gas mileage Mechanical vs Vacuum Secondaries

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LXguy said:
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?



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?
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relax, ain't bashing you or really even disagreeing with you.
this was the middle of average hwy/street-but based on cruising cause if I lead footed it all the time I'd get 8-10mpg.lol
yes everything was figured in.
The high initial and close tuning/lean as possible..see I will drop primary jetting to where it surges just a lil on the hwy, then go up 1-2 sizes depending the weather and only ran a PV in the front,
jetting was 70-72primary and 82 secondary.

The last time I figured it driving to fresno which the grapevine isn't very nice to gas milage with it's numerous inclines and sht road conditions.
despite contrary belief ...mopars get good milage..wayyyy better than a chevy.
 
chuckwagon2004 said:
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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Sorry Holley only has one model with mech secondary and one pump :read2:
 
Hey:

For the record my point was never that a double pumper couldn't be driven on the street with acceptable efficiency.

I was trying to answer the OP who asked if it was true that double pumpers get worse mileage than vac secondaries (even if it only gets 1mpg worse mileage, that's still worse, yes?). I answered in the affirmative, and tried to explain why, and what is required to be done to get a double pumper to function correctly once it has been jetted down for economy.

My response about Quickfuel carbs may not be completely accurate as I have only ever owned two of them. It is possible that they have other models that would be more readily adapted to efficient street use. My response is accurate to my carbs though.

I have found that Bob2000 site to be very helpful over the years. I think that diagram can be huguely helpful to people starting out. I didn't want him to get a bad rap because of a misunderstanding about something I posted.

These are facts.

My opinion is that if I weren't going to race a car at least semi frequently, I'd run a vacuum secondary carburetor on it. A good vacuum secondary is so much cheaper and easier by comparison, especially when you live in a climate where the density altitude swings by 3500 feet or more from May to October.

I run a vacuum secondary Holley on the car when I'm done racing for the year, and when I get it out of storage in the spring. The rest of the time, it wears a double pumper.
 
LXguy said:
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/



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.
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That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.

BTW, the density altitude here ranges from 4500 to 9800 ft., and that's here in Denver. Drive to the mountains and it's a whole 'nother ball of wax.

The DP vs. VS argument is really combination specific and much depends on the ability of tuner himself.
 
LXguy said:
Hey:

For the record my point was never that a double pumper couldn't be driven on the street with acceptable efficiency.



I run a vacuum secondary Holley on the car when I'm done racing for the year, and when I get it out of storage in the spring. The rest of the time, it wears a double pumper.
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I know, good info when it's needed.
 
ramcharger said:
That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.

BTW, the density altitude here ranges from 4500 to 9800 ft., and that's here in Denver. Drive to the mountains and it's a whole 'nother ball of wax.

The DP vs. VS argument is really combination specific and much depends on the ability of tuner himself.
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exactly!
 
ramcharger said:
That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.
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OK. Sure. For clarification, I never said it didn't come with a provision for a power valve. I said it didn't come with a secondary power valve.

BUT, if you're going to use the thing for racing, you don't want a power valve in the secondary for the same reason you run jet extensions. You don't want the fuel running away from the opening on launch.

Also, its easier to control secondary enrichment when its all done through the jets of course.
 
A lot of good info and tips in this thread, but to answer the original question. If tuned correctly and depending on how your engine is built and how you drive, a vacuum secondary can get better mileage than a DP, but it won't be that noticeable unless you have a 4spd which would be a considerable difference since you are basically accellerating 4 times for every one of the auto and with the dual accelerator pumps = "a lot of gas".

To answer someones question above, "why run a DP if your not racing a lot" it all depends on your engine, a vacuum secondary carb will not work, or at least work correctly if you are running upwards of 300 duration.
 
I really like lxguy's response as it is close to what I plan. I have a 750 holley vac sec with a proform carb body, a box stock 650 dp, and a thermoquad that I will be trying. I like to think of it as a carb for all seasons. I know this may seem excessive but all the carbs were used and picked up very reasonably. Actually I have as much in all three carbs as what the 650 dp would cost brand new.
 
a double pumper primary throttle blades open 40 per cent or degrees before the linkage begins to open the secondaries..

vacum secondaries..you can change the spring in vacum cannister to delay the opening of the secondaires...
 
Ramcharger, it's kinda rude to just ignore it when your mistake is pointed out and evfen more rude to make excuses while still not admitting you are wrong. LXguy was right on target.


Originally Posted by ramcharger
I don't know who "Bob" is, but the power valve is not open under cruise conditions.

Originally Posted by ramcharger
That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.
 
Mileage is mostly in your right foot.

350hp 360 with a 2500 converter and 3.00 gears got 20 mpg on the highway with an out of the box 4779 750 DP, no jet changes. I thought that was pretty good.

In my experience with the proform vac sec main bodies, you better have some air bleeds handy. Most chrsyler SB applications will be too lean at cruise, where the air bleeds and Idle Feed Restrictor play the biggest role.

If you want a good primer on tuning for cruise and part throttle applications, go to the innovative motorsports site and look at the tuning articles.These are the people that have the LM1 O2 reader out there.
 
mopar340dave said:
Ramcharger, it's kinda rude to just ignore it when your mistake is pointed out and evfen more rude to make excuses while still not admitting you are wrong. LXguy was right on target.


