Vacuum Secondaries

pmuller9

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There have been several 300 six engine performance engine builds with large valve, mild ported heads and street performance cams that are producing smooth, wide power bands but feel like they are lacking the low end kick that was expected.

I have not used a vacuum secondary carburetor on a 300 six because it responds well to mechanical secondaries without a hint of bog.
I would like to hear from those that are using Vacuum Secondary and Mechanical Secondary carburetors.

I would also ask if those of you that have Vacuum Secondaries if you wouldn't mind putting a screw and nut in the secondary linkage slot (or some other method) so the secondary is forced open after part throttle to see if the engine response changes.

Thank You in advance for you help with this research.
It will be a help to all with present engine builds and with future plans.
 

wsa111

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Too many variables to answer. A large carb with an automatic trans, a carb with vacuum secondaries would be the best choice. Holley offers a spring kit. Start with the stock spring & if no flat spot try the next lighter spring.
A small 4 barrel might be better off with mechanical secondaries especially with a stick shift.
Of course the A/F on the primaries & the secondaries also influences your options.
For street use i prefer vacuum secondaries, because the carb only supplies what the engine requiries.
 

jason832

Well-known member
This is a long post, but I've mostly ran a combination nobody has. Real interesting.

-300, 85 f150, 4 inch lift, 4x4, np435, 3.55 gears, 35inch mud terrains, 2100rpm@62mph
-h519p cheap hyper pistons
-91 octane required (any less it won't shut off)
-estimated 8.8 static compression
-Stock rods, bolts, mains, crank all that stuff
-Stock 84 head (with screw in studs, Clifford springs, stamped steel rockers, milled 0.060)
-Clifford shorty headers, true dual exhaust with magnalow mufflers you can see through
-Clifford intake
Comp 268 cam installed with comp gears dot to dot
-Street demon 625 cfm
-Stock duraspark 2 ignition

The truck is daily driven here in Canada and does a bit of everything. Towing 7500lbs, bogging through ponds, heavy summer traffic, 20hr drives even plowing over 4ft snowbanks. I have a wideband AFR meter to tune it, keep that in mind. Its with me everyday and my only ride to town regardless of temperature outside from 85*F to -40*F.

The carb is a 625 cfm street demon carb with a tuning kit (78-82 primary jets, 7 or so metering rod sets, 5 springs, all for the primaries). The primary side of the carb is tiny, about 1/3 of the total cfm and uses triple stack boosters giving it incredible atomization velocity and response. The mechanical secondaries are huge and tuned with just jets. Now to the tuning....

With the smallest 78 main jets, the second leanest of about 7 metering rods and second leanest of 5 springs on the primary side. The secondaries are way lean about 8 jest smaller than out of the box.
It cruises at 16.5:1
Very light acceleration about 14:1
Fully open~200cfm primaries 12.5:1
Full throttle above 2000rpm 12.5:1

Now for drivability. I have trouble with the electric choke not closing fully in Canada's January cold (linkage jamming or the spring not moving), but with the choke fully closed it starts instantly (you know its cold when the oil pressure doesn't come up to full for about 20seconds and it starts driving in neutral). Despite the 268 cam, the tiny primaries and triple stack boosters makes throttle response faster than you can smack the pedal and pulls harder than the stock 300 everywhere above 1000rpm. Below 2000rpm the secondaries will drown it but above 2000rpm it twists the truck and pulls hard. When I let off the gas in third at 4500 the truck shifts and the shifters move about 6 inches, it pullllls. I never really measure milage cause I drive it like a lunatic, but I've measured 14L/100km (16.8 USMPG) towing a 2200lb car trailer at 60mph for 4 hours of highway. Not bad for a 4 inch lift and 35s.

Long write up, but I'm very happy with this off setup. I tried a 500cfm summit carb (the primaries sucked and it had several problems). The 625 cfm carb is a bit overkill and I would not recommend it unless you have a wideband AFR meter and patience, but everyone that drives it or comes for a ride is stunned.

Next post will be a picture of the truck (two different paint jobs), link to the build thread and the odd carburetor NOTICE THE SMALL PRIMARIES THEY ARE GREAT ON A DAILY DRIVEN 300. Hopefully I will get my hands on one of gramps SR heads to play with and chassis dyno.
 

jason832

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pmuller9

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Thanks Jason.
Great info!
For the longest time 300 six builds have been under cammed and now most are being assembled with 268 cams and larger with very good results.
Now I believe we are limiting engine response by under carbureting these engines.
It sounds reasonable to have the secondaries controlled by vacuum but I believe the 300 six responds better with early opening secondaries.
Let the driver control the carburetor's secondaries instead as the above post by Jason demonstrates.

Looking forward to more responses that either support or bust this line of thinking.
 

Max_Effort

Well-known member
I like mechanical secondaries. I like 1:1 linkage ratio carbs on a road race, circle track or other engine that is driven aggressively at mid to higher RPM's.

