Carburetor Choice Advice...

[Screaming Metal]

What kind of gears and Trans you running??????????????????

 

[FastEddie59]

Hello Engine 24355, That is very good combination, I've had a couple like that. If you are running an upgraded ignition like an HEI with a module and hotter coil or an MSD style box with good quality wires, that will burn way more fuel than a stock set up. On a drag race engine if you go from say an old style dual point to an MSD system you will come close to going up 10 on the jets to get back to the same fuel/air ratio it was before. The cam is rather long on duration and has a fairly radical lobe centerline so it will take a lot more carb than you think. If you really want a vacuum secondary carb buy the Summit 750 you mentioned, heck of a buy for the money. If you want something that you can really get close to optimized go with a 650 to 700 cfm double pumper, the 700 for sure if running an upgraded ignition. The formula that another Bucketeer posted is a good rule of thumb, roundy round guys swear by it, but I have yet to see it work from a drivability stand point, fine if you run at wide open throttle all day, but you need more fuel than that to make the car really pull to get you off the line and make a strong transition to get to wide open throttle and have good throttle response and pull strong from a roll like if you are passing on a two lane road or it will feel flat. I agree with RPM's rule of thumb, and with a good ignition that motor should make at least 350 hp, if not closer to 400 and that would dictate a 750 to 800 cfm carb, keep in mind that a 325 horse factory motor with a Q-jet was 800 cfm. Thats my two cents worth again, add a dollar to it and you can get a cup of coffee, LOL! Good luck on getting it sorted out, Eddie

 

[Wild Mango]

A vacuum gauge is a good tool to let you know if you can flow enough air with a given setup. With your leg firmly hung in it, all barrels wide open, as that sweet babe revs on out the vacuum will slowly sneak up as the revs build.
I.E. The pressure drop across the engine needs to increase so as atmospheric pressure can stuff more air into it. The pressure drop will go up in a roughly linear relationship with the RPM as long as the flow is not restricted adversely.
However, if you have a flow impediment, like too small venturis, restriction will show as an increase in manifold vacuum past the linear increase, usually it rises quite sharply as the optimum point is passed.
SAFETY NOTE: If you are doing this test on the road, take an observer to watch and note the vacuum and RPM while you concentrate solely on driving..................

 

[terrymac]

This is some advise that Mike gave me when I was having trouble.
Here's a really good rule of thumb to use for carburetor selection -

Quote:
(Cubic Inches X Maximum RPM)/3456
All the "I've got a pair of 1050's on my 327" guys are going to wade in here screaming at me, but lets look at your application.

You have a 355" motor. What's the maximum RPM you see with this combination? Do you ever screw it up to 6,000?

(355 X 6000)/3456 = 616 CFM

Maybe you're brave and you run it up to 7,000? 719 CFM

8,000 RPM? 821 CFM.

9,000 RPM? 924 CFM.

10,000 RPM? 1027 CFM. Ahhh, now we're finally reached the flow capacity limit of your carbs.

But let's start looking at this realistically. What is the maximum RPM on this motor in normal driving conditions? I bet you rarely run it up to 4,500. That's 462 CFM.

300 of duration means a lot of overlap. A lot of overlap means a loss in manifold vacuum. A loss in manifold vacuum means a weak signal to the carb/s. Carbs are pressure differential devices and when a carb doesn't see much differential, it gets lost. So the carb is going to be sitting there waiting for air speed to pick up through the venturis before it starts to understand what the heck is going on. And that isn't going to happen until the motor starts to get upstairs.

I've been watching this happen for more years than I can count. A customer walks in the door with a wad of cash in his hand. His buddy told him to get a set of double-hump heads, a set of 12.5:1 pistons and a 302 camshaft, a tunnel ram and 2 750 CFM double pumpers for his street-driven 4X4 pickup with 33" tires and 3.08:1 gears.

What do you mean I will have to run race fuel in this engine, I plan to pull my boat to the lake with this truck. What do you mean I'm going to have to put lower gears in? Won't that kill my mileage on the way to the lake? And why do I need a looser convertor? And I think the stock valve springs will be OK, they still have 85 lbs of seat.

