power valves in racing carbs

[chevyart]

like to hear your thoughts on using a power valve in the front only of a racing carb, so you dont load up while driving around the pits. like to hear arguements for and against thanks
chevy
art

[johnracer]

OK, here's my thoughts. I really don't think you'll load up enough to matter at the track. If you run it a lot at home however, you'll load it up. I ran power valves in my sbc with a 950hp for just that reason. Since I went bigblock and a Dominator, I hardly ever fire it up at home. Just to unload and warm it for oil changes and valve adjustments, so I don't run pv's.
The only downside I can see is the potential for a backfire damaging a pv or just one failing. They really aren't necessary on a race car if you can resist the urge to annoy your neighbors!!

[john858]

i agree,not needed as for the neighbors just rev it up and cleaner out if it starts to load up :lol:

[johnracer]

i agree,not needed as for the neighbors just rev it up and cleaner out if it starts to load up :lol:
I like the way you think.....

[dcarr511]

On the side of using one ( or both ), you will go through the burnout, staging process cleaner and driving through the pits / return road.

For those that are concerned about damage during a backfire ... how often do your engines backfire ?

I was talked into running them front and rear in a 9375 with the 3rd circuit blocked and I didnt have any issues with it nor have the 2 people who have borrowed it and used on 3 differant engines.

I have a cup made up that fits over the rear PV on the float side and holds enough fuel to prevent it from going lean.

I also know of a N/A Altered that runs approx.1.15 60' and ran power valves front and rear. He might jump in on this thread if he reads it.

[chevyart]

thanks so far for posts. my car is strictly a drag car and will never see the streets. also would like to say I am using a 3.5 power valve, which is supposed to take longer to open, so I can idle better, as opposed to a 6.5 or 8.5, which kick in a little sooner and are usually used on the street.I really dont understand how vacuum and the power valves work, but from what i read i know to use a real low number power valve for a drag car carb. can the last post who was in favor of the power valve explain this whole vacuum-power valve thing to me as it is like calculus to me, and i really dont understand it thanks again fellas
chevy
art

[jmarksdragster]

I have run with and without, really no difference in drivability on an engine with a fair amount of cam. While some get away with it in the rear, why use one? On a drag engine the secondaries are almost always closed or wide open. Just one more thing that can screw up. Primaries I would use on a milder street strip car, but on a race only drag car with a fair amount of converter, I would leave it out. You have no chance of potentially inconsistent metering without. A circle track or road race car, it's a different story.

[sc3314]

Street car leave them in. All out race car pull them out.

Dave

[john858]

i just look at it this way, there is enough things that can go wrong with a racecar so if you can get rid of potenional problem get rid of it,even if it may never happen just one last thing to worry about.good luck

[dcarr511]

Power Valves are held shut by vacuum.

When the Vacuum drops below the rated value of the Power Valve ie: 3.5", 6.5" etc. the Power Valve will open and stay open until the vacuum signal is above the rated value.

Typical Drag Race cams have a ton of overlap causing a low Vacuum Signal to begin with thats why a lower rated PV is used.

During the Burnout and Staging the Power Valve will likely remain shut and the Carb will only be receiving fuel from the Main Jets which are smaller ( than when run w/o a PV ) so it will be running leaner.

The other guys are absolutley correct in saying that running without them is one less that can go wrong. I was just pointing out that they can be used as you asked for both sides.

I will always run them in the Primary Side regardless. The Secondairy's I might try one ... I might not.

A PV will not affect Idle ( unless it is damaged, ruptered by backfire ) regardless of the rated value as your not pulling fuel from the boosters at idle.

[chevyart]

hey dcarr511 think i am a little confused by your last post. you say with a low number power valve (around 3.5) it will not be opened yet doing the burnout. My car is over (or around) 5000rpms doing the burnout. shouldnt the power valve be open long before the 5000 rpms. If it was not then i would probably be lean at that point. the staging process, which is around idle, i can see the power valve still being closed. Im really leaning towards using the power valve in front but i just want to be sure i am thinking this out properly and using the right power valve. moter is 355, ported iron heads, 12.3 to 1 compression, and a crane roller around .625 lift and the duration around 265-270 @.050, and a 5000 stall converter. i use a powerglide with 5.61 gears with 31 inch tires and come through the traps around 7700 rpms. im using proform center on a 750 holley with 74 jets in the front with a 3.5 power valve and 84 jets in the rear without a power valve. does this sound about right for my carb setup. i really appreciare you input on this as i am really concerned about getting the right combo. thanks again Chevy Art

[nova77]

i had pro systems build me a carb for drag racing and he put a power value in it -- works great

[mopar1968]

I run a dominator on a small block and i took them out just fatten up the jets 2 sizes.

[jmarksdragster]

I run a dominator on a small block and i took them out just fatten up the jets 2 sizes.

Unless it was already rich, this is a bad idea. Losing the PV generally requires 6 to 10 more than with with one on a Dominator.

[mopar1968]

:? :? your saying to increase the jets 6 to 10 sizes up without power valves? i have never ran power valves.

