the difference between the vacuum secondary and a double pumper carb is the vacuum secondary carb has a smaller main jets and a larger power valve restriction where the double pumper is just the opposite with a larger main jet and a smaller PVR and that is why the MPG are worse when street driven

 

You do realize the 72 Holley compared to the 70 Holley was made leaner and the performance gain may be partially due to the 650 not dealing with emissions compromises?

 

Just to add, having spent lots of quality (tuning) time with my beloved Holley 650 Street HP, the hesitations are gone and crisper throttle response attained. Primary jets got smaller to increase afr at cruise. Primary squirters increased to 0.040" to eliminate the hesitation in part caused by the larger plenum of the Team G single plane. Still more intimate time required but it's running darn good now. The biggest hurdle in overcoming the hesitation for me was timing curve. And that started with "what does my engine like for initial timing and how do I find it?" I found an article published by Burton Engines(?). Best bit of tuning advice I've ever read for an engine. My ideal initial was found to be 22* using his simple method using a vacuum gauge. The problem then is reducing mechanical advance from 20-24* down to 12* in my case. Easiest but more costly solution is a programmable ignition system. Worth every penny. Then it was onto the Holley carb for adjustments. But most novices aren't willing to go to these lengths and even some more advanced guys. If you settle for factory 12* (smog) initial advance recommendations, your missing out, lots.

 

Wrong again. Apples to oranges.

 

My 427 gets twice that, easily. Hell, I can get 6-8 mpg on a track day. You're either doing something wrong, or have something wrong with your setup.

What's your timing curve? Does your vacuum advance work? What's your A/F ratio at cruise speeds?

Something needs correction IMO.

 

Oh hell, you just opened a can of worms now.

(I'm a fan of using a vacuum gauge for tuning also. )

 

Prior to the swap, I rejetted the 780, changed the PV's based on the engine vacuum at driving conditions, recurved the distributor mechanical advance, etc. The 780 worked ok, but still felt sluggish at the lower rpm range. (full disclosure; I'm used to the performance of my Cobra, so anything that doesn't snap my neck feels sluggish. lol) The engine feels much more responsive now with the 650.

Yeah, the original 780 may have an internal issue, not 100% sure. But I am sure that performance went up, not down with the carb swap. And no, I don't drag race it, so I have no top speed numbers to confirm. But for the street driving that I do, the 650 feels "right" to my seat of the pants. :-)

 

I used to laugh at the guys just using a vacuum gauge! But this Burton gentleman explains exactly how to do it but the qualifier is a means to retard mechanical advance which can be done mechanically or electronically. Otherwise, it'll have way too much total advance. Can of worms? Yeah, maybe. But that's when we start to split hairs when there is only a few more hp to squeeze out. I leave that to guys who make their living racing where its really needed. I'm happy with anything above 90% of maximum power potential.

 

Holley Carb size calculator pretty much agrees with your assesment
Check it out


https://www.holley.com/products/fuel...s/carburetors/

 

No... it does not agree with what he says.. but it does agree with the formula I gave and what the rest of the general consensus from thoes that actually know

type in the formal I gave.. it works..
I went to the Holley site and it also stated I ned over 1000cfm for my car
and a stock to mildly upgraded 350 it shows 600-670cfm carbs.

 

Well, the OP has certainly gotten lots of info. If I was him, I would remember all this data and take a 650 and 780 carb to his dyno session and see for himself.

Good luck,
walt z.

 

This thread has got to be one of the worst for having some of the most inaccurate information on carbs I've ever seen. There is some good info too, but for some reason people on this forum don't want to listen to people like Jim (427HotRod) who have done the testing and knows what he is talking about. There are others also who get get it.

(Edited)

I'm not going to bother putting my 2 cents in because nobody seem to want to learn anything anyway, they just want to push their opinion.

Mike

 

I will quote an old Lars statement..."Most carb problems are timing problems". The above statements about getting the timing curve correct before messing with the carb are dead on the money.

There's a gazillion ways to get a particular fuel curve. Just because a carb may have a smaller or larger jet....or a different PVCR doesn't account for the fact that air bleeds, booster design, emulsion tubes etc will likely be different also. I had a custom 1250, two circuit with a PV, progressive linkage Dominator built by the Holley custom shop because such an animal didn't exist. I had the good fortune of spending time with the guy who literally designed the fuel curve and parts combos of all the Holley's we've come to know and love. He'd been there forever. He built this carb and since our engine wasn't together yet, he sent it to a big name engine builder in the East to run it on his dyno on a big motor for final tune info. Holley didn't have anything healthy enough on their dynos at the time. When I got it and dyno'd it..it ran dead on...right out of the box. No lag, cruises fine on a nasty 598" pump gas engine with a 6 speed trans. Guess what? When I looked inside it had #100 jets on all four corners AND PV's. Plugs burned dead clean.

