Carb Tech

Aliate X · Jun 15, 2005

Hey guys I was planning on doing a 357 swap into my 93 5.0 LX and was wondering about carbs. Ive already pretty much decided to go with a 650 Holley double pumper, but what does all this stuff mean? I know the numbers mean CFM but what about double pumpers, chokless, wouldnt you want a choke? And all these other terms I hear. Thanks for the help guys.

70lightning · Jun 15, 2005

I dont think a DP is really needed for a street car, a 670 holley SA would be a great carb, go with vacuum secondaries.
http://www.holley.com/HiOctn/ProdLine/Products/FMS/FMSC/0-80670.html

95GTAODE · Jun 15, 2005

It all depends, a DP will work well with a 5 spd, while a VS is preffered for an auto.

A "Double Pumper" is reffering to the fact that it has 2 accelerator pumps in the carb. 1 in the primary circuit and 1 in the secondary. These are also called mechanical secondary carbs.

Vacumn Srecondary carbs rely on manifold vacumn to operate the secondarys as opposed to the linkage on a DP.

srothfuss · Jun 15, 2005

IF you live in a cold area (snow belt) you will need to get a carb that has a choke.

As everyone has mentioned so far, a DP is going to kill your Throttle response and MPG. A Vacuum secondary carb will do just as good as the Mechanical, but you will get so much more part throttle response.

CrazyCobra · Jun 15, 2005

To run a DP or VS will depend on the rest of your combo. The DP will work well on an engine with plenty of compression, large intake plenum volume(usually a single plane), low gear ratio(numericlly high), with a stick or lots of stall in an auto.
The VS is the more "street" friendly carb, not needing the above to perform well, as it(holley) relies on Bernulies(SP?) principal, which is as velocity increases pressure decresses. So at WOT the air increases velocity as it passes a small port in the passenger side primary ventury it causes a low pressure in the secondary control diaphram allowing the secondary throttle blades to open. This is desirable in a street engine lower comp, smaller intake, higher gears as a DP would tend to bog. However this slower response to open the secondaries will actually hinder throttle response on the first combo.
Hope you get somthing from that.

Aliate X · Jun 16, 2005

Thanks everyone, that really helped me out a lot. I thought DP's were meant for the street, I will be building a fairly mild 357. Id like a mean cam, maybe 3,000rpm and up or so. Will be running 4.10s with 10:1 compression. It will be 80% street car, what do you guys think?

crazypete · Jun 16, 2005

A mean cam and you will suffer with any carb. Carbs depend on vacuum signal to operate and you will need to compensate by cranking up idle speed, usually by drilling holes in the throttle plates with a wild cam.

That aside. Smaller carb is better. A huge carb will slay your throttle response, idle and basically everything but flat out racing. You will hate your car if you need to drive it on the street. It will have not bottom or midrange and hesitate at every light.

The DP is for flat out racing. A vac sec is for everything else. With a DP, you can floor the car at a standstill and both throttle plates fly open but the air looses all velocity. The dp compensates by throwing not one but two shots (hence the double pumper) of accelerator pump down the throat of the carb but in practice it does not do that well and the car still wicked bogs. With a vacuum secondary, you can only throw the primary open at once and the engine controls based on vacuum requirements alone, when and if the secondaries open. Much better system.

I second the holley 670 street avenger. One of the best carbs I have ever seen. Has every bell and whistle you could ever want on a carb and even the factory settings are almost right on. But lets say you have a 347. You probably wont spin past 6000 rpms so 347X6000/3456 = 607 CFM. A 570 Avenger would be enough if it is only a streetcar and the throttle response would be insane. I have 3 carbs: 570 SA, 670 SA and 600 DP and the 570 vacuum sec takes the prize hands down. The 670 if you rev past 6K regularly but you will sacrifice some lower end.

Just some food for thought.

Aliate X · Jun 16, 2005

crazypete said:
A mean cam and you will suffer with any carb. Carbs depend on vacuum signal to operate and you will need to compensate by cranking up idle speed, usually by drilling holes in the throttle plates with a wild cam.

That aside. Smaller carb is better. A huge carb will slay your throttle response, idle and basically everything but flat out racing. You will hate your car if you need to drive it on the street. It will have not bottom or midrange and hesitate at every light.

The DP is for flat out racing. A vac sec is for everything else. With a DP, you can floor the car at a standstill and both throttle plates fly open but the air looses all velocity. The dp compensates by throwing not one but two shots (hence the double pumper) of accelerator pump down the throat of the carb but in practice it does not do that well and the car still wicked bogs. With a vacuum secondary, you can only throw the primary open at once and the engine controls based on vacuum requirements alone, when and if the secondaries open. Much better system.

I second the holley 670 street avenger. One of the best carbs I have ever seen. Has every bell and whistle you could ever want on a carb and even the factory settings are almost right on. But lets say you have a 347. You probably wont spin past 6000 rpms so 347X6000/3456 = 607 CFM. A 570 Avenger would be enough if it is only a streetcar and the throttle response would be insane. I have 3 carbs: 570 SA, 670 SA and 600 DP and the 570 vacuum sec takes the prize hands down. The 670 if you rev past 6K regularly but you will sacrifice some lower end.

Just some food for thought.

