening too quickly?

A bog due to momentary lean mixture.

Some of you may remember POCA articles from a decade ago- a bright yellow '73 GT5-s clone that I got in my shop to 'straighten out'. It had so many issues that I did a couple of articles on the 58 problems I found.  On that engine, a vac-sec carb simply Would. Not. Work.  It came in with a brand new Holley Avenger (vac-sec) carb. I could NOT get rid of a mid-range bog during  acceleration.

Tried swapping springs, jet blocks and even float bowls (center feed vs end feed). I tried SIX more vac-sec carbs of various sizes and List Numbers, with nearly  identical results. Finally I took the car to Mallory's local shop for some expert advice. The dyno tech took one look at it and said, "Holley doesn't mention it but all Avengers are CA compliant emission carbs. You'll NEVER get that thing to function like you want on a performance 351-C".

Based on his advice, I bought a 'tuner' 700 double-pumper from Bob Oliver at Competition Carbs, and bolted it on. Instantly, no more bogs, more power and better fuel mileage.  His carbs have some 24 separate changes to the new carb as-received. It worked so well, I bought a second tuner-carb for our Pantera, and it worked better than what I had (I'd tried my old carb on the yellow car; no help). A week late we drove to 'Vegas for a Fun Rally with the new carb and got 22 mpg at an avg mph well above 70! Then Judy ran the track event and was all smiles at the power increase and drivability (900 miles trouble free round trip).

Over 40 yrs of ownership and two engines (first was OEM; the last of which I built) with the same Pantera, I have learned to defer to real experts in certain areas. My advice is to either spend every evening in your garage fiddling with your setup & going to bed smelling of 91-octane until you get lucky. Or buy a tuner carb and immediately enjoy the results. A third possibility is a late throttle-body-EFI setup. I have no direct experience but they are supposedly marginally better for power AND mileage than tuner carbs, for 3X the carb's price & complication. And are still not fool-proof on street or track. Good luck.

@forestg posted:

A bog due to momentary lean mixture.

Thanks Forest.

I just drove the car across town and there was no bog.  Thus, I plan to try the next lighter spring.  If it does not bog then I will try the lightest.

I now have the lightest spring (white) installed.  The carb has good manners when driving with others around.  I took the car to a more isolated place for some testing and I am impressed with how the vacuum secondaries operate.  Unlike a mechanical secondary carb the vacuum secondaries begin opening predictably at part throttle.  The car chirped the tires going into second gear.  There needs to be plenty of Venturi velocity for the secondaries to open.  The car needs to be in a higher RPM range and a lower gear.  For example, the secondaries will not open when going 30 MPH in third gear even if you floor it.

Thanks all for your guidance with this.

Marlinjack,

any pictures from the Pontiac by hand ?

like to see them.

Simon