I have a Holley Street Avenger 670 which comes stock with a 6.6 PV. I've been doing some research on how to properly size the PV and am finding way different theories out there. (I have a standard tranny by the way) Here are two that I found:

1st - Holley Tech says determine vaccum at idle and then divide by 2. Example: If vaccum at idle is 10inHg then you would need a 5.5.

2nd - The Mortec Carb Site says to determine vaccum at idle and then subtract 1 to 2 from that and that's your power valve size. Example if vacuum is 10inHg then you would need a 7.5 or 8.5.

These two techniques arrive at very different PV #s for the same scenario so I'm thinking one is probably more correct than the other.

Which technique do ya'll think is more accurate/correct in finding the proper PV size??

Also, anyone know why that carb equation (it's on the Mortec site and I've seen in many other places also) to determine proper CFM seems to come up with way smaller carb than I see most people run. For example, it says that a 350 running up to 7,000rpm only needs like a 650 cfm carb.....This just seems ludicrous as most of these guys I've seen are running at least a 750cfm if not an 800cfm carb. They can't all be wrong!

They were both right, just each gave a basic rule for different uses...

For performance the bottom line is you need a PV rated minimum of 2 below your in gear idle with an automatic and 2 below the idle with a stick. This will insure the PV doesn't open during idle and transition.

Now after that it's part of your tuning even though the holley rules of thumb don't say so. The power valve is part of the power circuit and opens based on engine vacuum. As you go WOT the engine vacuum signal drops and when it gets below the PV rating the PV enriches the main jets feed. You can delay or advance it by using a lower or higher rated PV.

Typically for street use it's best to just half your idle vaccume (in gear idle for auto trans) and call it good. there are a lot of other adjustments to give you good performance out of your carb...

PV is a fuel enrichment device to the main circuit. Most daily driver street car main jets are set up for lean economy at cruise. There are guys here that size them by neither of the 2 methods you mentioned, and I agree with them. They size it based on vac at cruise, on the theory that you want the PV closed at cruise because you are after lean economy, but when you put your foot in it you want some enrichment. The PV opening is equivalent to 4 jet sizes.
To me the 1/2 or -1 is just a rule of thumb. You want the PV tailored to your particular vac at the point you think you want extra fuel and you don't want it clobbering things at other points in the game. If I was sizing a PV for a street car I'd be eyeing a vac gauge as I was passing cars, going hmmmm yup about the time I see 7 or 8" vac is when I hammer it. But it's a moot point for my carb, I don't have squat for vacuum, and it either idles or runs WOT. Or gets took apart and played with:)


Carb sizing is engine specific as well. A hot 350 could very well be efficient enough to need a 750. But most 350's do great on a 650. Junk heads, average hyd FT cam, small headers, and a 750 is a waste. Impossible for that much air to go through it. Put a 650 on it and get a better signal across the boosters and the manners improve.
My ZZ4 with a 650 4777-7 ran hi 11's.

There are guys here that size them by neither of the 2 methods you mentioned, and I agree with them. They size it based on vac at cruise, on the theory that you want the PV closed at cruise because you are after lean economy, but when you put your foot in it you want some enrichment. .....If I was sizing a PV for a street car I'd be eyeing a vac gauge as I was passing cars, going hmmmm yup about the time I see 7 or 8" vac is when I hammer it.

I'm one of "them guys."

I absolutely believe that niether the main jets or the power valve could have ANY affect on idle. The main circuits are not flowing any fuel, how can they affect anything?

If you don't agree, go pull the primary main jets completely out, button her back up and start her. Betcha a C note she idles just fine. In fact, if you (slowly) open the throttle, she may go to 3000 rpm free reving on the transfer slots, with no effect at all from having a giant pair of holes where the main jets should be.

I set my primary mains for the best lean cruise I can get, and then set my PVCR size and power valve opening point to get the enrichment when I need it. I have a wideband and a vacuum gauge in my dash to help with that.

Years ago, during the "bad gas days" I ran a very high compression 400 SBC, and spent all summer tuning my holley double pumper. She cruised at about 18" of vacuum @ 3200 rpm @ 60. If I fed her throttle to pass (but not enough to kick her down to second) she would ping. Going to a 12.5" PV killed the ping. Idle vac in gear was 8".

