My engine pulls 15-16" @ idle, 10" moderate throttle, and 4.5 -5 WOT. Tests done with driving the car in all gears. This is w/a 750 DP.

Rebuild kit I just got comes w/the 6.5 valves. I'm putting on a 800, and wondering if the 6.5 is good, or should I get a 9.5?

Thanks fellas.
Vince

Patient everbody...one at a time please.

I'll take a shot. Use the 65 PV. If you want to, hook up a vacuum gauge and watch it as you drive. That will help you decide if you need to change.

I'll take a shot. Use the 65 PV. If you want to, hook up a vacuum gauge and watch it as you drive. That will help you decide if you need to change.

Thanks Pat. What would happen as far as vaccum when driving that would indicate the correct/incorrect PV?

Vince

Get a feel for how much throttle it takes to drop the vacuum to the PV opening point. With a higher vacuum PV, like a 95, you may find the PV is opening under fairly light throttle like a slight grade. You don't want that, your just wasting fuel. The 65 PV is the factory part for a reason. It works for just about all applications.

Get a feel for how much throttle it takes to drop the vacuum to the PV opening point. With a higher vacuum PV, like a 95, you may find the PV is opening under fairly light throttle like a slight grade. You don't want that, your just wasting fuel. The 65 PV is the factory part for a reason. It works for just about all applications.

Understood. Holley is listing the PV for my carb as a 12,21. Any idea what that is? Also, the airbleeds on the primary side are larger than the rear. Is that normal?

Vince

Vince, for example, you could run a 105 PV. When it went mid throttle the PV will kick in. You'll feel that in the ol azzdyno! Also,you might be able to dial back primary jetting for enhanced mpg with that configuration. Also might need less pump shot. Try it, you may like it, but it will require a retune.

As Pat mentions, a vacuum gauge is indespensible. You want the PV to hit right when it needs mid range power, just prior to the point of tipping in the secondaries , but not pulling up a moderate grade. Always set my carbs like that. Makes for an excellent street carb with smooth power transitions. Caveat: never tuned one for ET, just street driving/economy.

I think the 12,21 is for the footnotes in the catalog. The 12 is PN 125-85 secondary PV. 21 is PN 125-65 primary. My catalog is 5 years old but the footnotes probably haven't changed.

As Gene hinted at, there are mods you can make to the carb to increase mileage without compromising power. One is to increase the size of the PV Channel Restrictions and run a higher value PV. That lets you decrease the jet size for economy yet still have adequate fuel when the throttle is opened up.

I don't know about the airbleeds.

I think the 12,21 is for the footnotes in the catalog. The 12 is PN 125-85 secondary PV. 21 is PN 125-65 primary. My catalog is 5 years old but the footnotes probably haven't changed.

As Gene hinted at, there are mods you can make to the carb to increase mileage without compromising power. One is to increase the size of the PV Channel Restrictions and run a higher value PV. That lets you decrease the jet size for economy yet still have adequate fuel when the throttle is opened up.

I don't know about the airbleeds.


Thanks gentlemen for all the help.
Vince

Vince, I aint no carb Guru but heres my take. If your seeing manifold vac at anything other than zero under load / wide open throttle, that would indicate a restriction. Carb may be too small. I do not know your engine combo, but I am assuming that it is a mild cam based on your vacuum at idle of 15-16". With idle vac being what it is the 6.5 PV would be about right. Base line / rule of thumb is PV rated at half of idle vac.

You can always look up the list number at the Holley web site and configure the carb to its stock configuration for jetting and PV. If the carb has an unknown background it may have been tweaked by some Guru who may or may not know what ends up. If that carb has screw in air bleeds, I would definitly insure that they are the stock configuration as well.

I was running a Holley 770 avenger carb on my combo, and only make about 6" vac at 1000 rpm idle. It was jetted 74 / 84 with a 6.5 PV in the front, no PV rear. I have an Autometer vac guage in my dash. Always showed ZERO manifold vac at WOT. Are you sure your getting WOT with your carb linkage on your 750DP?

Here's what puzzles me about the PV thing. At what engine rpm does the carb come off of the idle curcuit, and start flowing via the main jets on the primarys? While the carb is running on the idle curcuit only, the PV regardless of its rating should not come into play.

I have thought in the past that regardless of engine RPM, if the manifold Vac was less than the rating of the PV, the PV would open an flow additional fuel. I do not think that is the case.

Can any one shine some light on that issue?

Vince, I aint no carb Guru but heres my take. If your seeing manifold vac at anything other than zero under load / wide open throttle, that would indicate a restriction. Carb may be too small. I do not know your engine combo, but I am assuming that it is a mild cam based on your vacuum at idle of 15-16". With idle vac being what it is the 6.5 PV would be about right. Base line / rule of thumb is PV rated at half of idle vac.

You can always look up the list number at the Holley web site and configure the carb to its stock configuration for jetting and PV. If the carb has an unknown background it may have been tweaked by some Guru who may or may not know what ends up. If that carb has screw in air bleeds, I would definitly insure that they are the stock configuration as well.

