How do I know what jet and power valve to use on my car?
Ok. So we have idle squared away and the car moves at low speed just fine.
Now, how to select jet and Power Valve (pv)?
Obviously the first step is to figure out what the original carb came with.
The above is simply a range, rarely ever does the requirement fall out of this range. Can it? Yes. But normally only with a very specific engine or usage.
In the case of a List 1850 or 2804/2805 600cfm carbs, the stock jet is a 66 and the PV is a 6.5
Can we just throw these in there and hope for the best?
Of course, and 95% of the time it will work out just fine.
But how to dial it in and get it closer to perfect?
Lets review first.
Once off the idle circuit, all fuel needs are met by the Main Jets and the Power valve.
Immediately off idle, as the idle circuit looses power or it’s contribution of fuel to the increasing airflow becomes minimal, we need a new fuel source. So the boosters begin to really flow. All of this fuel is coming from the main jets.
The boosters are almost always flowing vapor due to air flowing through the main well, but it is negligible. So in an ideal world, as the idle circuit is becoming lessened, the fuel vapor from the boosters is increasing. This should provide a seamless transition.
Right, back on topic, as the main circuit activates and the boosters begin to flow we in the car are at a steady cruise. All available fuel is limited by the jetting. Air is admitted into the mainwell via the HIgh Speed Air Bleed, this air serves many purposes.
It adds air to the fuel which lightens it and makes it easier to flow. The lightening also makes it more receptive and responsive to changes.
Due to the varied draw of the engine that the boosters see, this helps shape the fuel curve.
Now a typical fuel curve shows cruise and light throttle applications as desiring a lean a/f ratio, whereas the full power a/f ratio needs to be considerably richer.
How to achieve this?
The jet size is sized for this lean cruise.
Auxiliary systems like vacuum advance can also help the burn rate due to this lean mixture.
Where does the extra fuel come from for wide open throttle?
The Power Valve!
The power valve is simple a device which confounds internet carburetor experts by being the simplest and most misunderstood piece of equipment.
Seriously. The PV is an on/off switch, nothing more.
The power valve sees engine vacuum through a tiny hole in the baseplate. As an on/off switch it is held SHUT by this engine vacuum.
It opens only when engine vacuum drops sufficiently to allow it’s opening.
There is a spring that helps determine this opening rate.
The stamped number of 3.5, 4.5, 5.5, 6.6, 8.5, 10.5 etc is the vacuum needed to still hold the power valve CLOSED.
So we have a 6.5 PV. While cruising, we open the throttle further. As we continue to open it vacuum drops. At some point around 6.5in of vacuum the PV opens.
Now, it’s always nice to check the actual opening rate as with most any modern thing, accuracy is apparently not that important.
Sadly PV are again, just an on/off switch. It would be really handy if they progressively opened over a 5in of vacuum range, but that isn’t how it works. In a Holley style carburetor the fuel curve needs to be tailored with bleed air, etc.
So imagine if you will the scenario above. If we had a 3.5 PV installed, we would have to open the throttle considerably further in order for the PV to open.
Many people seem to view this backwards. They feel with a rowdier engine, they simply must put a 3.5pv in because their engine is racier.
This isn’t always the case, it simply depends on the vacuum where the engine needs this extra fuel.
Camshaft design and engine load have a far greater effect than anything.
The load differences between a 4200lb car with 3:1 rear gears vs a 2800lb car with 4.30:1 gears is great.
These are the sort if things you need to think about.
So the PV opens, how does fuel get into the engine?
Beneath the PV are two small holes. These are called “Power Valve Channel Restrictions” or PVCR. These admit fuel directly to the mainwell. These are separate from the main jets and work in conjunction with them.
PVCR+MainJet=total fuel admitted.
Again. The PVCR ONLY feed fuel into the mainwell. So idle vacuum, and whether the PV is open or closed at idle is quite irrelevant. Same reason you can idle just fine even if you took the jets out…. It simply does not matter until the main circuit gets flowing.
