alswagg

I am contemplating on ordering a new carb, the Holley street avenger. However I still want to get good millage like the Quadrajet gets. I am replacing the heads with Vortec versions as well as the intake manifold. any recomendations as per which CFM would be best? I am leaning toward the 570 myself. Thanks Al

bashcraft

I'd lean that way myself.

SanDiegoPaul

your mileage will go way down with anything Holley makes, remember: the oem carb has tiny primaries!

capevettes

The spreadbore Holley replacement carbs also have the small primaries and can be bolted directly to a factory spreadbore intake. I dumped my Q-Jet and put in the Holley and everything improved, including my gas mileage.

SanDiegoPaul

ww!! you are absolutely the first person that I have ever heard got better mileage with Holly!
Not doubting you, but I am in shock.

birdsmith

I would think a 570 to be on the small side for a 350 inch motor, even a mildly cammed one. It would work if you're not going to push it real hard but it'll likely starve for air at the upper rpm's. CFM rating doesn't really affect gas mileage; you've either got the right size carb or you don't. With a Holley the things that most affect mileage would be having the correct jet sizes, power valve sizes and squirter sizes. If these are optimized you should be able to approach the kind of MPG figures you'd get with a well-tuned QJet.

c3corvette77

As long as you stay away from mechanical secondaries and stick to vacuum secondaries you can do ok. Must be tuned properly though. Mechanical secondary double pumpers push fuel through both accelerator pumps with every twitch of your foot. My 750 Holley mechanical secondaries is not a mileage machine. Gas guege is a smile dial. As long as I have enough gas to get to the next station I'm good.

bashcraft

Assuming 100% volumetric efficiency, a 570 cfm carb is good to around 5600 rpm.

SanDiegoPaul

LOL 100% SURE ...

REMEMBER the oem carb was much larger...

capevettes

Honestly, I think I would attribute alot of it to the fact that the Q-Jet was old and not performing properly. I never liked them anyway, especially on cold starts. I put a 650 Holley with mechanical secondaries on my 68 437/390 and I feel like I bolted on 100 hp The car is no worse on gas than my small blocks as long as I keep my foot out of it, which is hard to do. It will loaf along on those puny primaries and is very efficient.

The 650 CFM is plenty of carb for the stock L36. Alot of people overcarburate small blocks. In 66 the 427/390 came from the factory with a four barrel that was under 600 cfm and it did not starve for fuel. The smaller carbs actually flow better and give better throttle response and great low and mid range torque. Unless you live at 6,500 RPM's the smaller carb is the way to go on the street.

bashcraft

So what's your point?

Petes73

I always wondered about the spreadbore gas milage. Since CFM is not only a function of the size of the the hole but also how much restriction there is (throttle plate) wouldn't a larger hole partially open flow the same as a small hole fully open? I know it is not quit apples to oranges, but if an engine takes a certain amount of gas to run at a certain rpm under a certain load what differance does the size of the carb make if it is set up correctly. I can see on the top end you can not flow what is not there, but around town I am having a hard time with the concept.

Slow day at work and inquiring minds wlould like to know.

Duane4238

I replaced my Q-jet with a Holley spreadbore back in 1976 or so. It was a direct replacement with vacuum secondaries. My car was more responsive, ran much better, and got a little better fuel mileage than the Q-jet. Some years later I replaced the Holley spreadbore with a Holley Double pumper. With my 4 speed trans. there was a marked performance difference and the mileage hasn't changed, except when I get into it for some fun. The main reason people's mileage drops with the Holley is because they enjoy driving and drive more aggressively with the new carb.
Duane

alswagg

I ordered the 570 cfm Holley street avenger. Should be at my shop tomorrow. What put me over was in the marine world where I seem to live in. All small blocks with Holleys are running the 600 cfm 4160 big blocks are running the 650 cfm 4150. These are engines running 5300 rpm. My car rarley ever sees 5,000 rpm. Thanks Al

Vette5.5

Not sure the way you're using the car, but hope the new Holley work's out well. Agree, Q-Jet's are hard to set up properly, but a much more sophisticated design than most. Generally easy to spot Holley equipped car's on the street, as those are the one's in the rear view.

Manuel Azevedo

Basically you are correct. But what does happen around town is if the bores are to big you keep the plates closed way to long and create a stumble because your still in the idle circuit and not pulling enough air through the booster to pull the fuel needed to increase speed, that's where the double pumpers came in to cover that flat spot and just ruin mileage. So the small bores of the spread bore work very good on the street while the others are not very good at all, meaning the only way the bigger bore carbs get rid of the stumble is to over feed the engine with those big accelerator pumps. (poor mileage). The whole CFM thing about this is too big that is too small is Basically just talk without any understanding about the needs of an engine. One poster here came up correctly that 570 CFM was good to about 5600 rpm which is all that is needed. If you put too much CFM capacity you slow down the air by the boosters and make the engine sluggish! The bottom line is to run the smallest CFM carb that flows what is needed at top rpm, and jet it properly and you'll run just fine, better than most really! Sorry for the long wind here!

jb78L-82

You can count me as number 2. I have had a 4175 Spreadbore 650 CFM Holley (just rebuilt it in Dec 2010) on my car since 1985 and everything got better than with the Q-jet including mileage. The 4175 has small primaries just like the Q-jet and gets very good mileage just like the Q-jet as long as you stay out of the secondaries. The 650 CFM in my car is more than adequate even at 6,000 RPM which is rare these days on a 34 year old original 66,000 miles L-82 4 speed. I generally don't rev it past 5,200 RPM since that is where the L-82 makes the highest horsepower! As I have said before, in my opinion, the Q-Jet is way overly complicated for what it needs to do. The simplistic design of the 4175 is a thing of beauty and I thought of that thought every minute I was rebuilding it. Try that rebuilding a quadrajet!

Manuel Azevedo

Totally agree, I run the 4175 also and works very well and you can buy what ever parts you want or need almost any where, try that with a Q-jet!!!.

Vette5.5

Very true, as Holley's are very simple design's, easy to tune and lot's of part's availibility. Fact is, well set up Q-Jet's will leave these eating dust. Not looking for an argument, as easy sometime's work's fine. Maybe not the best way if willing to do some tweeking.