I was running a Holley 770 avenger carb on my combo, and only make about 6" vac at 1000 rpm idle. It was jetted 74 / 84 with a 6.5 PV in the front, no PV rear. I have an Autometer vac guage in my dash. Always showed ZERO manifold vac at WOT. Are you sure your getting WOT with your carb linkage on your 750DP?

Here's what puzzles me about the PV thing. At what engine rpm does the carb come off of the idle curcuit, and start flowing via the main jets on the primarys? While the carb is running on the idle curcuit only, the PV regardless of its rating should not come into play.

I have thought in the past that regardless of engine RPM, if the manifold Vac was less than the rating of the PV, the PV would open an flow additional fuel. I do not think that is the case.

Can any one shine some light on that issue?


Craig,

The vaccum #'s I was getting was base on a 750 DP. I just got a used 800 DP and currently am freshening it up. If this doesn't lower the vaccum #'s at WOT I'll go w/the 850. These 454HO's demand alot of fuel ya know ...

I do not think that a 750 CFM carb should or would cause the Vac restiction you are seeing, unless you are not operating at WOT. The 770 carb on my 500 horse 461 shows ZERO vac at WOT

I had my wife floor it and I checked...definitely WOT.
Vince

where on the engine/ carb do you have the vac guage hooked up to. i'm running a 750 dp on a solid lifter 248/258 at .050 .575 lift 427 with 6 inches at idle but zero at full throttle, something isn't right with your reading

where on the engine/ carb do you have the vac guage hooked up to. i'm running a 750 dp on a solid lifter 248/258 at .050 .575 lift 427 with 6 inches at idle but zero at full throttle, something isn't right with your reading

Tee'd into vac advance line (manifold vaccum).

Maybe the heads or intake creating a restriction (don't flow well?).

Vince

FWIW:
Right from the Holley Web site.
I don't agree with their statement regarding Big cams, as it's been proven time and time again, that with BIG cams, a power valve about Half the manifold yeilds best results.
Their recomendation for 2" below just doesn't work in a high performance application.
For a Mild engine, or mostly street application, this is pretty good info.
Their should be ZERO vacuum at WOT.
I think you'd be just fine with a 6.5
http://holley.com/HiOctn/TechServ/TechInfo/TI-222A.html

I had my wife floor it and I checked...definitely WOT.
Vince

While the engine was running and car in neutral? With engine off have her floor it it and then see if you can move carb linkage to further open the throttle bore blades.

I think that you would see a difference in your vac guage readings with the engine under load / being driven as oppossed to " trans in neutral / park".

I do know that a vac secondary carb will not open the secondarys when engine is not under a load. Try putting the guage in the car and monitor it while your wife is DRIVING it at WOT. That would be pretty interesting in its self!!!!:)

I would focus on why there is a vac restriction at WOT. PV aint got nothing to do with that for sure.

What kind of exhaust system are you running? Just curious...................

Test was done is 2-3rd gear between 5500-6000rpms, had vaccum guage taped to window.

Check out this thread:

http://www.chevelles.com/forums/showthread.php?t=23880&highlight=carb+sizing

Let me answer Craig's question first as best I can. While I can't answer exactly at what RPM the carb transitions from the idle to primary side, its a function of throttle blade angle and velocity through the carb. So, if you theoretically had an engine only pulling 6" of vacuum at idle, but the throttle plates had been drilled to allow air, or the secondaries are opened a bit, ( or you have 3 cabs on top to idle through), even though the PV is now OPEN, the airflow through the venturies is not great enough to pull any fuel from the boosters.

Many times with big bad ass cams it is great enough to pull through since the engine may need to idle at 1000+ rpms. Here the SIMPLEST solution is to drop PV size to below idle vacuum. There are other ways to handle it, but that's the easiest way.

Vince, oh yeah, if you're seeing that much vacuum at WOT, then the carb is much too small. Had the exact same situation with my 427 and 750 at 7000+ rpms. An 850 cured it, but did pose a few tuning issues for street ease. They way I handled that is by using the higher rated PV trick in front and playing with jetting/pump shot untill the off idle was as smooth as could be with an 850 carb atop a small 4.33 ci low tq/high hp "l88" type motor running shallow gear.

BTW, you should know that no amount of tuning would make that 850- dp economical. its just ate gas no matter what, I'd lean it out and then it start coughing lean which pump shot wouldn't cover etc. So, my conclusion is that 850 DP are gas hogs no matter what you try, esp on a small street engine sicne the "holes" in the carb are big enough to require much fuel to cover them in a small displacement application. Tune it to be easy to drive, you're using fuel to cover the "largeness" of the beast. 2500 rpm sucks whn you need that largeness at 7200 rpms. ( the solution was tripower for me).

