Carb Mods

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vht
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Carb Mods

Post by vht »

I have an 850 on my .070 over 427 BBC. 4 corner idle but doesn't have replaceable air bleeds, annular boosters plus it came with no choke horn out of the box. Engine has 10.5 to 1 comp, 650 lift hyd roller, and Liberty heads, oval port, 305cc, 2.25 intake valves. I was thinking of changing the main body to increase cfm and get changeable air bleeds, what do you all recommend? I see you can get 950 and 1050, I seen on a mopar forum, a guy picked up et and mph with the 1050, 440 cu in in a heavy car. Also which booster, seems like the down leg is preferred? My carb was brand new in the box but it was a few years old, never installed. How are the Proform main bodies, china?
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Re: Carb Mods

Post by mag2555 »

What facets of its current performance are you wanting/ needing to make improvements on, strictly top end hp?

What Intake are you running?

Have you had the car on a chassis dyno?

Do you have a way to monitor the AFR?

Do you have a Carb spacer under it now, and if so what style/ type?

What rpm is the motor being taken to?

From what I have seen with the cid of your motor and the way it's built I would not step up to anything bigger then a 950 model,especially if your pretty well into the down hill side of the motors torque band by 4000 / 4500 rpm.
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Re: Carb Mods

Post by vht »

Mid range and top end seems off, I had a tunnel ram with 2 600 carters on it along with square port iron chevy heads and it seems to run about the same with the better heads and 1 holley. It don't pull as hard on top end as it did before. I have an eldebrock dual plane with a 1 inch open spacer now but the intake will be changed, that was the only oval port I had at the time. I'm going with another intake, single plane first and then see how it does but I was toying with the idea on the main body since carbs are so high right now. Also it has more compression than it did before, it was only 9.0 to 1 and when I changed heads, I put a set of JE pistons in it to get the compression up.
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Re: Carb Mods

Post by mag2555 »

Sorry I missed one important part, the boosters, they need to go , so a different tad bigger carb might be in a workable price range if you sell off that 850.
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Re: Carb Mods

Post by 1980RS »

vht wrote: Tue May 11, 2021 9:51 pm I have an 850 on my .070 over 427 BBC. 4 corner idle but doesn't have replaceable air bleeds, annular boosters plus it came with no choke horn out of the box. Engine has 10.5 to 1 comp, 650 lift hyd roller, and Liberty heads, oval port, 305cc, 2.25 intake valves. I was thinking of changing the main body to increase cfm and get changeable air bleeds, what do you all recommend? I see you can get 950 and 1050, I seen on a mopar forum, a guy picked up et and mph with the 1050, 440 cu in in a heavy car. Also which booster, seems like the down leg is preferred? My carb was brand new in the box but it was a few years old, never installed. How are the Proform main bodies, china?
When I tested my peanut port engine a couple of weeks ago the 461 like the 1000 Mighty Demon with down leg boosters over my A/D booster Mighty Demon by about 1/2 tenth and 1 mph. Could be because this 1000 really runs well with the Air Gap.
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Re: Carb Mods

Post by rustbucket79 »

VHT, it appears to me you are throwing $100 bills at this engine in the pursuit of improving performance without accurately testing the results of all these changes. For instance, your head change might have been an improvement but other parts are holding it back, or alternatively the new heads are horrible and the heads they replaced were actually better.

I do a bunch of carb swapping on the dyno, and it is a rare situation where one carb is significantly better than another unless it’s defective or severely undersized for the application. The carbs you’re talking about, I would expect 10 to 20 HP, is that good value for the money?

Honestly a day on the dyno with perhaps a carb or 2 to try besides yours, maybe a different intake, couple of spacers, along with timing and jetting would be money well spent IMHO. FWIW I’ve corrected many problems on the dyno due to valve float, distributors, wires, plugs, manifolds, carbs, jetting, timing, etc that would have never been discovered nor corrected on the street or track. Talk with local dyno shops and find one that is genuinely interested in perfecting your combo. He’ll likely have a few carbs to try as well. Good luck!
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Re: Carb Mods

Post by steve cowan »

1980RS wrote: Wed May 12, 2021 8:47 pm
vht wrote: Tue May 11, 2021 9:51 pm I have an 850 on my .070 over 427 BBC. 4 corner idle but doesn't have replaceable air bleeds, annular boosters plus it came with no choke horn out of the box. Engine has 10.5 to 1 comp, 650 lift hyd roller, and Liberty heads, oval port, 305cc, 2.25 intake valves. I was thinking of changing the main body to increase cfm and get changeable air bleeds, what do you all recommend? I see you can get 950 and 1050, I seen on a mopar forum, a guy picked up et and mph with the 1050, 440 cu in in a heavy car. Also which booster, seems like the down leg is preferred? My carb was brand new in the box but it was a few years old, never installed. How are the Proform main bodies, china?
When I tested my peanut port engine a couple of weeks ago the 461 like the 1000 Mighty Demon with down leg boosters over my A/D booster Mighty Demon by about 1/2 tenth and 1 mph. Could be because this 1000 really runs well with the Air Gap.
have you ever put a vacuum gauge on the intake and looked at how low manifold vac is across the finish line??
i wonder if a airgap pulls on the booster to hard if both carbs were equal apart from booster size which will ,say 50cfm loss with annular booster
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Re: Carb Mods

Post by F-BIRD'88 »

Put the Tunnel Ram back on.

