750dp to big for a 351w?

04mustang70 · Jul 7, 2008

Is a Holley 750dp to big for a 351w? My motor has 10:1 compression, aluminum intake, headers and mildy ported iron heads. I would guess 350hp. I have a 600 vacuum secondary holley on it and it runs fine. Picked up a used 750 double pumper and was wondering if I should even waste my time trying it. What do you think?

rbohm · Jul 7, 2008

a 750cfm carb is fine for a 351w, but i wouldnt use a double pumper unless i was going racing.

STSFCTN67 · Jul 8, 2008

What are the specs on your cam? You may be creating a bog monster like I did when I started with a 750. It was a bog on the line but a bat out of hell on the highway.

04mustang70 · Jul 8, 2008

Whats the difference between a douple pumper and regular carb? What are the pros and cons? Also the pros and cons of vacuum advance versus mechanical?

rbohm · Jul 8, 2008

04mustang70 said:
Whats the difference between a douple pumper and regular carb?

a double pumper carb has a second accelerator pump located ont he secondary side of the carb to pump fuel in when that side of the carb opens up. double pumper carbs also have mechanical secondaries which open with the primary throttles.

What are the pros and cons?

the pros are no bog when the secondaries open, and amazing throttle response. the cons are excessive fuel consumption because the secondaries open with the primaries.

Also the pros and cons of vacuum advance versus mechanical?

adding vacuum advance toa street engine allows for extra ignition advance under low load, high vacuum conditions to aid in fuel economy and low speed performance. a mechanical advance only distributor doesnt allow this advantage.

DK832 · Jul 11, 2008

I use a 750 DP on my 351W (10.5 comp, Edelbrock RPM heads, X303 hyd. roller cam) and the response is awesome. Mileage is still around 17-18 mpg on the highway @ 80 with no OD and a 3.70 rear end. The carb was modified by the builder (11 psi fuel pressure etc) so I wouldn't know how a stock behaves....

jikelly · Jul 11, 2008

DK832 said:
I use a 750 DP on my 351W (10.5 comp, Edelbrock RPM heads, X303 hyd. roller cam) and the response is awesome. Mileage is still around 17-18 mpg on the highway @ 80 with no OD and a 3.70 rear end. The carb was modified by the builder (11 psi fuel pressure etc) so I wouldn't know how a stock behaves....

What is your city mileage?

D.Hearne · Jul 11, 2008

DK832 said:
I use a 750 DP on my 351W (10.5 comp, Edelbrock RPM heads, X303 hyd. roller cam) and the response is awesome. Mileage is still around 17-18 mpg on the highway @ 80 with no OD and a 3.70 rear end. The carb was modified by the builder (11 psi fuel pressure etc) so I wouldn't know how a stock behaves....

11psi fuel pressure? that's double what it should be and an accident waiting to happen. Hope you've got fire insurance on your car.

zookeeper · Jul 11, 2008

I agree, I seriously doubt you have 11 lbs of pressure at the carb without having gas spew out of the vent tubes like "old faithful". And getting 18mpg with your combo at 80mph? With no od and 3.70 gears? I think your fuel pressure gauge and your speedo were calibrated by the same guy. It's just not possible, no way, no how.

D.Hearne · Jul 11, 2008

I can see 18 mpg at 80, with the right cam. My 331, when I had it in my 89 Ranger, would pul down 16 mpg on a 50 mile R/T commute running 55-75 mph, as long as I kept my foot out of the secondary carbs. Driveline was a Toploader 4 speed backed with a 3.73 geared 8.8 with 28" tires. Loose 800 lbs off that and 18 would have been possible. Curb weight was 3800 lbs with me in it.

zookeeper · Jul 12, 2008

The difference between your combo and his is like apples and oranges. You were essentially running a 2 barrel on a smaller motor with much taller tires. He's claiming to get 18mpg out of a 351 with a double pump 4 barrel, and while I don't know his rear tire size, I'd suspect that like most Mustangs, 26 inch tires are more realistic. I've been there and there's no way a motor running 4,000 rpm on the freeway with a 750 DP Holley is getting 18mpg.

D.Hearne · Jul 12, 2008

The key to doing it will be keeping his foot out of the secondaries. That and the tire diameter. Disconnect these and he could well get 18 mpg with the right cam and combined with 27" tires.

zookeeper · Jul 12, 2008

I see where this heading, so before it turns into another "...well, maybe if he did this..." arguement, I'll just repeat my point, there is no possible way a vintage Mustang with a 351 and a working 750 DP is going to get 18mpg at 80mph.

brianj5600 · Jul 12, 2008

Is that US gallons or Imperial gallons? 80KM per hour or 80mph? 18 miles per US gallon at 80mph does seem unlikely.

Zookeeper, there are things you can do to a DP to get gas mileage up there. Out of the box they are very thirsty, but a little drilling and a re-jet later they can do quite well. I am hoping for close to 20 mpg out of mine and still run 10's in the 1/4.

