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 ..about PVCR size and Powervalves... 
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Joined: Mon Dec 15, 2014 5:41 am
Posts: 57
When calculating PVCR jet size..
Is this correct??

Jet square for best MPH (1/4mile). Go down in main jet size for cruise A/F ~ 14.0:1
Calculate: Best MPH jet area - Best cruise jet area = Area of PVCR jet ??

Power valve:
Vacuum at given mph/rpm, lets say at 55mph/3200rpm vacuum is 14hg. -Ex. a 6.5Hg power valve or is that to low or to high (and why)?

Thanks :thumbl:
/Tom


Sun Apr 23, 2017 4:07 pm
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 375
Location: Phila. Penn.
PVCR size
Basically yes. My sense is it depends a bit on the carb whether it ought to be jetted square.
Also, a variable in Holley main jet flow is the chamfering. So the area you calculate will be close but assume it will need to be tested over at least once.
Personally I've done the procedure opposite way, same concept. Seemed easier with a street vehicle. I use the powervalve for the quartermile runs or dyno. Then see if they work or can be made leaner on the highway. I was rather surprised when I found the leanest best dyno run jetting was too lean to cruise over 60 mph. 8) :O The engine surges, revving and then dying, that's too lean. Drove 55 mph the rest of the way home since I didn't want to change jets on the side of the highway. So for that combo, the Holley PVCR was pretty much on target the way it was made.

What the cruise AFR number will be is hard to know.

[url]Power Valve Opening Point[/url]
There seem to be a couple of ways to ballpark where the power valve should open. One is to make part throttle runs by "crowding" (holding) a vacuum number. First 14", then 12", etc until the PV kicks in. When the times (or acceleration) drops, try a PV that opens sooner. In other words, acceleration should increase with more throttle. At minimum use a PV high enough so there's no flat or dead spot as you go deeper with part throttle.

It's probably legit to guess that an engine that's cruising at 14" may need a lower PV opening than one that cruises at 16"Hg, but others with more experience can say better than I.

According to Shrinker, the real technical way to figure it out is with a dyno and a 4 or 5 gas exhaust analyzer. But few of us have access to such tools!


Sun Apr 23, 2017 7:08 pm
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Joined: Fri May 20, 2016 5:18 am
Posts: 117
Hello Tom. So you got out of your IGLO as well.
Her is a post from Tuner last week
Quote:
I would drop the MJ in large steps, 3-4 sizes at first, until you find the lean limit for the part-throttle range with vacuum above the PV opening point, then go back in small steps to the leanest jet for good part-throttle performance, and then address whether the PVCR needs to be adjusted as a result of the jet change to maintain correct WOT A/F.

Quote:
Power valve:
Vacuum at given mph/rpm, lets say at 55mph/3200rpm vacuum is 14hg. -Ex. a 6.5Hg power valve or is that to low or to high (and why)?

You have to ask the engine by try and error, your combo like you suggested your self might be 8,5 to 6.5 range or perhaps something ells, you just have to ask the engine
Take care Tom


Mon Apr 24, 2017 5:12 am
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Joined: Mon Dec 15, 2014 5:41 am
Posts: 57
Thank You for the posts. I wanted to make shore Im on the right track in my thinking before I continue with the carburetor.

Tobbe* -Yes, its been a long winter and its fun to get to work on the car again and get it running, also racing season is soon here :thumbl:
Take care!

/Tom


Mon Apr 24, 2017 8:33 am
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Joined: Sat Jan 16, 2016 10:06 pm
Posts: 38
I used to assume that jet area should be equal when moving fuel from the main jet to the power valve or from the intermediate jet to the main jet, etc.

I recently did some dyno testing and proved to myself that I have been wrong all these years. I found out that jet area is not equal to jet area. That is, the carb "pulls harder" on some circuits than on other circuits.

So now I've decided that everything needs to be tested independently. You can start by assuming that jet area = jet area but don't be surprised to find out that it doesn't.


