[johnkimball]

I've got a 350 with a slightly modified '87 truck block, lunati voodoo cam and lifters, performer RPM manifold, and 750cfm speed demon carb. I remember reading somewhere that switching from a 750cfm to a 600cfm drastically increases gas mileage without much decrease in horsepower. If this is true: how much difference in power would there be? would it be THAT beneficial to gas mileage? What carb model would be the best for me?

For the record, this car is my daily driver, and the current 10-12mpg gets expensive. Thanks a lot for any help.

 

[Mike Feudo]

It will have better throttle response and should get better mileage (especially if you drive the 750 hard) but don't expect a giant gain. If the 350 is a mild motor you shouldn't see much change in power but if the motor makes HP at 7000 RPM you will notice a power drop off. A Q-jet is the best of both worlds. It's a 750 CFM with tiny pri. throttle bores that gets excellent mileage if you stay out of the sec.

 

[Motorhead62]

Yes you will get better mileage with a 600 cfm carb. Getting mileage is about driving with a light foot also. You will loose power on the top end. I swaped a 750 for 600 Edelbrock on my El Camino and gained a couple MPG. Yes my throttle response and drivability is better also.

Good Luck

 

[northern 396]

I've tried both a 750 and 580 Holley on my 396. They both run fine, with no noticeable difference in power. Most engines, even with 400 hp or so, can't use all the capacity of a 750.

Regarding mileage, I couldn't say. Based on my experience, on a 396, a 580 cfm Holley only gets about 10 mpg. The 750 can't be much worse.
David

 

[Unclepennybags]

Switching to a 600 should help you out. You might pick up a m.p.g., maybe two.

I don't know what the rest of your combo is, but between a good spark curve, smartly tuned reasonably sized carb, and an ignition system that is in tip top shape you should be able to get 15 m.p.g. I was getting no less than 15 m.p.g. with a 1969 Qj, and up to 17 with a 1974 Poncho QJ driving my 67 back and forth to work. No overdrive.

Mike

Mike

 

[70_FathomBlueMalibu]

Somehow I knew that Pennybags (Master of Mileage) would chime in on this post.

FWIW, I got 14 mpg with my 355 hyd. roller engine (fairly nasty street hyd. roller, btw....as in 79* of overlap), a 3310 Holley 750 V.S. carb and a 700R-4 with a nonlockup 3500 converter. I had it tuned pretty good, but could have been much better.

 

[vrooom3440]

You may get better mileage from a 600 but only if you tune it well.

The 600 will have higher venturi air speeds which will provide better fuel atomization. Better atomization leads to better fuel distribution. Better distribution means you can run it closer to the lean point without any cylinder(s) going too lean.

But for maximum affect you are also talking about maximum tuning and for that you need a wideband O2 sensor. Seat of the pants is not going to get you where you want to go if maximizing mileage is your goal. Likewise the O2 will help you minimize your power loss from the 600.

I was reading recently a suggestion that max economy would/could run mixtures in the 18:1 range depending on engine and ignition. According to this source the limit is being able to ignite the mixture which is where the ignition came in and made a significant difference. This lean burn was also supposed to improve efficiency by producing less power so more throttle opening could be used with less vacuum. It was an interesting concept but I suspect there are some significant "but"s and "maybe"s involved.

Steve

 

[Guest]

How many realize that the L-79, 350hp/327 motor ran a 585 cfm holley??

As for gas mileage, AND POWER, you can't beat a GOOD Q-Jet, imho!!

pdq67

Best Car Insurance | Auto Protection Today | FREE Trade-In Quote

 

[Unclepennybags]

If it's something that you are going to be driving a LOT and keeping for a long time, get overdrive. I've seen guys here who pull down 24 m.p.g.'s on a long cruise ---- with a carb. It's an expensive mod though, so it takes a lot of miles to pay for itself.

If you don't have overdrive shoot for at least 15 mpg. It's a reachable goal, and you'll save 50% off your fuel bill if you started at 10 m.p.g.

A 600 cfm will get better fuel economy than a 750 as long as both are in a comparable, decent state of tune.

It's been my experience that an out of the box Qj will beat an out of the box Holley for fuel economy. Of course, there is no reason why a Holley shouldn't get almost as good fuel economy if you take some time and carefully optimize your jet and pvcr sizes.

Lastly, don't forget the importance of keeping the choke on for the minimum practical amount of time and adjusting your idle mix properly. Idle mix is an important part of the equation for around town driving.

Mike

 

[Guest]

Years ago when I pulled my junk301 motor to install my hopped up 409 "W" motor, my old RR Buddy glammed onto my stock 350 carb. and intake for his H-D Gimmy 4x4, 3/4 ton P/U.

I told him it was shot but he said whatever b/c it had to do better then what he was getting with his stock Q-Jet..

He came back later and told me his P/U's gas mileage went from 8 to 12 mpg with my old worn out Q-Jet carb and intake.

pdq67

Best Car Insurance | Auto Protection Today | FREE Trade-In Quote

 

[JimM]

Picked up the latest CHP mag yesterday, first atricle I read involved tuning for mileage using an LM!, working the idle feed restrictions and idle and main air bleeds. Made some good points, including that MOST cruise is happening on the idle circuit, and that the idle mixture screws ONLY control mixture at idle, anytihng above idle, you're running on the transfer slots, and that mixture is controlled by the IFR's.