Exactly. The PV is the on/off. The PVCR is the size of the passage and determines how much fuelit will flow. That's another cool thing about a lot of modern carbs as some include a replaceable 'jet' for the PVCR amongst many other things, which allows you to screw in a bigger or smaller restriction vs having to drill or worse yet, fill then drill.

Off the top of my head, and don't quote me, if you have a PVCR thats larger than .080", you'll need to get High Flow Power valves as the standard ones won't flow enough fuel and thus becomes the restriction, which it isn't meant to be.

Sounds right SB. I am at .086. I experimented with standard PV's (Pri and Sec). Gained .3-.4 AFR at WOT (leaned up) vs the high flows that were in it. Went from 6.5's to 4.5's. Got a hell of a lean spot to boot during transition. I definitely needed the high flows and 6.5's to open a bit sooner and overall flow the amount the PVCR's wanted.

Another thing I will note: When adding a good amount of PVCR (Above 90) I recommend staggering the PV's vac rating by 1-2". This smoothes out the transition rather than gaining all the extra fuel at once. In a drag setup who cares... Marine definitely good to have curves rather than cliffs. My 2 pennys

#1,10.5 power valve is not hard to get.jeg,s,summit and just about any speed shop has them.#2,as i said earlier,just changing the firing order has no bearing on the amount of vacuum an engine will have.bb,s statement[the more you swap the more vacuum it will pull]makes no sence to me.no matter what the firing order is if nothing else is changed the amount of vacuum prodeced will not change.please explain how the firing order control,s the amount of vacuum the engine will produce.SB,what is tour take on this?i am asking this because you are usually on top of stuff like this.

I stopped using those 2 window std power valves years ago.

Like you guys say, the pcvr restrction has a huge effect on fuel flow to the engine. The misconception that if you block a power valve , you need to add xxx jet sizes, is vague. Without knowing rhe pcvr total area, its just a guess. A .035 pcvr is tiny compared to a .097 restrction!

In regards to the firing order swap on a big block chevy,

Back when GM was doing development on the big block engine series, i wonder if they just decided 18436572 looked good on paper, or did they maybe do some testing on firing orders? Over the 40 years of development and improvements on the big block chevy, did not one engineer at GM, suggest a different firing order ? Its not like it would have required major retooling or anything.

I have never measured any vacuum readings on a LS firing order big block, but , id like to think if it offered some significant increases in idle quality, vacuum, etc , GM would have taken advantage of it many many years ago.

I know the 8.1 truck engines had LS firing orders, but they were also a symmetrical cylinder head too. Seems like most of the bigtime engine builders ive talked to about the ls firing order on a old school bbc, offers very little advantage if any in "most" applications

Im with mike tkach, as i cant understand why a firing order swap alone, would make an engine pull much more intake manifold vacuum? Its not registering

Sound sells many things. Think Harley Davidson and think "I want a heavy thump at idle." :smile:

I actually have a lot of info on the firing order swaps but I need some time to put it into better english than my attempt about an hr ago that I never posted because of brutal english flow. I'll work on it in a bit.

I agree the double swap cam does sound different. Can't comment on the vac difference (different engine). Just did this in my 632 in Camaro. Can hear the difference

https://youtu.be/hd9ygKmYZ2c

My engines have the c swap 5/3, 4/7 cams. I don't notice a difference in the sound. Maybe others can hear it. This video is the last one
https://youtu.be/cYe5ChvBpwY

By staggering the PVs are you talking about doing, for example, a 4.5 in the primaries and 6.5 in the secondaries?