Keep you pv opening point 2 inches below idle vacuum and part throttle cruise in high gear.you do not need the added enrichment at these points.If the two do not align..then select from the idle vacuum first.I would install a vacuum gauge and record all #,s while idling and driving thru ranges and different engine loads.With all the data,you can sit down and make a relatively close estimate as to what would work best with your combo.There is plenty of avenues to trim the carb.Main jets,high/low air bleeds,pvrc etc.The holley/mortec specs can not factor in your unique combo..just like the recommended cfm calculators.

Holley Tech is correct!.....The stock power valve in a Holley carburetor is rated @ 6.5"Hg and is good for engines producing 12" Hg or more of vacuum at idle.....Anything less than 12" Hg at idle will require a different power valve and other minor adjustments such as opening the secondary throttle valves slightly.....As an example, if your motor, equipped with a manual tranny, is pulling 7" Hg of vacuum @ idle then divide that figure by 2 and install a 3.5" Hg power valve.....You may or may not have to readjust the air-fuel mixture.

If you don't agree, go pull the primary main jets completely out, button her back up and start her. Betcha a C note she idles just fine. In fact, if you (slowly) open the throttle, she may go to 3000 rpm free reving on the transfer slots, with no effect at all from having a giant pair of holes where the main jets should be.



I agree.

That's a great idea Jim. Why dintya post it to my "free 305" thread? I've already basically done that, playing around. Look thru the videos in my sig. You'll find my ZZ4 with a 1050 Dom. Also my ZZ-4cylinder with the same 1050 Dom. I was trying to prove you can't overcarburate but you can overjet, and over-something else but I don't have the word. De-signalize? hehe

Pullin the jets out of one is right up my alley:D

Since you guys are big on expermenting has someone got a vacuum gauge in the car and seen their vacuum at cruise or part throttle go below their idle vacuum? I know vacuum responds a bit different under load but other than an initial drop when you free rev it all I've seen is the vacuum increase when bringing the rpms up.

Jim I also agree the jets don't play directly in the idle circuit but the fuel that gets drawn through them feeds teh idle circuit. I replaced my float and needle and seat after a jet change because it loaded up and I assumed I damaged one of the 2 items. After resolving the loading up I also was able to richen my idle mixture by a little more than 1/8 turn and gained 2" of vacuum and lost the rich smell out the back in doing so.

I'd like to think the jetting didn't effect my idle circuit (that's what I've always believed) but unless the float level which was right before as it is after or the needle and seat change caused the difference.

I think what needs to be remembered is that the idle circuit pulls it's feed from the main well just like the booster. A jet change doesn't force more fuel to the idle circuit (or booster) but makes more fuel available to it. I think that allows a weaker vacuum signal on the idle circuit to pull in more fuel than it would with a smaller jet. Wolf - reference back to my carb thread and what we talked about jetting carb size and cubic inches... Both the idle and power circuits pull from the jet, the jet doesn't push the fuel, that's why you can remove teh jets and still idle.

Dennis, what you ran into is the single biggest factor in a carb. The level of fuel in the bowl effects EVERYTHING coming out. The float level being off, or a leaky needle and seat, effects the "head" of the system, and will dramatically change the mixture. You for sure described the effect of a leaky needle and seat.

As for the old analogy, flow thru a pipe ius governed only by the smallest part.
In this case that would be the idle jet.

I know Jim but after replacing the needle and seat and adjusting the float to where it was resolved the problems.

My comparison is 65 jets changed to 70's and I was able to richen the mixture screws and not have the rich idle smell I had with the 65's and the mixture leaned out more... I also lost about a 100rpm surge at idle...

Wolf - reference back to my carb thread and what we talked about jetting carb size and cubic inches... Both the idle and power circuits pull from the jet, the jet doesn't push the fuel, that's why you can remove teh jets and still idle.

;)Deuce, er I mean Dennis;) I know, and agree. That's why I want to do the silly speriment.:D That kinda stuff makes me happy:)

And Jim's pipe restriction theorem is true.

As far as your thread goes Dennis, I was sincerely trying to contribute to it. Dirt was reading me wrong. I'm sorry you had to shut it down.

It's OK, I got some pictures I'm gonna make of his car (as soon as I buy a Hot Wheels) sitting at a greater than 47° angle Pre-Launch on top of a carb:D. That's how much angle it takes to put the fuel level to the TOP of the secondary jets.

[quote=DjD;869963]Since you guys are big on expermenting has someone got a vacuum gauge in the car and seen their vacuum at cruise or part throttle go below their idle vacuum?

