Carb CFM.. Increase?

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Ivoryk3ys78

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I think I know the answer to this but I figured it couldn't hurt to ask.

I bought a Proform 650 Race Mechanical secondary carb for a pretty healthy 318 last year. I really loved the carb. The car started right up and ran great with it and was very responsive and I ran an A/F guage and it seemed pretty dialed in OOTB.

I am building a very healthy 408 now. It will have 254@050 mechanical and Trickflow heads. I am wondering if anything can be done at a reasonable cost to get that one to flow a lot better? I think 850 is ideal for my build.

I see a new proform 850 mechanical race is 600 and i'm sure thats what I will end up doing and I hope sell this one and get some money recouped.
 
Track car?
Amount of street usage?
I’d say yes.
 
It is setup like a track car but will mainly be driven on the street. It is in a 72 Duster and it does not get driven much so I was not concerned with it being what people generally consider nicely streetable. Hughes 3500 10.25 converter, 4.30 gears 27" tall tires and it will have 28" when it does go to the track, I expect the engine to make at least 550. I certainly know the 650 would be a restrictor plate on it. I think 850 probably would be about right.
 
You would be surprised how much more or less carb size makes a difference on the dyno.
I have a 289 Ford that's. 030 over with many modifications that made 455hp and 445 tg but I wanted to try three carbs on it. The shops dyno carb made the most power with the numbers stated a Quick Fuel dp 750 next was a 850 Holley that made exactly the same numbers and finally a little 600 Holley do that made 8 hp less cost the same in tq.
The one thing the gyno cant tell you is throttle response and I believe that the 600will out shine the others

Run your 650!
 
Naw, not until you increase the venturi size. That means another main body.....and the jets and all the other hooha that go with it. I would just get another carburetor.
 
I will go the opposite, ran my 383/509 cam engine with 4:56 and 3800 stall with a heavily tuned Holley 750DP for years and happy. Mainly strip use.

Went the other way for the street, dropped to 3:55 and went up to Eddy 800AVS Thunder. Car launches as hard and over all drives just as well with better throttle response and drive ability. Plugs look better as well.

Nothing changed in the setup but a higher gear ratio. All stock 69 a-body 383 exhaust in both variances. Single plane Eddy TM-6 intake.
 
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289 cubes vs 408 cubes.

The little engine was gulping down all it could.
The bigger engine will be better off with the 850.
I would it be against a 750, but not smaller.
 
Thanks for the suggestions guys. Thanks RustyRatRod yea that pretty much seals it. Hopefully I can get some out of the 650. It has hardly been run.
 
254* is a pretty big cam. IDK what size it needs on the track, but around town the 650 will be a blast.
Since your 1-2 shift rpm is gonna be around 6200;with 4.30s you might hit 44mph in first, then the shift will drop to 1.45/2.45=59.18% so about 3660rpm, and then away to 60@~5000...
3.23s will take you to 60 at the top of first.
Since the tires will be spinning the whole way, I think it would be hard to say which gears will be quicker to 60, on the street.
I get; 3.23s will cruise at 65~2600@zeroslip, so maybe 2700 with that 3500TC..and
with 4.30s, 65~3470 @ zero-slip
Since you already have that 650, I'd give it a workout; on the street anyways..
I ran 4.30s on the street one summer. Then sold 'em.That was too crazy for me (manual trans).
Happy HotRodding
 
I am running a 1050 QF 2 size bigger air bleeds on the primaries, 416 same set up as yours. The responce is like a off and on switch. 3650 weight low to mid 10's at a high elevation track. I have 750 850 and they were to small on the top end. It would run out of steam on the big end. I have a Comp series 9755s on a stock 318 with headers and wiend intake. runs great. I was always a believe in large CFM. six packs were 1250 . on a stock 340.

Duster with a 1050 at Nuemedea 3650lbs street driven for over 8 years tried them all. dyno's are not the real car in actual atmosphere. But they are a base line. We dynoed my motor with a 1000 BLP. we then tried a 750 proform, and also a 950 holley 4 corner.

I tried them all on my car with my right foot and two rich lean guages. The 1050 quick fuel was a little rich on the bottom until I went to the larger air bleeds. Everyone will have their opinion. I am telling you what worked for me. And I wasn't afraid to spend the money to have fun and test what worked. I have $5000 into trials of carbs and some loaners and what I can tell you bigger was better in all cases. The engine will only draw what it needs. I have the time slips and many years of street racing to back what I have learned.

My motor was dead with the recomended 750 from the experts. That 318 with the 750 9755s comp series is here right now and it is a happy snappy little thing. I had two 750's on a iron headed 340 with a 465 lift comp cam. On a truck that I pulled a trailer with. with 38 inch tires. I was told by the experts it would never work. It would turn all 4 tires in drive ,Yes it had throttle pressure so the wife could drive it. Borrow carbs to see what works if you need to. Books don't know your car. install rich/lean guages they don't lie.



