hey guys with whats in my sig I am trying to get better gas milage without changing the cam cause thats big job. well If i take off my edelbrock 750 and put an edelbrock 650 on do you think it would hurt my performance any? on desktop dyno i loose liek 2 horse. also woudl my gas milage go up any? or only liek 1 or 2 mpg? thanks

 

If they're both tuned properly and you're not driving around at WOT all the time I doubt you'd notice any difference at all in mileage between the two.

 

Won't notice a performance difference either. If you want to improve mileage by altering the carb, all you can really do is lean the primary's. The carb is only one part of the mileage 'equation'. Overall, you need the engine to be as efficient as possible at the rpm you need to get the mileage at--example, if you run 1600rpm at 60mph and 60mph is usually how fast you drive, the engine needs to be efficient at 1600rpm. A mild cam would be a big step in the right direction it that case. Of course, having a good ignition system with optimum timing is important. So is a light right foot, a light car, tires aired up, alignment perfect, transmission/clutch working correctly, etc...

 

I'm not sure I agree. The concept is to keep the carb as small as possible to keep the air velocity as high as possible. This increases the boost venturi signal, and also increases the vaporization of the gasoline liquid as it mixes with the air (remember, liquids don't burn).

With lower velocities, the mixture has to be kept artificially rich to compensate for the reduced vaporization in order to maintain actual burnable mixture ratio, which is required to maintain throttle response, power, etc.

The q-jet doesn't have small primaries for no good reason.

 

At cruise though the throttle blades will be mostly closed though anyway, so I don't know if there'd be much difference in velocity through the primaries.

 

The velocity past the throttle blades will be consistent, regardless of carb size, because the area that is opened is the same regardless of carb size.

However, the flow area through the actual venturi will be more with a larger carb, so at a given throttle blade open area the flow velocity in the venturi will be lower with a larger carb. Same for the booster venturi.

 

Ok, I see what you're saying. Are the primaries bigger on the 750 than the 600, or is it just the secondaries?

 

I couldn't find any net data to confirm or deny this, but I was under the impression that these are "square bore" carbs, and therefore an increase in CFM would be accomplished by increasing the bore of both the primaries and the secondaries.

 

I think "square bore" just refers to the spacing between the bores, but the primary and secondary bores can still differ slightly.

I looked up the sizes, the 600 has 1 7/16" primaries and 1 3/16" venturis. The 750 has 1 11/16" primaries and 1 7/16" venturis. The secondaries on both are 1 11/16" with 1 9/16" venturis. I suppose there could be some slight difference in mileage then.

 

:hail:

They are also called "square flange", which is probably more descriptive.

I knew some of the old (as in '60's) Carters had smaller primaries than secondaries, but I didn't know if that carried over to the Performer series. Interesting that the CFM difference between these two is all in the primaries.

 

being efficient is the key. i drove from arizona to nebraska at 80 mph at 1500 rpm and got 27 mpg. that motor was a 350 with 305 heads and a tbi yes it was a tbi but still who hear gets 27 mpg with a 350?

 

However, if the 750 and the 650 are both 'tuned' correctly, the mileage difference will be very small if it's detectable. Going from a 750 to a 650 isn't going to fix a 'bad mileage' problem.

 

the 750 has larger primaries and secondaries than a 600, I know, I had them both sitting next to each other.

I just switched from a 750 to a 600, and I do get quite a bit better milage, and I think it's faster even, especially at part throttle and off the line.

 

That's great milage for a 350!!!

H*ll, I had a 90 Cavalier Z24 3.1 that barely got that.

But hey.....that's what TBI's are designed for, right??

 

I have had a 750 Edelbrock Performer and now I have a 600 Edelbrock Performer on my car. I noticed about a 5 mile increase in gas mileage going to the 600. I drive between 60 and 65 mostly at around 1500-1800 in 5th. I didn't lose any power, I gained power! The engine had a lot more grunt off the line and a lot more throttle response while driving.

 

I'm no MPG guru by no means so I won't go there. With all my combinations last season, I noticed with the VS carbs when I ran the 600cfm it was real snappy off the line and pulled pretty good till about 4K, then it kinda faded. Stuck on a 750cfm VS, off the line petered out a little bit, but more than compenstated at 3K on up. With a smaller carb you'll lose out on upper rpm hp cause the motor wants to be fed.
The only thing I know about mpg was when I had the 600 and 750 VS I could street drive it around for quite awhile before refilling, as soon as I stuck the 830 DP race carb on...what does MPG stand for again. 1-1/2 hrs and I'm lookin for a gas station.
I think the smaller carb will be better suited in 89Formulas application as an all around carb for a daily driver.
Most of you probably have read this and I personally experienced it, when running a bigger carb you "usually" have to jet up, otherwise you'll create a lean condition because of the additional air flow coming in. Granted the theory is geared more towards WOT applications.