The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

I doubt that more carb / more carb opening is going to add anything. But I've also learned to test everything...
 

BigBlue94

Famous Member
I'm pretty well a carb newbie. I have a small amount of tuning experience on a quickfuel 450 vacuum secondary carb.

Engine is 309ci with a 9.75:1 SCR. Dynamic at 7.47:1.
Large SI valves in head with bowl work.
Clifford intake and long tube headers
351w 'sportsman N' step dish pistons
Schneider 140H cam.
Manual transmission.

Itll run on 89 octane, but likes 91 better. I run 53 main jets here at 1000' elev. 6.5 PV. Normal AFR at cruise is 16.5:1 The secondaries seem to open at WOT a bit before 3000 rpm.

In low range, I have more power than I know what to do with, even down low at 1500 rpm

I've never driven a mech secondary carb
 

guhfluh

Famous Member
I've run both the Holley 600 vacuum secondary and the Edelbrock Performer 500. I tune with a wide band also. I've chosen to stick with the Edelbrock for so long because of its drivability on both the primaries AND secondaries being much better than the Holley 600, but also because I was able to get the secondaries to come in much sooner with the Edelbrock than with the Holley while being able to keep it from stumbling. With the Holley, I couldn't get them to open much at all without a stumble and no way that I know of to be able to tune fuel at the transition when they open. With the Edelbrock, it took extensive modifications too the secondary air door, the secondary emulsion tubes, air bleeeds, etc, etc. and the AFR still isn't right, but the lean stumble is pretty much gone and the secondaries come in at about 2500rpm. I'd really like to have a more adjustable carb, like the Edelbrock AVS2 that I might be able to get tuned even better, as the one I have still isn't perfect.

I have always felt like it could always use the airflow much earlier and can still notice a big power increase whenever the secondaries open AND the fuel ratio is just right. For me, it seems like that rpm should be somewhere between about 2000rpm and 2500rpm. This is why I've really been wanting to try a large throttle body and EFI, where throttle body size doesn't need to be a restriction and doesn't affect fueling.

I also want to mention Canadian 91 octane and US 91 octane fuels differ from what Ive seen tested. US being able to make more power without detonation, preignition, pinging, run-on, etc. I use US 10% ethanol 93 octane. I've run US 100% gasoline 89 octane before and it has wanted to ping, diesel and run-on. I didn't try to tune around it or change plugs to fix it, just went back to 93.
 

THE FRENCHTOWN FLYER

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Max_Effort":vi527k9l said:
I like mechanical secondaries. I like 1:1 linkage ratio carbs on a road race, circle track or other engine that is driven aggressively at mid to higher RPM's.

The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

I doubt that more carb / more carb opening is going to add anything. But I've also learned to test everything...
x2
 

Harte3

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"Let the driver control the carburetor's secondaries..."

That is the key to running mechanical secondaries. My first carb was a Holley 450 Economizer with mechanical secondaries. It was configured similar to a Quadra-Jet with small primaries and larger secondaries and very lean running. When I stomped on the pedal from low speeds it had a hard stumble/kick-in unless running at 2500 rpm or more. I learned to push the throttle quickly to 2500 rpm and then could floor it from there without getting the stumble. I run a 465 Holley with vacuum secondaries now and even with the quickest spring in the vac module it does not stumble when stomping the pedal.
 

F-250 Restorer

Famous Member
Max_Effort":3ezbkug7 said:
The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

That would make sense. The 750 cfm Quadrajet flow 180 cfm on the primary side, and that was used on small and big block engines. The 800 cfm Quadrajet flowed about 210 cfm on the primary side. That would seem to indicate that smaller primaries are sufficient.
 

pmuller9

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SteveP":1iji8tiq said:
The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.
156.25 @ 90% VE

That is only part of the story.
The 300 six has 34% more cylinder volume than the equivalent V8.
While the average volume flow may be the same for a given rpm and VE, the peak flow and "Gulp" volume is much higher for the six.
This is one of the reasons the 300 six works well with larger than expected carburetors.

This is also why I suspect a properly tuned mechanical secondary carb has a better engine response than a vacuum secondary.
The mechanical secondary tuning is also more sensitive to having the proper mechanical advance curves with cams that have higher valve overlap.
This is all in the context of street driven engines.

The info collected here is very good and hopefully will show a trend as the thread continues.

Would any of you with vacuum secondaries be willing to temporarily modify the linkage so the secondary is forced open early by the primary throttle?
 

sandboxer

Well-known member
Anyone have experience with a 450 double pumper? I need to decide on my induction soon, so this thread is of the highest importance at this point for me.

10:1
Schneider 142F
300 rods

I have two Carter BBD (285 cfm) carbs that are an option to a 4 barrel mechanical.

The 450 double pumper is a distinct option though.
Thanks
 

THE FRENCHTOWN FLYER

Moderator
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SteveP":mmlz8mip said:
The CFM requirement of the 300 is low... maybe 175 CFM at 2000 RPM.

