The most important question has not been asked!!!!
Why is idle vac so low??????????????????????????????????
My guess: timing at idle is too low, & with that cam it will need 25-35*. If it is less than that, idle quality, & tip in performance will NEVER be as good as it could be.
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What? No sir. His cam is more mild than the one in my 340, he does not need 25° of
idle advance. Not even close. I do just fine with 18°, which is about where I need to be so my transfer slots are covered correctly. I doubt that minimum fuel consumption at idle is anyones goal here. Total timing advance with mechanical and vacuum all in is a totally different story.
I really want to know if you've ever tried to hot start a 440 with 35° of advance at idle. Because even when I was up at 22° on my 340 the basic denso starters aren't happy with a hot start. Which is a real pain in the *** for a street car. You've seen where people say to advance the timing at idle until the starter kicks back on a hot start I assume? Or does you book not mention that? Oh, wait, it literally says "though not commonly realized". Wonder why?
Wrong carb bud. No metering rods in a Holley. No step up springs.
And again, yes, you can add timing advance at idle which helps with a large cam. But that is not a cure all, there are limits and there is a balance. And really the OP's cam isn't radical at all. His in gear vacuum is a little low for what he's got but none of this is extreme or radical.
This carb came with the 64 jets, which I thought was fine since the original 4160 holleys came with the 64 jets. I do have jets ranging from 61-70, I also have the holley kit that includes different squirtter sizes, accelerator pumps, and cams. I do not have a stock of PV's (just a 6.5 sitting on the shelf)
Is there a way to determine when I need to go bigger/smaller jets? Besides reading vacuum, is there a way to determine PV sizes? Is there a section of the carb I should be adjusting first (jets, accelerator pump, pv)? I currently dont have an AFR gauge and dont have the means to add in the O2 sensors close to the exhaust manifolds.
Take a look at the Holley videos you linked and go from there. Based on your car, carb and cam my
extremely basic advice is that an 8.5 PV is probably a little too high, and the 64 jets are a little too small. I would try going down to a 6.5 PV, and up to 68 - 70 jets to start. If you decrease the PV rating you will need to increase the jet size, especially with a higher PV rating because it will be open quite frequently.
So for tuning, if you've got no throttle in at all, you're in the idle circuit. If you're in part throttle, you should be in the primary circuit for steady state. As you first tip in to part throttle from idle, that's the transfer slots. As you first add throttle from steady state, there's a pump shot in addition to the primaries. From part to mid throttle, steady state, with lighter loads you should be all primary. The PV should come in with acceleration or added load (as the vacuum drops), and bridge the gap between the primaries and the secondaries opening up. You've got vacuum secondaries, so they come in with further vacuum drop under load or heavier acceleration.
I would work on the primary jets first, since the primary jets will control most of your off idle mixture. Then you tune the pump shot and PV to deal with different levels of acceleration and load.
To me, a power valve shouldn't be selected based on a vacuum reading. It should be selected based on when the engine wants the extra fuel like Carnut said in post #18. There's no magic number since it's dependent on so many external factors - vehicle weight, gearing, etc.
This is true, but the generic estimate of using the idle vacuum helps with putting you in the ball park for the cam you have. If you have a lower idle vacuum than "standard" you will likely need to bump the PV rating down as well. If you have a fairly stock engine than the out of the box 6.5 PV should be close. And then you drive the car and determine where the PV is coming in and adjust from there.
I have a nice uphill section of freeway I use, plus my AFR gauge, to figure out where the PV is coming in. It's a bit easier though for me because I have mechanical secondaries, so, I know the throttle position when those start to come in.
So I guess all these people who have ethanol friendly carburetor kits should throw them all away? lol
Good grief man.
It doesn't matter whether the kits are "ethanol friendly" or not. My point was that the stoichiometric ratio for alcohol and gasoline are totally different. So the jet sizes, PV's etc are all very different from gasoline to ethanol. Stoichiometric for gasoline is 14.7:1, pure ethanol is 9:1. E85 is ~9.9:1.
There's a reason that most Holley's come with a 6.5 PV, because most gasoline engines will work well with that PV. If you have a more aggressive cam, then you need to LOWER the PV rating so it's not open more often than it should be.
If you have to get your PV out of an alcohol kit because you're running a 12.5 PV, it indicates you're not using the PV and jets as intended. With an average cam and compression ratio with gasoline you should be looking at a PV below 8.5-9, which is literally what Holley says, you can follow the links the OP posted above if you want. And I dunno, they build the carbs, so they might have a decent idea on that. And really, unless you're doing something really radical a couple steps in one direction or the other from the off the shelf settings should cover it. So if you need to double the off the shelf settings, well, either something isn't right or your combination is really radical. Which doesn't apply to the OP really, his combo isn't that crazy.
