mechanical secondaries vs vacuum secondaries
Thread Starter
Racer
Joined: Apr 2008
Posts: 319
Likes: 7
From: Rochester NY
mechanical secondaries vs vacuum secondaries
I'm about to purchase a different carb and I've had so many mixed reviews in regards to whether I go with a Mechanical secondary or a vacuum one.
I mainly will drive my modded 77 on the street to cruises, etc with maybe a few trips to the drag strip... I do mean a few, but I'm looking for a nice set-up where I can feel the power.
My issue is that the vette I purchased last year is an auto and it has a B&M stall conv but it's only a 2000 holeshot, but I have 3.73 gears...
Many people are telling me that the mechanical secondaries will be fine, but I'm not sure.
I know C3's are heavy cars in general, but mine has no AIR, Lighter wheel, alumn heads, be cool alumn radiator, no power windows or seats, alunm intake man, headers, no cats, lighter leaf spring, no spare tire or mount, etc. so in general my car should be much lighter -which I know would be helpful for mechanical secondaries...
So what do you all think?
Thanks-
I mainly will drive my modded 77 on the street to cruises, etc with maybe a few trips to the drag strip... I do mean a few, but I'm looking for a nice set-up where I can feel the power.
My issue is that the vette I purchased last year is an auto and it has a B&M stall conv but it's only a 2000 holeshot, but I have 3.73 gears...
Many people are telling me that the mechanical secondaries will be fine, but I'm not sure.
I know C3's are heavy cars in general, but mine has no AIR, Lighter wheel, alumn heads, be cool alumn radiator, no power windows or seats, alunm intake man, headers, no cats, lighter leaf spring, no spare tire or mount, etc. so in general my car should be much lighter -which I know would be helpful for mechanical secondaries...
So what do you all think?
Thanks-
Drifting
Joined: Aug 2005
Posts: 1,682
Likes: 3
From: League City TX
If feeling the power is what your after, a mechanical secondary carb will give you more of that feeling.
You will have to be careful on how large of a carb that you buy though since your converter is darn near stock.
You will have to be careful on how large of a carb that you buy though since your converter is darn near stock.
Le Mans Master
Joined: Nov 2008
Posts: 5,691
Likes: 12
From: columbus oh
i have always installed vac. secondaries for auto cars that are primarily street driven
there is really nothing to be gained with mechanical carbs vs. vacuum except for instant rear throttle plate opening and poorer mileage
properly tuned vac. secondaries will run right with mechanicals for the street, you just don't feel the initial surge and sound which most performance folks really like.....good luck on your decision....
go with the holley street hp version if you don't mind the chokeless set up.....
there is really nothing to be gained with mechanical carbs vs. vacuum except for instant rear throttle plate opening and poorer mileage
properly tuned vac. secondaries will run right with mechanicals for the street, you just don't feel the initial surge and sound which most performance folks really like.....good luck on your decision....
go with the holley street hp version if you don't mind the chokeless set up.....
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
I take the time to properly set up Q-Jet's for my street build's. Don't bother looking in the R/R mirror for the other guy's anymore, as know they're back there somewhere. The small primary's on these carb's really help around town torque/throttle response, and when the large vacuum secondaries phase in, hold on.
Le Mans Master
Joined: Oct 2007
Posts: 7,436
Likes: 961
From: Tennessee/Rhode Island
Vacuum Secondaries all day for a street driven machine! Mechanical secondaries will give you the "bog". For a basically stock motor, I would go with the Holley 4175 650 CFM spreadbore, it's Holley's replacement for the Qjet.
I did not notice any more power from the quadrjet when I replaced it years ago with the Holley 4175-They both had equivalent power just the Holley idled much better, started easier (cold start), was much easier to custom tune, ran better at cruise, and had about the same mileage. The reason I replaced the qjet which is a good carb IF you can get it to run right, was that the Holley's design is MUCH simpler for what a carb needs to do than than the Qjet which in my opinion is way to complicated for a relatively simple task!
I did not notice any more power from the quadrjet when I replaced it years ago with the Holley 4175-They both had equivalent power just the Holley idled much better, started easier (cold start), was much easier to custom tune, ran better at cruise, and had about the same mileage. The reason I replaced the qjet which is a good carb IF you can get it to run right, was that the Holley's design is MUCH simpler for what a carb needs to do than than the Qjet which in my opinion is way to complicated for a relatively simple task!
Thread Starter
Racer
Joined: Apr 2008
Posts: 319
Likes: 7
From: Rochester NY
From what I've heard I don't think a Holley 4175 would work in my case- as my car is not too stock. What I have is the following - maybe this will help?
What I have in the car now is:
A 1973 Chevy 4 bolt 350 engine bored over .30 and it has a set of Brodix aluminum heads (170cc intake ports, 69cc combustion chambers, 2.02/1.60 valves, 1.25 valve springs.)
Edelbrock Performer air-gap RPM intake - not installed yet.