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Don't buy a carburetor based on CFM. It's just crazy. Your 650 probably flows way more than you think.

Anything bigger than a 1.400ish Venturi and you'll probably not like it. You can get a carb that flows nearly 900 CFM with that size Venturi.

Can you see why you don't pick a carb based on CFM? It's essentially a number from a flow bench at 20.4 inches of water.

Pick by Venturi.
 
Okay maybe it is crazy. If it is though I am not in the minority for having an understanding that it is not. It is the only way I have ever heard/read about a carburetor being chosen. I am willing to concede it may well be wrong but it is certainly the common practice of selecting a carb.

I suspect the proform 650 can likely flow some more because I know they have made some improvements and it is a race carb but it is still based on 650cfm at 1.5hg I would assume.

David Vizard has a good write up on selecting a carb size. It is based on the old formula that everyone knows but he puts a modifier onto it based on your heads and camshaft duration @050.

Techtips - How to Select the Perfect Holley Carburetor for Your Car

Now if my 650 can flow a lot more then what it is rated at though then I think that is great to hear.
 
YR is telling you venturi size for not just flow but the throttle response with it. The old way, as you say, via cfm rating, was right for the intended drag car purpose that also fit (reasonably well) hot street cars with the older Holley design carb.

If you grab an old school Holley, I’d say no less than 750 for the engine. If you grabbed a modern body worked over carb, you can do more with less, or should. The trouble is prying money out of peoples pocket for the best of anything they deem over the top for the build or what they think they can do themselves.

If you want to drop 1,000 or 1,200 or whatever for a single 4bbl. carb, be my guest. That’s your call.
 
I don't think anymore then the 600 or an 850 that i mentioned early on in this thread when I asked if getting my 650 to flow more was really much of an option.
 
If your 408 was able to allow the atmosphere to fill the cylinders on every revolution,to 100% atmospheric pressure, it would draw in 408/2=204 cubic inches of air on every revolution, which is .118 cubic feet.
At 4000 rpm this is 472 cfm.
At 5000 it is 590cfm.
At 5500 it is 649cfm.
At 6000 it is 708cfm.
At 6400 it is 755 cfm.
Your 254* cam might torque peak at about 4000, and this is about where it might allow 100% filling. It might even allow more than 472 cfm; perhaps as much as 520 @110%VE.
But as the rpm goes up it will be more and more difficult, especially if the engine is forced to ingest hot underhood air. Or is pumping exhaust thru straws. By 6000 it might be down to 95% or less, and that 708 might be just 673cfm
What this means is that you could have dual 750cfm carbs on there, and the engine will still only pull 673@6000 thru them..... because that is all the cam and combo,is gonna allow.
Now if you restrictor your engine as you call it, with a 650, which is rated at 650 at a certain pressure differential, that 650 with different pressure differential is gonna flow a different cfm.

But I gotta re-iterate, on the street, with street tires in stock tubs,and no traction aider; with your 4.30s and 3500TC; you are gonna be spinning all the way thru first gear; so you might as well have a 2bbl on there. 60mph in second gear and not spinning,if that is possible, will be around 4900rpm which being 590cfm at 100%VE / 650 cfm at 110%VE. So,in your combo, on the street, the math seems to indicate that a 650 will be just fine.
Now if you take second gear to 6400/almost 80 mph, well then she is gonna want a 755 times whatever VE your engine can generate there, say 90%, and that would be ; 755x.90=680cfm.

Now; of course, we all know Mopars don't follow this logic, and they some how magically want carbs that are at least 20% bigger. So if that's true you better put an 816 on there, cuz if you don't, the engine will lay down after ~4000, and the guy on a moped in the next lane, is gonna surprise you. lol.
Bottom line is; give the 650 a chance. Whats it gonna cost you?
 
Ivoryk3ys78 said:
Okay maybe it is crazy. If it is though I am not in the minority for having an understanding that it is not. It is the only way I have ever heard/read about a carburetor being chosen. I am willing to concede it may well be wrong but it is certainly the common practice of selecting a carb.

I suspect the proform 650 can likely flow some more because I know they have made some improvements and it is a race carb but it is still based on 650cfm at 1.5hg I would assume.

David Vizard has a good write up on selecting a carb size. It is based on the old formula that everyone knows but he puts a modifier onto it based on your heads and camshaft duration @050.

Techtips - How to Select the Perfect Holley Carburetor for Your Car

Now if my 650 can flow a lot more then what it is rated at though then I think that is great to hear.
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That formula was wrong when it was developed. I know this is hard to hear, but it's wrong. Always was. Always will be.

