What happens if you put a carb that is rated at say 850cfm onto a 305 or 350? Just curious. I imagine it would bog something fierce and run way too rich unless you jetted it way down. What would happen when the secondaries open? Just some things like that...

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

It will bog very easily, and have terrible throttle response.

It will actually tend to run leaner tho. Same size engine=same air moved, but through larger bores it will produce less vacuum signal, hense suck less fuel.

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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
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ICON Motorsports

 

Jester have you rematched Pab's TBI car? I don't remember but didn't you lose to it a while back? OH, by the way I am planning either an SEFI or Carb swap in the future on my 92. Today I'm Leaning toward Carb Because I don't feel like dropping the tank an changing the fuel pump and fuel lines, plus harness and blah,etc...

 

No, Pablo never beat me. The last time we went to the track I was ahead by almost a second. he's gotten faster since then with tuning, but never enough to beat me.

But, he probably would beat me now simply because my engine has taken a $hit. The 250,000+ miles have caught up with it, and it's lost almost as much power as it has oil.

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"American made baby. 100% American iron. The muscle among the masses. My hero. Yep, you can take your ergonomically designed, space age, computer controlled, 4 door, cup holding map lighted split double wishbone split fold down retractable cargo covered moon roof piece of transportation and keep it. For I have felt the thunder. And I know the difference!"
JSP Motorsports
ICON Motorsports

 

Hello Biochem,

An 850 Holley is way too big for a 305. Remember, 305's don't rev very high and they don't fill the cylinders very well above 4000 rpm. A 650 is a big as you will need on a 305.

For a 350, only use an 850 in an automatic if you have steep gears (3.73 or higher) or a high stall converter. For a stick car, you will still need the steep gears.

If you have a real bad-boy car (headers, intake, cam, forged pistons & ported or aftermarket heads) you can make it both purr and roar with a slightly smaller carb.

Fuel atomization and mixture velocity are the key.

 

Couldn't of said it better. Just to expand a little. The small bore design of the 305 is what holds it back by making it have smaller valves. This is why the chevy 327 and the ford 302's so far out perform the 305.

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1984 WS6 Trans Am Hartop
Former L69 Car under restoration
1984 Trans Am T-tops
4-bolt main 350, performer intake, headers, Holley 650, T-5, hayes clutch, dual elec. fans and 3.23's.
Daily driver and restoration
13.98 @ 101

[This message has been edited by 84FTA (edited January 24, 2001).]

 

Thanks for the info. I have a Holley 750vac on the pretty healthy 357ci engine. I didn't think it was too much for it, but just wanted to make sure that anything "wierd" couldn't be attributed to over-carbing...

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1984 z28 w/ a 357 cu in. monster engine which is looking like the posterchild for Edelbrock with the exception of the Holley 750vac... all the suspension stuff... 9-bolt posi disk is in...

-=ICON Motorsports=-

 

LOL sorry 84FTA but I've never had any comparable 302s or 327s outperform my 305 yet, so I have doubts about your theory.
Anyway I will quote David Vizard as saying "there is no such thing as too much carburetor, just not enough signal at the boosters"
As long as you keep signal high at your primary metering then all is well even with 850+cfm on a 305.
And whoever said it was right about the bigger carb actually making you leaner in most cases because of the common loss in signal.
Granted it does take more experienced carb tuning to make the bigger carbs work well on the small engines, and usually with very little performance payoff.
So it is better to take the easy way and keep the cfms on the minimum side of the scale for the cubic inches to be fed.
If what you're wondering is if it's likely an 850 carb will perform better than a 600 carb on your 305, then I say it is highly unlikely to help at all, but more likely to cause severe drivability problems without the proper adjustments.

sorry just rambling
I'm into carb tuning


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White 1986 Irocz, 305/383 with Edlebrock Performer-RPM intake and Performer #1407 carburetor, +110hp shot of crack, 700R-4 tranny, 3.25:1 rear, Mcreary Road-Stars, single 3" Borla exhaust, Linginfelter-TPI camshaft pulls 17" vacuum solid. N/A runs 10.9 @124, Crack-runs 10.3 @135... haven't run at track since Oct-99

 

Well ODB, all I can say to you, is that you have never run a properly set up 302... it will wax a 305, especially at the big end, and especially if it has enough gear.

A car will almost always run faster with too little carb than too much. A 750 vac sec is fine for a 350, although the car might be faster with a man sec 650.

If you put too big of a carb on a small motor, it will run lean, because the smaller motor moves less air through the venturis, which will produce a smaller "signal" to the main system, which will feed less gas. This is not well understood it seems. On the other hand, it may idle very rich, because idle fule feed is based on manifold vacuum; and since a larger motor (which the larger carb will be calibrated for) requires more fuel at any given manifold vacuum, the small motor will get flooded at the same vacuum. So you end up with a confusing mess to try to tune: black smoke and fouled plugs at idle, flat spots and bogging in the midrange because of low velocity through the venturis, and lean-out at high RPM because the carb thinks the motor is doing less work than it is actually trying to.

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"So many Mustangs, so little time..."
ICON Motorsports

 

yep yep you are right, never denied that stuff.
Can you say tunnel-ram & dual quads on a street 350?

just simply going a little big CFM-wise on a mild cammed street engine with a dualplane style street-intake usually results in little harm, and can be adjusted fairly easily. However the same smallblock with a singleplane + monster camshaft my produce such weak signal at low-RPM that it's not worth trying to tune.
The dual quad setup is usually a nightmare at low engine speeds.

And you are right that an undercarbed engine can often outrun an overcarbed engine because of stronger torque at the shift recovery RPM = quicker ET's. Not always, but yeah it happens a lot.

Oh yeah I will let you know how my 305 runs N/A when I get it out again. Should be a good run for the 302s.

later,



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White 1986 Irocz, 305/383 with Edlebrock Performer-RPM intake and Performer #1407 carburetor, +110hp shot of crack, 700R-4 tranny, 3.25:1 rear, Mcreary Road-Stars, single 3" Borla exhaust, Linginfelter-TPI camshaft pulls 17" vacuum solid. N/A runs 10.9 @124, Crack-runs 10.3 @135... haven't run at track since Oct-99

 

"The OBD" what is a "305/383"?

 

I swap between the 305 and 383 engines in the same car. Lets me race with one while tweaking on the other,
that's all

 

Tal you did major seriously under-carb that one!!! I think in your case the carb was so small that its fuel capacity was exceeded even more than its air flow capacity was, which leaned it out and caused your meltdown.

Of course you didn't say what kind of 1000 miles that was , if it was 200 passes or what! But still, a 750 mechanical or 85 vac sec should suit that combo well.

Sounds like ODB has some version of "the best of both worlds" going on there... not a bad idea actually, if you have the time to do the swaps.

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"So many Mustangs, so little time..."
ICON Motorsports