Lunati cam – 224/224 duration@ .050, .460/.460 lift. Crane 1.5 ratio aluminum roller rockers, Crane chrome moly pushrods, TRW forged pistons. Exhaust -Hooker headers no cats -through 40 series Flowmasters.
350 auto trans has a B&M Hole shot 2000 stall converter and 3.73 gears.
What I have in the car now is:
A 1973 Chevy 4 bolt 350 engine bored over .30 and it has a set of Brodix aluminum heads (170cc intake ports, 69cc combustion chambers, 2.02/1.60 valves, 1.25 valve springs.)
Edelbrock Performer air-gap RPM intake - not installed yet.
Lunati cam – 224/224 duration@ .050, .460/.460 lift. Crane 1.5 ratio aluminum roller rockers, Crane chrome moly pushrods, TRW forged pistons. Exhaust -Hooker headers no cats -through 40 series Flowmasters.
350 auto trans has a B&M Hole shot 2000 stall converter and 3.73 gears.
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Race Director
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From: Corsicana, Tx
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Personally, I use mechanical secondaries on all this street toy stuff whether auto or stick. You'll have to tune both. The vacuum won't open fast enough and the mechanical *may* open too fast. If anything you play with pump shooters..no big deal..but may not be needed. You've got good heads and a realtively mild cam. Best part of mechanical is you control it 100% with your foot. Only opens as far as you want it to.
It's preference, vacuum secondaries will work OK...but if the mechanicals are tuned right they WILL run better on the street without the *rubber band* feeling of vacuum secondaries.
JIM
It's preference, vacuum secondaries will work OK...but if the mechanicals are tuned right they WILL run better on the street without the *rubber band* feeling of vacuum secondaries.
JIM
Melting Slicks
Joined: Mar 2005
Posts: 2,838
Likes: 10
From: Austin TX
Personally, I use mechanical secondaries on all this street toy stuff whether auto or stick. You'll have to tune both. The vacuum won't open fast enough and the mechanical *may* open too fast. If anything you play with pump shooters..no big deal..but may not be needed. You've got good heads and a realtively mild cam. Best part of mechanical is you control it 100% with your foot. Only opens as far as you want it to.
It's preference, vacuum secondaries will work OK...but if the mechanicals are tuned right they WILL run better on the street without the *rubber band* feeling of vacuum secondaries.
JIM
It's preference, vacuum secondaries will work OK...but if the mechanicals are tuned right they WILL run better on the street without the *rubber band* feeling of vacuum secondaries.
JIM
I'm a DP guy all the way if you have the gearing which you do (TH-350 + 3.73's out back).
If you've compared both a vacuum secondary vs. a mechanical, I believe there's no way you'd choose the VS carb. The response of the DP Holley is unbeatable.
And agree that a 650 cfm DP would work pretty good here or a 700 DP.
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
at the moment i think your border line to go either way. it would be much of a muchness with vac V mech...but put in a 2500 stall and a DP will be very usefull and i think you will be flying.you will then have the gears and stall for a 750dp but at the moment your engine package is going to like a 650dp so thats my choice.
Race Director
Joined: Nov 2002
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From: Taylor Michigan
Race Director
Joined: Dec 1999
Posts: 11,094
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From: Sierra Foothills CA
With no other changes I went .35 quicker and picked up around 2-3 MPH on my trap speed- far stronger with great drivability. At the time I was running a stock torque converter so it wasn't like I had a high stall to cover up any carb problems. Granted, the carb had some nice work done to it that helped performance but the point is that it has always run superbly and there has been no downside to running it with my TH400. It flat works! The only negative is that it uses more fuel but with my combo that simply isn't a priority to me. So, conventional wisdom isn't necessarily the best option.
Last edited by Les; Dec 24, 2010 at 12:21 AM.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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A 750 DP Holley is about the most universal carb ever made. Works well on everything. I ran one on my '82 GT Mustang with a 302 that was otherwise dead stock back in '82. Sucker was perfect. Used same carb on I don't know how many engines from that 302, through a 440 Mopar and a 454 Chevy. In fact it's still on the 440.
A dual plane *likes* a larger carb. For more details read the tech section on Edelbrock's website.
Don't get too hung up on the calculators. You don't want the carb maxed out and causing a restriction at WOT.
JIM
Thread Starter
Racer
Joined: Apr 2008
Posts: 319
Likes: 7
From: Rochester NY
Alright - I talked to a shop who rebuilds carbs and I was going to get a 650 DP mechanical secondaries -and they said that based on my set-up that I should get a 670 holley Street Avenger (vacuum secondaries) - they said that my auto with a low stall
(2000 stall) along with my 3.73 gears would not be enought for the DP.
I'm confused - everyone is suggusting something different - will a 650 DP work on my vette or will I regret getting it and have to go back to a vacuum secondary carb? Could it or will it hurt my transmission since I only have a 2000 Stall?
(2000 stall) along with my 3.73 gears would not be enought for the DP.
I'm confused - everyone is suggusting something different - will a 650 DP work on my vette or will I regret getting it and have to go back to a vacuum secondary carb? Could it or will it hurt my transmission since I only have a 2000 Stall?