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Author Topic:   Vaccum Sec vs. Mechanical
MrXerox
Gearhead

Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 08-05-2001 03:29 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
In what application and or circumstances should you use one over the other? I have always heard that all the 1/4 mile cars should use mechanical. However, I have seen some use vaccum secondary carbs with good results...?

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chips67
Gearhead

Posts: 651
From: louisville, ky, usa
Registered: Jul 2001

posted 08-05-2001 05:18 PM     Click Here to See the Profile for chips67   Click Here to Email chips67     Edit/Delete Message   Reply w/Quote
im not an expert but i can give you my opinion. if im running an automatic trans i'll go with a vaccum secondary. since the trans is going to be doing its own thing anyway i wanna be able to tune the transition from my launch rpm. unless of course you are talking about a radical machine thats mostly strip, less street. for cars with manual trans i always go with a manual secondary. as with everything in performance, which one would work best depends on.......variables. get your variables in a row and call someone you trust, tell them what you are planning on running and they can help you. right now the question is too general, there is no answer. both carb type work excellent in the right application.

------------------
67 coupe, 650dp and rpm intake on 5.0 with afr 165 heads, 4 speed, 4.11's.....best so far is [email protected] in 1/8 mile with 1.91 60ft. time.

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MrXerox
Gearhead

Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 08-05-2001 06:47 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Ok, I will fill in some of the variables
71 Mach, 351C, forged flat tops, quench 4v heads (fully worked and ported), X-cellerator intake, solid roller 244 @.050 .623/.623, FULL MSD ignition, C-6 Auto with TCI 3000-3500 stall. I purchased and rebuilt a 850DP with mechanical secondaries and the motor does not seem to like it at all. I am not sure what is wrong but I borrowed a friends 750DP mech. sec off his car last night and it ran a WORLD better. Anyway, I am in the market for a 750 now and was wanting some suggestions as to what would work best for my combo...

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chips67
Gearhead

Posts: 651
From: louisville, ky, usa
Registered: Jul 2001

posted 08-05-2001 10:32 PM     Click Here to See the Profile for chips67   Click Here to Email chips67     Edit/Delete Message   Reply w/Quote
with that auto and the 3000 stall id run a 750 vac. secondary carb. there maybe something wrong with your 850 or maybe its not tuned right. you know all about setting one up right? with that cam and heads i guarantee if it has original holley squirters, pump, cam, power valve, and jets it aint right. a new 750 vac. sec. carb, get the right power valve, a lighter secondary spring, and read your plugs for the right jet setting and you'll be doing fine. now am i wrong or are 351 4v heads ready to make peak power over 7000 rpm without porting? if thats right you either need a 5000 stall and a bullet proof bottom end or a milder set of heads.

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67 coupe, 650dp and rpm intake on 5.0 with afr 165 heads, 4 speed, 4.11's.....best so far is [email protected] in 1/8 mile with 1.91 60ft. time.

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clevelandstyle
Gearhead

Posts: 1035
From: Connersville, IN
Registered: Jul 2001

posted 08-05-2001 11:05 PM     Click Here to See the Profile for clevelandstyle     Edit/Delete Message   Reply w/Quote
I would go with a 750 mec secondary. Your engine looks like it will handle it fine. The 850 might be a little too big. My opinion is that a mechanical carb will out do a vaccum carb any day, even if it's with an automatic trans as long as you have a decent stall converter. Your converter ain't bad and I assume this is a street car. If you plan on doing some drag racing, you will need a higher stall to get good 60' times.

I use a modified 800 mec on my 351C with a 4500 stall on the street and the strip.

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MrXerox
Gearhead

Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 08-06-2001 01:03 AM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Well, I am positive I rebuilt the 850 correctly using a Holley kit. However, I am a newbie when it comes to troubleshooting carbs. I have one of those Edelbrock air/fuel monitors hooked up on the collector of the header and it said I was running lean (car had evidence of this as well) and I had it jetted 80's in the front and 84's in the rear so I reversed the jets and it started out reading ok but once the motor warmed up it read lean again and I could never seem to get it to idle at a decent rpm, it always wanted to run at about 1200 or so. When we put my friends 750 on it idled great and air/fuel was reading about 14-14.5 to 1 (perfect) and he only has 78 jets in the front...what gives? What are the symptoms when your carb is too big and you dont get enough signal?