Wed Apr 26, 2017 5:23 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
Andy is right, you need to treat each situation individually. Obviously, in comparison to the Main Jet, the flow path though the Power Valve is somewhat convoluted and the L/D (Length/Diameter Ratio) of the Power Valve Channel Restriction is not the same as the MJ. The PVCR orifice is longer than the MJ orifice.

Because engines pump air in discrete pulses, intake pressure (vacuum) and carb metering signal also pulses. The air and fuel flow through carburetors is not constant velocity, even if the engine RPM and throttle positions are constant. Further, no two engines, particularly of the hot rod variety, are the same. A large production run of "same" engines are very similar, but we have all seen outliers, some run better than others.

This situation of different PV system flow paths, combined with different engines carb metering pulsation behavior, results in a situation where if you wanted to factor area ratios to calculate PVCR size vs. MJ change, each different engine situation would require a different Coefficient of Flow factor be used in the calculation. The Coefficient of Flow for each different case would need to be determined by empirical means, testing, trial and error, taking notes, crunching numbers, too much like doing homework, may as well just test and see what works ..... oh .. well ...

Cliff's notes on the above: The only way to know MJ vs. PVCR for sure is try it and see. You can calculate area ratios, but add a few % to the PVCR to compensate for the convoluted flow path through the valve and the longer L in the PVCR L/D. Different engine and carb combinations will require a different % fudge factor, the only way to know what is correct is try it and see.


Thu Apr 27, 2017 1:46 pm
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Joined: Wed Dec 18, 2013 2:23 pm
Posts: 118
andyf wrote:
I used to assume that jet area should be equal when moving fuel from the main jet to the power valve or from the intermediate jet to the main jet, etc.

I recently did some dyno testing and proved to myself that I have been wrong all these years. I found out that jet area is not equal to jet area. That is, the carb "pulls harder" on some circuits than on other circuits.

So now I've decided that everything needs to be tested independently. You can start by assuming that jet area = jet area but don't be surprised to find out that it doesn't.

Anything more specific re your findings that you can add?


Thu Apr 27, 2017 9:26 pm
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Joined: Mon Dec 15, 2014 5:41 am
Posts: 57
I thought Id do an update..

I put in a 8.5hg powervalve (15-16Hg vacuum@3000rpm), #81 MJ and a 1mm diam. PVCR (about .033). Worked out nice, highway speed about 55mph@3000rpm A/F 14.1, god enough for me for a base setup. Problem was at WOT the A/F went rich, into 10.5. This had to do with a mistake I made. I had a problem with the carb going really lean at tip-in throttle. So I changed out the Transferslot jet, from .047 to .051. That did not cure the problem and made the carb run rich at WOT. I should have changed the IFR... well Im learning as Im doing...

Changed back to the .047 Transfer slot jet and just opened up the mixture screws, changed to a #61 Idle bleed.. and the tip-in issue almost went away, but then the carb runs a little rich at idle (A/F 13.2 with 7-8Hg vacuum). Good enough for me to make the drive for a photo shoot for a Swedish car magazine (Gasoline)... Gonna go up in IFR from .037 to .038 and more if need be and probably a Idle air bleed change as well...

Im so happy that Im finally starting to better understand this carb tuning stuff.... Its nice to get a positive result once in a while instead of banging my head against the garage wall with frustration...

Carb setup.

MJ #81 with .033 PVCR, 8.5hg PV
SJ #87
IFR .037 going .038 -->
TS .047

IAB #61
MAB #25

Thanks!
/Tom


Mon May 15, 2017 4:26 pm
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Joined: Wed Apr 03, 2013 8:06 pm
Posts: 169
What carb is this?

Some thoughts, I can't see that TSR change making it go that rich at WOT. I would think with an idle vacuum of 7-8", I would want a PV in the 6.5 range, although obviously the 8.5 will be closed at your 3000 rpm reading of 14". The .033 seems a very small pvcr for a carb with 81-87 jets. Just comparing these things to what would seem "normal" or "average" I guess. Are the main circuits active at your cruise rpm?


Wed May 17, 2017 10:02 am
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