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Topic Review (Newest First)
01-30-2013 07:05 PM
spinn
Quote:
Originally Posted by ap72 View Post
Well, velocity in the runner can impact volume so it becomes important only because of its effect on volume and condition. If the heads can effectively maintain a high quality of intake charge and fill the cylinder then the velocity can be lowered and you will make more power (hence larger ports are used when going after racing power numbers).

velocity in the carb can have a significant impact on "condition" depending on how well the carb can atomize and disperse the fuel- as the quality of the carb goes up the need for high velocity goes down, you can then run larger venturi and have more volume for a given pressure differential across the top and bottom of the carb. Hence a good carb will have lower velocity with better condition and more volume.

velocity is only one way of many to affect volume and condition (which is what you're really concerned with).
Disagree, I will get back to this. Part of it, if you left out all the Will, Only, Really's . You bring up some topics of discussion.
01-30-2013 06:59 PM
ap72
Quote:
Originally Posted by vinniekq2 View Post
velocity where?
in the carbs venturi or in the intake runner?
Well, velocity in the runner can impact volume so it becomes important only because of its effect on volume and condition. If the heads can effectively maintain a high quality of intake charge and fill the cylinder then the velocity can be lowered and you will make more power (hence larger ports are used when going after racing power numbers).

velocity in the carb can have a significant impact on "condition" depending on how well the carb can atomize and disperse the fuel- as the quality of the carb goes up the need for high velocity goes down, you can then run larger venturi and have more volume for a given pressure differential across the top and bottom of the carb. Hence a good carb will have lower velocity with better condition and more volume.

velocity is only one way of many to affect volume and condition (which is what you're really concerned with).
01-30-2013 06:53 PM
vinniekq2 velocity where?
in the carbs venturi or in the intake runner?
01-30-2013 06:39 PM
spinn
Quote:
Originally Posted by ap72 View Post
That's not at all how it works.

velocity is irrelevant, VOLUME and CONDITION is.
disagree wait.
01-30-2013 11:45 AM
496CHEVY3100
Quote:
Originally Posted by snakebit68 View Post
Bingo! Thats what I'm saying. Glad to hear you worked it out.
Your welcome.
01-30-2013 11:24 AM
snakebit68
carb

Bingo! Thats what I'm saying. Glad to hear you worked it out.
01-30-2013 11:08 AM
bowtie44s I have the bog taken care of, I went from the white to the long yellow. I was just wanting the secondaries to open at a lower rpm and that created the bog. I guess I was thinking about it wrong. The engine can't use more air if it can't burn the fuel.
01-30-2013 09:13 AM
PatM So, I'm no carb expert, but I was thinking along the lines of 496chevy3100. If this was my truck, I'd look to stiffen the secondary diaphragm spring. It's really easy and quick to do, not to mention cheap. A REALLY stiff spring would virtually lock the secondaries out. I think driving on the primaries only might be a pretty good test. If the bog is because too much air is entering at low RPM when the secondaries open, tuning with this simple spring might also offset the rich condition that the OP says he is experiencing.

PatM
01-30-2013 08:23 AM
cdminter59
vacuum secondary or douple pumper

I have always ran a single plane intake Victor JR. and 750 DP (383) or 850 DP (454) on my engines. But both of these engines had long duration mech. roller cams, 3500 stall converters, and 4.11 in my 67 Camaro 454 and 4.56 in my 75 Camaro 383. Neither of these cars were daily drivers. I have my truck as my daily driver.
01-30-2013 08:23 AM
ap72
Quote:
Originally Posted by spinn View Post
A towing application , you want velocity. Opening the secondaries up wide from low rpm will kill the velocity and need for fuel. At load the engine is hungry, but that sudden decerease will bog.
That's not at all how it works.

velocity is irrelevant, VOLUME and CONDITION is.
01-30-2013 08:18 AM
spinn A towing application , you want velocity. Opening the secondaries up wide from low rpm will kill the velocity and need for fuel. At load the engine is hungry, but that sudden decerease will bog.
01-30-2013 07:53 AM
snakebit68
carb

