850 carb. what size power valve to use and why?

[2ndgear]

I have a holley 850 carb. and wondering what size power valve I should use. The engine ideals at 950-1000 rpm and have 8 inches of vacuum. Some people said it should be 1/2 of the ideal vacuum which would be around# 4. What happens if I would use a # 6.5 instead. Would there be a bog at wide open throttle or what problem would a 6.5 pv cause instead of a 4 pv? Just trying to see what would work the best. 294 cam,445 stroker,rpm performer manifold,alum. heads, msd. 18 degrees at ideal and 36 total, 4-speed,389 gear,28 inch tire, blue cam front and brown cam back. 82 jets front and 78 jets back,both 31 squirters. Thanks in advance

[machoneman]

The 4781-5, if this is your number, came with 80 primary and 78 secondary jets. I'd start there or post up the actual part number on the carb.

Just like jetting, the correct PV can often only be chosen through some experimentation as to what your combo likes best. With good gearing, your cubes, a stick tranny etc. it's doubtful that your 6.5 would make the car fall on it's face at takeoff. But, make that a 390, auto, etc. and yes it would!

The 1/2 of your idle vacuum ( a 4) is a good place to start. Once that and your jetting is dialed in, don't hesitate to swap out a 4 for say a 5.5 or  a 6.5 for testing.

[chilly460]

As a trend, the 6.5 may make it a bit rich in transition as it'll start to open almost immediately with your relatively low vacuum.  4.5 should be closer, but as stated it's a bit of experimentation.  You have cubes and gears on your side so it should be a bit less picky in transition and can handle more fuel most likely.  There shouldn't be much difference at WOT, the PV is really just there to help in part throttle from running off the idle circuit to the full enrichment, at WOT they'll both be fully open. 

[My427stang]

I would offer that 1/2 idle vacuum is generally too low of a number and therefore too late.

When a PV number is too high, fuel comes in earlier and can be rich.  When it comes in late, the engine will experience a lean condition under load.  Rich is safer by far.

Remember, a PV cannot open at idle and provide fuel without airflow over the boosters, and cruise vacuum is not the same as idle vacuum.  I would start with a 6.5, and in some cases "may" delay that to a 5.5, but those cases are rare.

Now I haven't had a -5 in my hands, but if it is 80 primary and 78 secondary, what I would do is run a 6.5 PV and 78s in the primaries, and plug the secondary PV and go with an 86-88 jet out back.  In the secondaries, a PV can become uncovered under acceleration and go lean, something you do NOT want in full 4 barrel boogie.  That is about where I start with most 850s, and your combo should be happy with that

[preaction]

Ross, would there normally be a PV in the back and front ?

[My427stang]

4781s came both ways.  If you see a split in jetting, 80 primary, 88 secondary, then there is no PV out back.  If it is square jetted (same front and back) then it has a PV. (At least every one I have every seen)

Remember, the booster has two fuel sources, main jet and PV channel restriction.  If the primaries and secondary throttles are the same size, at WOT they have to get about the same amount of fuel from somewhere. If the booster has a PV, it gets fuel from the PV operation and the jets at low vacuum and adequate airflow, if it doesn't have a PV, it needs a bigger jet to compensate

I have seen LOTS of HP Holleys with secondary PVs too.  My 1000 HP on my 4x4 had a secondary PV, but I plugged it too

[2ndgear]

The number on the carb. is 4781-3 which came with 82 jets front and 78 jets rear? 30cc front pump and 50cc rear pump. Cam # 427 hole 1-front and cam # 380 hole #1.

[Tobbemek]

 Some people said it should be 1/2 of the ideal vacuum which would be around# 4.

Some people said it should be 1/2 of the ideal vacuum which would be around# 4.

  including Holley ,Summit and all of the popular magazines over the years, sorry to say
but that's not the proper way to shoes att what point HG the PV should start to open.

What happens if I would use a # 6.5 instead. Would there be a bog at wide open throttle or what problem would a 6.5 pv cause instead of a 4 pv

The "size" number of the PV indicates att what intake vacuum it starts to open and provide fuel throe the PV channel restrictions and the size of the PVCR determinate how much fuel. So att WOT it don't  matter 4- 6.5-10.5 they all are open to the PVCR.
 
