850 cfm proform carb on 318!!!!

[rumblefish360]

The problem with what you said is that the carb in mention is a TQ which has the smaller primary. This makes it alot easier to tune and drive over a similar cfm rated square bore design carb. The larger primary makes it harder to tune and keep in step with the throttle openings under various conditions. Also, the bigger carb is set up for a bigger engine by Holley, not for a smaller engine. This will require a bit of retuning everything on the carb.

I did such things some time ago and the bigger the carb, the bigger the pain in the butt it is to get it where it needs to be, or at least done right all across the RPM range and driving conitions. This is also why most people recomend a smaller carb for street , like a 600, a 750 for most heavy hitters.

 

[1wild&crazyguy]

4 corner idle will have less trouble idling compared to a vac sec with 2 corner.

The dyno is not the car, if the gears aren't there....it will suck till 35-4000k

Im building a 1050 annular quick fuel for my 410, but the 750 demon works pretty good too....
what is it they say about volumetric efficiency again ?

40 shooter on a 750 with a 318, must have some convertor....but then again I see a lot of black in the primary....hmmmmm, shot is covering for something else, covering a whole lot..................or is this a just a past history of back fires..............oh wait, Im sounding like someone else with all these ''............'' LOL


I knew a guy with a 850 on a 340 4spd who was real good at tunning, even he said his 850 blubbered a lil till 35-4000.....makes sense with the fuel just bouncing off the side of the boosters like a water fall in order to compensate for the major lag in main jet initiation, weak signal from too small cid/too large a venturi.

I bet a 950 runs better than a 850 on the same motor.

compensating the rest of the combo/gears in order to run a big carb, thats more than just a tailored curve.

just thinking out loud...

 

[JoeDust451]

I agree with moper on this, the larger carb can make more HP, but at lower RPMs it can't mix the fuel as well as a smaller carb (if its improperly sized too big), the signal is weaker, the air flow is slower, & by the time you jet down the curcuits to lean it out it becomes an issue, mainly once it gets into the main curcuit & WOT, You need to have the head flow & cubes to properly fill the area of volume to get the best results, because if this was the case that any carb would do on any cubic inch engine for all around perf., there would only be "ONE" carb fits all, Its been proven time & time again that a properly sized carb will give the best all around perf., larger carbs can work better in all out drag racing, But there is still a limit, most depends on the other aspects of the combo, gears/stall speed/shift points ect., but for a primarily street driven car, a smaller car (but not too small) will give the "best" overall drivability & power.

You can't compair Engine dyno HP/Tq too accual in car results, Even though you make alittle more HP/Tq on the stand, doesn't mean you'll run faster when its pulling a mass.

It all comes down to speed of the volume of air & the ability of the area of volume of that air & fuel to burn properly in all areas of the RPM range, Larger carbs work better at higher RPMs because thats where the signal gets stronger, a perticular engine is only going to draw in the amount of air that it needs, no more, no less, its what you do with the air that makes the difference, but the fuel speed will useally stay the same on that perticular carb, if you cannot burn the amount of fuel volume for the amount of air in a given area in the RPM range, you'll have less results, If you need to jet a carb up too much with all other areas being in good working order, you can bet it could use a bigger carb, if you have to jet a carb down too much, its most likely too big, i'm not talking a few jet sizes, i'm talking 6-8 jet sizes.

 

[JoeDust451]

IIRC, a '71 340 comes with an 850 Carter from the factory. So jokes on them!!

Your talking "spreadbore" vs. "squarebore", two different animals, can't compair apples too oranges!!

 

[moper]

With the Thermoquad arguement you're not just the spread bore deal, but a mechanical secondary Holley to a air-valve secondary Thermoquad is apples to oranges. The T-quad has the capacity to flow 800 or 850 cfm depending on the throttle bores. But the engine only uses what it needs, even if all the throttle plates are wide open. Same principle applies to a vacuum secondary Holley, and both the counterweighted AFB and air-door AVS Edelbrocks.

 

[1wild&crazyguy]

Anyone ever notice the booster design of the thermoquad...that helps.

How about the secondary booster setup.. oh wait there isnt one, only piss tubes...

 

[moper]

Yeah, Carter's got better throttle response and mileage as a result of the 2 and 3 stage boosters in those tiny primaries... The reason the secondaries don't have boosters is the air door...

 

[dgc333]

FWIW, I have a fairly stout 360, ~375HP. I have had 600, 670, 725 and 750cfm carbs on it. By the butt dyno I don't feel any difference in top end power between using the 670, 725 or 750 but either the 600 or 670 have it all over the bigger carbs in off idle through mid range in throttle response. I settled on the 670 as the best all around carb for my engine, plus it gets 5 mpg more than the bigger carbs.

I think I read the article you are referring to. If I recall the dyno runs started at 3500 rpm and went up, no info in the rpm range a street car speeds the majority of it's time. Dyno runs are done at WOT so they don't tell you anything about how the engine is going to run at part throttle when you are mostly running on the idle and transition circuits.

Another thing to consider going to a carb that is much larger than what the manufactuer recommends for an engine is going to require a lot more tuning to get it to run exceptibly. It won't be a main jet change, you will be playing with air bleeds, squirters and likely the emulsion jets on the metering block.

 

[darndart]

I agree with you 100%. A few years ago I did a similar test on my 73 Dart Sport bracket car. I normally run a 750 ProForm double pumper carb. One night at the track a buddy offered me his 950 ProForm to try out. I pretty much told him, he's crazy that's way too big. But for kicks we tired it anyway. The very first pass I knocked off a tenth from my best ever run with the 750(it's been on my car 7 years, plenty of tuning) with a couple jet changes I shaved a tenth and a half total! The crazy thing is, it was all in my 60 ft & mid track, mph was the same!!:wink:

 

[JoeDust451]

I agree with you 100%. A few years ago I did a similar test on my 73 Dart Sport bracket car. I normally run a 750 ProForm double pumper carb. One night at the track a buddy offered me his 950 ProForm to try out. I pretty much told him, he's crazy that's way too big. But for kicks we tired it anyway. The very first pass I knocked off a tenth from my best ever run with the 750(it's been on my car 7 years, plenty of tuning) with a couple jet changes I shaved a tenth and a half total! The crazy thing is, it was all in my 60 ft & mid track, mph was the same!!:wink:

Thats a very simple analogy, The 950 carb is "smaller" then the 850, if you don't beleive it, look it up (ventury size), thats why the 950 worked well, it has 1.3/8" venturies too a 1.3/4" base, they give a much better low-end signal resulting in better 60ft. times.

The 850 has 1.9/16" venturies too a 1.3/4" base, BIG difference.

The 950 is basically just a 750 installed on an 850 1.3/4" base, of coarse it tapers out to match, so your getting a better cone / \ effect picking the air speed up as it enters the ventury, this really isn't anything new, guys modified there holley 750s back in the day to mate to the 1.3/4" base, the 750 has a 1.11/16" base.