650 dp vs 750 dp in a 408

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Early a body

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I tried a 650 dp on my 408 after having a edlebrock 750 that i really did not like. What a difference it pulled much stronger. Then I put a 750 dp on because it makes sense that the 750 would be better than the 650 given the cubes. Nope the 650 absolutely out pulled the 750 dp. Both carbs are new, within the last 3 years. What gives? I have not messed with the jets or PV of either carbs. Is it simply because the smaller carb provided more torque because its smaller? Completely surprised at the difference in performance between the carbs.
 
Car weight
Stall speed
Gear ratio
Cam specs
Intake manifold

The 750 should have killed the 650.

The other question is what two carbs are we talking about.
 
750 DP ran the best time (ET proven) on my stock 360, even when I had my stock 360 heads.
 
It really doesn’t matter what you have in the car !! If it runs better with the 650 !! Well it runs better !! Had a friend years ago decided he needed a 850 dp. So he bought one ! The car went from 6:80 1/8 to 7:05 1/8 ! He put his 750 vac sec back on ran 6:80’s again . 62 Chevy 2 383 small block ! Sometimes just leaving the carb alone is worth a lot !! And sometimes trying different things makes your car go faster !!
 
I've been trying to decide on a Eddy 650 or 750 for my 68 340 Eng.20 over, 10:5 Comp,Eddy heads, air gap intake, Stock exh.manifolds with 2.5 Exhaust ,4 spd. , 323 gear. I thought 750 would be too much but others say it would be fine. Car is a driver. Being a driver I'm still leaning toward 650.
In your case have you run them both in the 1/4 mi. ?
 
Rmoore said:
It really doesn’t matter what you have in the car !! If it runs better with the 650 !! Well it runs better !! Had a friend years ago decided he needed a 850 dp. So he bought one ! The car went from 6:80 1/8 to 7:05 1/8 ! He put his 750 vac sec back on ran 6:80’s again . 62 Chevy 2 383 small block ! Sometimes just leaving the carb alone is worth a lot !! And sometimes trying different things makes your car go faster !!
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It’s 408 INCHES. That’s why I asked the OP what I did.

There is no excuse for the 650 being the better carb. I’d like to see the time slips as well.

Either there was something wrong with the 750, or the MPH went up but the ET went down (more HP less bite) or something.

A decent, modern 750 today will flow 930-950 CFM easily. That’s better than the used up 4779 series carbs. And yet, they make more power and drive better than the 4779 carbs.
 
if it's the 650 Thunder it's a Hell of a carb...it would be my choice and should pull hard on the street!
 
It sounds like the OP wants instant gratification, bolt on & go, it doesn't work that way. If you want the optimum performance, you have to work for it. My suggestion would be to play with jetting & read the plugs. Keep a log & only make ONE change at a time so they know what has changed ET & MPH wise each time.
 
Early a body said:
I tried a 650 dp on my 408 after having a edlebrock 750 that i really did not like. What a difference it pulled much stronger. Then I put a 750 dp on because it makes sense that the 750 would be better than the 650 given the cubes. Nope the 650 absolutely out pulled the 750 dp. Both carbs are new, within the last 3 years. What gives? I have not messed with the jets or PV of either carbs. Is it simply because the smaller carb provided more torque because its smaller? Completely surprised at the difference in performance between the carbs.
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What you are more then likely experiencing is the smaller venturis that are providing a higher velocity to better atomize the fuel in the air. This Leeds to a quicker air fuel charge for a snappy throttle and a better burn at low/mid engine speeds. The more the atomized the fuel is at the mid and lower speeds, it should make more torque.

If you had time slips and ET.’s... it would help. State of tune as well. IMO, the 750 should beat it all around.
 
Thanks everyone

Both carbs are holly 4150 DP's only a few years old. i have not touched anything in these carbs, the jets and squirters are the as they are when purchased. i have not run at the track, there is no track anywhere near me anyways.

Just looking to see if anyone has run into this before and if they knew what in the heck was up.

I am not even sure why I thought of doing this in the first place.
 
You don't need a race track, take your car out & drive it normally, take it home, check the plugs to see what they look like. Try changing jets depending upon rich or lean condition, but only one step at a time & keep records of the changes & how the plugs react.
 
Early a body said:
Thanks everyone

Both carbs are holly 4150 DP's only a few years old. i have not touched anything in these carbs, the jets and squirters are the as they are when purchased. i have not run at the track, there is no track anywhere near me anyways.

Just looking to see if anyone has run into this before and if they knew what in the heck was up.

I am not even sure why I thought of doing this in the first place.
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The 750 is most likely pig rich. Some proper tuning and the 750 should run the 650 into the ground.
 
i talked at length with Edelbrock tech. and they assured me the 650 Thunder is by no means a small carb.The carbs' design and operation are more important than it's cfm rating. I don't think the op is racing through the big end at 6500 plus rpm.
 
I agree with rumble. 650 has a bigger signal and better butt dyno on the street. Nothing wrong with that.
 
mderoy340 said:
I agree with rumble. 650 has a bigger signal and better butt dyno on the street. Nothing wrong with that.
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The slightly greater restriction at high rpms could also be helping fuel distribution.
 
mderoy340 said:
650 has a bigger signal and better butt dyno on the street. Nothing wrong with that.
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I agree. On the street, (100% street driven) I purposely run a carb a little small. The extra 10/20 hp is not really missed at all but the increase in throttle response is great.
Kendog 170 said:
I've been trying to decide on a Eddy 650 or 750 for my 68 340 Eng.20 over, 10:5 Comp,Eddy heads, air gap intake, Stock exh.manifolds with 2.5 Exhaust ,4 spd. , 323 gear. I thought 750 would be too much but others say it would be fine. Car is a driver. Being a driver I'm still leaning toward 650.
In your case have you run them both in the 1/4 mi. ?
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This would be my near perfect example. I don’t know what you have for a cam, but if it was a small street cam, I myself would do the 600/650. Yes the 750 would also be just fine.

On a racing a car or a monster street striper, ummmm, Yea, different story. IMO, spacecowboy has it going on there. Let it breath deeply! Burn don’t think his build is like the OP’s for some reason.... LMAO
SPACECOWBOY said:
My race fuel 414 has fastest et with a 950.
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He just said that it pulled harder. That is subjective and not anything you can measure. It is not a race car. We don't know anything about the car except it has a 408. Not much info. CR, heads, Cam, intake, exhaust. gearing, tire size, vehicle weight and on and on. Just a lot of assumptions when giving "advice" Carburetors work on demand. The actual flow depends on many variables. Remember that you can only flow in as much as you can flow out. Engine is just an air pump.
 
If the demand is the only thing that matters a 1000 cfm would be perfect for every vehicle under 500 hp...even a slant six!
Carburetor design and operation are more important than size.
 
Mattax said:
The slightly greater restriction at high rpms could also be helping fuel distribution.
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Also vaporization and homogenization as well amongst other things

ir3333 said:
If the demand is the only thing that matters a 1000 cfm would be perfect for every vehicle under 500 hp...even a slant six!
Carburetor design and operation are more important than size.
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I agree. The engine wants what it wants and has to operate over a wide range of rpm which is where the smaller carb will shine. Also different designs and booster types produce different fueling styles which some engines prefer over others.
 
If you are in NV...anyplace that is over 2000 ft in elevation the carb will need to be rejetted...
 
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