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jkilroy
Gearhead

Posts: 1808
From: Vicksburg, MS
Registered: Dec 99

posted 08-06-2001 08:38 AM     Click Here to See the Profile for jkilroy   Click Here to Email jkilroy     Edit/Delete Message   Reply w/Quote
I concurr, that 850 is way too much carb for your 351C, unless it is an all out track machine that will only see WOT.

------------------
Jay Kilroy
68' Fastback GT 390
"No such thing as a cam thats too big"

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 08-06-2001 09:47 PM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
I agree, a 750 would be your best choice. The one to get is the 4010 series with the annular boosters. These are an almost carbon copy of the great Autolite 4100 4 barrel. This is what I am using on my Capri, and I'm running 6.90's in the 1/8.

In my opinion; and this is a minority opinion, but has been formed by hands on experience, not theories or parroting what the car magazines tell you; a PROPERLY setup vacuum secondary carb will give pretty much identical performance to a PROPERLY setup mechanical secondary carb, only it will be much easier to manage on the street, not to mention you will pass far more gas stations before having to fill up.

------------------
'85 Capri
Cleveland powered
carburetted
6.95 1/8th
@94 1/2 MPH
1.49 60 ft.

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steve'66
Gearhead

Posts: 8826
From: Sonoma,CA,USA
Registered: Mar 2000

posted 08-06-2001 10:03 PM     Click Here to See the Profile for steve'66   Click Here to Email steve'66     Edit/Delete Message   Reply w/Quote
We run a vacumm secondary 750 holley on the 351w. Use the lightest yellow spring and it'll open the secondaries when it's ready for the extra cfm's! We launch off a two step now, so the squiters aren't used anyway. BTW it's a 4 speed car.

SteveW

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MrXerox
Gearhead

Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 08-07-2001 12:05 AM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Great info guys! I am curious as to why a Vac. Sec would be more fuel efficient than a mechanical unless you really got on it anyway to open the rear barrels? and is it true that auto trans equipped street cars would like the vac. sec. better than a mechanical? I can kinda see the logic behind it...?

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 08-07-2001 05:47 AM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
The vacuum secondary will be more fuel efficient because in normal driving you will only be running on the front 2 barrels unless you really need the CFM.

With a mechanical secondary, every time you get into the throttle a little too deeply, not only will the squirters be delivering additional fuel, but also the secondary barrels will be opening a bit, delivering fuel.

If you can drive very conservatively, and not get the throttle into the secondary pickup point unless you need to, then there would not be much of a difference. Easier said than done when you have a car that actually "runs" on the street.

------------------
'85 Capri
Cleveland powered
carburetted
6.95 1/8th
@94 1/2 MPH
1.49 60 ft.

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bluestreek
Gearhead

Posts: 1724
From: Athens,GA
Registered: Jul 2001

posted 08-07-2001 12:14 PM     Click Here to See the Profile for bluestreek   Click Here to Email bluestreek     Edit/Delete Message   Reply w/Quote
I have ran a 760 CFM mechanical on my little 289 with the same 1/4 mi results as a 600 vac sec.. I had to do some serious custom tuning on it to get it to be responsive.. You stated that it was idling too high and lean.. This could be caused from a vacuum leak, a hidden vacuum port that is not plugged, or rear butterfly that is opened too far.. Since you rebuilt the 850, I assume it was used.. Tuning a used 850 without any history of it's modifications can be real tricky at best.. I would highly recommend starting with a fresh new 750 DP or Vac. Sec... That way you know " the rest of the story", when you start tuning..

Good luck,
Dan

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MrXerox
Gearhead

Posts: 348
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 08-07-2001 04:39 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Hey Alex, care to share some of your infinite wisdom on this?

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