Originally Posted by ramcharger
I don't know who "Bob" is, but the power valve is not open under cruise conditions.

Originally Posted by ramcharger
That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.
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my 4779 has the plug in the rear but when removed still has no pvcr holes, kinda irritated me when I was gonna try & square up the jets.
 
kielbasa said:
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)...
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As noted in the answers... Carbs can come a variety of ways, from non-adjustable emulison bleeds to adjustables in 3,4,5 steps... If there is a flat road, and a steady right foot, and two identical carbs (meaning both same size throttle plates, same booster design, same manufacturer, same either non-adjustable, or adjustable and tuned by someone that knows how to tune...) there will be no difference in milage in the adjustable tuned ones, and minor advantage in the vacuum secondary if we're talking about Holley's because of the sizing of emulsion bleeds. PVCR does nothing in steady state, and any increase in size will mean a more sensitive power valve. Not more fuel per se, but more sensitive. If you change any of the parameters the whole "apples-to-apples" deal goes out the window. IMO, vacuum are for engines with cams of moderate duration. DPs are better suited for engines where horsepower is a top prerequisite rather than economy.
 
mopar340dave said:
Ramcharger, it's kinda rude to just ignore it when your mistake is pointed out and evfen more rude to make excuses while still not admitting you are wrong. LXguy was right on target.

I think it's kind of rude to think I was ignoring LX when in actuallity I had worked 21 days in a row and was trying to do dishes, clean around the house, deal with an employee who just lost an inlaw and was trying to figure out how to get bereavement PTO, etc.

Don't make assumptions.


Originally Posted by ramcharger
I don't know who "Bob" is, but the power valve is not open under cruise conditions.

Originally Posted by ramcharger
That's a plug in there and it can be removed and replaced with a power valve, they don't come without a provision to add one like my holley 4777.
Click to expand...

Both of these statements are 100% true. All QF metering blocks also have changeable PV restrictors if needed. Also, on my 4777 I never had an issue with stumble on tip in after the correct size squirter nozzle was installed. At cruise, I'm getting about 12 MPG. Not bad for a full time 4WD (all four wheels are driven all the time) 5600lb truck on 33x12.5's and 3.55 gears with the aerodymanics of a brick. Before the engine was rebuilt and I was using the factory 2 barrel I was getting about 9 mpg. By comparison, my 2001 fuel injected 5.9 Dodge ram part time 4wd with 4.11's was getting 14 mpg at cruise. The ramcharger would smoke that truck anywhere in the rpm range too.

The intake manifold and carb are actually too small for this engine too. It breathes hard even up here in the rare air.

Like I said before, MPG and driveability with any given carb and engine combo depends entirely on the build and the skill of the tuner. I can wood the accelerator on this 360 at idle, in a way heavy vehicle with automatic trans with a less than optimum rear gear and it just plain moves. Even holley says this can't be done, I'm saying it can. Most likely a lot of people couldn't pull this off, I'm just saying that it can be done without resorting to using a drill. Here's a video and you can even hear the squeak of the pedal when I mash it.
[ame="http://www.youtube.com/watch?v=nA4AdsTlMto"]YouTube- 6th Ave Blast[/ame]
 
Is that your defense? You were too busy? You even posted that bs right there as if a plug constitutes a power valve.. whatever man. You don't have time? You post at least five times every day for over five years but you did not have time?

A plug is not a power valve and he never said that power valves are open under cruise conditions. What are ya, a politician? LOL
 
kielbasa said:
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)...
Click to expand...


The problem is…… "assuming steady state driving with a constant throttle opening"…… Doesn't happen in the real world.

Vacuum secondary is an on demand system (it delivers when the engine asks for it) and will generally give better MPGs in real world driving (all things being equal).


Mechanical secondaries are just that, mechanical. When you reach a given throttle position (regardless of engine demand) it starts dumping fuel (all things being equal).

This thread became far more complicated then it ever needed to be.
 
mopar340dave said:
Is that your defense? You were too busy? You even posted that bs right there as if a plug constitutes a power valve.. whatever man. You don't have time? You post at least five times every day for over five years but you did not have time?

I post when I can, to try to help. I don't always have time to search for every post I've made. And no, I didn't have time.

A plug is not a power valve and he never said that power valves are open under cruise conditions. What are ya, a politician? LOL
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"Bob" said they were open under power conditions. The opposite would be a no power situation. Can't cruise with no power. What are you, an appointed cabinet member?. :-D

My point with the power valve is that can just be simply screwed in and the orfices are even adjustable. No carb is going to work for every application on every engine off the shelf. Just because a carb comes with, say, 68 jets..... Would you never change them even though you may be running rich or lean? Most carbs don't come with jet extensions, so they should never be used?
 
:thebirdm:You are ridiculous.

"Power conditions" is WOT or close enough to it. "Cruise" is just part throttle unless you're runnin 513/1 gears on the street..

Stop being an *** and admit when you are wrong.

My car came with an AM radio but I'm not running around telling people that it came with a cd player just because I can add one.
Your analogy is idiotic.
Refer back to the first sentence of this post.:thebirdm:
 
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