750 Dp [Archive] - Nova Tech

: 750 Dp


brian oneil
Aug 29th, 08, 09:40 PM
I need a holley expert. What is the difference between a 750 Douple Pumper and a 850 Double Pumper besides the obvious 100 CFM's and base plate. Jets and squirters????? Whats the difference between anular and drop discharge nozzels?????

brentphatrat68
Aug 30th, 08, 02:35 AM
well to put it short.(and Big dave could do this better) the boosters are lower in the venturies to promote better fuel adamization. when you go big cfm you lose signal so thay drop the boosters. as for jets thay stay the same. But the squriters have extenstions and can be center or outer discharge. But then we get into dominator class. why do you ask?

brian oneil
Aug 30th, 08, 06:12 PM
with my 408 small block super victor manifold and i have a .555/.554 solid lift cam with 11.5.1 compression figuring i may need a 850 and if it was feasible to transform my 750 into a 850.

brentphatrat68
Aug 30th, 08, 06:34 PM
I had a 750 dp and what i did was send it to a carb shop in CA. and thay made it flow 926 cfm and tuned it to my Eng. Thay did alot of stuff that i couldn't. and i still have it today. the best carb i ever owned. i will send you the link so you can see what thay do.
i also know that your 750 can work fine on your eng. It just needs the right tune.

brentphatrat68
Aug 30th, 08, 06:38 PM
here is the link

http://www.customcarbs.com/Home.htm

check out the info questions. thay get very detailed.
let me know what you think.

brian oneil
Aug 30th, 08, 06:48 PM
ok, i will check it out :)

Big Dave
Aug 30th, 08, 07:08 PM
Difference between a 750 DP and an 850 DP Holley is in the diameter of the throttle bores (at the venturi), the 750 is slightly smaller in diameter.

Difference between down leg and annular discharge is the annular discharge has a secondary venturi that enhances the vacuum signal to the main well for better atomization of the fuel. The fuel leaves the booster in a ring as opposed to a single hole pointing to the center of the bore. An annular booster allows you to run a bigger carb on the street with better throttle response and fuel economy. Down side is that huge ring in the throttle bore obstructs high speed laminar flow at WOT.

So for your high compression stout geared solid roller race car I would recommend the down leg booster as you will be spending a lot of time at WOT with that cam. Jets are the same for every Holley. Squirters are designed about the same except for the two center squirt Holley carbs designed for tunnel ram use (the 660 cfm and 850 cfm).

Holley makes racing carbs that have four corner adjustment for better fuel distribution in the manifold, and have adjustable air bleed screws to allow you to customize your air flow for your motor (compression, port size or more accurately port velocity, and finally your cam's duration which determines your vacuum at idle. These carbs also have the air horn curved for optimal air flow with no choke at all. You can still find Holley 950 cfm three barrel carbs that were built for race use back in the sixties. With some modification they will flow up to 1150 cfm and still retain the 4150 format. A 4500 series Holley has additional baggage that you do not want with street use (such as the linkage falling into the engine if not properly maintained), as they are not designed to have a smooth transition from idle to WOT as they were for racing only.

Big Dave

brian oneil
Aug 30th, 08, 07:35 PM
Difference between a 750 DP and an 850 DP Holley is in the diameter of the throttle bores (at the venturi), the 750 is slightly smaller in diameter.

Difference between down leg and annular discharge is the annular discharge has a secondary venturi that enhances the vacuum signal to the main well for better atomization of the fuel. The fuel leaves the booster in a ring as opposed to a single hole pointing to the center of the bore. An annular booster allows you to run a bigger carb on the street with better throttle response and fuel economy. Down side is that huge ring in the throttle bore obstructs high speed laminar flow at WOT.

So for your high compression stout geared solid roller race car I would recommend the down leg booster as you will be spending a lot of time at WOT with that cam. Jets are the same for every Holley. Squirters are designed about the same except for the two center squirt Holley carbs designed for tunnel ram use (the 660 cfm and 850 cfm).

Holley makes racing carbs that have four corner adjustment for better fuel distribution in the manifold, and have adjustable air bleed screws to allow you to customize your air flow for your motor (compression, port size or more accurately port velocity, and finally your cam's duration which determines your vacuum at idle. These carbs also have the air horn curved for optimal air flow with no choke at all. You can still find Holley 950 cfm three barrel carbs that were built for race use back in the sixties. With some modification they will flow up to 1150 cfm and still retain the 4150 format. A 4500 series Holley has additional baggage that you do not want with street use (such as the linkage falling into the engine if not properly maintained), as they are not designed to have a smooth transition from idle to WOT as they were for racing only.

Big Dave

here is the link

http://www.customcarbs.com/Home.htm

check out the info questions. thay get very detailed.
let me know what you think.


Brent I am surprised they dodnt ask for my mothers name in the info they wanted :).