DK832 · Jul 12, 2008

Sorry guys, it's a fact (and sorry for the thread hijack!). Repeatedly measured full tanks going up and down CA and Denmark/Sweden. Speed usually 65-80 mph on TomTom GPS also supplying driven distance.
JW Performance C4 (1:1 3rd), 3.70 9" rearend with TrueTrac, 295/30R18 Kumho tires
351W built by Joe Beghosian (Fresno, CA, ex Indycar engine builder), SVO block, forged everything, FRPP X303 hyd. roller cam (specs http://www.google.com/url?sa=t&ct=r...z9R7VkFtEOZNew4WQ&sig2=9YtcpXr1W4hSBmt9DU9Pbg), roller rockers, Edelbrock RPM heads, JE pistons @ 10.5:1, dual plane FRPP intake, 750DP also built by Beghosian. 91 octane (US octane rating that is).
Hooker ceramic Super Comp headers into dual 2½" exhaust w H-pipe & Magnaflows
Curb weight 3600 lbs with ½ tank of gas & me.
Temps in the 60-85°F range. Comes out at around 7,5 km per liter every time = 17.64 miles per US gallon
(Online Conversion - Fuel Consumption Conversion). But i agree, it doesn't take a lot of sprints or overtakings to ruin it!
I very rarely measure city mileage but around 9 mpg usually. Gas is more than $9 per gallon here so I dont go pedal to the metal all the time!

The carb guys here told me to get a new gauge when they saw the 10-11 psi so this is the second gauge showing it. I'm not a carb guy so you can tell me a lot about why it shouldn't work - I'm just thrilled about the way it does!

LILCBRA · Jul 12, 2008

As far as the mileage goes, I don't think 18 mpg would be out of question, depending on the build of the engine, tire size, weight, etc. Since we don't know that, it is hard to prove or disprove. Here is what I have in a 2900# car: 1968 302 shortblock .030 over with flat tops. Trick Flow Twisted Wedge heads. Melling 290/300 duration, 214/224 @ .050, with .507/.533 lift with the 1.7 roller rockers. Offy 360 intake, metallic ceramic coated headers, and a holley 650 double pump carb. I recently replaced the 4 speed with a T5 from a 95 Mustang GT and have an 8" 3:55 rear. I have 245-50/13's and with this setup, I get between 17-18 cruising between 65 and 75 on the highway. I suppose if I went with a taller tire, I could probably get better. As far as if a 750 DP is too big for a 351 Windsor, it again depends on engine build. I would say try it out and tinker with it some. You can re-jet it, try different squirters, and different pump cams. Your mileage will go down more than likely, but if thats not what your after, don't worry about it. Worse case scenario is you put your 600 back on, since you know that works well.

zookeeper · Jul 12, 2008

Here's the only thing I know, when my car was stock, with a good-running 289, a stock 2 barrel carb, 2.79 gears and 26" tall tires, I routinely drove it to work and school. At 60mph, it could get 20mpg. If I went faster, say 65 mph, the mileage dropped off to 18mpg. My wife's stone-stock '88GT will get 21-23mpg if the cruise is on and you don't go over 65 or so. If you go faster, again, the mileage drops off. The car typically gets 20mpg during normal freeway driving. Now, compare that to a guy who claims to have 11lbs of fuel pressure (no way in hell) and drives 80mph with a big carb and mechanical secondaries, and steep gears, and it gets roughly the same mileage? There really is no truth on the internet anymore, is there?

D.Hearne · Jul 12, 2008

DK832 said:
Sorry guys, it's a fact (and sorry for the thread hijack!). Repeatedly measured full tanks going up and down CA and Denmark/Sweden. Speed usually 65-80 mph on TomTom GPS also supplying driven distance.
JW Performance C4 (1:1 3rd), 3.70 9" rearend with TrueTrac, 295/30R18 Kumho tires
351W built by Joe Beghosian (Fresno, CA, ex Indycar engine builder), SVO block, forged everything, FRPP X303 hyd. roller cam (specs http://www.google.com/url?sa=t&ct=r...z9R7VkFtEOZNew4WQ&sig2=9YtcpXr1W4hSBmt9DU9Pbg), roller rockers, Edelbrock RPM heads, JE pistons @ 10.5:1, dual plane FRPP intake, 750DP also built by Beghosian. 91 octane (US octane rating that is).
Hooker ceramic Super Comp headers into dual 2½" exhaust w H-pipe & Magnaflows
Curb weight 3600 lbs with ½ tank of gas & me.
Temps in the 60-85°F range. Comes out at around 7,5 km per liter every time = 17.64 miles per US gallon
(Online Conversion - Fuel Consumption Conversion). But i agree, it doesn't take a lot of sprints or overtakings to ruin it!
I very rarely measure city mileage but around 9 mpg usually. Gas is more than $9 per gallon here so I dont go pedal to the metal all the time!

The carb guys here told me to get a new gauge when they saw the 10-11 psi so this is the second gauge showing it. I'm not a carb guy so you can tell me a lot about why it shouldn't work - I'm just thrilled about the way it does!

The X303 is your saving grace here. I've got the Z303 in my 331 and it too is the key to the fuel economy at highway speeds. These are wide LSA cams geared toward EFI motors, but they work outstandingly with a carb.

750dp to big for a 351w?

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