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Cruise vacuum vs. cam duration? #2728384
12/30/19 07:49 PM
12/30/19 07:49 PM
Joined: Apr 2006
Posts: 3,749
West Plains, MO
DrCharles Offline OP
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DrCharles  Offline OP
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West Plains, MO
While I wait for my Dart's rear end rebuild, and I'm out of town anyway, I was wondering about cruise vacuum readings since I plan to start playing with power valves and PVCR diameters.

My 451 has a 272 @.050 mushroom cam, and idles at 7" vacuum at 1050 rpm. It'll be turning about 3000 rpm at 60 mph (295/50R15 tires, 3.91 gears).
Any idea what vacuum will be in a level cruise at 60 mph? work
Or is there no way to predict it, other than actual measurement?

Re: Cruise vacuum vs. cam duration? [Re: DrCharles] #2728403
12/30/19 08:37 PM
12/30/19 08:37 PM
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Benton, IL.
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DaveRS23 Offline
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Benton, IL.
I'll start; 15" +/-.

But the most important thing to figure out right from the start is whether you cruise on the idle circuit or on the main circuit. That, even more than the cruise vacuum, will be the most important factor.

If it cruises on the main circuit, you will have a much easier time dialing in the low speed transitions. But if it cruises on the idle circuit, you will have a bunch of compromises to hash out.

Most of mine end up cruising on the idle circuit. Which means I need the PV to open early, so I usually have a 10.5 PV.


KOS
Re: Cruise vacuum vs. cam duration? [Re: DaveRS23] #2728409
12/30/19 09:05 PM
12/30/19 09:05 PM
Joined: Apr 2006
Posts: 3,749
West Plains, MO
DrCharles Offline OP
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DrCharles  Offline OP
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Posts: 3,749
West Plains, MO
A good point. I do have a wideband AFR gauge on the dash, so I can try jetting down the primaries a bunch and see if the cruise AFR goes up or stays about the same... scope
It's an old 4780 (800 double pumper, 2 corner idle) which I already know is a bit undersized. When I get tired of screwing around with it and still not getting optimal driveability, I'll send my twocents to Dom and have him build me the right carb to bolt on and go!

I already know the PV is way off. I think it's a 4.5 or even a 3.5, because I listened to the oft-repeated (and wrong) advice of "half the idle vacuum". Two inches below the cruise vacuum, or at least a 10.5, seems to be a much better starting point from my researches...

Re: Cruise vacuum vs. cam duration? [Re: DrCharles] #2728487
12/31/19 10:10 AM
12/31/19 10:10 AM
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Benton, IL.
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DaveRS23 Offline
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That 800 is probably undersized for your combo so it is probably going to cruise on the mains. The Double Pumpers are competition calibrated carbs, so the cruise will be fat. You may start by stepping down 4 sizes on the mains to get a feel for how far you will need to go. And if it does cruise on the mains, the PV opening point will not need be as quick as it would if it cruised on the idle circuit. But having said that, I personally like an early opening PV. Better a bit rich that a bit lean on the transition. No stumble or hesitation that way.


KOS
Re: Cruise vacuum vs. cam duration? [Re: DaveRS23] #2728495
12/31/19 10:48 AM
12/31/19 10:48 AM
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Posts: 965
Ohio
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jlatessa Offline
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Conversely, should you change PVs until you get a stumble at tip-in then go back one step?
This without an O2 sensor.

Thanks, Joe

Re: Cruise vacuum vs. cam duration? [Re: jlatessa] #2728500
12/31/19 11:34 AM
12/31/19 11:34 AM
Joined: Jan 2003
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Benton, IL.
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DaveRS23 Offline
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That sounds good in theory. And would work any time the weather conditions are the same as when that tune was done. But the reality is that trying to get that sharp of a tune for a car that will be driven in different weather conditions will mean that sometimes it will probably be too lean. And sometimes too rich. But our engines are much more tolerant of a rich condition than they are of a lean one. So to be ready for the varied atmospheres that will be encountered, the transitions will need to, unfortunately, be a little bit on the rich side.

Unless the OP wants to adjust his tune to the weather conditions on any given day.


KOS

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