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mechanical advance 250ci 


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Posted: Wed Jul 25, 2007 8:20 pm 
Getting Side Ways
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VIC, Australia

ahh the holley saga continues :roll: why not just throw on a pair of 750 dominators and be done with.

here is a simple widely used formula: CARB CFM = CU.IN. X RPMS divided by 3456 X VE%

formula explained below

One general rule of thumb uses a formula to determine the CFM requirements of your engine. It goes like this: You need to know the CUBIC INCHES of the motor. You also need the maximum RPMs the motor will be spun to. Finally you also need the VOLUMETRIC EFFICIENCY PERCENTAGE (VE%) of the engine. The first two items (CUBIC INCHES and RPMs), are relatively easy to determine. The engine VE% is another matter. If an engine could use all of the air it ingested, it would have a VE% of 100%. Many performance engines reach this level. Certain race engines can actually exceed this and reach a VE% of over 100% at certain points in their RPM range. Most production engines and most street performance engines have VE levels below 100%. In fact, stock, production, low performance motors will fall around 75%-85% volumetric efficiency.

So if you had a stock, low performance production motor of 350 cubic inches and you wanted to spin it to 5000 rpms max and it had a VE% of 80%, the formula would determine a required carb CFM of 405 CFM. If you had a warmed over street performance motor of the same size, but it was capable of 7000 max rpms and it had better heads, camshaft, headers and a performance intake that raised the VE% to 95%, the formula would give you a minimum required carb CFM size of 673 CFM.
Posted: Wed Jul 25, 2007 8:20 pm 
Getting Side Ways

Posts: 6449

Joined: 11th Nov 2004

cjh wrote:
tickford_6 wrote:
cjh wrote:

Because the Mechanical advance would still take it up there.
A 600 Holley works well on a 250 Xflow, with the right headwork and cam, and exhaust with the dizzy done right.

it's not realy the best setup. 600cfm is far to much. a 450 or if you want a vac second a 465 would be better. but even at %100 efficiency the engine doesn't need that much carburation. you might loose a few hp up top (like 3 or 4) witht he smaller carb but the midrange and low rpm manners will be far improved.

It was making over 170 RWHP with a 2.92 LSD & single rail and needed a bit more tuning to achieve more. Low down torque was not a problem, it would smoke 265/70 15's.
Also raced a BA XR8 Auto, twice, and won.

that might be the case but it's still twice the carburation the engine needs




Posted: Wed Jul 25, 2007 8:48 pm 
Fordmods Newbie

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Power: 203 rwkw

Location: mELBOURNE
VIC, Australia

pfft 600.... thats no where near bigenuf...
Posted: Wed Jul 25, 2007 11:40 pm 
Oompa Loompa

Age: 33

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Location: bathurst
NSW, Australia

i found the 500 has been a major improvment even though it hasn't been tuned yet. i don't think the 500 is to big as it isn't a stock 250.

flat top pistons 60th
street/race cam
huge valves
major port n polish
full balance ect ect. :lol:
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