Just like picking a carb by CFM rating. I'm sure if you look around you can find the Venturi size for your carb. If it's between 1.375 and 1.400 run it. Ignore CFM ratings and math that uses RPM/VE/displacement to pick a carb. You'll be undersized every time.

As an example...I'm looking at buying a carb that has 1.375 Venturi but flows 930 CFM at 20.4 inches of water. How do you explain that? It's because the shape of the carb is way more important than size.

That's what I'm saying to you. Skip CFM to pick a carb. Find out what Venturi you have and if it falls within the above, run it.

If it's a track only car, and you have the gear and converter for it, you can easily run a 1050 Dominator. Won't be nice for the street. But at the track it will be quicker.
 
IMO,
One of the reasons it's wrong is because , to the carb, a 408 is not a 408. The formula ignores the total chamber volume. If you have a 10/1 engine then the chamber size is 408/8/10-1=5.67cubic inches (~93cc), so the carb is seeing (408/8 plus 5.67)x8=453 ci of volume ,on that 408.
So then, that formula more or less works;
6400x453/3456=839cfm
I know it ain't 100% correct because at 8/1 Scr, the volume jumps to 7.286 cubic inches making the 408 into a 466, which would require a 944cfm carb...... which makes no sense.
The formula works off the swept volume, and is the best we have.
But again, IMO , that is where some of the error is.

For my 11/1Scr 367 it works pretty slick; {(367/8/11-1)plus 367/8}x8=404ci, and
404x6400/3456=748 cfm. And I love my 750DP.
 
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Doesn't the relationship between the throttle body and venturi size have a lot to do with it? The amount of signal strength your boosters get, etc depending on throttle body size?

yellow rose said:
That formula was wrong when it was developed. I know this is hard to hear, but it's wrong. Always was. Always will be.

Just like picking a carb by CFM rating. I'm sure if you look around you can find the Venturi size for your carb. If it's between 1.375 and 1.400 run it. Ignore CFM ratings and math that uses RPM/VE/displacement to pick a carb. You'll be undersized every time.

As an example...I'm looking at buying a carb that has 1.375 Venturi but flows 930 CFM at 20.4 inches of water. How do you explain that? It's because the shape of the carb is way more important than size.

That's what I'm saying to you. Skip CFM to pick a carb. Find out what Venturi you have and if it falls within the above, run it.

If it's a track only car, and you have the gear and converter for it, you can easily run a 1050 Dominator. Won't be nice for the street. But at the track it will be quicker.
Click to expand...
 
Ivoryk3ys78 said:
Doesn't the relationship between the throttle body and venturi size have a lot to do with it? The amount of signal strength your boosters get, etc depending on throttle body size?
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Yes. But the Venturi diameter is the biggest influence for performance and driveablity. And the shape of the main body. Look at the Holley junk we had when I was young. That idiotic choke tower was in the way. I was milling those off in high school. And it still sucked because you still had that huge square, flow killing area around the primaries.

That's why I say don't get locked into CFM rating. There is so much more to it than that.
 
Ivoryk3ys78 said:
Yea.. apparently there is.. It's a whole different way of thinking about it.
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Your gettin it! Call up a few of the higher end carb places and give them your information and ask a question or two.
You can simply get a basic Holley or Edelbrock and say, that 750/800 will do and it will or grab something different.

A lot depends on what your doing and of course, how thick your wallet is. Is it worth the expense?
All your call.
 
Ivoryk3ys78 said:
from this forum where they are discussing the main body that people get from proform to modify their holleys they are saying the venturi from that main body is 1.39 in primary and secondary and it is the same from 650 to 850.
Proform 67100C 650-800 CFM Holley Carburetor Main Body

My carb is Proform Race Series carb 67199. So based on the part numbers I think mine is the 1.39 venturis

Proform Race Series Carburetors 67199

Proform Main Bodies for Holley Carburetors 67100C
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If yours is the 1.39 Venturi and a 1.75 throttle blades that's plenty. If it's the 1.687 throttle blades and you think the carb is acting too small, you can buy a 1.75 base plate, blend the bores to match and run it.
 
I’d start off with the 650, see what vacuum level it pulls at full throttle run. And make an educated guess to how much larger you need if any.

The cid x rpm / 3456 = cfm
Is a good appropriate estimate of how air will be consumed by your engine but the problem is that number has no direct relationship with how carbs are measured. An 850 carb doesn’t move more air than a 650, say you engine consumes 750 cfms of air and fuel. Both 650 and 850 will pass 750 through just at different vacuum levels, “restriction” 650 will probably be above 1.5 hg and 850 should be below 1.5 hg. Giving more or less pumping hp loss.
 
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