Big Dave, I am running a 4779 with 4 corner idle circuit that was flowed at 790 CFM's for my previous set up.
I am running a solid lift cam, non roller type. I switched to a super vitor manifold along with the cam swap. I am now only guess at 575HP for tis set up. Could be wrong, usually am :yes:.
I will run the 1/8 tomorrow with this new cam and intake and see if I can better my best of 7.3.
Most pepole i talk too with 400 with large cams all run 850 CFM carbs and was wondering from the guru's out there if thats what i need to be doing?

Big Dave
Aug 30th, 08, 07:46 PM
Well I have a few Dominator carbs lying around (and 850's both vacuum and double pumpers).

http://i45.photobucket.com/albums/f63/3454545/DSC02818.jpg

But I put an HP Street series 750 cfm vacuum secondary carb on top of my 406 that makes a dynoed 500+ lbs-ft. of torque at 3250 to 4650 RPM and nearly 320 lbs-ft at 2400. It makes 513 gross horse power at peak but I didn't build it for the oval high bank but street use.

http://i45.photobucket.com/albums/f63/3454545/DSC02821.jpg

Big Dave

brian oneil
Aug 30th, 08, 07:57 PM
Why would you want to put a dominator carb on a dual plane?

Those numbers are impressive. I have to go by what desk top dyno tells me. I know generally to remove about 20% and thats probably fairly accurate.

What do you all reccomend for carb size???

http://www.novas.net/forums/picture.php?albumid=9&pictureid=162

brentphatrat68
Aug 30th, 08, 08:16 PM
Very nice gentelmen

Big Dave
Aug 30th, 08, 08:16 PM
Dominator will work equally well on a dual plane as it will on a single plane. It has no idea as to what it is bolted to. A dual plane can make with in three percent of the peak horse power of a single plane manifold, but a single plane gives up all bottom end and most mid range torque making them next to useless for a street driven car.

You can not have a carb to big for racing. On the street an oversized carb results in rotten throttle response, poor idle quality and bad mileage. Is this a race car or a street car (the two are mutually exclusive).

Big Dave

brian oneil
Aug 30th, 08, 08:27 PM
This will soend 95% of the time at the drag strip the other 5% which will come later at local shows not more than 10NM from my house. I do understand the non mutal for street versus strip. If this was a street car I would be using a 3310. I have two of those carbs now.

Understand that a carb has no idea what manifold it is bolted to as well. Just seems like over carb for a dual plane. I am sur though in the next few years, some one like Edelbrock will make a dual plane manifold work to 7500 rpms and make it competitive to the single plains.

I have been told though to 6500 rpms, there probably no better manifold then the edelbrock air gap. I have one of those two.

this motor will leave at approx 3500 to 7000 rpms. I will not turn it to 7500 which the cam is good for. Heck i may even short shift it to 6800 even farther. My crank which is a scat cast crank is only good for 550HP and 7000 rpms, forges H beam rods and forged pistons i am not too concerened with. So, I am already pushing the crank beyond its limits with this set up

Big Dave
Aug 30th, 08, 09:55 PM
Racing any SBC will do well with an 850 (Chevy put them on the Z-28 302 that shifted at 9,600 RPM). Most all 355 and larger SBC motors will respond better with the 850 cfm carb than a 750.

Street cars have to be able to idle, and then smoothly transmission to off idle to full throttle. To do that there has to be a vacuum signal in the manifold available for the carb to boost (that is what the booster in the venturi does).

Single plains allow all of the ports to interact as each cylinder has the intake valve slam shut the pulse runs up the runner to reflect off other pulses and it creates turbulence. A dual plane physically isolates half the motor with the cylinders paired by firing order to allow the pulses to reinforce each other for a steady vacuum draw. So a single plain and a big cam and a big carb all make street driving unpleasant at best, but for racing none of that is important. You leave at high RPM and only go up from there.

Big Dave

blumrlnoutfitters
Sep 12th, 08, 02:40 PM
Topics like this are why I love reading this forum. I am usually a lurker, but I had to chime in here. I am about to be purchasing a new carb for my '74 355 Drag Nova. I was convinced a 750 would provide plenty of flow and response. I guess I should look into an 850 now with my new info. The car is a strip only car and is not/will not be street leagl. It currently has a Holley 670 cfm street avenger (needs a rebuild and was on the car when I bought it) installed that is very inadequate for the cam and heads on the car. After some research, I am pretty sure I am going to purchase my carb from Bigs Performance. After giving them detailed info about my setup, Jesse recommended one of their Stage 5 Double Pumper 750 carbs. I figured if they have been in the performance business since 1962 it would be hard to argue with his recommendation. Has anyone had any dealings with these guys before and how was your experience?

Big Dave
Sep 12th, 08, 08:04 PM
Carb guru in my neck of the woods was a fellow by the name of Barry Grant. He closed shop over in Clearwater, FL and moved up the road a piece to Dahlonega, GA. I get by without a tuner now adays.

Big Dave