Start at the opposite end of things. How are you planning to drive the car? On the street? Long distances, are they a possibility? If so, then you are likely wanting to use pump gas. So a lot of compression is a no-no. Where is the motor going to spend the majority of its running time? At 3,000 RPM? Then build everything with that in mind, so the engine can be happy at 3,000 RPM. That means 'think small'. Small runners in the heads and intake. Small valves. Small cam profiles. Small carbs.

Need a visual to see what I'm talking about? Go outside, take the nozzle off your garden hose and turn the water on. Go ahead, turn it on full blast. How far is the water shooting out of the hose? Now, take your thumb and reduce the opening at the end of the hose. What just happened to that stream of water? Air does the same thing in a runner as water in a hose.

Same with intake manifolds. Take a plastic soda bottle and pull a vacuum on it with your mouth. See the bottle collapse? Now, take a five gallon jug and pull a vacuum on it with your mouth. Your vacuum ability gets lost with that additional volume, doesn't it?

Again, the bigger-is-always-better guys are going to flame me for this one, but don't let them fool you. If I just spent $10K on a sheet-metal intake and a pair of whiz-bang, double throw-down Dominators, then that is going to be the hot lick too. I don't want you to see me fuming because my investment won't work.

Do you want to have things look good? Or do you want performance to rip your head off?

Makes for some very good reading. I wish I had seen this before I bought my carbs but with a lot of tweaking things are good now

 

[engine24355]

[quote name='Screamin' Metal']What kind of gears and Trans you running??????????????????[/quote]

I am running at 350 with 2K stall and 273 gears.

 

[engine24355]

Thanks for all of the help and advice here guys. I am really stoked about all the potential, but want to be smart about it also. My goal is to have a streetable car that has some power on tap if I need it. Obviously, I want this motor to run efficiently so finding the right carb is essential. I am an idiot when it comes to carbs. I have worked on them for years, but never really understood the fine tunings and jetting like I should. Therefore, I need all your help I can get. So, based on my engine specs and information I have given, keep the suggestions coming. At this point it sounds like I need to go at least 650 or maybe even higher. Again, not too familiar with the jetting etc. so I need as much help as possible. Sorry, most of my work has been on dirt track cars that ran two barrels that did not have much tuning capability. Thanks again!!

 

[Screaming Metal]

I am running at 350 with 2K stall and 273 gears.

Hey Engine......the 2.73's are highway gears.......you can leave the line a little above idle and got a good healthy free reving engine.......a lot of cam...........3.23's or 3.55's would be better with a 700,735, or 750........you'd have to have at least 3.73's to honk with the bigger carb., I'd thing a dual feed 650 with vacc. secondaries and get a holley trackpack, that way you can spend a afternoon leaving black marks on the road tuning it..............One word of advice......don't put too light of a vacc. spring in the rear 2 barrels or your gonna have a big bog.......Idle her up some and you can change your accelerator pump cam. With that engine you might even want to go with a crows foot squirter nozzle........

 

[one finger john]

Look in the want adds and find a good used Edelbrock Performer with a good used 500 cfm manual choke Edelbrock carb. Or better yet, Performer/450 vacuum secondary Holley.

 

[engine24355]

What do you guys think about the Holley Street Avenger line? I actually just came back from one of my favorite and trusted speed shops and they suggested running a 670 avenger for my application. I like the features it has, but everyone has their own story. Any thoughts?

 

[Screaming Metal]

What do you guys think about the Holley Street Avenger line? I actually just came back from one of my favorite and trusted speed shops and they suggested running a 670 avenger for my application. I like the features it has, but everyone has their own story. Any thoughts?

Go for it......if you have 20 or 30 CFM more than you need at WOT.......you can tune your 2 secondaries to come in later where your engine can use it....20 or 30 won't kill ya unlessyour up into the Dominators.....hee-hee....when I'm on the line up there racin.......if it blows, like the old saying goes.......'We can't go any faster if we don't try!'