Never with my combo.Part throttle and cruise double my 6 hg at 900 rpm idle speed.I have 4.5 pv,s with 74/74 jets in the hardcore 870 dp.
I am with you on introducing more fuel into the main wells.Would that not tax the low speed air bleeds and fatten up the emulsified a/f mix if the idle mixture screws are not trimmed to compensate?

If there's none flowing other than what the idle jets flow how will it fatten it?

hmmmm, when yer doin my 'speriment, Tim, why not try some playdough in the main airbleeds too?

Heck, maybe tomorrow I'll pop the rods outa the ede and fire her up? Lots easier than wrestling the jets out of a holley!

I don't have an online picture to post but I got my holley book out and started studying the diagrams. To simplify everything first realize that except for the accl pump all fuel is moved through a carb by a vacuum signal once the engine is running.

So from there think of a drinking straw as a jet. Now get one of those fat ones at McDonalds that let you drain half the cup in one suck of the straw and compare it to someplace else with a narrow straw. With the same amount of suction (vacuum) you get more volume from the fat dia straw than you do the narrow dia straw.

Jets don't open and close they are just a straw that fuel is sucked through.

For the vacuum gauge, every engine I see at work has a manifold pressure gauge. Load increases, manifold pressure increases(=vac drop in an NA engine). Right down to 2"HG or up to your wastegate setting whichever you have.

On automotive, the only ones I've had vac gauges in are trucks. Be a waste of a gauge on the wolf. Extra weight:D.

On a light duty half ton unloaded Chevy PU w/ stock 350 I had 14" at idle, 10" in gear, and 8 " at cruise. It was a Edelbrock carb with stock springs. 373 gears and used all the gas you could buy, and ran like a bat outta hell for a stock engine. But I'm seeing your point Dennis and I hadn't thought of it along the lines of tune, really. Because all my junk is 100% loaded and we tune at 100%. But I've always said the only reason junky built car engines last any time at all is they have no load on them at all. AN NA engine at 5"HG is at 75 to 80% load. You'd be hard pressed to find anything outside a truck that sees that more than a few minutes a day. Yup. Goes right back to tuning for cruise economy and enrichment from there.

Oh, and we consider an NA engine at 10" HG to be at 25% load or less.

Interesting numbers, Tim... kinda wierd tho. vac went down at rpm?

My "tractor 383" pulls 15" at idle in neutral, 18 at cruise. Virtually any movement of the throttle will cut that 18 in half. 5" accelerating liesurely thru the gears is not uncommon, and she's see less than that passing from 60 in fifth at say half throttle (that's pulling from 60 to 70, 1800 to 2100 rpm, against a 2:1 final drive.)

The vacuum gauge and wideband on the dash are kinda fun. I can clearly watch the needles pull at 8" for example, mixture goes from 13.5 to 12 right at 8".

And no, I haven't tuned this carb with the gauges, did with the ole 600 ede, but this 800 is a stopgap for this season only and I didn't take the time, just tranfered the jets rods and springs from the old carb to this one.

hmmmm, when yer doin my 'speriment, Tim, why not try some playdough in the main airbleeds too?

Heck, maybe tomorrow I'll pop the rods outa the ede and fire her up? Lots easier than wrestling the jets out of a holley!


I'll do any speriment anybody wants... Long as we don't hurt my Vortec heads and roller cam setup I just got for free:). I have to come up with another Vortec intake first. The 305 doesn't get the one off my ZZ.

Interesting numbers, Tim... kinda wierd tho. vac went down at rpm?



Vac goes down (pressure goes up) with load. Of course rpm is a factor, but a minor one.
Watch this....

I start a V-16, let it idle at 6 or 700 rpm until things are happy. Bring up RPM to max 1400. At idle and low load MP was probly 7 or 8" HG (we have huge latent loads sometimes ... think of it as idling in gear with a stock stall vs high stall converter). At full rpm but still unloaded my MP is probly about + 1 or 2 positive. I load it. MP goes up to + 22 to 23 # positive. RPM has never altered except when I took it off idle. Vac dropped(= pressure increased) as load increased.:)

It's just showing you that car engines are idling most of the time. Idling at high or low rpm, still idling. Sure Jim, your MP goes to 5" when you gig it. That's why we have hot rods. Granny don't do dat.:). Not too often. Go Granny Go:D