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Andreas 07-27-2000 10:39 AM

Dp vs. vacuum carbs
Whatīs the difference between a double pumper carb and an ordinary vacuum controlled carb? Dpīs are supposed to be better suited to light street vehicles, or so Iīve read, but to me it sounds like itīs more up to the gear ratio (if the engine can turn the wheels without much hassle, so to speak).
Mpg aside, which one is the better performer? Reason Iīm asking is because I can get a Holley 800 dp for a very good price......and if it just sucks gas with no gain in performance I donīt want it![img]/wwwthreads_images/icons/smile.gif[/img]


If a Blazer runs smooth, it isnīt really a Blazer[img]/wwwthreads_images/icons/crazy.gif[/img]

Scoobydoo 07-27-2000 10:55 AM

Re: Dp vs. vacuum carbs
DP have manual secondarys, and should ONLY be used with a manual tranny! (since you have more control of your rpms)
If you use it with a auto tranny you will dump too much gas down it's throat before your rpms are high enough, and it will stumble... (cough, cough, stutter, stutter, spit, spit, VAROOM!!! off you go). a vacuum secondary doesn't open until your rpms are up so problem solved.

Andreas 07-27-2000 11:07 AM

Re: Dp vs. vacuum carbs
Aha. Thanks for your quick reply Scooby (hereīs some snacks....good boy..gooood boy[img]/wwwthreads_images/icons/wink.gif[/img])! Although I like the "VAROOM" part I think Iīll follow your recommendation and stay with vacuum. Again, thanks !


If a Blazer runs smooth, it isnīt really a Blazer[img]/wwwthreads_images/icons/crazy.gif[/img]

Andreas 07-28-2000 10:29 AM

Re: Dp vs. vacuum carbs
Sorry for bringing this to the top again, but I have one more carb question for you all.
What the h*ll is a predator carb? I know how it looks, but I donīt know the function of[img]/wwwthreads_images/icons/smile.gif[/img] I know there are two versions of them and Iīve read that they require much fiddeling around before they behave well. So, whatīs the idea behind that design?


If a Blazer runs smooth, it isnīt really a Blazer[img]/wwwthreads_images/icons/crazy.gif[/img]

Scoobydoo 07-28-2000 02:19 PM

Re: Dp vs. vacuum carbs
I don't know how they work, but I do know NONE of my racing buddys will use them. (they all use seriosly re-worked/re-flowed holleys) but I have heard the preditor works about like a toilet[img]/wwwthreads_images/icons/smile.gif[/img] FLUSH [img]/wwwthreads_images/icons/shocked.gif[/img] VAROOM!!! get the idea! [img]/wwwthreads_images/icons/blush.gif[/img]
Its either a little gas or alot of gas and not much in between!
Around here NOBODY uses them![img]/wwwthreads_images/icons/frown.gif[/img]

RolngThun 07-28-2000 03:19 PM

Re: Dp vs. vacuum carbs
I tried one of those junk toilet bowls for almost a year.
I did nothing but remove it and install my Holley and went a full second quicker.
They basically work like the back half of a Q-jet. Big air doors on top attached to a cam through linkage, the cam is what meters the fuel. It comes with three cams, 1,2,and 3, 1 being the leanest, 3 the richest. It uses a double check ball instead of a needle and seat. The street version has an adjustable idle circuit.
They are very forgiving, you will never get a bog out of one. But they don't make any power either.

**DONOTDELETE** 07-28-2000 07:37 PM

Re: Dp vs. vacuum carbs
I tried a Predator once, and took it of after a day of trying to make it run good. I could get awssome throttle response out of it, but when I took it for a drive, it was way down on horsepower. I know of a couple of people that trie them on mud boggers, and ended up with Holley dominators. I think the Predator is to simple to be tunable enough in racing applications.

Thanks, Ray

chevota 07-29-2000 07:11 PM

Re: Dp vs. vacuum carbs
Something to consider when choosing a vacuum carb is the secondarys will always open late, and never open all the way. It doesn't matter if you use the lightest diaphram spring, because the secondarys need vacuum to operate, if there is vacuum there is lost power. Plus there is no enrichment circuit on the vacuum carb for the secondarys.
A double pumper is the choice to have. You have complete control of your carb at all times, and max power is possible. Using a double with an auto trans is not a problem, you just have to demonstrate some foot control. I use an 800 dbl holley with my auto and I wouldn't have it any other way. And I can floor it from an idle and it does not stumble. I've tried at least a dozen other carbs and none perform as well as that 800. You said you can get one cheap? If you don't want it, I may be interested. Corey

P.S. Dbl or vacuum the mileage is the same.


**DONOTDELETE** 08-01-2000 03:16 AM

Re: Dp vs. vacuum carbs
the vaccume they use is gennerated in the venturi's, not the manifold, and on a dyno they will usualy show the same output, the big differance is that the doubble pumper will be down on torque if used a wot and low rpm..

looking for a new diesel powered blazer

chevota 08-01-2000 01:06 PM

Re: Dp vs. vacuum carbs

True, but the vacuum port is located a bit down the barrel past the venturi. It needs manifold vacuum or a serious amount of flow past the venturi to generate a significant vacuum to open the secondarys. In either condition the seconderies should have already have been engaged.
True, a Dbl will be down at wide open @ low R's, but that is its ONLY drawback, and it is easily cured with foot control. Also the secondaries have an enrichment circuit, and a power valve to help control fuel mixture at variable vacuum/rpm/opening rate conditions.
A vacuum carb relies on vacuum to control these conditions, an easy cure, but at a cost in power, especially accelerating.


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