156.25 @ 90% VE

I doubt that the 300 has a vol eff of 90% anywhere in the speed range of the engine. Maybe 80% tops. So about 139 cfm?
 

pmuller9

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sandboxer":1h93acp8 said:
Anyone have experience with a 450 double pumper? I need to decide on my induction soon, so this thread is of the highest importance at this point for me.

10:1
Schneider 142F
300 rods

I have two Carter BBD (285 cfm) carbs that are an option to a 4 barrel mechanical.

The 450 double pumper is a distinct option though.
Thanks
My last build had a 232/232 .050" duration .534" lift cam with a big valve ported head.
I had very good results with a Quick Fuel HR 650 cfm carb, mechanical secondary.
I would recommend a 650 cfm carb for your engine also.
 

F-250 Restorer

Famous Member
I have run ten different carbs, from 465 to 800 cfm. All but one, the 465 Holley, were run exclusively with a c6 and a small rv cam. Of those I have to say that the 500 Eddy and the 600 cfm Summit produced the best driveability. The performance provided by the 600 Summit, was in a class of its own. The two Quadrajets I ran both had tiny primary venturi, but with the auto trans I could not tell a difference in throttle response between those and the Summit 600.

Now, however, I have a manual trans and an engine with a cam that has much more duration, and the 465 Holley seems to leave me with a low end lack of zip. I was hoping that my trusty Quadrajet might cure the low end lethargy. Of course in the back of my mind I'm wondering if I need a carb with larger, higher flowing primaries. We'll see.

Pmuller: I'm a few days from putting on the QJ. But once I do, it is a simple matter to adjust the secondaries to open at different rpms. I'd be happy experiment with opening the 'toilet flushers' at varying points. The QJ has an adjustable spring that determines how much vacuum is needed to open them. The secondary butterflies are opened by the throttle linkage, but no fuel is metered out until the air flap begins to open. I'll just have to rig up something to only allow the flaps to open a small bit. I don't want to get the full 600 cfm secondaries popping all the way open and drowning my engine!
 

sandboxer

Well-known member
pmuller9":ejucslal said:
sandboxer":ejucslal said:
Anyone have experience with a 450 double pumper? I need to decide on my induction soon, so this thread is of the highest importance at this point for me.

10:1
Schneider 142F
300 rods

I have two Carter BBD (285 cfm) carbs that are an option to a 4 barrel mechanical.

The 450 double pumper is a distinct option though.
Thanks
My last build had a 232/232 .050" duration .534" lift cam with a big valve ported head.
I had very good results with a Quick Fuel HR 650 cfm carb, mechanical secondary.
I would recommend a 650 cfm carb for your engine also.
Thanks. I’m reading mixed messages throughout this thread, as everyone has a different setup, but I’m definitely worried about carb overkill.
I spoke with Tom at Stovebolt and he was adamant about using the Holley/Weber progressives (3 of them) for drivability. I hear all the other voices of experience as well, so it’s a lot to digest.
I tend to agree with the notion that 80% VE for the 300 is probably near the top end of things, so I was planning on staying in the 400 cfm range.
I’ll pick up a 650 though and try it. I’m worried about used ones that have been “tuned”....
 

pmuller9

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Sandboxer
Average flow is not the issue and is a poor way to measurer the flow requirements of the induction system.
The 300 six has 50 cu inch cylinder volumes, equivalent to a 400 ci in. V8.
It is the peak flow, not the average flow that is the more important consideration with a big six.

The other factor is the intake manifold plenum volume.
The less the plenum volume the more the peak flow responsibility is shifted to the carburetor.

A big valve ported head 300 with a big cam still provides a lot of low end torque.
If the carb with less than 600+ cfm is used the upper end of the torque curve falls off early and the power band is shortened with no real gain in low end torque.

Here is another testimony.

AbandonedBronco;18702899 said:
I've often wondered about this, because in every formula I've ever seen, cylinder size is never a component. Unlike aftermarket stuff, which is almost always geared toward V8s, carb CFM size is used for every engine, with all configurations.Then, as all the pistons are moving up and down, it creates a steady airflow in through the carburetor, not individual pulses through the carb for each cylinder. It's simply pulling in 300 cubic inches of air (minus whatever for vacuum) for every other RPM.That's where the number 3456 comes from in the CFM formula. It's just converting cubic inches to cubic feet, then multiplied by 2, since a 4 stroke only pulls in air every other rotation. 12 * 12 * 12 * 2 = 3456

There's no accounting for number of cylinders or their size.That said, what you say always seems to be true. I've always had better luck with carbs that are bigger than the 300 should be using. [highlight=yellow]My 390cfm had way less performance than the 600cfm I had on there.[/highlight]Most have better luck with 500 cfm Edelbrocks than they do 200 - 300 cfm carbs.Etc.Makes you wonder
 
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