[jmarksdragster]

6 to 10 when removing a power valve. A 1050 typically runs 86 to 88 with power valves, 92 to 94 without. 3 to 4 more on an 1150, 6 to 8 more with a 1250. Modified Dominators may be a few off either way

[cepx111]

Ok, I have to disagree with this post at least part of it anyway.

" During the Burnout and Staging the Power Valve will likely remain shut and the Carb will only be receiving fuel from the Main Jets which are smaller ( than when run w/o a PV ) so it will be running leaner "


Durning the burnout at least mine anyway I'm almost always have the throttle at wide open or pretty close to it which would mean there is zero manifold vacuum and any power valve if installed would be open not closed since vacuum holds a PV closed not open.

Now I agree in staging it should be closed but this is where I get concerned with running a PV especially when "STAGGED", if you foot brake like I do while your torquing up the motor while STAGGED there is a chance that engine vacuum can drop enough to let the PV open or start to open which can cause a part throttle rich condition. Some people run a PV with a lower rating to try and combat this but this is primarily the reason I dont run one. Why take a chance on having a stumble or monkeying with my ET consistency?
Now if you run a box class you'll have it hammer down when stagged so it wont matter then.

JMO> Charles

[chevyart]

hey cepx111 that is the post that really answers my original post and especially my second post questioning the power valve being closed for burnout. my second post describes my setup. if i go with a power valve in front and use the tranny brake always over 3000 rpms, which power valve number should I use thanks chevy art

[cepx111]

Honestly since your almost over the threshold @ 3000rpm the 3.5 valve should be fine, but ultimatley you'll have to experiment to see exactly what the car wants so dont be afraid to make minute changes from weekend to weekend and see what works best for your particular set-up.

Note: I had a similar set up like yours on a 350 smallblock race only motor, Holley 750 dp with a proform center section, but I went with no power valves and had the carb jetted 82 square, worked really well for me.
Goodluck, Charles

[chevyart]

hey cepx111 thanks again for your last post. guess what. I got my carb form my buddy who really knows his stuff and it was e3xactly like yours except he had the jets 80 front and84 rear, no power valves. I was the rocket scientist who changed it to 74 with 3.5 in front and 84 in rear. so if i go back to his original 80-84 it will be just about the exact same as yours. do you think I should go 83-83 square, as most guys recommend making the jets the same. pretty sure i will just go without the power valve, aand go with what you suggest as my jet size. thanks a million to you and all the other guys who took yhe time to help me with this important part of my engine tune. chevy art

[cepx111]

Your welcome Art, anytime we can be of help dont hesitate to make a post.
Probably 83 square would be a good starting point if you go without the PV's, just keep playing with it going up or down on the jets 1 or 2 jet sizes at a time and see what makes the car the most consistent.
Note: make only one change at a time and always run what you brung making changes inbetween rounds is a no no unless its absolutely necessary.
I prefer to have mine square and a tad on the lean side.
If you bracket race like I do and especially if your race at night where it gets cooler as you get into the later rounds a fat or rich carb will give you fits trying to keep it dialed in because as the night air cools it becomes more dense which helps the engine makes more power so it will usually make the car really start picking up between rounds, but if you have it a tad on the lean side it doesnt do it as much.
To help with a possible off idle stumble on a carb thats alittle on the lean side I always used a 50cc acc pump on the primary only which will give you super crisp throttle response.

I hope this helps and goodluck, Charles

[Scooterz]

CPEX: I am confused about power valves too, but I have a very healthy street motor. I have a QF 950 with primary 78 & 3.5 PV. Can't remember secondary set up. Anyways, I want the motor to be a little better for street, because I will go to the track one or twice a year. The motor was in the car when i bought it & it is pretty healty... the trouble is, I just want a little better low end performance/ better idle w/ less loading. Do you think a vacumn advance would help? I don't have one now... its all MSD & 6AL. Would you mind telling me about low end efficentcey? Its ported & polished, 10.2 comp, Team G, BBC all roller w/ solid lift Lunatti. It has 3500 Stall & 488's. Any feed back appreciated. BTW- I drive it on the street occasionally as a toy only.

[chevyart]

hey cepx111 gao your last post.learned another thing, which is contrary to the way i always set up the carbs. you said to use the 50cc accelerator pump in the front, and i assume the 30cc(smaller) in the rear. is this all the time, or just when you set up the carb for one special setup. could you explain. thanks chevy art. PS this forum is a real trip. thanks for the ride art

[cepx111]

Yeah anytime you mix big cams and single plane manifolds a 50cc shot on the primary is almost manditory as with some squirter changes. Now putting one on the secoundaries can be a preference or a tuning aide, like I stated before I prefer a leaner carb set-up and thats why on that particular set-up I didnt go square on the pumps not to mention I was running a nine inch slick where traction was marginal at best and I didnt want to take the chance of braking the tires loose.
A double shot off the line just might make you go from a lean stumble to a rich stumble, so make little changes one at a time seeing what works and what doesnt, keeping waht works and removing what doesnt.
And dont make the mistake of trying to cover up something that doesnt work by changing something else, theres a happy medium to everything and finding it is the key to tuning correctly, again to your particular set-up.

Goodluck, Charles