The best way to find out the "size" opening point in HG your combo need is with a vacuum gauge in side the car when driving and find att what intake HG under load, by trail and error would be the best PV fore your particular combo, 6.5 to start with! yes why not. 
And in PRI side with 80MJ plugg SEC PV and use 86MJ to start with.

[plovett]

I run a Holley 4781.  I'll give you my setup just for reference.  I use 80 jets front and rear.  I run power valves, front and rear.  I use 3.5" power valves front and rear.  I use size 35 squirters front and rear. 

My cam is bigger at 252/260 vs. 248/248.   So I have lower idle vacuum.  I haven't actually checked my vacuum in years, but it seems like it was in the 7-8" range at maybe 900 rpm.  I have since turned my idle speed up.  It now idles around 900-1000 rpm cold and 1200-1300 rpm hot.  It seems to work well with this setup.

My guess is that since your cam is a bit smaller you will want a bit "larger" power valve.  However, the displacement of your engine will affect this, too.  You said you have about 8" of vacuum at your idle speed. 

Based on all this, my guess is that you will like something like 4.5" power valves.  I would consider going square on the main jetting, 80 and 80 just like stock. 

JMO,

paulie

[plovett]

More thoughts:

You're at 445 inches with a 248/248 cam.  I'm at 432 inches with a 252/260 cam.  So it's likely that my 3.5" power valves are not optimum for your combination.  That's assuming they're optimum for my combination.  I would say that 6.5"  power valves are not optimum either.  I think you want somewhere in between.  So,  I bet your combination will like 4.5" or 5.5" power valves.  I'm leaning toward 4.5".   

I really believe in square jetting on a square carb.  Staggered jetting cannot be ideal unless there is something else wrong, in my opinion.  When all four throttles are fully open, all four of the holes should have the same needs, in terms of fuel, unless something wacky is going on.  Or if the intake manifold is doing something strange.

JMO,

paulie

[2ndgear]

Thanks for all the info. First I will square the carb. 80 jets all around. 2nd I will try 6,5 pv then to 5,5 pv and then plug rear, no pv and change them jets in the rear to 86 or 88. Thanks again

[My427stang]

I really believe in square jetting on a square carb.  Staggered jetting cannot be ideal unless there is something else wrong, in my opinion.  When all four throttles are fully open, all four of the holes should have the same needs, in terms of fuel, unless something wacky is going on.  Or if the intake manifold is doing something strange.

JMO,

paulie

Paulie, nothing wrong with your statement, but all four holes can have the same fuel even if you run different jets. It is just sourced from a second channel when you have a PV circuit.  The front needs a PV because there are combinations of throttle movement and load that need the fuel, and different driving conditions that want to be leaner. 

For the most part, those do not exist as much on the secondary side, as you generally don't work the secondaries like you do the primaries.  Aside from possibly road racing, usually you are in the secondaries or out combined, and it doesn't apply here, but a vac sec will generally come in a little softer too which acts a bit PV like for the rears. 

Something to think about, it's at a far less extent, but  keep in mind that head pressure, or the fuel pressure feeding the jets and PVs due to gravity and acceleration, on the secondaries is going to be lower as the fuel is trying to walk away from the jets and PV, but on the front is walking toward the circuit.  This can slightly affect timing of the circuits just like float level can.

Not saying you are wrong, and starting with the factory square jetted setup runs well in 99% of the carbs, but in the vehicle these are tuning tools, not bandaids. 

Question for you on the Cougar, are you jetted richer than factory jets?  3.5 is a very late PV, my hunch is you have a lot of accel pump and a rich main that compensates.  FWIW, my favorite starting point on a 4781 is 80/88 6.5 or 5.5 front, back plugged, dual 30cc pumps with pink cams in #1 and no larger than 31 squirters.  They usually run pretty clean there

[2ndgear]

Hi Ross

Thanks for your respond but would you use the pink cams in front and rear and both 30cc pumps. Now what I have is blue cam front and brown cam rear,30cc pump front and 50cc pump rear. Just trying to understand a good starting point. I know everyones engine likes something a little different.

[plovett]

I really believe in square jetting on a square carb.  Staggered jetting cannot be ideal unless there is something else wrong, in my opinion.  When all four throttles are fully open, all four of the holes should have the same needs, in terms of fuel, unless something wacky is going on.  Or if the intake manifold is doing something strange.