Again...unless you look at every aspect of a carb, you're just blowing in the wind trying to compare jetting etc between two of them. It means nothing.

JIM

 

100%

Neal

 

Back in high school I took 2 years of auto mechanics, my tuneup class teacher said almost the exact same thing and it stuck with me and through the decades I have found it to be very true. He said " Almost all carb problems are ignition problems."

Mike

 

Perhaps we need a sticky thread on all things Holley carburetor, penned by those in the know. I only know some basics, but probably just enough to keep my car from exploding into a fireball. What say the experts? I wanna learn!!

 

Wander over to Speedtalk.com and sign up. There are a couple of guys like jmarkaudio and tuner who have posted a ton of info about Holleys, and there are lots of other threads by pros in the engine building biz which you may like as well.

 

Yeah speedsmoker. That would be great. I started this thread and I've learned a lot.
Thanks
John

 

When it comes to old carburetors for street engines nothing beats the Rochester QuadraJet as it has small primaries for a high venturi velocity and huge secondaries for increased cfm. That's why it was used on engines as small as the inline six Pontiac to the large 500" Cadillac. Their small center fuel bowl was their main drawback but the Edelbrock's "Thunder" and Holley's "Street Demon" air valve secondary carburetors solved that problem as they have dual fuel bowls as well as an air valve secondary to prevent the power killing "over throttle" condition at low rpm's.

 

There are just so VERY MANY variables in this......any recommendation based in "engine size" is at the very best a "STARTING POINT" for average or typical only.

Holley has, for as long as I can remember, had a very basic chart view you can go to for help on sizing.

First up is your Cubic inch displacement, then expected High RPM limit, and finally the REALISTIC efficiency of your Build......lot's of folks here suggest 85% for mild street builds and upwards of 100% for fully thought out, planned and perfectly machined and matched parts.

Over the years since I first saw this, over 30 years ago ... , I figured getting maximum efficiency would be tough to achieve.......head porting and parts matching was an actual talent only available to the very best builders ....certainly not the typical build.

Today with computer programs and computer guided machining process........ a lot more people are getting closer to that elusive efficiency of 100%.........but one thing hasn't changed.

With Carburetors......you have to make all kinds of compromises the EFI crowd only scratches their collective heads and wonders why we still use this one dimensional method of delivering fuel and air to a motor.

No matter what engine we are talking about.....all find their way into different vehicles and they are rarely just single use vehicles.

Carbs are WONDERFUL and really easy to nail down in a single purpose trailer it to the 1/4 mile race track car.

But the dual use car will always require you to decide what you prioritize. No engine is one size fits all when looking for a Carburetor.

Generalization for starting point is probably the best anyone can recommend.

IMHO......

vacuum carbs generally speaking will deliver the better MPG and are far more forgiving of not quite the right size for this application.

Double pumpers are without question the better choice for the car owner that needs and wants that cover shot because yanking open at full throttle and maximum performance with out consideration of other priorities is what the owner wants.

Sizing happiness varies on typical use too.....smaller carbs will just feel crisper and deliver better throttle response at anything less than a run to red line in most cases. The carb that will run best and deliver best performance to red line is going to be bigger in CFM than the best choice for every day use.

In the end ......ALL OF THIS......is why people spend the money to convert to EFI.

But the idea of the all too typical 350 Corvette with one size of carb as the perfect choice?

Never gonna happen........one 350 Corvette and it's typical mild build for street use might be best off with a vacuum secondary 600.........and conversely the crazy high HP 350 in a car prepared for the drag strip and RPM that test the limits of durability on super expensive bottom end parts might want a 1000CFM for all I know.....it's owner might take it to the track on a trailer and not even care it it won't make any power at all bellow 3,500RPM.

I'm not here to SLAM anyone else's opinion or experience......I just know that for our own street driven SBC a 650CFM Double pump has gotten the most use.......at the drag strip I've swapped in a 750CFM with success so small as to almost not be worth it.

YMMV........and I may learn that a 750CFM Double pump might actually make a meaningful difference on the larger displacement 383 recently installed in my Corvette but I won't know until next summer.....for now the car is parked for the winter.