Hopefully your 800 is a bit kinder. But I would certainly encourage you to bump the primary PV to a 095 or 105 depending on what the vacuum gauge reads whille you're driving, suign the uphill grade example above. ( Example, a real gas drain that doesnt help driviability is in a clutch car , if the PV is rated above where the vaccum drops when the clutch is let out in a normal start, sucks down the gas 1-2 mpg measured - which is why I have to run a 085 tops in my app now with a slightly hairy cam).

While you have the front of the carb off to replace the PV to a 105, drop jet sizes by 2. Now, that's just a guess. Sometimes an increase in jet size nets better driveability and economy since the A/F ratio allows less throttle opening. But you may find that leaner primary jetting with a higher PV and a tweak of the pump shot gives you a BOTH a smooth transition to the secondaries and decent high vacuum cruise economy.

You have the jets, I encourage you to do a bit of fiddling with it. IMHO, you make your own rules when tuning a Holley atop a specific application. You have to be willing to play around a bit for the best result. Remeber that primary jets, accelerator pump shot ( both volume and timing ), and power enrichment are all part of a simultaneous equation, tweak one and another may need attention until they all work together as one blending into the secondary circuits.

GO FOR IT! :thumbsup:

Oh yeah, the real good news. If everyhting is working well, adn you're pulling that kind of vacuum, your heads, cam and intake are WAY doing their job!! Must be a well designed powerplant, good VE to suck that nicely! GIVE IT SOME AIR! The flow capacity of a 3" paper air cleaner is just marginal at this level btw.

Let me answer Craig's question first as best I can. While I can't answer exactly at what RPM the carb transitions from the idle to primary side, its a function of throttle blade angle and velocity through the carb. So, if you theoretically had an engine only pulling 6" of vacuum at idle, but the throttle plates had been drilled to allow air, or the secondaries are opened a bit, ( or you have 3 cabs on top to idle through), even though the PV is now OPEN, the airflow through the venturies is not great enough to pull any fuel from the boosters.

Many times with big bad ass cams it is great enough to pull through since the engine may need to idle at 1000+ rpms. Here the SIMPLEST solution is to drop PV size to below idle vacuum. There are other ways to handle it, but that's the easiest way.

Vince, oh yeah, if you're seeing that much vacuum at WOT, then the carb is much too small. Had the exact same situation with my 427 and 750 at 7000+ rpms. An 850 cured it, but did pose a few tuning issues for street ease. They way I handled that is by using the higher rated PV trick in front and playing with jetting/pump shot untill the off idle was as smooth as could be with an 850 carb atop a small 4.33 ci low tq/high hp "l88" type motor running shallow gear.

BTW, you should know that no amount of tuning would make that 850- dp economical. its just ate gas no matter what, I'd lean it out and then it start coughing lean which pump shot wouldn't cover etc. So, my conclusion is that 850 DP are gas hogs no matter what you try, esp on a small street engine sicne the "holes" in the carb are big enough to require much fuel to cover them in a small displacement application. Tune it to be easy to drive, you're using fuel to cover the "largeness" of the beast. 2500 rpm sucks whn you need that largeness at 7200 rpms. ( the solution was tripower for me).

Hopefully your 800 is a bit kinder. But I would certainly encourage you to bump the primary PV to a 095 or 105 depending on what the vacuum gauge reads whille you're driving, suign the uphill grade example above. ( Example, a real gas drain that doesnt help driviability is in a clutch car , if the PV is rated above where the vaccum drops when the clutch is let out in a normal start, sucks down the gas 1-2 mpg measured - which is why I have to run a 085 tops in my app now with a slightly hairy cam).

While you have the front of the carb off to replace the PV to a 105, drop jet sizes by 2. Now, that's just a guess. Sometimes an increase in jet size nets better driveability and economy since the A/F ratio allows less throttle opening. But you may find that leaner primary jetting with a higher PV and a tweak of the pump shot gives you a BOTH a smooth transition to the secondaries and decent high vacuum cruise economy.

You have the jets, I encourage you to do a bit of fiddling with it. IMHO, you make your own rules when tuning a Holley atop a specific application. You have to be willing to play around a bit for the best result. Remeber that primary jets, accelerator pump shot ( both volume and timing ), and power enrichment are all part of a simultaneous equation, tweak one and another may need attention until they all work together as one blending into the secondary circuits.

GO FOR IT! :thumbsup:

Oh yeah, the real good news. If everyhting is working well, adn you're pulling that kind of vacuum, your heads, cam and intake are WAY doing their job!! Must be a well designed powerplant, good VE to suck that nicely! GIVE IT SOME AIR! The flow capacity of a 3" paper air cleaner is just marginal at this level btw.

Thanks for the insight.

I'm a little afraid that the 850 might be a little big, thats why I'm trying this 800. I'm considering the Demon 850 annular to help w/response. GM does recommend an 850 for the ZZ454, which is my motor w/aluminum heads.

I was looking at the Holley website, and I noticed that the 650,700,750, and 800 all use the same size throttle bore diameter/plates (1-11/16). Its when you get to the 850 that the bore size increases to 1-3/4.

Lets see what happens.
Vince