Thanks a lot pete that really helps, the holley 570 street avenger looks like the way to go. I didnt want to run like a super wicked cam, but just something that would scare people a little and be fun to drive.

bluevenom867 · Jun 16, 2005

Whats you idea of a "mean" cam?Are you gonna run a single plane or a dual plane intake (single plane intakes can run up to 100cfm more carb than the same motor with a daul plane intake)?

My previous combo was alot like your,I had a 357.It had 10.5:1 compression,and Crane hyd roller cam (224In/232Ex 2 .050, 542In/563Ex @valve w/ 1.6,112* LSA),and had TFS Track Heat heads and a Victor jr.That motor I ran with a 750 Speed Demon (mech secondary).Id shift bout 6-6200rpm's and on the shot (usualy 150hp shot) I would shift around 6500rpm's.

And Ill tell you this,thats car drove just find on the street.

So what im basicly saying is choose your carb based on what your maximum air flow requirments will be.Dont let your carb be resticting you.

bluevenom867 · Jun 16, 2005

Oh,I forgot to mention.

I also had a 3500rpm 10" stall converter and a C4 in that car.It ran with 3.73 gears.

edit:And mostly was drove on 26" slicks or E/t streets (cheater slick).

Aliate X · Jun 16, 2005

bluevenom867 said:
Whats you idea of a "mean" cam?Are you gonna run a single plane or a dual plane intake (single plane intakes can run up to 100cfm more carb than the same motor with a daul plane intake)?

My previous combo was alot like your,I had a 357.It had 10.5:1 compression,and Crane hyd roller cam (224In/232Ex 2 .050, 542In/563Ex @valve w/ 1.6,112* LSA),and had TFS Track Heat heads and a Victor jr.That motor I ran with a 750 Speed Demon (mech secondary).Id shift bout 6-6200rpm's and on the shot (usualy 150hp shot) I would shift around 6500rpm's.

And Ill tell you this,thats car drove just find on the street.

So what im basicly saying is choose your carb based on what your maximum air flow requirments will be.Dont let your carb be resticting you.

Well its going to stay N/A, and Im not sure what type of intake to run, whats the advantages and disadvantages of each?

By a mean cam I really just meant something that has a more than average lopey idle to it.

bluevenom867 · Jun 16, 2005

What kinda intake your gonna run depends somewhat on your heads.If you got stock heads or something like GT40's or Edelbrock performers.Than I think the best choise for you would be a performer rpm and a decent sized cam (220ish In/230ish exhaust,valve lifts around .530 maybe a little more,an run it on a 108-110 LSA).A motor like that will need a 650,no dout about it.

bluevenom867 · Jun 16, 2005

Also,it depends alot on your rear gear and tire size,and what kind of trans your gonna run.

Heres something I dont get...crazypete,why are you so worried about "low-end" and your runing 4:56 gears?

crazypete · Jun 16, 2005

Simple my good man!

I have an automatic. I need things to happen from 600 rpms on up. Any inconsistencies in throttle response or holes in the powerband and the car doesnt ride smooth. I also dont visit the track or really rev my motor to 5K or higher on the street anyways. I'm out of the powerband up there and the auto will shift anyways before then.

I need it to pull gingerly out of the driveway, slap some honda kids around then ride nice and smooth on the highway and have good torque for climbing hills. I was even thinking of putting some kind of truck cam or RV cam in here, something that emphasizes low and midrange.

Things break at high rpms/high loads. Mechanical parts break, seals burst, drivetrain shock becomes a factor and any mickey mouse quickfixes become liabilities. I got into the habit of tuning the car to do great things on even 2/3 throttle. Results: I still kick all the local kiddies' butts and my almost 150K plant hums right along as if it was new. My 130K hand rebuilt transmission shifts solid. Different mentality. I win little stoplight races on throttle response and midrange alone and I still have a very streetable combo.

But this is beside the issue. If a car bogs off the line with a certain carb, that means something. Sure it runs. I'm sure the peak numbers look super good too. But not everything is about peak numbers. There's lots of numbers inbetween. But I confess, the 570 is probably too small for a 347 stroker. I will say that with a 570, the car will have so much throttle response and quick revs and low end torque. Isnt that what strokers are about? Torque? Theyre not the high revver motors (they can be but usually are not).

My 670 "felt fine" on "the street". I didnt realize that it wasnt the perfect carb until I tried the 600 then the 570. Wow. There is a huge difference between "feels fine" and "perfect carb". I have to be careful not to chirp the tires just pulling into the garage!!

bluevenom867 · Jun 16, 2005

So your runing a 302?

bluevenom867 · Jun 16, 2005

Thats the differance.

Me and you got (well I had) two differant set-up's completly.

Im a little more radical with my set-ups.

Dark Knight · Jun 16, 2005

670 street avenger, 570 will be way to small.. and you can run a bigger carb on a dual plane, not single..

the 670 with an RPM or stealth intake would work good.. not sure if they make the stealth for the 351 though

bluevenom867 · Jun 16, 2005

Dark Knight said:
670 street avenger, 570 will be way to small.. and you can run a bigger carb on a dual plane, not single..

the 670 with an RPM or stealth intake would work good.. not sure if they make the stealth for the 351 though

No,you can run a bigger carb on a single plane.

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