JMO,

paulie

Paulie, nothing wrong with your statement, but all four holes can have the same fuel even if you run different jets. It is just sourced from a second channel when you have a PV circuit.  The front needs a PV because there are combinations of throttle movement and load that need the fuel, and different driving conditions that want to be leaner. 

For the most part, those do not exist as much on the secondary side, as you generally don't work the secondaries like you do the primaries.  Aside from possibly road racing, usually you are in the secondaries or out combined, and it doesn't apply here, but a vac sec will generally come in a little softer too which acts a bit PV like for the rears. 

Something to think about, it's at a far less extent, but  keep in mind that head pressure, or the fuel pressure feeding the jets and PVs due to gravity and acceleration, on the secondaries is going to be lower as the fuel is trying to walk away from the jets and PV, but on the front is walking toward the circuit.  This can slightly affect timing of the circuits just like float level can.

Not saying you are wrong, and starting with the factory square jetted setup runs well in 99% of the carbs, but in the vehicle these are tuning tools, not bandaids. 

Question for you on the Cougar, are you jetted richer than factory jets?  3.5 is a very late PV, my hunch is you have a lot of accel pump and a rich main that compensates.  FWIW, my favorite starting point on a 4781 is 80/88 6.5 or 5.5 front, back plugged, dual 30cc pumps with pink cams in #1 and no larger than 31 squirters.  They usually run pretty clean there

Don't worry Ross.  I don't think my tuning skills are very good, much less perfect.  I was just throwing my combo out there for reference. 

I totally understand the jetting change if you plug the power valve.  I was referring to a carb with both power valves.  That is how mine is.  But, I've never tried plugging the rear power valve.   It may work great. 

I also tend to fixate on WOT, LOL!  Transition states are not something I think about too much unless there is an obvious problem.   

And finally yes, I likely have more accelerator pump shot than I need.   I can't remember what pump cams I have on there.  I will look at it.  But still 30cc pumps front and rear. I could likely juggle things around a bit and get a better combo, but what I have works well, doesn't foul plugs, doesn't seem to detonate, no hesitation anywhere that I can feel.   The fuel economy sucks.......

I am runnning stock jets in my 850, all the way around.  I did run smaller jets on the dyno, but that was with a 4 hole spacer, not the clover leaf I have currrently.  Also a different camshaft now, and a higher idle speed.   And I lowered the power valve number.   I'm up for trying to make it better, but don't want to hijack a thread.

paulie

[My427stang]

Hi Ross

Thanks for your respond but would you use the pink cams in front and rear and both 30cc pumps. Now what I have is blue cam front and brown cam rear,30cc pump front and 50cc pump rear. Just trying to understand a good starting point. I know everyones engine likes something a little different.

So, I look at enrichment using an accelerator pump as a hatchet versus a power valve which is a carving knife.  I try to minimize as much pump shot as I can, however, pump shot is required, so don't let me infer it's bad.  It's just not metered or atomized as fine as something coming through a booster or transition slot.

First, I don't see a reason for a 50cc secondary pump unless you are running alcohol.  I haven't seen one that needed it yet, although I suppose a tunnel ram or very low vacuum heavily ported Victor with a big open spacer could like one, and the area of the primaries are the same as the secondaries so I don't see a reason for it with a square bore carb on gasoline.  If you need 50 cc, up to 66% more fuel depending on cam, using gasoline, seems like there is something wrong.  So I use dual 30cc pumps.  That being said, if I had a good carb I likely wouldn't take it apart just to start.

Second, the blue cam up front is a little different.  I like that cam because it comes on quick and pumps a bit more.  Sort of counter to my original statement, but it often can offset a marginal transition a little, especially in HP series and silver bodied carbs (like the Avengers) that have a funky emulsion well that isn't as responsive.  Hatchet carving a turkey, but it's one of those that seems to work well when the engine wants it.  When I use those, I use the smallest shooter I can, usually a 28-30 on the 4781, because I like it to come in hard and hang for a bit, as opposed to a big shooter that will dump fuel

My recommendation:  If this was a known good carb and ran well the way it's set up, that is where I start.  If it is a new carb, as delivered is where I start.  If it is an unknown carb or needs a rebuild, I generally return to as delivered for every passage and adjustment, however, in that case, if I had a pump arm, cam, and housing, I would swap to a 30 cc pump out back.