 

[FastEddie59]

Hello Engine 24355, The 670 should work great, but the Summit 750 would too and as they are both vacuum carbs the motor will only take as much as it is able to pull, both have many good features and are adjustable, but I would give the edge to the Holley because it has an easier design to work on, quick change secondary spring kit, and a rear metering block so changing jets will be easier, even though it is more expensive. Buy the dual line model, the center hung floats will work better in your application, and the parts for a Holley are more readily available, Eddie

 

[bucketkids]

What a huge amount of info on this thread. Thanks everyone!!! I love this place

 

[engine24355]

I have read some bad reviews on the Holley SA line so I really could use your help here. I like the features of the carb, but if real world application is bad I need to move on to something else. That said, many of the reviews I read or from a while back so I can only hope that Holley has listened and made some improvements.

Thanks!

 

[Screaming Metal]

I have read some bad reviews on the Holley SA line so I really could use your help here. I like the features of the carb, but if real world application is bad I need to move on to something else. That said, many of the reviews I read or from a while back so I can only hope that Holley has listened and made some improvements.

Thanks!

If you listen to 'some' bad reviews........you'll probably want to toss Holleys.....cause of the power valves! Ha! Just gotta learn to tune them......get a dual feed carb with center hung floats 650 or 670 cfm,........just go get a older Holley Number, some of them they're really pround of........usually I get the older stuff and rework it all.

One of the Holleys or one of the generics....you can tune them for most anything.........sometimes you get a bad one.....but thats with any make. Edlebrock hasn't got the bugs worked completely out yet.....I think its in the quality control.....maybe the shippers dropping them.....I don't know.....some folks have had a few problems....me personally....until 'I' have a problem........I won't say anything negative about them.

Now.....If your gonna flog the crap outta the 355......and do alot of performance driving..........a 650 doublepumper would be the ticket......you can always detune the rear 2 slightly.............Just my 2 pescos.......

Ted has alot of experience in this type of stuff....he used to work on all these typa cars.....I'd be anxious to hear his recommendations.........he knows his stuff..............

 

[engine24355]

Thanks for the advice guys. I picked up the 670 street avenger with electric chock and vacum secondary. I thougt it would be worth a shot so we will see what happens. Still need to pick up a few things like a fuel line and air cleaner before I can get the new intake on. Stay tuned....

 

[Screaming Metal]

Hey Engine.....let us know how she turns out.....and how she runs!!!!!

 

[engine24355]

Well the new intake and carb setup is up and running. Still tuning in and getting it just right, but overall it is running good. One question I have is with the electronic choke. I have a 670 Holley Street Avenger with the E-Choke on it and I understand how the choke works (metal coil etc.), but I do not understand how this effects the idle (or does it). I have always used manual chokes or the gas pedal to warm up a car so this is new to me. Can someone explain. Thanks guys, I will post some pics when I can...mocked up and running, but still need to get new fuel line, air cleaner, etc. on before complete.

Thanks!!

 

[Rick]

If you follow the shaft in your choke plate you will see a rod going down to the bottom of the carb.At the bottom you will see a little cam devise and your throttle shaft has a widgit on the end with a screw on it.Ok when your chock is parially or all the way closed the screw clicks into flats on the cam devise and raises the idle till the choke releases it then your idle goes to the setting you made where your gas peddle goes to.You can adjust your choke high iddle with said screw on the throttle shaft widgit.Hope this helps.

 

[engine24355]

If you follow the shaft in your choke plate you will see a rod going down to the bottom of the carb.At the bottom you will see a little cam devise and your throttle shaft has a widgit on the end with a screw on it.Ok when your chock is parially or all the way closed the screw clicks into flats on the cam devise and raises the idle till the choke releases it then your idle goes to the setting you made where your gas peddle goes to.You can adjust your choke high iddle with said screw on the throttle shaft widgit.Hope this helps.

Thanks for the information Rick. I think I see what you are talking about. Seems like the problem I am having is that the engine runs okay at idle and then it just drops down and tries to die after a couple of minutes. I have not confirmed yet, but it seems like the choke is droping off before the engine is warmed up enough. However, the engine is at normal temp when this happens. Anyway, I need to mess with more and figure out. Thanks again!