gas mileage Mechanical vs Vacuum Secondaries

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kielbasa

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I've always heard that vacuum secondaries get better mileage, wondering if anyone knows how true that statement is?
Just curious, I've been trying to figure out why that would be the case (assuming steady state driving with a constant throttle opening)...
 
Your right foot makes all the difference... Whether or not your secondaries are vacuum-operated or mechanical, if you open them your gas mileage will suffer. Really you should choose secondary actuation based on the rest of your engine/trans combo.
 
At steady state assuming they are the same size and jetted the same there would be little if any difference. The difference comes from the fact that double pumpers come jetted richer for performance have dual accelerator pumps and alot of times one is a 50cc instead of just a singe 30cc on vac carbs. Sometimes the boosters are different as well.
 
The PVCRs on a double pumper are sized for performance/race apps rather than street stuff, so mileage will be worse with a double pumper even with moderate driving.

This (and the solutions to it) are covered in the "Super tuning and modifying holley carburetors" book.

Supposedly the 750 Double Pumper Street HP carburetors have a "street calibration" so mileage should be better with it than a regular double pumper.


Steve
 
yes, when you're on and off the gas, I can see mileage suffering when the accelerator pumps are constantly pouring fuel every time you mash your right foot....that's kinda why i asked about steady state driving, just cruising down the interstate for example....
i understand the secondaries would be open (mechanical), but I guess I can't see that in itself burning more fuel (vs running just off the primaries, which would have to be open more to be running the same speed down the highway)...

maybe my real question is, is the engine more efficient using two small venturis vs four larger ones (that aren't open as much)?
 
kielbasa said:
yes, when you're on and off the gas, I can see mileage suffering when the accelerator pumps are constantly pouring fuel every time you mash your right foot....that's kinda why i asked about steady state driving, just cruising down the interstate for example....
i understand the secondaries would be open (mechanical), but I guess I can't see that in itself burning more fuel (vs running just off the primaries, which would have to be open more to be running the same speed down the highway)...

maybe my real question is, is the engine more efficient using two small venturis vs four larger ones (that aren't open as much)?
Click to expand...

A double pumper has a progressive linkage so all four barrels are not open at cruise.
 
yep ramcharger, and from the dp's Vs's I 've played with they are usually on the lean/small side, so I'm not really banking or putting too much significance on the pvr's that the other guy mentioned, they are open during low vac/power mode anyways with the vac too high at any real cruise speed to open them.

it's all in the foot.

btw...unless it's a special app/ or large cfm holley..the pri/sec r both 30cc pumps.

and know that the vac sec will open the secondaries if vac is high enough, given only as much as the slot/linkage will allow.
 
kielbasa said:
Just curious, I've been trying to figure out why that would be the case (assuming steady state driving with a constant throttle opening)...
Click to expand...

The reason for this is because double pumpers are jetted richer than vacuum secondary carbs. As a result, the PVCRs are smaller. You can jet the double pumper down to get better mileage at cruise/steady state, but then it'll run lean at WOT. To fix the air/fuel at cruise and at WOT you have to jet the carb down and drill out the PVCRs.

It is the calibration that Holley ships the carbs with that makes them less efficient (they are after all, racing carburetors). Not the mechanical secondary itself.
 
Here's a snippet from this guy's site:

http://www.bob2000.com/carb.htm

Double Pumpers and Gas Mileage (or lack thereof...)
Have you heard people complain about the gas mileage they get with performance-type double pumper carbs? There is a reason that the 0-4776 through 0-4781 double pumpers get bad gas mileage. It's the jets! Surprised? Holley sizes the jets and air bleeds on these carbs so that they run on the rich side at cruise speeds. They make more power this way, at the detriment of gas mileage. These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations.
The way to do this scientifically is to measure the diameters of the openings of stock jets and PVCR's using drill bit diameters. Then calculate the total area of all the openings, and add them up.
A = 3.1415 x dia x dia / 4
Decrease the main jets for proper cruise mixture, and enlarge the PVCR's until you get back to the original area of all the openings. This way, your cruise mixture will give you good gas mileage, and under power, the engine will have proper mixture.


