Need Advice On Choosing Carburetor , New Build

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850 double pumper, or QuickFuel 850.

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There is nothing wrong with an 850 DP with stock jetting as new! Carbs are calibrated to give 12.5 to 13. to 1 AFR at wide open throttle and 14ish at part throttle. carbs do not know what is under them only how much air is being pulled through it.

Anyone who washes the walls has problems and no crab tuning and fixing skills. washing the walls would indicate floats to high, blown power valve, bad gaskets and so on.

870 is a good choice if you intend to do some racing then I would lean to a 850 DP. new or at least properly rebuilt and set up.

many get a carb then their expert buddies talk them into jetting it up 10 sizes cause it's a "race motor" I could tell you stories.

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There is nothing wrong with an 850 DP with stock jetting as new! Carbs are calibrated to give 12.5 to 13. to 1 AFR at wide open throttle and 14ish at part throttle. carbs do not know what is under them only how much air is being pulled through it.

Anyone who washes the walls has problems and no crab tuning and fixing skills. washing the walls would indicate floats to high, blown power valve, bad gaskets and so on.

870 is a good choice if you intend to do some racing then I would lean to a 850 DP. new or at least properly rebuilt and set up.

many get a carb then their expert buddies talk them into jetting it up 10 sizes cause it's a "race motor" I could tell you stories.

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Have you ever plugged in a wide band while running an 850 DP? There is no way they cruise in the 14s OOTB.

Most of the carbs we are talking about will be 13ish at WOT, so that is not an issue with either a street or a race oriented carb.

What do you think is the difference between a race oriented and a street oriented carb? Nearly all carb companies build carbs for street duty and other carbs for the track.

There is a reason for that. Most of our street cars (not all, but most) will like 14.5-14.7 for a cruise ratio. And on the cruise circuit is where a street engine spends a good part of it's time. Even most of the street carbs will start a little on the fat side of that just for safety's sake.

But any race/track oriented carb will be considerably fatter than that. A lot of times in the 13's at cruise. And that will wash down the walls and foul the gas. Guaranteed!

Bottom line: either style of carb can be made to work good. One will just start off a lot closer than the other.

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This is why we read plugs and install widebands. I`ve always put bigger than suggested carbs on my junk including an 850 double pumper on my 11-sec 360 and it wanted more jet and went faster after givin it what it wanted and no way are you washing down cyl. walls at 13. afr......not even 11`s from what I`ve seen. Bigger cams get away w/a richer mixture due to overlap and increased duration and my car will not tolerate 14.- 15. cruising mixtures w/my 275-280 dur. @ .050..........just not happy there and it`s way happier cruisin in the 13`s w/perfectly clean NGK # 8`s...............

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The double pumpers are a race oriented carb and are very, very rich for street driving. They work good but will wash down your cylinder walls and foul your oil without a good bit of re-work on the transition and cruise circuits.

850 is a good CFM range for 451". But a more street oriented carb would be better suited to the majority street time the car is intended for.

Either way, some tuning will be in order. But a street carb should be closer OOTB than a race carb for a predominately street car.

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Holley has made many different jetting and circuit changes on a lot of there HP DP carbs, the 4781 in particular. I have three of them now, a 4781 no dash #, a 4781-2 and a 4781-7. The -7 is a lot better at part thtrottle on my wideban than the other two. As I agreed to on Brian post, a new 4781 with the curent dash # should be awesome, the Holley 950 HP works very well on the street also

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