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 Fresh 540 Log 
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Joined: Fri Feb 08, 2013 2:37 pm
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Ron Gusack wrote:
jmarkaudio wrote:
Ron Gusack wrote:
I thought if manifold vacuum was over 1.5 Hg at WOT the carb was to small and if it was under .75, it was to big. Am I way off base here? Was that ever the theory? If not, how the hell did I come up with that? Isn't 1.5 Hg the standard to measure carb cfm?

So this means having manifold vacuum logged won't tell me if the carb is sized right. Vaporization? In my quest to make the car consistent, I'm learning how little I know. Spending more and knowing less, what a world.


1.5" HG is the standard for a 4 barrel, that translates on the flow bench to 20.45" H2O. What the ideal vacuum for any engine will be depends on the efficiency of the induction, the efficiency of the combustion chamber (including the piston top), and how the cam profile interacts with them. An engine that has a well designed combustion chamber, an induction system that not only keeps the velocity high but also allows for equal distribution to the cylinders, and a cam profile that has the proper timing for the engine combination and RPM range will thrive with a lower WOT vacuum reading. The lower the vacuum, the lower the pumping losses and the more HP the engine makes, up to the point vaporization and distribution quality is hurt. It comes down to filling the cylinders equally and with a homogeneous mix of air and fuel, and then burning it at an even pace in the chamber.

Oh and one more, when you start pushing these limits the ignition quality becomes increasingly important, having the most energy available to burn the air and fuel can make a difference between having a fair amount of random misfiring or having minimal and making more power. Years ago we disconnected the MSD, running on points only, to see if a problem we had was with the MSD. The next pass the engine was popping, which a new set of plug cleared. The MSD was able to fire less than ideal plugs. A high output ignition and coil, and even a higher voltage battery can make a difference.

So I really have no way of knowing what would be an ideal manifold vacuum for my out of date engine. I have noticed that the cfm formula for my engines has always given me less performance than a larger than recommended carb. The formula would say X cid at X rpm and X ve would need an X cfm carb. Then I'd put roughly that cfm carb on it and slow down 2 tenths. Do an a-b-a and get the same results. I've done that many times over the years.

There is no “ideal” intake vacuum, except the lowest you can get and still make the large carb(s) work. Too bad there wasn’t intake vacuum information from the Carb Dyno Shootout.

The Carb vs. CFM formula was devised in the time when Holley and Edelbrock were beginning to market the single carb intakes and double pumpers in the early ‘70s to people they knew just wanted to bolt-on-and-go, plug-and-play, no worries mate…. The formula obviously doesn’t apply to dual carb intakes and they somehow make more power than single carb setups in spite of it.

A few pages after the carb selection CFM chart is a chart showing a pair of 660s on a tunnel-ram 350 SBC with 0.2” H2O at about 4000 RPM, and if you ever have been-there-done-that with that setup on the street or in a boat we all know it runs like a scalded dog.

Again, there is no “ideal” intake vacuum, except the lowest you can get and still make the large carb(s) run well, like you want.


Wed Apr 10, 2013 5:20 pm
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Joined: Thu Mar 28, 2013 6:09 am
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Mark,Bruce
When you guys did all the Dyno testing with E85 and gas did you log any vacuum during the session???


Wed Apr 10, 2013 8:43 pm
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Joined: Sat Feb 09, 2013 10:46 pm
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WE log vacuum whenever we can.

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Wed Apr 10, 2013 9:28 pm
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