I got a very nice result using the Proform 750 HP style carb body upgrade on a old 700 cfm 4778 holley. But thats on a 750.
Your return will likely be less than, on your 850.

Might try out a simple Stub Stack on that carb for effect.
(IMHO its more likely the hyd roller cam is the Cork in the system, not your 850 carb..)
Testing manifold vacuum @WOT thru the traps is a idea.
Need a MAP sensor to read small changes +/-.

But if you do want the big top end charge back, re-install the Tunnel Ram.
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Re: Carb Mods

Post by FC-Pilot »

If your 850 is an older style it might actually have a larger Venturi than a newer 950. Before you start making big changes do your homework as you might get burned.

On a side note, on a 350 we dyno tested, we squeezed every last HP we could out of a custom built annular 750. Then dropped on our dyno 750 with down leg boosters and picked up over 30 HP on the first pull. That showed me that we were on the small size for the RPM we were spinning it. If we were using an 850, we would not have seen that big of a drop, if any at all. My point is, if the carb is on the small side by a little, then annular will choke it even more. If sized well for the engine, you probably would not be able to notice the performance difference without a time slip or dyno. Now having said that, one may have better driving manners than the other, and sometimes that is more important to me than the added 4HP.

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Re: Carb Mods

Post by 1980RS »

steve cowan wrote: Thu May 13, 2021 2:42 am
1980RS wrote: Wed May 12, 2021 8:47 pm
vht wrote: Tue May 11, 2021 9:51 pm I have an 850 on my .070 over 427 BBC. 4 corner idle but doesn't have replaceable air bleeds, annular boosters plus it came with no choke horn out of the box. Engine has 10.5 to 1 comp, 650 lift hyd roller, and Liberty heads, oval port, 305cc, 2.25 intake valves. I was thinking of changing the main body to increase cfm and get changeable air bleeds, what do you all recommend? I see you can get 950 and 1050, I seen on a mopar forum, a guy picked up et and mph with the 1050, 440 cu in in a heavy car. Also which booster, seems like the down leg is preferred? My carb was brand new in the box but it was a few years old, never installed. How are the Proform main bodies, china?
When I tested my peanut port engine a couple of weeks ago the 461 like the 1000 Mighty Demon with down leg boosters over my A/D booster Mighty Demon by about 1/2 tenth and 1 mph. Could be because this 1000 really runs well with the Air Gap.
have you ever put a vacuum gauge on the intake and looked at how low manifold vac is across the finish line??
i wonder if a airgap pulls on the booster to hard if both carbs were equal apart from booster size which will ,say 50cfm loss with annular booster
Last time I looked the 1000 was less than .5 inches and the 850 A/D carb was a little more but not over an inch. What about Dominators? most all of them have A/D boosters and I think only the early ones had regular boosters.
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Re: Carb Mods

Post by vht »

I've seen what these heads will do on the dyno on a 396 and a 468, they work great. I know the boosters are hurting the cfm, that's why I was looking at main bodies, I figured I'd buy one with the down leg boosters. The cam pulled good to 6800 before with the tunnel ram and as far as selling the carb and buying another one was my first thought but I figured it'd be cheaper going the main body route. What intake would you all recommend? I've been looking for a used one, Strip Dominator or something close to it. The Speedmaster intakes look good but I've heard bad things about them.
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Re: Carb Mods

Post by C.A.R.E. »

VHT be careful with the Oval port Strip Dominator (300-5 ?). Testing with 8 oxygen sensor have shown that the center 4 cylinders run rich and the 4 outer ones run lean. Edelbrock Victor Oval port made more power and torque on a street 496.
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Re: Carb Mods

Post by 1980RS »

C.A.R.E. wrote: Thu May 13, 2021 10:02 pm VHT be careful with the Oval port Strip Dominator (300-5 ?). Testing with 8 oxygen sensor have shown that the center 4 cylinders run rich and the 4 outer ones run lean. Edelbrock Victor Oval port made more power and torque on a street 496.
My buddy said the same thing when he ran his. I ported the plenum on mine and run a 2" 4 hole tapered spacer all 8 cyls were spot on when looking at the fuel ring on the plugs. I wonder if extending the runner dividers would help the lean rich condition on that intake manifold. I know the 300-5 was quicker and faster over my ported Dart oval port intake manifold in testing.
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Re: Carb Mods

Post by vht »

Thanks for the info, that's why I don't like dual planes, they seem to have a problem with that on BB chevies. I've seen good results with the new Air Gap but don't have any personal experience. One guy on YB made like 625 with a 454 using one.
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