Along these lines, changing jets to change idle fuel is worthless. It just doesn’t have an impact.
Now I know someone will say “but I changed my jets and the idle got lean.”
Possible that this occurred. But more likely you had a gasket leak or source of unmetered fuel that you unwittingly fixed by removing the metering block and reinstalling.
So we have the idea behind us of function, the how and why, etc.
How do we determine perfect jetting, PV, pvcr?
Jetting is for cruise fuel first and foremost.
So go for a ride.
Lets say we have an 1850 with 66 jets.
Ride goes fine.
Drop jets to 63 and go for a ride. Hrm. Engine has a mild lean spot at cruise and with light throttle changes.
This is often noticed by a light surging or bucking. Sometimes this is difficult to notice, or only occurs at a certain throttle opening or car speed.
So we need to jet up until all of this is gone and the engine performs best.
Yes. Air/fuel meters, checking plugs, etc are all fantastic guides to help determine this, but lets assume you have nothing but a car and a well calibrated ear.
This is the simplest way to know what your “lean limit” in jetting is.
Altitude, air temps, etc do indeed play into this, so take those into account.
Proper distribution also matters.
If two cylinders are lean, the engine will respond lean. We need to richen for all to allow proper running sadly. In an ideal world, we would port the intake and heads perfectly so distribution is perfect to each cylinder…. But almost none of us have the tools available to do so. As such we are pretty much just working with a “best case scenario” for this.
Alright!!! We found in our example that a 65 jet produced the best results. Cool, we are done right?
No. We still need to determine where and how much fuel enrichment we need at heavier loads. So lets go for a ride again. This time we need a vacuum gauge.
We can watch the gauge as throttle increases and visually see what the vacuum is.
To determine where we need the PV open, we either need to have a wideband Air Fuel gauge or we need to install a PV plug.
With the a/f meter we can visually see where the PV opens and change PVs until we find the right one. In this case the right one is a PV that covers any lean spots by being open, yet is not open so early as to be richening the engine when it is not needed.
With a plug in place, we can load the engine until you again get a surge or lean spot. You can also do this with a plug and the a/f meter.
Please do not do this at the track or under heavy load as you can damage the engine.
We are just doing an experiment to see and feel at what vacuum the engine needs more fuel.
As always, safety first. It is always safer to have the fuel a little too early, or to be slightly too rich. Literally no damage comes from an engine that is a few a/f points too rich, but damage may occur if too lean. As they say tho, “lean is mean” and this is true to a point. They should really say “the a/f ratio required for a particular load makes power.” But I guess that isn’t as much fun to say.
I will repeat this, “Opening PV early wastes a tiny bit of fuel and power, opening a PV late creates a lean miss and in extreme circumstances, engine damage. So always plan to open it earlier than needed.”
Sizing Power Valve Channel Restrictions:
This is one I kinda hate to discuss, but I need to, even if only for academic purposes.
Jetting+pvcr=total fuel at fuel load.
This really needs to be done on a dyno, or very carefully. Leaning an engine out at wide open throttle is very bad for an engine.
At the track, you aren’t really concerned about cruise, so simply jetting to best speed makes sense.
In the case of a drag strip only car, it’s totally likely that there is no need for a PV.
On a street car, you spend so little time at WOT, that being too rich or slightly too lean for 2 seconds really shouldn’t matter. So Pvcr adjustments aren’t ideal.
A street car should always have a PV for obvious reasons. Full power fuel is wasted at cruise. This fouls plugs, lube oil, and washes the cylinders down excessively.
As such, I think altering these to aim for perfection is awesome…. But I would not advise it unless you have a particular situation where it is needed.
However, finding your proper jetting and PV opening is just good tuning and can certainly help dial in an engine.
Of note. As always. You need to do this at your maximum operating temperature. Timing advance must be smooth and appropriate for load and speed.
I will also mention, if your ignition system is in any way compromised or not 100% none of the above maters whatsoever. Always assure it’s proper function before touching a carb.