Cheers


Steve
 
I don't know who "Bob" is, but the power valve is not open under cruise conditions.
 
the blades aren't open much to make hp for a 60mph cruise.

if it's milage we are talking, the street dp's are fine with the leanness not being enough to cause damage to the engine but decent 'for what it is' fuel economy.

this true.

Hey the other way is lose the PV for a plug and go up 10 jets, but then of corse...
 
LXguy said:
Here's a snippet from this guy's site:

http://www.bob2000.com/carb.htm

Double Pumpers and Gas Mileage (or lack thereof...)
Have you heard people complain about the gas mileage they get with performance-type double pumper carbs? There is a reason that the 0-4776 through 0-4781 double pumpers get bad gas mileage. It's the jets! Surprised? Holley sizes the jets and air bleeds on these carbs so that they run on the rich side at cruise speeds. They make more power this way, at the detriment of gas mileage. These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations.
The way to do this scientifically is to measure the diameters of the openings of stock jets and PVCR's using drill bit diameters. Then calculate the total area of all the openings, and add them up.
A = 3.1415 x dia x dia / 4
Decrease the main jets for proper cruise mixture, and enlarge the PVCR's until you get back to the original area of all the openings. This way, your cruise mixture will give you good gas mileage, and under power, the engine will have proper mixture.


Cheers


Steve
Click to expand...
good info - soooooo, this begs the question,why wouldn't Holley do this and market a street oriented double pumper? I would be willing to bet the majority of double pumpers sold are used for the street than "drag racing at the strip"....
the above solution sounds like a win-win situation for a street application....or is this what the aftermarket (Bigs, QF, etc) end up making from a base Holley?
 
LXguy said:
OK, where does it say that the power valve is open under cruise conditions?
Click to expand...

Right here "These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations."
 
LXguy said:
Supposedly the 750 Double Pumper Street HP carburetors have a "street calibration" so mileage should be better with it than a regular double pumper.
Click to expand...

http://www.holley.com/0-82751.asp

The Quickfuel carbs (and the others I imagine) are jetted pretty aggressively also, and don't have a secondary power valve at all (they up the rear jet sizes even further). If you have the secondaries open at all at anything other than WOT on those, its halfway to fouling plugs.

I really don't know why anyone would run a double pumper if they aren't going to be racing quite a bit. Street performance is going to be nearly identical-probably better with the vacuum secondary given the difficulty of properly tuning a double pumper - and the tuning and mileage on a vacuum secondary is going to be easier/better.

The big advantage to a double pumper in my book is that you can leave the line with all the throttle blades wide open. There's no analog for that on the street as all you'll do is blast the tires.

Maybe its "manlier" to run a double pumper? LOL

Steve
 
ramcharger said:
Right here "These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations."
Click to expand...

That says "under power conditions" meaning WOT. Not cruise conditions.

If you put leaner jets in the carb, you get your better fuel mileage under cruise conditions, but when you got WOT and the power valve opens, the small PVCRs can't supply enough enrichment with the smaller jets and you get a lean condition.
 
Heres the guy's diagram about carb circuitry:

fgrph.gif
 
LXguy said:
http://www.holley.com/0-82751.asp

The Quickfuel carbs (and the others I imagine) are jetted pretty aggressively also, and don't have a secondary power valve at all (they up the rear jet sizes even further). If you have the secondaries open at all at anything other than WOT on those, its halfway to fouling plugs.

I really don't know why anyone would run a double pumper if they aren't going to be racing quite a bit. Street performance is going to be nearly identical-probably better with the vacuum secondary given the difficulty of properly tuning a double pumper - and the tuning and mileage on a vacuum secondary is going to be easier/better.

The big advantage to a double pumper in my book is that you can leave the line with all the throttle blades wide open. There's no analog for that on the street as all you'll do is blast the tires.