Keep in mind that when under load your carb is dumping alot of fuel through the system, but your RPM's arent going to build fast enough to burn all that fuel when towing, resulting in poor fuel economy. I had a similar issue on one of my work trucks. We switched it over to an Edelbrock 750 with vacuum secondaries and increased our average fuel mileage by 2 MPG and it towed much smoother. Of course, to be honest, we may have gotten the same results by fine tuning the Holley 750 we had on it to begin with.
01-30-2013 07:02 AM
496CHEVY3100
Quote:
Originally Posted by bowtie44s View Post
Hey guys, I'm sure you've answered this 100 times. I know this isn't a hot rod question so I appoligize. I have a mildly built 454, I'd guess around 400hp. It is in a heavy 1 ton truck with a 5 speed manual. I use this truck to tow a good bit and just cruise around. I have a 750 holley vacuum secondary on it now. It does pretty good for cruising but when loaded I have to get the rpms up enough for the secondaries to open. I'd rather have a douple pumper so if I push the throttle enough I can open the secondaries at a lower rpm. I tried a really light spring in the secondaries but without the pump shot it makes it bog when they open. Every place I read says you don't want a double pumper for towing but no one says why. Would it be a bad idea to switch to a dp? Also would it kill my fuel mileage? I'm getting about 11mpg empty with the vs... pretty good i think for a 6600lb truck.
put a firmer spring back in start with yellow then adjust firmer until bog goes away ,,secondarys opening to soon causing Bog ,probably end up with brown,
01-29-2013 04:34 PM
ap72
Quote:
Originally Posted by bowtie44s View Post
I have a 6.5 power valve. I am pulling right at 20" at idle (yea, mild cam.) I have been watching the vacuum while driving. Cruising in OD i'm around 12-15". When I punch it, it quickly drops below 5 so I haven't noticed any problem with the power valve. I haven't watched vacuum with a load, only empty. I know quick fuel makes a 4 door high flow power valve. I'm not sure what I have, I never paid attention. I just checked it for tears and figured it was good. Is there another way to get more volume of fuel other than a "high flow" power valve? How would I know if I need more fuel on the power valve circuit with out an afr gauge? It doesn't feel or sound lean under load.
you don't need a high flow PV and switching to one wont change a damn thing, what needs to be adjusted is your PVCR's (power valve channel restrictors), on a stock metering block they're just a drilled orifice, on a HP metering block they are tapped and threaded for brass inserts that can be drilled and adjusted the same as jets or air bleeds.

how you can tell if you need one is that you should have about a 15:1 AFR when cruising, and when under high load about a 12:1 AFR. your jets will dictate your cruise AFR and your PVCRs will kick in to add the additional fuel needed for high load. Of course air bleeds and transition metering and even the idle circuit also all factor in- which is why you pay a good carb guy the money for making it all work together correctly.
01-29-2013 02:26 PM
bowtie44s
Quote:
Originally Posted by ap72 View Post
them opening sooner really isn't going to change a lot, it'll help some, but it'll be very slight. I would also look into properly tuning your PV circuit, its the most overlooked and one of the most important. your high load tuning should be done with this circuit NOT the jets as is so often done.

I have a 6.5 power valve. I am pulling right at 20" at idle (yea, mild cam.) I have been watching the vacuum while driving. Cruising in OD i'm around 12-15". When I punch it, it quickly drops below 5 so I haven't noticed any problem with the power valve. I haven't watched vacuum with a load, only empty. I know quick fuel makes a 4 door high flow power valve. I'm not sure what I have, I never paid attention. I just checked it for tears and figured it was good. Is there another way to get more volume of fuel other than a "high flow" power valve? How would I know if I need more fuel on the power valve circuit with out an afr gauge? It doesn't feel or sound lean under load.
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