Maybe its "manlier" to run a double pumper? LOL

Steve
Click to expand...

Where in the world do you get your information? QF carbs come with their billet metering blocks with power valves on both the primary and secondary side, plus you can get either 4 or 5 circuit adjustable emulsion channels. Another plus is that they come with 3 linkages for the secondaries, super progessive, progressive and 1:1 for racing.

If someone knows how to tune a holley style carb, a DP works great on the street with excellent manners.
 
LXguy, what it looks like on paper to you and how it actually runs are 2 different things.
'not talking about the chart above'
I've been running run of the mill holley street DP's for a long time, and compared to the vac secondary carbs, there really isn't a diff noticed at the pump, but the ET's sure are better with the DP's and 'my combo's' which have to be streetable since I drive primarily on the street/hwy's.

I was getting 15mpg with my 10.1 344sb .528 solid w/+5*, running 24*int/32*total 3.73 gear, 4 spd , ported X heads, w/weiand single plane-750 double pumper.

w/no vac advance and could of only added w/vac adv 6* more before it would clack upon hard xcelleration, so maybe there was another 2mph left...17mph?

the issue with pvcr's is app dependent, as in...if milage is your ultimate goal.
 
ramcharger said:
Where in the world do you get your information?
Click to expand...

I got my information from the sheet that came with my Quickfuel carb.

Here is the same information on Summit's website:

http://www.summitracing.com/parts/QFT-Q-650/

ramcharger said:
If someone knows how to tune a holley style carb, a DP works great on the street with excellent manners.
Click to expand...

You are correct. Mine runs great. But that's a big IF-most folks can't tune one of these to save their life. And if I wasn't racing, I wouldn't bother with it. And my fuel mileage would probably be better also.
 
ramcharger said:
Right here "These are competition carbs, and they are supposed to work this way. Why don't we just put leaner jets in them to get mileage? Because the PVCR's are small, and under power conditions, the carb will supply an overall lean condition. What you can do is lean the jets out, then enlarge the PVCR's to compensate for power situations."
Click to expand...

yep and would read better posted as ''hey, if you wanna squeeze all the milage you can out yer dp...then you'll have to up size the pvcr's when you lean out the jets so you don't run too lean under wot/hard xcelleration where the pv/pvcr's work''.

good stuff to know when really getting into it.

ME? I would simply change to a more aggressive billet metering block kit.
but consider if the carb is on the small side..you may be on the big side of the stock jetting and never have any issue with being too lean at wot.
 
1wild&crazyguy said:
LXguy, what it looks like on paper to you and how it actually runs are 2 different things.
'not talking about the chart above'
I've been running run of the mill holley street DP's for a long time, and compared to the vac secondary carbs, there really isn't a diff noticed at the pump, but the ET's sure are better with the DP's and 'my combo's' which have to be streetable since I drive primarily on the street/hwy's.
Click to expand...

Well, "notice" is sort of vague. The mileage on my hot-rod could vary pretty easily by 3-5 mpg before I'd notice. I don't drive them a lot. I hot dog when I've got them out. Unless you were specifically tracking it, a change of 1-3 mpg is going to be imperceptible, and if you were specifically tracking it, you'd drive in a pretty sedate fashion, which would obviously close the gap between the two.

How much ET difference is there? Are you letting off the gas to shift the manual or are you powershifting?

1wild&crazyguy said:
I was getting 15mpg with my 10.1 344sb .528 solid w/+5*, running 24*int/32*total 3.73 gear, 4 spd , ported X heads, w/weiand single plane-750 double pumper.
Click to expand...

Oh come on. Is your speedo corrected and verified for the different gear and tire you're running? How many tanks did you average? Any highway driving in there?
 
Just curious how the thread got changed to double pumper tuning? I though it started out about vac vs mech secondaries. Everyone is putting some good info in here though :D I would say difference between the two is not much if the carbs were tuned the same and adjusted right. super tuning holley carb book is about the most info I have seen on these carbs. I dont think they miss anything. Lots of info and a good parts list as well.
 
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